Help - 807 Technical problem

[sonata149]

I've finished building an SE 807 circuit with 6SJ& as driver. Can someone please explain why on one of the 807s cathode I'm reading 64 volts while on the other it is spot on at 21v (with 400 ohms as cathode bias)? Anode and screen voltages are the same as for the other valve (around 310v).

In my fault-finding trials I (a) swopped both output and driver valves and (b) isolated the problematic 807 from its driver stage but I still get the same high voltage reading on the cathode.

One other bizarre feature is that if I touch the grid leak resistor (1K) with the DVM probe there is a hefty spark on the output transformer!

I hope I can get some feedback which will lead me to solve this problem.

Thanks and regards to all.

Joe A

[Original Burnedfingers]

Can you post a schematic?

[sonata149]

ee

[Original Burnedfingers]

I can't even see it

[sonata149]

Sorry burnedfingers! Somehow the image is too large to post. I reduced with bad effect. I sent you an email for your address so I can send to you via my email. as I'm still having problems, can you reply to my personal email. Thanks for your patience.

[Original Burnedfingers]

I'm having a bad day... haven't received an email yet either.

[Miles Prower]

Originally posted by sonata149
I've finished building an SE 807 circuit with 6SJ& as driver. Can someone please explain why on one of the 807s cathode I'm reading 64 volts while on the other it is spot on at 21v (with 400 ohms as cathode bias)? Anode and screen voltages are the same as for the other valve (around 310v).

In my fault-finding trials I (a) swopped both output and driver valves and (b) isolated the problematic 807 from its driver stage but I still get the same high voltage reading on the cathode.

This looks a lot like a case of a gassy 807. If there's 64V at the cathode with cathode bias, that means way too much current. If this isn't being caused by a leaky coupling capacitor (and there are some of those audiphool PiO capacitors out there that are quite leaky) then suspect gas. You might even be able to see a purple/blue glow filling the space between the anode and cathode. I had a gassy 807 that did the same thing, except this was with fixed bias, and there was no way to get the bias to settle down. Plate current just went up and up. Of course, just because you don't see that thyratron-like glowiness doesn't mean that gas isn't causing a problem there.

One other bizarre feature is that if I touch the grid leak resistor (1K) with the DVM probe there is a hefty spark on the output transformer!

I betcha you didn't have a load on that OPT. Touching the grid lead will send a pulse-like signal to the plate. If the OPT isn't loaded, that current spike will cause a tremendous voltage. The OPTs operate as current step-up transformers. If it doesn't have a load, it will still try to pull the same current, and since:

V= IR,

V= I(inf)= infV

Of course, it'll never see infinite voltage since something's sure to poof before it ever gets anywhere near that high. I never run any VT amp with an OPT without a test load. Even if the volume control is all the way down, you can't be sure you won't get an oscillation, followed shortly thereafter by:

[ray_moth]

It used to be common practice with tube amplifiers to connect a resitor of ~1k permanently across the OPT secondary, as a safeguard against accidental disconnection of the speakers.

[sonata149]

Hi again, this is proving to be my most frustrating project, tho' I've learned a few things - leaky capacitors and gassy tubes - TNX a lot burnedfingers and 'foxy' Miles Prower.

The circuit is a variation of JC Morrison's design here: http://tinpan.fortunecity.com/saints...er/6l6-se.gif. My valve complement is a 5B/255M (an 807 variant) driven by 6SJ7. Also as my OPT doesn't have a UL tap I'm feeding the screen from a 250V Zener string from the HT tap on the OPT. Screen and cathode voltages on the o/p valve measure OK.

Problem started when I tried to measure the voltage at the anode of the 'SJ7. Almost immediately there were sparks followed by small fire at the anode pin of the 255M valveholder. I switched off at once and tried again to measure the same voltage but this time on the 'SJ7 of the other channel. Same result only this time with more spectacular (!!) pyrotechnics inside the o/p valve.

The problem seems to be on one channel only and linked to the OPT transformer. BTW I always test the amplifier with the speakers connected to the OPT secondaries.

What do you think? -- Thanks!!

[Miles Prower]

^^^^

Would you have another site for the schemo? That link leads to one of those gawdawful Fortune City sites where I get a "page not found", and thier search feature is utterly useless.