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Post subject: '64 Twin Reverb voltage question
Posted: Wed Mar 05, 2014 10:14 am
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Aspiring Musician
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I'm working on a friends '64 Twin. A while back I replaced all the electro caps, couple of burned up 470 grids (I actually replaced all the 470k's and 1.5k's) and retubed/rebiased the amp. It was fine for a couple of months and then the Vibrato channel only volume got erratic, with some volume at 0, none at 5, etc. I replaced the pot with the right 10k twice and it cleared up the erratic volume, but now the volume on this amp is considerably less than it should be. It's not distorted, just lower output. All the voltages are close I guess, most are 20 to 30v less than the schematic, PT is 332vac. But pin 1 of V5 is 426v. Pin 3 and 8 have 0v. Pin 6 is okay at 372v. I did short the Tremolo jack but the reading is all over the place of course then. I also swapped tubes. I have new power tube sockets on the way also. The original speakers have a ton of miles on them, but both equally as loud.

Any help would be greatly appreciated.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Wed Mar 05, 2014 10:43 am
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upnorth wrote:
It's not distorted, just lower output. All the voltages are close I guess, most are 20 to 30v less than the schematic, PT is 332vac. But pin 1 of V5 is 426v. Pin 3 and 8 have 0v. Pin 6 is okay at 372v. I did short the Tremolo jack but the reading is all over the place of course then. I also swapped tubes. I have new power tube sockets on the way also. The original speakers have a ton of miles on them, but both equally as loud.

Any help would be greatly appreciated.


1.) PT of 332VAC is fine.

2.) Pins 3 & 8 of V5 (tremolo/vibrato tube) = 0 VDC is is not good. You have no or very little current passing through V5. Is the tube in, when you are doing these measurements?

3.) What are the voltages on the plates (BOTH pins 1 & 6) of V5? They must be within tolerances of the schematic (+/- 20%).

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

HTH! Good luck with the fix. :mrgreen:


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Wed Mar 05, 2014 12:41 pm
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BMW2002Ti wrote:
2.) Pins 3 & 8 of V5 (tremolo/vibrato tube) = 0 VDC is is not good. You have no or very little current passing through V5. Is the tube in, when you are doing these measurements?


I'm thinking cathode bypass cap, Steve (both sides of V5).

Arjay

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Wed Mar 05, 2014 1:23 pm
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Aspiring Musician
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All the tubes are in when I'm measuring.

Here are all the readings:

V1 - pin 1 = 265v, pin 6 = 255v, pins 3 & 8 = 1.9v
V2 - pin 1 = 263v, pin 6 = 255v, pins 3 & 8 = 1.9v
V3 - pin 1 = 426v, pin 8 = 9.2v
V4 - pin 1 = 240v, pin 6 = 244v, pins 3 & 8 = 1.9v
V5 - pin 1 = 426v, pin 3 = 0v, pin 6 = 372v, pin 8 = 0v,
V6 - pin 1 = 230v, pin 3 = 98v, pin 6 = 238v

I did put new 25/50's Sprague's on the whole board when I did the re-cap.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Wed Mar 05, 2014 6:46 pm
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Arjay is correct. Check both the 25mfd/25VDC (just replace them with 25mfd/50VDC lytics) and the 2700-ohm and 100k-ohm resistors tied to the cathodes of V5. Pull one side of the resistor's lead and measure the DC resistance. If they are beyond 15% of spec, replace them.

Check DC voltages on pins 3 & 8, afterwards.

Good luck! Keep us informed. :mrgreen:



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Post subject: Re: '64 Twin Reverb voltage question
Posted: Wed Mar 05, 2014 8:24 pm
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Thanks, I'll try that. I did replace the 25/25's a couple of months ago but will re-check them or replace them. I do have some new 25/50's to use. I'll lift and check the resistors also.

Thanks for the help and I'll let you know the results. Can't have this old Twin sounding like this.


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Wed Mar 05, 2014 10:25 pm
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upnorth wrote:
I'll lift and check the resistors also.


+1

Also a possible issue given that you've got zip at V5's pin 3 and pin 8 and the cathode-bypass caps are relatively new. Check them both -- both visually for signs of geriatric fractures or thermal-related damage and using a DVM to ascertain value drift (if any).

Arjay

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Thu Mar 06, 2014 9:12 am
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Figured I might as well post the patient. It's been at a few gig's in it's time.

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Thu Mar 06, 2014 9:54 am
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What's the date codes on those Jensen's?

The amp is a rough specimen indeed but by no means a goner. Once you get the chassis squared away, a little TLC will do wonders for the cosmetics.

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: Thu Mar 06, 2014 10:27 am
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Aspiring Musician
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1964 on the speakers. 220410 and 220420, C12NSA's. They need to be re-glued or re-coned. The surrounds/gaskets are loose from the frame. I explained this to him and am working on him to get some replacements for now and have these worked on. I've used Mojo's aged grill cloth on other amps and it looks pretty good.

I'm not a very good photographer but here it is on a couple of amps. I need to loosen one staple above the Fender logo on the DRRI.

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Post subject: Re: '64 Twin Reverb voltage question
Posted: Thu Mar 06, 2014 12:57 pm
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Aspiring Musician
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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?


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Thu Mar 06, 2014 3:56 pm
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Always a possibility with the older amps. I've encountered many marginal sockets over the years. Decades of baking eventually takes its toll on the dielectric insulator which often fosters arcing within the assembly.

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: Thu Mar 06, 2014 6:47 pm
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upnorth,

I know this is a silly question, so bear with me. Is the vibrato circuit on? That is, did you switch the footswitch? Before taking voltage measurements.

Thanks!


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Thu Mar 06, 2014 7:23 pm
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Aspiring Musician
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I don't have the foot switch so the only thing I could do is use a shorted RCA jack I made up. When the trem is active (I can see it working) the readings are all over the place on pin 1, when off, they're as described earlier.

I'll swap out the socket tomorrow just to see if that could be it.

I really do appreciate the help and ideas, thank you. Getting thin on top, don't want to start pulling hairs out. 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!


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Post subject: Re: '64 Twin Reverb voltage question
Posted: Thu Mar 06, 2014 7:40 pm
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Maybe test the triplet disc caps in the circuit. The 0.02/0.01/0.01mfd caps in a row. Also, perhaps the the opto-coupler is bad?

What are the voltages on pins 3 & 8 of V5, with the vibrato/tremolo circuit on?



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