5E3x2 half power switch and CxRk questions

[Josh_B]

I have been running my homebuilt Mission 5E3x2 for over a year with a 270 ohm/22uf Cathode resistor setup. When Mr. Collins had this amp on his bench, he said it liked the GZ34 rectifier best, so that's what I use.
Yesterday, I decided to install a half power switch. I used two 10 watt 250 ohm resistors and 25uf caps to split the inside and outside pairs of 6V6s.
It sounds great at both settings and nothing is red plating. Heat doesn't seem to be an issue either. Plate voltage is still around 377 volts, but the tubes are idling at 38-40 ma.

Do I need stiffer resistors, or will this setup be OK? It seems a little hot, but I always heard that if nothing is red-plating, and it sounds good, it's probably ok.

[Chris / CMW amps]

Was the 270/22u also used for TWO tubes or for FOUR tubes?

(as a not native english speaker it's not totally clear)

If both posted values are used for TWO tubes than 250 ohms will bias somewhat hotter than 270 ohms.

Hope this does help a bit. Can't speak for Bruce, he's a great guy, maybe contact him (also)?

[Josh_B]

The 270/22uf was used for FOUR tubes.

Now, each set of two tubes has their own 250/25uf.

Yep, Bruce is a GREAT guy! Thanks in advance to everyone for advice and help!

[Chris / CMW amps]

Theor. seen: double the r-value and half the C-value when converting that from shared pairs to sperated pairs.

Try a 330 to 470 ohms value resistor 1st (one for each pair). Without knowing the voltage on the cathodes it seems kinda hot biased so to say, looking at your posted currents and voltages.

And yep, Bruce is a really nice guy!

[Josh_B]

Chris,
The Cathode voltage is about 21 volts on all four tubes. What should it be?

[tubeswell]

21 or 22V sounds about right, but it depends on the plate current. With cathode biased 6V6s, they can idle around 12W (14W for JJ6V6s) quite happily for a long time. If you run them higher, you won't get much use out of them before you have to replace them.

This is how I work out the plate dissipation

1) Without any signal running through the amp, measure the cathode voltage. Then divide the cathode voltage by the cathode resistance (you should measure the actual cathode resistance in Ohms here) to get the idle tube current. This will be the current for all the tubes that are hooked up to that cathode resistor. (divide that tube current by the number of tubes, to get the idle current per tube. Then subtract about 2mA - allowing for screen current - to get the idle plate current per tube).

2) Then subtract the cathode voltage from the plate voltage to get the plate-to-cathode voltage per tube. Now multiply the plate-to-cathode voltage by the idle plate current obtained in 1). That is the plate dissipation (in Watts)

[bnwitt]

Boy you must have been running those 4 6V6GT's really cold with one 270R resistor. A 150R would be more like it in a typical 4 6V6GT configuration.

270R per pair should be fine. follow tubeswell's instructions to determine the dissipation wattage per pair. For two tubes I might subtract 6ma for the screens though. Also, don't forget that when you divide by the number of tubes you're assuming that the tubes in the set are pulling evenly. So, still watch for red plating. For Cathode bias I like to set the tubes at about 90% of max.

[Josh_B]

With a GZ34, I get 15 watts - a 5U4 drops it to 14 watts.

So do I need to increase the resistors to 300, or maybe 500 to get down to 12 watts?

[bnwitt]

What is your plate voltage at pin 3 of each set of tubes?

Set1= ____VDC
Set2= ____VDC

What is your ohm reading across each cathode resistor with the amp unplugged and drained?

Rk1= ____ohms
Rk2= ____ohms

What is your voltage drop across each cathode resistor with zero signal?

Rk1= ____VDC
Rk2= ____VDC

[Josh_B]

Thanks very much! I will get the GZ34 settings tomorrow. (I'm on a 24-hour paramedic shift)


With a 5U4GB:

Set1= 19.6VDC
Set2= 19.6VDC

What is your ohm reading across each cathode resistor with the amp unplugged and drained?

Rk1= 247ohms
Rk2= 250ohms

What is your voltage drop across each cathode resistor with zero signal?

Rk1= 365VDC
Rk2= 362VDC

[bnwitt]

Well if all tubes are even you're running about 14.4 watts per. That's a little over max. You'll probably shorten your tube life a little with that. I'd probably go to 270R or 300R on the Rk's

[Josh_B]

Rk's of 300 and bypass caps of 10uf with the 5U4GB rectifier put it down to 12 watts. It sure sounds good to me!!
Is 12 watts a good goal for NOS GE 6V6GTAs?

[bnwitt]

Rk's of 300 and bypass caps of 10uf with the 5U4GB rectifier put it down to 12 watts. It sure sounds good to me!!
Is 12 watts a good goal for NOS GE 6V6GTAs?

Yes. The goal is to keep your tubes from red plating and to get a tone you love. It sounds like you have accomplished both. 12 watts is 85% of max which is plenty cool and if the tone sounds good then you’re done. You can literally set the tubes at max with cathode biasing unlike fixed biasing as the cathode bias method self adjusts to changing signal level. Fixed biased tubes do not self adjust and should be set at around 70% of max dissipation for each tube type at a zero signal input so there is head room for higher signal levels when they happen. I say it’s time to stop fiddling and start spanking the plank.

[Josh_B]

Swwwweeet! Thank you for all your help, everyone!

[chuckb]

Hey Josh_B, do you have a schem for that half power switch? I'm looking to put one on my 5e3.

Chuck.