I am having some trouble with my Dynaco ST 70. Only getting output from one channel. I've tried some of the basics: Checking speaker wire and interconnects, checking the speakers themselves, switching the inputs, etc. Switched the output tubes and the 7199's-no dice. Checked bias and the left side was a bit high at 1.7, the right side was 46.3(!!)
Turned the amp off. Any clues from the experts? Your time and help is greatly appreciated.
paul

 Re: i Channel Dynaco st70 - Joe Curcio 07:52:35 05/22/03 (5)
 Re: 1 Channel Dynaco st70 - paul cbc 20:09:25 05/22/03 (4)
 Re: 1 Channel Dynaco st70 - Joe Curcio 22:17:40 05/22/03 (3)

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 Re: i Channel Dynaco st70 - Joe Curcio 07:52:35 05/22/03 (5)
 Re: 1 Channel Dynaco st70 - paul cbc 20:09:25 05/22/03 (4)
 Re: 1 Channel Dynaco st70 - Joe Curcio 22:17:40 05/22/03 (3)
 Re: 1 Channel Dynaco st70 - paul cbc 18:57:07 05/23/03 (2)
 Re: 1 Channel Dynaco st70 - paul cbc 18:43:45 05/25/03 (1)
 Re: Why 3 watt cathode resistors? - Russ57 10:26:20 05/28/03 (0)

If you find that it has opened (high resistance)it is probable that one of the output tubes on that side went into saturation (high current). If you don't wish to risk further damage to the output tubes it's probably time to replace the output tube sockets and associated solder connections (and wire).

Also, the 1.7 volt bias (on the good side) is indicative of a problem with the bias circuit. You may want to try implimenting the "bias expansion tweak" that I have posted on the dynaco-doctor site. You should also check (and possibly change out) the selenium rectifier.

Let us know how it works out

Thanks for the input. I will check the resistor. A couple questins: Is it necessary to replace all 4 output sockets at once or just the chanel in question?(2) Have I toasted my output tubes? Is the 1.7v bias on the right side that high-a serious problem??

Many thanks for your help and the site.
-paul

It's hard to say if the output tubes (on the side with the high bias) are defective - probably not. You have placed them in the other channel (I think you said) and they functioned well so they're probably OK. If they have been harmed with the saturation problem as I suspect, the worse that can happen is that you will probably fry the replacement cathode resistor again. BTW, you can replace the 15.6 ohm resistor with a 10 ohm / 3W resistor but if you do it is _mandatory_ that you set the bias to be 1.0 volts (as opposed to 1.56 volts)

1.7 is a little high (representing 54mA per tube) but is not too serious. However if this is the bias you are getting with the bias pot fully counterclockwise, this indicates a problem with the bias supply - you should look into the bias tweak or updating the bias supply.

Thanks for the continued help. The side with the 1.7v bias reading is not opened up fully counterclockwise. I will report back when I check the resistor.

Thanks again!
paul

Indeed the left side 15.6 Ohm resistor had failed. Replaced both sides with 10 Ohm 1 watt(until I can order 10 ohm/3 watt) resistors and set the bias to 1.0v both sides. It's working! Of course if it happens again it's on to the tube sockets you spoke of.
Thank you for your help!!
paul

Russ