Before going farther, do you have any way to check the voltage gain of the amplifier?
The assumption, that the amplifier with the lower gain is defective, may not be correct.
It is possible that the amplifier with the higher gain could be bad instead. The feedback
may not be connected, or else the feedback resistor could be wrong.
The mono amp with gain corresponding to the spec, should be considered to be the good one.
It seems from the description that the gain difference could be 6dB to 10dB.
If there is a schematic, it would help if you post it here.
The assumption, that the amplifier with the lower gain is defective, may not be correct.
It is possible that the amplifier with the higher gain could be bad instead. The feedback
may not be connected, or else the feedback resistor could be wrong.
The mono amp with gain corresponding to the spec, should be considered to be the good one.
It seems from the description that the gain difference could be 6dB to 10dB.
If there is a schematic, it would help if you post it here.
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Issues with M125 after Auto Bias install
Read this thread might help. Original M125 use only 2 grid 1 resistor and 2 coupling cap, but when you use autobias (ccs) board, it becomes 4 grid 1 resistors and 4 coupling cap, that is where the confusion comes in....maybe there is an options to use only 2 grid 1 resistor and 2 coupling caps as is done with the original even after auto bias in installed..I can not recommend which is better, but you can get it to work.
Read this thread might help. Original M125 use only 2 grid 1 resistor and 2 coupling cap, but when you use autobias (ccs) board, it becomes 4 grid 1 resistors and 4 coupling cap, that is where the confusion comes in....maybe there is an options to use only 2 grid 1 resistor and 2 coupling caps as is done with the original even after auto bias in installed..I can not recommend which is better, but you can get it to work.
Attachments
This picture matches the schematic I have in the documentation that came with the kit: https://i.pinimg.com/originals/51/66/75/5166753eea85c6413a7f761311fbe6a7.jpg
As far as measuring voltage gain, I would need guidance as to what to measure but would be happy to take the measurements/
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The readings are taken on pin 5 of KT88 and ground.The readings are taken on pin 5 of KT88 and GRD on auotbias board.
I have measured 2 differences with the amps. The resister at R28 on the driver board (same resister installed in both as far as stripes indicating they are 10 ohm resisters)
The right (quieter) amp reads 1.4 ohms, the left reads 10.0 ohms.
And measuring from pin 5 of the output tube sockets to the associated GRD on the autobias board, V1 and V2 on the right (quieter) amp read 1K. The other 6 readings are all 100K.
So this gives me some things to work on...
The right (quieter) amp reads 1.4 ohms, the left reads 10.0 ohms.
And measuring from pin 5 of the output tube sockets to the associated GRD on the autobias board, V1 and V2 on the right (quieter) amp read 1K. The other 6 readings are all 100K.
So this gives me some things to work on...
The driver broad has been changed since some years ago, the new one has LM334 instead of resistor in LTP. Are you allow to post the schematic and part lists here? However I can find driver schematics for ST70/ST120, not sure they are the same.
https://img.usaudiomart.com/uploads/large/587629-7b4d123f-dynaco_vta_tube_amplifier_kit.jpg
The pcb version is found here Dynaco MK3 PCBs.
https://img.usaudiomart.com/uploads/large/587629-7b4d123f-dynaco_vta_tube_amplifier_kit.jpg
The pcb version is found here Dynaco MK3 PCBs.
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Sounds like your getting somewhere.
You could do this with a meter though, if your DMM/meter is half decent. If you have an old speaker or better a biggish 10r resistor connect this on the speaker terminals.
Connect a signal source to the IP of the LHS channel of amp.
The next bit is more tricky, turn the volume control all the way up, try a 10mV 1khz sinewave or if using a speaker, 400hz.
Next what do you measure on V1 g1? Next measure the anode of V1.
Next measure the anodes of the phase splitter.
Next you might measure the anodes of each KT88, but as this is the first time you've done this it might not be a good idea.Just do the first stages for now.
One thing I would check are all those switches that SW between triode & pentode mode, do this whilst the amp is un powered though, just check continuity/DC resistance for now,
Andy.
Have you got a sig gen & scope? If so it's just a case of connecting the sig gen to the IP of the amp, then going though each stage with a scope probe to measure Vac signal.
You could do this with a meter though, if your DMM/meter is half decent. If you have an old speaker or better a biggish 10r resistor connect this on the speaker terminals.
Connect a signal source to the IP of the LHS channel of amp.
The next bit is more tricky, turn the volume control all the way up, try a 10mV 1khz sinewave or if using a speaker, 400hz.
Next what do you measure on V1 g1? Next measure the anode of V1.
Next measure the anodes of the phase splitter.
Next you might measure the anodes of each KT88, but as this is the first time you've done this it might not be a good idea.Just do the first stages for now.
One thing I would check are all those switches that SW between triode & pentode mode, do this whilst the amp is un powered though, just check continuity/DC resistance for now,
Andy.
Koonw - this schematic looks like the one I have in the documentation (looks old): https://i9.photobucket.com/albums/a78/Bob01605/M-125schematic_2_NV.jpg
VTA parts list is here: tubes4hifi amplifier KITs page
VTA parts list is here: tubes4hifi amplifier KITs page
Thanks Andy, I can use a laptop as a signal generator. I do not have a scope, but since I now own a few pieces of tube gear I think I will read up on recommended models and look for a deal on a used one.
There is no such schematic on tubes4hifi, may be you should send email and ask for it. Only for better advising you, I don't need it.
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Today I received my order which included LM334s and replaced the LM334 on the driver board. That was the issue. My sound stage is no longer skewed strongly to the left. It is where it should be. The two monoblocks now play at the same volume. Thank you all for your help and advice. I really appreciate it. This is a big relief (this was really bothering me).
Alashikata - your advice to contact Ian Eales was great advice. What a knowledgeable, nice, helpful person. Very generous with his time.
Alashikata - your advice to contact Ian Eales was great advice. What a knowledgeable, nice, helpful person. Very generous with his time.
Keep reading Ian Eales's website to improve M-125 to make it sounds better. On his site, he has a link that another person has similar problem with M-125 on tubes 4 hifi forum. I do visit his site once a while where I can apply his method to other amps.
That's good news, well done. The relatively complicated bias/driver board does complicate things but the procedure in post # 51 is agood way to fault find in valve amps. Try to get a good working Tektronix analogue scope,they were designed with high voltage use in mind.
Andy.
Andy.
I realize this thread is a bit old, but I made some mods to my 125's when I built them earlier this year. A couple were putting the RCA input jack on the front, and the power switch on the back. I just didn't like the routing for the power switch wiring. Also, I mounted the LM334's on the underside of the board. Not as effective as gluing to the side of that diode, but the temp should hold steadier inside the chassis I would think. I removed the covers on the Coppertop rectifiers and drilled holes near the bottom and a few around the top. This allows for better cooling of the resistors and thermistor.