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JWK ([email protected])
6/22/2002 9:26 PM
Need Phase Inverter Advice
I have one last step for rewiring my Traynor Mark2. Figuring out what PI to use. I've been doing load line plots all
afternoon with a 12AT7 chart using plate resistors of 47k, 33k, 27k. I followed Aiken's article but I have no idea
why one would choose the 27k load with higher bias current over the 47k load with much less current. I don't know
how to choose the value of the grid bias resistors. The Traynor Mark2 I have has a 12AX7 Marshall type PI except
for the 47k grid bias resistors. There's two in series on the input side and one on the other side. Huge difference
from the usual 1M. Then there is the 370k grid bias resistors on the Fender PI that uses 47k plate loads on the
12AT7 SF amps.
I'm going for clean without sounding sterile. I'm not having a presence control. I will have a pot for the NFB loop.
Cut control Vox style.
I've never seen an amp with 27k plate resistors on a 12AT7, but that doesn't mean it's not a good idea. Of course
then it might not be a good idea at all. I need some advice. Thanks.
John
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kg ([email protected])
6/23/2002 7:32 PM
Re: Need Phase Inverter Advice
hey john,
are we talking about a LTP type splitter, or concertina, or what? you didn't say...
are you bent on using 12at7? it IS a better driver tube...
how about the output stage, what are we talking here? 2xel34? what kind of bias voltage are you planning on
needing for the outputs?
gimme more info and i'll take you through the design choices...
ken
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JWK ([email protected])
6/23/2002 11:19 PM
Re: Need Phase Inverter Advice
Sorry. It was late and I was frazzled and didn't realize how little information I gave out.
Yes, typical Fender/Marshall type LTP. The circuit is pretty much a copy of a Marshall plexi with some slight
changes in component values in the tone stack and LTP (the 47k grid bias resistors).
Two EL34 tubes. 400-0-400 PT. Schematic says 540v B+ to the OPT. I might get a little less since I use R.G.
Keen's vintage voltage adaptor. With that I usually get around 115VAC. I don't know what that would do for bias.
Maybe -18, -19? The schematic says there's -46 at the bias pot.
I gather that the 12AT7 is the better driver tube, I just don't know why. I don't get how more current can help drive
the output tubes when all that great dc current is taken away by the coupling capacitors. My ignorance strikes
again.
I'm trying to stay away from "crunch" without getting anywhere near "sterile".
Thanks for the help.
John
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Greg Simon ([email protected])
6/24/2002 9:10 AM
Re: Need Phase Inverter Advice
I've often wondered on some of this too. They say a preamp is a voltage amp, and then the power amp is acurrent
amp...and a phase inverter needs to be able to provide lots of current to drive the power amp?
Well what is it that makes a particular circuit amplify current instead of voltage or vice versa? Is it the component
arrangement? Some magic?:)
I also don't really know how to calculate some of these things in the amp either to decide on which value of parts
to use. I have all the books I can find on tube audio, but many are too math intensive still (O'Connor's stuff)or
doesn't explain it in a way I can understand, or misses some tidbit of info that I need to understand it!
My project is probably too complicated too for a first project, but I'm not giving up on it now! My project is taking a
Conn power amp chassis with 3 power amps in it, (2x6L6, 2x7868, 2x6L6, each with it's own output transformer
and phase inverter) and building 3 seperate, footswitchable preamps to match. It will be really cool when I'm
done!!!...(if I ever get done)
Anyway, I'd love to know more if anyone cares to enlughten me on the aformentioned questions among others.
Thanks!
