1970s Ampeg B-15N, screeching oscillation after recap

[cobascis]

I just picked up a very clean (entirely untouched inside) Ampeg B-15 from the early to mid 70s. I did minor testing of the amp with all the old electrolytic, but had a recording session in the next few days, so I replaced all the electrolytic and the can cap with parts from FlipTops. I also installed a 3 prong grounded cable (abraded chassis and used a large hi wattage iron to solder ground to chassis right as it enters amp), and disconnected the AC outlet and the polarity switch. I did brief testing after the recap, and the amp sounded good. It wasn't until the session that I learned all was not OK.

Here's what happens:

• When I turn the volume past noon (with an input plugged in) a horrible oscillation/screech comes out of the speaker. The screech is fixed when I turn volume down below noon. At lower levels the amp seems to function normally.
• The entire amp is extremely microphonic, I can tap anywhere on the chassis and it is very loud. I tried all new power and preamp tubes, no change in microphonic aspect
• I tried another CE Can cap to see if it was a faulty cap. No improvement.
• The screen does not occur if I pull out the preamp tubes
• I've checked and reflowed solder on ground connections

What else should I check? Hoping it is something simple that I am overlooking, need to get this thing back in action for another recording session coming up. Happy to post voltages or anything else that might help track this down. Thanks!

Here is a video of the microphonic nature of the amp and what happens when I turn the volume up when an input is connected.

[passaloutre]

Is there a specific preamp tube that kills the oscillation? Got any spares

[cobascis]

Is there a specific preamp tube that kills the oscillation? Got any spares

I swapped out all 3 of the GSL7 preamp tubes with known good tubes and no change to the squeal. I also tried switching the order of the preamp tubes. No dice. Put new power tubes in as well just on the off chance that was a factor, no change.

[passaloutre]

Put them one at a time. Which one kills the oscillation?

[cobascis]

Put them one at a time. Which one kills the oscillation?

Removing V1 kills the oscillation in channel one, removing V2 kills oscillation in channel 2. If V1 and V2 are removed, no oscillation in either channel. If V3 is removed and V1/V2 are left in, no oscillation.

[sluckey]

Did the amp have this issue BEFORE you replaced the caps? If not, I suggest reexamining the cap replacement with close attention to the cap that feeds the preamp tubes. Look for loose connection or poor solder on positive and negative cap terminals.

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_B-15N.pdf

[cobascis]

Did the amp have this issue BEFORE you replaced the caps? If not, I suggest reexamining the cap replacement with close attention to the cap that feeds the preamp tubes. Look for loose connection or poor solder on positive and negative cap terminals.

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_B-15N.pdf

I believe it did not, unfortunately I didn't spend a ton of time testing the amp before I recapped it.  But I'm fairly certain I turned it all the way up with no issues.

I've triple checked the solder on the can cap, and replaced the can cap already so I'm sure it's not the connection at the can cap itself. But perhaps where it meets the board and powers the preamp section could have a bad solder joint. Looking at the schematic, it is the middle 40uf (middle from right to left) that powers the preamp. I will see where that connections and reflow that solder. Anything else I should try?

[sluckey]

Looking at the schematic, it is the middle 40uf (middle from right to left) that powers the preamp.

No. The 40µF on the left feeds the preamp tubes (V1 and V2).

The issue may not be directly related to the cap can replacement, but rather some collateral damage that occurred during the process

[cobascis]

Looking at the schematic, it is the middle 40uf (middle from right to left) that powers the preamp.

No. The 40µF on the left feeds the preamp tubes (V1 and V2).

The issue may not be directly related to the cap can replacement, but rather some collateral damage that occurred during the process

Got it. Just reflowed all solder joints where the cap is providing power. Also reflowed both preamp tube socket pin 5. Checked to make sure both those resistors were at around 470K, they were. Reflowed cap can ground. No improvement so far. Hmm...

[pdf64]

R37 supplies the preamp HT node. The symptoms align strongly with that node not being adequately decoupled, eg the 40uF cap connected to R37 either being bad or not properly connected.

It's interesting to note that the heater circuit looks to have a negative DC elevation (something would probably be needed to mitigate heater hum leaking into the signal path at the unbypassed input stage cathodes).

The standby is hot switching, its use will pointlessly stress the GZ34.