Soldano Lucky 13 (Leaking o/p board?)

[Simonsaysbang]

Has anyone come across a leaking o/p board (6L6) on a Soldano Lucky 13?

V9 and V10 bias nicely at -55v with a draw of 22mA/tube.

V7 and V8 bias in the sockets at -55v with tubes removed but draw 70 mA in test.

Swapped over tubes same result.

Replaced all bias circuitry as a matter of course including 1N4007. Original bias was at -35v.

220K resistors test fine. Lifted 0.022uf coupling caps, same result.

470R screens and 2K2 grids test fine.

O/P TX primary is nicely balanced at 18 Ohms each side of CT although no idea at the moment if it is failing in use so I might swap primary windings over although it might oscillate.

Removed, cleaned o/p board + re-tensioned and cleaned all o/p tube pins and re-flowed board

Original fault was V7 short and V8 red-plating as a result.

Hmmmm? I might replace all o/p valve holders with chassis mounted and point to point wire.

Just wondered if anyone else has experienced the same with this amp? I think it is about 14 years old?

[Enzo]

V7 and V8 bias in the sockets at -55v with tubes removed but draw 70 mA in test.

OK, but does that -55 stay that way with the tubes installed?

Replaced all bias circuitry as a matter of course including 1N4007.

Probably not necessary, especially if there is only one bias supply serving both sides of the amp. If the supply provides the -55v, it seems to be working.

This appears to come in 50w and 100w versions, yes? WHich do you have? Oh wait, duh, you have four power tubes, so I will assume 100w. In which case, you apparently have two sockets that do this? Have you tested them one socket at a time to see if the whole issue is coming from one rather than both on the offending side?

Wouldn't hurt to verify the cathodes are properly grounded, or if they have 1 ohm resistors or some equivalent sensing arrangement, are those intact and at value?

470R screens and 2K2 grids test fine.

What is voltage at screens?

O/P TX primary is nicely balanced at 18 Ohms each side of CT although no idea at the moment if it is failing in use so I might swap primary windings over although it might oscillate.

Frankly I wouldn't bother. Your idle currents are just that, idle. So if there were shorted turns, this isn't how they'd show up. And at 18 ohms, even a dead short wouldn't change things much.

Hmmmm? I might replace all o/p valve holders with chassis mounted and point to point wire

I recommend finding the actual problem before wholesale rewiring in hopes it might cure the problem. Knowing is better than hoping.

Just wondered if anyone else has experienced the same with this amp?

This sort of thing can happen to any amp, any brand.

[Simonsaysbang]

Hi Enzo, thanks for your reply.

Ok, the voltages V7-10 are as follows. I've re-biased down. Current draw on V7 and V8 is 32.8mA and 33.8mA. Current draw on V9 and V10 is 11mA and 10.3mA. All these voltages are with no pre-amp or splitter valves fitted and with coupling caps to 220K resistors lifted. Cathode grounds to chassis are good. Rechecked screen and grid resistor values to the actual sockets and they are correct.

1. 0
2. 11vac
3. 533
4. 533
5. -65
6. 534
7. 11vac
8. 0

[Enzo]

Just a thought, though this won't help the unbalance, with all those other tubes removed, the current draw off the B+ is reduced, so the voltage climbs. And that in turn affects the tube dissipation.

Tubes are voltage sensitive things, so if your circuit board were conductive, I would expect it to show up in voltage readings. However, you could put your meter probe on the board itself and see if there is stray voltage.

If the socket voltages are identical across the four, and the current anomaly does not follow the tubes, but stays with the sockets, I don't have a good suggestion at the moment.

[Simonsaysbang]

I agree Enzo.... This is weird. Just done the check for one tube at a time and the same results. I think I'll make up a little rig with four valve holders and connect to that and see how we go. I'm also going to discount pin 6 on the rig as it is only being used as board support on the amp o/p board although it is strapped to high voltage.

[Jazz P Bass]

Are there 1 ohm resistors on the output tube cathodes?
How are you reading the idle current to the four tubes?

Me, I would test one tube, in one socket, one at a time to identify the tube characteristics.
Then and only then I would move a known tube to the other sockets to see how it performs there.

If it is one pair of sockets that is off, I would start looking at resistors.
Screen R's come to mind as one item that will change the performance of a tube.

[Simonsaysbang]

Oh jeez.................................................... .......... I didn't see your reply Jazz but I have just figured it out now............. Doh! and doh again! I did swap the tubes over but I trusted the fitted TAD Bias-master sockets that I use to monitor idle current....... The meter is fine as I have now checked all inputs on there but one pair of Bias-master sockets are duff. Swap the Bias-master sockets from V9 and 10 to V7 ad V8 and low and behold the current drops back.... I'm off to bash my head against a wall............

[Enzo]

No, you eventually used troubleshooting to identify the problem. When all four sockets had the same numbers, that tends to point us outside the amp.

[Simonsaysbang]

True Enzo... Thinking outside the box as it were... A cup of tea helped! Cheers for your help Enzo and Jazz! Nice one