Princeton Amp with Dual bias adjust

[TurboGuitarMelton]

How would I add dual bias pots (one for each 6v6) with this circuit? I was thinking since it has bias vary tremolo I would need a dual gang 250kl tremolo intensity pot feeding each of the 220k resistors. Or is there a way to do it with a standard 250kl pot?


Thanks!

[PRR]

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[TurboGuitarMelton]

Thank you for the reply PRR. I dont see the bias adjust for each 6v6 in the schematic you posted. To do that I would separate the leads on the right junction on the schematic of the 250kl dual gang pot (from negative bias voltage) and add the individual bias pots in between the two right most intensity lugs and negative bias voltage. Correct? Like this -


So there isnt a way to use a normal 250kl pot?

[tubeswell]

Thank you for the reply PRR. I dont see the bias adjust for each 6v6 in the schematic you posted.

Look again - PRR's schematic shows 2 pots

[sluckey]

Thank you for the reply PRR. I dont see the bias adjust for each 6v6 in the schematic you posted.

Look again - PRR's schematic shows 2 pots

PRR's drawing shows two INTENSITY pots but no BIAS pots.

[sluckey]

Thank you for the reply PRR. I dont see the bias adjust for each 6v6 in the schematic you posted. To do that I would separate the leads on the right junction on the schematic of the 250kl dual gang pot (from negative bias voltage) and add the individual bias pots in between the two right most intensity lugs and negative bias voltage. Correct? Like this -

Your drawing will work although it would be better to add a filter cap to the wiper of each bias pot. Otherwise the INT pots may not completely eliminate the tremolo when turned to zero. You may also need to change the value of the 100K range resistor.

So there isnt a way to use a normal 250kl pot?

You will need the dual gang INTENSITY pot.

[tubeswell]

Thank you for the reply PRR. I dont see the bias adjust for each 6v6 in the schematic you posted.

Look again - PRR's schematic shows 2 pots

PRR's drawing shows two INTENSITY pots but no BIAS pots.

Right you are. It’s been a long day

[TurboGuitarMelton]

You may also need to change the value of the 100K range resistor.

Are you talking about the 100k on the tremolo speed pot or the 100k bias supply resistor?


Also I just noticed that PRR changed the 1M resistor to 470k. Why do I need to do that? Does it increase the strength of the tremolo?


How does this schematic look?


I appreciate all the help!

[PRR]

Look again - PRR's schematic shows 2 pots

Yes, I screwed-up bad.

Series pots to a grid won't change bias. Shunt pots on this hi-Z bias supply will interact.

Are we maybe asking for too many degrees of freedom? Wouldn't it be as simple to find a fairly well matched pair? Has repeated catastrophe in tube-world made that a lost dream?

[TurboGuitarMelton]

Series pots to a grid won't change bias. Shunt pots on this hi-Z bias supply will interact.

Are we maybe asking for too many degrees of freedom? Wouldn't it be as simple to find a fairly well matched pair? Has repeated catastrophe in tube-world made that a lost dream?

I think you are talking about the schematic I just posted? Right? Sorry if you're not I dont understand what you are saying entirely. If so - Would joining the two 15k resistors fix the interaction?


I wanted to make each adjustable because I have a lot of NOS 6v6's and none are matched. So Id like to be free to use what I have.

[sluckey]

Your last circuit should work with no interaction. I would add a filter cap to the anode of the diode for a total of three bias caps.

[Fresh_Start]

Would the bias voltages converge as you turn the Intensity pot up in that last option?


My brain may not be fully engaged yet this morning, but is there some way to diddle the bias voltage first for tremolo and then have a bias balance between where the tremolo is injected and the power tube grids?

[TurboGuitarMelton]

Bias voltages interacted with each other even with trem on 0. Tremolo worked fine (a little thumpy though) but canceled out the two individual bias pots. Haha. Well, I learned a lesson here. Simple is good as PRR tried to tell me. Going to change to long tail phase inverter and keep my dual bias adjust.


