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previous: jason what model of Supro is this? I have... -- 4/25/2000 9:19 PM

Re: Measuring B+ on 12AX7 - before or after plate load resistor?

4/25/2000 10:47 PM
beto	Re: Measuring B+ on 12AX7 - before or after plate load resistor? Its a model 24 - 2 6973 powertubes, 5y3 and 3 12AX7's with one 12 inch Jensen. 4 inputs. In Aspen Pittman's latest Tube book he has a schematic on page 636 for a Supro Model 6424 which I suspect is my amp as it has the same tubes and inputs and most of the component match. But here are the problems: (1) I am a newbie (2) The guy(s) who had it before me attempted to fix it...probably without the schematic. So when I purchased it I got it "as is": trem doesn't work, amp is very weak sounding (distorts real easily and is not loud). Now I have made some improvements. I found that the previous owner had stuck in a 6CZ5 (a possible replacement tube for a 6973) tube in V4, but did not rewire the socket. The 6CZ5 socket needs a jumper from pin 1 to 8 that the 6973 tube does not. So I put in the jumpers. Then I stuck in two NOS 6CZ5's and re-biased the amp (cathode bias...replaced the cathode resistor from 239 ohm to 300 ohm...idle current fell from 52mA to 43mA using transformer shunt method of measurement). The tubes were glowing cherry red. Now the amp sounds MUCH better with more headroom and more pleasant overdrive. My interest in the B+ stems from the schematic in Pittmans book. It lists the voltage at those two points on V1 as 110 and 200. Mine is considerably less at least at the one stage. So I hope to increase it back closer to the schematic. Tonight I am gonna stick the GZ34 in there and see what happens. Where did you find your Supro schematics?

4/25/2000 10:47 PM

beto

Re: Measuring B+ on 12AX7 - before or after plate load resistor?

Its a model 24 - 2 6973 powertubes, 5y3 and 3 12AX7's with one 12 inch Jensen. 4 inputs.

In Aspen Pittman's latest Tube book he has a schematic on page 636 for a Supro Model 6424 which I suspect is my amp as it has the same tubes and inputs and most of the component match. But here are the problems:
(1) I am a newbie
(2) The guy(s) who had it before me attempted to fix it...probably without the schematic. So when I purchased it I got it "as is": trem doesn't work, amp is very weak sounding (distorts real easily and is not loud).

Now I have made some improvements. I found that the previous owner had stuck in a 6CZ5 (a possible replacement tube for a 6973) tube in V4, but did not rewire the socket. The 6CZ5 socket needs a jumper from pin 1 to 8 that the 6973 tube does not. So I put in the jumpers. Then I stuck in two NOS 6CZ5's and re-biased the amp (cathode bias...replaced the cathode resistor from 239 ohm to 300 ohm...idle current fell from 52mA to 43mA using transformer shunt method of measurement). The tubes were glowing cherry red. Now the amp sounds MUCH better with more headroom and more pleasant overdrive.

My interest in the B+ stems from the schematic in Pittmans book. It lists the voltage at those two points on V1 as 110 and 200. Mine is considerably less at least at the one stage. So I hope to increase it back closer to the schematic. Tonight I am gonna stick the GZ34 in there and see what happens.

Where did you find your Supro schematics?