Testing 807's.
 

Not sure if this should be in testing or here....

Been testing some 807's today to try and get a matched quad. Out of around 25 807's I found three faulty ones.

The first iffy one was a JAN - USA made 807 with an OC heater, it was NOS, still in the box. Tried resoldering the pins, then when that didn't work I took the base off. There was still no continuity between the htr wires. On closer inspection through a magnifying glass, I saw that one side of the htr wasn't even connected to the bonding wire, in fact the bonding wire was over to one side, not even close the htr. See pic - NC htr RHS with bonding wire about 1/4" further to the right almost touching G1 bonding wire.

The next two behaved oddly, showing way over the top Ia, 140mA + , when they should have been around 30mA. Control grid not connected maybe, no, that was ok on both. After more fiddling about and controlling Ia by reducing G2 voltage I noticed the G1 bias voltage drop to 0v if I turned up G2 voltage, so a short between G1 and G2.

However the other is a mystery, no shorts detected but the valve just runs away till the anode glowed red. Again the control grid worked ok, so maybe a G1 cathode short?

Andy.

Re: Testing 807's.
 

The runaway ones might be air?

The electrons from the cathode collide with the air molecules knocking off electrons and creating ions that migrate to the grid. If there is sufficient gettering left to absorb the air they might recover if run hot for a while (not red!).

Re: Testing 807's.
 

Doesn't look holed, plenty of getter left and no whiteness.

A.

Re: Testing 807's.
 

I find on my AVO VCM MK4 that some 807s give randomly high anode current readings. I've always assumed that the reason is RF instability, but have never gone so far as to put a scope on them (VCM and scope live in separate sheds!). Presumably some feature of the internal VCM wiring encourages occasional oscillation, though why particularly on 807s I don't know.

I find that the general fix is to plug the valve into an actual amplifier and measure it in circuit.

Martin

Re: Testing 807's.
 

It doesn't appease the pain of a bad valve, but it is somewhat cathartic to 'see' the actual fault (and then wonder how it could have passed any form of basic testing).

I'm about to use an insulation resistance tester on a batch of diodes - that may be an easy way to identify leakage, although using a high grid leak in a standard test jig should be able to detect abnormal grid conduction.

Re: Testing 807's.
 

I found when testing the 807s for the amp I built, pictured, I came across several that gave unstable anode current readings in the valve tester but settled down and worked fine in use. The ones I used were bought from Russia. Out of 100, I only had one genuinely faulty one. Lucky, I guess!

Re: Testing 807's.
 

Andy, like other high slope valves, 807's are prone to parasitic oscillations, particularly in AVO VCM's. Hence seemingly high Ia readings. I always use a ferrite-beaded adaptor. And the Anode lead is also ferrited.

Regards, David

Re: Testing 807's.
 

WOW omegaman!!

I only use four at a time in PPP.

Joe

Re: Testing 807's.
 

Joe.
This one has 30!
1200 Watts....

Re: Testing 807's.
 

And that's just the heater power!

well, maybe with 813s..

David

Re: Testing 807's.
 

Those anode caps are great for hiding an anode stopper - just the ticket for 807's.

It took 17 valves for me to end up with a reasonable dual and a quad to idle at 460V/300V. I wanted the dual to both idle and mu match as it was going in to a pentode mode Williamson.

Re: Testing 807's.
 

The anode cap does have a ferrite in it, but I will stick the runaway valve in an amp and check for oscillation. My tester does have anti oscillation measures - ferrites on all valve base pins etc.

That's some amp Howard, did you do a write up and any chance of a look at the schematic? Looks an interesting amplifier. Finding reasonably matched valves is tricky. I matched two quad's, just about, on my homemade tester only on quiescent current, I haven't enough 807's to match to any tighter standard.

Regarding trying to match valves for a PP parallel OP stage, I did wonder about running the OP stage in a mixture of fixed and cathode bias to try and iron out any mismatch.

Andy.

Re: Testing 807's.
 

The concern I have with using ferrite beads/tubes in the top cap, or sleeving the wire going in to the top cap, is the part temperature could exceed 100C, and so the ferrite type would need to be confirmed as being ok.

Re: Testing 807's.
 

Andy,
I haven't particularly written anything up about it. Here's some details in brief.
The circuit is basically five Ampeg SVTs with the outputs paralleled into one transformer. There's five preamps and five phase splitters. Runs AB2. 700V on the anodes and 300V on the screens. The power supply is regulated and fully variable and is capable of 1kV at 3A. There's 3 massive IGBTs under the heatsink.
I used 22 Ohm 1W carbon resistors in each of the anode caps and with the mess of anode wiring on top, it's rock steady with no instability at all.
It drives four, 4X18 speaker cabs for bass on a Reggae sound system. Stand in front of those and it'll shake your spleen off its mountings....

Re: Testing 807's.
 

Interesting Howard. What sound system?

A.

Re: Testing 807's.
 

Andy,
If you need some more 807's, let me know.
Hamfair coming up next Saturday here.
Pretty cheap usually, if I pick up cases of tubes.

Re: Testing 807's.
 

Andy,
It's the Axis Sound System in Huddersfield.

Re: Testing 807's.
 

Ah, Axis, thanks Howard.

Andy.

Re: Testing 807's.
 

I'm up in Almondbury at the moment. Can't hear anything, so it must be off.

David