Recapping a Pro Reverb: Questions

[Blue Shadows]

Recap questions for those with more experience than me. I recently acquired a 1976 Pro Reverb in very clean condition, pristine I would even say.  It has the push/pull Master VOlume boost, but is *not* the ultralinear model, as it is 45 watts and has the 5u4 rectifier.  Everything is original so I will need to do some routine maintenance in terms of getting it up full working order.  It sounds great; only issues are it is biased a bit cold (30/31 mA on the 6L6s, whereas closer to 40 mA should close in on the 70% of max dissipation) and the tremolo is pretty weak.  I am hoping a simple recap will do the job.

I will definitely be replacing everything in the doghouse, filter caps and resistors. I was able to find 22uF 500V caps, which is close enough to the spec'ed 20uF. For the other two filter caps wanting 70uF, I found some 80uf 450V caps that I am thinking should also be close enough to be close enough. For the bias board, also getting a full swap, the closest I could find to the spec'ed 80uF 75V were 100uf 100V...is that still close enough? Technically a hair over 20% variance, but thinking it should still be "close enough"...although let me know if I am way off on that. And then from my readings I feel like there is a lot of debate over whether to touch any coupling caps unless they are not functional, and then also the cathode bypass caps, which I believe I identified correctly on the layout below, which I do plan on changing out but please correct me if I have misidentified those, or whether those "need" changing. Anything else for routine maintenance? I am hoping that this update will both normalize the bias and also bring the tremolo back to life, but if not I will also swap that tube (first) and then look at replacing the oscillator caps, which I believe are the trio of .01, .01, .022 caps in the middle of the board. If bias remains off I will look at installing a bias pot, as I have done that before. Still learning on this journey, so if I am spitting any blunders be kind, but I appreciate all of the knowledge to be gained from this board!

[HotBluePlates]

... I was able to find 22uF 500V caps, which is close enough to the spec'ed 20uF. For the other two filter caps wanting 70uF, I found some 80uf 450V caps that I am thinking should also be close enough to be close enough. ...

Close enough!

... For the bias board, also getting a full swap, the closest I could find to the spec'ed 80uF 75V were 100uf 100V...is that still close enough? ...

Close enough!  (Actually, slightly better)

[shooter]

It sounds great; only issues are it is biased a bit cold

fwiw;

last time I repaired one, I did some dynamic testing, found that biased normal, about 67% and playing hard, PA tube dissipation was averaging real close to "my" danger zone.  I backed it off a smidge closer to 60%

[Latole]

I never bias a amp at 70%, never over 60 % .
If you play loud, you may burn power tubes.

I test with guitar and I never hear a difference in tone,

[Blue Shadows]

Sure thing about the bias...I don't want it too hot, but by my maths it is currently set at about 48%, which I would consider to be pretty low.

Any thoughts on the cathode bias caps?  Did I identify those correctly, and are they worth the swap as well while I am in there?  Any other caps worth replacing (or not touching)?

[pdf64]

Sure thing about the bias...I don't want it too hot, but by my maths it is currently set at about 48%, which I would consider to be pretty low.

What's the rationale by which you make that conclusion?

Any thoughts on the cathode bias caps?  Did I identify those correctly, and are they worth the swap as well while I am in there?  Any other caps worth replacing (or not touching)?

All vintage ecaps should be checked as ok, including their ESR and DC leakage at rated voltage, or just replaced.

[Blue Shadows]

Sure thing about the bias...I don't want it too hot, but by my maths it is currently set at about 48%, which I would consider to be pretty low.

What's the rationale by which you make that conclusion?

Good question.  For now, the main thing that has caught my ear, which is not necessarily bothersome, is the fact that the amp starts to break up earlier than I would anticipate (about 4-5 on the channel volume).  My understanding, which may be flawed, is that hotter biases--depending on the amp--can keep the tone cleaner higher, whereas cooler biases help with earlier breakup, and people may adjust their bias accordingly to taste. 

[HotBluePlates]

... the amp starts to break up earlier than I would anticipate (about 4-5 on the channel volume).  ...

That's about what the 1966 Pro Reverb I used to own did.  I would consider that "normal" for most blackface Fender amps.

... My understanding, which may be flawed, is that hotter biases--depending on the amp--can keep the tone cleaner higher, whereas cooler biases help with earlier breakup ...

Typically the opposite.

    - Hotter Bias:  higher plate current by reducing the negative grid-volts.  The output tube distorts with a smaller signal.

    - Cooler Bias:  lower plate current by increasing the negative grid-volts.  The output tube accepts a larger input signal before distorting.


Overall, a tube will distort when the peak drive signal equals or exceeds the grid-bias voltage (absent a special driver circuit).  So increasing the (negative) grid-bias voltage keeps the output tube cleaner, longer.

[Blue Shadows]

Learning.  Appreciated!  Mine is, however, a Silverface.  I know that the Blackface circuits are known for earlier breakup, but I thought that the silverfaces of this line were supposed to more closely resemble a lower watt Twin in terms of keeping it cleaner until higher volumes, but perhaps that is all relative to the decibels...not sure I have ever got a twin up to 5 on the dial...

But I should add that I am also not really sweating the cooler bias, I've just always read and heard that many individuals tend to keep them hotter than 48% max, but maybe not after all.  I also am not intending to "clean up" the sound necessarily, it just breaks up earlier thatn I would have thought, but it is a good sound all in its own.

Once I get the caps changed I will re-check everything and if the bias is in the same neighborhood I will just leave as is.

[shooter]

not sure I have ever got a twin up to 5 on the dial...

It's not for the faint of heart!
as a tech, I test at full volume, monitoring PA currents.  You can do it with a dummy load, since i'm in the middle of nowhere, playing poorly at 10 just makes me smile.

[Calboy]

On the board it looks like 25uf cathode capacitors connected to v1,v2v,v3 and 5uf connected to v5.
It might be worth changing the filter caps and bias caps first and then revisiting the cathode caps on the board if needed.
And adding a bias pot if needed.
For a 1976 fender, others might replace all the electrolytic caps at the same time.