Edit: meant to say Going to change BACK to long tail phase inverter to use up the extra triode and keep my dual bias adjust.

[mresistor]

Changing to LTPI is going to change the character of the PR though.  Tubes don't have to be biased exactly the same to sound good. In fact sometimes a little mismatch sounds better.

[pdf64]

… Going to change to long tail phase inverter and keep my dual bias adjust.

I don’t see how changing from a cathodyne to an LTP is going to help with anything in that regard?

[mresistor]

The distortion when driven just sounds different to my ears, everything else equal (which is probably not perfectly possible)


To quote a friend you all know well or know of 

[pdf64]

… To quote a friend you all know well or know of

… The LP will hit the power tubes harder (due to phase inverter gain) …

Dunno about that 
If, as with the Princeton Reverb, we’ve got 2 12AX7 sections to play with, they can be configured as a CC gain stage and cathodyne. Total voltage gain from CC grid to cathodyne outputs probably over 50, if cathode is bypassed and no global nfb.
Or they can be configured as an LTP, with total voltage gain from the LTP signal input to its outputs being about 30.
It looks to me more like the LTP option has less gain.

Regarding the overdriven scenario, the output valve control grids will probably clip way before the cathodyne or the LTP.
Depending on the output valve operating point, the degree of overdrive may need to be quite significant before they clip.

[TurboGuitarMelton]

… Going to change to long tail phase inverter and keep my dual bias adjust.

I don’t see how changing from a cathodyne to an LTP is going to help with anything in that regard?

I was saying that in regard to the unused triode. But I may just keep it  with the cathodyne PI anyway. The overdrive tone is amazing as is! 

[TurboGuitarMelton]

The distortion when driven just sounds different to my ears, everything else equal (which is probably not perfectly possible)

Actually I can confirm this! (From my own ears)


I initally had it as a LTP and the amp was louder and broke up a lot later on the volume knob. It was a smoother Overdrive.


The Cathodyne PI is a little more raw. Its only being driven by 2 gain stages so even on 10 it never gets raspy. Although theres a lot of Sag. (Which I like!)

[PRR]

....voltage gain from CC grid to cathodyne outputs probably over 50, .....voltage gain from the LTP signal input to its outputs being about 30. It looks to me more like the LTP option has less gain.

That 1.5:1 difference is easily made-up somewhere else.

The difference comes in grid-blocking. The cathodyne will putt different signal to each power tube and it can get large. It can grid-block, deeply, and asymmetrically. The LTP with the big power bottles is more likely to just-barely grid-block, controllable by the player. (With EL84 it does tend to block-out.)

[pdf64]

… Going to change to long tail phase inverter and keep my dual bias adjust.

I don’t see how changing from a cathodyne to an LTP is going to help with anything in that regard?

I was saying that in regard to the unused triode…

What unused triode?
Your Princeton has 2 12AX7, so 4 triodes, 2 are used as CC gain stages, 1 as a trem oscillator, 1 as a cathodyne.
Apologies for the erroneous reference to a Princeton Reverb previously 

....voltage gain from CC grid to cathodyne outputs probably over 50, .....voltage gain from the LTP signal input to its outputs being about 30. It looks to me more like the LTP option has less gain.

That 1.5:1 difference is easily made-up somewhere else.

Indeed, I was just pointing out that for most practical applications, the quote from robrob may be misleading.

The difference comes in grid-blocking. The cathodyne will putt different signal to each power tube and it can get large. It can grid-block, deeply, and asymmetrically. The LTP with the big power bottles is more likely to just-barely grid-block, controllable by the player. (With EL84 it does tend to block-out.)

Isn’t all that mostly just due to the LTP tending to have higher output impedance though?
And hence might the above issues attributed to the cathodyne be resolved by the use of higher value grid stoppers to the output valve control grids, and a grid stopper for the cathodyne itself?
As per Merlin’s suggestions http://www.valvewizard.co.uk/cathodyne.html