Download Extra Gain.pdf

Saturday, December 22, 2001 10:59:29 AM
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Mark Lavelle ([email protected])
12/18/2001 12:38 AM
reduce gain or spill extra to ground? (long)
I spent the weekend prototyping a preamp w/reverb - a mix of a Spitfire input stage, Brown Princeton volume &
tone, BF Princeton reverb & 6G15 reverb controls. Once I cured it of thinking it was a radio ;), I ran into a few
spots where I had more signal than the next stage could handle.
I knew that I could reduce the gain of a stage by adjusting the cathode & plate resistors. I also figured out
(doh!) that I could just add a resistor to ground before the next grid to 'throw away' the excess voltage. I ended
up doing both, mostly because I wasn't comfortable with really large value cathode resistors and still had some
gain I had to lose.
In my eagerness to just get the darn thing working without horrendous clipping, I wasn't paying much attention
to what all this may have done to the tone. So this seems like a good time to step back & think about what I did,
before diving into the real fine-tuning. Some questions:
1. What are the tonal consequences of reducing the gain of a stage?
2. Is there any significant risk of getting an audible filter effect from a 'pull down' resistor at the grid of the next
stage?
3. Generally, what are the advantages and/or disadvantages of each of these approaches to adjusting interstage gain? Other than wanting to have lots of gain going into the tone stack, it seems to me that there's never
any need for more than 'enough' (however much that may be).
4. How long can I get away with being ignorant of impedance issues? :)
Feel free to add your own questions (and answers)!
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Mark Lavelle ([email protected])
12/18/2001 9:13 PM
Re: reduce gain or spill extra to ground? (long)
I thought posting the schematic might help: http://home.pacbell.net/logimark/verbproto1a.pdf
(and it's a good excuse for bumping this back to the top! ;))
R4 was changed from 1.5k to 10k to reduce the gain of the 1st stage, and R24 was added to reduce the
amount of gain going into the tank driver. B+ is around 320V.
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tim
12/18/2001 11:44 PM
Re: reduce gain or spill extra to ground? (long)
Mark, very nice looking schematic...
Wish I could answer some of your questions, but often times I find myself doing the same thing... throwing
away unwanted/extra gain through a resistor or cap+resistor to ground. I am not sure of the consequences,
from a theoritical standpoint, other than I've read sometimes that it affects "loading" of stages. My approach
usually is to just "go for it", I'm usually not pushing the tubes very hard anyway (haha). And usually the results
are sonically pleasing. (Or in some cases very pleasing).
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Saturday, December 22, 2001 10:59:29 AM
Good questions! And good luck getting some answers.
Tim
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Mark Lavelle ([email protected])
12/19/2001 6:24 PM
Re: reduce gain or spill extra to ground? (long)
My approach usually is to just "go for it", I'm usually not pushing the tubes very hard anyway
(haha). And usually the results are sonically pleasing. (Or in some cases very pleasing).
I hear you there - right now I'm really liking the way the dry signal distorts when the volume knob is in the top
half of its rotation. OTOH, the distortion in the reverb isn't cutting it...
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jpl ([email protected])
12/19/2001 3:00 AM
Re: reduce gain or spill extra to ground? (long)
Mark,
Changing R4 to 10k lowers the gain but raises the bias. This may result in asymmetrical clipping in the first
stage.
R24 lowers the gain of the first stage through loading but may also affect the high frequency response of the
reverb driver.
Both of these results may or may not be *bad* but they may be audible.
For some guidelines on design check out: http://www.aikenamps.com/CommonCathode.htm
Some other techniques to "loose some gain"
1. Interstage attenuator--proper selection of resistance values and compensation can help minimize loading
and high frequency roll off.
