Biasing a ST-70

Hello Folks:

This is my first post, and there is wealth of information on this forum.

Anyway, I have a ST-70 amp that was slightly modified by the prior owner. There are no power take off (PTO) sockets installed, but the pior owner installed two inputs. From what I see in the guts the wiring does not appear to be modified at least from the standpoint of the biasing.

I have been an owner of guitar amps for a while so I do most of my biasing the somewhat old fashion way and that is taking reads for both plate voltages and idle current. Based on what I read, a stock ST70 amp using the PTO for the voltage read should be around 1.56 volts. Assuming the cathode resistor is accurate at 15.6 ohms then the result is an idle current of 0.10 mA. My first question is the idle current the total for two EL34 tubes? I believe it is but I want to make sure. If it is then a single EL34 is at 0.05 mA (assuming both tubes are matched). Based on a plate voltage of 410 volts (using the value in the factory issued instruction manual) would result around 20.5 watts per tube. Of course this was based on the lower AC voltages of the 1960s. So my second question is this calculation approach correct? The third question is the bias pot used to adjust both tubes on the same side or is one pot for both tubes in the back and the 2nd pot for the front tubes?

Thank you

The bias pot is used to adjust both tubes on the same side.

You should verify that the 15.6 ohm resistors are still in tolerance. If not dynakitparts.com has them.

These days the recommended setting is 40ma to 45ma per tube. You won't hear a difference and your tubes will last longer.

Thanks Ken. Yes, confirming the actual resistance of the resistor is one of my to do items. But I also want to make certain in my understanding is correct with regards to the other questions.

Can I simply pull the tube and take the read of the resistor or will it give a false reading since it is still connected in circuit?

Nothing else is connected to the top of the 15.6Ω resistor except the cathodes and a suppressor grid, so unless a tube is fatally shorted inside, you don't have to remove them to take that resistance measurement.

GP49 wrote:Nothing else is connected to the top of the 15.6Ω resistor except the cathodes and a suppressor grid, so unless a tube is fatally shorted inside, you don't have to remove them to take that resistance measurement.

Thank you for confirming.

Bob Latino wrote:To find the value of the main bias resistor on a stock ST-70 (with the amp off) just take a RESISTANCE reading (instead of a DC voltage reading with the amp ON) between the bias measuring point and chassis ground. If the front octal tube sockets on the front of your amp have been replaced (as you mentioned) by two inputs, then almost assuredly one input is connected to chassis GROUND and the other is connected to pin# 8 of one of the output tube sockets for that channel.

As Hawaii.ken has mentioned, the 1.56 VDC mesurement that Dynaco recommended across a 15.6 bias resistor gives a bias current of 100 miliamps for TWO output tubes or 50 milliamps for each tube of the pair. Conventional wisdom now is that 50 milliamps is a little high for a modern EL34. If you use about 1.25 VDC as your bias reading then that would equate to about 40 milliamps of bias current for each output tube of the pair.

Bob

Thank you Bob for confirming.

Actually the 2nd input in the front panel is for the other channel and not ground. But I got what you are saying. Yes I would agree that 50 mA @ 410 volts is a bit high.

Re: >"Yes I would agree that 50 mA @ 410 volts is a bit high."

With today's higher line voltages, if you measure the actual plate voltage from pin #3 to chassis ground on any of the output tubes on an original Dynaco ST-70, I would be willing to bet that your voltage is in the 430 to 440 volt VDC range.

Bob

Bob Latino wrote:Re: >"Yes I would agree that 50 mA @ 410 volts is a bit high."

With today's higher line voltages, if you measure the actual plate voltage from pin #3 to chassis ground on any of the output tubes on an original Dynaco ST-70, I would be willing to bet that your voltage is in the 430 to 440 volt VDC range.

Bob

Yup' I bet you are right on that one. Need to lower the plate dissipation before pumping some blues through it. Thanks again Bob.