Princeton Reverb

SEK_Hakuna · February 20th, 2008, 05:03 PM

Can anyone show how to install an adjustable bias in a Princeton Reverb? Torres Engineering offers a kit with instructions for a modest price, but...........

After my post on the 5E3 volume mod from Mission Amps, I thought perhaps some TDPRI members may have some info and layout diagrams for the mod. I don't understand schematics very well and, well....they really confuse me to be honest. Layout diagrams work for me and that's how I successfully constructed 6 tube amps, ( with a little help from some great fellas here).

I own a Fender Princeton Reverb II and found instructions for adding a bias adjustment at this website -> http://www.geocities.com/princerev2/ and had good success with that. I want to add this feature to my PR just to play around with different settings.

I found this http://www.el34world.com/charts/bias_conversions.htm info but it's not for the PR amp. The layout looks similar though.

micpoc · February 20th, 2008, 07:46 PM

At the same site, there is this:

http://www.el34world.com/charts/Biascircuits.htm

Scroll down for PR info.

Dacious · February 20th, 2008, 07:48 PM

Edit: Beat me to it.

SEK_Hakuna · February 20th, 2008, 08:42 PM

How'd I miss that?

Thanks, I can do the mod from that information.

Dacious · February 21st, 2008, 11:35 PM

I just did this mod to my amp - it had a 110K range resistor and a 22K bias resistor (as measured on analogue and DMM). The tubes were pulling 40-45ma each (!). These JJ 6V6s must be tough! This pair is also pulling a lot of current, it seems. I have a socket adaptor that lets me measure current at pin 8.

The new 240 volt primary power transformer I installed was running pretty hot, too. It actually dropped about 10 volts B_+ over the original US primary with stepup transformer I was using. I put in a 27K resistor and the 50K trimpot and it biased up right just off the bottom stop, measures about 32K cumulative.

I've pulled them down to about 18-20ma at 410 plate volts, and the amp 'seems' happier - still plenty of poke, nice cleans etc. The JJs seem well able to take the abuse.

I have a feeling QC was not too good on this amp from Fender - it's got a centre tap to the 6.3 volt circuit, but it still had the double 100 ohm resistors off the heaters, and the values of the bias resistors aren't to spec. Unless they were using some really low-current tubes. I'm thinking from the near perfect condition this amp was a low-volume, home use unit that never got cranked, which is the only reason it never cooked it's power tranny.

SEK_Hakuna · February 22nd, 2008, 12:37 AM

I biased my amp manually a while back but can't remember what values I used. I purchased some more resistors today but my local supplier did not have a bias pot. I'll check Radio Shack tomorrow. I hope I don't have to order one off the net....geesh...shipping will be expensive for a $2.99 item.

It's a good thing you checked yours huh? So often we assume things are okay when they really aren't. Hmmmmmm...I wonder if my Oldsmobile is gonna crap out on me tomorrow? LOL

Paul G. · February 22nd, 2008, 07:31 AM

As long as you're in there, put 1 ohm resistors between the cathodes and grounds.

P.

SEK_Hakuna · February 22nd, 2008, 10:00 AM

Does that mean add a resistor to pin 8 of the power tubes to ground? That's what I did on my PRII to make checking voltages much easier.

Dacious · February 22nd, 2008, 11:16 AM

Just be sure they're precisely 1 ohm or your mv reading will be out by whatever decimal factor they are. And obviously if one tube has a 1.2 ohm and the next a 1.0 you are going to get a mismatched reading. Those sandbox resistors seem to suffer more variance. I had to measure a dozen to get two that read the same. Ended up not using them.

I use Doug Hoffman's bias-checker design which allows a DMM to be direct inline with pin 8 and read the ma current between cathode and plate - as accurate as your DMM is. I've been told it is prone to oscillate and indeed it did so with one amp, a Laney Marshall-clone if I wasn't careful. But it seems fine with my Fenders.

Rechecking mine tonight it was 20-22ma @ 415v, so I dropped it to ~16/16.5@445. This still highish, but the JJs seem to work fine with it. It takes longer to unship the chassis than to adjust.

That works out to just over 14 watts at the plates.

http://www.el34world.com/charts/biascurrent.htm

JohnnyCrash · February 22nd, 2008, 03:58 PM

I did that mod to a PR I built into a Champ chassis. Added bigger iron and threw 6L6's in there. I used 1 ohms to bias terminals in the back of the amp, so I can check and change bias without pulling the chassis.

As usual, nothing in life is ever 100% accurate, so don't get too hung up on "exact" bias. Get it in the ballpark and then adjust to taste.

SEK_Hakuna · February 22nd, 2008, 09:30 PM

I got the Bias pot installed and soldered in the 1 ohm cathode resistors. While I was in there I tidied up a few of the wires and tucked everything in out and around.

I biased at 15 ma which, after a few sound tests at other voltages, seemed to be the sweet spot. It's really nice to have an easily adjustable bias to get it just right.

I really like this little PR clone. One sweet amp for sure.

