EF86 Woes

[Maxwell_E]

I've just finished final voltage checks on my Tode and everything is in great shape. Except the EF86 filaments wont come on. I double-checked the resistances all around, plugged it back in and powered on, still nothing. I thought maybe it was a dark tube, or the screen does a great job of hiding it but I'm not getting anything. I went ahead and did the Voltages checks and it came out great all around and I know the tube is getting a solid 6.1 because the EL84 is lighting up no problem. All solder joints are good and I just wanna make some noise. Is there anything obvious that I'm missing?

[Paul Birkeland]

If all the voltages are good, the EF86 is lighting.

The EF86's we supply don't offer much (if any) of a glow, but their operation sets the bias for the EL84, so you would really notice if the tube wasn't functioning!

For more light, look for a used mesh plate EF86 on eBay.

-PB

[Maxwell_E]

Ok, thank you, I was really worried. I haven't had any experience with EF86's before so I thought I was just missing something simple. I just remembered I gave you guys all my good Tele's, I should have held on to one of them

[willspeed]

I had the same worry - I thought I had a bad tube, since I saw 6.3V at the filament heater wiring, and the tube felt cold. Seems that these EF86s run pretty cool - at least compared to the EL84 which gets hot enough to feel thru the wood case!

[Paul Birkeland]

Yeah, the EF86 is current starved in this schematic, running several tenths of a milliamp at 20V on the plate and a little more on the screen.  If the tube is left to run for 10-20 minutes, it should be slightly warm, but even the EF86 in the Eros circuit runs fairly cool to the touch.

[willspeed]

Just out of curiosity, I know the EF86 is a Winged-C tube, but I couldn't tell what the EL84 was.

[Paul Birkeland]

The EL84's are Russian Military old stock (for now).