New amp build finally started. JTM45 with KT66.

Personal semi-rant:
How ironic that what got me into doing my own maintenance, modding, and eventually building guitar amps, was an event that featured an oscilloscope.

Many years ago I had a Carvin Legacy combo that I bought used. Shortly thereafter it developed some serious problems. Brought the amp to a local well known electronic technician. The guy was not a guitar player, but he was a properly educated e-tech that just about all the music shops in this county would refer to. He fixed the amp, and his method of biasing an amp was to use an oscilloscope. The amp blew about a month later.

Pulled and sent the chassis to Carvin. They replaced the fried pcb that houses the power tubes, and reset the bias to the standard Carvin-cold bias. Carvin also told me that they suspect the amp failed because the bias was set way too hot... Hmmmm... :hmmm:.

This is about the time I began getting involved in forums and searching for answers. Learned to bias an amp with a meter and a small screwdriver. Also learned what it feels like to get zapped, ha ha!

Even if I had an oscilloscope and knew how to use it, I would not use it for final bias settings. Rant over!
 
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When I got home from work yesterday, there was a care package from Alaska and also some more Piher resistors that I found online.

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Got to work soldering, and thought I would finish up the amp. Found the strain relief for the AC cable was missing. Hunted all over, and couldn’t find it. Had to order another, as I knew I wouldn’t be anywhere You-Do-It Electronics this week. That will give me a bit of time be sure I’m happy with everything before turning on the power.

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I used some insulation off of a wire to stick on the Ohmite resistor to make sure it didn’t come in contact with anything it should not be.

7EA21DEF-1A57-4118-A4DA-0C1A6D9B9414.jpeg
 
SG John said:
When I got home from work yesterday, there was a care package from Alaska and also some more Piher resistors that I found online.

View attachment 49637

Got to work soldering, and thought I would finish up the amp. Found the strain relief for the AC cable was missing. Hunted all over, and couldn’t find it. Had to order another, as I knew I wouldn’t be anywhere You-Do-It Electronics this week. That will give me a bit of time be sure I’m happy with everything before turning on the power.

View attachment 49638

View attachment 49639

View attachment 49640

I used some insulation off of a wire to stick on the Ohmite resistor to make sure it didn’t come in contact with anything it should not be.

View attachment 49641
Click to expand...

That's Purty... Nice looking guts...
 
I think it's done. I'm waiting to hear from one of the Amp Club guys to see if he can let me into Tim's house while he's away to use the dummy load, Variac, and oscilloscope this weekend. I hope I didn't screw it up at all.

Finished_01.jpg


Finished_02.jpg




I have a late seventies enclosure laying around, so I thought I would use it temporarily. Unfortunately, only one mounting hole lines up. I'll use it for now, and think of plan B later.


Finished_04.jpg



Finished_03.jpg
 
Spent about five hours troubleshooting the amp today. As we brought up the voltage, it would blow the fuses at 90 VAC. Found that I incorrectly wired the 500mA fuse block and stand-by switch. Fixed it, and was able to get it up to power.

Then, the phase inverter let us know it was wired backwards, so I swapped the wires and resistors going to pin 5 of V4 and V5. Then, we found mismatched voltages at the 82K and 100K resistors. Ends up, the 82K was 820 Ohms. Also saw that the 27K resistor next to the 8K2 resistor failed, and only measured 4K.

After all that was taken care of, we could get sound, but it was quiet. We still had voltages that were much too high. The 16uF capacitor that is supposed to be about 375 volts is about 560 volts, and everything on the input side is too high. Also, the 32/32uF filter cap is almost 700 volts, not 468. All the resistors seem to have correct values now.

Hoping to go back to Cambridge next Saturday to have Scott help me some more. In the meantime, I'll check under the board tomorrow or Monday to make sure everything is correctly done. It does all ring out ok, but it doesn't hurt to look.

Finished_02_edit.jpg
 
CoyotesGator said:
Did you make a voltage chart?
Click to expand...

No, we know everything is about 200 volts too high up to that point. Phase inverter has the 7 volt mismatch at 235 and 242 volts, and the the power tubes were 465 volts at pin 3.

We were under the gun to finish up. I'll probably check the voltages again after looking under the board and write them down. I just don't want to have the power on too long, so I don't fry anything.
 
I wouldn’t worry about the PI being a little off but higher voltages elsewhere (preamp) might lead me to believe that a tube isn’t conducting.

Are you positive about the value of the resistors in the PI?
 
syscokid said:
This mismatch of 7 volts is perfect. I think this is what you want.
Click to expand...

According to my past notes from other builds and repairs with comments by IvN and Plexi, that is perfect. It’s the stuff in the green frame of my last photo and the filter cap that are off.

One of the lugs on the filter cap has wires coming from the choke, B+ center tap of output transformer and standby switch(my notes are downstairs, so I may be wrong).

That cap is too high. I’ll ring stuff out again tomorrow and look under the turret board.
Everything else seems to be ok. I’ll run it for brief periods to get some numbers after I check the turret board.
 
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