Very high plate voltage on 6l6

[allears]

I have built a small SE amp -12ax7 and 6l6- and it works although there is some distortion at high volume. The problem is it seems to have a ridiculously high voltage on the plate. The voltages shouldn't be that high , it's around 350 volts on the plate socket before I plug the valve in , but it goes off the scale ( 600 volts) of my meter when a valve is inserted..it has actually killed my meter ! I don't want to do the same to my next meter - is this a known effect of something , bad valve or opt? The meter was still reading normal screen voltages and preamp B+ voltages while this was happening before it died.

 

[JonSnell Electronic]

Sounds like it is oscillating at HF so not able to hear it just enough on the anode to destroy any DVM that has no protection.
6L6 will stand 600volts on the anode as long as the screen grid is held down at around 250 - 300v.
In short your meter is probably reading DC with HF on it.

 

[ArcticBrew]

Can you show us a schematic diagram of your circuit?

Steve

 

[allears]

Sounds like it is oscillating at HF so not able to hear it just enough on the anode to destroy any DVM that has no protection.
6L6 will stand 600volts on the anode as long as the screen grid is held down at around 250 - 300v.
In short your meter is probably reading DC with HF on it.

Thanks for the reply , I think you are probably right - I'll take it to a friend with a 'scope and check.

 

[kodabmx]

I don't see why it would oscillate with a cold 6L6... Do you mean it goes to 600V when it warms up or really as soon as it's inserted?

 

[JMFahey]

Many schematics have a warning, with an arrow pointing at the plate: "do not measure here"

Not kidding.

To check plate voltage, measure it at the other end of the OT primary,at the supply rail where there is NO Audio signal (hence not oscillation either):

In my early days (decades ago) I killed a couple Digital meters (new and expensive way back then) so I learnt the lesson.

WARNING: you may easily kill the scope also.

Use an attenuator from plate to ground: 900k:100k and measure from ground to the junction, then multiply readings by 10.

Make the 900k out of 9 * 100k in series, or at least 2 * 470K 1/W or 1W , not for dissipation but for voltage rating.

This is basically a safe DIY 10:1 high voltage probe.

I don't see why it would oscillate with a cold 6L6... Do you mean it goes to 600V when it warms up or really as soon as it's inserted?

he didn´t say that.

 

[kodabmx]

He said "when a valve is inserted"... That means to me the power is on, the B+ is 300V, the cold tube is inserted, the B+ is 600V immediately.

 

[6A3sUMMER]

A complete and accurate schematic is very valuable towards successful troubleshooting.

Any thing else is just like a blindfolded archer, who is rotated an unknown number of degrees, and then asked to hit the target.

The above are just my opinions, and my experience.

 

[56085]

Were you playing music when you measured the plate voltage?

 

[allears]

Hi , thanks for all the replies , as far as I can remember it was high as soon as I switched on , the meter blew after I'd been playing for a minute just to check that the amp was still working. I will draw up a schematic and check at the other end of the primary and use a divider..all this needs to wait till I get another meter in a few days , so I'll post my results then.

 

[56085]

The anode voltage of a power tube loaded with an output transformer will swing above B+, and at full power will swing to about 2 x B+. That's why I asked if you were playing music when measuring.

 

[JMFahey]

Yes; and even worse if amp oscillates because there we are dealing with high Q resonances, large value inductors, etc.

Think of a TV fly-back or a car ignition coil

 

[kodabmx]

Yup. George said he's measures 2kV peaks from one of his guitar amps I think... Fully clipping OD distortion though - you won't find this normally in a hi-fi amp. B+ was around 500V if I remember correctly. Inductive kick isn't it called? That's why you see freewheeling diodes across relay coils driven by transistors, right?