Struggling With EL84 Bias Problem

[Cosmo]

I just completed a Tremolux clone based on the 6G9 EL84 circuit. Note this is uses EL84s, NOT 6L6s. The only mods I made were to add a MV, tube buffered parallel FX loop, and variable bias circuit. The amp is built from the 6G9 schematic, but with the three mentioned mods.

The Weber bias calculator says I should be at 21.1ma. The problem is that I can't get the bias up to even 1ma. It seems to want a voltage greater than my minimum -18, and that doesn't make sense to me. Now here's the squirrelly part: I had accidentally installed 1K cathode resistors, not 1R. With the wrong resistor I was able to set the bias.

I added probe jacks to each EL84 cathode and each cathode has a 1R/2W resistor to ground. The bias circuit is tapped off one side of the B+ and provides -18 to -40 VDC. The schematic calls for -22.5. Both the plate and screen voltages are 398VDC. The PT, OT, and choke are Mercury Magnetics.

I have removed the cathode resistor and tied directly to ground. I've tried a different sent of tubes. And I've tried staring at the amp for two days. None of those things have helped!

Any thoughts would be very much appreciated!

[Firestorm]

What kind of DMM? You are setting it to read millivolts, not milliamps right?

[Tillydog]

Note this is uses EL84s...

....The bias circuit is tapped off one side of the B+ and provides -18 to -40 VDC.

....Both the plate and screen voltages are 398VDC

The EL84s will need something like -12V to bias correctly.

I'm not an expert, but your plate / screen voltages sound awfully high to me for EL84s. I suppose the voltage will drop a bit once the bias current is flowing, but you might want to double-check.

Andy

[Cosmo]

I've tried both of my meters. One is a junk auto ranging Extech, and the other is a quality FieldPiece. No joy with either.

[Cosmo]

The EL84s will need something like -12V to bias correctly.

I'm not an expert, but your plate / screen voltages sound awfully high to me for EL84s. I suppose the voltage will drop a bit once the bias current is flowing, but you might want to double-check.

Andy

My reference is the Fender 6G9 schematic. It shows the plate/screen voltages at 398 and the bias at -22.

This is my second EL84 build so I have little to no experience with these tubes. The first ended up with a cathode bias. I'll set the -VBias mid-point at -15 and see how that works.

[Cosmo]

Problem solved. Thanks to your help!

The -VBias was too high. Once in the -12VDC range things settled down. I guess the 6G9 schematic with the -22.5 is not to be trusted with EL84s.

Thank you so much guys! You solved in minutes what I wasn't able to do in two days.

[Tillydog]

Glad it worked out

Andy

[tubeswell]

Hope you've got good quality EL84s in there if you're running close to 400V on the plates