JCM900 Rebuild into JCM800

[Mr_bibbles]

Do'h! I realized from my last post that plan was just putting us back to square one, with Bias voltage present on the tubes in standby mode, but with 200ish Volts on the HT while in standby., since the HT was being half-wave rectified in standby....

What if we used both halves of the DPDT, one to disconnect the half of the secondary winding, and the other to interrupt the HT after the rectifier? The downside is this leaves HT on the reservoir caps in standby... Is this potentially damaging or dangerous? I can't think of a reason why, but I always assumed standby kept ALL HT out of the amp, including the caps.

I wired it up on a limiter and I get great bias voltage on standby, and great HT voltage on the tubes / OT off standby...

[The Dude]

LOTS of amps leave the filter caps charged in standby. Look at a Fender Twin, for example. It's not an issue if your caps are rated high enough voltage. B+ will be slightly higher with no load.

[Helmholtz]

Do'h! I realized from my last post that plan was just putting us back to square one, with Bias voltage present on the tubes in standby mode, but with 200ish Volts on the HT while in standby., since the HT was being half-wave rectified in standby....

Was that with power tubes in?
Wondering if that also happens with a JCM 900.
Does the amp produce sound in standby?

[Mr_bibbles]

LOTS of amps leave the filter caps charged in standby. Look at a Fender Twin, for example. It's not an issue if your caps are rated high enough voltage. B+ will be slightly higher with no load.

Awesome thanks! I Turns out sticking with the two 500V caps in series for the reservoirs came in handy then, should be able to handle it no problemo. As a last precaution I will install a parallel 47K resistor across the AC interrupter as shown at the bottom of the page here.

[Mr_bibbles]

Was that with power tubes in?
Wondering if that also happens with a JCM 900.
Does the amp produce sound in standby?

No valves installed yet until I make sure the voltages are nice and safe, so not sure if would make a sound. I'm guessing the 200V on on the tubes is not the case with the 900 as it has no balancing resistors across the filters? Honestly not sure, but that's the only difference I can see from the schematic?

[Helmholtz]

No valves installed yet until I make sure the voltages are nice and safe, so not sure if would make a sound. I'm guessing the 200V on on the tubes is not the case with the 900 as it has no balancing resistors across the filters?

Now that makes a huge difference.
As you said 200V on the tubes, I assumed the tubes were in.
I couldn't see how there could be a B+ of 200V with a real load (i.e. tubes).
I assume that voltage will collapse with tubes in, so everything would be fine.

[Mr_bibbles]

Hey, got busy with life for a bit but completed the amp! It’s loud and it sounds like a jcm800 However…

There is a 120hz hum that rides the signal, and gets louder as I turn up the preamp gain. Considering it’s not constant and affected by the early controls, this is probably not a “global” filter issue, right? Can anyone give me tips about what to troubleshoot?

1. I triple checked grounds
2. Filters are wired correctly, double checked terminations
3. there are two cap cans without balancing resistors to compensate for the lack of center tap… could this be the culprit?

[pdf64]

The grounding arrangement in the triode drawing doesn't look great to me.
Merlin's multiple local star with single chassis connection guideline makes most sense to me http://www.valvewizard.co.uk/Grounding.pdf

Several Modulus layouts show how it can be implemented
https://download1077.mediafire.com/n...800+LAYOUT.pdf

[Mr_bibbles]

The grounding arrangement in the triode drawing doesn't look great to me.
Merlin's multiple local star with single chassis connection guideline makes most sense to me http://www.valvewizard.co.uk/Grounding.pdf

Several Modulus layouts show how it can be implemented
https://download1077.mediafire.com/n...800+LAYOUT.pdf

Considering I am modifying the Power Supply and using a non-CT HV tap, this seems like a likely explanation.... I have all of the FIlters leading to a single star ground, but I am using a ground bus across the back of all potentiometers, mounted directly to the chassis.... according to the Merlin page these symptoms could be explained by Power Supply noise finding it's way back into the audio path through the loop? (See 15.9)

Solution: Remove the Chassis-connected ground buss from all preamp connections, and have it terminate at a SINGLE point where the filters are grounded.

I'll try this, thank you.

[loudthud]

You might try this first. Move any preamp filter cap grounds from the star to the back of pots ground buss.

[Mr_bibbles]

You might try this first. Move any preamp filter cap grounds from the star to the back of pots ground buss.

Thank you Loudthud (And thanks PDF as well!)

It was indeed ground loop noise inducted back into the preamp, separating the preamp filter ground into the pot buss cured all! Even so, decided to create star ground for Bias, Filters (x2), Preamp, Transformer, etc. The only thing no included in the main star is the 120VAC Earth pin.

This thing is sounding mighty, hum floor very low. Thanks to all the kind members who helped with their time.