Need a second set of more experienced eyes on ‘71 super lead

[Chris4189]

Guys,

My buddy bought a ‘71 1959 that needs some TLC and I’m trying to help him with it. The amp hasn’t been modded but someone has definitely been in here at some point. I’ve replaced some resistors on the board in order to go from 6550’s to EL34’s, changed the NFB tap and resistor and removed the death cap and installed a 3 prong plug. I’ve double checked the screen filter wiring and it looks right to me but when I went to bias it, I’m long getting like 230 volts at the plate. Tried a known set of good tubes and same thing. I’m at a loss.

[SteveD]

Snip those silver 'Hunts' caps and discard for a kick off.

[Chris4189]

Snip those silver 'Hunts' caps and discard for a kick off.

Don’t you have to have the snubbers?

[SteveD]

Snip those silver 'Hunts' caps and discard for a kick off.

Don’t you have to have the snubbers?

They're actually across the AC input to the recifier and just cause bother more often than not.

It's the first thing Marshall Service would do if they had it in. They are simply not required.

If you want to fit snubber caps. solder 1n/1kV polyester caps across each individual rectifier diode. Lose the Hunts caps asap. They may even be the cause of your issue.

[Chris4189]

Consider them removed then. I thought they we snubbers. Still learning

[Chris4189]

Snip those silver 'Hunts' caps and discard for a kick off.

Don’t you have to have the snubbers?

They're actually across the AC input to the recifier and just cause bother more often than not.

It's the first thing Marshall Service would do if they had it in. They are simply not required.

If you want to fit snubber caps. solder 1n/1kV polyester caps across each individual rectifier diode. Lose the Hunts caps asap. They may even be the cause of your issue.

Do I need to replace them with anything or remove them and drive on?

[Chris4189]

Removed them and I’m back up to 500 volts pins 3 and 4 V4-V7 but I have no negative VAC on pin 5 on V4 and V5 :( V6 and V7 are both showing -39.8


I have an old quad of EL34’s on my work bench and just as I figured V4 and V5 red plated like a boss while V6 and V7 were fine.

I double checked and the grid leak resistors measure out at 220k on the nose, PI output coupling caps measure out at .22 uf, grid stop resistors measure out at 999k and the orange and green wires coming off the board are intact and run to the 1.5k’s.

[SteveD]

Snip those silver 'Hunts' caps and discard for a kick off.

Don’t you have to have the snubbers?

They're actually across the AC input to the recifier and just cause bother more often than not.

It's the first thing Marshall Service would do if they had it in. They are simply not required.

If you want to fit snubber caps. solder 1n/1kV polyester caps across each individual rectifier diode. Lose the Hunts caps asap. They may even be the cause of your issue.

Do I need to replace them with anything or remove them and drive on?

Don't replace them with anything. It seems they were indeed the problem for pulling your HT voltage down.

[SteveD]

Removed them and I’m back up to 500 volts pins 3 and 4 V4-V7 but I have no negative VAC on pin 5 on V4 and V5 :( V6 and V7 are both showing -39.8


I have an old quad of EL34’s on my work bench and just as I figured V4 and V5 red plated like a boss while V6 and V7 were fine.

I double checked and the grid leak resistors measure out at 220k on the nose, PI output coupling caps measure out at .22 uf, grid stop resistors measure out at 999k and the orange and green wires coming off the board are intact and run to the 1.5k’s.

That should be negative dc voltage on pin 5, not VAC by the way.

You've obviously lost the negative bias voltage on V4 and 5. Are those 1k5 resistors ok? You need to check all wiring, joints and components from the 220k that feeds V4 and 5.

[Chris4189]

Yes, I meant to say VDC.

Yesterday, I checked the grid leak resistors, grid stop resistors, coupling caps and they all checked out fine. I then measured the legs of 220k’s and noticed the leg of the 220k on the green wire side had no VDC. Lifted the board about a 1/2 inch and all tubes showed -50 VDC. Tighten the board down problem comes back. I hate to think about it because the board is wired tight and there isn’t much play in it but I’m thinking the wire has an issue.

[Chris4189]

Found the nick in the wire and put some shrink tube around it and everything is back within speck.

The pilot light was burned out so I got a replacement but it’s a 110-120 volt one. How in the world do I hook this up? I wired per the directions in the metro forum: solder red wire to bottom buss wire of on/off switch and black wire to mains fuse tab. When I do this the light comes on as soon as the amp is plug into the wall and the light goes off when the on/off switch is switched to on.

I don’t know if it matters but this amp has a polarity switch. The cap has been removed and a 3 prong cable installed but they 120 wire off the PT is going to the little board where the cap was.

[SteveD]

Are you in the USA?

You need to identify the 120VAC primary tap and common. Red and orange respectively I believe. Check it with your meter.

Remember, the chassis is upside down when you're working on it, so bottom can mean the contact closest to you.

[Chris4189]

Yes sir.

Red goes to the board where the death cap was and the common (orange) is on the bottom buss wire running through the polarity switch.

[SteveD]

So, if you are in the States, you'll find the 120V you need on the mains switch. Just make sure you connect onto the switched side, otherwise as you've already seen it'll illuminate as soon as it's plugged into the wall.

Check the various terminals with your meter to make sure.