6c45

[bcherry]

Used as a driver tube for a 300b, any opinions or experiences of setting the plate current for best sound? It will dissipate 7w and the data sheet shows 50ma at 200V. I have it at about 10ma which is just on the edge of it's linear operating range.
Brian Cherry

 

[J Epstein]

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

 

[bcherry]

Hi J,
Thanks. I'm also using a battery bias, 2.6V and 200V on the plate. What value grid leak resistor?
I used it to replace the 6SL7 and really like the sweet sound and grip it has. It's been said that it get a little grainy/glary at high plate currents.
How did you calculate the effective load the plate choke presents?
Brian

 

[J Epstein]

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.

 

[bcherry]

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.
Brian

 

[Antonio Tucci]

6C45P driver stage and low-pass filter

Is it possible to obtain a 12dB low-pass filter properly adjusting the characteristics of plate choke and/or those of cathode resistor in a driver stage employing a 6C45P (receiving an input signal of about 2-3 volt)?
If so, may that be utilized in a biamping systems ?