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From:
Date:
Subject:
Mark Lavelle
Page 1 of 8
([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 inter-stage 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)!
From:
Date:
Subject:
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.
From:
Date:
Subject:
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...
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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).
Good questions! And good luck getting some answers.
Tim
From:
Date:
Subject:
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...
From:
Date:
Subject:
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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From:
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Subject:
Mark Lavelle
Page 3 of 8
([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! :)]
From:
Date:
Subject:
Mark Lavelle
([email protected])
12/31/2001 2:22 AM
"Verberator" prototype update
Finally got in some good chunks of tweaking time on this the last few days...
All your comments were pretty much on the mark. The distortion that was bugging me wasn't
clipping in the 1st two stages (they do clip, but in a nice way! ;)) - the problem was a ~5kHz
oscillation that was being picked up by the reverb driver tube. I'm pretty sure that's mainly
because I've still got all the leads on the tranny at full length, but I was able to tame it (for the
purposes of my proto) by dropping the plate voltage there.
The new schematic is available here:
http://home.pacbell.net/logimark/verbproto1a.pdf
Thanks to all...
From:
Date:
Subject:
Adam Alpern
([email protected])
12/31/2001 5:04 PM
Re: "Verberator" prototype update
Hey Mark,
How do you like the sound of that reverb circuit?
I've been looking for a nice reverb circuit with dwell, tone & mix controls that could be easily
incorporated into an existing amp, and yours looks nice and clean. It looks like with minimal
tweaking yours could maybe even be done with only 1 tube if you used a 12DW7 (I'm short on
space for more tubes :) )
Thanks for sharing!
-Adam
From:
Date:
Subject:
Mark Lavelle
([email protected])
12/31/2001 6:16 PM
Re: "Verberator" prototype update
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How do you like the sound of that reverb circuit?
Now I'm very happy with it. It's got a lot of range, from subtle & dark to dripping & bright. When I
build it into the Verberator combo next year (tomorrow? ;)) I'll try to raise the plate voltage of the
tank driver to get more drive/dwell, but it's quite usable "as is" for anything but insanely drippy
surf...
I've been looking for a nice reverb circuit with dwell, tone & mix controls that could be
easily incorporated into an existing amp, and yours looks nice and clean.
Why thank you, sir, but really, it was nothing! I basically took the 6G15 controls & pasted them
into the standard BF combo reverb design.
If you do build this into something, be sure there's another gain stage between the mix & the PI.
This circuit is only putting out about 2V p-p at the "preamp out" jack. I'm running it with another
volume control after the mix & before that last stage (partly because my front-end volume control
gets the sound pretty crunchy in the top 1/4 of its range).
Thanks for sharing!
NBD: to me, that's what Ampage is all about.
Happy New Year!
-- Mark
From:
Adam Alpern
Date:
1/1/2002 4:24 PM
Subject:
([email protected])
Re: "Verberator" prototype update
Why thank you, sir, but really, it was nothing! I basically took the 6G15 controls &
pasted them into the standard BF combo reverb design.
Well, at the very least then your schematic is nice and readable! A lot of schematics out there
(Fender included) are packed so tight that after looking at 12 of them in a row I can't tell the
difference between the B+ lines and ground anymore without tracing out the whole thing! I try to
avoid that effect as much as possible in my schematics too.
-Adam
From:
Mark Lavelle
Date:
1/1/2002 6:04 PM
Subject:
([email protected])
Re: "Verberator" prototype update
Thanks again - I know exactly what you mean about too-tight schematics. It drives me crazy when
they're packed so tightly as to obscure the 'meaning' of the circuit. But back then they didn't have
OrCad, of course... :)
From:
Carl Z
([email protected])
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Page 5 of 8
1/2/2002 3:07 PM
Re: "Verberator" prototype update
God I love being right! :D :D
Time to start tweaking mark. Have fun!
Carl Z
http://www.zwengelamps.com
From:
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Subject:
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
From:
Date:
Subject:
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
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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...
From:
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Subject:
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
From:
Date:
Subject:
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?
From:
Date:
Subject:
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
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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...)
From:
Date:
Subject:
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 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
Page 8 of 8
([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...
From:
Date:
Subject:
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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