JCM Mini Clone Issues

imgumby001

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Continuing in the same clockwise fashion with the power on and the Standby switch in play mode, the next pin has a blue wire that goes to R1 (10k). What is the DC voltage from that pin to ground?
Pin 3-2.66vdc
4-3.6vac
5-3.6vac
6-195.8vdc
7-0v
8-1.84vdc
9-3.56vac

Should I keep going for all tubes? Thanks bub!!
 

imgumby001

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V2
1-279vdc
2-152.2vdc
3-154vdc
4/5-3.6vac
6-153.3vdc
7-36mv
8-1.05vdc
9-3.6vac

V3
1-198vdc
2-22.5vdc
3-37.8cdc
4/5-3.6vac
6-206.3vdc
7-21.1vdc
8-37.84vdc
9-3.6vac

V4
1-22.2mv
2-3.6vac
3-401vdc
4-381vdc
5- -40.3vdc
6-n/a
7-10.78vac
8-22.1mv

V5
1-22.3mv
2-3.6vac
3-399vdc
4-380.5vdc
5- -40.3vdc
6- n/a
7-10.78vac
8-22.3mv

All readings taken in full play mode, tubes fully warmed, with the kobalt dt-218 and verified with fluke 323.
 

Ten Over

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Cool. The heaters look a little bit high, but other than that those voltages all look good. What is the AC voltage between 4/5 and 9 on any pre-amp tube?

There is no effects loop in this amp, correct?
 

imgumby001

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Cool. The heaters look a little bit high, but other than that those voltages all look good. What is the AC voltage between 4/5 and 9 on any pre-amp tube?

There is no effects loop in this amp, correct?
7.2vac on the heaters. And yes sir, there is no loop.
 

Ten Over

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Should I hook up the ol variac and pull her back a bit?
I would. Tube failures increase with higher heater voltages.

The power transformer is for 110V. Supplying it with 117V would explain the high heater winding voltage and the high bias winding voltage. You might be able to get the proper bias voltage when the heaters hit 6.3V with the variac.
 

imgumby001

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I would. Tube failures increase with higher heater voltages.

The power transformer is for 110V. Supplying it with 117V would explain the high heater winding voltage and the high bias winding voltage. You might be able to get the proper bias voltage when the heaters hit 6.3V with the variac.
I’ll give her a try and let you know. Thanks for all you time. It has been a real battle with this project. No other amp build has been so ridiculous. Much appreciated!!!!
 

imgumby001

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I’ll give her a try and let you know. Thanks for all you time. It has been a real battle with this project. No other amp build has been so ridiculous. Much appreciated!!!!
So, just as an update, I hooked her up to the variac. Pulled back about 5% of the voltage to get the numbers to the circuits spec. Sounded ok at best, and red plated as soon as the gain and master were turned up. I’m going to grab a pair of jj’s and see if the higher current handling makes any difference in the circuit, but I’m not holding my breath. Good news is if it doesn’t work I’ve got a bunch of parts to make into robrobs jcm800 20w build!!

Thanks again for everything. You were a big, big help just taking a look at all my junk!!!
 

imgumby001

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I would. Tube failures increase with higher heater voltages.

The power transformer is for 110V. Supplying it with 117V would explain the high heater winding voltage and the high bias winding voltage. You might be able to get the proper bias voltage when the heaters hit 6.3V with the variac.
So, just as an update, I hooked her up to the variac. Pulled back about 5% of the voltage to get the numbers to the circuits spec. Sounded ok at best, and red plated as soon as the gain and master were turned up. I’m going to grab a pair of jj’s and see if the higher current handling makes any difference in the circuit, but I’m not holding my breath. Good news is if it doesn’t work I’ve got a bunch of parts to make into robrobs jcm800 20w build!!

Thanks again for everything. You were a big, big help just taking a look at all my junk!!!
 

Lowerleftcoast

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FYI, KLD shows the heaters powered from a 12.6 VAC with a center tap. There are times to meter the VAC from a tube pin to chassis and there are times to meter VAC the tube heater is actually *seeing*.
Please provide the power tube heater VAC readings with your probes on pin2 and pin7.
Please provide the preamp tube heater VAC readings with your probes on pins4/5 and pin9.

The power tubes are using one leg of the 12.6 volt secondary with the CT. The preamp tubes are using the other leg of the 12.6 volt secondary and the CT. It is possible the voltage to the power tube heaters is different than the voltage to the preamp tubes heaters. Inquiring minds want to know. hah.
 

Lowerleftcoast

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The measured bias voltage should not result in red plating. Something is probably changing (shorting/opening) which results in the red plating.

I don't see a wire end in the turret where the 220k resistors meet the bias circuit, although they must connect to get the bias voltage reading.

Keep an eye on the power tube pin5 voltage (bias voltage) for drastic changes. Chop stick.
 

Ten Over

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FYI, KLD shows the heaters powered from a 12.6 VAC with a center tap. There are times to meter the VAC from a tube pin to chassis and there are times to meter VAC the tube heater is actually *seeing*.
Please provide the power tube heater VAC readings with your probes on pin2 and pin7.
Please provide the preamp tube heater VAC readings with your probes on pins4/5 and pin9.

The power tubes are using one leg of the 12.6 volt secondary with the CT. The preamp tubes are using the other leg of the 12.6 volt secondary and the CT. It is possible the voltage to the power tube heaters is different than the voltage to the preamp tubes heaters. Inquiring minds want to know. hah.
He had 7.2Vac at the pre-amp tubes and 7.18Vac at the power tubes with no variac.
 

Ten Over

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So, just as an update, I hooked her up to the variac. Pulled back about 5% of the voltage to get the numbers to the circuits spec. Sounded ok at best, and red plated as soon as the gain and master were turned up. I’m going to grab a pair of jj’s and see if the higher current handling makes any difference in the circuit, but I’m not holding my breath.
Were the Master and Gain turned up enough to get maximum output at the speaker?

Did both tubes red-plate?
 
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dan40

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The OP never mentioned the dreaded squeal from having the primaries reversed but is there a chance that positive feedback is causing the tubes to redplate?
 

Ten Over

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The OP never mentioned the dreaded squeal from having the primaries reversed but is there a chance that positive feedback is causing the tubes to redplate?
Yeah, I was considering ultra-sonic oscillation as a possible cause. Not only reversed OT primaries, but also the lead dress. You can get away with way more when using PCB's, so maybe converting to hard-wired has caused problems. I was also curious about C23 (470pF) going to ground from the V4 grid stopper. Did they do that because of an oscillation problem even with a PCB?
 




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