Questions about my Princeton Reverb

[sparky]

The AB1270 schematic, which my PR goes by (although it was made a bit before December) shows a .002mf (2000pf) cap in parallel with the 220K resistor at the reverb footswitch jack. What is the reason for this cap? The AA1164 schematic doesn't have it...

Also, after poring over the schematics, it seems that the only differences in the AB1270 circuit and the AA1164 circuit are this cap, a different value cap in the bias cirucit (50uf/70VDC, rather than 25uf/50VDC), and that a couple of plate resistors are changed from 100K, 1/2 watt to 100K, 1 watt. Am I missing anything else?

What would I gain by changing these components to the values in the AA1164 circuit?

[stokes]

That 2000pf cap is likely a suppression cap for parasitics.The 50uf/70v is an upgrade from the 25uf/50v.Either will work,the 50uf will work better.A 100k 1watt will take more "heat" than the 1/2 watt,but wont affect anything else.Cant put my hands on the AB1270 schem at the moment,but if it is a Silver Face as opposed to the AA1164 Black Face there are likely other differences,in the amp itself but wont necessarilly show up on the schem.Some early Silver Faces still had some Black Face components and wiring during the transition from Silver to Blackface.To make things even more confusing some late Blackfaces have Silverface traits as they used all the parts on hand and there are some amps with a mix of circuits and faceplates.

[Enzo]

Or possibly it is there to reduce the likelyhood of the reverb return picking up local radio stations.

[Richard]

Could it be there to avoid a "pop" when switching the reverb on/off. I've heard that the Princetons and smaller amps didn't receive as much of the dreaded CBS treatment as the larger ones.

[Enzo]

My first impression is that that would be counterproductive, but I couldn't rule it out.

[sparky]

Thanks, guys...

I'll probably just leave those things well enough alone...

I just got the amp and am getting ready to recap it, change the power cord to a 3-prong, and disconnect the ground switch. I'm thinking of adding a standby switch and a bias pot, as well....

I always hate to mess with an amp that hasn't been touched (this one still had the original tubes), but it needs some general maintenance. The bass response is very mushy and that should tighten up with a cap job, I think.

[stokes]

Just found a schem for the AB1270 and like Richard said this amp doesnt look the same as most other SF amps.You should already have a bias pot.Dont "hate to mess with an amp that hasnt been touched".Tubes and caps have a limited life span and need to be changed.Some of the worst sounding vintage amps are the ones that were left in a closet and not used in 20 years,they look great but sound like crap.I would suggest using 40uf for the first 2 caps,this will further tighten things up,and dont go with those cheapo Jap caps,get good quality Spragues.The 3 prong cord and standby switch are good ideas as well,especially the cord.That cap on the grid of the reverb output could be for any or all the reasons given here,I would leave it alone.Priority would be the cap job those caps are definately done.

[sparky]

Where would the bias pot be? The bias board is in the usual place, but there's no pot there, only a 27K resistor. I was thinking of installing a 10K pot with the resistor soldered to it -- using Doug Hoffman's layout.

The Sprague caps should be here in a couple of days from Mojo (AES was out) and the can should be here by Friday from AES.

Thanks for the advice on the 40uf caps; I may give that a try.

I'm pretty sure that the PR is a fixed, non-adjustable bias. I've been inside the amp and through the schematic and there's no pot. I was gonna add the 10K pot and move the 27K resistor over to it as Doug Hoffman suggests...

[Tom Phillips]

I've never seen a stock Princetone or PR with a bias adjust pot. I have added them as you discussed though and with the proper parts you can make it look stock to someone who isn't real familiar with the Fender guts.
I have taken that cap off the reverb return with no ill effects. However, I can't state that I've seen a specific improvement just because of the removal of the cap since I usually remove it as part of other repair / refurb work.

[Amp Kat]

However, I can't state that I've seen a specific improvement just because of the removal of the cap since I usually remove it as part of other repair / refurb work.

Like Blackfacing them which is what most people ask for IMO.
If your going to do all of the other mods you may as well just go ahead and blackface it unless you don't feel up to it.

[sparky]

I'm up to it...

As I said in the first post, after looking at the schematics I really don't see any differences other than those I've mentioned.

Are there more changes to make in Blackfacing the amp?

[stokes]

Sorry,my mistake,I didnt look very close,the pot I was looking at is the tremolo,I was looking for the typical balance pot that most SF's have.You would have to add the bias pot.

[Amp Kat]

Yes, that's right Stokes and that circuit is quite a strange one as I think it's the only one that is biased quite like that. I wonder if turning the Intensity pot has any effect on the bias ? Sparky if you look at the Deluxe AA1172 and A1270 you'll see the 2000pf caps on the reverb jack as well and that circuit is very much like the B1270 even the 5U4 but it's bias circuit is along the typical true bias array and the Intensity pot seperate. Man that's your call as to changing all that. I play a AA1164 Princeton all the time and it's a beaut of a gem sound wise. I'm not sure what this amp will sound like with that tremelo/bias topology but the bias set could be a problem as opposed to how much easier it could be.

[sparky]

Amp Kat,

So are you saying that you wouldn't add the bias pot? Or that it would be much easier to bias with the pot (my thoughts). Of course, I could check it and find that it's working fine, as is.

I'm not getting into any of that until I get the recap done, though....

[Amp Kat]

Amp Kat,

So are you saying that you wouldn't add the bias pot? Or that it would be much easier to bias with the pot (my thoughts). Of course, I could check it and find that it's working fine, as is.

I'm not getting into any of that until I get the recap done, though....

Well the problem is the circuit only uses 1/2 of the 12AX7 to form an oscillator where the typical blackface uses a neon bulb/varistor and 1 whole duo triode to drive it. You could most likely add a pot where the 27k resistor goes but I'm not sure it wouldn't interact with the intensity pot unless you totally isolated it and that is part of the oscillator circuit. I'd try it as it would be quite simple and yes much better than changing the resistor every time. The amp using that topology may give it a unique sound from being so different and you may like it better.