74 Marshall Lead Pot issues?

[PeterJ_22]

Hi guys.
Ok so looks like I can't win with this amp!

So I was given a set of nos RCA 6ca7's so I decided to give them a go. I noticed there were no 1 ohm resistors from pin 8/1 on the power tube sockets so I installed them. using 1w 1ohm carbon resistors as I had no metal film on hand.

I also changed the 32x32uf mains filter cap to a 50x50uf to tighten it up a tad.

As I tried to bias the tubes it acted weird i could turn the pot till about 24mA then it jumped to 120mA.

I decided to change the bias resistor from stock 47k to I think 27k? or 33k? so I could get the bias in range.

Now both tubes are biased at 42mA, my amp is only running at 370v so I figure that's about right as its lower voltage then my other amps at around 450-500v.

Ok so amp sounds great but heres the issue.

All the tone pots are scratchy as (assuming they have dc on them?) and the treble pot cuts out at around 8 and kills the signal?

I've attached a link to a video demonstrating what I mean and a bunch of super high res pics of the amp (since I have rebuilt it a few days back)

any help would as always be greatly appreciated!

Cheers guys - Peter

http://s1289.photobucket.com/user/Peter ... t=3&page=1" onclick="window.open(this.href);return false;

[neikeel]

The cutting out with increased setting on the treble or a gain pot usually suggests parascitic oscillation, particularly of it is preceeded by a whine or screetch.

Scratchy pots can be helped by stripping and cleaning them but most are carbon tracked and do wear. A scratchy presence pot when using the 5k/0.1uF is normal due to some dc. (your original presence pot of 25k with 4k7 and 0.68uF was designed to ameliorate this).

[PeterJ_22]

It's just interesting as the pots were 100% fine before I changed the bias / tubes / added 1 ohm resistors to power tubes - ground, and changed a filter cap.

Also no noise what so ever once the treble cuts out. its like the signal just goes

And the volume pots both make no noise and can be cranked to full no dramas

Also all pots are 6 month old CTS pots so should be fine condition

[PeterJ_22]

The amp actually sounds great its just when I move the pots or push the treble beyond 8

[neikeel]

The amp actually sounds great its just when I move the pots or push the treble beyond 8

Check your lead dress of your grid wires and control wires first.
Make sure the plate wires are low onto the chassis and the grid wires slightly lifted.
Do you have grid stoppers on pin 5 of your output tubes? Of not try them (4k7 or 5k7) and restest.

[PeterJ_22]

Yea got 5k6 grid stoppers

[stef]

Screen grid resistors?

[PeterJ_22]

Should all be there check the photos I uploaded

[PeterJ_22]

I went over it today and everything looks like it should, I can't find any issues yet its still doing that with the pots?
Anything on the pics look out of place?

The only things I changed from when it was working fine till now is the Mains / screens can caps, the bias resistor (next to trim pot) and adding 1 ohm resistors to the power tubes. So I wonder if one of those did it?

Would dropping the 47k bias resistor to 27k (to get the bias in range) have an effect like that?

[neikeel]

Does the presence setting affect when the treble cuts out?

[PeterJ_22]

A very small amount.
The volume doesnt go from full to nothing it fades out over like 1 degree of the dial.
Presence control affects it slightly

[PeterJ_22]

Ok so this is interesting.
I measured the DC voltage on the pots with my fluke multimeter and hear are the values:
Order is Left - middle - right lugs looking from the back of the pot.

Presense: 9 / 0 / 0
Bass: 17.5 / 47 / 43
Middle: 0 / 17 / 17
Treble: 48 / 195 / 195
Volume 1: 0 / 0 / 0
Volume 2: 0 / 0 / 0

Tone stack has a huge load of dc on it especially the treble.
Even testing the treble pot caused sparks to come off the input (right) lug


Crazy... soo where does that leave the amp? Leaking cap somewhere?

[PeterJ_22]

So I thought to make sure it wasn't a tube issue to try all new power tubes / preamp tubes and it made no difference.

Also now the amp is constantly making that weird noise

[PeterJ_22]

Ok seemed to of worked it out / fixed it.

Looks like it was the 0.022 coupling caps coming from V2 leaking DC.
Put some new sozo ones in and seems to be fine with no dc on the tone pots etc.

I'll keep testing etc and make sure its fixed !

[PeterJ_22]

And now its all bolted back together after playing for an hour faultlessly its squealing with the gain cranked... sigh it really doesn't like me this amp.