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Post subject: Issue with Princeton Reverb
Posted: Sat Oct 20, 2012 11:59 am
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Hi, I'm working on a princeton reverb. It had a strange metallic hum at loud volumes at the speaker. I got audio output too, just with this metallic hum in the background. I checked voltages and they were low. Schematic says 420vdc at centertap of output transformer and 410vdc at plates of output tubes. I'm getting around 250 volts at these points (not exactly but very low around 250). So I thought I might have bad components. I combed through the amp and replaced a few out of tolerance components including a couple resistors and capacitors. I replaced the can style filter cap in the power supply as it had one cap out of tolerance as well. I thought this might remedy the metallic hum, but no such luck. So, at that point I figured maybe the problem was the rectifier tube, because I have proper voltage at the output of the transformer secondary into the rectifier tube, but low voltage at the cathode of the tube supplying dc voltage to the filter caps. So I got a complete brand new set of tubes for this amp as I do not yet have a tube tester and that remedied the metallic hum, but I still have the low voltages at the above mentioned test points. I should mention that the amplifier sounds fantastic now and just seems like it's working perfectly, but these low voltages have me very concerned. Can anybody share their experience and offer me some insight on this peculiar problem. Thanks.


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Post subject: Re: Issue with Princeton Reverb
Posted: Sat Oct 20, 2012 12:29 pm
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Welcome to the Forum!

How are the voltage readings, now after the change-outs?

You know, it sounds like you have a mis-aligned voice coil in the speaker. Maybe rubbing on the permanent magnet and causing the noise. Have you tried another speaker?

Good luck!


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Post subject: Re: Issue with Princeton Reverb
Posted: Sat Oct 20, 2012 1:30 pm
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Wich Princeton reverb ? I have one schematic AA1164. Is this the only one ?

You have a short circuit in this amp in power supply line . That is why voltage drop.

If you remove all the tubes except rectifier and voltages is not better ;

1- Isolate power supply from tubes circuit first to see if voltage go up. Keep filter caps in circuit and disconnect wire going to tubes 's circuit. Check voltage.

2- If not better disconnect Output Transformer primary B+ tap .

Metalic sound could be the speaker as BMW write but also a short output transformer

Do only those two tests and come back with your voltages mesurement.


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Post subject: Re: Issue with Princeton Reverb
Posted: Sat Oct 20, 2012 5:18 pm
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The schematic I'm using is supposedly the original shipped with the amp DWG NO. 045427 REV J. That's all the identifying information that exists on this particular schematic.

Thanks a lot for the direction. I pulled all tubes except rectifer tubes and voltages went way up. In fact, now the voltages are about 100 volts too high! Around 516 volts at cathode of rectifier to centertap of output transformer, and same at plate of output tubes.

Does this sound normal -- the high voltage? And what about the tubes? I just installed all new tubes into this amplifier. Does this indicate that one of the tubes is pulling too much current for some reason pulling these voltages down?


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Post subject: Re: Issue with Princeton Reverb
Posted: Sat Oct 20, 2012 5:52 pm
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It's not uncommon for reading points to be higher with the tubes pulled. Less load & voltage drop. Plus, your house voltage maybe higher than the voltages used when your amp & its schematic were made.

Also, be sure to have the speaker connected when you are doing these measurements.

Good luck!


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Post subject: Re: Issue with Princeton Reverb
Posted: Sat Oct 20, 2012 7:19 pm
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thanks for the awesome replies BMW200 and Stratele52. I did a little more poking around and found that I don't have -30v at the junction where two 220k ohm resistors parallel off to the grids of the two output tubes. Rather, I have zero volts at the point! I think this would cause that huge voltage drop because the grid isn't negative enough with respect to the cathode to properly restrict the flow of current to to the plates of the output tubes. I'm needing to bring the grid voltage down. I'm gonna dig around some more but can use all the insight I can get. Thanks


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Post subject: Re: Issue with Princeton Reverb
Posted: Sun Oct 21, 2012 1:55 am
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kwkened wrote:
thanks for the awesome replies BMW200 and Stratele52. I did a little more poking around and found that I don't have -30v at the junction where two 220k ohm resistors parallel off to the grids of the two output tubes. Thanks


1- No tubes amp give 516 volts ; this normal , good.
2- You have no bias negative voltage !!!
You did not see your outputs tubes red plating ?????? They must.

