JTM60 Low Distorted Output Volume drops.

[gtrplayr1976]

Hi Folks. I'm working on a JTM60 for a friend. It had no sound when I got it ,but it powers up ,and tubes all glow. Tubes have heater voltage. I tried the most common thing with these by adding jumpers on the fx loops. Still nothing. I took the PS board out ,and checked solder joints ect. Found one screen grid open ,so I replaced both of those, all the tube sockets(they were deformed), The 47uf ,and 22uf caps, bridge rectifier, broken extension speaker jack. After that I got a signal ,but very low ,and when I turned the volume up even that dropped lower. I ended up replacing the main speaker jack as well ,and this time the signal got louder as I turned it up ,but when I got to about 6-7 on the MV the volume dropped off (also thought I heard a click at this time) Both channels act the same. I have more testing to do tomorrow. Picture is how I jumped the speaker jacks. I figured it was the same as a single fx loop as far as connecting the cables. Thank you for any info !

[The Dude]

1) Run the effects send to another amp to see if your preamp is working. I suspect it is, but be sure.

2) If sockets were deformed and screen resistor was burnt, it's pretty likely you have at least one bad output tube. I'd install a new set, BUT FIRST check voltages on your tube sockets and make sure you have correct voltages, especially bias voltage. I believe those amps have a bias circuit that is capacitor coupled from the transformer and those caps can fail. You don't want to ruin a new set of tubes.

3) What do you mean by "I jumped the speaker jacks"? I'm not sure what you're talking about there.

4) Please post a schematic or schematic link of your amp.

[gtrplayr1976]

The Dude
1) Run the effects send to another amp to see if your preamp is working. I suspect it is, but be sure.

2) If sockets were deformed and screen resistor was burnt, it's pretty likely you have at least one bad output tube. I'd install a new set, BUT FIRST check voltages on your tube sockets and make sure you have correct voltages, especially bias voltage. I believe those amps have a bias circuit that is capacitor coupled from the transformer and those caps can fail. You don't want to ruin a new set of tubes.

I'll do these tomorrow ^^^ I'll check the voltages I know the plate voltage is 517VDC ,and bias current is 34 on one tube ,and 38 on the other. I'll check negative voltage tomorrow.

3) What do you mean by "I jumped the speaker jacks"? I'm not sure what you're talking about there.

What I mean was I put an instrument cable between the send ,and return of both ^

4) Please post a schematic or schematic link of your amp.
Is there a way to post a PDF? Here's a link to the schematic.

https://www.thetubestore.com/lib/the...-Schematic.pdf

Thanks for the help !

[gtrplayr1976]

So you were right. I connected the send to another amp ,and was way more volume. I then checked the voltages on the power tubes , they all seemed normal. 511 plate volts , 504 on on screen grid, -44.9 on control grid, heater voltage good. I tried all new tubes ,and still little output ,so I tried to check the OT ,but I want to be sure I connected it correctly. I induced 10VAC into the secondary side ,and read the voltage on the primary side ,BUT I may have had the neutral in the wrong spot. I had it connected to ground ,and moved the hot (line) to each peg on the secondary ,and read voltages. Thinking I did it wrong I moved the neutral to the common on the secondary ,but voltages were erratic. I couldn't get very good readings. The voltage in was jumping all over ,and the voltage out was erratic as well?

[gtrplayr1976]

1) Run the effects send to another amp to see if your preamp is working. I suspect it is, but be sure.

2) If sockets were deformed and screen resistor was burnt, it's pretty likely you have at least one bad output tube. I'd install a new set, BUT FIRST check voltages on your tube sockets and make sure you have correct voltages, especially bias voltage. I believe those amps have a bias circuit that is capacitor coupled from the transformer and those caps can fail. You don't want to ruin a new set of tubes.

3) What do you mean by "I jumped the speaker jacks"? I'm not sure what you're talking about there.

4) Please post a schematic or schematic link of your amp.

I think the OT is bad. I ran low voltage into the secondaries ,and the voltages were erratic ,so I took the one out of my JTM30 (same OT),and put it in the JTM60 it was WAY better ,but still didn't seem loud enough ,so I tested that OT ,and the voltages on the primary side weren't close to equal ,and still low. I think I had around 50VAC on one ,and 15 VAC on the other. I have new transformers to install ,so we'll see.