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Post subject: Fender Princeton Reverb Silverface Restoration Help
Posted: Sun Dec 08, 2013 10:35 am
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Hi guys! I'm new on this forum and decided to join after reading a few of your posts...

I've had this amp for a few years. It was given to me by a friend who left the country and couldn't take it with him. I used the amp for quite a while with no problem..but also with no knowledge of the risks of running the amp like it was, because it had been modified.

Don't quite know if the mods on the amp are normal mods people do on a Princeton Reverb or some are and some are not...

The amp was running always since i got it with 6L6. Up to now when I've slowly started to learn a little bit about tube amps and how they work...the only thing I knew is that the amp had been modded to run with 6L6...that the rectifier was switch to solid state and that the amp seemed to have been modded with an add of an extra transformer. That's what I knew.

I used it for quite a while and to be honest the sound was quite impressive. It was loud, hot...very SRV if you know what I mean.
I updated the speaker a while ago because the one that the amp had wasn't the original...and a friend of mine sold me a Weber C10Q which i was digging a lot.

As i said before...I was using the amp, without knowing...until one day, while playing, the power transformer started leaking really badly and then the amp stopped working.

When I looked the fuse, which I haven't checked before (completely my fault)..it had a 25A fuse!!!!
Anyway...now the amp doesn't work and every the time I tried to turn it on..the switch of whole electrics in the house went down!

I was messaging BMW2002Ti about one of his posts I read...about restoring a princeton too...and he suggested me to post this on the main forum with some pics of the amp without having been touched.


I guess the first thing I want to do and know...is how to check if the power transformer is dead (which I guess it is)...or not.
I would appreciate some help guys because I know a bit of electronics but I'm not an expert and I'm willing to take this restoration as a small project to learn more about it. I guess the best thing to do would be to restore the amp to origins...maybe with the mods to a blackface princeton reverb?

Have to say I also have a Fender Twin Reverb blackface for 1965 on which I've also been working these last months since it had also been badly modified...so I guess I don't need a Fender Princeton on steroids anymore..but a good sounding amp.

So here's the amp with some pics...

http://www.dropbox.com/sh/lkmo7ek47lw3dco/KIy53qedM0


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Post subject: Re: Fender Princeton Reverb Silverface Restoration Help
Posted: Sun Dec 08, 2013 11:42 am
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
Welcome to the forum ,

Real mess this amp :(
You need a qualified tech to retrofit like a original Silferface. Or simply to be safe .
You can learn here how to be qualified to works on this amp , too many mods.

Running an amp built for 6V6 with 6L6 could damage Power Transformer . 6L6 heater need more current then 6V6 . Some Power Transformer can handle it , some not .

To check if your PT can , you must use a voltmeter and read AC volt on 6L6 heater . Must be 6. 3 at best but not less than 6 volts.
If not , put 6V6 and check 6V6 bias .

No new wiring need for 6V6 to 6L6 so you can retrofit to 6V6.

Also , check the value of the fuse , be sure the power is same as write on back panel ( 2A ? )
Too many people when fuse blow put wrong value and damage the amps.

I have always amp to fix and 1/3 time fuse is to powerfull.


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Post subject: Re: Fender Princeton Reverb Silverface Restoration Help
Posted: Sun Dec 08, 2013 4:18 pm
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Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
isrvel, Thanks, for posting those photos and explanation of where you stand with this amp.

1.) The extra tranny (so cleverly held down with the metal strap :oops: ) seems to be powering the 12AX7 heaters. Actually, whoever did this mod should have used the separate tranny to heat the 6L6GC's.

2.) The original power tranny is a stock OEM Princeton Reverb iron. Death of this PT was inevitable, as those 6L6GC draw twice as much heater current as 6V6GT's.

3.) The black square capacitor is an old-style WIMA-like Euro cap. Added with the blue axial cap, as an after-thought to the main power supply filtering. Very poor layout.

4.) There are two snubber caps on the solid-state diode rectifier. You got me why this is a worry for a low powered amp.

5.) There are yellow square bypass cap on each of the 12AX7 signal tubes. Why? Dunno. However, I suspect this amp was very unstable after the mods were done. The caps were added to limit high frequency response that often induces instabilities.

6.) There is a separate cap can on the top of the chassis. This replaced the original cap can (whose wiring has been bypassed). But it's a 32/32mfd at 500VDC cap. Not the correct one for either the PR or any stock dual 6L6GC Fender amp. I think this value can is used in some Marshall PSU's.

7.) There are some crazy bailing wires around the sockets of the 6L6GC's. I dunno what they are there for. Maybe to absorb stray electrons from a UFO? :lol:

8.) The 25-amp fuse was used, because the owner got tired of changing out the stock 1-amp slo-blows, after the modification. Tells you something about the mods. :!:


This is an example of mods that were popular, back in the 1980-90's, when used PR's were readily available at $150. Ppl hot-rodded many to get that over-the-top "Santana" Mesa Boogie tone. I do not know of one of these modded amps that survived more than a few years of gigging, before catastrophic disaster sent them to the junk pile --- nevermind reliabilty. That went out the window, with every "hot-rod."


Post Mortem:

You could revert this amp back to its AA1164 roots, but you'd need to strip off ALL the added gobblegook parts and start from scratch. Don't even test the PT. It's toast. Bad thing is the OEM is a multi-voltage PT. I think only Hammond and Mercury Mag make a replacement. You may need to go to a set voltage PT and bypass the selector switch.

The output tranny maybe suspect, too. As its primary winding is for the current of two 6V6GT's. Not two 6L6GC's. Toss the solid-state mess of a rectifier abortion. Replace with the OEM 5U4GB or GZ34.

Get a good replacement cap can. With OEM specs. I like the CE 30/20/20/20 at 525VDC can. Get one that has a bit more capacitance in the first filter stage. Most NOS Sprague and Mallory cans ran about 25-50% higher capacitance than rated, in their first stage. Many newly made cans have tighter tolerance and may actually fail prematurely, due to this fact.

Redo all the heater lines. The mod was a terrible hack job. Use good single-strand, green insulation copper wire.

Inspect & replace the two 6V6GT octal sockets, if they look worn or overheated. Get a good socket for the added tube rectifier.

Inspect and redo are wiring that has been modified from OEM layout.

Rebuild that bias supply. It looks like the resistor is non-stock, paralleling the bias supply cap, prolly to compensate for the 6L6GC's.

Now... you can start looking at coupling caps, resistor values, and bypass caps.

In other word, you need a COMPLETE overhaul. If you have the parts or can get them easily --- and have the time plus a good solder station --- it maybe a nice Winter project. If this were a Deluxe Reverb, it maybe worth your while in resale value.

How does the cabinet look? Or is it going to require similar amount of sweat-labor?

There are better used PR out there --- if a small & reliable amp is what you need. But, if you want to learn more about amp layouts & philosophy. The art of soldering. In addition, have access to parts. This maybe a test bed project. Albeit, one that requires a lot of work.



http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Fender Princeton Reverb Silverface Restoration Help
Posted: Thu Dec 12, 2013 6:41 pm
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Joined: Sat Jul 03, 2010 2:18 pm
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Musta scared the OP. Sorry.

Gonna have to stop channeling the junior novelist in me...

:lol: :lol: :lol:


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