79 JMP 2203 Odd Preamp Voltages

Chubbs

Member
Today I went through my JMP as it seems to be lacking something and discovered some odd preamp voltages. The amp is 99% stock with despite some drift in resistor values. I measured all voltages with a wall voltage of 115v. Compared to the Metro Amp manual, some voltages seem unusually high and some unusually low, any input is appreciated.

V1

Pin#1= 146v

Pin#2= 0v

Pin#3= 2.67v

Pin#4= 2.85v

Pin#5= 2.85v

Pin#6= 127v

Pin#7= 0v

Pin#8= 1.8v

Pin#9= 2.8v



V2

Pin#1= 82v

Pin#2= 0v

Pin#3= 0-1v

Pin#4= 2.8v

Pin#5= 2.8v

Pin#6= 150v

Pin#7= 148v

Pin#8= 150v

Pin#9= 2.8v



V3 (Some readings were bouncing between two values)

Pin#1= 123v

Pin#2= 113/10v

Pin#3= 201/42v

Pin#4= 2.9v

Pin#5= 2.9v

Pin#6= 119v

Pin#7= 119/10v

Pin#8= 201/42v

Pin#9= 2.8v
 

Chubbs

Member
Original filter caps? Your voltages do seem low..
No, all the can caps and bias caps were replaced since they were old and sounded quite bad, stock values though. I did go back and measure everything again and my plate voltage on pin1 of v2 and v3 were 147 and 204. The plates on pin6 of v1 and v2 were also up to 198 and 280. Pin 6 of v3 was still low at 103v.
 

glpg80

Well-known member
V1 and V2 look fine. V3 is PI and I don’t know those as well by pin voltages just by staring at it.

4, 5, and 9 are all heaters and since each triode is wired in parallel, you’ll see that each side when added together looks to be around 2.9*2 or 5.8V. The heaters are AC and to get true AC reading you’ll need a true RMS AC measurement capability. When you remeasure with that from 4-9 and not 4 to ground and 9 to ground, you’ll find it’s close to 6.3V.

As for the rest of the voltages, they all look correct to me for 115V. I set mine for 118V and cut the difference between 115 and 120.

Maybe check your bias and see if your tubes are biased too hot and pulling B+ down? They honestly don’t look bad at first glance when accounting for component drift as well. If something is up I don’t see it, ignoring the PI.
 
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Chubbs

Member
V1 and V2 look fine. V3 is PI and I don’t know those as well by pin voltages just by staring at it.

4, 5, and 9 are all heaters and since each triode is wired in parallel, you’ll see that each side when added together looks to be around 2.9*2 or 5.8V. The heaters are AC and to get true AC reading you’ll need a true RMS AC measurement capability. When you remeasure with that from 4-9 and not 4 to ground and 9 to ground, you’ll find it’s close to 6.3V.

As for the rest of the voltages, they all look correct to me for 115V. I set mine for 118V and cut the difference between 115 and 120.

Maybe check your bias and see if your tubes are biased too hot and pulling B+ down? They honestly don’t look bad at first glance when accounting for component drift as well. If something is up I don’t see it, ignoring the PI.
You were right, I checked the bias correctly and each tube was at 25w, dropped it to 17watts and it's much better.
 
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