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Post subject: Re: Bias setting
Posted: Sun Jan 06, 2013 7:28 pm
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It is the vibrato channel.

I just got through checking voltage and found a problem at the 500pF cap, voltage is 50. Pulled it out and it measures 0.49nF. The voltages on the 12AX7 was off. Hopefully this cap is all that is wrong.

Should I replace all the disk cap?

Thanks!

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Post subject: Re: Bias setting
Posted: Sun Jan 06, 2013 8:01 pm
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If it ain't broke, don't 'fix' it.

Replace only what can be determined to be out-of-spec.

Arjay

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Post subject: Re: Bias setting
Posted: Sun Jan 06, 2013 10:27 pm
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MattGuitarKy wrote:
It is the vibrato channel.

I just got through checking voltage and found a problem at the 500pF cap, voltage is 50. Pulled it out and it measures 0.49nF. The voltages on the 12AX7 was off. Hopefully this cap is all that is wrong.

Should I replace all the disk cap?

Thanks!


Coming to this thread late with three comments and one question.

Comment #1: No one has posted a schematic of an AB763 Tremolux, so here it is.

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

Comment #2: 0.49 nf = 500 pf. 0.49 nf is within 2% of the 500 pf value. Damn good for a ceramic cap.

Comment #3: The only 500 pf cap I can find is one of two coupling caps for the phase invertor which should have no impact on the voltages for anything in the vibrato channel.

Question: Where are you measuring 50 volts?

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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 6:55 am
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Oops, thanks for the n to p correction.

For clarity, this issues if for the Dual Showman. I have a schematic, thanks for uploading the tremolux.

Voltage is measured at the 500pF & 1M resistor, (pin 2 of the 12at7 tube). I read it should be 95V.

Thanks for your help.

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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 8:43 am
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Matt, Pin 2 of the 12AT7 is the signal grid (or input) for the reverb stage. I really don't think it should be 95VDC. If it's reading this high, it seems that the 0.02 mfd coupling cap off the second half of the input 12AX7 (7025) in the Vibrato stage is leaking DC.

The signal grid of any gain stage tube should be in the mV DC range. Or you risk damaging the tube. Maybe, go back and check those values again. I really doubt the reverb would work with 95VDC on that grid.


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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 9:38 am
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BMW, the voltage measure 50V. I am reading the schematic at 95V. (Tremolo)

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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 9:57 am
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Side Note: Going Donkey-Bonkers trying to get a post, posted.. here... :lol: :lol:

WHOOPS! Wrong AB763 circuit. I thought this was the reverb section. Not the vibrato/tremolo section (as the Tremolux has no reverb :oops: :oops: ).


Lemme catch a few zzz's and get mback to Nethanderthal stage beofre attempting anymore posting (let alone diagnosing).

Sorry, about that.

PS: Where was that 95VDC point? Thanks!


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


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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 4:10 pm
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All right. I am confused. What amp are we talking about now, the Tremolux or the Dual Showman?

Tremolux:

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

Dual Showman:

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

The 500 pf cap is the coupling cap feeding the + input (grid) of the 12AT7 phase invertor at pin 2(7).

Where are you measuring 50 VDC? The schematic show 95 VDC at the cathode of the 12AT7 PI which is pin 3(8).

None of this has anything to do with the tremolo circuit.

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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 7:19 pm
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Dual showman.

50VDC at the 500pF & 1M resistor connected to pin 2 of the phase inverter.

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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 7:28 pm
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MattGuitarKy wrote:
Dual showman.

50VDC at the 500pF & 1M resistor connected to pin 2 of the phase inverter.


What do you read at the junction of the two 1 megs, 22k, and 470? Should be 93 VDC there. Have you verified the resistor values in the PI tail?

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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 9:20 pm
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It is 90V at the junction of the two 1 megs, 22k, and 470. The voltage around the phase inverter are fine, sorry for the confusion.

For the 12ax7 for the tremolo.
Pin # V measured (V spec)
P1 438 (280)
P2 -50 (?)
P3 0 (2.1)
P4&5 2.5 (?)
P6 380 (380)
P7 -50 (?)
P8 0 (17)
P9 2.5 (?)
All caps and all but one resistor connected directly to this tube meet spec. The 2700 resistor measured 3500. Is this enough out of spec?

Thanks!

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Post subject: Re: Bias setting
Posted: Mon Jan 07, 2013 10:22 pm
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MattGuitarKy wrote:
It is 90V at the junction of the two 1 megs, 22k, and 470. The voltage around the phase inverter are fine, sorry for the confusion.

For the 12ax7 for the tremolo.
Pin # V measured (V spec)
P1 438 (280)
P2 -50 (?)
P3 0 (2.1)
P4&5 2.5 (?)
P6 380 (380)
P7 -50 (?)
P8 0 (17)
P9 2.5 (?)
All caps and all but one resistor connected directly to this tube meet spec. The 2700 resistor measured 3500. Is this enough out of spec?

Thanks!


Well, first of all, if you are only measuring 2.5 volts (this should be AC) at pins 4 and 5, you have a heater circuit problem as these should have about 6.3 volts AC on them.

If you are meauring 438 VDC at pin 1 (plate) vice 280 VDC, you have a serious problem in that circuit.

If the 2700 ohm cathode resistor is a 10% tolerance and measures 3500 ohms, it is way out of spec (2700 +/- 270 ohms).

I don't understand how we went from troubleshooting a power tube bias problem to a phase invertor problem to a vibrato circuit problem.

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 7:35 am
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Well, here's an example.

MattGuitarKy wrote:
...Having trouble with the DS. Plate voltage is -45.5V not - 53V and bias range is 0- 20mv. The amp is noisy at idle somewhat a 60cycle hum and a buzz. Any help is greatly appreciated. House wiring is good, verified with tester and the Tremolux.

Thanks


In a previous post, the amp was noisy upstairs and quiet downstairs. The OP doesn't seem to know exactly what he is measuring, nor how to compare to the schematic. No way you are getting a negative voltage on the Plate, and the negative grid voltage will not necessarily be exactly as the schematic shows, especially afetr the bias has been adjusted.

With the "tremolo" off, the voltage readings at the Plates for that tube are kind of meaningless and that circuit would not cause the noise the OP is describing.

My bet is that the amp is fine and the OP is determined to screw it up. :wink:

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 7:53 am
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I agree. If you are getting more than mV DC (+ or -) values off ANY signal grid of any tube --- maybe time to look at the meter settings and then the coupling caps, prior to that grid.

BTW, can anyone use the COLOR option for FONTS? Just to right of posting area?


Thanks!


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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 8:30 am
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...

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Last edited by MattGuitarKy on Tue Jan 08, 2013 11:04 am, edited 2 times in total.

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