Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SleepLess on January 12, 2013, 12:42:56 pm
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Hi.
I have just finished a 1987 build. I'm supposed to have about 185VDC on pin 6 of V1 and I'm only getting half of it, about 97VDC. The amp works but that voltage is not right. I have another 1987 I can compare it to and I have just measured that same pin I get 187VDC.
So far I have changed the 0.022uF cap (circled in red), changed the V1 tube, gotten rid of the bright cap (circled in red). All these three moves didn't change the voltage I'm getting.
I measured the 100K resistor that is linked to the 0.022uF cap and it reads 98K, so it's good.
All the other measures are within specs... If you have any ideas about what to change/swap to get the proper voltage on pin 6, please shoot!
Here's a picture of the finished build. I have installed a PPIMV. An FX Loop will be added when it will be working 100%. Thanks!
(http://img594.imageshack.us/img594/4900/copiedemg3818.jpg)
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Is the voltage correct on V1 Pin 1?
If so, what is the voltage at the power rail side of the plate resistors for V1 pin 1 and V1 pin 6? Same electrical point - should be the same.
Disconnect the .022 cap from the plate connection V1 pin6. Any change?
Something in those three questions should jump out at you.
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How much B+ is on the other side of the plate resistor? What voltages do you have on pins 7 and 8? What voltages do you have on pins 1, 2, and 3 of same tube? What resistance value do you measure from pin 8 to ground? What resistance value do you measure from pin 3 to ground?
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OK, V1 readings with power tubes pulled out, amp fully on:
Pin 1: 129V
Pin 2: 0V
Pin 3: 0.84V
Pin 4/5/9: 3.12VAC
Pin 6: 98V
Pin 7: -0.5V
Pin 8: 0V
Resistance on pin 3 to ground: 0.8K. Resistance from pin 8 to ground: 0K
Tell me if you need more! I'm at it tonight!
Thanks for your help!
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Resistance from pin 8 to ground: 0K
It should read 2K7, 2700Ω. I clearly see a 2700Ω resistor. Something is shorting out the resistor. Maybe the cap is bad.
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Resistance from pin 8 to ground: 0K
It should read 2K7, 2700Ω. I clearly see a 2700Ω resistor. Something is shorting out the resistor. Maybe the cap is bad.
OK thanks Sluckey. I will check this. The resistor leads were barely long enough to go round those large Russian caps, this one in particular. Maybe the contact is not good enough. I will first unsolder it and add some resistor legs cut-offs to make it longer. If this still doesn't work, then I'll change the cap...
Thanks!
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Got it!!!! The 2K7 resistor legs weren't long enough. I added some wire and now I do have 187VDC on pin 6. Problem solved!!!! Thank you guys, I LOVE this board!!!!
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Looks like both resistor leads were touching the metal shell of the cap?
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Looks like both resistor leads were touching the metal shell of the cap?
Yeah, this or the resistor legs were barely long enough to make a real good connection on the cap legs... It's all good now. Too late to fire it up and play but I'll do that tomorrow and if everything's good I'll launch the FX Loop installation... Thanks Sluckey!