Need Help diag. 102 Build

[fusionbear]

Hello gents, I have a 102 build on my bench that came in for repair. Someone else built it (overall a nice job!!!). It used an old bandmaster as a platform and is using 7591 power tubes (which are wired correctly). I repaired a lot of things on it and it sounds great except if you dime the master and bring up the gain pot in the clean channel. I get these weird artifacts upon the initial attack and then they fade away. Some of the things I have already done are as follows:

1. Re-soldered filament connections that had come loose.
2. Biased FET circuit to proper levels.
3. Changed V1 cathode resistors and caps to 2.2k/5uf, 3.3k/5uf
4. Re-flowed solder on whole board.
5. Re-made several "iffy" solder connections on tube socket terminals.
6. Cleaned with 99.99% Alcohol using cotton tips
7. Checked bias on 7591's to 65% dissipation when SS rectifier is on. (Has Tube/SS switch).

If you use the amp like most of us do, there is no problem, but its owner usually begins with the Master dimed and brings up the gain as needed.

Any help would be appreciated.... I am pulling my hair out.....

[aflynt]

Could the OD be bleeding into the clean? How is the OD relay wired?

-Aaron

[fusionbear]

Could the OD be bleeding into the clean? How is the OD relay wired?

-Aaron

I will double check that. Also, to hopefully give a better description, if you play a humbucker neck pick up and hit a low note it almost sounds like a speaker with coil rub, but it fades. If you keep the master down, this problem does not come up. I tried different cabs to be sure it wasn't my speaker and then played my JTM45/100 Igor build through them to hear the loud clean tones and there was no artifact coming from the speakers. Maybe bad tubes, but I don't have a spare set to test with....

[aflynt]

Weird. I've had those shorting jacks in old Fenders produce a similar kind of sound when they loosen up before.

-Aaron

[fusionbear]

Weird. I've had those shorting jacks in old Fenders produce a similar kind of sound when they loosen up before.

-Aaron

I will check that out as well....

Thank you!

[amplifiednation]

Does it have pre/power loop jacks? If you don't have a scope you can use those jacks and work with another amp to try to isolate the problem. At least then you can try to figure out if it's before, or after the master volume.

[Guy77]

I had a similar problem once "weird artifacts upon initial attack" and it had to do with the Phase Inverter. One of the pins on the PI tube (12ax7) was damaged. It was pin 6.

You might have checked already but just thought I would let you know.

Cheers

Guy

[CHIP]

On the subject of shorting jacks, I once had a similar problem on the loop shorting jacks. Cleaning the contact point on the shorting jacks did the trick.

[Baddog01]

See my next post:)

[Baddog01]

I hang out at another forum and here is something that might help you.


I've done three BMR chassis to combo conversions/complete rebuilds where I initially intended to keep the original OT (an 022848). In all three amps, once the phase inverter was redone as an AB763, with the 1M grid resistors and 82k/100k plate loads, the NFB went somewhat squirrely. The NFB hookup polarity was correct (as the amps would howl like banshees when the polarity was reversed), but with the NFB hooked up 'correctly' they would exhibit bursts of approx 28k Hz oscillation on sine wave peaks, coming in and worsening as the amp was pushed harder.

Acoustically, this sounded exactly like a freaking coil rub in the speaker. It's funny, because I noticed the problem first in one of the three amps, and thought I did have a bum speaker. I even sent it off for a recone, because my verification was that the speaker did the same thing when driven by another amp. Yup, the 'other amp' was another of the three BMR conversions with the 022848 in it. Doh! I finally got things sorted out and wound up replacing the OTs in all three with nice Hammond units. For grins, I compared the primary inductance of the 022848 with that of the replacement Hammond OT. I don't have the numbers anymore, but the 022848 had ridiculously low inductance by comparison. If you look at the layout drawings for some of the amps using that OT, you will see that Fender added some phase compensation to the NFB loop in the form of a cap shunting the NFB resistor. (The cap doesn't show up on any schematic, just the layout drawings). So something funky was going on with that particular OT that they were trying to whip into shape (at least enough to be able to sell the amps using it).

[fusionbear]

I appreciate the tip and I think you are right. This amp has 7591's. As soon as converted to 6l6GC's, the problem lessened significantly. What value of cap did you see across the nfb resistor? I would like to give that a try to control it even better. I even tried a fresh set of 7591's to no avail. Those seemed to "amplify" the problem (couldn't resist the pun....)

The rest of the amp seems fine. If I feed the preamp out to my other amps, no issues at all.

BTW, by swapping back to 6l6's, the classic delicious feedback, even on the clean channel, is back. Which makes me think it is a OT issue...

[Baddog01]

It wasn't my fix, I just remembers it from another forum so I decided to post it here. I can go back and read the whole thing and see what I can find out about the capacitor value. Do you have the same transformer as described in the post?

Glad I could help
Keith

[Baddog01]

I looked at a few bandmaster reverb schematics and I did see a .01 across the feedback resistor in one of the Silverface amps. I guess I'd try that and see what happens. I'm guessing that was fenders attemt a fixing th issue.
This schmematic and layout shows the capacitor alcross the resistor in the layout bu not the schematic. I can make it out but it might be a .02. Hmmm
http://schematicheaven.net/fenderamps/b ... _schem.pdf

Keith


Here is a good schematics and layout website

http://schematicheaven.net/m-index.html

[fusionbear]

The OT is a T026 from Allen amps. A great OT, but not really high output. I finally figured out what is going on, I think. If I use 6V6's, no problem at all. If I use 6l6's, a little bit of problem, but almost gone. The 7591's accentuate the problem drastically. So, it may be a saturation issue.

I am going to experiment a little more and call it a day...

[Baddog01]

Sounds like your in the right track.