Linear conchord restoration, too higher voltage on one power tube. Help Please!

[nedhogan]

Im currently restoring an old Linear Conchord. They where used in the UK as a poor mans Vox ac30. I have replaced many of the components and loaded her up with tubes. A 5w 470 ohm resistor on one of the power tubes keeps over heating due to a very high voltage present at that stage of the circuit. I cant figure out why though! I'll try and post the schematic and maybe someone can suggest something. Im wondering if the voltage could be coming through one of the dc shielding caps or something..... any suggestions welcome.

[Steve Conner]

Either that EL34 is worn out, or the 0.1uF coupling capacitor that feeds signal to its grid is leaky. Swap the EL34s and see if the problem follows the tube or stays with the socket.

The 0.1uF caps are very commonly leaky, though. The cure is replacement. The new ones should have a nice high voltage rating like 600V.

[nedhogan]

Thanks for the quick reply.
I hope that much voltage hasnt damaged the el34 because theyre brand new. I'll replace all the coupling caps and hope for sound the next time I turn it on!

[Enzo]

One test would be to remove the tube from the socket, and see if there is any DC voltage on pin 5. There should not be. If there is, then we have the leaky cap discussed above.

[nedhogan]

All coupling caps replaced and no more problems!..... Except maybe that the volume output is barly any higher than the input signal
Gonna have to do study the schematic and try and figure out what could be wrong, because nothing looks wrong inside the amp as far as I can see. The only things that are original are the resistors and the last 3 DC smoothing caps.
Was thinking I might wire some 12ax7s in anyway, rather than the ef86s.

[nedhogan]

Think Ill keep the EF86s, I'd heard wrongly that its hard to get distortion out of them. Also I've narrowed the problem down to V1 not working but it must be something to do with that socket because the tubes work fine when swapped. All the voltages are there... its wierd.
Definatley going to put a master volume on this amp though once its sorted. One like on an ac30 me thinks 500k pot between the two el34 inputs with the two 330ks uprated to 1Meg should do it. ?

[nedhogan]

This old thing sounds better than my ac30! Just had a band practice and it sounds so bluesy!

[walkman]

Ive one of the myself . I've reworked it to be like a Dr Z route 66 , but im still working on the tone stack. Sounds wonderful

[nedhogan]

Strangely when I put the new tone stack caps in I put the two bass caps (5nf and 50nf) in the wrong way round. It sounded great, when I realized I changed them round to find I didnt like the sound any more and swapped them back! By the look of it the wave form it is more like an ac30 tb when they are backwards, and the right way round it looks very hifi orientated. Give it a try. OR just put a new tone stack in it.

[nedhogan]

If you download the duncan tone stack calculator the "james" circuit is the same if you tweek the values and rearrange the components in the amp a little. Also it sounds better if you put a 20k res where the calc has the default 180k one.

[nedhogan]

Right can anyone suggest how to get more distortion out of the pre amp and more headroom from the PI because at the moment it gets plenty of dist but only from the PI.
I want V2 to distort a bit more and as far as the PI goes I dont understand them no matter how many times I read about them, if I lower its cathode res Rk will I get more headroom?

Here's a pic of what I've done so far...