Help needed, urgently!

[Oddvar R]

Sounds like you simply need to swap the OT primary plate leads.

Hi SDluckey, I did that, but the other way made such noise I couldn't turn it on, the amp might have walked away.

[Oddvar R]

So this must be some error in the bias circuit, I have swapped the 1k resistor with a 8.5k resistor, (Brings the voltage down to-38ish) but when i again swap it for a 6.8k resistor the hi pitched noise appears a lot later when the volumes ar cranked. (The bias is then -40ish.) So I wonder what actually determinse the bias voltage in all, is there a other parts that dertermines this? The voltage fro PT is 50v?

[martin manning]

The bias voltage is determined why the 1k resistor and the 47k to ground. I would keep the 1k and make a voltage divider (18k from the 100u cap and 33k to ground), or better yet a pot (with a 47k shunt), so you can adjust the bias easily. Something like this.

[Oddvar R]

The bias voltage is determined why the 1k resistor and the 47k to ground. I would keep the 1k and make a voltage divider (18k from the 100u cap and 33k to ground), or better yet a pot (with a 47k shunt), so you can adjust the bias easily. Something like this.

Hi and many thanks, what knowledge. That was a great improvement. The hi pitch is now not showing up before almost to full. So I guess it's playable.
I have ordered 50k pots awhile back, but post is slow thee days.

[Oddvar R]

Could anyone look at these voltage readings and see if anything seems totally wrong. I had sound in this for a little while, but then again, no sound?

[Oddvar R]

Also some voltages elsewhere.

[martin manning]

Here's what I see in your preamp voltages.

[Oddvar R]

So now it seems like the amp is working. It has been a long drive this one. The problems have been, at times, impossible to comprehend, but what I take away from it, is that one problem doesn't have to be rooted in another. So, after having redone the bias after Martin Mannings kind help, it worked a while but then stopped. That was due to a loose ground wire in the grounding one of the bass capacitors. Then I guess the tube sockets had it's own problems, because the V1 and V2 tubes went leaky, all at different times, 3 or 4 tubes went that way, so I changed all the power sockets. And twice two EL34's red plated and caused a big bang/fire.

So thank you to great inputs from Amp Garage people.

[martin manning]

It has been a long drive this one...

I think this build was challenging because the layout is unusual, and the connections in the drawing are difficult to follow. I would have had the schematic close by, and be using both documents to be sure of each connection. A bias supply that has no adjustment capability is something to be careful with. The AC secondaries from an aftermarket power transformer might not match the original, meaning your plate and bias voltages could be different, and of course the variation in newly manufactured tubes often means they don't necessarily bias at the expected voltage. An adjustable bias is very useful for a safe start-up, since you can power up with the bias voltage set very low, putting the power tubes in cut-off. Then you can bring it up carefully while monitoring plate current.

[Oddvar R]

The bias voltage is determined why the 1k resistor and the 47k to ground. I would keep the 1k and make a voltage divider (18k from the 100u cap and 33k to ground), or better yet a pot (with a 47k shunt), so you can adjust the bias easily. Something like this.

I have ordere 50k pot to install it like you explained earlier. I was looking at it just now. Since I am communicating in a language that is not my natural one, some words can be a bit difficult to understand. By "shunt" you mean connected to on of the outer pins on the pot?

[martin manning]

No, from the wiper (middle terminal) to ground as I drew it above. That means you can go from having the resistor in parallel with the pot element (~24k to ground from the 18k), or out of the circuit at the other extreme for the full 50k to ground. I guess you could call that an adjustable shunt.