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May 17, 2022, 05:09:14 PM

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JTM60 Combo rectified diodes fried

Started by Psabin8951, November 05, 2019, 04:15:27 PM

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DC voltage readings for power tubes pins 3,4,5, & 8


Start at the start.  Pull the power tubes and check voltages at the socket pins.  Pins 3 and 4 of each should measure B+, pins 5 should measure bias voltage.  If those look reasonable, THEN install the power tubes.

Plug a cord into the speaker jack set for the impedance you will be using.  Now measure resistance from tip to sleeve.  You are measuring through the output transformer secondary.  Do you see a nice low resistance?  or do you see an open?


Ok DC voltages with ref to dc gnd are:-
P1 0v
P2 0.1v
P3 0.46v
P4 0.46v
P5 -42v
P6 0v
P7 0v
P8 0v

P1 0v
P2 0.1v
P3 0.39v
P4 0.39v
P5 -42v
P6 0v
P7 0v
P8 0v
Resistantance across tip and sleeve with tubes in is 138k


Well your voltage readings show no B+ on pins 3 and 4, so that alone will kill sound.  You did have the amp fully powered?  NOT on standby?

Do you have some other tube amp?  Turn it off, plug a cord into the speaker jack output, and measure resistance.  I expect to see an ohm or two tops.  138k is an open to me.


Perhaps the output transformer primaries are both gone, assuming there is B+ at W102 now (given the new power transformer and B+ diodes) and it's not a Standby switch problem instead. 

Maybe best not to have power tubes in with screen voltage but no anode voltage.

Interesting that the PT was taken out.  Did you confirm the fuse values were as per the schematic (500mA for F102 on HV secondary, and 1.6A for F201 on primary) ?


Ok standby switch was causing an issue.
I'm now getting sound but quiet and distorted on all channels.
I've changed all the tubes and no difference.
Ideas please?


Quote from: Psabin8951 on January 31, 2020, 01:49:46 PM

Resistance across tip and sleeve with tubes in is 138k
I assume that is the resistance reading on the secondary of the output Tx?
Well if so then the output Tx is open,, sounds like a complete failure and taken out both tx's and power valves all at once. :'(
To the best of my understanding;
If the amp is trying to drive such a high output resistance (130k) then you may end up just repeating the same destruction. Never run a valve amplifier with a high or no load resistance you will blow it all (As above)
Pull the Valves and repeat the DC voltage testing, as you did before,, if they read ok then check the Output Tx.