Tube Guitar Amp: preamp stage gain - how to 'adjust'?

[Wharfcreek]

I've done a few threads lately about some little tube guitar amps I've been building. I've completed 4 so far, and all have been slightly different in terms of available parts, chassis, output tubes, rectification, and even 'circuit' to some level. However, all are similar in that they are all single ended units, either 6V6 or EL84, and use just a single 12AX7 on the front end. They're all based on the old Fender 'Tweed' Princeton or Champ circuits. Nothing fancy going on!

These amps run about 150 volts to the 12AX plate, and use a 1500 ohm resistor for the cathode bias. In building them, I've played around with a 'tone' control, feedback vs 'no' feedback, a bypass cap on the 12AX cathode resistor, and even changing coupling cap values. All this has resulted in variables that have either 'improved' or 'detracted' from the amp's performance (in my opinion). My last build was closer to the Champ circuit in that it was a volume control only unit. And, to my delight, this amp actually sounds the best. However, it's also really sensitive in that it's 'breaking up' well before it's getting loud. I'm attributing this to both an 'input' sensitivity issue, as well as perhaps some issues with adjusting the levels of gain through the 12AX. That's where my question is: Can I 'adjust' something on the 12AX to perhaps lower this gain some and reduce the immediate drive response...or simply 'tame' the amp some, and if so, what is the recommended process?

Thoughts and suggestions are greatly appreciated. Tom D.

 

[savatage1973]

Have you experimented with a lower gain tube like a 12AU7 or 12AT7?

 

[Wharfcreek]

Yes, and the results were pretty good. However, I rather looked at that as a 'band-aid' type 'fix'. It did reduce gain, and I actually cycled through a 12AT, a 12AY, a 12AU, and a 12AZ.....all of which produced differing results. In the end, I'm not sure but what you're mentioning isn't the best way to address the issue in the long run. It DOES give the amp more 'flexibility' to leave it like this.....with the AX producing all kinds of gain....which someone might really like. BUT......tube-rolling isn't 'teaching' me anything. I'd like to learn just what's defining these parameters.......how to 'adjust' them, and the affects that the changes make. Honestly, I only see a couple of different possibilities here; either you adjust the voltage at the plate, OR....you adjust the cathode-to-ground resistor which I believe is what is setting the bias on the tube. As I said above, there is a 1500 ohm resistor on the cathode....both of them. If I started 'guessing', I'd perhaps go with something like an 1800 or 2K....which I believe would cool the tube off some. But, I don't know if this is perhaps already the issue? I do know that in some 'stereo' amps the cathode resistors are like 820 ohms with upwards of 140V on the plates. From this I'm thinking that the tube is then running such that it's producing maybe even more gain......and maybe 'not' 'clipping'? In other words, pulling more current at the cathode might clean the amp up vs going the other way.....and I'm just not sure? So....hopefully someone else will know, and maybe give some advice about it. In any event, it's a 'learning' experience for me......and the building of these amps has been great fun. Kinda has me 'playing' again....though still as pitiful as ever...lol Tom D.

 

[IceFyre13th]

Everything you need to know....

www.guitarstudio.tv/documents/Designing-V-T-Amplifiers.pdf

 

[Wharfcreek]

Wow!! 318 pages!! This is not the 'short paragraph' or two that I hoped for. More like a class in a Community College condensed into 'book' form! But, I suppose it could replace a few nights in front of the TV!! And, for the record, I appreciate the info. I just have to confess that I tend to get several pages into stuff like that and find that I'm both over my head, and my inability to understand it all causes me to just dismiss the rest of it. I mean really...... 318 pages? That's a LOT of info to digest, particularly just to address one or two questions.....yes? But, again, I appreciate it, and I'll give it a try. I'm just not that good with that much info all at once. Wish I was.............. TSD

PS: I just read through the index, and it looks like skipping around through that document is maybe do-able. In any event, it's clearly a well-put-together package.....and I will probably enjoy reading much of it! Thanks again!! Tom D.

 

[IceFyre13th]

No Problem, and yes skipping around is a good idea....but you now have a reference to just about any question.

While not a "main-stream" book I found it to be very useful

 

[gadget73]

5751 is closer to a low gain 12ax7 than the rest of those tubes. Those all bias different and such. A lot more than just gain differences between them.