This a short circuit like I write before.

This can destroy your output tubes.

Don't plug output tubes in the amp until you fix bias circuit . You can fix that and test without output tubes in the amp. Don't power ON amp too long at 500 volts this is over filter caps maximum voltage.


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Post subject: Re: Issue with Princeton Reverb
Posted: Sun Oct 21, 2012 7:50 am
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Ok, thanks. Luckily, knowing the amp has issues I've never left it on longer than a minute at a time. I did notice yesterday when trying to toubleshoot the problem that the output tubes got very hot! I'll see if I can figure out why I don't have bias voltage and post the outcome for others who might run into this problem. Thanks again for the direction.


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Post subject: Re: Issue with Princeton Reverb
Posted: Sun Oct 21, 2012 9:59 am
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Kwkened, you have a B1270 circuit PR. The circuit within the red line is the Bias Power Supply. You need to go through the diode, caps, and resistors. As well as connecting lines. Most of this circuit sits on a separate breadboard, under the power lamp.

You can see it in photo, on the upper right of this chassis shot. BE CAREFUL OF THE POLARITY OF THE DIODE AND CAPS, in this part of the circuit.

Good luck with your amp.

Image

Image


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Post subject: Re: Issue with Princeton Reverb
Posted: Mon Oct 22, 2012 5:28 am
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I saw that circuit there on the schematic and see the line going up from the intensity knob to the grid of the output tubes. I see that circuit you outlined there and need to check it out inside the amp to hopefully find the culprit. I did replace that big capacitor there, the one that on the schematic you sent me is 50mfd 70volts. On my schematic (amp) it actually reads 75mfd 80volts. I'm wondering if I'm reading that right, because I notice that its listed differently than the other caps in the amp. Other caps have the capactitance above the voltage with no line in between them. This one cap however has the capacitance above the voltage like this 75/80, instead of like this 75 80. I definately need to make sure I installed the right component there.

Thanks again for all the help.

Kaiser


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Post subject: Re: Issue with Princeton Reverb
Posted: Mon Oct 22, 2012 10:10 am
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75mfd/80VDC would be fine. Be sure the polarity is correct. Positive (+) to ground. Measure voltages at various points (after the diode) to be sure the VDC is NEGATIVE.

You have any photos of your amp?


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Post subject: Re: Issue with Princeton Reverb
Posted: Mon Oct 22, 2012 10:35 am
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Why do you replace the capacitor ? I did not read that you take some voltages readind and you saw it is not good. ????

Better to read voltage at the diode, before (AC) and after ( DC) . No bias negative voltage , (as you write ) it is not the capacitor fault.

It's the diode or 100k resistor defective or wire or solder in the line from transformer secindary to tube's grids


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Post subject: Re: Issue with Princeton Reverb
Posted: Mon Oct 22, 2012 11:00 am
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I had replaced the capacitor earlier because it was out of tolerance. It was out of value. Now I'm just wanting to make sure that I read the schematic properly. My schematic says 80/75. I'm assuming that's 80mfd over 75volts?

Thank's Stratele52, that makes good sense. I will check at the diode and resistor and make sure there's no short from transformer secondary to grid.

kwkened


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Post subject: Re: Issue with Princeton Reverb
Posted: Mon Oct 22, 2012 11:16 am
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You have a good value , almost any cap over 75mfd and 65 volts will work . A 100 MFD have chance to quiet bias voltage. But what you put is OK.

Better to fix your bias trouble first before rebuilt the amp . Cap no need to be exactly on .


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Post subject: Re: Issue with Princeton Reverb
Posted: Tue Oct 23, 2012 5:27 pm
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I'll try to post a pic of the amp BMW2002ti. I'm going to post some online. Good news, I was able to get the bias voltage restored. It had a bad solder connection between the diode and capacitor in that circuit. Unfortunately I still have very low voltage when I plug in the output tubes. I don't understand, because theses are brand new tubes -- the whole set. Again, the voltages seem okay without the output tubes.

Thanks again.
kwkened


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