New Member Dynaco ST-70 bias voltage

Hello Everyone I'm new to the forum and have recently aquired an early 60's version of the ST-70 with all original tubes.
I got into vintage tube audio about 15 years or so ago and have mostly had integrated Fisher, Eico, Scott and Bogen amplifiers which I most of the time restore to stock, I did own a Stereo 70 about 7 years ago, but ended up selling before doing any work on it.

I'm currently in the process of restoring my newly aquired Stereo 70, I've installed the CE 80/40/30/20 quad cap and replaced the selenium rectifier with a 1n4007.
I've also replaced the tw0 50uf caps in the bias supply with 68uf 100vdc.

My quad cap voltages at about 118 vac are all relatively close with only about a 7-10 volt difference from the specs, although I had to drop the 22k down to about 14k to get the 305 vdc as per spec.

The voltage that I'm wondering about is the negative voltage at pin 5. I'm only getting about -25 for one pair and -26 for the other. I have nice range with the bias pots and can dial it in to the recommended 1.56 v or drop it a little and still get a lower than spec negative voltage at pins 5 or 8.

The original tubes all tested "good" on my tube tester although I'm not sure how accurate this would be from my old Precision 10-12 tube tester.

I've done searches but haven't really seen anything concerning this particular issue?

Should this be something to be concerned about? Is this maybe running the tubes a little hot?

Any thoughts would be greatly appreciated!

Thanks!

No worry about the voltage on pin 5 as the current across the tubes are within spec ( reflected by the fact that the voltage across 15.6 ohm is 1.56 volts.
The difference you see is that one pair of tubes has a slightly lower Gm then the other, that is perfectly normal.

Okay, thanks for your input!

I was just asking because the manual states that this voltage should be -32vdc and I'm getting -25 per pair and now -24 for the other pair.

I did read further in the manual and found that it states that this voltage can be within  +/-20% of -32vdc so I guess I'm right on the lower end.

I measured the grid resistors and found that all have drifted up to between 285-290ohms wondering if I should just replace these?

Chris E. wrote:Okay, thanks for your input!

I was just asking because the manual states that this voltage should be -32vdc and I'm getting -25 per pair and now -24 for the other pair.

I did read further in the manual and found that it states that this voltage can be within  +/-20% of -32vdc so I guess I'm right on the lower end.

I measured the grid resistors and found that all have drifted up to between 285-290ohms wondering if I should just replace these?

Do replace them. 250k is reasonable.

Okay, thanks Peter.

I did replace them and also replaced other out of spec resistors on the driver board as well as 4 of the coupling capacitors.

I decided to run the amp for a while keeping an eye on the bias voltage which held steady at the recommended 1.56v, I will probably lower that a bit in the future.

After a couple of hours I noticed that the power transformer was very hot as well as the new quad cap that I installed.

Is this normal with this amplifier? I know any tube amplifiers will generate some heat but how much is normal with this particular amplifier.

One other thing, I checked the two 15.6 ohm bias resistors before running the amp, and got different readings at different times. last check they both read 16.7 ohms each?

Thanks for your help!

The power transformer will be hot. The can cap should not be hot by itself, it may be heated by the surroundings. Keep an eye on it.

Yes I'm hoping it is just the radiant heat coming from the tubes and transformer that is causing the cap to get warm, I will keep my eye on it.

I'll probably run this on my variac at about 117v and decrease the bias voltage to see if it runs a little cooler.

At some point I will invest in the upgraded power transformer and a new matched quad ouf output tubes.

Thanks again for your help!

If you decrease the bias the B+ voltage will rise ! A small change is ok, but i would keep at 1,5 Volt
at the bias msrmt tap.

If the can cap leaked before ( and got warm) chances are that it has been better adapten now and won't be worm anymore ( process called formation where an oxide film is created )

I really hope it didn't leak because it's a new 80/40/30/20 can!
Thanks again