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Re: 7189s in Dynaco ST-35 - Ouch!

Posted by Ivan303 (A) on July 21, 2002 at 12:49:18

In Reply to: Re: 7189s in Dynaco ST-35 - Ouch! posted by steven oda on July 21, 2002 at 12:20:37:

The only time I recall using 7189A's in my ST-35, I watched the voltage across the cathode resistor and the PS voltages very carefully. IIRC, the pin in question is not used on the Dynaco ST35, at least on my system, but we must remember this is a kit amp, so anything is possible.

Another issue is, while the 7189A is capable of more power, the critical issue might be "does the it draw more current in this particular cathode self bias circuit?" If it does, we are toast. If not, perhaps no problem. Again, IIRC, I kept close tabs on the voltage across the cathode resistor just to make sure all was OK.

If you look at the schematic of the ST35, it runs the EL84 at its max specified limit, perhaps even a bit over the limit. No problem, back in the days when Amperex EL84's were cheap and plentiful.

I have increased the value of the single 90 Ohm Sand Cast Cathode Resistor to about 12Ohms and have cut the plate dissipation of the output tubes by just a bit. Still sounds great, and hopefully those NOS Amperex EL84's will be with me for a while longer.

Additionally, the Dynaco ST-35 has a very under spec. power transformer that you can fry eggs and bacon on. This identical transformer frequently fails in the nearly identical SCA-35 integrated amp, just from the load of a few extra small signal tubes. I really think what happened, as described in the above post, might have been due to unbalanced quad, where one tube runs away and before you know it the voltage drop across the 90 Ohm cathode resistor exceeds the tolerances of the 35V bypass cap and KERBOOOOM!

Just a guess.

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Dynaco Doctor

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