Adding a Negative Feedback loop on a 5C1

[alparent]

I realy like the effect it as on a 5F1, can it be done on a 5C1?
Any info would really help. Very green in tube amp building.

[loudthud]

The pentode preamp tube limits the options. That leaves feedback to the 6V6. Some Fenders use feedback to the power tube grid, but they are push pull amps IIRC and the Volume control presents a problem. The last option is feedback to the 6V6 cathode. Disconnect the cathode resistor from ground and connect it to the speaker. Don't try it unless you have an oscilloscope. It may oscillate no matter which way you change the phase.

[alparent]

Thanks for the info. But since this is my first amp build, I'll stick to the schematic for now

[Lauri]

Here's a schematic of a 5c1 style amp with adjustable negative feedback.
champ.sch.svg

Voltage to 6SJ7 screen grid is taken from 6V6 cathode and there's a 1k potentiometer in series with the cathode bypass cap to adjust the amount of negative feedback.

[pdf64]

Does that NFB arrangement enable the 6SJ7 to have sufficient gain for the application, with such a low screen grid voltage (ie dc coupled to 6V6 cathode)?

[Lauri]

Does that NFB arrangement enable the 6SJ7 to have sufficient gain for the application, with such a low screen grid voltage (ie dc coupled to 6V6 cathode)?

With 6V6 cathode fully bypassed the 6SJ7 gain will be approximately 150 (43.5dB).

[pdf64]

Is that measured or calculated?
If calculated, have you factored in the likelihood of reduced transconductance due to the low screen grid voltage?

[Lauri]

150 gain is my conservative estimate. In a similar circuit I measured almost 200 gain.

[alparent]

So I redraw your propose changes to the original 5C1 schematic (Just to make sure I understand)
Did I get it right?

[Lauri]

Yes you got it right. You'll also probably want to change the 250k plate resistor to 330k or 470k.

[Lauri]

If you have the amp built already you don't even have to remove the 2M resistor. Just remove the 0.05µ bypass cap, connect the screen to 6V6 cathode and add the 1k potentiometer.

[alparent]

was this ever built?

[Lauri]

Yes, I built this kind of an amp long time ago. The only difference was that it didn't have the adjustable NFB.

[Lauri]

I built a test circuit to make sure this thing works correctly and it does. I tried different values for 6SJ7 plate resistor, 270k sounded the best. With 300V plate voltage and 260V screen voltage for 6V6, output power was about 4W with 5k plate load and 3W with 10k plate load. 6SJ7 had 150 gain with 270k plate resistor and 260V supply voltage.

Here's a sound clip demonstrating the NFB control in action.
champnfbtest.ogg

[alparent]

Talk about going the extra step!
Thanks a lot for all this work.
So where you able to get good clean sound with the alot of NFB and cranking up the volume?

Can't believe you did this last night!
Never built an amp before, so for me this is remarkable.

Now I have to decide what speaker to use !?

[Richard]

I like how you threw that together like most people throw together a salad. Nice demo, too.

[Lauri]

So where you able to get good clean sound with the alot of NFB and cranking up the volume?

In the sound clip I made volume was fully cranked up and the 1k pot on the 6V6 cathode was used to adjust the distortion/volume.

Now I have to decide what speaker to use !?

I would use at least 10" speaker. I've never heard a 6" or 8" speaker that sounded good.

[alparent]

I don't want to stray too far from the original tone.