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4*6V6 Raa?

10/15/1998 6:47 AM
MArkus Torvinen
4*6V6 Raa?
I`ve been thinking of build a power amp using 4 6V6īs in class a, using fixed bias. What is recommended primary ohmages for O.T. What is bias voltage here?
10/15/1998 7:15 AM
Steve M.

Hi Markus,  
There was a post by John Arcc on the Vintage Guitar BBS stating that he had built an amp with 4 6V6s. I asked him what the primary impedence and plate voltages were and he responded that the Raa was about 5K ohms, and the plate voltage was around 445V. He couldn't remember the bias voltage, IIRC. The post should still be there if you want to check it out. Hope this helps.  
10/15/1998 8:34 AM
Kevin Timm

Really anywhere between 3 and5 k should be alright. Voltatges and biasing is totally up to you I personally wouldn't go much above 400 because it's really hard to find 6V6'S that are reliable above that.
10/15/1998 10:17 AM
J Fletcher
If you are going fixed bias, the B+ will have to be somewhere in the 300 volt area, otherwise you'll be way over the max plate dissipation spec. The bias voltage can be determined by experimentation, I'd guess somewhere in the region of -16 volts, but that's just a guess. I'd put in a trim pot so it can be adjusted. Adjust to the 1/2 way point between "cut-off", and "saturation". Measure the plate current draw to make sure that you're in a safe operating region. I'm not aware of any fixed bias class A amps, any that I've seen are cathode biased. Don't know why though....
10/15/1998 12:06 PM
J Fletcher could look in a tube manual, like I'm doing now, and you'd see this...Single Tube Class A1 Operation..Plate 315v..Screen 225v..Grid -13v..Load resistance 8k5 ohms..output 5.5 for 4 tubes in push pull (8k5 + 8k5)/2= 8k5 ohms (hope that math's right)...output power 22 watts...
10/15/1998 2:06 PM
Reid Kneeland

>so for 4 tubes in push pull (8k5 + 8k5)/2= 8k5 ohms (hope that math's right)...output power 22 watts...  
Extrapolating from single-ended to push-pull isn't that simple.  
First of all we're all assuming he meant a push-pull amplifier (from his reference to "Raa"). He *could* build a single-ended amp with four parallel 6V6 tubes (think "Champ on steroids"). We're also assuming pentode connections. Anyway... for a 4-tube 6V6 push-pull amp, 8.5K is too high. You were correct to divide by two (because there are two tubes on each side of the output), but you don't necessarily multiply by two in going from single-ended to push-pull. Also, the power output will more than double when you go to push-pull, so you can expect more than 22 watts.  
Here's a better way to approach this: look at typcial operation data for a *push-pull* class A amp (2 tubes), and divide the load resistance by 2 (for 4 tubes). Such data can be found right here at Based on that data, 4K is about right, with 285V on plates and screens, and you'll get about 28 Watts out. I wouldn't go higher on plate voltage if you care about tube life.  
10/15/1998 8:00 PM
J Fletcher
In my tube manuals, those specs at the www.firebottle are shown as Class AB1, not Class A. Also, the example I mentioned had the screens at considerably lower voltages than the plate, I think this raises the required load impedance for full output power. You're right, I did extrapolate from single tube specs, and am not too surprised if I'm off base. Wouldn't be the first time. Why doesn't one just multiply by two (the load impedance) to go from single ended to push pull , everything else being equal, ie plate,screen,grid voltages ? ....Jerry

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