Hi, I am very pleased with my modded original amps. PC-4 has been treated to modern parts and the amps sound great. For a while I've wanted to wire individual tube bias pots for the EL34's. It looks as simple as seperating the output tubes and adding another 13.5 ohm resistor and bias pot to V2. Without worrying about mounting the extra pot whatelse would I need to do?

Hi Bob,
I did meantion not to worry about the extra pot, I will remove the power octal and mount a plate to it with the pot and 2 pin jacks for V2 & V3 which I'll label. If I can't fit the jacks I will have to drill 2 holes along side the power socket/V3 biaspot hole thus devaluing, even more, the already hole enlarged(WBT nextgen RCA's and Acme fuse holder)chassis.
I say let my children worry about selling these baby's at the estate sale in 50 or 60 years.

It is not difficult to construct a complete new bias adjust resistor string with two resistors and a miniature pot, on a tag strip. The pot can be soldered directly to the tag strip; no need to drill a hole, since bias adjustment isn't done often and the mounting of the pot need not be ultra-sturdy. The tag strip can be mounted on the top of the chassis using one of the screws holding the circuit board. The screw also serves as its ground and the new bias string is wired up to the 100K resistor in the bias circuit of one tube; the original one feeds the other tube and can be left intact. You have to separate the joint connection between the two 100K resistors where the original bias connection is made, easily done by lifting one end of a 100K resistor to tie in your new bias adjust string anyway. At the output tube sockets, separate the cathode resistors, using two of exact value; if you can't find a matching one to the one already in the amp, get two new matching ones instead. Value is not critical, anything around 10Ω is fine. If you don't use the Biaset octal socket for anything else, connect the two cathode resistors to different pins on it so you can measure the bias current on both tubes without having to probe under the chassis when setting bias.

I have done this on one of my Mark II and will do the other one next time it comes out for servicing. If I had thought to take pictures when I did it, I'd try and post them here!

With this dual bias adjust, I set one tube for the desired idle current at its Biaset point (50mA into an 11Ω resistor is 0.55 volts) and then connect my DVM across BOTH the Biaset points, then adjust the other tube until the meter reads ZERO volts.

I'm interested in the circuit mods needed to bias the individual tubes. I want to mod a stock ST-70 with a setup similar to what is used on the DIYtube board, where there is a "balance" type control used to adjust the bias between the tubes and a bias voltage level control used to set the overall bias current. The mods should pretty much the same for a Mk IV and an ST-70.

Since the ST-70 chassis was already drilled for additional pots,I'm not worried about the decreased value. I like the ease of zeroing out the difference between the two tubes, then setting the overall bias current.