Greg Simon
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kg
6/24/2002 2:08 PM
ok, here's the way you design it from scratch. (long)
john,
first off, you're not THAT ignorant... you've got some good ideas here! :)
you're right in that the DC blocking caps prevent the output stage's grids from drawing any DC from the driver
stage. still, there are advantages to a robust driver.
the at7 makes a better driver because of three reasons: a) it has a lower plate impedance, which means that it will
lose less voltage when loaded down by a lower load impedance, b) it is capable of higher current operation than
an ax7, and c) it is capable of higher plate dissipation.
as far as advantage a), the lower plate impedance, with an at7 driver you can use lower value grid return resistors
on the output stage, and not lose a bunch of signal voltage. it may be helpful to think of the voltage source model
of a tube, where there is a perfect voltage source equal to mu*Vin, which is in series with a resistance equal to rp.
the load is connected from the "top" of the modeled rp to ground. clearly a voltage divider is formed between the
rp and Rl. historically the ideal is to make Rl at least 10x rp, which keeps you in the safe area. the at7's rp is
around 15k, whereas the ax7's is around 60k, or about 4 times higher. for best performance from an ax7, you
should load it with at least 60k*10 or 600k... for the at7 that drops down to 150k, which is much better.
the advantage of the lower value grid returns say 150k vs. 600k, is that your output stage will be more stable and it
will also be quieter.
advantage b) is helpful to prevent slew rate issues. the slew rate is the required voltage swing per unit time. the
required current to prevent slew rate limiting is a function of signal frequency, signal amplitude, and driven load
capacitance... as any of those three factors goes up, so too does the required current to prevent slew rate limiting.
to boil it down to a nutshell, all capacitances must have a current flow into them before the voltage across them
changes. since the grid has an associated capacitance, signal current MUST flow into and out of the grid in order
for the voltage on the grid to change. the driver stage must supply this signal current to prevent slew rate limiting.
since the driven capacitance is part of the current equation, when driving pentodes or BTs (which have very small
Cgp, and thus very small miller capacitance) the slew rate is rarely an issue. triodes, otoh, have a considerable
Cgp and thus a significant input capacitance. the way to account for this is to have a decent amount of current
flowing in the driver stage, say at least a few mA--to some extent, the more the better. the at7's 15mA max plate
current is better suited for this task than is the ax7's 8mA ip max. (for more info on this, see my webpages.)
this leads us into advantage c) which is the higher plate dissipation. if you're running the driver tube at a higher
current, to preserve the signal headroom you will also need to run at a decent plate voltage. the product of the two
is plate dissipation, and the at7 gives you more leeway here than the ax7 does--2.5w vs. 1w.
now, if you want a clean amp, you want to design the driver stage "right," which is to say for maximum undistorted
signal swing and lowest output impedance. the above design goals are a step towards that ideal. to put it into
practice, let's start using some math.
i suggest you derive the driver's b+ node from as high a voltage as you can. the absolute maximum b+ for the at7
is 550vdc, which gives you room to play with. let's assume a 500vdc supply.
let's assume that you want to pull 10mA of current through the at7. seems like a lot, but it will give you much better
performance that way (remember, gm goes up and rp goes down--two good things--as ip goes up).
given the at7's max Pa of 2.5w, let's derate that a bit for tube longevity... say down to 2w. that gives you a max
Vak of 2w/0.01a or 200vdc.
now, we know that we're drawing 10mA per section through the at7... and our b+ is 500vdc, and our Vp is 200vdc.
that means 300vdc @ 10mA is being dropped across the plate load resistor... ohms law gives us 300vdc/0.01a or
30k. closest match is probably 33k, which is fine.
that 33k resistor will be burning off 300vdc*0.01a or 3w of DC power. add to that the AC power dissipation and
you're looking at at least a 5w rated resistor, maybe 7w if you like things conservative.
to determine the effective grid bias necessary on the at7, we need to consult tube data. we have an ip of 10mA
and a vp of 200vdc:
there's an operating point given that is vp=200vdc and ip=11.5mA... that's close enough. the slightly smaller
expected ip will just mean a slightly more -ve bias voltage.
the given bias voltage on the data sheet for that operating point is -1vdc. you can expect your grid bias to be very
close to that--let's say -1.1vdc.
