JCM Mini Clone Issues

imgumby001

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Hello all!!

This is my first post here, so I’d like to say thank you to everyone, and that I’m glad to be here. I’ve been a lurker for a long time, and some may recognize me from other forums/groups.

I have an ongoing issue with a JCM 800 type circuit. Firstly, the amp has no power. It’s listed as 25w (2 6l6), but seems to be putting out about 1w. Secondly, it red plates near any type max vol/gain settings. Adding bass near max also starts a red plate. The circuit has a pentode/triode half power switch that does nothing, so I’m not sure if my issues are related, but I don’t really need it, so I’m also wondering if I can delete it from the circuit and how to go about safely removing it. My apologies up front for my ignorance.

Thanks all!
 

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Lowerleftcoast

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Welcome to TDPRI.

Are you able to work with high voltage safely? We will need voltage measurements.

Pictures of the power transformer connections, pentode/triode switch connections, 6L6 socket connections, may be helpful.
 

2L man

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Connect guitar and booster/OD pedal directly to FX return. If amp stay quiet problem is after FX return.

First measure bias. Sometimes what look red plating to musician eyes is normal filament glow but your observation looks red plating.

Then problem can be OT, output jacks, wiring or impedance selector or perhaps loudspeaker?
 
Last edited:

Ten Over

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Instead of tearing things apart without any real evidence that there is a actually something wrong with it, let's focus on one thing until we're satisfied that it is working correctly.

The bias circuit would be a good one to start with. I think you should be getting -26V to -40V at pin 5 on the 6L6GC's, but you are apparently getting something different over on EL34world. Can you confirm your voltages at pins 5 and at the junction of the 220k resistors with the amplifier in standby mode? Also, I would like to know the AC voltage on the bias winding.
 

imgumby001

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Here are some pics. Please keep in mind this started its life as a pcb, which was a pos, and was quickly ruined by a rookie. I salvaged what I could, and built my own turret board.

As far as current readings, all readings taken with no input signal and all knobs turned to zero. Clearly it is not in stand by.

V1
Pin2- 1.8vdc
4- 201.6vdc
7- 2.7vdc
9-244.5vdc

V2
Pin2- 1.0vdc
4- 201.6vdc
7- 2.7vdc
9- 284.6vdc

V3
Pin2- 38.2vdc
3- 24.1vdc
4- 210.4vdc
7- 38.3vdc
8- 25.3
9- 203.1

V4
Pin3- 403.8vdc
4- 386.6vdc
5- -46.9

V5
Pin3- 403.8vdc
4- 387.3vdc
5- -47vdc

All heaters measure good to go. Please ask for anything else you thing is important. I’m more than happy to take readings 10 times if needed. Thanks!!
 

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imgumby001

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Instead of tearing things apart without any real evidence that there is a actually something wrong with it, let's focus on one thing until we're satisfied that it is working correctly.

The bias circuit would be a good one to start with. I think you should be getting -26V to -40V at pin 5 on the 6L6GC's, but you are apparently getting something different over on EL34world. Can you confirm your voltages at pins 5 and at the junction of the 220k resistors with the amplifier in standby mode? Also, I would like to know the AC voltage on the bias winding.
Pin5 is -46.9vdc v4, -47vdc v5. The bias winding is labeled 25v I believe. I get 29.8 when measured where it meets the diodes.
 

Ten Over

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Here are some pics. Please keep in mind this started its life as a pcb, which was a pos, and was quickly ruined by a rookie. I salvaged what I could, and built my own turret board.

As far as current readings, all readings taken with no input signal and all knobs turned to zero. Clearly it is not in stand by.

V1
Pin2- 1.8vdc
4- 201.6vdc
7- 2.7vdc
9-244.5vdc

V2
Pin2- 1.0vdc
4- 201.6vdc
7- 2.7vdc
9- 284.6vdc

V3
Pin2- 38.2vdc
3- 24.1vdc
4- 210.4vdc
7- 38.3vdc
8- 25.3
9- 203.1

V4
Pin3- 403.8vdc
4- 386.6vdc
5- -46.9

V5
Pin3- 403.8vdc
4- 387.3vdc
5- -47vdc

All heaters measure good to go. Please ask for anything else you thing is important. I’m more than happy to take readings 10 times if needed. Thanks!!
I think you need to take a second try at reporting V1, V2, and V3 voltages.

I'm getting just about bleary eyed from looking at those pictures, but I think I'm getting the idea of how it goes. The pentode/triode switch looks right to me and the V4 and V5 voltages are consistent with a properly wired switch.

The bias is set cold and you may have to change R55 to 27k in order to get the desired bias voltage. But I wouldn't worry about that just yet.

What is the DC voltage across R24C and R24D at this idle? It should be in the mV range.
 

imgumby001

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America
I think you need to take a second try at reporting V1, V2, and V3 voltages.

I'm getting just about bleary eyed from looking at those pictures, but I think I'm getting the idea of how it goes. The pentode/triode switch looks right to me and the V4 and V5 voltages are consistent with a properly wired switch.

The bias is set cold and you may have to change R55 to 27k in order to get the desired bias voltage. But I wouldn't worry about that just yet.

What is the DC voltage across R24C and R24D at this idle? It should be in the mV range.
I’m reading 1.14ma across both at idle.
 

imgumby001

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Age
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Location
America
18.4mv &18.1mv respectively. Sorry, had my meter set to ma on accident. Thanks for all your help bub. I’m at my wits end with this thing.
I think you need to take a second try at reporting V1, V2, and V3 voltages.

I'm getting just about bleary eyed from looking at those pictures, but I think I'm getting the idea of how it goes. The pentode/triode switch looks right to me and the V4 and V5 voltages are consistent with a properly wired switch.

The bias is set cold and you may have to change R55 to 27k in order to get the desired bias voltage. But I wouldn't worry about that just yet.

What is the DC voltage across R24C and R24D at this idle? It should be in the mV range.
I just ran all the numbers on v1-3, everything came out near what I’d measured previously. What are you not liking? Is there anything you’re not seeing that should be there?
 

dan40

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Posts
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Location
Richmond Va
As Ten Over noted, your readings on your preamp tubes are very odd. Pins 4/5 and 9 are filament pins and should be reading around 6.3v AC. The rest of the pins will be a DC voltage reading. Pins 3 and 8 are the cathodes and should only be a volt or two. Pins 1 and 6 are the plates and should be 150vdc or higher. Half of the v2 tube is wired as a cathode follower so the numbers will be a little different there but all of your sockets are showing very odd readings.
 

dan40

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Posts
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Location
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Your preamp sockets appear to be wired correctly so I'm guessing that you may have gotten the order of the pins wrong when reporting the voltages. I did notice a missing 100k resistor that is normally wired across the v2 socket on the JCM 800 circuit. It normally connects across pins 1,6 and 7 but I'm not seeing it in your pics. Here is our member Rob's JCM 800 layout which shows the resistor across the v2 socket.

 




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