Quicksilver Bias Problem

[Original Burnedfingers]

Having intermittent bias problem with one of my 60watt 8417 converted mono block amps. With the bias set at 100mA hot it will sometimes change to 50-55mA. The bias pots were changed when I converted it a while back. A small hum is also heard from the speaker when this happens. I can turn it off wait 2-3 minutes and turn it back on and have it behave ok or it can rear its ugly head and change its bias again after its been on a while.

Any ideas?

Thanks
J

[audiobot]

Is one of the output tubes bad? That would cause loss of half of current and hum from resulting loss of 2nd harmonic cancellation of power supply ripple.

Michael

[Original Burnedfingers]

Michael

That is surely worth a try. I will switch the output tubes and see if the problem goes to the other amplifier and report my findings.

Joe

[audiobot]

Could be an intermittent solder joint in the amp circuit as well. Any crackle when it drops back?

M.

[GaryB]

I'm not sure I know whats meant by a converted monoblok. Does it still run 8417s or have you switched to another tube? If its still running 8417s, then stability problems are a well known phenomena and are caused by 8417 tubes that draw too much grid current. Certain 8417 tubes work well (Sylvanias are the ones I remember) while others cause the instability you mention. I think I remember GE's had this problem.
You can fix it by modifying the amp to be more tolerant of higher grid current. All you really need to do is lower the grid resistor and optionally to increase the value of the coupling caps to keep the same LF roll off. Details are documented on the Triode Electronics website, along with other possible mods.

http://www.triodeel.com/8417.htm

If its not running 8417s, then I'd try swapping tubes between channels and see if it moves with the tubes. Could be a leaky cap or a a bad solder connection as others have mentioned.

---Gary

[stormy]

Hi,
is 1 of the valves starting to glow red when this happens,
could be runaway on a bad valve.

if the amp is more stable when biasing neerer cutoff then I
think u need to get new valves.

just a guess



Regards stormy

[Original Burnedfingers]

GaryB

The amps are running KT88's now instead of the original 8417's. I still have enough 8417's but I save them for special listening

Stormy

No glowing red, both valves look normal.


audiobot

Michael, the problem followed the valves to the other amplifier. I installed another pair of KT88's and the problem was resolved. Never saw this problem before. Its one I will remember from now on.

Thanks to all.

Joe

[Charles Hansen]

The internal steel leads of the tube are soldered inside hollow brass pins on the tube base. Sometimes this connection is poor (intermittent) and you need to re-solder this connection. It just takes a minute or two.

[Charles Hansen]

Originally posted by burnedfingers
Never saw this problem before. Its one I will remember from now on.

Joe

I guess I wasn't too clear in my previous post. I've seen this happen before, and it is a bad solder joint inside the tube pin that is causing the problem. The internal steel lead exits through the bottom of the brass sleeve, and is trimmed flush after being soldered. Steel is not the easiest thing to solder, so it's not that uncommon to see a bad connection here.

Instead of throwing the tubes away, you can simply re-solder the connection. Since you probably don't know which pin is bad, add a little bit of solder to each one. Be sure to touch the tip of the iron only to the tip of the brass sleeve -- you want to avoid getting any solder on the side of the brass sleeve, where it would change the dimensions of the pins.

(This procedure only applies to tubes with an external base like most power tubes. Miniatures like the 12AX7 have a different system whereby the internal wire is the same piece of metal as the external pin.)

[Original Burnedfingers]

Thank you Charles I will give it a shot and see what happens.

Joe