EL34 vs 6L6

[bal704]

I'm finishing up a SLO clone. I've wired it up for 6L6 (or 6V6). It's got 2 power tubes, with pin 8 to ground and a resistor between pin 1 and pin 5.

I've been running it for a couple of days, and it sounds great. Today, I decided to try some 6V6 tube in it, and as I"m removing the power tubes I installed, I noticed they're EL34's. I had them in 6L6 boxes, and mistakenly installed them.

My understanding of EL34's and 6L6's is that they are interchangeable as long as pin 1 and pin 8 are tied together. I'm not doing that in this case.

Why am I getting sound out of this amp? A really good sound btw.

[pompeiisneaks]

Pin 5 connected to Pin 1 means your input is at the last grid before the anode as well, albeit reduced due to the resistor. It's likely going to work but be a bit odd, and at high volumes may be problematic as you'll be really pushing a strong signal into the tube. It may also be dangerous. Much better to tie pins 1 and 8 together if you want to use EL34's. That's the correct expected operating use.

~Phil

[strelok]

I'm not 100% certain on this but as long as you're not over driving the output section and you're running fixed bias its not really a problem. Pin 1 is connected to the suppressor grid (G3) on El34's. Its purpose is to reflect secondary emissions from the plate. It does this so long as its potential is at 0 volts or less. If the amp is fixed bias there will be a negative voltage on it. However if you drive the output stage into overdrive then the signal may swing up to 0v or possibly a little higher depending on the grid clamping effect, in which case for that portion of the cycle you may get a bit of grid current and the suppressor grid probably isn't designed to handle that. Additionally for that portion of the cycle the tube is functioning effectively as a tetrode and you may get oscillations.

Its one of those things that may or may not be a problem at some point. But good practice would dictate that pin 1 and 8 be tied together thus ensuring g3 is always at the same potential as the cathode and will always serve its purpose properly. If you want to run EL34s I would strongly recommend tying pin 1 and 8 together and then just solder the grid stopper directly to pin 5 and the wire feeding it. If you don't like the idea of having the connection floating open up in the air you could put some heat shrink around it or you could put a terminal strip on the nearby screw mount for the tube socket and connect the resistor and wire to that.

[pompeiisneaks]

Yeah strelok's right, i didn't explain what I was saying very well but instead of supressing it becomes more like a beam tetrode instead of a pentode, and you'd have some of your signal on that pin 1 grid that's supposed to be negative. If you've got a place you can connect the pin5 resistor, say a terminal strip, that gives you back pin1 to jumper to 8 and you can use either tube type, (6v6,6l6,EL84)

~Phil

[bal704]

Thanks for the tips.

With my limited knowledge of this topic, I was surprised it worked at all.

[Stevem]

I Must say here that am concerned that you are building amps and working with deadly voltages and you did not notice the physical difference between a 6L6 and a 34!

[Leo_Gnardo]

With my limited knowledge of this topic, I was surprised it worked at all.

And I'm surprised it's still working. First time I swapped EL34's for 6L6 I also didn't make any changes on pin 1. The amp sounded great for about half an hour then the EL34's just plain died. Nice Siemens pair too. . So don't hesitate, ground your pin 1. If necessary just stand the grid stop resistor at pin 5 up on its mounting tab Marshall style, and you may want to change that resistor to 5K6 like Marshall or some nearby value.

Also take note some old Traynor amps run their EL34 pin 1 to raw negative bias supply voltage. There may be some advantage to that but I haven't been able to figure out what it might be.

[Ten Over]

Also take note some old Traynor amps run their EL34 pin 1 to raw negative bias supply voltage. There may be some advantage to that but I haven't been able to figure out what it might be.

If you have G3 hard wired to the negative bias supply voltage (not the wiper on the bias pot.), the EL34's won't red plate if the bias pot. fails or if G1 becomes disconnected. I have heard that a negative suppressor grid improves the performance of an EL34, but I have never verified that and I'm not sure how I would go about it.

[Stevem]

The other concideration when using 34s in a amp that was designed for 6l6s is the excess PI drive signal that the 34s don't need as they have a better gain factor .
This will greatly change the ratio of preamp distortion to output stage distortion and you can loose the added dynamic range that the 34s have due to them not being beam Tetrodes like the 6L6s!