Setting the gain on an ST-70

Hey everybody!  I'm new here, but I've been an ST-70 owner for years and reading the posts has been nothing short of enlightening.

I've got a new setup that requires having gain trims at the power side of the equation and I'm wondering about the best way to match gains on the two channels of my ST-70.  They aren't miles off ~ 1-2 dB.  The gain difference doesn't follow the 7199 tubes or the EL34's.  I'm considering just replacing all of the resistors as they are old carbon comps.  I just thought I might put trimmers in somewhere to get it as tight as possible and adjust for variation in the tubes.

The filter caps have been replaced by an SDS board underneath (long ago by a previous owner).

Thanks,

-d

Thanks so much Bob!

I'm going to order a new board. In the meantime, I pulled the carbon comps out and they are off by about 200 ohms. I have some 1% 1K metal film resistors on hand. Is 1/2 watt enough?

-d

The 1/2 watters have been in there cooking for an hour and they seem to be fine. I don't really measure much of a voltage drop across them. The channels are now 0.58 dB different, which is better -- certainly good enough for now, until the new board and parts show up. I have to say that I am amazed by this amp every time I listen to it and in my measurements, it's razor flat from 20Hz - around 2K when it starts it's gentle descent, hitting -3dB around 18K. It's a little flatter up top on the 16 ohm tap. I run it in triode mode, which may be the cause of the roll-off. I don't really know.

Anyway, thanks again. I'm glad to know you are out there.

-d

Bob always has great insight to the issues on Dynacos. I want to point out two more items.

The 1000 ohm feedback resistor is critical but also must be looked at (see schematic) in relation to the 47 ohm resistor that is part of the pentode’s cathode resistor. The 1000 ohm and 47 ohm resistors together form a voltage divider that sets the dB of feedback. Not just the 1000 ohm resistor. These 47 ohm resistors will go out of spec to so some of your deviations may be from having them mismatched channel to channel.

The plate and cathode resistors for the pentode voltage amplifier and the phase splitter section are also critical. The 620 ohm cathode resistor adds to the 47 ohm to ground to set the bias voltage. If that plate resistor is also off the gain of that tube will be hard to match to the other channel. It is tough enough to get the tubes to be consistent with each other but having these resistors out of spec is a sure way to not have channel balance.

Many replace all the resistors with metal film as they have excellent tolerances. I like the carbon film types as they also can have great tolerances but still use carbon in their construction. Much is written about each type and either will help your channel balance out. If the unbalance persists you may have some tube or capacitor related problems.

Thank you very much, WLT!

I hand matched new resistors in those positions and now the channels are 0.05 dB different with the dummy load! I'll do some listening throughout the day, but what a difference that made in the measurements. The 620's were close and the 47's were close, but the ratios were not as the 620 on one channel drifted high and its corresponding 47 drifted low, while the other channel was the opposite. That's a good lesson in circuit theory (the real reason I tinker anyway).

I guess my last question is about bias set. Should I adhere to the 1.57V marked on the chassis? I've read a few places that that might be a bit hot and 1.4-1.5 might be more appropriate.

Again, my thanks to you both!

-d

Search earlier posts here and you will find this discussed in detail.

Advantage of leaving at 1.57VDC – High current thru output tubes. Sets the amp far into class A at idle. Will have the lowest measurable distortion.

Disadvantage at 1.57 VDC – pushes output tubes hard which shortens their life. Not a problem in 1959 with output tubes costing $2-3 at the local drug store. Today tubes cost much more and no one likes to swap them on a regular basis. Turning down the output tube current helps and the minor amount of increased distortion seems to be inaudible.

My system. An EL34 has a plate dissipation of 25 watts (depending on make etc.). Per much internet discussion use 70% of that for derating (think longevity). That number still gives good sound. My Dyna ST70 uses no isolation transformer or thermistors. Accordingly B+ is around 465 VDC. Watts = VxI. Solve for I = 25 x .7/465= 38mA. Many guys use 40 mA and that is reasonable. Two tubes share one 13.5 ohm resistor so V=IR = 2 x.04 x13.5= 1.08 VDC. If you want to use 1.2 or 1.3 VDC that is OK as well. Try several settings and see if you can hear a difference. This is a feel good number vs. cost of output tube replacement. No right or wrong answer. Others may disagree and I can understand most of their arguments.

Lowest THD (measured with my Minilyzer) on my system yields 1.45V at the bias checkpoint. I'm going to live with that until I do a full rebuild.

Also, one quick question: I think I have a 15.6 Ohm resistor, making the calculation 2 tubes x .04 Amps x 15.6 = 1.25VDC. Is that correct?

OK, really last question...I feel like I should take out the mono/stereo switch. It just seems like more room for sonic imbalance, regardless of the quality of the soldering or components. Am I dreaming?

This is my last day of tinkering before I have to get real work done, but it's been a good one and an educational one. I thank you all.

-d

My bad. You are right about the 15.6 ohm resistor. I had the MK IV schematic up which uses 13.5 ohm. Same number crunching so 1.25 VDC or so is the right calculation. Your 1.45 VDC is fine. Try listening to slightly lower settings and see if you can hear a difference. I doubt it. Actual distortion readings can be reassuring as well.

My vintage ST 70 is playing now and sounds great. Enjoy