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i lost my bias ADJ. on my amp. i unhooked B+ and set it at-50v. when i put
tubes back in,the left one takes off to orange. i swaped tubes same thing.
i lost control of the trim pots. turning ADJ up/down do nothing.
shannon my son has photos/will send when he has time
thanks

audioray wrote:i lost my bias ADJ. on my amp. i unhooked B+ and set it at-50v. when i put
tubes back in,the left one takes off to orange. i swaped tubes same thing.
i lost control of the trim pots. turning ADJ up/down do nothing.
shannon my son has photos/will send when he has time
thanks

Hi Audioray,

Do you mean you swapped the tubes from the other monoblock or just from left to right (but the left one always starts thermal runaway)? Were you re-biasing with different tubes when this happened? To be honest, I don't recall offhand what the bias for KT88/6550's might be, and if switching from EL34s to KT88s, you might need to set the initial bias much lower, eg -60VDC.

Shannon

i was re-biasing sed el34 tubes and the left tube ran away. now i have no bias adj. could it be a resistor problem.

audioray wrote:i was re-biasing sed el34 tubes and the left tube ran away. now i have no bias adj. could it be a resistor problem.

Hi Audioray,

To confirm that the bias is working, test the bias voltage at the socket side of R17 and R18 (these are the 1K grid stoppers). Without the secondaries connected, you should be able to verify these and smooth operation of your adj pots. If all this looks good, then look at your left octal socket. Unfortunately, the currently available ceramic PCB ones are not the greatest, and see if pin 5 (the grid pin) is a little dodgy. Also, if the plate connection at pin 3 was dodgy, your screen would go thermal as well. Basically look to see if they are too wide to make good contact with the tube base pin.

Shannon

let me clarify
my eiclones were on top of my Zhorns speakers. i bumped into one of them.
amp hit floor. broked one tube el34. replaced broken tube/re biased. seems
to work fine. problem is other amp. left tube ran away[started to turn orange] shutdown fast.
reset -50volts at pin 5 so ok.changed to new tubes. Now i have no control of
the bias pots l/r. to set bias. i checked the things in your last post. im lost at
the moment.

audioray wrote:left tube ran away[started to turn orange] shutdown fast.
reset -50volts at pin 5 so ok.changed to new tubes. Now i have no control of
the bias pots l/r. to set bias. i checked the things in your last post. im lost at
the moment.

Hi Audioray,

So you still can measure -50V on both the left and right pin 5 (the grid) but the adjustment pots don't do anything? Or now you measure nothing on the left and right?

Shannon

shannon
i replaced the el34 sed with my 6ca7eh tubes. now i am conntrol again.
dont understand why el34 acted that way. i have bias adjustment again.

audioray wrote:shannon
i replaced the el34 sed with my 6ca7eh tubes. now i am conntrol again.
dont understand why el34 acted that way. i have bias adjustment again.

Hi Audioray,

I go back to my 'pin size' theory coupled with the sub-par ceramic sockets. Thermion has tried some from AES that he likes much better and they grab the pins more securely.

On a more eclectic idea, you could try it with the feedback disconnected to see if the tube is actually oscillating.

Shannon

Ditto. I've blown up a couple of caps because a tube was not drawing power due to a bad socket. It's can be hard to track down because the cap goes before you can do your measurements. Yellow_Light_Colorz_PDT_21

Almost as frustrating as creating mirror image mono amps and finding that the 2A3 grid is glowing on one side because you wired it backwards.

In addition to some of the sockets not gripping, sometimes a tube has a poorly soldered pin. You can look at the pins, and if it looks suspect, resolder it. Make sure you scrape it smooth with a knife afterward.