Cathode bias for D style amps

[ampedup]

I have a Dlite style amp with 6v6's and the usual fixed bias. I saw a thred on Hoffman's site about cathode switching for power reduction(here's the thread: http://www.el34world.com/Forum/index.php?topic=4401.0)but you must have cathode bias. Some people there seem to really like this for attenuation. Any of you guys try cathode bias and how much does it change the sound of the amp?

[heisthl]

I've tried this on 2 different occasions and it's not as magical as it sounds. The tone loses girth similiar to using a master volume.

[ampedup]

So you did the thing where you disconnect one or both of the power tube cathode bypass caps? I've tried many different ways to keep the volume down and retain some tone, (power scaling,weber mass, a couple one watt amps, L pad)I was hoping this might be a viable solution. Oh well. Can anyone comment on cathode bias with a D amp?

[Structo]

I just installed this on a little 2 x 6V6 amp.

Like Henry said it is less that spectacular.
I was expecting a huge volume drop but it isn't there.

Remember that at half power you are only dropping 3 db.
But since I drilled a hole in the chassis, it stays.

This is the diagram I followed.

I don't think I would convert a D'Lite to cathode bias. It's the wrong amp for it IMHO.
BTW, I have a D'Lite 44.

[heisthl]

So you did the thing where you disconnect one or both of the power tube cathode bypass caps? I've tried many different ways to keep the volume down and retain some tone, (power scaling,weber mass, a couple one watt amps, L pad)I was hoping this might be a viable solution. Oh well. Can anyone comment on cathode bias with a D amp?

Cathode bias is not the way to go if you want it to sound like a Dumble. You could try the "poor man's" power scaling and add a large resistor (I like 470K) between B+3 and the PI plate resistor's common feed point (trimmer), it lowers the perceived power but does change the character of the amp in a retro kind of way.

[ampedup]

thanks for the replies, I think I will scrap this idea.
I recently put the OD trimmer on the back so I can fiddle with it but it doesn't seem to matter what combo of OD,trimmer, volume, OD level, the distortion kind of always sounds the same, sort of fizzy, I thought at higher master volume settings (which normally is too loud) it would go away. I've never been able to get that clear singing OD with feedback that the Dumble is so famous for. what kind of settings do you usually use for that.

[tubeswell]

Cathode bias isn't innately good or bad - it depends what you're after, and how else the amp is set up. If the amp is set up with an LTP inverter and some highish plate voltage (around 380-400), then cathode biasing sucks IMHO (tried it and didn't like it).

But cathode bias can sound really good if you lower the plate voltage (with 6V6s go to 340-350 ish) and then have the tubes biased to run flat out at idle (say with a 220R-250R cathode resistor). 2 x 6V6s PP going into 8k load resistance, and keeping the screens at about 30-50V below the plate voltage, with a driver and cathodyne inverter (like a tweed deluxe) can sound really good.

[talbany]

My 2 cents worth... Cathode biasing 6V's in a D-Style amp is pretty meaningless......A pair of 6550's or KT88's cathode biased is in my opinion worth a switch....


Tony VVT

[ontariomaximus]

I have good luck with 6V6 cathode bias D style amps. The one mentioned is based on the HRM minimalist. It's a comment from a customer:

"Anyway, I just wanted to let you know that I'm absolutely thrilled
with the amp you built me - it's an absolute joy to play! You
totally nailed what I had in mind, and then some. While I had
originally intended this amp to be used primarily for cutting leads in
my home studio, I brought it to the "big studio" to show off to my
engineer and we've been using it for all manner of work on my record
since. Of course the first thing we did was jack my strat straight in
and overdrive it, but the real surprise was the clean tone. I never
meant to run this amp clean, but I was pretty damn proud when my
DRUMMER piped in with "that thing sounds better than the black one".
The "black one" being drummer speak for one of the army of blackface
fenders we had been using for prior cleans.

This little monster is one of the most responsive amplifiers I've ever
worked with. No chord manglers need apply - you built a true guitar
player's amp - awesome pick dynamics, string definition and response
to the knobs on the strat. It's definitely got that "singing"
tonality that I was looking for when I asked you for a low-wattage
"dumble style". The rig I''m using pretty much exclusively right now
is the strat through a keeley comp and a fulltone full-drive into the
Phaez. Between the "mod" switch on the amp and the various boost and
whatnot on the fulltone, the amp runs clean to crunch to lead like
butter..."

Before I sent the amp to the guy I documented what I did, because I knew it was something special. The output stage was 6V6 cathode bias, approx 370VDC, 230 ohm cathode R, 1000uf bypass cap.

Just a small shred of evidence that cathode bias can be desirable. FWIW.

[Structo]

What effect does using a huge bypass cap have on cathode bias?

I usually see around 25-50uf there but have seen amps with much higher values.

[tubeswell]

What effect does using a huge bypass cap have on cathode bias?

I usually see around 25-50uf there but have seen amps with much higher values.

Very full sounding

[heisthl]

On 6L6 ouputs anything much over 50 makes it sound (and Feel) a lot like fixed bias.