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RE: I'm a sucker for a pretty face........ But I need biasing help

Posted by dls123 (D) on November 10, 2014 at 15:54:55

In Reply to: I'm a sucker for a pretty face........ But I need biasing help posted by colebearanimals on November 10, 2014 at 09:00:55:

Hi Paul,
It appears to be an early version of the 200. The schematic posted and the other one I found show the 200-B or C and it has a bias adjust pot for each channel. This is clearly an early version with just one pot for both. Of course the usual drill about rebuilding the power supply for safety applies. Bringing it up slowly on a variac.... and of course, having a load on the speaker terminals whenever you run it. Either a 10+ watt rated 4, 8, or 16 ohm resistor on each channel or a pair of speakers. If you want to roll the dice, then put the tubes in, put a speaker or dummy load on it and bring it up slowly on a variac. Once you hit 115 volts then look for any output tube that is red plating.....if you see that or smell bad things then pull the plug. If you get that far then you can figure out the biasing as below.

So it uses the 6GW8 which is a triode and a pentode in the same bottle. The triode of each tube is used as the driver/phase splitter and then it drives the pentode output section. The two schematics I have looked at show a 47 ohm cathode resistor on the pentode (output) section of each tube. These 47 ohm resistors then connect to a 10 ohm resistor to ground. At the head of that 10 ohm resistor is a connection to that bias test point. My guess is that if you measure the resistance from the bias test point to the chassis you will get about 10 ohms. However, the schematic posted there are two bias pots and a switch to measure each channel's bias. So each channel had a 10 ohm resistor. You may have one resistor for all 4 tubes and you have no way of knowing how well matched they are using a single test point. However, you have the 47 ohm cathode resistors from pin 7 of each output tube. So open your amp and see if there is indeed a 47 ohm resistor connected to pin 7 of each output tube. If there is then you can measure the voltage drop across this when it is idling and use Ohm's law to determine the idle bias current on each tube. You will need the plate voltage. That will be from pin 6 to ground of each output tube. You really only need to measure one in each pair. Schematic says 320 volts DC. It may be higher due to modern wall voltages, or lower because we don't have the right schematic, and of course all parts have aged. But it will be in the 300- 350 volt range most likely. Maximum plate dissipation for that tube is listed at 9 watts. You want to run it at 70-80% of that at idle. So 80% of 9 watts=7.2. So I think you want about 7 watts. 8 watts tops. So if you have 320 volts on the plate and you want 7 watts, the power = voltage x current, so current = power/voltage. 7 watts/320 volts = .022 amps = 22 mA. So if you are shooting for about 22 mA per tube then you can now measure the voltage drop across that 47 ohm resistor on each cathode. You need to measure the resistance with the amp off to see how far out they are from 47 ohms (or they may be a different value because we don't have the right schematic). But assuming they are 47 ohms, then to get .022 amps you need.. Voltage = current x resistance, so .022 amps x 47 ohms = 1.03 volts. So you can turn that bias adjustment pot until you read about 1.03 volts drop across that 47 ohm resistor. In reality, there is about 5% of the dissipation on the screens so you are being about 5% conservative setting them this way, which is not a bad thing. Of course, assuming all 4 cathode resistors are the same value, you will probably read quite a variation in the 4 tubes. So you have to set the bias so that the hottest tube is no more than about 1.03-1.05 volts and hope the coldest tube is somewhere near that. You can swap tubes around to match the pairs better. More sophisticated designs have individial pots per tube or at least you can dial each pair up and down and then balance each pair with another pot. This one appears pretty primitive, so as Eli said, you need a reasonably matched quad. Perhaps yours are though.

So pop the bottom plate and check pin 7 of each output tube and see if there is a 47 ohm or other value resistor attached. Measure all 4 and note their values. Then bring it up slowly on a variac unless you are really feeling lucky. If it runs without red plating then measure the plate voltage on pin 6 of an output tube on each channel, one probe on the tube pin, the other to chassis for ground. Note the voltage which is probably in the 300-350 volt range. Then measure the voltage drop across that cathode resistor. You will need to put a probe on either side of it, not one to ground, because it is doubtful one end of that cathode resistor is grounded based on the schematics I have seen. Meaure that voltage drop on all 4 tubes. Then adjust the math above for the voltages you measured and see what the dissipation of the 4 tubes measures. Then adjust that bias pot so the hottest tube is running no more than 8 watts as calculated above.


Good luck,
Don

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