2.Local Feedback--some amps (Dumble etc.) use this to control the gain of a stage, check here:
http://www.aikenamps.com/FeedbackAmp.htm
3.Split plate load-- this is where the output of a stage is obtained through a pair of plate resistors rather than
the bottom of the typical 100k plate load
In general a lower gain stage will have better bandwidth or high frequency response.
jl
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Mark Lavelle ([email protected])
12/19/2001 5:19 PM
Re: reduce gain or spill extra to ground? (long)
Thanks for the pointers!
I don't know why, but I seem to have a mental block about checking Randall's site - I knw it's there, but always
for get to check it. Same apparently goes for the split plate load idea - I read the threads about it with great
interest only a few months ago! [...but I'm too young for senior moments! :)]
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Saturday, December 22, 2001 10:59:29 AM
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Chris W ([email protected])
12/19/2001 10:38 AM
Re: reduce gain or spill extra to ground? (long)
Hi Mark
Pretty cool concept !
Some ideas and questions :
What PI and output-circuit are you using ?
Try lowering the 2nd stage gain ( Rc : 1k8 - 10k , unbypassed ) and upping the 1st stage gain and defarting it
( Rc : 820 - 2k7 , Cc : 470n - 4u7 ) .
R18 will also lower the gain of the non-effected signal . I suggest redesigning/tweaking the tone/mix-circuit
after the reverb-recovery-stage . You can also try removing R19 and change R18 to 470k .
Your dwell-control-circuit will lessen the amount of bass on higher settings ( that's cool ) but it's also loading
( through that 470p input-cap ) the 2nd stage output .
Good luck and let us know !
Chris
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Mark Lavelle ([email protected])
12/19/2001 6:16 PM
Re: reduce gain or spill extra to ground? (long)
Thanks. The ideas for this just sort of fell together from spending a year hanging out here on Ampage and
talking to my brother the surf fanatic about what makes for a good reverb...
In the prototyping phase, I'm using the power amp from my 'Tonerator' (based on O'Connor's Tonnes of Tone
stuff: see page 3 of http://home.pacbell.net/logimark/tonerator_i_schematic.pdf). The final design is still up in
the air, but I'm thinking of using the Tonerator MV & PI in front of a pair of p-p EL84s. Tremolo is also a
possibility.
It has occurred to me that part of my problem is that my plate voltages are about 50% higher than in the circuits
I'm stealing from, so I think I'll be trying the split plate load approach to taming the gain. I'll also try your ideas
re: the cathodes of those 1st two stages.
I'll probably try some of your tweaking suggestions in the reverb area, too, but I'm going for a very Ferder-y
thing there and probably won't stray too far from the original designs. The tone & mix actually work pretty well
as is, but I do need to fix up the dwell. My next pass at that is going to remove R24 and put the dwell pot
between two resistors so that the load adds up to 1M like in the Fender internal reverbs. Sizing those two
resistors right should let me put the dwell pot in the sweet spot.
I'll report back after the next soldering session...
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Jose
12/20/2001 12:01 AM
Re: reduce gain or spill extra to ground? (long)
At ax84 you can read how the split plate load method is used in the P1 to reduce the gain without loosin highs.
Read here: http://www.ax84.com/media/ax84_m35.pdf
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Saturday, December 22, 2001 10:59:29 AM
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Carl Z ([email protected])
12/20/2001 5:28 AM
Re: reduce gain or spill extra to ground? (long)
Mark;
That amp should NOT be clipping with any reasonable guitar signal input. Without lifting the bypass cap you're
not losing any gain in the first stage, just hosing the bias point. You're probably seeing somewhere around 5v
output from the first stage, which means your second stage won't start to clip till around 2/3 rotation. the output
from this thing is probably around a solid 15v, maybe more, which is knocking the stuffing out of the reverb
driver stage. I might consider attenuating the signal to the dwell control then adding a stage to the driver and
bootstrap! You want a big clean peaky signal going to the tank.
Disconnect the reverb tube from the circuit and see how it sounds. pull the tube and see how it sounds, then try
disconnecting the wires. I'd be willing to bet the verb is what's causing your distortion problems.
