ST-70 low bias after blown fuse

ADClosson

ADClosson

Member
I recently noticed a hum in the left channel of my completely refurbished ST-70. I read a thread suggesting to swap the pairs of EL34s to see if the hum moved channels. I did this without re-biasing (perhaps foolishly?) and upon powering up the fuse blew. When I replaced the fuse, I could not detect any bias voltage. I then replaced the rectifier tube, which restored bias voltage, but unevenly. I cannot get the left side of the amp to bias—it runs high, around 2.75 VDC. The right side biases fine. What could I have damaged?

Besides the transformers, choke and bias pots, all other electronics were replaced at the time I purchased it. The amp was fitted to run 6gh8a driver tubes, otherwise it is in stock configuration.

Thank for the help!

Aaron
 
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Check the EL34 cathode resistor. If too much current is drawn this resistor will evaporate, saving more
expensive components.
 
Open or high resistance resistor would definitely make it show high voltage while not actually drawing high current. Can check this without even opening the amp, just measure resistance from the test point to ground with the amp off. Stock is 15.6 ohms, if its more than about 16.5 (allowing for meter lead resistance) its bad.
 
gadget73 said:
Open or high resistance resistor would definitely make it show high voltage while not actually drawing high current. Can check this without even opening the amp, just measure resistance from the test point to ground with the amp off. Stock is 15.6 ohms, if its more than about 16.5 (allowing for meter lead resistance) its bad.
Click to expand...

Left channel (the problem channel) does read higher, around 17.5. It's also a bit unstable, running up and down. Which resistor would be the culprit?


peterh said:
Check the EL34 cathode resistor. If too much current is drawn this resistor will evaporate, saving more
expensive components.
Click to expand...

Do I have to cut one resistor lead from the circuit in order to get an accurate reading from my multimeter?

Thank you all.

Aaron
 
ADClosson said:
Left channel (the problem channel) does read higher, around 17.5. It's also a bit unstable, running up and down. Which resistor would be the culprit?




Do I have to cut one resistor lead from the circuit in order to get an accurate reading from my multimeter?

Thank you all.

Aaron
Click to expand...
You don't need to disconnect the catchode resistor to measure it, just measure via the biaset outlet and ground ( power off of course)

If it measure 17.5 AND is unstable, replace. dynakitparts.com has new ones.
 
ADClosson said:
Which resistor would be the culprit?
Click to expand...

If you flip the amp over, it will be the 15.6 ohm resistor, usually on the rear-most output tube sockets. In this pic its the white resistors on the left and right edge, basically centered top to bottom. Your specific resistor may look different, these are the original style from "back in the day".

Dynaco_underside.JPG
 
I visually inspected the cathode resistors, which are a similar style as yours, Gadget73. All looked well, and when I retested the resistance via the bias outlet it read a stable 16.5 on both sides.

So I reinstalled the tubes, fired her up, waited 15 minutes and attempted to bias. This time, no problem. I had room to adjust the previously high channel down within spec. No clue what changed. Perhaps a tube wasn't seated properly?

In any event, I now know more about these 15.6 ohm bias resistors—helpful in the future. Thanks again for all your input. These are beautiful machines, although to a novice they are somewhat mysterious. Thanks for dispelling a bit of the mystery.

Aaron

IMG_1065.jpg IMG_1066.jpg
 
Not mine, thats a random internet pic. I don't own an ST-70.

Just for confirmation, what does your meter read if you just connect the leads together? If its somewhere about 0.9, then those resistors are bang-on, which is a good thing.

Improperly seated tube could be it. Could also be a bad connection on one of those resistors. If the solder is cracked you'll get flaky results, same if the screw where that ground lug attaches is loose or if there isn't a good connection to chassis because of the paint. Looks like its been scraped so you're likely OK there.
 
Well, I resolved the bias issue but am now back to my original problem. After listening for 25 mins, the noise in the left channel has returned. It's a "crackle" type of noise, and it's present even when I turn off the preamp. These are a different set of EL34s and a different rectifier than when I first detected the noise, although the driver tubes are the same. Perhaps that's the issue? Right channel is silent.
 
ADClosson said:
Well, I resolved the bias issue but am now back to my original problem. After listening for 25 mins, the noise in the left channel has returned. It's a "crackle" type of noise, and it's present even when I turn off the preamp. These are a different set of EL34s and a different rectifier than when I first detected the noise, although the driver tubes are the same. Perhaps that's the issue? Right channel is silent.
Click to expand...
You can pull the 7199 tube without harm. If it crackle is still there the problem is few components around the EL34 tubes ( coupling cap, bias circuit. If crackle disapperas teh problem is in the board. Try swap the
7199 and see if crackle moves to other channel.
 
Just checked the resistance on my ST 70, bias outlet on the front of the amp to ground. Looks like both are at 19 ohms. Meter leads are about 1 ohm. This means that both bias resistors are out of spec and should be replaced, correct?
 
The fact that both measure the same makes me wonder if the meter is lying. I've rarely had 2 resistors drift by the same amount in an amplifier before. Those resistors are some type that is very stable long term, so generally unless they are roasted open they tend to stay put.

Happen to have any known good resistors in the 15-20 ohm range to check the meter against? Failing that, how fresh are the batteries? Digitals tend to get goofy when the batteries are low.
 
You were right. Digital meter was off. Just checked more carefully with my Simpson - with fresh batteries. Looks like it’s 16 ohms for both resistors. So I think it’s ok. Thanks for the help.
 
check the condition and tightness of all your power tube sockets. Dirty and loose socket pins are the cause of many older tube amp and preamp issues!
 
Thanks. Besides the tube sockets, need to do a lot to the amp. It’s completely stock but has a lot of sentimental value. Belonged to a close friend/mentor who passed away a number of years ago. I’ve been going through the forum posts and learned that I should be running a 5AR4 instead of a 5U4GB (which i understand is at 1/2 power), need to replace all the caps in the power/bias supply, including the quad cap. Also need to replace the 7199 board (thinking about Curcio but would like to leave it stock - I have probably a dozen or more 7199’s in my basement - bought them years ago whenever they popped up at garage sales, etc). Just had Maggie LRS delivered. They sound terrible with the ST 70 as is. I think it has a lot to do with the 5u4GB and the low power. So now have the speakers running off a Carver M1.0t (a garage sale find for $15…..). The speakers sound very good with the Carver but I would like to get the ST 70 running correctly and try them with the Maggie’s. I’m kind of new to this, or rather haven’t had my hands in audio equipment, in a long time, so have a lot to do to reorient myself. The forums have been very helpful.
 
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