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Post subject: Re: '64 Twin Reverb voltage question
Posted: Thu Mar 06, 2014 8:52 pm
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upnorth wrote:
Wish I had a scope. I could point the 3 X 9 at it but my wife would lay an egg when she heard the shot!


LMFAO!!!

KA-POW!

Don't forget to police up your brass.

:mrgreen:

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 9:34 am
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This morning I changed the socket and it did the same thing. I did check all the wires for cont. when I had them off. With the tremolo on the readings are close but very erratic/up and down, but peaking close to spec. on all measurable pins. It's funny, because when I measure pin 6 the ticking stops, but the roach keeps flashing. (DMM to ground) Must be acting as a capacitor.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 2:21 pm
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You are measuring VDC, right? Not VAC.

This is a short aritcle on got from RG Keen's site about "ticking" tremolo/vibrato circuit fixes. Addressing the wiring layout really makes a BIG difference. Try to keep the wires from one-half of the 12AX7 physically away from the other half. If they must cross, try have them cross at 90-degree angles. Like the layout of the socket wiring in this Bassman.


Image






Fixing the "Ticking" in Fender Tremolo circuits

This is from the great electronics guru R. G. Keen.


• Fiberboard contamination: Dust, dirt, and junk can let the LFO signal leak into the audio path. Vacuum the dust and dirt away, and if it still persists, remelt the wax top and bottom with a hair dryer.

• Solder blobs from eyelets touching insulating board: Sometimes excess solder drips out the bottom of an eyelet and can intermittently contact the insulating board, can cause ticking. Remelt the eyelets and examine the board underneath for any blobs dripped down.

• Funny ground on some SF Fenders; On one of the signal tubes, the cathode cap was placed on the tube socket, and wired to a ground lug on the vibrato cancel jack instead of across the resistor on the fiberboard. The vibrato shares this ground line, and can the vibrato current can cause audible ticking in the audio path. Rewire the cap to another ground or relocate it to the board.

• Poor Signal wire layout: signal wires run too close to vibrato leads can pick up the LFO signal. Move them around and see if the ticking goes away.

• Bad repair/replacement foot switch cable: the Fender foot switch cable is not two conductor; it's single conductor shielded, plus single conductor. The reverb wire is shielded, vibrato wire is not. This keeps vibrato out of reverb. If you retrofit with two conductor shielded, you get vibrato ticking onto reverb audio.

• Sharp tick in vibrato oscillator: On neon/LDR Fenders, on the neon bulb side of the module there is a 10M to one side of bulb, 100K to the bulb; from the 10M straight across the board is the gnd point of the LDR. Put a 0.02 cap from 10M/bulb to the ground point; this works by filtering the output of the oscillator.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 4:47 pm
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I'm measuring DCV's and V5 on my earlier photo looks to be laid out well. I'm really not concerned with the ticking as much as the voltage issue, but thanks for the article.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 7:22 pm
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upnorth wrote:
Okay, I changed both 25/50's and the resistors. Same thing.

436v at the point where the 10M, 100K and 220K meet below the roach.

426v where the 220K and pin 1 meet. That should be 300v. The 220K is well within tolerance.

Now here's a stupid question. 300v is supposed to be coming from pin 1 on the tube. 17v are supposed to be going to pin 8 and 2v to pin 3. Could it be a shorted socket?


300VDC is applied, by the PSU, to the anode plate (pin 1) of one of the triodes of the 12AX7. 380VDC to the other anode. 2VDC and 17VDC (pins 3 & 8..) is the voltage difference between ground and the cathode of each half of the 12AX7. These are the voltage differential caused by the electrons passing from ground through each cathode. They are attracted to the anode, because of the voltages applied to the anodes.

You can have 300VDC on the anodes and 0VDC measured voltages on pin 3 & 8, if the tube is dead. Or there is discontinuity between the tube's cathodes and ground.

Signal is AC. And rides on top of the applied DC voltage. The signal is amplified by the attaction of the passing electrons wrt the AC waveform. As it passes from the control grid to the anode.

So, I take it that pins 3 & 8 are still reading zero voltage, even with the vibrato switch on?


