4X6V6 Cathode Bias

[Bruce / Mission Amps]

So I put back the 1/2 watt 100 ohm resistor on the 6.3v CT. It got rid of a bit of hum and brought my voltage down some. I also switched from the 120v tap to the 125v tap which also lowered my plate voltage. Finally, I went with a 5U4GB which of course also lowered my plate voltage....

I think I'm ok with either rectifier. I kind of like the headroom/response with the GZ34 better...

The DC bias on the filament voltage should have ZERO effect on the plate voltage, so either I read that wrong, you are mistaken, it is wired wrong or you measured something incorrectly.
And, if you really want this amp to kill.. try keeping the 5U4GB for now but change the 2700 ohm screen resistor to around 1500 ohms and drop the cathode bias completely using a fixed bias setup instead.
Set the bias supply so the four power tubes are idling at around 9-10 watts each.
With fixed bias, you can probably run the GZ34.
The PT has a low voltage tap for a fixed bias supply.
All you need is one 1N4007, a 10K resistor, a 25K trim pot and a 68uF to 100uF 100v filter cap.
Short the cathodes of all the power tubes to chassis ground, or where ever the first filter cap is grounded.
Make sure you lift the two grounded 220K grid load resistor for the power tubes and apply the negative bias voltage there.
There are lots of other tonal tweaks to turn this into a great amp....

[Bruce / Mission Amps]

So I put back the 1/2 watt 100 ohm resistor on the 6.3v CT. It got rid of a bit of hum and brought my voltage down some. I also switched from the 120v tap to the 125v tap which also lowered my plate voltage. Finally, I went with a 5U4GB which of course also lowered my plate voltage....

I think I'm ok with either rectifier. I kind of like the headroom/response with the GZ34 better...

The DC bias on the filament voltage should have ZERO effect on the plate voltage, so either I read that wrong, you are mistaken, it is wired wrong or you measured something incorrectly.
And, if you really want this amp to kill.. try keeping the 5U4GB for now but change the 2700 ohm screen resistor to around 1500 ohms and drop the cathode bias completely using a fixed bias setup instead.
Set the bias supply so the four power tubes are idling at around 9-10 watts each.
With fixed bias, you can probably run the GZ34.
The PT has a low voltage tap for a fixed bias supply.
All you need is one 1N4007, a 10K resistor, a 25K trim pot and a 68uF to 100uF 100v filter cap.
Short the cathodes of all the power tubes to chassis ground, or where ever the first filter cap is grounded.
Make sure you lift the two grounded 220K grid load resistor for the power tubes and apply the negative bias voltage there.
There are lots of other tonal tweaks to turn this into a great amp....

[LarryLarry]

I thought it odd as well that the 100 ohm resistor on the filament CT would have any bearing on the plate voltage. I mainly put it back because it got rid of a bit (minor amt) of hum. I think it changed the plate voltage around 5v, I'd have to pull it again and measure to be sure.

I have considered going to a fixed bias but I'm worried that the amp will be too loud or too clean for what I'm using it. This is filling the volume/headroom gap between my 18 Watt or 5E3 and my JTM45 or JCM800 50w MV. In this capacity it is working in my Bad Co tribute band. I am curious though since I could get the wattage down that it would still work.

Bruce, can you elaborate a bit on what effect going to a fixed bias would have on overall volume/headroom/distortion given my needs for this amp?

Thanks!

[stokes]

That 100ohm from the heater CT to the cathode of your power tube should actually serve to reduce the hum from the filaments,it floats your CT ground at 24vdc above ground,I find this very effective and do it to all my cathode biased amps,although I find it is better to use the 2 100ohm resistors from each leg of the heater and disregard the CT.I dont see how removing this would raise your plate volts,tho.The 2 5E3's I built I just worked from a schem someone gave me years ago,it can be found on the Fender Amp Field Guide now.I have modded it many times and experimented with different mods over the years.I settled on the basic circuit with the Paul C mod to the PI and upped the filter caps to 100uf on the main and 50uf on the screen,the preamp caps I left the same values as the original on the advice of G.Weber.I have .02 coupling caps throughout and a 5V4 rectifier.On the 4x6v6 unit I ended up using one 500ohm adjustable wirewound Ohmite I forgot I had in one of my parts drawers,for all 4 tubes.I have them idling at about 13 watts.Although they are no longer "true" 5E3's,I am very happy with the tone and extra headroom I get.In the 2x6v6 model I have 1x12" EVM12L and the 4x6v6 has 2x 12" 50 watt Eminences that I dont know what the model is,but they have red cones and sound great,but I will eventually put 2 EV's in that as well,I have one,I will get another and replace them,the EV's are incredible in these amps.

[stokes]

Boy,I am one slow typist,you guys posted three replies while I was typing my last one.I guess my two finger typing method just dont cut it.

[LarryLarry]

Do you take the 2-100ohm artificial CT to a 6V6 cathode or straight to ground?

[Bruce / Mission Amps]

Do you take the 2-100ohm artificial CT to a 6V6 cathode or straight to ground?

Either, but biasing the filament supply with a small DC voltage is a very very old trick used in tube ham radio gear and old hi-fi stuff from the 40's 50's and 60's. This is the way ALL my amps have been built for the last 20 years and is the way the Mission 5E3 is and I also did this in the 5E3X2.
If you have a center tapped filament winding, it is a waste of a resistor and time to do anything but either ground it or, with a cathode biased PA, connect it to the cathode of the power tubes with a 47 to 100 ohm 1/2 watt resistor.

[LarryLarry]

Thanks Bruce. I'm thinking of doing the 1/2 power thing as well. I'll probably first do the dual 330 ohm 10 watt resistors and bypass them with 100uF/100v caps so I can pull 2 of the tubes and see if it's worthwhile putting it on a switch.

[LarryLarry]

Thanks for the clarification Bruce!

If I want to be able to pull 2 tubes for the 1/2 power thing and my OT primaries are going to the plates of V4 (tied to V3) and V6 (tied to V5), do I tie the cathodes of V3->V5 and V4->V6 and run a 330 ohm 10w bypassed with a 100uF/100v from each of the tied pairs?

Sorry 'bout the run-on-sentence...

[LarryLarry]

Bruce - I did convert it to a fixed bias and I definitely like it better! What other cool things can I do to this?

[LarryLarry]

Bruce - So I did convert my 5E3x2 to a fixed bias and I like it

You mentioned some other cool tweaks for this amp?? I tend to run the single tone control wide open. I'm wondering if I need a more robust tone control section...

[Bruce / Mission Amps]

Bruce - So I did convert my 5E3x2 to a fixed bias and I like it

You mentioned some other cool tweaks for this amp?? I tend to run the single tone control wide open. I'm wondering if I need a more robust tone control section...

The problem with the super simple tweed tone control is that you really can not turn the bass down... only make the amp more bright or less bright.
For a start, you should try changing cathode bypass caps in the preamp to limit the absolute gain at low freqs.

All my 5E3 kits come with my handmade eyelet board that has an extra eyelet in the preamp section so a builder could dump that 820 ohm||22uF combo on the cathodes of the first preamp tube .
Try splitting the two common cathodes and use two 1k5||22uF caps for cathode biasing and bypass, but use something like 2.2uF to 4.7uF max for the cathode bypass around the 1k5 resistors.
Reduce the coupling caps from the first preamp stage to something smaller like .022uF to .047uF.

[LarryLarry]

Thanks for the info Bruce, it is greatly appreciated

I was looking at the JCM800 layout and noticed some similarities in concept with your suggestions. I will do some experimentation!