Issue with a Clone Hybrid Hiwatt/Ampeg

[blackwhaleamp]

The tail resistor is indeed 220k. I need to measure with the scope, but I think the LTP is enough to drive the KT88. There is no NFB loop around the power amp. Should I put it?
I removed V1 and V2, and the oscillation still there. If I remove V3 or V4 it dissapears. Then, I decided to leave all the tubes and start to remove the plate voltage. So, if I also removed the power for V3 at R31, the oscillation still there. Same case if I remove the power for V4 at R33. Another thing about V4, only half tube is used. The other half is no connected, but the fillament at PIN 5 is connected.

[nevetslab]

I would put the NFB loop back in.. I was wondering if you lifted the tube connections for C6 and C17 briefly from the Presence Control, does the oscillation go away? Removing either V3 OR V4, the oscillation goes away, you said. Still makes me wonder about the Presence circuit and the absence of the NFB in the most recent circuit diagram.

[blackwhaleamp]

Just checking, is the LTP tail resistor R43 really 220k? Even if it's 22k, are you sure that the LTP can put out enough signal to fully drive the KT88 (ie to Vg1-k = 0, on peaks)?
And there's no global NFB loop around the power amp?
Does removing V4 stop the oscillation?
If not, then V3?

I think you are right. I measured the signal after the OT, and I saw 15V with 8 Ohm. That's only 28W. I was expecting to see at least 35V.

Do you think that I should put another tube right after the LTP, or should I change the LTP configuration?

[pdf64]

I think that a smaller value for R43 would allow a greater potential signal swing at the LTP plates.
Why is it such a high value?

[mhuss]

Agreed, you're losing a lot of your potential voltage swing across that 220k resistor. And is a 12AY7 the best choice for driving a quad of KT88s? Perhaps a 12AT7 would be better?

[nevetslab]

I think the 12AU7 would be a better choice...those are used in the Ampeg SVT-CL drive circuit off of the cathodes, with the bias adjust circuit on the 12AU7's Grids. If you need even more current, ECC99 do that well. Taller tube, same height as an EL84. Ashley's BTA-400 uses that tube to drive 8 KT88's off of it's cathode

[blackwhaleamp]

I think that a smaller value for R43 would allow a greater potential signal swing at the LTP plates.
Why is it such a high value?

It was a typo on the schematic. I didn't realize. I changed the resistor for a 22k and now it works properly. There is no pluk-pluk-pluk and the output power is around 200Wmax as expected. Now, I just have one remaining issue. When I put all the pot at max, there is a buzz noise coming up. As soon as I decrease the Master to 8, the buzz goes away.

As for the ground, I separate the pre-amp from the power stage. I use a single bare wire that goes all the way left to right. This is attached to a ground star close to the input jacks. I did the same for the power stage, but the power star is attached close to the power jack. I read that this is the best way to avoid ground loops.

Do you have any idea why I get this buzz noise, and how can I get rid of it?

Thanks

[pdf64]

Does the presence control setting affect the buzz, eg bring it on at a higher or lower master vol setting?
Do the power tubes draw more plate/cathode current when it's set to 'buzz' mode?
It would be very beneficial to include 1 ohm current sensing resistors in the power tube cathode 0V returns.