Removing a cathode bypass cap is a quick and easy way of dropping gain. The other is to reduce the size of the plate resistor. You'd either have to increase the cathode resistor, or just reduce the voltage feed to keep the plate voltage proper.

 

[Wharfcreek]

I have no 5751s to play with......and at the current price for them, I don't see myself buying any very soon!! But, I'll keep my eyes out for one or two, just as 'reference' tubes. Hopefully I'll be able to get a good one....... As to the different 'bias' points on all the other tubes, I was rather thinking that in using them as 'drop-in' replacements in a 12AX circuit like this, the tube's characteristics with respect to bias and gain are part of why you do it this way. In other words, if you dropped in a 12AU for the 12AX, then went ahead and 'optimized' the circuit for the 12AU....... you might as well just 'adjust' the circuit with the 12AX in place and save yourself the cost of the 12AU.... if that makes sense. As to the bypass cap, I do plan to play around more with those, to see what affect it has. That, and perhaps the feedback part of the circuit. One of the total of 4 amp builds was the 'KIT' that AES sells......the MOD102+ I believe it is. You can go on AES's web site, search the kit, and when you find it, you'll be able to bring up the schematic of that amp as well. While not an exact copy of either the Princeton or Champ, it also uses a 12AX7 and an EL84 output in SE configuration, as well as has some similar voltages. Kind of hard to tell about these voltages as Fender didn't publish any......so, 'specs' on the circuit diagrams are all 'contributions after the fact' by those who have worked on the amps in the past, made measurements, and posted them to the schematics. But, going back to the MOD102+, it had both Treble and Bass controls (which were useless!!) as well as some 'pull' functions for 'mid boost', 'bright', and 'turbo' modes......all equally useless except for perhaps some distinction in the 'bright' mode). Anyway, that amp has NO feedback......unlike both the Fender circuits. So, the 'with' or 'without' aspect of these builds has been interesting as well. Thinking about making it a 'switchable' function on the final product.

6 - thanks for that chart! I'll go over it more tonight, but late for work now......so.....gotta go!!

TSD

 

[gadget73]

keep in mind, i don't operate in the guitar amp world. I look at tube substitutions as something to be avoided because they will not be operating as designed which will usually reduce performance and add distortion. Guitar amps just aren't used and designed in a manner my little low distortion brain can function with.

 

[kward]

The equation for gain of a standard "common cathode" triode gain stage is this:

Stage Gain = (mu * RL) / (rp + RL + Rk(mu+1))

mu = gain factor of tube (12AX7 = 100, for example)
RL = Plate load resistance
rp = internal (dynamic) plate resistance (for a 12AX7, rp is between 60k and 80k usually, depending on where it is biased)
Rk = Cathode resistance (Rk = 0 if cathode is fully bypassed)

This equation fully describes what goes into delivering the gain of the stage. As with any division problem, you can make the result smaller by making the numerator smaller or the denominator larger, or both. The parameters that are easy to adjust to do this are the external resistors RL and Rk.

Mathematically, now it can be seen why unbypassing a cathode causes gain to go down, because Rk is in the denominator, so the denominator gets larger when unbypassing Rk, thus making the result smaller.

Mathematically, now it can be seen why increasing RL has an overall effect of increasing gain since RL multiplied by mu is in the numerator, which contributes more than just RL in the denominator.

Adding negative feedback into the cathode of a common cathode gain stage has the effect of reducing the open loop gain by the amount of feedback added. If you add 12 dB of feedback it will reduce gain by 12 dB at the stage where the feedback is returned.

Slightly more tricky but you need to know it: If you reduce RL but do nothing else, the stage will be biased at a different point. THis will reduce gain certainly, but may put the bias point in a place you don't want it. Equivalently, increasing RL, will increase gain certainly, but doing nothing else, will also re-bias the stage at a different point. So if you Increase or decrease RL, you usually also need to increase or decrease (respectively) the rail voltage powering the stage, so that the quiescent voltage drop across the tube remains the same, thus keeping the bias point the same.

That's basically all you need...

 

[kward]

One more thing that sometimes confuses...

