Low Voltage

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Kcz5o
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Low Voltage

Post by Kcz5o »

Can anyone tell me if there is an issue with these voltages? I put together a JTM45+ kit using tube depot instructions but used a different power transformer. I think what I used should have higher voltage rather than lower voltage. The amp uses el34s. I will attach the voltage chart and the power transformer pdf. Thanks for all the help I've received from the forum.
JTM45+ Voltage.pdf
p-tm45p_datasheet.pdf
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TriodeLuvr
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Re: Low Voltage

Post by TriodeLuvr »

I'm a newcomer here, but I've been in audio/electronics for many years. Most of of those voltages aren't critical and aren't likely to degrade amplifier performance. The most significant effect potentially is that the reduced voltage on the anodes of the output tubes will limit output power somewhat.

Jack
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Re: Low Voltage

Post by JMPGuitars »

TriodeLuvr wrote:
Sun 12/13/20 1:18 pm
I'm a newcomer here, but I've been in audio/electronics for many years. Most of of those voltages aren't critical and aren't likely to degrade amplifier performance. The most significant effect potentially is that the reduced voltage on the anodes of the output tubes will limit output power somewhat.
It's not just output power; a large variance in plate voltage will also change distortion & tone characteristics. Depending on the tube, it can also contribute to unwanted crossover distortion levels, or other issues.
Kcz5o wrote:
Sun 12/13/20 10:19 am
Can anyone tell me if there is an issue with these voltages? I put together a JTM45+ kit using tube depot instructions but used a different power transformer. I think what I used should have higher voltage rather than lower voltage. The amp uses el34s. I will attach the voltage chart and the power transformer pdf. Thanks for all the help I've received from the forum.
Please post your schematic so we have a reference.

Thanks,
Josh
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TriodeLuvr
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Re: Low Voltage

Post by TriodeLuvr »

Crossover distortion won't be an issue if bias is adjustable. And the difference in anode voltage is only about 10%. Line voltage varies more than that in many areas. Definitely not worth changing out the PT unless an exact clone is the goal (in which case, a different transformer probably wouldn't have been used). Not trying to start an argument, but the sound will almost certainly be very close to the original kit.

All that aside, I'm curious why the OP believes the replacement transformer should deliver a higher, not lower, voltage. Maybe something is pulling it down.

Jack
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Re: Low Voltage

Post by JMPGuitars »

TriodeLuvr wrote:
Sun 12/13/20 2:19 pm
Crossover distortion won't be an issue if bias is adjustable. And the difference in anode voltage is only about 10%. Line voltage varies more than that in many areas. Definitely not worth changing out the PT unless an exact clone is the goal (in which case, a different transformer probably wouldn't have been used). Not trying to start an argument, but the sound will almost certainly be very close to the original kit.

All that aside, I'm curious why the OP believes the replacement transformer should deliver a higher, not lower, voltage. Maybe something is pulling it down.

Jack
I didn't specify the tube, crossover distortion can also be affected by the PI plates.

I'm not arguing either (only adding information), and I also don't know why the PT is in question. If the voltages are off, then the voltages can obviously be adjusted within the circuit (assuming the cause isn't because of a bad component or bad soldering, in which case it needs fixing, not adjusting).

Either way, we would need a schematic and build photos to form informed opinions about this specific case.
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Kcz5o
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Re: Low Voltage

Post by Kcz5o »

Thanks everyone. I will attach a schematic. I readjusted the bias as it was a bit low. I seems to sound good. I was just wondering if the voltage differences were an issue. Thanks
jtm45plus_schematic_v4.pdf
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Re: Low Voltage

Post by JMPGuitars »

Kcz5o wrote:
Sun 12/13/20 3:12 pm
Thanks everyone. I will attach a schematic. I readjusted the bias as it was a bit low. I seems to sound good. I was just wondering if the voltage differences were an issue. Thanks
If the amp sounds good and there are no issues, then the voltages being lower isn't an issue. If the voltages were too high, that could be an issue. The only voltages that aren't really safe to be lower than spec is the heater voltages.
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Kcz5o
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Re: Low Voltage

Post by Kcz5o »

Great! Thanks
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TriodeLuvr
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Re: Low Voltage

Post by TriodeLuvr »

I see now why you didn't expect a lower voltage with the transformer you used. Have you checked your line voltage where the amp is plugged in (with the amp turned on)? Will your voltmeter allow measuring the AC voltage across the secondary (one-half at a time might be safer). Were there any other changes to the power supply, such as the filter caps? The lower voltage probably isn't too important, but now I'm curious.
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Re: Low Voltage

Post by Kcz5o »

You just triggered a thought. I powered it up and took measurements using a dim bulb limiter with like a 250 watt bulb. Would that make a difference I haven't checked the wall voltage. All components are exactly like the schematic. I did make one mistake while bench testing. I had placed a clip with a resister from ground to the filter cap and forgot to take it off at one point during testing. I had it hooked up to the limiter and the light got real bright and the amp started to smoke. I killed the power immediately. I don’t think the filter cap was damaged as the amp is dead silent.
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Re: Low Voltage

Post by geoff 1965 »

I did the same once,I measured my voltages through a bulb limiter and had a voltage drop of about 20V so posted my concern“to the amusement of the senior amp techs!”
post your power tube voltages as well as the preamp tubes and your B+ at first capacitor.
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Kcz5o
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Re: Low Voltage

Post by Kcz5o »

I will have to remeasure my voltages without the limiter. I should be a constant source of entertainment for the senior amp techs :-)
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Re: Low Voltage

Post by TriodeLuvr »

Kcz5o wrote:
Tue 12/15/20 12:08 pm
You just triggered a thought. I powered it up and took measurements using a dim bulb limiter with like a 250 watt bulb. Would that make a difference I haven't checked the wall voltage. All components are exactly like the schematic. I did make one mistake while bench testing. I had placed a clip with a resister from ground to the filter cap and forgot to take it off at one point during testing. I had it hooked up to the limiter and the light got real bright and the amp started to smoke. I killed the power immediately. I don’t think the filter cap was damaged as the amp is dead silent.
Yes, the limiter is almost certain to be the issue. The amp smoked?? That can't be good. Was the clip on the main filter cap just off the rectifier? If there's nothing visible, you need to discharge the caps and sniff around the chassis to see what smells. Might be the supply diodes if the switch was set to solid state. I wouldn't let this go, even if it seems to work OK. You need to determine what smoked.
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Kcz5o
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Re: Low Voltage

Post by Kcz5o »

It was on the filter cap that is hooked to the choke. I rechecked to voltage at the plates for the power tubes and it was around 460. Schematic says 482. Rebiased as the bias was a little high when I rechecked. It seems to work fine on both tube rectifier and when I switch to the diodes. Not sure what else I need to look for.
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Re: Low Voltage

Post by Kcz5o »

It was the connection at the filter cap that smoked. Sorry, went back and read my reply. Nothing appears burnt.
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