A pre-emptive thanks to my learned colleagues.
This is a problem with a Marshall Artiste 50w from about 1978.
It came in with several problems which I have worked through (V1 plate resistor, and Bias rectifier diode). The amp is now going but with a very, very low output (4.8VRMS across the speaker).
The B+ voltage was very low (360V where I was expecting 420V). I replaced all the original filter caps and the main rectifier diodes. B+ is still 360V). Next I measured the P/T secondary (with rectifiers out of circuit) and it was reading 570VRMS (Fluke 87V) where the specs on the Classic tone site had me expect 690V http://www.classictone.net/40-18023.pdf . That’s down about 20%.
Question #1: I thought Rectified DC = 0.9xVRMS. Clearly I am wrong. I don’t understand why a secondary voltage of 690VAC would result in a B+ voltage of only 420VDC according to Marshall. Removing the HT fuse didn’t see any notable increase in B+.
Here are relevant readings with all tubes installed:
(and in text for, just in case picture doesn't load)
Measured location Expected Value Actual Value
P/T Secondary Vrms 690 570
V @ rectifier VDC 360
V5plate 420 358
V5 screen 420 349
V5 grid -30 -25 (maxed)
A4A plate 175 153
A4B plate 170 141
A3A plate 210 171
A3B plate 125 98
A2A plate 140 113
A2B plate 100 88
A1A plate 140 67
A1B plate 105 92
Question #2: Could having a V1 plate as low 67V be the cause of the low output?. Looking at the 1KHz sine wave at the speaker output shows a slight distortion on the top half of the sine wave.
Question #3: It looks like the P/T is a couple of windings short of a picnic. Is there anything you recommend I check before breaking the bad news to the customer.
This is a problem with a Marshall Artiste 50w from about 1978.
It came in with several problems which I have worked through (V1 plate resistor, and Bias rectifier diode). The amp is now going but with a very, very low output (4.8VRMS across the speaker).
The B+ voltage was very low (360V where I was expecting 420V). I replaced all the original filter caps and the main rectifier diodes. B+ is still 360V). Next I measured the P/T secondary (with rectifiers out of circuit) and it was reading 570VRMS (Fluke 87V) where the specs on the Classic tone site had me expect 690V http://www.classictone.net/40-18023.pdf . That’s down about 20%.
Question #1: I thought Rectified DC = 0.9xVRMS. Clearly I am wrong. I don’t understand why a secondary voltage of 690VAC would result in a B+ voltage of only 420VDC according to Marshall. Removing the HT fuse didn’t see any notable increase in B+.
Here are relevant readings with all tubes installed:
(and in text for, just in case picture doesn't load)
Measured location Expected Value Actual Value
P/T Secondary Vrms 690 570
V @ rectifier VDC 360
V5plate 420 358
V5 screen 420 349
V5 grid -30 -25 (maxed)
A4A plate 175 153
A4B plate 170 141
A3A plate 210 171
A3B plate 125 98
A2A plate 140 113
A2B plate 100 88
A1A plate 140 67
A1B plate 105 92
Question #2: Could having a V1 plate as low 67V be the cause of the low output?. Looking at the 1KHz sine wave at the speaker output shows a slight distortion on the top half of the sine wave.
Question #3: It looks like the P/T is a couple of windings short of a picnic. Is there anything you recommend I check before breaking the bad news to the customer.
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