If the stage on which you are tweaking gain is included inside a feedback loop, adjusting stage gain has much less effect of changing overall forward voltage gain because feedback compensates for it (that's one of the purposes of feedback). So if tweaking RL on a stage inside a feedback loop, forward voltage gain will change incrementally, but not to the value that you would calculate with the gain equation. If you make that same change in a stage that is in open loop (not in a feedback loop) it will have exactly the amount of gain increase or decrease as stated by the equation, because there is no loop feedback to compensate.

 

[Wharfcreek]

K, ….. WoW!! That's some 'fancy' math there!!…...lol Oddly, I think I understood it......or 'most' of it anyway. In essence, 'increasing' the plate load resistor, or removing the cathode bypass......both will 'decrease' gain. I don't think you covered this directly, but perhaps the comment about Rk = '0'..... would that apply is the cathode resistor was never bypassed in the first place.....but if the value of this resistor were altered to a lower resistance, or is it actually the reverse....where 'increasing' the resistance would reduce the gain? I think Gadget was referencing this in his post #8,... but I wanted to get the same clarification then....but these other few posts rather beat me to it.

Thanks, TSD

 

[kward]

Wharf, If the cathode resistance is bypassed, Rk goes to zero in that equation above, and stage gain will go up, relative to where it was when Rk was not bypassed.

It was not my intent to show the equation to calculate exact stage gain, but rather to show where the resistors are in the equation so you can see how increasing or decreasing them (or bypassing them in the case of Rk) affect gain going up or down.

But since we now know the equation, we can use it in thought experiments on an imaginary 12AX7 gain stage.
Consider an imaginary baseline 12AX7 stage where RL = 270K, Rk = 1.5K, rp = 68K, Calculated stage gain in the baseline is 55.

• Now keep everything in the baseline the same except change RL to 470K. You will see that gain goes up to 68.
• Now keep everything in the baseline the same except change RL to 100K. You will see that gain goes down to 31.
  
• Now keep everything in the baseline the same except bypass Rk (make Rk go to zero). You will see that gain goes up to 80.
• Now keep everything in the baseline the same except change Rk to 3K. You will see that gain goes down to 42.

 

[Wharfcreek]

That was to be my next question 6. In looking at your diagram, the plate resistor is what I believe is labeled 'Rp'. So, if I'm understanding this correctly, the B+1 is fed into the plate resistor (RL)..... and the cathode resistor (Rk) is the 'fixed' connection between the cathode of the 12AX and Ground. The Rk is generally bypassed with some level of electrolytic cap, mostly seen by me as being something from about a 25uf/25V to perhaps a 100uf/60V. As perhaps a side-bar here.....is the voltage of this cap particularly critical if maintained to at least 1.5X the estimated voltage at this point in the circuit? From what I recall seeing, there's usually not more than a volt or two on a 12AX7's cathode. And, I might add, I've NEVER seen a 12AX with it's cathode going straight to ground. So, perhaps another discussion on the subject of cathode resistor bypass caps is of value......but right now I'll stick to the just the two resistors; plate and cathode.

So, in all these amps, the plate resistor is a 100K. I'm not quite sure about 'rp'....but if I do understand it correctly, this should be a 'constant' for the 12AX7 as I would think that this is part of the measurable 'specs' on the tube itself. Hence, the 68K used in the imaginary baseline above is a 'constant' in any calculation involving the 12AX7, yes? So, assuming you're going to respond 'yes' to that....then the variables here are going to be the Plate Resistor (RL)...which seems pretty consistently applied as a 100K, the B+2 feeding the plate resistor....which is roughly 150 to 170 VDC, the Cathode resistor, which is 1.5K in all these builds, and the application (or not) of a cathode bypass cap, as well as it's value. The MOD uses a 22uf/25V cap on both sections of the 12AX. The Princeton has only the first 'input' section cathode resistor bypassed with a 25uf/25V, and the Champ has neither section's cathode resistor's bypassed.

As I said, I'm not sure I completely understand the reason 'why?' the cathode bypass increases 'gain'....but leaving that to the side for the moment, I completely do NOT understand 'why?' the 'increase' in resistance level of the Plate Load Resistor (RL) would cause the 'gain' to go 'up'? To me this is just contrary to what my 'logical mind' tells me 'should' be happening!! I say this because as I think about the concept of a 'resistor' controlling voltage within a circuit.....I see a larger resistor impeding voltage to the plate. As such, with a 'lower' plate voltage, seems to me that with everything else remaining the same, the 'gain' should be correspondingly reduced as well....since the voltage is now 'lower' to start with. So, since the opposite is what 'really' happens...... I need to understand 'why?'!!! What's happening within the tube that causes a reduction in plate voltage to result in an 'increase' in output from the tube?

