Self-inverting PPP Output

for the DIY ST35, the Dynakit and every other PP EL84

Self-inverting PPP Output

Postby 20to20 » Sun Sep 30, 2012 7:12 am

I came across an interesting article in a '61 EW describing a self-inverting 6BQ5 amp. Have been mulling over building a 6DZ7 PPP setup with an ST-70 chassis and outputs but with a different PT, driven by one 12BZ7.

This seems to be a variation on the LTP type driver circuit that sometimes (if not every time) needs a load resistor tweak on the grid driven half, to get it equally balanced.

Anyone tried something with self-inverting outputs?

TIA
Last edited by 20to20 on Sun Oct 14, 2012 9:07 pm, edited 1 time in total.
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Postby EWBrown » Sun Sep 30, 2012 2:03 pm

Look up the "OddWatt" self-inverting circuits, these have several references on this forum. The shared cathode "resistor" is an LM317 or an IXYS IXCP10M45S or similar CCS regulator device. With a simple shared cathode resistor, there would be current and voltage fluctuations, and that would resultin some nasty distortion products.

From my experience with paralleling the two pentodes in each channel of a 6DZ7 SE amp, the grid resistor has to be 150K or less, with 200K and above, they get very unstable and start to oscillate at RF frequencies., due to the relatively high combined transconductance. This would need an SRPP driver stage, perhaps 5695 would be good, or a 12AT7, with no bypass cap on the lower triode's cathode resistor.

For PSE operation, I used an SRPP 6CG7 driver, operating at 4-5 mA, with RK+RA = 1K, with very good results, and an 0.47 uF (0.33 uF would also be good) interstage coupling cap. The Self-inverting PP or PPP design would require about 2X the driving voltage to the top 6DZ7's control grid.

These particular dual power pentode tubes (6DZ7 and the similar 6DY7) were intended and designed for PP Pentode service, with a single tube, then the two control grids are driven 180 degrees out of phase, it may be better to connect them as so that each 6DZ7 is connected in PP, rather than in parallel, as you have them, but then, the downside of that would be that you would not have the option of ultra-linear or triode-strapped operation, due to the SGs being internally connected together . Seriously, four separate 6BQ5s / EL84s (or 6GK6s) would be a lot easier to work with, and a lot more circuit-flexible. 6DZ7s ar eharder to acqire,and generallya LOT more $) $) $) $) , too.

A 1K grid stopper for each control grid, and 100 to 220 ohms grid stopper for the screen grids is almost mandatory for stable operation.

HTH, hopefully this is not too confusing of an explanation, I could have probably written it alittle better :/ ;) (lol) .

/ed B
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Postby 20to20 » Sun Sep 30, 2012 7:06 pm

EWBrown wrote:Look up the "OddWatt" self-inverting circuits, these have several references on this forum. The shared cathode "resistor" is an LM317 or an IXYS IXCP10M45S or similar CCS regulator device.

With a simple shared cathode resistor, there would be current and voltage fluctuations, and that would resultin some nasty distortion products.


The way the author of the article (and circuit I assume) described it, specifically warning against bypassing the Rk, that the Rk was the source of the signal for the inverting tube. It seems to make sense that the feedback there would help reduce distortion. And a bit of gain, too, of course... He says the voltage swing at the cathode around the grounded grid is what makes the inverting tube function.

EWBrown wrote:From my experience with paralleling the two pentodes in each channel of a 6DZ7 SE amp, the grid resistor has to be 150K or less, with 200K and above, they get very unstable and start to oscillate at RF frequencies., due to the relatively high combined transconductance. This would need an SRPP driver stage, perhaps 5695 would be good, or a 12AT7, with no bypass cap on the lower triode's cathode resistor.


Your old posts alerted me to the 6DZ7's sensativity on the Rg. I saw the data sheet note after that. (Note to self... read entire data sheet if time allows. Note wierd stuff...)

EWBrown wrote:For PSE operation, I used an SRPP 6CG7 driver, operating at 4-5 mA, with RK+RA = 1K, with very good results, and an 0.47 uF (0.33 uF would also be good) interstage coupling cap. The Self-inverting PP or PPP design would require about 2X the driving voltage to the top 6DZ7's control grid.


The author's circuit used a 6C4 (1/2 12AU7 equivalent) to drive the single grid. Since the 6DZ7 grid needs a lower value Rg I thought a 12ax7 equivalent could handle it. I have some 12BZ7's I thought I'd give a shot because they are taller, therefore, look cooler. Ih be damned! I only have to use use ONE! AAHHHAAAHAHHAAAAAA! ( Mad scientist laugh, circa 1928...)

EWBrown wrote:These particular dual power pentode tubes (6DZ7 and the similar 6DY7) were intended and designed for PP Pentode service, with a single tube, then the two control grids are driven 180 degrees out of phase, it may be better to connect them as so that each 6DZ7 is connected in PP, rather than in parallel, as you have them, but then, the downside of that would be that you would not have the option of ultra-linear or triode-strapped operation, due to the SGs being internally connected together . Seriously, four separate 6BQ5s / EL84s (or 6GK6s) would be a lot easier to work with, and a lot more circuit-flexible. 6DZ7s ar eharder to acqire,and generallya LOT more $) $) $) $) , too.


I have a few. Mostly aquired for @$25 ea. delivered. If you consider they are two vintage 6BQ5's then I've gotten them for $12.50 per section.

EWBrown wrote:A 1K grid stopper for each control grid, and 100 to 220 ohms grid stopper for the screen grids is almost mandatory for stable operation.

HTH, hopefully this is not too confusing of an explanation, I could have probably written it alittle better :/ ;) (lol) .

/ed B


Grid stoppers noted!

Your posts are always worth the time to work through. I'm easily confused.

My main focus of the question above was to get comments on the idea of a load resistor in the top half plate circuit, if necessary, to find a balance with the inverted half's lower gain. Ala LTP driver circuits.

Edit: The author claims the output is well balanced so maybe the bottom line on this circuit is that whatever flows through the cathode of the grid driven half will also flow through the grounded grid half. And being push pull having some imbalance, if any, is a minor anomaly.
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