if you're sharing a cathode resistor (which you should be) then you have the plate currents of both tubes pulling
through it. that means 20mA of current. in order to generate say 1.1vdc out of 20mA of current you need to have a
resistance of 1.1vdc/0.02a or 55 ohms. this is your shared value of Rk, common to both halves of the at7.
unfortunately this is a low value--too low to rely on to provide enhanced current sharing and phase splitting.
ideally at this point you would have a -ve bias rail... this will allow you to avoid having to float your whole splitter up
on a larger tail resistor to enhance the phase splitting. for example, if you had a -100vdc rail, you would size the
shared Rk the exact same way, except that instead of dropping 1.1vdc across it, you would drop 1.1+100, or
101.1vdc across it. your new value for shared Rk would be 101.1/0.02, or about 5k, and it would connect both
cathodes to the -100vdc b- rail. this would make the splitter much more accurate, since it would more closely
approximate the ideal current sourced tail of a long tailed pair.
the advantage to the +ve and -ve supply is that the grids of the splitter remain referenced to ground... iow the two
grid return resistors go from g1 to ground, and you don't need to cap couple to preceeding stages (if they're
ground referenced, of course). NOTE: a quick little rule of thumb... if you've got a symmetrical +ve and -ve supply,
i.e. +500v and -500v, you can simply make the shared tail resistor half the value of the plate load resistors and
everything works out perfectly.
now, let's assume you DON'T have a -ve rail handy that will handle 20mA, so you have to float the splitter up off of
ground. in this case, you will be eating into your voltage headroom a bit, so you'd better have the volts to spare.
500vdc of b+ is plenty.
let's assume you'd like to stick with the 5k of shared cathode resistance from the above run-through. now, you
lose the ground reference to the splitter's grids. you keep the 55 ohm shared Rk. at the bottom of the 55r shared
Rk comes the 5k to ground. the grid return resistors for the splitter come back to the junction of the 55r and 5k
resistors. you will need to capacitively isolate the grids from the rest of the circuit so that their DC voltage is
preserved.
just as before we are losing about 100vdc across that 5k resistor, except instead of the volts coming from the -ve
supply we are now cutting into the single, +ve supply. this means we must account for that in the plate load
resistor's value... whereas before they were each passing 10mA and dropping 300vdc, now we want them to drop
200vdc @ 10mA. their new value will be 200vdc/0.01a, or 20k. probable closest match is 22k, or you could series
up two 10k resistors for each plate load.
either one of the splitters (double supply/single supply) will provide excellent performance and more than enough
voltage swing output to drive a pair of el34/6l6 tubes to clipping. try to keep the grid return resistors for the output
tubes about 10x the rp. (if you look back at the operating point of 220vp, 11m5A on the datasheet you will see that
the rp is a nice, low 10k5. that means you could go with grid returns as low as 100k and hardly lose voltage.)
sizing the coupling caps to the output tubes follows the familiar F = 1 / 2 pi r c formula for an LF breakpoint. you
will notice that the one bad thing about low value grid leak resistors is that the coupling capacitance must be made
larger to achieve the same LF -3dB point. as diy, we don't care too much about that.
hopefully i have provided enough detail and methodology so that even if your particulars are different you will be
able to go back and substitute values more applicable, and get useful results. let me know if anything is unclear or
needs better explanation.
ken
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Dave James ([email protected])
6/24/2002 3:34 PM
Re: ok, here's the way you design it from scratch. (long)
Ken,
Thanks for the info to John. I found it very interesting!
After a many, many month hiatus from my amp projects (and Ampage), I've been thinking about using triodes with
a lower inherant Rp for the tone-stack driver and pre-driver to the power tubes.
I played around with Mr. Duncan's tone-stack calculator, inserting Rp values for the 12az7 tube and noticed an
improvement in overall gain through a typical F-type circuit - makes sense. And now after reading your 12at7
discussion, I understand why the 12at7 has been used in past designs.
