Sunn Sentura II low frequency oscillation

[booj]

When the volume is turned past about 2, what sounds like a 60hz powerful farting oscillation takes over and swamps out everything.
>When the output tubes are taken out, it doesn’t happen. The signals at the grids look normal.
>It has been extensively re-wired with new parts, both capacitors and resistors, but although a bit sloppy, seems to be done accurately according to the schematic, which also seems to be accurate.
>I tried swapping the output transformer leads to the plates of the output tubes, only to be greeted by a horrible howl from Hell.
>I suspect the main filter cap is compromised, but cannot confirm it with a cap checker. It’s a JJ multi-section 40/20/20/20 @ 500 volts rating. The power supply puts out about 585 volts. It reads good as far as capacitance on my 878A LCR meter. The “D” and “Q” readings are normal. Is it possible it’s breaking down just enough to belch out this horrible low frequency noise and then look normal to a cap checker? It’s totally dis-connected at present, and I don’t really want to hook it back up. I suppose I’ll have to re-do the power supply filter section and try it again, but I’d appreciate any feedback I can get on this. Thanks!

[booj]

I may have a clue. After fabricating a high voltage 4 section cap to use, it did the same thing, but, as I was re-tracing the wiring I noticed the screen grids are connected to 1K 1 W resistors, which are tied together, then to a power supply lead connected to point B (wrong). The output transformer has been replaced and is not a screen tap type. So now I have another question. should I just tap off of point A and use a series resistor of say, 400 ohms to another filter cap and use that as a screen supply? Otherwise I could just add that 400 ohm resistor to the branch in series and add another cap.

[booj]

Tried a 470 ohm in series after point A with an extra cap. Does the same thing. guess I'll have to swap out output trans. next.

[g1]

The screen arrangement that was there didn't seem "wrong" to me, and I don't think it could cause the issue you have.
Don't swap out the OT, swap the primary leads around.

[Chuck H]

Don't swap out the OT, swap the primary leads around.

From post #1

I tried swapping the output transformer leads to the plates of the output tubes, only to be greeted by a horrible howl from Hell.

I'll put my money on one or more bad grounds for the power supply.

[booj]

Okely dokely. Another clue. I swapped the output transformer for a Bandmaster OT. Aside from a little hum from my inadequate home made multi section cap (4 x 5uf) The amp worked normally. Wonderfull. Now I get to tell this guy his output tranny is bad, plus he has to replace the multi-section JJ filter cap.

[booj]

the output transformer is a power transformer Dynaco 464003. I guess that would explain it. I feel like Harvey Lembeck in Beach Party. Why me?

[Chuck H]

There's something you don't see every day!?! I guess that might explain the lack of screen taps

I'll recommend an off the shelf Hammond. The more common models are reasonably priced and have screen taps. Also virtually indestructible. Pick one rated for about half the amps wattage. That's a tip from a tech at Hammond I spoke to. He said you could even go lower if you wanted to. You may still notice that the half rated OT is larger than average for a guitar amp! I'm currently using a 1608 (rated for 10W) in a 2xel84 amp that I crank all the time. I use an attenuator and have made the occasional mistake with the load not being hooked up for a moment. The transformer survived just fine where others may not have. So, no fragility issues with the half rate suggestion. Incidentally, the Hammond equipped amp sounds better (to me) than an identical amp with a custom Heyboer.

EDIT: Well I just looked up the Hammonds and it looks like the 1645 or 1650k would be your unit. The problem is that you need the easy wire secondaries so the model numbers change to 1645A or 1650kA and cost a bit more. Either unit is about 135 clams (ouch) Still, not sure where you'll find a suitable UL OT for less. The 1645A is a 5k primary and the 1650kA is 3.4k.

[soundmasterg]

Triode Electronics sells transformers that fit the Dynaco Mk3 and also fit the Sunn 200S type amps, which would be any of the two x 6550/KT88 amps.

They aren't too costly either.

Greg

[Gtr_tech]

Replace the low voltage lytics while yer in there....the ones for the bias and trem circuits. They can and *will* cause problems.

[booj]

Just to clear up a couple of things for anyone who happens to have a Sunn Sentura II and is looking on line for help. I got her up and running. Because of some weird form of dislexia, I got turned around and mistook the power transformer for the output transformer. I mean things get flipped around when you flip them over, right? Anyway I had the guy order a new output tranny, and as I prepared to install it I realized my mistake. but...it wasn't working so what the heck. I installed the beautiful new Triode Electronics Sunn Sentura output tranny, with the screen taps and all... direct replacement. Then...it did exactly the same thing. Low frequency howl. Panic ensued. That "special kind" of panic that happens when you make a mistake and may have to pay big bucks and long hours recovering from. But I knew I had all the right parts and I knew it wasn't my wiring job. Or was it? After a couple of hours I realized I had connected the 16 ohm unused tap to ground instead of the the feedback loop. I also cleaned up a couple of other ground points. I completely re-did the feedback loop circuit. Finally, it was working perfectly.