HUM! Need some help...

[MHProd]

You mean the wires going from the 220Ks to Pin5 right?

I believe they are not reversed. Check the first picture I posted on page 2. The blue wire goes to V4.

[Littlewyan]

Could be the wires going from the grid leaks to pin 5 or could be the output transformer primaries. Its possible that when you installed the PPIMV you actually had the wires reversed and the amp worked fine, now that you've put them back to how they should be they are out of phase and instead of negative feedback you've got positive feedback.

Try switching either the wires going from the 220k grid leaks to pin 5 of the output tubes or the OT primaries. This would explain the hum and the fuse blowing. Especially on a JTM45 as it had a LOT of negative feedback, enough to kill an amp if reversed and made into positive feedback.

Edit: This would also explain why you only get the issue with both the PI and Power tubes installed. Without one or the other there is no feedback loop.

[MHProd]

YES!

I did reverse the wires on Pin 5 and it worked ! I originally built the amp with the PPIMV and VVR so you were right, something was backwards !




Unfortunately there seem to be a second issue after all:

I played the amp for a few minutes and it worked and sounded great.

I put it on Standby and came back to it later to play some more. When I switched the Standby, I saw a spark going in the recto and then no sound.

Unplugged everything and found the 500mA fuse blew...

Replaced the fuse and installed a different recto. The amp worked but was nowhere near as loud and distorted much earlier. Also noticed the sound started to drift away. Turned everything off. Installed my bias probe to check the B+ and bias. All good. Removed the probe, turned the amp back on and this time both fuses blew instantly. This one is because I'm an idiot and put the recto (JJ GZ34) that sparked back in...

I'm out of 1/2A fuses so it might take a few days before I can work on the amp again.

It's frustrating because the amp sounded great!

I'm wondering if the Mains electrolytic is bad... What could make the volume drift very slowly? It's almost like a cap isn't charging properly..? Power tubes seemed fine. No redplating, stable bias.

[matt h]

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[Littlewyan]

Could be the choke caused a high voltage spike which fried the recto. Hell I blew a mains fuse once by being lazy with the standby switch and accidently holding it in the middle and my amp has no choke. So maybe dont be too concerned just yet.

I'm bit confused with the story tho, at what point did you reinstall the bad recto by accident?

[MHProd]

After checking the B+ and tube bias. I changed the recto as well for some reason and put the bad one in. I do stupid things like that every once in a while...

I have to figure out why the amp distorts so much so early and why the volume fades.

[Littlewyan]

So its still doing that after installing a new recto? You need to measure the plate voltage of all tubes, heater voltage and maybe screen voltage on the output tubes as well. As the volume fades it'll be interesting to see if any voltages change.

[matt h]

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[MHProd]

Well, I finally sorted it out! Replaced the cap for the screens and it works great now. Hope it stays that way! Thanks again guys for all the help!

[matt h]

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[matt h]

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