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A little help with 6v6 screen needed.

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lazzer408

lazzer408

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Joined 2010
#1
I need some advice on where to tie the screens on some 6V6GTs. I've seen schematics with it tied to ground, tied to plate, resistor to plate, and resistor to ground. The amplifier is push-pull. What are the advantages and disadvantages of the different methods?

Another quick question. I have a UTC output transformer from a single-ended PA amp that has taps labeled B+, 2.5k, 4k, 7k, 10k. Is there a way I can use this in a push-pull amp? Are any of the taps close to what I need? Could I place my B+ at 2.5k and use their B+ and 10k to my 6V6 plates? The transformer is rated at 10w which is what I need to drive the 8" speaker I have.

This amp will be used for guitar practice.
 
#3
Apply the plate supply in the normal fashion to the OT. (I'm assuming this is single ended) You have three other options as far as the screen grid goes. Firstly, do not ground it. The normal pentode operation of the 6V6 is if the screen gets it's own decoupled power supply node (a resistor and cap after the plate supply) which is generally called B+2.

Next, you could triode 'strap' the pentode by directly attaching the screen to the anode. This will reduce the over all power level and give a different tone. Lastly, ultra-linear is where a secondary tap is applied to screens (typical values are 40%). I have no idea how it will respond on your transformer though.

Easiest would the first option. It is also a typical guitar amp setup.
 
#4
RTF671 said:
Apply the plate supply in the normal fashion to the OT. (I'm assuming this is single ended) You have three other options as far as the screen grid goes. Firstly, do not ground it. The normal pentode operation of the 6V6 is if the screen gets it's own decoupled power supply node (a resistor and cap after the plate supply) which is generally called B+2.

Next, you could triode 'strap' the pentode by directly attaching the screen to the anode. This will reduce the over all power level and give a different tone. Lastly, ultra-linear is where a secondary tap is applied to screens (typical values are 40%). I have no idea how it will respond on your transformer though.

Easiest would the first option. It is also a typical guitar amp setup.
Click to expand...

I mentioned it's a push-pull amplifier. Can I tie both screens together with a single resistor to plate supply? I have a 6L6 amp that does it that way.

aardvarkash10 - Could I wire it up for testing or is the impedance to low? I do have a 100v line transformer but it's not very large. I've read about wiring them in ultralinear somewhere. That would give me a place to stick my screens.
 
#5
Sorry about that. I guess disregard that last post then. I skimmed your original post.

Now with that in mind, ideally a P/P OT is balanced. To get as close as possible in your setup you may wany to attach each plate to each end (10k and original B+ end which will be 0k) and the normal B+ will go in the middle (ish!) at the 4k line. With a mismatch like that you may be able to run different tubes in the power section. Also the PI may need adjustment or the power tube's bias (or anode voltage) to deal with the mismatch.

And yes you could supply the screen voltages from the plate line with a resistor although it may be noiser than if it had its own node. Make sure to make the resistor big enough to drop some voltage. In order for a pentode to be functioning as one it needs a screen less positive than the plate (but more than the control grid). Higher voltages maximizes power, which in your case you don't need.

There! Hope that's more helpful this time 🙂
 
#6
RTF671 said:
Sorry about that. I guess disregard that last post then. I skimmed your original post.

Now with that in mind, ideally a P/P OT is balanced. To get as close as possible in your setup you may wany to attach each plate to each end (10k and original B+ end which will be 0k) and the normal B+ will go in the middle (ish!) at the 4k line. With a mismatch like that you may be able to run different tubes in the power section. Also the PI may need adjustment or the power tube's bias (or anode voltage) to deal with the mismatch.

And yes you could supply the screen voltages from the plate line with a resistor although it may be noiser than if it had its own node. Make sure to make the resistor big enough to drop some voltage. In order for a pentode to be functioning as one it needs a screen less positive than the plate (but more than the control grid). Higher voltages maximizes power, which in your case you don't need.

There! Hope that's more helpful this time 🙂
Click to expand...

I just tested this transformer backwards, with an ac supply into the speaker tap. I have B+ to 2.5k = 26.5v, and 2.5K to 10k = 26.5v. The 2.5k tap seems to be the center. Is 2.5k enough for a pair of 6V6? Maybe not ideal but it should work fine for a practice amp? 😕

You guys will actually get to hear this one when it's done. I figured out how to build distortion with multiple stages. The next step was to add a power amp to it.
 
#7
hey dog! Umm, the opts are different - a pp is not gapped (the iron forms a complete magnetic circuit) while an se is gapped to stop it from saturating with dc current. The gap makes it a VERY inefficient pp opt, even if you can get it to balance.

I'd go with a cheap and popular edcor g series open-frame opt. Cheap as chips for you guys in the upper 48.
 
#10
My power transformer seems to have developed a short. Looks like I'll have to buy transformers if I want this to work... which I won't. So in the trash it goes unless I stumble across another transformer. 🙁 Anyone have an extra power transformer kicking around? Then, what are the odds it will fit my chassis that I JUST had bent out of aluminum. This is so disappointing. Seems to be my luck lately. Everything goes well and at the last moment -something- has to go wrong. Damn you Murphy! -shakes fist at the sky- 😡
 
#11
and the normal B+ will go in the middle (ish!) at the 4k line.
Click to expand...

The middle of a 10K transformer is 2.5K! The impedance ratio is the square of the turns ratio so 2.5K is the center tap of a 10K transformer. A 10K center tapped transformer is OK for a P-P 6V6 amp, in fact is it on the high side of "normal". The gap will not be a problem either, it just might kill some of the low frequencies, but the lowest note on a guitar is about 80Hz, so that is OK too. You generally can't use an ungapped P-P OPT in SE mode, but an SE OPT will work for P-P if it has the right windings. The only issue might be winding resistance since the higher impedance taps might have been wound with thinner wire. If the resistance measurements between the two halfs are close, try it.

Anyone have an extra power transformer kicking around?
Click to expand...

Maybe, what kind of 6V6's are you going to use, tube or SS rectifier.
 
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