Weird voltages on JTM45

[jwarlock]

Hello guys, my name is José, recently I was modifying/rewiring my old JTM45 build in order to take out some annoying hum noises that -I think- were product of a faulty build when I was a teenager. Years and years have passed, not playing a single note on my amp, mainly because it’s too loud for my room/house, but last week I felt the need to finish that amp.

I'm attaching the layout that I’ve used (I’ve made the amp around 2010, so this is a newer layout with very minor component modifications), it has a ½ power switch on the power tubes that I found very interesting. All measurements have been made with this switch on the full power position.

On the first attempt to measure voltages I’ve made a terrible mistake, connecting all tubes and turning on the power and stand by switch without plugging the speakersl, then I’ve seen some weird lights and heard some noises on the power tubes, I turned off the amp immediately. And went to read how to take measurements since my electric knowledge is very rusty in 10 years of not practicing/studying.

I’ve checked 7 times all the connections, looking for possible short circuits, and everything seems to be ok. The following voltages are DC unless otherwise noted. All control pots are turned to min, and bias pot is turned to the most negative voltage (-61V) at the end of the 56k next to that pot.

After plugging the KT66 tubes, when measuring the bias it reads 5/6 mV, and it does not change if I turn the bias pot.

I'm attaching some photos of the board, sorry for the mess, originally all cables were soldered under the board, but in this rewiring (after checking several times) I just thought that it would be better soldering on top of the board, it just helps to visually check everything. Once I have everything running, I hope to make things pretty again!.

2 clarifications. As you can see, I have a 16+16 cap, one end is going to the 8.2k (as per layout), and the other is replacing the axial capacitor that would have been installed on the board connecting to pin 6 of V2. Another thing, my light indicator is a 220v one since I couldn’t find a 6.3v for the filaments, and is connected to the ON switch.

I suspect that the power tubes are broken. If you guys need me to take more measurements I’ll take them, just tell me what tubes should be plugged and if load is required.

Any help will be most welcome and appreciated, thanks in advance guys!

[Reeltarded]

Heya Jose!

When you say you turned the amp on and it was unloaded (no speakers) and you say there were lights and sounds.. so you played through the amp and truned the volume way up because it made no sound at first?

If the tubes looked like lightning and they made sounds like rattlesnakes.. the output tubes are done. They would even light up but with signal there will be a huge electrical storm when they have signal.

Did the amp make smoke or smell like ozone?

We are going to have to start from the start. Honestly, you should follow the version of your exact amp's schematics and layout. I am sure that version is available. The half power switch is useless to you because you have a different power transformer (not to mention that half power doesn't make any difference in noticiable volume, it only softens the attack, honestly)

My advice is to start at the inputs and rebuild the amp before doing anything else. There are a couple reasons.

You may have never had proper bias. That makes hum. If you ever ran the amp for any length of time with bias problems it might have eaten the tubes.

You can pull the tubes and go back to the start-up instructions provided by Nik and take a look at voltages. They are all important. There are a couple of those voltages that may appear ok with a meter but when the amo is tubed and loaded the tubes go insane with fire and noise again.

It's not easy to know until you get it ready to start-up and do the testing.

[xtian]

Heya Jose!

Why, you're amazingly coherent, and haiku-free today! Daylight savings looks good on you.

[jwarlock]

Heya Jose!

When you say you turned the amp on and it was unloaded (no speakers) and you say there were lights and sounds.. so you played through the amp and truned the volume way up because it made no sound at first?

If the tubes looked like lightning and they made sounds like rattlesnakes.. the output tubes are done. They would even light up but with signal there will be a huge electrical storm when they have signal.

I did not play, the amp it was on my bench (I dont remember the position of all the pots, but volumes were at minimum), I was in the process of rebuilding, so as soon as I turned on the std by switch those lights and sounds ocurred, I turned off the amp as soon as I could. Maybe it it as you said the output tubes are done (or maybe they are fine but other things are broken), but what can I do to make sure that those are the only things that broke?, when I place my order for components I just want to be sure that I'm not missing something, since I have to order from another country.