SEK_Hakuna · February 24th, 2008, 09:42 PM

I had to dig this thread out of the hole.

Five posts up I commented about the possibility of my Oldsmobile crapping out on me. Well....yesterday it happened.

A coolant fitting made of cast crap zinc something rotted thru and spewed anti freeze on the engine. I go to take it out and half the threads are stuck inside the engine block. It literally rotted off inside the threaded hole. I spent 3 hours today trying to get the broken off piece out so the new one can go in. It got dark and I'll have to finish up tomorrow. If I don't get this thing fixed I'll be out of an Oldsmobile.

At least my amps work good.

Dacious · February 25th, 2008, 08:58 PM

I used my Princeton last night at a band practice. I was on '5' where it would break up nicely with my Thinline, the other guitarist was using a 59 reissue Les Paul into a Gibson GA30, the bass player his upright into a Gallien Kruger 400 with a drummer.

Easily, easily kept up - in fact they asked me to turn down as I was overpowering the 400w/side and 15" speakers PA, so I plugged into the second input which thinned the sound a little.

At the end of 3-4 hours the power tranny was cool enough for me to touch it with comfort. Big improvement over the stock one which was almost blisteringly hot the week before.

It seems the more robust windings of the Weber (extra amp on the 5v and 6.3v and 30-40ma on the secondary) help it to keep it's cool.

I don't know what the speaker is apart from a 10" ceramic blue-label Emi with a 1980 date code, but it has plenty of output and no lack of bass!

Its only a little box consuming 14 watts but boy, is it loud!

SEK_Hakuna · February 25th, 2008, 09:51 PM

Sounds like you had a good time. When a guy has confidence in his equipment it seems to help him play better.

I have a Jensen C10Q in mine. It is unreal how much low end these PR's have. Especially for a 10" speaker. I played my bass guitar thru it one nite....just to loosen up the speaker a bit. I was amazed how tight it sounded. Low volume practice is very possible with the PR.

February 20th, 2008, 05:03 PM	#1 (permalink)
SEK_Hakuna Tele-Meister Join Date: Jul 2003 Location: Kansas Age: 52 Posts: 182 15 Photos	Princeton Reverb Can anyone show how to install an adjustable bias in a Princeton Reverb? Torres Engineering offers a kit with instructions for a modest price, but........... After my post on the 5E3 volume mod from Mission Amps, I thought perhaps some TDPRI members may have some info and layout diagrams for the mod. I don't understand schematics very well and, well....they really confuse me to be honest. Layout diagrams work for me and that's how I successfully constructed 6 tube amps, ( with a little help from some great fellas here). I own a Fender Princeton Reverb II and found instructions for adding a bias adjustment at this website -> http://www.geocities.com/princerev2/ and had good success with that. I want to add this feature to my PR just to play around with different settings. I found this http://www.el34world.com/charts/bias_conversions.htm info but it's not for the PR amp. The layout looks similar though.

February 20th, 2008, 07:48 PM	#3 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,622 35 Photos	Edit: Beat me to it. __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

February 21st, 2008, 11:35 PM	#5 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,622 35 Photos	I just did this mod to my amp - it had a 110K range resistor and a 22K bias resistor (as measured on analogue and DMM). The tubes were pulling 40-45ma each (!). These JJ 6V6s must be tough! This pair is also pulling a lot of current, it seems. I have a socket adaptor that lets me measure current at pin 8. The new 240 volt primary power transformer I installed was running pretty hot, too. It actually dropped about 10 volts B_+ over the original US primary with stepup transformer I was using. I put in a 27K resistor and the 50K trimpot and it biased up right just off the bottom stop, measures about 32K cumulative. I've pulled them down to about 18-20ma at 410 plate volts, and the amp 'seems' happier - still plenty of poke, nice cleans etc. The JJs seem well able to take the abuse. I have a feeling QC was not too good on this amp from Fender - it's got a centre tap to the 6.3 volt circuit, but it still had the double 100 ohm resistors off the heaters, and the values of the bias resistors aren't to spec. Unless they were using some really low-current tubes. I'm thinking from the near perfect condition this amp was a low-volume, home use unit that never got cranked, which is the only reason it never cooked it's power tranny. __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

February 22nd, 2008, 07:31 AM	#7 (permalink)
Paul G. Tele-Holic Join Date: Mar 2003 Location: Rhode Island Posts: 885	As long as you're in there, put 1 ohm resistors between the cathodes and grounds. P. __________________ Listen to your mother

February 22nd, 2008, 11:16 AM	#9 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,622 35 Photos	Just be sure they're precisely 1 ohm or your mv reading will be out by whatever decimal factor they are. And obviously if one tube has a 1.2 ohm and the next a 1.0 you are going to get a mismatched reading. Those sandbox resistors seem to suffer more variance. I had to measure a dozen to get two that read the same. Ended up not using them. I use Doug Hoffman's bias-checker design which allows a DMM to be direct inline with pin 8 and read the ma current between cathode and plate - as accurate as your DMM is. I've been told it is prone to oscillate and indeed it did so with one amp, a Laney Marshall-clone if I wasn't careful. But it seems fine with my Fenders. Rechecking mine tonight it was 20-22ma @ 415v, so I dropped it to ~16/16.5@445. This still highish, but the JJs seem to work fine with it. It takes longer to unship the chassis than to adjust. That works out to just over 14 watts at the plates. http://www.el34world.com/charts/biascurrent.htm __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

February 20th, 2008, 07:46 PM	#2 (permalink)
micpoc Tele-Holic Join Date: Mar 2007 Location: Louisiana Posts: 675	At the same site, there is this: http://www.el34world.com/charts/Biascircuits.htm Scroll down for PR info.

