Princeton Reverb Bias Circuit

[jim]

I have this real nice Blackface Princeton Reverb in my shop.  Its owner wants the "authentic" Fender tone.  The old tubes are drawing ~10ma, the new pair of Phillips tubes I want to use draw~30ma of bias current.  Doug's library details how to convert the fixed bias to adjustable either using a 10K pot or
50K cermet pot.   After I change out the old elctrolytic there will be room on the bias board for the mod.  Is it cool to simply epoxy the cermet on edge onto the bias board?   I would love to remove the "ground"
switch and place a 10K pot there for external adjustment but the owner would not like that--so the adjustment pot needs to be inside the chassis. Is there a secure and more elegant way to add an internal adjustment pot?  Jim

[TubeGeek]

Doug sells a small 10k or 50k pot for bias.  It is small enough to be used on the bias board and not require drilling a hole in the chassis or epoxying to the chassis.

http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=PotsKnobs&ORDER_ID=161254089

[Willabe]

That's what I used on my BF PR for the bias, works fine.


                            Brad     

[jim]

Cool--If i solder one pin to an eyelet it will keep it still.  I'll try it.  Thanks

[sluckey]

Cool--If i solder one pin to an eyelet it will keep it still.

That's what I did on an old '57 Harvard. One pin soldered in an eyelet, the other pin bent and soldered to a 24KΩ resistor. I was gonna use hot melt glue but it wasn't necessary.

Here's a pic...
http://home.comcast.net/~seluckey/IMG_0617.JPG

[Fresh_Start]

I serviced an old Princeton Reverb a while back that had a bad filter cap on the bias board.  Instead of mucking around with the old board, I cut the old one out, put it in a baggie for the owner, and put in a new board with much better filtering.

Cheers,

Chip

[HotBluePlates]

I have this real nice Blackface Princeton Reverb in my shop.  Its owner wants the "authentic" Fender tone. 

Guaranteed that amp already has "authentic Fender tone". I've owned 3 or 4 of those over the years.

IF your meter can measure capacitance, check the value of all the low-voltage electrolytics in the amp (the ones in the brown cardboard). Be sure to write down what values you found for each cap. You will almost certainly find they've dried up somewhat and dropped well below 25uF; in fcat, they're probably more like 5uF. That's really not a problem, and in my opinion makes these amps sound better. Some folks purposely use 1-5uF bypass caps in the preamps of these amps for a clearer low end.

You may consider replacing the existing bypass caps, especially if they have bubbling on the ends, but you'll want to preserve the original "dried up" values to retain the amp's tone.

Other than that, a rebias (which it seems you're working on), as long as the speaker is a good original Jensen C10Q, you're just about set. Get some RCA preamp tubes (or Telefunken, if the owner has a source of them cheap) and that amp will sound golden.

Interestingly, my first tube amp was a '67 blackface Princeton Reverb. I didn't know anything about amps at the time, but I knew it sounded better than any solid-state amp. It was a couple years before I was even aware that all the preamp tubes in my Princeton were Telefunkens...

[Fresh_Start]

HBP has it right IMHO.  I would leave well enough alone except where you see an obvious problem, component values are WAY off, or there's DC voltage where there shouldn't be any.

It took me a while to find it, but I wrote down all of the actual component values in the Princeton Reverb I worked on.  The biggest problem was leaking coupling caps - found by checking for DC voltage where there shouldn't be any - and a bad electrolytic filter cap on the bias board.  However, the reverb mixing resistor which should be 3.3 meg had drifted up to over 3.7 meg and that was dumping too much of the dry signal.

Someday (not today) I'll mark up a Princeton Reverb schematic with the voltages and component values I found.  FWIW the cathode bypass caps were as follows:
V1a - 19uf
V1b - 17uf
V2a&b - 12uf
V3a - 11uf
V3b - 6uf

For some reason, I didn't blueprint the tremolo section.  Probably figured that it either worked or it didn't.

Oddly enough, the Treble, Bass and Volume pots all appeared to have drifted downward in value (216K for Treble, 198K for Bass, and my notes for the Volume pot don't make sense).  At the same time, cathode resistors all drifted up somewhat but plate resistors seemed close to spec.