Thanks again,
DJ
From:
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JWK ([email protected])
6/25/2002 8:30 PM
Homework done - Now some questions
Ken,
Thanks for the great information. I printed it out and between that and the Aiken article I learned a lot. Still some
missing pieces in my mind, but I got enough to figure the phase inverter circuit.
I figure on 500v B+. That's dropping about 30v or so from the screen B+ supply (more on that later). I'm shooting
for about 6mA bias current. The Fender BF PI gets about 2mA and the SF circuit gets about 4mA - that's both
sides counted in. I figure shoot for 6mA. 34k plate resistors (two 1 watt 68k wired in parallel to give me a 2 watt
34k - is this right?) I think about 370v should be good, so 130/.012=10834. Call it 11k. That's the tail resitor. I need
about a 220 ohm to get 6mA of current at 370v, so a 110 ohm bias resistor for both sides. From the data sheet it
looks like a 150k grid return resistor looks about right.
Is this correct so far?
To capacitively isolate the grids, do I put the caps in the same place as they are in Fender/Marshall circuits? ie.,
on the input it's from the signal source and wired to the grid along with one end of the grid return 150k resistor. On
the other side it will be wired to the grid with the other 150k resistor, the other end being wired to the ground side
of the tail resistor. Correct?
Is the feedback resistor part of the "tail" resistance? On Fender this is a 100 ohm, on Marshall this is the 5k pot.
As I understand it, these proportion the amount of NFB with the resistor wired to an OPT tap. I'm wondering if on a
Marshall this makes the tail resistance 15k (the 10k tail resistor and 5k pot presence control are in series to
ground) and on a Fender 22k100. I'm guessing it has to be factored in, so my tail resistance would really be
11k100 (I'll use the Fender feedback circuit) which will give me about 365v supply instead of the 375v. Correct?
On an unrelated note, I need to know what the hell to do about the extremely high screen voltage I'm going to be
stuck with. Originally the circuit had a 470 ohm dropping resistor from the plate supply, filtered with 40uF, then
another 470 ohm resistor common to both screens. With the low current draw of the screens, still about 525v B+
according to the schematic which pretty much agrees with my own calculations. Installing a choke and putting on
individual screen resistors will bring the voltage up even closer to the plate voltage; 530v - 540v. The RCA manual
says 425v is max for EL34 and 450v is max for 6L6GC. I know we stretch it in guitar amps, but that's a lot. These
amps were originally designed around 7027A tubes, but they did run off of 6CA7 for the later years. I'm planning
on buying a pair of JJ E34L or JJ 6l6GC tubes. I have no idea what to do here. What can I do to go easy on the
tubes? Will this burn them out fast? Even with 1k5 resistors there's hardly any voltage drop. Suggestions/advice?
Thanks a bunch for the help. If the circuit I planned looks good and the screen voltage issue is resolved, I'll get
those parts ordered and get myself a working amp!
John
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stephen conner ([email protected])
6/26/2002 9:58 AM
Screen voltage issues
Your screen voltage is very high indeed for EL34s. This is by no means uncommon, a lot of old Marshalls use high
screen voltages like this, and the tubes basically last about a week. (That's why Jimi Hendrix changed to 6550s)
But what's important is the screen power dissipation, screen voltage*screen current. The screen current is low at
idle and increases as you drive the amp harder. By putting in a larger screen resistor, even though you don't see
the effect at idle, when you crank it to 11 the screen supply will sag more and this will limit the dissipation. Of
course when you're dimed the whole power supply sags considerably which helps too.
You can check your screen dissipation by looking through the holes in the plate structure, if the screen wires glow
a dull red when you're cranking full volume, no problem, but if they go orange or white you're in trouble. You need
to increase the screen resistor, or...
If you are feeling real smart you might want to do what Hiwatt and Traynor did and connect the EL34 suppressor
grid to the negative bias supply instead of ground. It supposedly sucks up electrons that would otherwise ricochet
off the plate and hit the screen, thus reducing the screen current.
