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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 9:46 am
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BMW2002Ti wrote:
...You may want to remove the aftermarket wiring and re-establish the stock setup....


+1

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 9:51 am
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" The aging PT may not have sufficient reserve current to run the heater elements in a 5U4GB. "

Arjay, I use the 5U4GB just to test the suspect rectifier. I typically use a known, good US made rectifier (usually a 5U4GB or 5Y3GT) --- as I know these fit the vintage US made sockets well. And perform to a consistent voltage.

I just use them for that test. For a few seconds to get the readings. Under no load. No volume.

I used to use a Mullard GZ34, but that rectifier became too precious for testing purposes. I guess one could use one of those JAN-Philips ECG "5VA4GA" labeled 5AR4...

BTW... When testing rectifiers, I pull all the other tubes. So, at least the output tubes aren't loading the PT.


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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 11:03 am
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BMW2002Ti wrote:
I just use them for that test. For a few seconds to get the readings. Under no load. No volume.


Copy Lima Charlie.

That makes sense now. Or pick up a decent Sov-Tek GZ34 or two and restrict their use to "bench-check only" tubes. No way would I hazard a precious Mullard when troubleshooting either.

Arjay

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 7:00 pm
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Here are the pictures of the OT and PT that were requested.
Image
Image
I think this may be the filter cap that Arjay was talking about, it is still there, it has just been bypassed. I guess somewhere down the line somebody determined the original was no good and bypassed it instead of replacing it (not sure who, I did not look in the amp when I bought it, and would have not known anyway).
Image
Image

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 7:20 pm
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Thanks, for the photos. First off... you need to take voltage measurements. Compare to the circuit you have (which is prolly an AA964).

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

Second do as Arjay sez, pull those aftermarket main filter caps, which aren't even grounded to a single star point. Replace with a new cap can. I suggest the CE 30/20/20/20 at 525 VDC,

http://www.tubesandmore.com/products/C-EC30-20X3-525

Third, check the layout (bottom of schemo link) for mods. Restore to stock. Esp that feedback circuit.

The Princeton is a reliable, tone-master in stock form. Hot-rodding these amps most often leads to unstable amps.

Recheck the voltages, once restored. Including idle bias & plate voltages of the output 6V6GT's. You should be good to go. Provided the tubes are ok.


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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 7:25 pm
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BMW2002Ti wrote:
Hot-rodding these amps most often leads to unstable amps.


+1

The only mod I performed on mine (aside from rearranging the "furniture") was to add a standby function using the now-superfluous ground/polarity switch on the rear panel. It's been rock solid ever since I've owned it.

Arjay

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 8:03 pm
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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.


Just to be clear, this is Pin 8 on the Rectifier tube, right? Assuming there is a drop in voltage that corresponds with the cut out, what does this tell me?

Also, I started testing a few of the components, and the resistors on the input jacks are reading below 40k when they should be 68k? I did not check too many of the others, but should I look through for things like this too?

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 8:19 pm
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Check before starting speculation of what-if. Pin 2 is the DC output of the rectifier.

The resistors on the input are connected in parallel with nothing plugged into the jacks, so you should read below 40K, probably close to 34K if they are good. Plug a guitar cable into input 2 with no guitar connected, to separate the input resistors, then check each one.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 8:52 pm
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shimmilou wrote:
Check before starting speculation of what-if. Pin 2 is the DC output of the rectifier.


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This confuses me then. Am I reading the layout wrong?

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 9:10 pm
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:oops:

Many of the schematics for the Princeton have it backwards while the layout on all are correct, it is pin 8. Sorry! I should have known that. Red and yellow wires on the same terminal.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 10:16 pm
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Just a feeling, but I suspect that the problem is in the connection for one or more of the positive leads of those added power supply caps. In particular, where the red wire from the rectifier goes under the board, or maybe even the yellow wire next to it where it goes under the board. The connection for the red wire from the rectifier to the board doesn't look great. :idea:

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 10:18 pm
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Shimmy, re: rectifier. Couldn't you read either pin 2 or 8 and get similar voltages? One side should be conducting every 180-degrees of the sine-wave. In a near equal PIV. Right?

Kinda like one hump equals one pin and the other the other pin --- in tis diagram?

http://www.freewebs.com/valvewizard/bridge4.jpg


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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Wed Aug 15, 2012 11:45 pm
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Both half-cycles can be read from either pin 2 or pin 8, since both halves go through the Cathode which is connected to the heater at pin 8 (so, yes, similar voltage on either pin). The two Plates split the duty, each conducting on every other half-cycle, but the Cathode sees both half-cycles. In the case of the indirectly heated Cathode of the 5AR4, I would think that it's a bad idea to pull the DC output current through the heater by using pin 2 for the output. If a directly heated Cathode (filament) like a 5U4GB, then it wouldn't matter which pin is used, 2 or 8 would have the same output.

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Thu Aug 16, 2012 8:11 am
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So it turns out it was the Neg FB Loop switch. All I did was put the loop back in and I could push the amp as far as I wanted without it cutting out.

I am going to remove the switch all together, is there anything more useful than that (or the ground switch, since the AC has been exchanged for a grounded plug) that I could use that switch for?

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Post subject: Re: 1965 Princeton BF (no reverb) - Cut out issue
Posted: Thu Aug 16, 2012 8:20 am
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I used that "orphan" switch to add a standby function to both of my Princetons. Basically, it's removing the B+ voltage from the power-tube plates when engaged. A totally non-invasive and reversible mod.

Arjay

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