Mesa Subway Rocket Low Volume

[52 Bill]

That is what he wants, to increase bias voltage and decrease idle current.

Maybe the cap is bad?
Or some issue with the bias probe? Does it use voltage range or current range on your meter?

Oops! My bad, I thought he was trying to decrease the voltage.

It may be that -16 volts is all the circuit will produce. What voltage is on each side of the 33K resistor?

[Pedro Vecino]

With a bias circuit so stiff I would go straight to raise the value of 2M2 resistors between the grids and ground. Until 4M7, 6M8 ... to raise the voltage bias.
It also has a free 70VAC secondary. You could build a much more flexible bias circuit with it.

[The Dude]

Let's start by measuring the actual bias voltage. Do you get -16VDC or so? How much AC do you get? Before we modify, let's make sure it's working correctly to start with.

[Pryde]

Let's start by measuring the actual bias voltage. Do you get -16VDC or so? How much AC do you get? Before we modify, let's make sure it's working correctly to start with.

The bias voltage is -15VDC. I am getting this amp back to my friend for now as he needs it. He has been playing it for years and says it goes through a lot of power tubes. Reading other posts it seams the circuit is designed to cook the bejeezus out of the EL84s. I will get it back from him soon and try to get the bias current down somehow.

[The Dude]

......He has been playing it for years and says it goes through a lot of power tubes. Reading other posts it seams the circuit is designed to cook the bejeezus out of the EL84s......

Understandable. That bias current is around double what I would say is acceptable.
Interesting that the amp's manual has two and a half pages about bias. Yet, they still don't seem to understand it themselves.

[g1]

It also has a free 70VAC secondary. You could build a much more flexible bias circuit with it.

I think Pedro has the best solution, with the added bonus you don't have to worry about messing up the -3V supply.