Mojotone JTM45 build issues-RR

[rrozell]

To all:
I've been off this forum for a while, but still playing with guitar tube amps. For informational purposes, I bought a JTM45 kit from Mojotone, and I'm regretting it every day.
I thought I would save a couple of bucks...silly me. It took them weeks to send me the electronics kit, and after about 6 weeks the cabinet still has not shipped. I understand parts can sometimes be hard to source, but the web site said, "usually ships in 3 days"...
then I got the kit and there were virtually no instructions, just a couple of poorly drawn, even incorrect, diagrams . Buyer beware. I got the thing built somehow and one of the EL34's would not glow. OK. Stuff happens, but how could it be a matched set if one tube doesn't even get hot? To be fair, the rest of the kit seems to be pretty true to the original Marshall and that's why I elected not to send it back.
Long story short, I have the amp working on a used set of EL34's from an old Carvin I have, but the output is weak and distorted and buzzy. I have swapped out the ecc83's with 3 new EH 12ax7 tubes and got the same results. I don't have a new set of EL34's, but the ones I have in there came out of a working amp. I'm hoping the weakness and buzziness is due to my not having set the output tube bias current yet, but I'm skeptical that's my only problem. On pin5 of both EL34's I have -35.5 VDC. I have 488 VDC on pin 3 of both EL34's. That seems too high to me, but I'm not an expert. This kit uses a GZ34 rectifier tube. All of the tubes are JJ Electronics and seem of questionable quality to me...obviously. The voltages off the primary transformer are 717VAC, 5.16 VAC and 6.84VAC. I put a meter on every resistor and capacitor before I soldered them in place, and they all were well within tolerance. I own a pretty good capacitance meter that I used for that.
Any suggestions or comments would be appreciated.
Thanks,
Russ Rozell

[Chuck H]

Probably a wiring error. Have you traced signal through the amp yet? Weak, distorted and buzzy output on a new build sounds like perhaps one side of the PI isn't delivering or there is an incorrect bias condition on one of the preamp stages. But that's a rough guess with very little basis. Another rough guess would be that the power tubes are over biased or not drawing enough current for some other reason. This would explain the high plate volts. Have you measured actual idle current yet? Do you have any other bench gear besides a DMM? Scope, signal generator, etc.?.

[rrozell]

Chuck,
Thanks for your response. I may very well have messed up something wiring-wise, although I rechecked all of the connections. I also don't have much faith in this wiring diagram since I've found several errors on it so far. I wired the input and output jacks they way it was drawn only to find out the input and output (speaker) wiring was shorted to ground that way. Somewhat frustrating. I got those part corrected using a Ceriatone drawing, which was much clearer. I had the amp on for quite a while last night and added 1 ohm resistors from the 8 pin to ground of the EL34s'. However I was getting basically 0 millivolts across the resistor. I thought my meter was messed up and it was late so I called it quits. No current? Maybe you are right. I went today to retest with an amp meter, but found out that the 8 pin socket for the GZ34 had basically disintegrated. I guess from heat, but it didn't melt. It just crumbled like a cookie. I don't have another 8 pin socket so I ordered several from Tube Depot today along with two Mullard EL34s', and another GZ34. I have an oscilloscope from work thatI can trace this out with and a little drum machine ti generate a signal.I'll figure it out eventually.
Russ R

[Chuck H]

the 8 pin socket for the GZ34 had basically disintegrated. I guess from heat, but it didn't melt. It just crumbled like a cookie.

I may sig that.. LOL

We can get back to this when your up and (kinda) running again.

[rrozell]

"Sig That"?
Did I say something funny?
I've attached a scan of the drawing they sent me. It looks close to the original Marshall, but the inputs and outputs are wrong. Maybe there's something else obviously wrong that I missed...
RMojotone JTM45 drawing.pdfR

[rrozell]

Tube Depot told me they shipped my stuff today that I ordered yesterday. Now that's service. I went down the street to a local guitar shop to see if they had a socket, but they don't fix amps any more, so no parts. I should have a new set of tubes and a replacement socket by Thursday. Thanks for your help.

[rrozell]

I got my parts and I replaced my rectifier socket. I now have a new GZ34 and socket and new EL34 output tubes. Still have 488 VDC on pin 3 of the EL34s and also have no bias current. Zero. I checked the wiring against the drawings Mojotone sent me and I can't find where I messed up anything. The output tubes appear to not be pulling any current. Any suggestions>
Russ Rozell

[Chuck H]

On the power tubes, what is your screen voltage (pin 4)? What is the resistance from pins 8 and 1 to chassis?

[rrozell]

On pin 4 of the EL34s I have 0.158 VDC. The resistance from pins 1 and 8 to ground is 1.1 ohms. I have a 1 ohm power resistor on there now for the biasing procedure...or 1 resistor per EL34 anyway. In both cases it's 1.1 and 1.2 ohms to ground respectively.

[Chuck H]

Fine on the cathode resistors. They can live there. No need to ever remove them and your reading there is fine. But pin 4 is the power tube screen grids. They should be at a VDC just lower than the plates (though your plates read high right now because the tube isn't drawing current). I would check now to see if you have preamp voltage. Pins 1 and 6 of any preamp tube. If you don't have preamp voltage I would suspect there is a fault in the power rail, a wiring error or possibly even a bad choke. If you do have preamp voltage there is a fault in the circuit after the choke that exclusively feeds DC to the screen grids.

[rrozell]

Mojotone JTM45 schematic.pdfChuck,
I think I found it...with your help, of course...the 1k power resistor feeding the grids is open. I checked it before I soldered it in and it was good! Pre-amp tube voltages were there. I found a discrepancy between the schematic they sent me and the drawing. The drawing shows a 470 ohm resistor across pins 4 and 6 of each EL34 which is fed bus voltage through the 1k resistor, but the schematic does not show that (unless either pin 4 or 6 is a no connection pin).
I followed the drawing they sent me. I'll attach the schematic. The drawing preiously attached to this thread.

[loudthud]

Pin 6 is not used on the EL34 so it can be used as a terminal to connect a resistor to a wire. It is sometimes used to provide support for the 5.1K stopper resistor on pin 5.

[rrozell]

OK. Cool. Unfortunately, I replaced the 1k resistor with the closest thing they had at Radio Shack (4, 1k, 1 watt resistors in series/parallel for approximately 1k 4 watts), powered up and I popped the primary fuse after about 15 seconds. I don't have another fuse, so I'll have to continue tomorrow. Hopefully the fuse was just damaged from initial testing, but I'm worried something popped the original 1k resistor. Something did melt the original rectifier socket, although I had the thing powered up for quite a while while I was testing. I don't see anything obviously shorted to ground. If something was really wrong electrically, I would think I would have been popping fuses from the outset.
Oh well, such is life in electronics...

[Chuck H]

That's definitely an indication that somethig is wrong. What would fry the rectifier socket and twice the screen feed resistor is a mystery to me. I'm inclined to think there must be a mistake (or more than one) in the wiring. The drawing looks good. Even if you've checked it ad nausium, check it again. And keep checking until you find the error. It would be a good idea at this point to pull the rectifier tube and take readings for the PT secondaries to be sure nothing is damaged.

[rrozell]

I'll go through it again. The replacement screen feed resistor is still ok. I did have a bad EL34 in there the first time I powered up. Everything points to something in the output tube section. Nothing got noticeably hot or burned except the rectifier tube socket and I didn't notice that until later. I did have audio output which was weak and distorted which tells me the input section must be ok, but I'll recheck.