Let's just clear that hurdle....then on to the next; the bypass caps!! We can deal with the cathode resistors after that as well.

Thanks guys!!

Tom

 

[kward]

I completely do NOT understand 'why?' the 'increase' in resistance level of the Plate Load Resistor (RL) would cause the 'gain' to go 'up'?

Let's remind ourselves of the definition of voltage gain: the output voltage swing divided by input voltage swing.

Consider two extreme cases. One extreme is where the plate load resistance is zero. If the plate load resistance is zero, you have a cathode follower. In a cathode follower, there is no signal generated at the plate, or in other words the plate swings zero volts because the plate voltage is held constant by the power supply. If the power supply is holding the plate voltage constant, the plate can't change voltage no matter how the input voltage changes, therefore there is no voltage swing at the plate and therefore no gain. Plate current swing, on the other hand, is the largest it's allowed to be since there is no plate resistor impeding the flow of current. (However the dynamic plate resistance, or internal plate resistance does contribute to impeding current flow).

The other extreme case is where the plate load resistance is very large, say 1 MΩ. Here the voltage at the plate is allowed to swing its maximum amount. In this case gain is maximized because plate voltage swing is maximized given some input voltage swing. Current swing at the plate, on the other hand, is small because the plate load resistance is so large it impedes nearly all current flow. But even with small current flow, the plate load resistor being so large will drop a lot of the rail voltage at peak input signal swings. At the opposite peak of the input signal swing, it puts the tube nearly into cutoff so nearly zero current flows, so very very little voltage drop occurs across the (large) plate load resistor. If the plate load resistor is consuming a lot of the rail voltage on one half cycle of the input, and then nearly zero voltage drop on the other half cycle of the input, then the plate voltage must be swinging opposite of that: A lot of rail voltage consumed across the plate resistor equals little voltage at the plate. Little rail voltage consumed across the plate load resistor equals a lot of voltage at the plate. Thus the two extremes of voltage drop across the plate load resistor force the plate to swing from min to max volts, thus conditions are such that the plate can swing maximum voltage, thus maximizing gain.

So from these two extreme cases, it can be seen that we need a plate load resistance to allow the plate to swing voltage. Thus the value of the plate load resistance is directly proportional to the voltage gain of the stage: larger plate load resistance equals more voltage gain. Smaller plate load resistance equals less voltage gain.

In the limit, the most voltage gain that can be eked out of a common cathode wired triode stage using a single large plate resistor is about 90% of the mu of the tube. For example if I used a 680K plate load on a 12AX7 stage with bypassed cathode, the gain equation previously discussed says gain would be 90.

 

[gadget73]

To look at it another way, the power you get out of a given tube is fairly constant. Power is volts * current. If you want more voltage swing, you have to decrease the amount of current. Higher plate resistor means lower current. Lowering the plate resistor value decreases the amount of voltage swing possible, but it increases the amount of current.

 

[gadget73]

You can also just put the cap across the resistor, it works out the same as long as the caps are in good shape. As an added bonus, its typically less voltage across the cap and it won't have any voltage surge at turn-on since there will be zero drop until the tube begins to conduct.

 

[triode17]

Tom- Allow me to look at the overall here. My new thing is to 'simplify' problems. You first have to determine if the preamp is overloading, or the power amp is overloading. I happen to have a '66 Fender Blackface Vibro-Champ. The preamp 12AX7 is before the volume control so the gain is preset and not controlled directly, as built. I believe today's guitar pickups are stronger in output than in 1960, so overload is more of an issue now. But let's assume the preamp IS overloading, why don't you simply build an attenuator and install it before the preamp. 2- 1 Meg Ohm resistors in series should work. Connect one end of the pair to the guitar signal, the other end to Ground and pick off the signal between them to connect to the first control grid circuit. That will provide 6 dB of attenuation, or half the original signal. Now listen for how the amp. responds using the same guitar.