I am having problems with mighty midget. The amp has been working but all of a sudden blew an output transformer in the right channel.
I purchased new output transformers (edcor gxe10-8-8k)
I installed the new transformers but the right channel starts heating up in about 30 seconds even without tubes. slight scratching noises in right speaker. small burn near r16 in board (arc over??) . checked filter capacitors, most resistors in the vicinity,
Can any body give some guidance where to start next?
test equipment is limited to a DMM. and digital cap checker
thanks in advance
mike
I purchased new output transformers (edcor gxe10-8-8k)
I installed the new transformers but the right channel starts heating up in about 30 seconds even without tubes. slight scratching noises in right speaker. small burn near r16 in board (arc over??) . checked filter capacitors, most resistors in the vicinity,
Can any body give some guidance where to start next?
test equipment is limited to a DMM. and digital cap checker
thanks in advance
mike
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with no tubes installed (transformer gets hot quickly)
cathode voltage is 0 on good channel and 0 on bad channel
plate voltage is 174 on good channel and 10 on bad channel
cathode capacitors and resistors are good. First things I checked.
Probably in the power supply?
the voltages were checked with NO tubes installed
cathode resistor goes from pin 9 to ground with no other connections .
I have verified that cathode resistor and bypass capacitor on both tubes were good.
I did not install the tubes because the transformer on the bad channel heats up very quickly. I can disconnect the transformers and check again if you wish.
I am very glad to have your help.
cathode resistor goes from pin 9 to ground with no other connections .
I have verified that cathode resistor and bypass capacitor on both tubes were good.
I did not install the tubes because the transformer on the bad channel heats up very quickly. I can disconnect the transformers and check again if you wish.
I am very glad to have your help.
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Please: 1.Check resistance between socket cathode pin and ground, must be 200 ohm. Check for short between socket pins. /But first, switch off an amplifier/..... 2.Check voltage on cathode resistor 200 ohm with tube /tester must be in DC mode/ for short time.... 3. Disconnect C11-0.22 uF and see what will happen..... 4. Disconnect R17 360 kohm..... 5.Change for a short time output tubes, from left to right channel..... 6. Resistance between first grid /pin 8/ and ground must be 221 kohm / amplifier must be switched off/.
With only 10 volts on the plate of the bad channel with no tubes in, it has to be shorted somewhere around there. Unplug the amp, discharge the B+ capacitors, and measure resistance, at the tube sockets of both plates.
The 174 on the good channel is lower than what is supposed to be there because the bad channel is shorted, from the op transformer to pin 11 of the 6T10.
You should have had the fuse blow first.
The 174 on the good channel is lower than what is supposed to be there because the bad channel is shorted, from the op transformer to pin 11 of the 6T10.
You should have had the fuse blow first.
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I will check all of the above tomorrow. I have 6 spare tubes which I have tried. the problem does not follow the tube. I hesitate to power the amp up with the transformers
attatched because the bad channel will heat up the transformer in about 30 seconds.
the amp is not built with the recommended power transformer. the one I used has 500 vac ct. the rectifier was modified from bridge to full wave with center tap to ground 330 ohm power resistor removed from the circuit. the amp worked for about 2 months before I heard a small chirp then silence in the bad channel.
attatched because the bad channel will heat up the transformer in about 30 seconds.
the amp is not built with the recommended power transformer. the one I used has 500 vac ct. the rectifier was modified from bridge to full wave with center tap to ground 330 ohm power resistor removed from the circuit. the amp worked for about 2 months before I heard a small chirp then silence in the bad channel.
Hi,
Good for you that you found the problem.
A few questions, not relating to your particular problem but to the construction of the amp.
How does it sound?
The 6T10 compactron is a strange beast. The output pentode-section does what it's suppose to but the other section is ...well, strange.
The weirdest plate-curves, absolutely non-audio imo.
Now, could this problem be solved by biasing grid-3 as with D6,7,9,10?
Will this square the curves up to something usefull?
Anyone has an idea? Or even better, does anyone have curves with this biasing?
rgds,
/tri-comp
Good for you that you found the problem.
A few questions, not relating to your particular problem but to the construction of the amp.
How does it sound?
The 6T10 compactron is a strange beast. The output pentode-section does what it's suppose to but the other section is ...well, strange.
The weirdest plate-curves, absolutely non-audio imo.
Now, could this problem be solved by biasing grid-3 as with D6,7,9,10?
Will this square the curves up to something usefull?
Anyone has an idea? Or even better, does anyone have curves with this biasing?
rgds,
/tri-comp
tricomp
the amp actually sounds good to me but is only the second tube amp I have built.
as for the more technical questions I don't have an O-scope.
Pmillett could answer any questions you might have as this amp is his design.
Pete Millett's DIY Audio pages look for mighty midget.
the amp actually sounds good to me but is only the second tube amp I have built.
as for the more technical questions I don't have an O-scope.
Pmillett could answer any questions you might have as this amp is his design.
Pete Millett's DIY Audio pages look for mighty midget.
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