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Post subject: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Tue Aug 14, 2012 4:21 pm
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I have a (as far as I can tell) 1965 Princeton, which has a modified cabinet, converting it to just a head unit. It also has a mid eq knob mounted in the Ext. Speaker jack.

The issue I am having is that when the input signal gets hot enough to get the amp just where I want it, it will cut out for a few seconds when I really start hitting high riffs or just generally really pounding away on the guitar. It will stay good as long as I use a lighter attack, but then it is not quite as dirty as I would like if I don't, so....

The place I got it from did some work with the rectifier, and the problem was mitigated, but it is still not quite there.

I have book knowledge of how amps are built and how the circuits work, but I have not had the chance to actually apply (and really learn) this stuff. I installed a mod kit (just a bag of parts with instructions) on my Carvin Vintage 16 a while back though, so I am comfortable getting in the amp to do stuff.

I have considered getting a solid state plugin for the rectifier, to see if that will give it the juice it needs to hit the spot I am looking for. If that will work, would anybody's replacement work, or are there stuff that really stands out in this field?

Any other ideas on how I might mod this thing to get just a bit more endurance would be great. Thanks in advance.


Some pictures of the Princeton, for science.
Image
Image

EDIT: Full album of images from all posts

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1965 Princeton (Head Mod)

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Last edited by duineAnaithnid on Wed Aug 15, 2012 7:02 pm, edited 1 time in total.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Tue Aug 14, 2012 4:36 pm
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I'd examine the chassis for any signs of a poor connection that becomes aggravated by sonic vibrations of the amp when driven hard. It could be a marginal solder joint, a fractured carbon-comp resistor, a ceramic cap that got toasty one time too often, or a loose tab on a tube socket. A dielectric probe (popsicle stick, chop stick, pencil, etc) may be of help when poking around a hot-and-exposed chassis. Even the on/off switch or a brittle heater wire could be considered suspect. Use caution.

I have an '80 non-verb Princeton that I also built into a piggyback configuration -- the world's loudest 12-watt amp......

Image

Best of luck, HTH

Arjay

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Tue Aug 14, 2012 4:39 pm
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Be nice to have a nice clear photo of the internal wiring side of the chassis (maybe a couple of shots). And the transformer (including the stamped numbers), cap can, tubes and socket side of this amp.

Not sure what circuit you have. Cosmetics (like the front plate and dials) can fool one.

Thanks!


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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Tue Aug 14, 2012 8:17 pm
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Retroverbial wrote:
I'd examine the chassis for any signs of a poor connection that becomes aggravated by sonic vibrations of the amp when driven hard. It could be a marginal solder joint, a fractured carbon-comp resistor, a ceramic cap that got toasty one time too often, or a loose tab on a tube socket. A dielectric probe (popsicle stick, chop stick, pencil, etc) may be of help when poking around a hot-and-exposed chassis. Even the on/off switch or a brittle heater wire could be considered suspect. Use caution.

I have an '80 non-verb Princeton that I also built into a piggyback configuration -- the world's loudest 12-watt amp......

Best of luck, HTH

Arjay


What would be the best way for me to check these things? Do you know of any good resources where I could get tips on diagnosing an old amp?

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Tue Aug 14, 2012 9:24 pm
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Yeah Duine, you could try to find" Vacuum Tube Guitar and Bass Amplifier Servicing", by Tino Zottola. Kevin O'Connor is another good author. Gerald Weber has CD's out for amp maintenance, but I haven't seen them. Agree with Arjay, if you're comfortable working on a live amp , then the chopstick probe is a good idea( you'll need a speaker load and no input). Beemer really knows his way around Princeton's, and there are a bunch of other fellas on this forum that could help you with specific amp problems. Kind of leaning towards a bad tube or dodgy solder joint. Art

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Tue Aug 14, 2012 11:32 pm
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What rectifier tube is in there now? I would try a new rectifier first.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 4:14 am
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shimmilou wrote:
What rectifier tube is in there now? I would try a new rectifier first.

Sovtek 5AR4, which the shop put in fresh for me, along with some other work on the rectifier.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 4:48 am
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As requested, here are some pictures of the guts.
Image
Image
Image

At first glance I noticed that some of the tube socket tabs jiggle... on more than one of the sockets too.

Second, where are the filter caps? I had the amp unplugged for a while, so I guess they drained, but I could not find any residual voltages in the amp before I started poking around.

EDIT: Regarding the vibration effect loosening something, the problem occurs whether or not the "head" is on a speaker or sitting on the floor with the speaker on a stand next to it.