Hope that helps,

Chip

[HotBluePlates]

Somewhere, I have a piece of notebook paper from 1996 where I wrote down all the parts values in my '67 Princeton Reverb. From what I remember, the bypass caps in my amp had dried up a lot more than in FreshStart's amp. It seems like 5-9uF was the norm.

[jim]

Hey Fresh--thanks for the quick lay-out, maybe I'll just do that.  It has a grounded plug and the cap can was replaced previously--a Mexican can, but that amp is quiet.  There is no hum, crackle, or hiss so I may leave everything else as it.  I'll use a RCA tall plate in V1, if I can find one in my collection that is quiet enough.  Thanks for all your help!  Jim

[Fresh_Start]

I'll bet HBP and others would be as interested as I am in the current values of plate resistors, cathode resistors, and cathode bypass caps.  If you can measure those values, would you please share them with us?

Chip

[jim]

I finished this amp.  I want to thank FreshStart for his bias board layout--it is exactly what I did.  Plate voltage is 450V. I adj bias to draw 22ma (like my own DR) The amp sounds great but breakup starts on  three. I bumped it down to 18 and there was no change.  I found a Mullard ECC83 in the phase inverter and switched it with the Groove Tube in V1 and its sounds a little better.  I have never played one of these old Princetons but I hope that early breakup is typical.  Now the tremelo is weak and as I recall the fix is to bias it colder?  Here is a pic:   Now all that remains is to relic the solder joints........just kidding.  Jim

[Willabe]

I have a BF PR and it doesn't start to break up on 3, that seems a little early?

What kind of guitar/PUP's are you using?


       
                                Brad     

[jim]

It has a replacement speaker -- a Weber 10F150 -- which checking his site, is one of his "aggressive,
early breakup" models.  Pushed with a SG....that explains the tone.   But just out of curiosity--where do you like your bias? 

[Willabe]

I'm not that fussy about it and I really should learn now to do the math for 70% dissipation, I think that's what the guys shoot for.

I've been setting it for about 35mA's per tube, but my B+ is sitting at around 420dcv with a GZ34 rectifier, which is the rec. tube listed on the tube chart inside the amps cab.


                   
                           Brad     


   

[jim]

Subbing a known good speaker cleans it up dramatically.  This Weber is really crunchy.  The trem is just OK--maybe I won't get any complaints.  There is a RCA GZ34 but B+ is a little high at 450V.  Oh well...I'm going to sew this box up--I am done.   Thanks for all the help  Jim

[Fresh_Start]

FWIW when biasing a Princeton Reverb, you have to take the tremolo into account.  After all, it's diddling the bias voltage.  What worked for me was to find the sweet spot for bias in terms of tone and then dial it just cold enough from there to get the trem working well.

Brad - don't worry about the exact percentage.  Anywhere from 60-80% maybe ok in your amp.  Just find the bias point that sounds the best in that range.  Be aware thought that if you're pushing 80% or more, the amp may eat power tubes like a Blues Jr. does.

Cheers,

Chip

[Willabe]

There is a RCA GZ34 but B+ is a little high at 450V.

Most PR's, AA1164, B1270, were shipped with a 5U4 and it was listed on the Fender tube chart inside the amps cab. If your amp was designed for a 5U4 and you put in a GZ34 that's why your B+ is a little on the high side.

The AA764 was shipped with a GZ34 and from what I've read not many were made that way. I don't think there's even a schemo for it from Fender?    

Brad - don't worry about the exact percentage.  Anywhere from 60-80% maybe ok in your amp.  Just find the bias point that sounds the best in that range.  Be aware thought that if you're pushing 80% or more, the amp may eat power tubes like a Blues Jr. does.

Thanks Chip, I don't try to run my amps as hot as I can.

I still should learn the math just as a referance/safty point though, wouldn't hurt.    


                   Brad      

[jim]

The plate voltage is 438 with the tubes IN.   oops.  I did reduce bias stepwise untill the trem came alive
and I left it at 16ma.  It still sounds excellent. Thanks for your help.  Jim

[Fresh_Start]

Best article I've found on biasing:
http://www.duncanamps.com/technical/lvbias.html

Cheers,

Chip