# Best way to reduce Voltage on a gain stage?



## bcmatt (Aug 25, 2007)

I am trying to fix up an amp that I built for my brother-in-law.
It is an 18Watt variant called the T-Rex. (replaces the trem channel with a high-gain TMB).
Anyways, I get a very unmusical squeal when turning up the t-rex channel. The Normal 18 watt channel seems quite fine. Also, there is a more subtle but unpleasant fizziness to the distortion on the T=Rex channel.

I'm realizing there is too much gain somewhere in the preamp of that channel (thanks to Keith for pointing this out). When I check the voltages at the plates, most are in the 180s and one is 256.

http://4.bp.blogspot.com/_4u9l5q2yFBA/Sp_q22knBOI/AAAAAAAAA18/bKCoZcE95RQ/s1600/18wTRexVoltages.JPG

So, I feel I found the culprit. From reading on the Tone Lizard site, he tells about how to adjust a gain stage and the one that I feel may apply, is reducing the value of the plate resistor. Will this lower the voltage, or just the gain? How are the two related?
Another suggestion it seems to have that might apply is using a snubbing capacitor in parallel with the plate resistor?

Any thoughts or advice?
Also, do my phase inverter plate voltages also seem to high?


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## Wild Bill (May 3, 2006)

bcmatt said:


> I am trying to fix up an amp that I built for my brother-in-law.
> It is an 18Watt variant called the T-Rex. (replaces the trem channel with a high-gain TMB).
> Anyways, I get a very unmusical squeal when turning up the t-rex channel. The Normal 18 watt channel seems quite fine. Also, there is a more subtle but unpleasant fizziness to the distortion on the T=Rex channel.
> 
> ...


Your link doesn't work for me.

Anyhow, is this a Marshall variant? If so, is that 256 volts found on the plate of a cathode follower that drives the tone stack? If so, that voltage is normal! The plate of a cathode follower is tied directly to the plate supply with no dropping resistor. The cathode is lifted up by a resistor of a similar value to a plate resistor. To compare apples to apples, you should measure the cathode voltage and subtract it from what you found on the plate.

Snubbing caps will cut highs rather than overall gain.

Can you provide a link to the schematic so we can see what's going on?

:food-smiley-004:


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## bcmatt (Aug 25, 2007)

sorry, my links to pics never work for some reason. The schematic is at the top of this latest blog entry here:
http://yeomansinstruments.blogspot.com/
I wrote the voltages in red.
It is a variant on the Marshall 1974x.

This high voltage is after the tonestack. I'll try to get the cathode voltages and report back today. I have to head to a memorial service right now though.


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## bcmatt (Aug 25, 2007)

Is anyone able to see the picture?
I'll try again right here, but these never work for me:








http://4.bp.blogspot.com/_4u9l5q2yFBA/Sp_q22knBOI/AAAAAAAAA18/bKCoZcE95RQ/s1600-h/18wTRexVoltages.JPG

So, I checked the voltage at the cathode for that last stage (where I get a plate of 256V), and I got 2.85 volts. 
Am I reading that right? When I flip the DMM to the red voltage reading setting, I get like 5.6 Volts. Is that normal?


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## Brennan (Apr 9, 2008)

Here, try this:

http://1.bp.blogspot.com/_4u9l5q2yFBA/Sp_q22knBOI/AAAAAAAAA18/bKCoZcE95RQ/s1600/18wTRexVoltages.JPG

Edit: ok, it seems you just can't hot link images from them. Just copy and paste the URL into a browser, and change the number at the beginning to any other number (ie, 3.bp.blogspot.com) and it will work.


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## bcmatt (Aug 25, 2007)

Brennan said:


> Here, try this:
> 
> http://1.bp.blogspot.com/_4u9l5q2yFBA/Sp_q22knBOI/AAAAAAAAA18/bKCoZcE95RQ/s1600/18wTRexVoltages.JPG
> 
> Edit: ok, it seems you just can't hot link images from them. Just copy and paste the URL into a browser, and change the number at the beginning to any other number (ie, 3.bp.blogspot.com) and it will work.


Weird! so that is the problem! Blogspot won't allow hotlinking. I need another way to host m pics, otherwise I will forever be frustrated by any picture linking in forums.


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## bcmatt (Aug 25, 2007)

How about this:









Cathode Voltage of 2.85V on V2b.

(horay for tinypic image hosting!!)


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## Wild Bill (May 3, 2006)

Something is screwy here. If you're getting a cathode voltage drop across the cathode resistor yet such a high voltage on the plate then something is wrong.

Check that 100k resistor feeding the plate. Is it really 100k?

The other possibility is that something is wrong feeding the grid of that stage. Is the 2n2 resistor shorted, allowing the plate voltage form the previous stage to leak through? Or a wiring mistake? Check the voltage on the grid pin. There shouldn't be any!

:food-smiley-004:


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## bcmatt (Aug 25, 2007)

Wild Bill said:


> Something is screwy here. If you're getting a cathode voltage drop across the cathode resistor yet such a high voltage on the plate then something is wrong.
> 
> Check that 100k resistor feeding the plate. Is it really 100k?
> 
> ...


I'm trying to make sure there are no wiring mistakes. I'm pretty sure there aren't, but I still am only human, and it has happened once before. I did just double check and it does seem to be right.

Things seem to be clean around the 2n2 cap and resistor.

The grid pin gives exactly 0 voltage.
The voltage across the cathode resistor is 2.88V. Is that too high or too low? What is this telling me?
Should I lower the 100k plate resistor? (it is 100k - I checked)

Thanks so far.


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## Wild Bill (May 3, 2006)

bcmatt said:


> I'm trying to make sure there are no wiring mistakes. I'm pretty sure there aren't, but I still am only human, and it has happened once before. I did just double check and it does seem to be right.
> 
> Things seem to be clean around the 2n2 cap and resistor.
> 
> ...