Edit 2: It also has the ground switch replaced with a Neg FB Loop switch, in and out

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Carvin Bolt+ Kit
1965 Princeton (Head Mod)

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 7:14 am
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The filter caps are -- apparently -- those three black tubular components arranged in parallel, next to the power tranny. They are totally inappropriate for a Princeton Amp. Who put that trash in there anyway......somebody from "RETARDS 'R' US"?

The standard filter cap array for a Princeton amp consists of a multi-sectional cylindrical can mounted vertically on the top of the chassis. Have a competent tech rip that trash out and install the correct part. You can find them at Tubes And More, Mouser, or on Ebay -- they run around sixty bucks.

Arjay

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 8:35 am
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Retroverbial wrote:
The filter caps are -- apparently -- those three black tubular components arranged in parallel, next to the power tranny. They are totally inappropriate for a Princeton Amp. Who put that trash in there anyway......somebody from "RETARDS 'R' US"?

The standard filter cap array for a Princeton amp consists of a multi-sectional cylindrical can mounted vertically on the top of the chassis. Have a competent tech rip that trash out and install the correct part. You can find them at Tubes And More, Mouser, or on Ebay -- they run around sixty bucks.

Arjay


Could that be causing the problem I am having?

Out of curiosity, what makes these no good for a Princeton? I am trying to learn how to maintain this amp and working on/building tube amps in general, so any technical info is appreciated.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 8:54 am
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Put a DC voltmeter on the output from the rectifier (negative lead to chassis), and monitor the voltage while the cutout occurs. That can be a clue as to whether the problem is in the DC supply or not. Don't worry about the pins on the sockets moving, that is normal for vintage sockets.

What "other work" was done to the rectifier?

Any difference when switching on/off the feedback?

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 9:01 am
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What is that switch next to the fuse? The one that has the two bundled yellow wires going to what looks like the GNFB points. Is that a global feedback switch?

Really, the dressing around gain & EFX stages should be as short as possible. To avoid instabilities. You may want to bypass that switch & see if that helps the issue. There maybe other mods done to the amp. In unstable amps, sometimes it's easier just to re-wire the thing back the OEM nominal values and layout.

Also, that extra carbon comp resistor off the power supply rail (the one sitting upright) --- maybe kinda susceptible to failure due to vibration.

Socket tangs should jiggle a bit, when the tube is removed. This helps insure good contact wrt different diameter pins. I know of some Sovtek and JJ rectifiers that had slightly smaller diameters pins. Small enough to have some issues with older US made sockets. Do you have a US made 5U4GB that you can try in the socket? Also, a little contact cleaner may help.

Also, once drained ---- the socket tangs can be tighten A BIT. I use a metal Dental Probe to tweak the tangs.


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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 9:05 am
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BMW2002Ti wrote:
Do you have a US made 5U4GB that you can try in the socket?


The aging PT may not have sufficient reserve current to run the heater elements in a 5U4GB. I would stick with the GZ34/5AR4.

JMOOC

Arjay

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 9:08 am
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shimmilou wrote:
Put a DC voltmeter on the output from the rectifier (negative lead to chassis), and monitor the voltage while the cutout occurs. That can be a clue as to whether the problem is in the DC supply or not. Don't worry about the pins on the sockets moving, that is normal for vintage sockets.

shimmilou wrote:
Any difference when switching on/off the feedback?

I will try these tonight when I get home from work

shimmilou wrote:
What "other work" was done to the rectifier?

I am not sure what was done. When I noticed the cutout issue, I took it to the shop (which has a great reputation in our area) where I bought it.

First they replaced the rectifier tube, but that did not fix the issue. They then gave it to their "amp guy" and the problem was mitigated some, but I am not 100% sure what he did (I will see if I can find the repair ticket when I get home and report back on that).

BMW2002Ti wrote:
What is that switch next to the fuse? The one that has the two bundled yellow wires going to what looks like the GNFB points. Is that a global feedback switch?

That is the in/out switch for the feedback loop.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 9:20 am
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duineAnaithnid, I'm not sure which iteration of Princeton circuit you have, but these push-pull amps usually need some feedback voltage to work properly. This is really true for the feedback onto a concertina (cathodyne) phase inverter (like most Princetons).

You may want to remove the aftermarket wiring and re-establish the stock setup. Good 6V6GT will overdrive just fine without circuit gimmicks. Just my opinion.

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

In the light of what Shimmy said. You may need to go through the solder points on the PS rail, rectifier, and other sites (esp those modified points). A good DVM, check list, and step-by-step approach is best.

Luckily, the Princeton is a very easy test bed to start out this procedure. We can all thank Leo and his foresight.


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