Tone Lizard amps have a schemo for this mod plus more tips on keeping your screens happy, in their Marshall
Myths section.
http://www.tone-lizard.com/lesson6.htm
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JWK ([email protected])
6/26/2002 2:21 PM
Re: Screen voltage issues
I never understood the rumour of Jimi changing to 6550 tubes for this reason. According to the RCA Tube Manual,
the voltage rating on 6550 screens is even less than EL34 (400v as compared to 450v).
Because of the Traynor layout, hooking up g3 to bias voltage is extremely easy. I just don't understand why this
would help the screens. The *only* possible reason I can see is that excessive screen current is, as you
suggested, caused not so much by the high screen voltage itself (after all, you can bias lower, right?) but from all
the electrons bouncing/escaping electrons from the plate. Maybe the high screen voltage accelerates the
electrons even more, exacerbating the problem that always exists but perhaps doesn't cause "problems" until a
certain percentage of escaping electrons is reached. Since many of us measure the current flow with the help of a
1 or 10 ohm resistor on the cathode, we only get the total tube current. We assume the proper screen current but
are unaware of disproportionate amounts. Not a problem with amps that have a B+ supply of 450v or less, but
becomes more and more of an issue as the B+ supply climbs above the recommended max voltage for the
screens. Just trying some intuitive logic with what I've learned so far. I'm hoping someone can jump in and verify
or correct this guesswork.
Thanks for the spark.
John
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phil.h
6/26/2002 9:06 PM
Re: Screen voltage issues
John and Steve,
Im running my el34s(mullards) screens at 525v straight from the OT CT via 1k resistors.
And while im ready to admit I havent got it sounding how I want yet. I cant say its been a reliability issue....yet.
Im running them fairly cold, (10.5w plate dissapation each.)which could be a factor.
Also I dont crank it that much.
Beacause it sounds like s**t.:D
cheers phil.h
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stephen conner ([email protected])
6/27/2002 12:50 PM
holy smokin screens batman
Hi guys
I did some work on this screen voltage issue when I built my first EL34 amp. To cut a long story short, I was using
a regulated 475V plate supply with zero sag, which treated the tubes pretty hard. Running the screens off this
same voltage with a 1K shared screen resistor gave me 75 watts before clipping out of two EL34s and the screens
glowed completely white hot. I thought this would wreck the tubes, so I cut the screen voltage down (with a
separate screen regulator) to the 360V recommended in the EL34 datasheet for a 50 watt application. That gave
me 55 watts before clipping.
I later wanted to try the amp with 6L6GCs. At the 360 screen voltage I only got 30 watts so I put it back to 475.
That gave me 55 watts with the new tubes. I put in a screen voltage switch so I can run either tube type. (I'm
thinking about a plate voltage switch too so I can run EL84s and 6V6s)
http://homepages.strath.ac.uk/~cnbp111/toaster.html
Steve C.
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JWK ([email protected])
6/27/2002 7:39 PM
Re: holy smokin screens batman
Good one. I wish I understood mosfets, zener diodes and all the tricks that go with them just a little bit. I went to
your site again and looked up the regulation schematic for your toaster. By comparing it to the "mosfet follies"
description on how to reduce B+ using a mosfet in conjunction with a zener diode, I can't help but think with a bit
more knowledge I should be able to use this for just a screen regulation. Keep the 530v B+ for the plates but
regulate the screens down to 450 or so. I would think that paired with the negative bias voltage on g3 would give
me a clean type amp with long tube life. Tried getting some stuff out of O'Connor's TUT2 but couldn't follow it. If
you have any hints on how to do this, throw them my way. Cool looking amp. I'm envious.
John
From:
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Dave H. ([email protected])
6/28/2002 11:36 AM
Re: holy smokin screens batman
On an unrelated note, I need to know what the hell to do about the extremely high screen voltage I'm
going to be stuck with.
If you don’t want to go to the trouble of making a regulated screen supply you could try this. The B+ is 530V. If you
add a 100V 5W zener in series with each screen it will knock the screen voltage down to 430V without the extra
voltage sag and loss of power you’d get by increasing the value of the screen resistor to drop 100V.
