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Post subject: 66 Pro Reverb oscillates on peaks
Posted: Sun Jun 27, 2010 7:58 pm
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I'm working on a friends Pro Reverb AA165. It was making all manner of strange noises so like a hundred times before I recapped it and checked the resistors which are all about + 10% which seems reasonable. It sounds really good now except when you pluck a note hard it breaks into an oscillation on the peaks which sounds like a bad buzz. I pulled the tremelo and reverb tubes so now only have the 7025s for the preamp and the AT7 phase splitter, same problem. Tried a different set of preamp tubes, would need to get new power tubes.


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Posted: Sun Jun 27, 2010 9:21 pm
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Does the oscillation persist on both channels?

If so, I'd check the voltages at V6 (the phase inverter) for an abnormally high reading which might indicate the tube was being driven into saturation due to a tired cap or carbon comp resistor.

If not, check the voltages through the signal chain of the affected channel for the same possible cause.

HTH

Arjay

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Posted: Sun Jun 27, 2010 10:38 pm
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IMO you will need to check the rectifier tube and power tubes, before moving on. I hate to bring it up, but did you check the the new filter caps before installation and bedding them in ? It seems the quality of some of our trusted cap manufacture's is falling a little short. Just sayin'. Art

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Posted: Mon Jun 28, 2010 10:51 am
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The B+ is high +30V but the line is normal at 115 VAC, tried a different rectifier. It will buzz on both inputs. However, this morning it is not doing it. Think I'll try to tighten the socket pins and resolder the connections.

Did not test the caps but adding more capacitance to the B+ doesn't affect it.

Thanks for the input


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Posted: Mon Jun 28, 2010 12:15 pm
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Being a Fender amp you probably dont have a snubber cap on pins 1 & 6 of the PI
Put a 47pf to 100pf cap on pin #1 & pin #6 of your PI tube or circuit board and tell us what happens

Also check your bias grid resistors (Pin's #5) of your power tubes with a meter to see if they are good. Should read 1.5K

If you increase these resistors to 2.2K that may cure this as well but you may loose a little shine in a fender amp / Not critical if it works

I would look at other things like especially preamp tubes first. Brand new or not (Hopefully you are using some nice vintage tubes) it depends on what brand of preamp tubes you have now. Some of the new tubes are crap right out of the box

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Posted: Mon Jun 28, 2010 3:50 pm
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Thanks, do you mean between pins 1 and 6 ?

I'm finding a few resistors pretty far off so I'm going to change those and see what happens,


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Posted: Mon Jun 28, 2010 4:14 pm
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Yeah take a 47 or 100 pf ceramic or silver mica cap and solder each leg one to pin 1 and the other to pin 6. If you have any oscillations coming from the PI coupling to the power tube it should stop that

Sometimes parts couple together electrically (In the air thing) and make an amp edgy and on the verge of or already squeeling out of control on certain notes. Though i like the sustain of being on the edge just not too much

Check the values of your bias grid resistors first though / 1.5K

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Post subject: found it
Posted: Mon Jun 28, 2010 8:59 pm
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thanks to each of you for your input. I finally found it. I discovered I could make it come and go tapping around the phase splitter circuit. Tapping anything around there made it pop loudly. I resoldered all of those joints and it is fixed.

Now that I found it I recall seeing the same thing about 25 years ago in a Fender bass amp.


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Posted: Mon Jun 28, 2010 9:40 pm
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Yep, I'd say that you're on the right track cleaning & retensioning those tube sockets. Getting the resistors back to spec is a great idea. I agree with Arjay, about checking the voltages and everything surrounding the PI. The aging process is tough on everything(me too). It may be nothing more than a solder joint in need of a reflow. IMO the snubber caps are unecessary if the amp is up to snuff, besides your problem is not a high frequency oscillation. But Triode makes a good point about the grid resistors on the 6L6's. I'd go with 470-1K5 ohm 2watt metal film. Good hunting and welcome to the forum. OOPS! Posted while you you were. That's good news ! Art

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Last edited by aclempoppi on Mon Jun 28, 2010 9:45 pm, edited 1 time in total.

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Posted: Mon Jun 28, 2010 9:44 pm
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aclempoppi wrote:
The aging process is tough on everything(me too).


It ain't the years, it's the mileage!

:mrgreen:

Arjay

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Posted: Mon Jun 28, 2010 10:40 pm
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I hear that!!! Did I say metal film ? Well that's wrong. How about metal oxide for the grid resistors ? Art

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Posted: Mon Jun 28, 2010 11:08 pm
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Indeed, those grid resistors on 6L6s take a real beating under prolonged loads. Metal oxides are much more durable than vintage carbon comps.

Maybe one day they'll make replacement arteries and heart valves out of metal oxide.

:mrgreen:

Arjay

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Post subject:
Posted: Tue Jun 29, 2010 7:13 pm
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Yep Arjay, they seem to be "the tool for the job" and hot damn they're flameproof ! Actually I put 1watt CC resistors in my Band Master, years ago when I changed the sockets. I was thinkin' vintage, vintage, vintage. But the sockets were ceramic. How bogus is that logic ? The Bassman has the metal oxide though. You know, I gotta stop taking my time with these posts and getting side tracked into doing something else, and then finishing them. I've done this a couple of times. Maybe I'll change my handle to Distracted. Art

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