HELP ME SAVE 74 SUPERLEAD!!

[Doug H]

ya, weird I've never seen a gut pic with resistors like those in it. As long as they are 1m.

just take the diodes out of the circuit and see if any problems go away. I wouldn't expect anything to change, but since you haven't found the issue yet...

[Doug H]

Maybe I am not seeing al the pics you can see but I see no flyback diodes on this amp PT. Usually see flyback diodes on OT.

Check the first power tube right under the master in the first pic I think it was.

[neikeel]

OK now I am really puzzled.

There is no V8 in this amp (V1-3 are preamp valves and V4-7 are the output valves.

The 'death cap' I see and the polarity switch, odd installation of mains cans , added/swapped reisistors for EL34 install.

The screen resistor on V4 has been replaced (looks like for like value, originally a Welwyn).

There are bias resistors on the output valves (look like small light grey MF, typically 1 ohm between pin 8 and ground (or between pin 1 and 8 and then to ground). Is that what you are talking about?

As suggested above by others you will probably find a dodgy joint. BTW hope you are replacing the impedance selector with same rather than ream the chassis and panel out?

[heckitar66]

My mistake, Neikeel. It's V7. It got auto corrected without me noticing. No pun intended Doug H was referring to the 1 Meg resistors at the tail of the phase inverter and the diodes we mentioned were across pins 2 and 7 of V4 and V7.

You are spot on with your assessment of the components you mentioned I'll be checking solder connections and shortening wires tonight. And lastly I ordered a Marshall impedance selector from Valvestorm to help clean up that mess.

Hopefully I'll have time to play through it tonight when I finish although I won't have the new clamps and selector switch for a couple of days.

[neikeel]

My mistake, Neikeel. It's V7. It got auto corrected without me noticing. No pun intended Doug H was referring to the 1 Meg resistors at the tail of the phase inverter and the diodes we mentioned were across pins 2 and 7 of V4 and V7.

Ah now I see them, odd not on all4 tubes and look pretty puny to me usually 1000v 3A rated for an amp like this, and why on the heaters?

[heckitar66]

I've only read about them on this forum so I have nothing to offer. I'm guessing they're not necessary or everyone would be using them?

[Doug H]

it's hard to recommend someone remove a safety measure. If the voltage spikes up cause of no load it gets over the reverse voltage of the diodes and the spike goes to ground instead of through your OT insulation, something like that. You don't see a lot of them but on paper it sounds like a great way to protect the output section.

[heckitar66]

I understand completely. Maybe it's not a bad idea to have them but I want to rule out everything I can.

[heckitar66]

I'm happy to say the amp is sounding much better! And very quiet.

- removed death cap and polarity switch wiring
- shortened all wires to v1, v2 and v3
- removed master volume
- removed flyback shunt diodes on v4 and v7
- moved purple nfb wire further away from power tubes
- cleaned tube sockets

I'll be putting the new filter cap clamps on and changing the impedance selector switch next. Also I'll try the stock value bright cap on volume 1.

The low end sounds much less fuzzy and compressed. If I listen close I can still hear some compression that doesn't sound quite right to me and the tone controls have very little effect. Any idea what might cause the tone controls to be so weak?

[Doug H]

maybe check for gratuitous amounts of DC on the tone pots?

[danman]

Make sure that your grounds are solid also.

[heckitar66]

First, thank you Neikeel and Doug H for getting me this far. It's sounding MUCH better. I got my parts from Valvestorm today but misordered the impedance selector so I'll have to wait for that

Doug H and Danman, I will check both of those things and post results later tonight. Thanks!

[heckitar66]

I have 140v dc at the presence control.

[Carbia]

140dc on the presence pot?? too high.

It's normal to have some DC on the presence pot (that's why presence pot makes a little noise when you turn it) but 170vDC y so high.

[heckitar66]

Agreed, Carbia. I'm going to take a look at that dual gang impedance selector tonight. I really wish I had the new one. I'll try to find out where the DC is coming from. Any other places I should be looking?