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Ok This i think is more about understanding this and whats going on
Mesa boogie mark 4 (b) Looking at the diagram for both A and B versions mine is closer to the B than the A
It came in for bad tubes and hum
One checking the bias voltage on the 6L6GC tube bases without tubes , the bias voltage was missing.
Traced it to 2 x open circuit 220k. Could also see that the 2 x 10,000uf 16v were leaking, Replaced all items
Tested without tubes, I had bias voltage on the tube bases , but its lower on V8 - V9
I can see by the circuit (-53V) supplys R322-321 then to V7-V6 but the supply to V8-V9 goes through
another 220K for each V8 - V9
mk4b.pdf
On this amp there are a couple of differences the 220k for V8-V9 is 150K and there is No 2.2k This is all original with no sign of any changes since manufacture. I have changed the bias caps as a matter of course
Amp is set for 240V line voltage
Bias voltage @ -53v
HT @ 430v
so what i have on the tube bases is V8-V9 is has about 44v and V6-V7 has about -47v ..without tubes
I have new tubes to fit , but at this point, im using a test set of 6L6's and the amp powers up and sounds ok,,
Without checking it im guessing that the bias will be higher on 2 tubes than the other 2 ??
Looking at other amp circuits most have 220k supplying all tubes either in pairs or quads
So why is this one different and for what reason or do i have an issue ???
many thanks
Mr A
Ok This i think is more about understanding this and whats going on
Mesa boogie mark 4 (b) Looking at the diagram for both A and B versions mine is closer to the B than the A
It came in for bad tubes and hum
One checking the bias voltage on the 6L6GC tube bases without tubes , the bias voltage was missing.
Traced it to 2 x open circuit 220k. Could also see that the 2 x 10,000uf 16v were leaking, Replaced all items
Tested without tubes, I had bias voltage on the tube bases , but its lower on V8 - V9
I can see by the circuit (-53V) supplys R322-321 then to V7-V6 but the supply to V8-V9 goes through
another 220K for each V8 - V9
mk4b.pdf
On this amp there are a couple of differences the 220k for V8-V9 is 150K and there is No 2.2k This is all original with no sign of any changes since manufacture. I have changed the bias caps as a matter of course
Amp is set for 240V line voltage
Bias voltage @ -53v
HT @ 430v
so what i have on the tube bases is V8-V9 is has about 44v and V6-V7 has about -47v ..without tubes
I have new tubes to fit , but at this point, im using a test set of 6L6's and the amp powers up and sounds ok,,
Without checking it im guessing that the bias will be higher on 2 tubes than the other 2 ??
Looking at other amp circuits most have 220k supplying all tubes either in pairs or quads
So why is this one different and for what reason or do i have an issue ???
many thanks
Mr A
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