Yet another St70 issue

[Omegaman]

From left field


When you replace the original selenium rectifier with a diode, the bias voltage adjustments sometimes go out of range. The reason for this is that a diode has more "forward voltage" than the selenium rectifier. There is an easy solution. On the 7 lug terminal strip to the left of the power transformer are two 10K resistors (between lugs 1 and 2 and between lugs 3 and 4) which set the "range" of the two 10K bias pots. These two 10K resistors are now too high for the proper bias range on an ST-70 with a diode. These two resistors should be dropped to about 5K each. A common cure is to parallel the two 10K resistors with two other 10K resistors giving an effective resistance of 5K.

Mr Bob Latino also mentions this in his writings

 

[s-petersen]

You could measure AC voltage across the first cap, if it's high, a bad cap is likely, like Gadget said...

 

[gadget73]

probably the temperature coefficient of the carbon track in the potentiometer causing it to change. Between radiant heat and the small amount of dissipation from the voltage divider action it does warm up somewhat. I know things change from say 5 mins after power to several hours, but none of my amps move enough for me to have ever actually looked into what specifically is drifting. I always assumed it was basically "everything". The parts in the bias and HV supplies and the tubes all change somewhat as they heat soak. And the line voltage is never 100% stable so that bumps things around to a degree as well.

Setting the bias to multiple decimal places and trying to keep it there will drive you nuts.

 

[Lavane]

Stereo 70 and at least one other brand of tube amp I have drifts more than hundredths or thousandths. They can drift by several tenths, but, as you say, AC mains fluctuations can account for some of it. I also found it varies with different tubes. I do think the MIL spec pot holds more steady. Funny thing is, the Stereo 35 with the individual bias adjust module with the little square blue mini pots adjust just about rock solid.

I have one of the ST-35 bias boards. I bought it already built. I haven't installed it yet. But those trimmers do work well. I've used that type to replace the old trim.ers used in SS amps to set bias and DC offset. They stay put. I'll have to check. They may be multi turn pots?

 

[J RR]

(PARTIAL) UPDATE: I installed a new can cap. I was supposed to have a multitude of resistors in today for a general replacement for all -- alas they are not in. Hopefully tomorrow!

 

[J RR]

And finnally the moment you've all been waiting for! Resistors were sent to the wrong house -- luckily that lovely citizen brought them over today. New cap in, new resistors. New readings!

Measurements (Pre, then post):
Can cap:
A (Square): 227VDC now: 284
B (triangle): 282VDC now: 359
C (blank): 324VDC now: 414
D (Backwards D): 352 now: 427

EL34 (VDC):
1- .5 now: 1.53
2- 1.08 now: 1.74
3- 322 now: 410
4- 323 now 413
5- -31 now -33
6- -31 now -33
7- 1.09 now 1.70
8- .58 now 1.52

In general the readings seem OK ... sections A and B are a bit weak. I'm biasing as we speak ... getting enough voltage out of there fine.

Anything seem super abnormal? Pin 2 and 7 are reading low on the EL, I believe ...

 

[J RR]

Set meter to AC volts, one probe on EL34 pin 2, the other on EL34 pin 7, and you should read about 6.4VAC.

Nodes A and B on the quad cap are well within reason but might still be a bit closer.

Try checking the test points indicated for the 7199 in the chart - same deal with measuring the AC value between 7199 pins 4 and 5. Check both of them.

Bias diode cathode should be 50VAC to ground, and the anode should be -65VDC to ground.

PCB eyelets 3 and 18 should be about 370VDC. Try checking the test points for the rectifier tube - also in the chart.

Is your buzzing gone?

Indeed the buzzing is gone! I believe the diode checks out ... I'll check the other points tomorrow. ... toddler is calling for all the attention. ... amp does sound great though.