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 7:24 pm
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Well I'm convinced (please correct me if I'm wrong) by my own "lack of knowledge" that the tremolo has to be engaged to get a proper voltage reading. It seems as though the fluctuation is on V5 alone for the tremolo. The voltage is pretty close on all the pins with it engaged, including pin 3 @ 1.9v's and pin 8 @ about 18v's. It does fluctuate on pins 1, 3, 6 and 8 but peaks close.

When disengaged, the voltage jumps on pin 1 to 400+, and drops on pins 3 and 8 to zip, but stays close on pin 6. That still puzzles me. I did end up replacing the 10M resistor because it read 13M lifted. A few others are pushing the limit. I've got some Fender style power tube sockets and some new resistors on the way. These sockets are pretty bad. I realize they have nothing to do with this problem but they need replacing..

Thanks again for the help.


Last edited by upnorth on Fri Mar 07, 2014 7:33 pm, edited 1 time in total.

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 7:29 pm
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Sorry BMW2002Ti. I was writing while you were posting. I do get close readings on 3 and 8 when the trem is on but it fluctuates.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 7:35 pm
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OK... widely fluctuating DC voltages could also be an opto-coupler issue. You may want to replace the opt-bug, just for the sake of completeness. As they are relatively cheap.

BTW... the AC voltage SHOULD fluctuated a little, with the vibrato switch on. This is how it works. By raising and lowering the AC signal.

Good luck, keep us informed. :mrgreen:


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 7:43 pm
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Thanks much, I'll replace it. Could a bad LDR cause the voltages to be off with the tremolo disengaged?


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 7:49 pm
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upnorth wrote:
Well I'm convinced (please correct me if I'm wrong) by my own "lack of knowledge" that the tremolo has to be engaged to get a proper voltage reading. It seems as though the fluctuation is on V5 alone for the tremolo. The voltage is pretty close on all the pins with it engaged, including pin 3 @ 1.9v's and pin 8 @ about 18v's. It does fluctuate on pins 1, 3, 6 and 8 but peaks close.

When disengaged, the voltage jumps on pin 1 to 400+, and drops on pins 3 and 8 to zip, but stays close on pin 6. That still puzzles me. I did end up replacing the 10M resistor because it read 13M lifted. A few others are pushing the limit. I've got some Fender style power tube sockets and some new resistors on the way. These sockets are pretty bad. I realize they have nothing to do with this problem but they need replacing..


As for a bad LDR (opto-coupler) can give you bad DC voltage (or intermittent voltage readings).
Thanks again for the help.


Yes. You would not expect much voltage on pins 3 & 8, if the vibrato/tremolo was turned off. As there would be little AC signal on the grids. Thus, little attraction of electrons from the cathode to the anodes. Signal is what causes the electrons in a tube to pass from the cathode to the anode. Increase in the overall flow of electrons = amplification.

Kinda like a gate or valve. Thus, the term "valves" the British use for tubes.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 07, 2014 9:27 pm
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Thanks for the explained info. Guess I should get Dave Hunter's book again. I've built some amps from kits and scratch, but only by the schematic. I need some more in depth study in this old brain.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Sat Mar 08, 2014 3:51 am
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You are welcome! Hope you are able to sort out the problems.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 14, 2014 12:30 pm
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Well, this Twin is being difficult. All the voltages are very close now that I replaced some parts. The original main problem was volume loss. My DR is much louder on 3 than this Twin is on 5. The bass - mid - and treble are all up on 6. It's not distorting until about 8 or so on the volume and then it's lousy distortion and still quieter than my DR on 3. I replaced all the tubes/biased, all the questionable resistors, new sockets, all new electrolytic caps, new Orange Drop caps, except the blues, new pots, including the bias pot. I also tried the speakers on a different amp and they're fine.

This amp had blown screen grid resistors the first time I got it. I'm sure it was tried a few times by people with just tube swaps, not realizing the internal damage. I replaced the resistors and all the electro caps at that point with a re-tube and bias. It was fine for a couple of months until the volume got erratic and dropped.

Could it be the PT? Any ideas?

Thanks,

UpNorth


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 14, 2014 12:37 pm
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Your power tranny seems to be performing to spec, given the measurements you've taken of the available voltages.

Have you checked the choke?

It's an oft-overlooked item when troubleshooting unusual symptoms.

Arjay

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Fri Mar 14, 2014 1:19 pm
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Being fairly new at all of this, I need to ask how would I check the choke. Would it read open/closed, ohms? Is there a way to check the OT also?

Thanks again


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