• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.


J Epstein

2002-02-08 7:24 pm
I've used this tube with fixed (battery) bias of 2.3V and plate voltages between 165 and 225.

Current comes out to about 30-35mA depending on the sample of tube you are using. This operating point sounds good to me. Applications are: as driver for a 2A3 loaded with a Lundahl 1668 plate choke, and in a line stage, loaded with a 5K:8 transformer.

The Munich guys have pointed out that 2.3 VBias is low for linestage use and they are probably right but the sound is good so I won't let it keep me up too many nights.


J Epstein

2002-02-08 7:24 pm
What value grid leak resistor?

With the linestage, I am using grid chokes (actually small line transformers wired with the windings in series.) With the power amp, I am using 18K but that is in parallel with a very low DC resistance coming from a transformer volume control so the effective DC resistance is only a few ohms here. I ran into a situation where 18K was too low a value for a certain preamp to feed, so I used something like 100K for that setup. The lower the better, but it depends what you are feeding it with how low you can go. A grid choke is ideal.

How did you calculate the effective load the plate choke presents?

Well, I didn't actually: first I had on hand a Lundahl interstage transformer and I tried it and it worked well. Then a friend was ordering from Lundahl a bunch of stuff and I piggybacked on his order and got the corresponding plate chokes.

The rp of the 6C45P is so low that you don't need a tremendously high inductance to load it: almost anything meant as a plate choke should do fine, I think.

I tend to use what I have on hand, at least at first. I really didn't expect the line stage to work, it's a bit extreme (high mu/huge step-down), so I just slapped it together. Then it turned out I liked it, so it just sort of stayed slapped.
I'm also coming from a transformer volume attenuator, but with 75K in the gridleak position. A lower value resistor to get a little more voltage on the grid is something I'll try. I just put a choke on the plate, 50h/250ma with quite nice results. Only dropped 7 volts so still 195V at the plate.
Lots of different opinions out there. Just reading a thread that says battery on cathode is worse sounding than usual cap/resistor and better to use on the grid. I'll have to go back to test that one out.
Thanks for your comments.