Ah, now I see! That 256v is normal. The cathode resistor is 10k, which means that the current through the tube is much lower than the usual stage. Less current means less voltage drop across the plate resistor. It's similar to a stage in the JCM800's.

Don't mess with the plate resistor value. It's not just a function of the voltage. A larger resistor would drop the voltage but put the gain UP! That's because you could develop a larger voltage swing across the larger resistor.

It looks like this circuit was designed to have tons of gain in the T-REX channel. If you want it cleaner, but still different from the other channel, I would replace those two 150k resistors driving that last stage with a 250k or 470k pot, or even trimpot. That would allow you to set the amount of signal driving that stage, just like a gain control. Use a trimmer and you could set it for the amount of gain that you liked.

:food-smiley-004:


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## bcmatt (Aug 25, 2007)

Wild Bill said:


> Ah, now I see! That 256v is normal. The cathode resistor is 10k, which means that the current through the tube is much lower than the usual stage. Less current means less voltage drop across the plate resistor. It's similar to a stage in the JCM800's.
> 
> Don't mess with the plate resistor value. It's not just a function of the voltage. A larger resistor would drop the voltage but put the gain UP! That's because you could develop a larger voltage swing across the larger resistor.
> 
> ...


Hey! Thanks Wild Bill. I believe that my issue is largely layout and lead dress problems because I had to wire the whole thing point-to-point using only terminal strips (I was tired of waiting for the MIA eyelet board material to arrive). So, if I can reduce these problems without having to basically start over on the build, I will be very grateful. 

So, for this pot idea, do you meant to just replace both 150k resistors with a single pot and not 2 pots set as variable resisters?
Would I just wire the wiper to the grid of the last gain stage and the two outside legs to the ground and 2n2 cap respectively?


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## bcmatt (Aug 25, 2007)

Wild Bill said:


> It looks like this circuit was designed to have tons of gain in the T-REX channel. If you want it cleaner, but still different from the other channel, I would replace those two 150k resistors driving that last stage with a 250k or 470k pot, or even trimpot.


It's not that I want it cleaner. I do want it to be pretty high gain; that's the point of the channel, I just want to get rid of the extra fizziness and squealing. So, I feel like it's probably just a bit too unstable at one of these points.

Here is a more recent diagram with more voltages taken:


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## Wild Bill (May 3, 2006)

bcmatt said:


> Hey! Thanks Wild Bill. I believe that my issue is largely layout and lead dress problems because I had to wire the whole thing point-to-point using only terminal strips (I was tired of waiting for the MIA eyelet board material to arrive). So, if I can reduce these problems without having to basically start over on the build, I will be very grateful.
> 
> So, for this pot idea, do you meant to just replace both 150k resistors with a single pot and not 2 pots set as variable resisters?
> Would I just wire the wiper to the grid of the last gain stage and the two outside legs to the ground and 2n2 cap respectively?


Yep! Wire the pot or trimmer exactly as you said. The two 150k resistors are just acting like a voltage divider, which is a potentiometer by definition.

I agree now that lead dress is probably your real problem. All that fuzz and fizziness could be oscillations. Often they are ultrasonic so you don't hear them directly. You hear their effects.

Point to Point takes some experience to prevent these problems. First off, signal wires should always be as short as possible. Wires should not run parallel for any distance, if you can. They should cross at right angles, so that the common area for capacitance coupling is just the dot where the wires intersect, rather than the surface area of the entire parallel lengths.

Also, with the usual common cathode gain stage, every stage flips the phase of the signal. That means that wires from the second stage ahead are in phase with the input of the first stage of the string. IOW, the output of the second triode stage of a 12AX7 is in phase with the input of the first stage. Signals in phase if coupled together can oscillate. Signals out of phase can't. You have to keep this in mind as you wire the stages.

Don't be afraid to use short lengths of shielded wire on grid lines. Some purists claim that you can lose highs from the capacitance but I think this is bigtime overkill. The capacitance of 6" of shielded wire will have mice nuts affect on the highs. We're not talking a 50' guitar cord here!

Besides, sometimes you have to make a trade. An amp that's stable and sounds great with a bit of shielded wire or an amp without it that is too unstable to use. You can't always get what you want!

You have to assume that if you are wiring a working circuit and you have these problems then you've got something else wrong. Lowering the gain is a last-resort method of solving things. It may not work anyway if you don't pick the specific stage that's oscillating. I've found that it works best if you first fix what's REALLY wrong rather than try to mod the circuit to compensate for what's screwed up.

I'd first try using a plastic pen or something to gently push wires and parts around, looking for any change in the oscillation, good or worse. That usually identifies the nature of a lead dress problem and lets you know where to re-run the wires.

Also, don't ground willy-nilly wherever's convenient! An audio amp is not like a truck where you can ground lights and such to any handy bare spot on the metal. You can get ground loops that exacerbate lead dress feedback problems. Do a google on star grounding. I've found that it's usually good enough to have a common ground for each stage, i.e. preamp, PI, output stages, etc. You can either then use the chassis as a return for those common points or you can run a ground wire from each of them to the common negative at the power supply filter caps.

Isn't amp building fun?:smile:

:food-smiley-004:


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## WCGill (Mar 27, 2009)

Removing one of the cathode caps (25/25) will reduce your gain also.


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## parkhead (Aug 14, 2009)

the lead channel is lifted from a Trainwreck express 

it shouldn't squeal unless you have an assembly error 

its a nice pre amp but needs to be built right ! 

do a google search on lead dress for Trainwreck express clones 

use as little wire as possible ! especially on the grids (inputs of the pre's) 

I did one in a bassman once ... 











p


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