Grid Bias Balance?

[Bluto!]

Hello Everyone, first post!

My friend asked me to repair his Princeton Reverb Reissue and it was fried up and down, so in lieu of trying to replace the transformers and hunting down what caused this catastrophic failure, I suggested that we gut it and do it properly, down to ceramic bases for the 12a* tubes. I have built mostly pro audio gear like this, so I have a basic understanding of the 'right' way to do this.

My question is this. On grid fixed amps, we have that voltage divider at the lower grid coming from the bias control. Is there any benefit to placing a 10k lin pot in the circuit, wiper to the bias current and the 2 ends to the 220k, to balance the bias voltage between the tubes?

I recall seeing it somewhere, or maybe it was in a dream. 

Does this matter for rock? The cleanest is not a design consideration, noise is. This is to become a studio amp. A personal goal is for it to outlive me with no repairs. 

Thank you for your knowledge!

[vampwizzard]

howdy! Welcome to the boards.

Theres really no benefit to bias balancing output tubes on guitar amps when we can get matched tubes at really affordable rates.

[sluckey]

Never seen a fried amp before. Could you post a pic of that?

I recommend don't modify the bias circuit. That will probably mess up the tremolo.

Hoffman has a turret board for the Princeton Reverb. Take a look. I highly recommend going that route if you can't fix the PRRI.

[shooter]

Never seen a fried amp before

find a peavey forum 
no pic's from the early tube amp pcb board days, but a couple were black crispy, one caught fire as I was watching the show, plenty of beer handy to extinguish 

[Bluto!]

howdy! Welcome to the boards.

Theres really no benefit to bias balancing output tubes on guitar amps when we can get matched tubes at really affordable rates.

I like this answer!

As far as getting the Hoffman board, I checked it out and I didn't want to use a can cap so I laid out something similar. I know I'm in the deep end, swim or sink!

Yes, this amp is fried. The power and output have windings that short to ground. There should be some DCR, not a beep!

Sluckey, you have lifetimes more experience than me and I'm here to learn. I'm going to leave that part stock, but for didactics, I don't see how tuning the voltage divider at the end of the bias chain would mess with the tremolo. The 2 220k resistors are 5% on the schematic so 210k-230k is within tolerance. They're also grid leaks, but I don't think that asymmetry there would matter. Can you tell me what's missing in my logic?

Thank you for the replies!

[sluckey]

Yes, this amp is fried. The power and output have windings that short to ground. There should be some DCR, not a beep!

That may not indicate any problem. Some of the PT and OT wires are ***CONNECTED*** to ground. Further testing needed.

I don't see how tuning the voltage divider at the end of the bias chain would mess with the tremolo.

Maybe not. I did say probably. Try it and see. You did notice that the tremolo is tied to the bias circuit?

[Bluto!]

Yes, this amp is fried. The power and output have windings that short to ground. There should be some DCR, not a beep!

That may not indicate any problem. Some of the PT and OT wires are ***CONNECTED*** to ground. Further testing needed.

I don't see how tuning the voltage divider at the end of the bias chain would mess with the tremolo.

Maybe not. I did say probably. Try it and see. You did notice that the tremolo is tied to the bias circuit?

In my initial inspection, I was able to take the PT out of circuit to test it. Primary is open, some secondaries are open, some short. The fuse was good but I didn't check the rating. Ooops. There's so much to catch! The DCR on the OT primaries are at 280R (2@140R) but the secondary reads 1.5Meg (it had been a minute since my initial diagnosis. I remember it being out of whack) . I really should jam some AC voltage through the OT and see where it lays, but 1.5Meg sound too high.

I'm thinking one of the rattling 6v6s caused a short.

As for the bias wiggle, yes. What it looks like to me is that the .1 cap gets charged in both directions with the LFO fluctuating the negative bias voltage with a positive DC voltage osc. Am I in the ballpark? I'm curious what the actual voltage swing of the LFO circuit is before and after the 1M (and how to figure it out if it's not to NASA). Also if its AC component reaches the output grids.  (Is it that the 1M-.02uf form a LPF?) The answers will all be there when it's built! Learn by doing.

Thanks for helping me understand.

[mresistor]

I recently replaced the stock circuit board and pots in a PRRI that had a filter cap go bad on the PCB.  Leaked all over the place. I measured out the length I could use in the amp, got the board material and turrets and layed out a HPR circuit on it.  I was considering doing it Allesandro style, but went with Hoffmans circuit.  I made the board long enough to mount 3 HV PS filter caps on the end and mounted the preamp HV filter cap nearer the input jack behind the pots and on the board. Did not use a cap can.  It worked out quite well.  Hoffmans layout has the bias pot on the turret board..  Allesandros uses the stock bias pot and if you view his conversions it is a little difficult to determine just where some of the bias components are located.  I was going to also use the stock bias pot, but dicided it was easier to use the trim pot on board then running wires over to the bias pot. Covered the hole up.  Those Fender board are pretty flimsy, and notice they are mounted with 8 or 10 standoffs because of that fact.  I used 4 or 5 on mine because the board material is very stiff. If you do decide to do an HPR conversion be advised you also have to drill out the pot holes on the chassis and faceplate to fit full size 3/8" mount pots if you use them.  But a step drill makes that pretty easy.

