need some advice around my suddenly non-functioning princeton

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noisepunk
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need some advice around my suddenly non-functioning princeton

Post by noisepunk » Thu Nov 25, 2021 10:53 am

i finally got around to buying a step-down transformer for my princeton. it's 300 watts, should be plenty, and i bought one that people seem to think is pretty good quality.

but now the amp that was working just before coming over isn't working. i mistakenly turned it on without the tubes (i had remembered installing them and didn't think to check), blew this fuse, and had to track down a new one. turned it on this morning when my fuses showed up and the fuse immediately blew again. i'd made sure to put the tubes back in in the order they came out, so i'm not sure what the problem is. in going to remove the tubes again just to check, i almost certainly cracked the housing on one of the 6V6s (really pissed at myself–they were good JAN tubes that probably still had a tone of life in them).

the amp was just serviced in late 2019 and should be in tip-top shape, so i'm expecting something dumb and easy is wrong. i just don't know where to start, and the ease of just taking it to someone is really appealing, but i'm strapped for cash and if it's likely to truly be something dumb and easy, i'd maybe like to start there.

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Larsongs
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Re: need some advice around my suddenly non-functioning princeton

Post by Larsongs » Thu Nov 25, 2021 9:28 pm

I’m not the guy to ask but I want to learn more about Amp designs & hope to build one soon…

Question; Is it standard practice to put 300 watt Transformers in a 12 Watt Amp?

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Re: need some advice around my suddenly non-functioning princeton

Post by sookwinder » Thu Nov 25, 2021 10:28 pm

So ... you turned on the amp will ALL VALVES not fitted? Including the GZ34 or 5U4GB ? I am assuming that the amp is either BF or SF.

I have a step down transformer that I can use for a number of US 120V vintage amps as well as my PIANET N.
It is set up in such a away that I can go 120 -> 240 or 240 -> 120
Did you by accident put 240V into the amp rather than 120V ?

Have you removed the chassis and JUST HAD A LOOK?
No power, no valves , no touch just a damn good look ?
Are there any burnt or smoldered areas? Do you smell burning or the remnants of burning?

Has the amp been "modernised" and the death cap removed ... a basic safety requirement in 2021?

Is it a Princeton or a Princeton Reverb?

See ... a valve amp can just sit there quite nicely till the end of time with the power on and all of the small 12A?T valves not fitted (just with the rectifier valve and the two power valves 6V6GT installed). I would think it could do the same with ONLY the rectifier valve fitted.

Did you put one of the 6V6GT power valves in the position that is for the rectifier valve by accident?
I am trying to understand why the housing of one of the 6V6GTs is cracked. To crack those bakelite valve bases takes a large amount of power. I can't think of any other way that would happen other than placing the 6V6GT in the wrong socket
relaxing alternative to doing actual work ...

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Re: need some advice around my suddenly non-functioning princeton

Post by johnnysomersett » Thu Nov 25, 2021 10:42 pm

Well, until recently, I used a relatively inexpensive 100w step-down for my VibroChamp - which uses the same transformer as a Princeton - with no issues at all so 300w should be plenty.

First thing I would check is the step down transformer. Put a multimeter into the output and check you're actually getting 120v coming out of it. That eliminates that potential point of error. Got 120v? Then the problem is in the amp somewhere.

Then, pull ALL the tubes and power it up. If the fuse is okay then the problem is the tubes, if the fuse blows then there's an issue with the amp itself - or the fuse. Dumb question but you're definitely using a slo-blo fuse, yeah?

IIRC you have a silverface Princeton so no death-cap etc to worry about.

I would quickly pull the chassis and check with a pencil or chopstick that nothing has shaken loose in transit as well...that could easily have happened and would be an easy fix.
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noisepunk
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Re: need some advice around my suddenly non-functioning princeton

Post by noisepunk » Fri Nov 26, 2021 4:24 am

good places to start, thanks everyone–i'll try some of this out over the weekend.

yeah, it's a '79 supposedly (if i remember correctly, the serial number is actually dated for '81, which is puzzling), so silverface, and non-reverb. definitely had the tubes in the right sockets–i made a sleeve for them out of foam and stuck them in serially before packing the amp away. this also isn't the first time i've pulled the tubes for one reason or another, not by a longshot, so i'm pretty confident beyond my quasi-failsafe that they're in the right places.

not sure how to explain the cracked tube–i was removing it and didn't have enough outward pressure on retaining clips (awkward angle, being a little impatient), so i probably was pulling too hard. i heard a small crack, and now the base feels loose :fp:

transformer is just a step-down–i triple checked there was no 240 switch before plugging the amp in initially.
Larsongs wrote:
Thu Nov 25, 2021 9:28 pm
I’m not the guy to ask but I want to learn more about Amp designs & hope to build one soon…

Question; Is it standard practice to put 300 watt Transformers in a 12 Watt Amp?
yeah, the amp itself wants minimum 200watts of wall power–most reading i looked at suggested going a bit up so the transformer isn't working as hard. different watts rating than what we're typically referring to in amp world.

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Re: need some advice around my suddenly non-functioning princeton

Post by noisepunk » Sat Nov 27, 2021 1:46 am

fairy magic.

i pulled the tubes, put in a third fuse, plugged the tubes in one by one checking to see if each lit up, and everything is working now...

it's possible i had the preamp and tremolo tubes swapped, but then group consensus is it shouldn't have blown the fuse when everything was removed, so i have no idea why it didn't work the other day.

thanks for the advice everyone!

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Re: need some advice around my suddenly non-functioning princeton

Post by sookwinder » Sat Nov 27, 2021 4:08 am

Yeah swapping a 12AX7 with a 12AT7 would not cause any issues electronically

Next time you turn it on just watch the valves and make sure they don’t red plate

Hopefully you’ll be ok
relaxing alternative to doing actual work ...

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