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|19th February 2014, 02:21 PM||#1|
Join Date: Oct 2009
KMD GV100D Low distorted output
I have a KMD GV100D on the bench at the moment. This is a hybrid amp with a solid state preamp and tube output section with 2 x 6l6's. Similar to Peavey or Music Man I believe!
I've spent quite a bit of time on it and still have a low distorted output.
It had bulged power caps which I replaced put new 6l6 valves in it.
Line out signal is hunky dory so pre amp is good. So problem lies in the output section.
I've been through all capacitors on circuit board they all measure fine with esr and cap meter. Same with all diodes and resistors measured out of circuit. I have replaced some electrolytics anyway.
Voltages on output valves are as follows.
Tubes are lighting up so I reckon filament voltages are good.
I have also cleaned and re-tensioned the output tube sockets.
Putting a sine wave into the input, there is a good sine wave out of IC9 op amp on pins 1 and 7. Now Iím a little confused when signal gets to T4 and T5 (TIP41C) .
Signal and voltages pretty much the same on both.
Base waveform ok (0.25VAC),
Emitter wave only positive half coming through (1.2VAC) ???
Collector Top half of waveform tailed off. (4VAC)
Any thoughts on how to get a proper signal running through this beast are very welcome!
PDF of schematic should be attached. Keeps timing out so
here's a link hope it works!
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Last edited by joebags; 20th February 2014 at 10:32 AM.
|19th February 2014, 10:45 PM||#2|
Join Date: Sep 2004
Location: Lansing, Michigan
61v on the power tube cathode looks right. 5v there looks wrong. So I suspect the driver transistor in that cathode circuit. Either it is mostly shorted emitter to collector or it is being held on at its base.
|20th February 2014, 10:37 AM||#3|
Join Date: Oct 2009
There was a typo. Pin 8 on V2 was 65V not 5V. Apologies.
I have also replaced the 2 x TIP 41C transistors.
There was a brief moment yesterday when I touched pin 8 of V1 that I got some proper signal to the speaker. So then I thought it might be bad connections coming from the tube sockets. I re flowed the board around the sockets and checked continuity.
I suppose it is still possible that these sockets need to be replaced.
If for example the output transformer was faulty. If I Input a sine wave I should still get a proper sine wave on the output valves. Yea?
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