[mresistor]

[Bluto!]

I recently replaced the stock circuit board and pots in a PR that had a filter cap go bad on the PCB.  Leaked all over the place. I measured out the length I could use in the amp, got the board material and turrets and layed out a HPR circuit on it.  I was considering doing it Allesandro style, but went with Hoffmans circuit.  I made the board long enough to mount 3 HV PS filter caps on the end and mounted the preamp HV filter cap nearer the input jack behind the pots and on the board. Did not use a cap can.  It worked out quite well.  Hoffmans layout has the bias pot on the turret board..  Allesandros uses the stock bias pot and if you view his conversions it is a little difficult to determine just where some of the bias components are located.  I was going to also use the stock bias pot, but dicided it was easier to use the trim pot on board then running wires over to the bias pot. Covered the hole up.  Those Fender board are pretty flimsy, and notice they are mounted with 8 or 10 standoffs because of that fact.  I used 4 or 5 on mine because the board material is very stiff. If you do decide to do an HPR conversion be advised you also have to drill out the pot holes on the chassis and faceplate to fit full size 3/8" mount pots if you use them.  But a step drill makes that pretty easy.

mresistor,

Nice build! This is almost exactly what I'm doing. Layout is similar, but I'm orienting C and D caps a bit differently. This may now change, as I'm sure it will once I get the real parts.  As of now, I laid out the bias right next to that 9k(?) pot.

I have some PEC pots laying around for pimp controls. They feel great. The same issue about holes applies to the Marshall style inputs I plan on using. I don't have the bit for the cap hole, which has led me down this beautiful rabbit hole.

[mresistor]

In hindsight  once the 25K bias pot was removed there was probably enough room to mount the HVPS filter caps in that area and I could have shortened the board and mounted them all on the chassis with some terminal strips, they don't all have to be in the same place. Next time I will reconfigure where the big caps go, because I feel the board could be shortend and moved more towards the PT so the wires connecting to the tubes are shortened and more direct. There is also the problem of standoff bolt positions to deal with.  It is advisable to mount the preamp PS filter cap near the preamp ckt. Doug has even mounted all the HVPS filter caps up between the board and pots.  I will say that once you decide to do this there is no turning back..
 I do like Allesandros method and want to try that sometime. His looks like an AA1164 circuit on a turret board but I have no idea what the turrets are that he uses.  And he isn't talking ..  LOL 

[Bluto!]

Please excuse the rounded wire paths. B+ will go above board, wrapped. Also, omit the bias balance and the input shields will go to chassis, not the preamp ground. I've modded the inputs, making the second inject after the tone, added a pre PI master volume, hum balance, a different voicing option for the first stage, output brake, feedback switch, slower trem, floated grounds, bleed resistor, and a tap there to DC float the heaters. 

I'm not asking for a work check, but if anyone sees anything gross, I'd appreciate it. Fun!

[mresistor]

Oh sorry   I see that was a 10K bias pot..  your pic cut off the pot connections..

[Bluto!]

Oh sorry   I see that was a 10K bias pot..  your pic cut off the pot connections..

Snap. I laid it out over the workspace. Note for next time. The top pot connections and the speaker f/b are cropped, but there.

[PRR]

> benefit to placing a 10k lin pot in the circuit, wiper to the bias current and the 2 ends to the 220k, to balance the bias voltage between the tubes?

How can that "balance the bias voltage between the tubes?"??

There's no current in the 6V6 grids! So no voltage drop. And no change-of-drop with change of resistance.

It will *seem* to make a difference while you poke a grid with a meter due to meter loading.

OK, there's always a current, and that's why 6V6 is rated 100k MAX grid resistor in fix-bias, so the bias does not get out of hand on a worst-case 6V6. They rarely specify actual grid current, but "bad" may be 0.5uA on a small tube and 5uA on a power tube. 5uA in all 10k makes a 0.050V change, which next to a nominal 37V bias is "nothing" (<1%).

There are ways to do this. Amps that live-or-die based on THD Meter readings have a trim so they can look good in the review.

We do NOT do this on guitar amps. A little lop-side unbalance ripens the soft tone. The bias has almost nothing to do with FLAT-OUT volume or tone. It is an extra part, to buy, install, trim, to fail, or get mis-trimmed by the next tech (tho this gimmick does no harm until the pot wiper loses contact, as it will).

[Bluto!]

PRR!

I've followed your posts way back at Prodigy Pro. 

Thank you for your insight. That all makes a lot of sense.