el84 bias...How hot?

[Chuck H]

Supposedly a quote from the designer:

Instead of using an inverter to put the out of phase signal on the 2nd output tube, you just ground its input...

And then:

If you look at each input tube's input grid with a scope you'll see inverted input signals.

I think this guy's trying to play "Ken Fischer" on us!!! (What? Too soon?!?)

Ok... Going to look over your image now.

[g1]

So grid is grounded through a resistor. Then I guess you would see an inverted signal there.

[eschertron]

I've looked at a couple of self-split designs. What strikes me is how much they look like a LTP driving a tranny instead of a couple of resistor loads. Am I far off here?

[Chuck H]

You know, this is why two heads are better than one. When devising a way to achieve bias for barfoden I hadn't considered using individual resistors and then a tail. Eloquent and easier to dial in than my idea. Nice.

[Chuck H]

That seems right. The thing that nags me is the question of what happens when the power tubes are clipping?!? I suppose it comes down to how it sounds as long as it continues to operate within safe parameters.

[Dave H]

So grid is grounded through a resistor. Then I guess you would see an inverted signal there.

We need the schematic. If the grid is grounded through a resistor seeing signal on the grid implies there is grid current. I have no experience with self split circuits but wouldn’t it need to be running in class A? (Which is presumably why it’s running so hot)

[Chuck H]

We need the schematic. If the grid is grounded through a resistor seeing signal on the grid implies there is grid current. I have no experience with self split circuits but wouldn’t it need to be running in class A? (Which is presumably why it’s running so hot)

That's what "I" was saying. It needs to run that hot to keep tube one's cutoff from the positive swing of tube two. Even then the asymmetry of a clipped signal with OT kickbacks as a grid signal for the fed power tube acting within a loop and it should be a hot mess. Unless the power tubes are never driven into clipping!?! In which case it's a clean 2XEL84 class A power amp. And that's not going to make more than about 10W, MAYBE. Certainly not 15 as the moniker implies. Of course the designer probably figured "class A + 2XEL84's = 15W, like an AC15."

Like I also said, it depends on how it sounds. Maybe it does clip, even strangely and sounds good doing it. But the implication that one of the grids is grounded and you can read signal on it and that the power tubes are run in series has me thinking that the designer may have implemented this circuit because he thought it would be slick rather than because it was a good idea for a guitar amp. Still no monitors or I'd try to find some sound clips on YouTube.

[jazbo8]

There are few variations for the self-split push-pull output stage, more details can be found in RDH4. It operates in Class A only, so 15W output is doubtful as mentioned.

[Dave H]

That's what "I" was saying...

It’s ‘great minds think alike’ then or ‘fools never differ’

I’ve just tried it in LTSpice by hacking up an AC15 circuit I had and with a 91 ohm ‘tail’ it clips asymmetrically at 10V into 8 ohm or 6.25W. I‘d call it a ‘short tail pair’. If I make the tail really long (constant current sink) it clips symmetrically at just over 12V or 10W. The non driven grid is grounded via a resistor and there’s no signal on it as you’d expect.

[frus]

found the schematic

http://i809.photobucket.com/albums/z...3selfsplit.jpg

it works very similar to LTP, the copy of input signal appears on the cathode(s) (since there's no bypass capacitor), and if you wiggle the cathode of the secod tube while keeping the grid grounded, it's the same as wiggling the grid while keeping the cathode constant

in my experience (scoping and of course playing my 18W build), output stage with no bypass capacitor works and sounds way different than with one

[Chuck H]

found the schematic

http://i809.photobucket.com/albums/z...3selfsplit.jpg

it works very similar to LTP, the copy of input signal appears on the cathode(s) (since there's no bypass capacitor), and if you wiggle the cathode of the secod tube while keeping the grid grounded, it's the same as wiggling the grid while keeping the cathode constant

in my experience (scoping and of course playing my 18W build), output stage with no bypass capacitor works and sounds way different than with one

Well... You found "a" schematic. That's not the amp in question though as far as I can tell. I think we get how it works too. The question is how to best cool the bias and still keep the right coupling balance. And secondly if that's a good idea. And thirdly if this is even a good idea for a guitar amp.

My favorite so far is g-one's schem with the two cathode resistors on a tail. The way I see it that could be improved by bypassing the independent resistors and leaving the tail unbypassed. That would allow for maximum transfer, lower the tail value and ultimately an overall 'more bypassed' cathode circuit.

[g1]

For the record, that is not my schematic, I stole it off the web. The person who posted it said it was a circuit they "came by" so I wasn't sure if they drew it up either.
Here's the link, there is a sound clip also:
freestompboxes.org ? View topic - Self Inverting Push Pull EL84 Amp

[Chuck H]

Dammit, I need to get my @#$! monitors back! I can't listen to a thing on my poot for a month now!

I knew you didn't design the schem. Why would you? It's a questionable choice for a guitar amp circuit. I've never designed a self split power stage either! I was just mentioning that the design was the most relevant to the solution (with "MY" upgrades of course ).

[Dave H]

Well...The question is how to best cool the bias and still keep the right coupling balance.

Replace the tail resistor with a constant current sink. That will make it clip symetrically then adjust the constant current sink until the EL84 dissipation is down to 12W. LTSpice says its now a 6W amplifier. If you need more power turn up the current and use a fan on the EL84s.

Edit.
I’ve just had an idea. Make the variable current sink a front panel control and call it ‘Power’ then you have ‘A variable power class A guitar amplifier with fixed B+ voltage’. Can I patent that?

[jazbo8]

Yes the clipping characteristic can be improved with a CCS on the tail if that's what you want - but I suspect the self-split PP circuit was used on purpose to provide asymmetrical clipping - if you are into that sort of thing . But it is a bit hard to understand... how much is saved with 1 extra tube to do a proper PI? Is Kustom that hard up for money?