Traynor YBA-1A

[gui_tarzan]

Ok, back to work on this one after I verified the schematic from part to part, schem to actual amp. Upon testing with the power tubes in place I'm getting these voltages and the pre-amp voltages (no tubes installed) are really high.

According to the schematic the output tube voltages look right but even though the resistor values are all in line the voltages are not. Any ideas?

[Jazz P Bass]

Why would you expect the preamp voltages to be correct if there are not any preamp tubes installed?

Without any conducting tubes, there is not going to be much of a voltage drop.

[gui_tarzan]

I didn't figure they'd drop over two hundred volts, I figured it would burn out the tubes with voltage that high. The data sheet says max. plate voltage is 250v.

[g1]

Resistors in an open circuit (no current) will not drop any voltage. So with no tubes and no current flow, you will get all your same (or close) voltages as at the power tube sections. Once you have the preamp tubes in, there will be voltage dropped across the power supply resistors, and the preamp tube plate resistors. Plus, no one pays attention to that 250V max. rating, many preamp tubes are run much higher than that.

[gui_tarzan]

*sigh*

Yes, that makes perfect sense. I really hate asking stupid questions, especially since the answers are so obvious once I see them.

[g1]

Don't feel bad, much worse to give stupid answers .

[gui_tarzan]

I need to remember what Enzo said yesterday - "the whole circuit".

[Chuck H]

Also worse to NOT ask stupid questions (if you have them). So... Ask 'em if you got 'em.

Once you're up and running I think you'd do well to try a guitar through it (turned up as loud as possible ) I had a YBA-1a (actually still do but it's become a mod platform) and the stock amp sounded great cranked up.

[gui_tarzan]

Much better. My pre-amp voltages are close to the readings on the schematic with the tubes installed. Whew. Another lesson learned. BUT, I still have a problem with it starting to red-plate on V4 (the first 6CA7) and it stays with the socket. More investigation...

[Chuck H]

Check for bias voltage (I know you knew that) and maybe clean the socket plugs. Re tension if necessary.

[gui_tarzan]

V5 (the second 6CA7) - Plate to cathode = 46ma, 520v

V4 - Plate to cathode = 45ma, 521v

Bias voltage went up (or down) to -51v, it was -44v.

[Chuck H]

-51V on the grid is less likely to red plate than -44V. The more negative the grid, the "colder" the amp is biased.

[g1]

It went from -44 to -51 on V4 socket only? Then there was likely a bad connection to pin 5 of that socket somewhere that was either fixed or is intermittent.

[nevetslab]

The power tube sockets on the YBA-1A are of the 'Forked' type....two-point contact, vs the wrap-around style. I've never been a big fan of the Forked contacts, and due to the age on my YBA-1A, I replaced them soon after getting the amp up and running (before any of the re-build work was begun) so I had reliable contacts. If Traynor had changed to the wrap-around contacts, those are easy to re-tension, using tiny Jewler's or Wiha bladed screwdriver tip. To re-tension the Forked-style, you have to remove the contacts one by one, which means you have to unsolder the wire(s) to each in order to extract them, then re-tension & clean, re-install and re-wire.

The preamp tube sockets can be both cleaned and re-tensioned.

[gui_tarzan]

My thoughts exactly. When I looked at them last night, one of the sockets looked a little funky and it happens to be the one that's red-plating. I'll just throw a new pair of sockets in and try it again.