Carl Z
http://www.zwengelamps.com
P.S. I've talked to several people and was told there probably wouldn't be much of a market for that verb unit I
was telling you about. You buy that notion?
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Mark Lavelle ([email protected])
12/20/2001 6:28 AM
Re: reduce gain or spill extra to ground? (long)
Carl,
As mentioned above, I'm pretty happy with the dry tone over most of its range. OTOH, I'm pretty sure I'm
seeing over 30V peak-to-peak on the plate(s) of V1 with this circuit (guess I should take another look with the
'scope) and I'm pretty darn sure that V2B is clipping when the volume is at 3/4 or so.
Could you explain that comment about the bias point? Are you saying that C1 negates the effect of using 10k
at R4?
You're right that the real problem is "knocking the stuffing out of the reverb driver stage." I don't want to add
another stage, though, so I'm trying to find the best way to feed V3 with an ideal level (got a ballpark number
on what that should be?). Does the scheme I mentioned at the end of my reply to Chris W sound plausible to
you?
-- Mark (I'll email you tomorrow about that other stuff...)
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Carl Z ([email protected])
12/22/2001 2:55 AM
Re: reduce gain or spill extra to ground? (long)
Mark;
I was just giving a ballpark guesstimate off the top of my head but 30v p-p sounds more than reasonable.
Ok, here's the bypass cap deal. Yes it negates the effect of the 10k resistor if you were looking to reduce the
gain of the stage. When you bypass the cathode you get the total possible gain of the tube down to the -3dB
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Saturday, December 22, 2001 10:59:29 AM
break point of the RC pair. The equation I like to use to calculate AC gain of a tube is...
Av = Rp||RL/(Zk+1/gm) where Zk=AC impedance in the cathode circuit. Note that if you slap a cap across Rk
your AC impedance goes to 0 above some particular shelving frequency, at which point the gain approaches
that of an unbypassed resistor, or Zk=Rk. Randall Aiken grinds through all the math on this using a slightly
different formula but the results are the same.
So, by using a large cathode resistor like you did you didn't actually reduce the gain but rather just current
starved the tube. Remember that a larger cathode resistor value increases the bias voltage allowing for more
clean headroom before the onset of grid blocking. Personally, I don't feel there's any need for a resistor this
large on the input stage of any amplifier. If you need to lose gain, attenuate via a resistive divider network or a
split plate load.
With regard to the reverb driver, you want to push as much current as possible and let the voltage fall where it
may. The output voltage of the tank is going to be in the neighborhood of 10mV p-p so you may want to adjust
the gain of the recovery stage accordingly as well as adding a mixing stage of some sort. You also want to
watch out for the damping factor on the tank. if there's not enough damping in the system the tank will clang
and distort no matter what you do. You just need to fiddle with values to see what sounds good to your ears.
You could also use some global feedback exactly like you'd see in a power amp to adjust the system's
damping factor. Lots of ways to skin the cat here but it'll probably cost you an extra tube.
Did that make sense or did I completely confuse you?
Carl Z
Zwengel Amplifiers
http://www.zwengelamps.com
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Mark Lavelle ([email protected])
12/21/2001 4:35 AM
Re: reduce gain or spill extra to ground? (long)
Forgot to ask: what level should I be shooting for to feed the reverb driver? And more generally, how does one
determine the appropriate level for feeding any given tube?
Thanks...
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Carl Z ([email protected])
12/22/2001 3:04 AM
Re: reduce gain or spill extra to ground? (long)
what level should I be shooting for to feed the reverb driver? And more generally, how does one
determine the appropriate level for feeding any given tube?
Thats what bias voltage is all about. Read my vacum tube basics paper then hit randall's triode gain
calculations paper and start crunching numbers. Generally, and I stress the word generally, 10v p-p should be
more than adequate.
Carl Z
http://www.zwengelamps.com
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