Dynaco ST-70 out of 12 year hibernation. First observations

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Pekka72

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After putting my ST-70 on my neighbors Variac with speakers connected and nothing smoked after bringing voltage from 30 to 110 so I figured it’s safe to bring home and play something through it. This is a stock ST-70, Sovtek 5AR4, a pair of 6GH8A’s in place of the 7199’s on the driver board (restored with new caps and resistors), RCA 6L6GC output tubes, and the multi-cap can was replaced. Issue I’m having is the left channel’s output is less than the right’s by at least 50%. I’m compensating for the difference with the balance knob on my Kenwood C1 preamp. Signal is from a Samsung CD player. Should I shut it down and swap output tubes around and reset the bias? I have at least a dozen GGH8A’s to switch in and out if that would help. Around 12 years ago this amp didn’t have such an issue. I should mention, with dead air in a passage there’s some slight hum in the right speaker too, only very faint hum in the left side.
 
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You could try swapping the driver tubes you have now in there sockets and see of the low channel switches to the other channel. :dunno: That would tell you if you have a really weak driver tube. If it doesn't you could then swap the power tubes in pairs and see if the problem switches to the other channel. You may just have a bad connection somewhere.
 
Yeah, and I'm not feeling the stock part.

"This is a stock ST-70, Sovtek 5AR4, a pair of 6GH8A’s in place of the 7199’s on the driver board (restored with new caps and resistors), RCA 6L6GC output tubes,"
 
I've seen 6L6GC's triode connected, driving the screen windings of a ST-70.

About 20 watts, Sounded fine.

But they did not look happy at that plate voltage...
 
Pekka72 said:
After putting my ST-70 on my neighbors Variac with speakers connected and nothing smoked after bringing voltage from 30 to 110 so I figured it’s safe to bring home and play something through it. This is a stock ST-70, Sovtek 5AR4, a pair of 6GH8A’s in place of the 7199’s on the driver board (restored with new caps and resistors), RCA 6L6GC output tubes, and the multi-cap can was replaced. Issue I’m having is the left channel’s output is less than the right’s by at least 50%. I’m compensating for the difference with the balance knob on my Kenwood C1 preamp. Signal is from a Samsung CD player. Should I shut it down and swap output tubes around and reset the bias? I have at least a dozen GGH8A’s to switch in and out if that would help. Around 12 years ago this amp didn’t have such an issue. I should mention, with dead air in a passage there’s some slight hum in the right speaker too, only very faint hum in the left side.
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The hum might be gone when you have installed matched pairs of EL34 .
As for the unbalance i suspect that some of the resistors have drifted ( has nothing to do with the tubes)
Most resistors can be measured in place ( 330k is one exception but unlikely to give that symptom). Replace the
ones that is drifted.
 
Both 7199 and 6GH8's are notorious for heater to cathode leakage.

Try replacing them with your spares to fix the low level hum.

Also the difference in basing between 6GH8 and 7199 is to improve inter-electrode spacing.

7199 was one of the last tubes designed by RCA specifically for audio along with 7355, 7868, 8417, 7027.

Edit:Forgot 7591.

Buck
 
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Buck Rogers said:
How did a ST-70 end up with 6L6's?

Not a good tube for that amp.
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My father had the amp at his place in New York when I moved back to Chicago. After leaving he sold all the tubes, a quad of JJ’s, the 7199’s, and my Mullard 5AR4. He replaced them all with tubes that “will work” and only last month gave me back the amp and claims to know nothing about the tube assortment. He’s 88 and has “foggy memory” selectively. I plan on replacing the output tube set next, then a driver board mod.
 
Still, I would swap the driver tubes to see if the hum follows the tube to the other channel. If not, then the output tubes. I personally wouldnt use 6L6 tubes in that amp. I would worry too much about one failing and causing some other issue.
 
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Pekka72 said:
Should I bias them lower than 1.56V?
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YES~! But at this point we have no idea what the "Test" resistor is? A 15.6 ohm is stock but at 1.56 volt that will make the 6L6's plate glow Very red.

Pictures... Pictures... Pictures...

You can keep the 6GH8's/driver board just do a listening test for hum.

Going back to EL34's will fix most problems. I'm using New Mullards and they seem to be fine.

Also check for excessive B+ filter cap size, Common modder mistake.

It's a great amplifier, One of the absolute BEST~!

Once you get things closer to "Stock" it will sound great and last many years.

Good luck

Buck
 
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The voltages and idle current aren't really a big issue for a 6L6GC, its just not an ideal plate load in UL mode.

Plate voltage around 450 and 50ma per tube is idle dissipation of about 23 watts plate plus screen on a tube good for what, 500v and 30 watts? Something like that.
 
Buck Rogers said:
YES~! But at this point we have no idea what the "Test" resistor is? A 15.6 ohm is stock but at 1.56 volt that will make the 6L6's plate glow Very red.

Pictures... Pictures... Pictures...

You can keep the 6GH8's/driver board just do a listening test for hum.

Going back to EL34's will fix most problems. I'm using New Mullards and they seem to be fine.

Also check for excessive B+ filter cap size, Common modder mistake.

It's a great amplifier, One of the absolute BEST~!

Once you get things closer to "Stock" it will sound great and last many years.

Good luck

Buck

PS:Upload pictures to www.Imgur.com then post them here. I keep lots of pictures hosted there.
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Why the added step when posting pictures? :dunno: There's an attach files button to the left of the post reply button below. You can upload directly to AK in your post from a smart phone, laptop, pad, etc. No need for a picture hosting sight. They tend to disappear over time. I've dug up so many older posts over the years where the pictures sent through a hosting sight are no longer in the thread. Just a thought. ;)
 
Lavane said:
Still, I would swap the driver tubes to see if the hum follows the tube to the other channel. If not, then the output tubes. I personally wouldnt use 6L6 tubes in that amp. I would worry too much about one failing and.causing some other issue.
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I was planning on getting a matched quad of Russian NOS 6P3S tubes. Why do you suggest otherwise and what would you recommend?
 
EL34, 6CA7, or KT77 tubes are appropriate for these. JJ makes a decent EL34. Of my collection they sound most like the Mullards that people pay through the nose for.

The 6P3S are about a 20 watt tube, not really up to the task besides the bad match for the load.
 
Lavane said:
Still, I would swap the driver tubes to see if the hum follows the tube to the other channel. If not, then the output tubes. I personally wouldnt use 6L6 tubes in that amp. I would worry too much about one failing and.causing some other issue.
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I’ll be swapping out the 6GH8A’s with the dozen others I have on hand. All passed the tests on my tube tester.
 
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