Dave
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JWK ([email protected])
6/28/2002 5:25 PM
Re: holy smokin screens batman
That would be a great, short term solution for using the EL34 tubes. So if I understand this, the screens would
track 100v less than the plates. The plates sag to 510v under some heavier load, the screens are at 410v, etc. Is
this correct? Another question would be why it needs to be a 5 watt zener. For one, I can't find 5 watt zeners in the
Mouser catalog with anywhere near this voltage. Would three 1 watt 39v zeners in series equal a 3 watt 117v
zener? That would be cheap, fast and easy for the short term. Thanks for the info.
John
From:
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Dave H. ([email protected])
7/1/2002 8:44 AM
Re: holy smokin screens batman
So if I understand this, the screens would track 100v less than the plates. The plates sag to 510v
under some heavier load, the screens are at 410v, etc. Is this correct?
Yes, it’s not as stiff as a regulated supply. It will track the plate voltage.
For one, I can't find 5 watt zeners in the Mouser catalog with anywhere near this voltage. Would
three 1 watt 39v zeners in series equal a 3 watt 117v zener?
I checked the Ampage tube data for EL34s and it says that they can have a screen current of 44mA (two tubes) at
max output. 22mA x 100V = 2.2W so two or three 39V 1W zeners in series would be OK.
Dave
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stephen conner ([email protected])
6/28/2002 12:49 PM
voltage regulation
If you like that Music Man kind of sound then regulating your screens is the way to go (but then I would say thatwouldn't I) You can use the circuit I showed for screen regulation, but it's a bit more complicated than it needs to
be. Here is a description of the absolute simplest screen regulator it's possible to make using one MOSFET and a
bunch of zener diodes. The zeners only carry a few mA so they don't need to be high-power: the ordinary 0.3W
(IIRC) size will do. If you're feeling retro you can replace the zener diodes for gas-tube regulators.
1. First get your reference voltage. Make a series string of zener diodes that adds up to the screen voltage you
want. (eg for 450V: four 100V zeners and one 47V) Connect one end (anode end) to ground and the other to your
original screen supply via a 47K 2W resistor. Put a 0.47uF 630V plastic film capacitor across the zener string. You
now have a stable 450V reference.
2. Now you buffer this to form your screen supply. Find a suitable power MOSFET (look in your digi-key or mouser
catalog for a 1 amp N-channel mosfet rated 600 volts or more) and connect the drain to the original screen supply.
Connect the gate to the hot end of the zener string (see step 1) via a 100 ohm resistor. (This is the gate stopper
resistor which kills RF oscillations) Also connect the gate to the source via a 12 volt Zener (cathode to gate)
Connect your screens to the source pin via the appropriate screen resistors.
Steve C.
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JWK ([email protected])
6/28/2002 5:42 PM
Re: voltage regulation
Stephen, thanks for the great info. I printed this out for future reference. I don't really know if I like the Music Man
kind of sound or not. I've never played through one. I only know them by reputation. Right now my main goal is to
get this up and running quickly while still being kind to the tubes. From what I've gathered in the past few days it
would seem that pentode operates best under a regulated screen supply. Whether this is best for guitar or not, I
don't know. One of my future projects is to use a tranny set I have from an older Mark2 with the even bigger OPT
from the 7027A days. This will be a KT88 amp with 6DS22 rectifier tubes. I might use screen regulation for that.
This Traynor I'm working on now is not intended to be permanent. I need something to play through so I can work
on my other projects and do them they way I really want, rather than constantly compromising with layout,
voltages, transformers, etc. This will give me the breathing room to plan out my own layouts and have my own
chassis made.
Thanks for the info and help.
John
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JWK ([email protected])
7/2/2002 1:00 AM
I have it - almost
1. The 12 volt zener that goes from the gate to the source. Is that directly on the gate or on the 100 ohm resistor
which is wired to the gate?
