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Doctor Olds 02-19-2012 12:30 PM

Another Dynaco ST-70 thread
I am not here all that often but I have another problem and I am pretty sure someone here can tell me where to look. I now have a problem with the ST-70 I have owned for about 25 years. The first 15 years or so were trouble free and other than checking the bias every once in a while this unit was trouble free. Then it died and I found the 5AR4 rectifier tube went and while testing the tubes the rest were weak so I replaced them all. The new 5AR4 failed within a couple of days so I began a thread here and removed 2 huge external caps added to help the bass response then replaced the can cap on the amp with a new CP one and installed new orange drops on the original board. It ran fine and sounded good but began to burn out the 5AR4 occasionally (every other year or so.) I installed a NOS (ouch) 5AR4 a few years ago which solved that problem. The unit developed a "hum" / both speakers in stereo mode so now I am back here......all the tubes are lit normally so I tried a spare 5AR4 just in case which did not help. When I checked the bias I found it was very low @ .44 VDC. I tried turning up the bias set (for just a second or two) and could only achieve .70 and the hum did not change either louder or quieter (I turned it down to where it was.) The unit is now down on my bench and I am wondering where to look first?

Any help would be appreciated. Doc

bktheking 02-19-2012 01:36 PM

The bias was low on both sides or just one? How old are the el34's? The buzz could be coming from a number of things, weak tubes, something wrong in the bias circuit, bad choke...

awise1961 02-19-2012 01:40 PM

I can't really tell by the photo, but did you replace the sellenium recitifer with a 1N4007 or similar diode already?

Al. Wise

bktheking 02-19-2012 02:01 PM

The sellenium isn't there, it would be on the center threaded screw, there is a black diode in it's place.

Doctor Olds 02-19-2012 02:09 PM

It was equally low on both channels (both were set at 1.2 to 1.2 vdc) and dropped dramatically to .40 when the hum appeared. I did not mess with the sellenium rectifier since the diode was already there. (It was probably changed when whomever before me added the huge caps externally.) I also did not change any of the carbon resistors or the two silver electrolytic caps inside the unit.

131east23 02-19-2012 02:11 PM

Maybe it's just hidden from view but I don't see a bias resistor on V7.

buglegirl 02-19-2012 02:22 PM


Not a exact ST-70 expert but what about those long silver caps underneath. Also the mustard colored Erie's on the driver board should be looked at.

Also clean and re-tension all sockets and verify all solder joints on the board.


bktheking 02-19-2012 02:33 PM

Bottom of the manual has test voltages, measure and repost. Focus on the rectifier voltages and el34's.

JonL 02-19-2012 02:33 PM

I think the fact that the tubes are drawing LESS current points away from it being a bias supply problem. To get less current, the bias supply would have to be more negative than normal. Don't know how that would happen. I wonder what the B+ is on the output tubes.

TerrySmith 02-19-2012 03:47 PM

Three things I notice from the pics:

1 The can cap is very underrated (475v) and is probably bad. It should be rated for 525v! Replace it with a NEW can cap or a cap-board.

2 Those Sovtek 7199's are known to be noisy and unreliable. Won't affect bias though.

3 When that chinese 5AR4 fails it's usually fireworks, so keep an eye on it!

Iguanas4me 02-19-2012 03:47 PM

Bad cap?
Check the bias supply filter (electrolytic) cap(s). The low bias voltage is most likely caused by excess cap leakage.

BuzzK 02-19-2012 03:51 PM

My guess -- just that -- is that the first section of the CE cap has gone bad, and that the B+ has dropped quite a bit. A read on the B+ will shoot a hole in that theory or confirm.

Doctor Olds 02-19-2012 04:33 PM

I have my original Dynaco Manual so I used the test there.

Here is what I got:
At the EL34's
1 & 8 = .47 /.41 dcv to chassis (same as bias)
2 to 7 = 6.5 / 6.1 acv
3 = 384 dcv (all)
4 = 385 dcv (all)
5 = -35.5 dcv (all)
6= -35.5 dcv (all)

At the can cap:
The square 328 dcv
The triangle 371
The half moon 256
Plain 375

At the diode
-78.7 dcv
58.0 acv

Printed circuit
#3 324 dcv
#18 324 dcv


8 & 2 = 375 dcv
3 & 6 = 387 acv


4 to 5 and 15 to 16 = 6.59 acv

Interestingly when I touched / wiggled the stereo / mono switch the hum changed so I clean it with de-oxit which helped the wiggle noises but not the hum. (neither wiggle nor cleaning helped the loss of bias.)

BuzzK 02-19-2012 05:02 PM

I show about 413 volts at the plates so you are running a bit low. Did you try a different rectifier?

bktheking 02-19-2012 05:05 PM

The half moon which is D should be 435V. D is fed by the rect tube and the main transformer. So either the can is going bad, rectifier is on it's way out or the main transformer is failing. Do you have another rectifier tube to try? The can looks new to me so doubt it's that. The rectifier tube seems to be what you've replaced most, but what causing tube to fail?

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