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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: 7189s in Dynaco ST-35 - Ouch! posted by Bryon on July 21, 2002 at 13:30:02:
Byron,To form new electrolytic caps, I use a Variac. But, a good idea is to use the old in-line light bulb trick, first.
First thing to do is replace the GZ34 (or any indirectly heated rectifier) with a 5Y3GT or a solid state plug-in type. All other tubes are in. Speakers are connected. Volume at zero. No inputs.
But before you begin, go down to the hardware store and buy an extension cord and a ceramic light bulb holder. About midway in the extension cord, cut ONE of the AC wires (black or white). Insert the ceramic light bulb socket, inline. Place a 40-watt light bulb into the socket.
Now, plug the amp into the extension cord’s female end. And the male end into the wall outlet. Turn on the amp. The light bulb should SLOWLY light up. If it goes bright immediately---TURN OFF THE AMP. You probably have a short between the amp’s AC input, the switch, the PT, the rectifier, and/or the PS filter circuit.
You’ll need to drain the PS caps (I use a 100 ohm, 5 watt resistor with alligator clips on both ends---clip one end to the chassis and the other to the (+) point or A+ point of the rectifier/PT.
If the light bulb lights up slowly, you can proceed to the Variac. Disconnect the amp from your homemade short tester. Plug the amp into the Variac. Turn the Variac up to 40 volts. If you comfortable with using a DVM probe in a live circuit, I’d check the VDC on the A+, B+, C+, and D+ voltages. See if their values are relative to the input voltage. If all seems reasonable, keep the Variac at 40 volts for an hour or two.
After two hours (or so), check those tap-off values, again (A+, etc). If they seem reasonable, turn the Variac up to 60 volts, for one to two hours. Continue this trend until you reach 120 volts.
Measure the tap-off values and the bias supply value (I suggest changing the bias supply filters and diode(s) at the same time, for fixed biased amps). The A+, B+, C+, and D+ should be within 10-20% of nominal value. But, more importantly the relative relationship between the tap-off points should be consistent.
The negative VDC read off the bias supply should be close to nominal value. This is the bias point for FIXED biased amp.
As long as the values as within tolerance and all seems well with the amp, you can go ahead and play the thing---once you’ve re-biased the power tubes.
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Follow Ups
- Re: 7189s in Dynaco ST-35 - Ouch! - steven oda 17:08:49 07/21/02 (4)
- Re: 7189s in Dynaco ST-35 - Ouch! - Bryon 17:56:38 07/21/02 (1)
- Schematic - steven oda 18:16:55 07/21/02 (0)
- Adden-dumb - steven oda 17:12:57 07/21/02 (1)
- BTW... - steven oda 17:14:05 07/21/02 (0)
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