2. There will be a choke after the plate supply. Do I put a filter cap on the choke connection or do I just wire that
side of the choke to the drain and put the filter cap on the source as shown in O'Connor's TUT1? Or both?
These are the two biggest questions I have right now after going over this in TUT1 in comparison to your
description. They both match up well but O'Connor does leave a lot out. It's more of an over-view and not
specifically for a screen supply. It's more geared for setting up the preamp voltage for lack of hum.
John
From:
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kg ([email protected])
7/2/2002 11:55 AM
Re: I have it - almost
the gate is very sensitive due to the high slope (gm) of the mosfet--therefore its only attachment should be its
stopper to prevent oscillations.
as far as ? # 2, i'm not sure i know what you're asking.. is it the placement of the mosfet for screens/preamp
filtering, before or after the choke from plate b+ node?
personally i would rip the choke right out of there and use it somewhere else. the mosfet series pass element is
more effective at quenching hum and improving regulation.
ken
From:
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stephen conner ([email protected])
7/2/2002 12:19 PM
Re: I have it - almost
1. The 12 volt zener that goes from the gate to the source. Is that directly on the gate or on the 100
ohm resistor which is wired to the gate?
I always put it directly on the gate. That way the resistor can limit current spikes which might otherwise blow the
zener. As KG suggested you need to keep the lead between gate and zener short- mount the diode right on the
MOSFET pins- otherwise it'll negate the effect of the stopper resistor.
2. There will be a choke after the plate supply. Do I put a filter cap on the choke connection or do I
just wire that side of the choke to the drain and put the filter cap on the source as shown in
O'Connor's TUT1? Or both?
If you use the choke (and KG is right- it's not needed- use it for your preamp supply instead) there must be a filter
cap on the choke connection. If you leave it out, first time the screen current changes rapidly, the choke will hit the
MOSFET with a huge kickback and exterminate it. You can put a filter cap on the source too, but it's not really
necessary- the regulating action gives the effect of a gigantic filter cap already. In my circuit I used a tiny 4.7uF
cap between source and ground.
Steve C.
From:
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JWK ([email protected])
7/2/2002 4:31 PM
OK, no choke required - final draft
No choke required for the screens since the regulator does the job. I won't bother with the choke for the preamp
since PSUD2 tells me my 7 henry choke with a 22uF cap does the same job as a dropping resistor with a 50uF
cap. Voltage ripple p-p of about .22V.
The plate supply and the voltage regulator will be wired to the same input cap.
I'm not perfectly sure about this, but from what I can gather from Stephen and kg's response is that the gate
stopper resistor will be wired directly to the mosfet gate pin and the 12v zener (static electricity protection
according to O'Connor) will be directly wired to this resistor with the "hot" side of the 375v zener string. The anode
side of the 12v zener will be wired directly to the source pin on the mosfet. A 450v 4.7uF cap will be on the source
to ground. Forgive me for going over this in this manner, but I'm strictly painting by numbers with the regulation
circuit. I have no idea right now how the zeners and mosfet are doing what they are doing. I'll have to look up the
basics of SS components in the near future.
I've received the tubes I ordered along with half the other parts (preamp caps and resistors, power supply stuff,
etc.). I'll go ahead and order the rest of the parts unless someone sees a problem.
Thanks for all the help.
John
From:
Date:
Subject:
kg
6/27/2002 8:10 PM
Re: Screen voltage issues
jon,
hooking g3 to b- rail will probably only help your screens IF it lowers your bias current. in other words you could
probably net the same effect by increasing -ve vg1 bias.
see these links to some scanned pages from spangenberg:
http://ken-gilbert.com/spang/spangenberg_vacuum_tubes--page_269.jpg
http://ken-gilbert.com/spang/spangenberg_vacuum_tubes--page_271.jpg
http://ken-gilbert.com/spang/spangenberg_vacuum_tubes--page_273.jpg
as the text points out, if too -ve a voltage is applied to g3 you MAY actually INCREASE ig2 due to a virtual
cathode formation in the space between g2 and plate.
hth
ken
From:
Date:
Subject:
JWK ([email protected])
6/28/2002 3:37 AM
Re: Screen voltage issues
Thanks for posting those. I saved them and went over them a few times. An awful lot of it I don't understand, but I
grasp the basic concepts.