February 20th, 2008, 08:42 PM	#4 (permalink)
SEK_Hakuna Tele-Meister Join Date: Jul 2003 Location: Kansas Age: 52 Posts: 182 15 Photos	How'd I miss that? Thanks, I can do the mod from that information.

February 22nd, 2008, 12:37 AM	#6 (permalink)
SEK_Hakuna Tele-Meister Join Date: Jul 2003 Location: Kansas Age: 52 Posts: 182 15 Photos	I biased my amp manually a while back but can't remember what values I used. I purchased some more resistors today but my local supplier did not have a bias pot. I'll check Radio Shack tomorrow. I hope I don't have to order one off the net....geesh...shipping will be expensive for a $2.99 item. It's a good thing you checked yours huh? So often we assume things are okay when they really aren't. Hmmmmmm...I wonder if my Oldsmobile is gonna crap out on me tomorrow? LOL

February 22nd, 2008, 10:00 AM	#8 (permalink)
SEK_Hakuna Tele-Meister Join Date: Jul 2003 Location: Kansas Age: 52 Posts: 182 15 Photos	Does that mean add a resistor to pin 8 of the power tubes to ground? That's what I did on my PRII to make checking voltages much easier.

February 22nd, 2008, 03:58 PM	#10 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	I did that mod to a PR I built into a Champ chassis. Added bigger iron and threw 6L6's in there. I used 1 ohms to bias terminals in the back of the amp, so I can check and change bias without pulling the chassis. As usual, nothing in life is ever 100% accurate, so don't get too hung up on "exact" bias. Get it in the ballpark and then adjust to taste.

February 22nd, 2008, 09:30 PM	#11 (permalink)
SEK_Hakuna Tele-Meister Join Date: Jul 2003 Location: Kansas Age: 52 Posts: 182 15 Photos	I got the Bias pot installed and soldered in the 1 ohm cathode resistors. While I was in there I tidied up a few of the wires and tucked everything in out and around. I biased at 15 ma which, after a few sound tests at other voltages, seemed to be the sweet spot. It's really nice to have an easily adjustable bias to get it just right. I really like this little PR clone. One sweet amp for sure.

February 24th, 2008, 09:42 PM	#12 (permalink)
SEK_Hakuna Tele-Meister Join Date: Jul 2003 Location: Kansas Age: 52 Posts: 182 15 Photos	I had to dig this thread out of the hole. Five posts up I commented about the possibility of my Oldsmobile crapping out on me. Well....yesterday it happened. A coolant fitting made of cast crap zinc something rotted thru and spewed anti freeze on the engine. I go to take it out and half the threads are stuck inside the engine block. It literally rotted off inside the threaded hole. I spent 3 hours today trying to get the broken off piece out so the new one can go in. It got dark and I'll have to finish up tomorrow. If I don't get this thing fixed I'll be out of an Oldsmobile. At least my amps work good.

February 25th, 2008, 08:58 PM	#13 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,622 35 Photos	I used my Princeton last night at a band practice. I was on '5' where it would break up nicely with my Thinline, the other guitarist was using a 59 reissue Les Paul into a Gibson GA30, the bass player his upright into a Gallien Kruger 400 with a drummer. Easily, easily kept up - in fact they asked me to turn down as I was overpowering the 400w/side and 15" speakers PA, so I plugged into the second input which thinned the sound a little. At the end of 3-4 hours the power tranny was cool enough for me to touch it with comfort. Big improvement over the stock one which was almost blisteringly hot the week before. It seems the more robust windings of the Weber (extra amp on the 5v and 6.3v and 30-40ma on the secondary) help it to keep it's cool. I don't know what the speaker is apart from a 10" ceramic blue-label Emi with a 1980 date code, but it has plenty of output and no lack of bass! Its only a little box consuming 14 watts but boy, is it loud! __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

February 25th, 2008, 09:51 PM	#14 (permalink)
SEK_Hakuna Tele-Meister Join Date: Jul 2003 Location: Kansas Age: 52 Posts: 182 15 Photos	Sounds like you had a good time. When a guy has confidence in his equipment it seems to help him play better. I have a Jensen C10Q in mine. It is unreal how much low end these PR's have. Especially for a 10" speaker. I played my bass guitar thru it one nite....just to loosen up the speaker a bit. I was amazed how tight it sounded. Low volume practice is very possible with the PR.