Problem with 2204 build

[fted]

Hi,

I am making a 50W Marshall JCM800 clone using the mhuss schematic: https://mhuss.com/MyJCM/JCM800_2204.pdf . So far things have been going pretty smoothly and I got the electric stuff finished during my christmas break. Since then I've concentrated on building the enclosure and haven´t even turned the amp on until yesterday. I was surprised to find that the amp had lost most of its sweet overdrive, even with the pre-amp volume on 10, the tone was mildly crunchy at best.

So I took some measurements and found out that the voltages on the first preamp tube were way off. Or more precisely, half of them: The V1b is fine, but on V1a plate voltage is around 185 (should be 296) and on the v1B cathode its 10.7 (should be 3.3).

Voltages on other tubes are fine and the V1 plate resistors and decoupling caps seem to be OK. The tube socket looks good too. I also swapped preamp tubes around to no avail. Any ideas?

[Stevem]

I would unsolder one end of all the resistors dealing with that tube and confirm there value is correct for starters.
With over 10 volts on the B side cathode that tube should be near burning up!

[xtian]

Welcome to the board! Measure plate and cathode resistors for V1a. Maybe you have a 100K resistor where it should be 10K? If everything measures correctly, maybe resolder to make sure connections are good.

[Coolhat]

I would unsolder one end of all the resistors dealing with that tube and confirm there value is correct for starters.
With over 10 volts on the B side cathode that tube should be near burning up!

Aaand there I go, messing up my very first post I should have written V1A cathode of course. I guess I got confused, because in the schematic B comes before A... But to clarify: V1A voltages are OK, it is V1B that is off (185v plate/10.7v cathode).

I have measured the resistors numerous times, but I´ve not unsoldered them, should that make a difference?

Welcome to the board!

Thanks!

[Coolhat]

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[Coolhat]

Ouch, it seems I have made double accounts! This is not going well at all...

[xtian]

How about post a photo of V1 components?

Since we're talking about V1b now, that 0.68uF bypass cap is also suspect--if it's partially shorted, that might explain the extra current. Lift one end and see if that changes the voltages at all, at plate and cathode.

[Coolhat]

No no no, I´ve done it again This is not my day, it is the V1A that is the problem! You must think I am an idiot and TBH, I would not blame you. But yes, the problem is with V1A, I swear!

How about post a photo of V1 components?

I don´t have the amp here at home, it is in my rented storage/workroom, but I´ll see if I have time to drop by tomorrow and snap some photos.

[Coolhat]

I woke up early and dropped by the workroom on my way to work. I measured all the resistors and caps around the V1 once again and everything was correct. I also measured the decoupling caps for DC leakage, but nothing there either.

However, when I measured the V1A grid (pin 2), I found a strange thing: the DC voltage started form zero, creeped up to 400 mV and then dropped to zero again, creeped up to 400 mV etc. So I clipped a 1Mohm gride leak resistor from the preamp volume pin 2 to ground. Lo and behold, the V1A voltages were instantly cured! Unfortunately the volume also dropped and the tone went from a mild crunch to completely clean, complemented by a loud hum.

I didn´t have time for more in-depth examinations, but at least this sheds some light on the issue. I´m guessing I´m dealing with grid leakage here?

[xtian]

V1a (pin2) should have a stable path to ground through the preamp volume pot. You should not need to add a grid reference resistor. That said, I can't imagine why adding a 1M resistor from pin 2 to ground would dump any volume. Something is wrong (obviously) with your circuit, and I'm still hoping you can post photos.

[Coolhat]

Hi again!

Dropped by the amp again and managed to snap few pics. I´m no photographer and it was difficult avoid reflections and shadows, but I hope these are of some help.



The whole amp in all it´s glory



V1 and V2

As you can see, it´s not the tidiest of setups, partly due to the small chassis, partly due to my inexperience. I used the Ceriatone layout ( http://www.ceriatone.com/ceriatone/wp-c ... iatone.jpg ) and there are couple of mods, most obvious being a switching system to choose between two master volume pots.

[xtian]

Wow. I mean. After staring at these photos for a while, I have no new insights for you.

[Coolhat]

Wow. I mean. After staring at these photos for a while, I have no new insights for you.

Sorry to hear that. I know it is butt-ugly and I can not really say anything else but I am still learning. This is the third tube amp I´ve built and the first one not from a kit. Perhaps I´ve bitten more than I can chew here, in hindsight I should have at least used a bigger chassis.

But it is what it is, so any help is greatly appreciated.

[Roe]

Just reflow all soldering connected to v1. double-check the grounding.
Try moving the shielded input cable directly to the tube socket. At least, the resistor should very close to the socket, without a long leg.

[Coolhat]

Try moving the shielded input cable directly to the tube socket. At least, the resistor should very close to the socket, without a long leg.

Yes, that makes sense, thanks. The reason I soldered it like that is because of the Ceriatone layout. I did have my doubts, but decided that there must be some wisdom behind it...

I´ll have more time to work on the amp during the weekend, I´ll try to go through every single component and joint.