The way I see it, I can "help" keep the screens comfortable with a lot of voltage sagging type circuits. A
combination of Weber's copper cap, higher screen resistor values and the 47 ohm resistor on the EL34 cathode
trick @ AC50.
But that wasn't the goal. Nice clean amp, lots of headroom/dynamics. NOT cold biasing with squish. I found a
schematic of a Musicman. EL34 tubes. 700v B+ on the plates and 350v for the screens.
I gotta find a way of understanding the voltage regulation section in TUT2 or this isn't gonna work for me.
Thanks again for the info.
John
From:
Date:
Subject:
JWK ([email protected])
6/28/2002 3:49 PM
kg - Need KT88 advice
I have three unmatched JJ KT88 tubes I bought for my future SE amp. I'm putting in separate bias pots in this
Traynor, so I might as well use them. It's a great solution electronically, but maybe not logistically.
With the present tube sockets the KT88's will be 3/16" apart. Because of the chassis layout the 12v fan I will have
to mount will be 3/4" away from the tubes. The fan will be running on rectified filament voltage, so somewhere in
the 8v - 9v range DC.
The only other option is to drill a new hole which opens up another batch of layout problems.
I know your KT90's are close to each other, but I don't know exactly how close and how close those two fans are.
What do you think of these physical operating parameters? Any problems?
John
From:
Date:
Subject:
stephen conner ([email protected])
6/28/2002 4:09 PM
Re: kg - Need KT88 advice
It shouldn't be a problem- the 50-watt Traynor circuit will only run the tubes at about half power, hence half the
heat dissipation. The minimum mounting distance quoted in the KT88 datasheet assumes the tubes are running
full power. If you've a fan blowing on them then I'd say no worries.
Steve C.
From:
Date:
Subject:
JWK ([email protected])
6/28/2002 5:58 PM
Re: kg - Need KT88 advice
But I don't have the 50 watt Traynor circuit anymore ;) The data sheet I read over on the Triode Electronics site
says a KT88 with 560v on the plates and 300v on the screens gets you 100 watts. I think that 540v on the plates
and close to that on the screens will be well over 50 watts. Traynor claimed 80 watts for bass. The OPT is bigger
and heavier than most 100 watt guitar amp OPT's I've seen, so I'm guessing... :)
I think you're right about the air moving over the tubes. A constant flow should keep things OK, but kg has first
hand experience with this type of set up so I thought I would ask.
Mostly I'm concerned about the fan causing noise electronically. 3/4" is awfully close.
John
From:
Date:
Subject:
kg ([email protected])
7/1/2002 12:22 AM
Re: kg - Need KT88 advice
hey john,
i run two 12vdc muffins @ 16vdc very close to the kt90s, which are actually touching each other in a couple of
spots. remember that electrically the power output tubes are pretty hard to disturb.. they're reasonably low plate
impedance and the gain is low WRT the 2ndary speaker output voltage, so i don't think you'll have a lot of
problems. the BAGA is quiet as death.
you would be amazed at how effective the airflow is at cooling... i'm with steve as far as that goes.
the key to big amps is always in the power supply. if you keep those 540v/300v rails steady you can easily make
100w with a deuce of 88's.
ken
From:
Date:
Subject:
JWK ([email protected])
7/2/2002 1:05 AM
Re: kg - Need KT88 advice
Excellent. Thanks again. While I'm in there I'll install a fan and do the regulated screen voltage supply. Then I can
run either EL34 or KT88 with only a bias adjustment. This started as a simple get-'er-up'n-runnin' project. I just
can't do that.
John