Did the amp make smoke or smell like ozone?

No smoke, I don't remember any smelling something weird, so I'll go out on a limb and say no to smell too.

We are going to have to start from the start. Honestly, you should follow the version of your exact amp's schematics and layout. I am sure that version is available. The half power switch is useless to you because you have a different power transformer (not to mention that half power doesn't make any difference in noticiable volume, it only softens the attack, honestly)

I couldn't find the schematic for my original build, but it was exactly like the 2006 version that I'm attaching now. With the added load selector of later versions and a 50k bias pot.The version that I've attached at the beginning (and even the 2020 version available online) is almost the same, with some minor changes on the rectifier and std by switch.

I've made some additional measurements. The first thing that I did was check again if I've made some mistake in wiring the std by. Everything is fine there. Then I've removed every single tube, and did some readings without load.

1. Measured the voltages on the rectifier tube socket, I think it looks fine.
2. After that, I've plugged the GZ34 and measured again, remember that no other tube are plugged. A voltage chart available online (I'm attaching that too) says that I should be getting 400 VDC on pin 2-8, I've got 462 VDC. And 320 VAC on pin 4-6, I've got 464 VAC. Maybe my readings are fine if we account that I have no other tubes connected, I don't really know.[/li]
2a. When reading the voltages coming to the power tube sockets (kt66 unplugged) I have around 70 VDC more than the reference of 400 on pin 3, pin 4-6 are very strange since I've got nothing.

If I follow what the assembly manual of TubeDepot's JTM45 says regarding voltage measurements, my preamp is fine; I can get from sections 8.1 to 8.3 with more or less the same readings (see voltage charts on my first post).
https://s3.amazonaws.com/tubedepot-com- ... lus_v5.pdf

[jwarlock]

1. Measured the voltages on the rectifier tube socket, I think it looks fine.
2. After that, I've plugged the GZ34 and measured again, remember that no other tube are plugged. A voltage chart available online (I'm attaching that too) says that I should be getting 400 VDC on pin 2-8, I've got 462 VDC. And 320 VAC on pin 4-6, I've got 464 VAC. Maybe my readings are fine if we account that I have no other tubes connected, I don't really know.
2a. When reading the voltages coming to the power tube sockets (kt66 unplugged) I have around 70 VDC more than the reference of 400 on pin 3, pin 4-6 are very strange since I've got nothing.

I was missing the 1k resistor that goes between the 8.2k and S1 (half power switch), that explained the missing voltages on pins 4 and 6 of the power tubes. After soldering that resistor I've taken those measurements again.

[Reeltarded]

so the voltages are good across the amp now? whole thing?

[jwarlock]

so the voltages are good across the amp now? whole thing?

I don't know. Should I put every tube, plug the speakers and read the voltages?, the values that I've provided look ok-ish?

Edit.
I tried to measure everything with all tubes plugged. Out of nothing I decided to test the cable that I was using to connect the speakers (an old Fender, about the age of my amp, 10 years), it was broken... So the times that I've taken readings with power tubes on, were without load.
Put a new cable on, and when I power on the amp (with standby switch closed) to take a reading of the power tube voltages, I hear a "hum" (not too loud, but not low either), and about 5 seconds later the 500mA fuse blows.

[TUBEDUDE]

Try it with the power tubes removed. Sounds like bad tubes or fried output xfmr.

[jwarlock]

Try it with the power tubes removed. Sounds like bad tubes or fried output xfmr.

With power tubes removed I get the voltages that I've shown on the last chart uploaded (only measured gz34 and the voltages coming to the power tube sockets). I suspected the OT may be fried, measured the resistance between primary cables and I get about 170 ohms, so I think it's still fine after all my mistakes. Is there any more readings that I can do to check if power tubes and/or OT are done?

[TUBEDUDE]

VThose voltages look good. Did you check the primary to secondary for a short, or from the primary to ground. Any evidence of arcs, carbon tracks on the sockets?