Princeton Supply Side Economics

[choker]

Hello all

I do hope you will excuse the pun, my problem is just as baffling to me.

I have a Fender AA1164 (w/5U4) that is ailing. I am puzzled by one particular reading that I hope someone will be so kind as to suss and explain to me.

I have replaced the can cap filter(s) and for several days all seemed well. Then I replaced a couple of bypass caps. Everything seemed OK.

A few days later while the amp was still on the bench I noticed some voltages had changed. Specifically, I was reading nearly 450v at the OT center tap. This itself was not terribly surprising as the amp has an upgraded PT (ClassicTone 40-18027)

What was surprising however was reading the same 450v at point D (hence ~420v on pin one plate of V1 as well as high voltages on other preamp plates). Yet all the 1-watt resistors (1K, 18K, 18K)appear to be reasonable.

I don't understand from where all this extra voltage at point D is coming. Could it be that the new can cap has failed? (the old can was GE 20/20/20/20@450v; the new can is CE 30/20/20/20@525v) Or might there be something else that is just not occurring to me?

Thanks for reading and any help is appreciated.

schem: http://www.spearfoot.net/sfpr/prince...1164_schem.gif

please ignore the artificial center tap on sshem

[Tom Phillips]

… I was reading nearly 450v at the OT center tap. This itself was not terribly surprising as the amp has an upgraded PT (ClassicTone 40-18027)…

I would be concerned about that. I would not expect that an “upgrade” should increase the voltage so much. 450V is stressful to the 6V6 power tubes.

… What was surprising however was reading the same 450v at point D (hence ~420v on pin one plate of V1 as well as high voltages on other preamp plates). Yet all the 1-watt resistors (1K, 18K, 18K)appear to be reasonable…

One situation that could cause this is that no current is being drawn through the 1 Watt resistors that you listed. In that case there would be no voltage drop across the resistance. Are you making the voltage measurements with the pre-amp tubes pulled out? Also, if the power tubes are not installed that could also account for the higher than normal voltage at the OT center tap.

[g1]

Have you tried another meter, (or battery for that matter)?
Where are you putting your black probe? Any bad grounds?

[choker]

Thanks for replies.

Readings have been done both with and without tubes in. Did not appear to make much difference.

Tried with two different meters and even put new batteries in meters just in case...

Black probe was securely on chassis. Nothing made me suspect ground problems previously. When I replaced the can I tested it briefly with a guitar. It was quiet and sounded good.

[drewl]

Does the amp work?

What's your wall voltage?
These amps were designed with ~115v ac wall voltage, nowadys it is commonly around 120-125v ac.

[Tom Phillips]

...Readings have been done both with and without tubes in. Did not appear to make much difference...

That's strange. With the tubes pulled I would expect just the range of readings you posted. Then, with the tubes installed and warmed up, there should be a significant drop in the voltage readings. Especially at the pre-amp tube plates. The high readings you are getting at the V1 plate are much higher than would be caused by high line voltage. In situations like this it's usually best to sit back and just think about it for a while and double check everything.

[g1]

Did you check the voltages after you replaced the can? Or is this something you just noticed? Is it possible the new can may not be wired up right?
Here is the link to the layout: http://www.spearfoot.net/sfpr/prince...164_layout.gif

[choker]

Wall voltage is 120.

The new can has one 30uf and three 20uf all rated @ 525. However, the 30uf is on the half-moon terminal. In a (misguided?) effort to use the 30uf as the initial reservoir, I reversed the wiring between the half-moon and the square terminals. After installation, the amp worked well and sounded good. Voltages seemed reasonable. e.g. 403v at plates of 6V6s; 233v at point D; 158v at pin 1 of V1.

After replacing the can and checking some voltages, I put the amp together and played guitar through it for a couple of days. It sounded good. I had pulled the amp and put it back on the bench so I could go through it and see if anything else needed doing. It sat (off) for a day or 2. When I got back to it, the voltages had gone weird.

Is this where I find out that my ignorance of cap internals has cost me a rather expensive can? Or was my strategy reasonable?

edit: I have not connected a speaker since noticing the high voltages. The output is connected to a dummy load while on the bench.

[supawah]

you're not pulling any current through the preamp maybe, look at the grounds from the board to the brass plates behind the pots and the brass plates two? grounds to the chassis. what's the voltages at the preamp cathodes?

[Tom Phillips]

My input is that nothing you described would cause either the high voltage readings or any other problem. The voltages listed in post #8 seem fine. Why you are getting much higher readings now is still a mystery. In these situations it usually turns out to be a test equipment problem or a measuring error. The fact that the amp works well right now seems to reinforce that theory.

[choker]

Sorry, I'm afraid I was not very clear.

The amp DID work fine after putting in the new can. I have not connected a speaker or tried to use the amp since seeing the high voltages. Right now the amp is on the bench connected to a dummy load. I suspect it is not working fine with those voltages.

Simple testing of the meter on some other stuff didn't seem to indicate a serious problem with the meter. Maybe I need to do some more elaborate checking?

So far, checking between chassis and ground points (including brass plate) shows continuity.

[Tom Phillips]

That clarification helps.
Are the heaters in all the tubes on? I.e. tubes lighting up and getting warm to the touch. I ask because, if there were a fault that caused the heaters to be off, then the supply voltages would rise similar to what you are measuring.

[choker]

Bingo!

I had assumed that since the pilot light was lighting, the filament supply was OK. But at your suggestion I found the tubes were not heating.

I disconnected one leg on the pilot and read 7 volts across the filament tap of the PT so it seems to be fine. I guess now it's a matter of tracing where it's getting lost.

Thanks very much. Any favored culprits for this problem, or is it just a matter of searching?

[Tom Phillips]

...Any favored culprits for this problem, or is it just a matter of searching?

Hey! Now we have something that makes sense. Did you wire the amp like the layout posted earlier? If the pilot light is on and all the tube heaters are off then the problem must be a bad connection or broken wire between the pilot light holder and the first 6V6 so look there by close inspection. The problem is now basically isolated to four solder joints and two wires. Or (with the amp power off and unplugged) use your meter as a continuity checker on those connections to find out where the open wire/joint is. You could also measure the heater voltage with the amp on. Same small section that holds the problem. Voltage is at the pilot light and not at the first tube in the string.

[choker]

Problem solved.

Due to a combination of my rather pathetic eyesight and an awkward angle, I had missed a poor solder connection on the lead from the pilot light to the 6V6.

I got in there with some good magnification and just started tugging on things. Lo and behold, one lead popped right out. Soldered it in properly and voltages seemed to go right back down around where they should be.

I want to thank you Tom and everyone who chimed-in. I had gone into this thinking of an excess of voltage as opposed to a lack of voltage drop. A subtle but important difference. This was a fine lesson for me.

I hope the day will come when I am knowledgeable enough to help others on this forum as you have helped me.