[Brett Buck]

The U70 I built a while back had a problem. It was working fine last time I saw it, and left instructions to check the bias every couple of weeks. Then it came back with "glowing red plate" syndrome and a funny sound in the right channel (where the glowing red tube was).

On further examination I noticed that the right channel bias pot was turned up much further than the left. Ah, I got it, replaced the tubes all around (had a fair bit of time and EH 6ca7 are reasonably cheap), turned the pots to about the same, fired it up. Fine. Let it warm up a while, and set the bias. Still fine. Set my VOM to record high and low voltage. As I turned to walk away and let it go for a while, I hit my knee on the table. Immediately it beeped at me. Showed about half a volt as the min voltage. Checked the bias voltage again, it was back to normal. So I tapped it, sure enough I could get the bias to almost drop to zero if I knocked on it horizontally. Then I wiggled the tubes, and sure enough the sockets were loose , screwing up the ground for the bias - since it's terminated at the tube socket.

    So I let it cool and removed the screws and slipped some sandpaper in between the "ears" on the sockets to sand off any oxidation and then screwed them back down tight. Set it back up, and now everything was pretty stable. But I got to thinking, and then took it back apart and ran a wire from the ground side of the bias resistor to the central ground point on each side. Now it doesn't care if the sockets are tight.

    I figure what happened was that the socket got a weak ground, and the bias was dutifully run up to 1.56V. Then it got jostled, and it grounded solidly, making the bias very high. And then wiping out the output tube.  By grounding it with a solid wire I think there's nothing in there that cares about the chassis ground any more and it should be more robust.

   Word to the wise.

     Brett

[trebejo]

Interesting... I got a glowing tube this week myself, and I was crushed. I'd heard about these tube amps and their issues.

There was no bizarre bias disparity before the glowing failure, but naturally I took off the bottom lid and looked at what you're describing. I don't think I have loose sockets. But I can't figure out what you connected to where. I see a resistor (160 ohm? It looks like brown-blue-red) going from a junction at the socket to Frank's input board. I take it that you connected a wire from that same junction to the chassis?

[avahifi]

There is a 1000 ohm resistor from Pin #5 of each output tube to the AVA audio board.

There is also a 15.6 ohm (or 16 ohm on more recent models) resistor going from Pin #1 and 8 on sockets V2 and V7, each to ground at the socket base.

This is the ground connection mentioned earlier.  If the hardware holding the tube sockets to the chassis are loose, this can provide a bad ground connection between the socket base and the chassis itself, resulting in faulty bias voltage readings.

On recent units the connection at the tube socket base is done with a toothed solder lug to provide a better long term reliable connection here.

There are no 160 ohm resistors used in the amplifier.

Regards,

Frank Van Alstine

[simon wagstaff]

"Interesting... I got a glowing tube this week myself, and I was crushed. I'd heard about these tube amps and their issues."

I will admit that perhaps there are "issues". I have had to replace tubes every few years and just recently had a rectifier tube go bad. Keep in mind that many of these U70's are based upon a chassis that just might be 10 years old or older, many home built from kits,  these things would happen. I am guessing that the current Ultravalve will be put together very solidly.

And in the end the results are definitely worth the effort!

[tgp06]

It would be helpful to the technically challenged among us if someone could provide before and after photos or diagrams to clarify this grounding modification- which wire is involved/added? where is the "central ground point" located? Most of the glowing tube issues I had went away with the installation of new tube sockets and checking all solder connections, but this change might eliminate those few that still occur in this ancient chassis.