Screen Drive Push Pull ideas

[whitelabrat]

Ok. Spending some time on Upgrade Island has cooled me off to the whole interstage transformer thing. I do intend to go off the ranch though. New idea.

6J6 --> Triode strapped 7W7's --> 2SK2700 (x4?) --> *AV5 (x4)

No tubes that cost more than a buck.

With PPP on the output I shouldn't need as much swing to get up to the OPT's 100w rating.

P.S. I notice that Antek has a new series of OPT's with a lot more taps on the primary and secondary. I apparently got the surplus older model, but it's good for this project.

Some related interesting reading: http://www.audiokarma.org/forums/sho...d.php?t=151142

[tubelab.com]

Some related interesting reading:

I don't have an audiokarma login so I can't see the schematics.

With PPP on the output I shouldn't need as much swing to get up to the OPT's 100w rating.

You have a 2022 ohm CT primary. To get 100 watts RMS you will need 635 volts P-P at each plate. This is ohms law regardless of how many tubes and what kind they are. Ohm will aslo tell us that the peak current will be about 1.25 AMPS, a bit much for one 6AV5, so two per side is about right.

I started out to build a big power screen driven amp to make use of the 400 watt Plitron toroidal OPT's that I got. The "universal driver board" was to be used to drive it. Another poster got me interested in morphing that board into a direct coupled driver that would be capable of driving a pair of 6L6GC's up to 30 watts or so. So now I have a driver board that uses a pair of 6SN7's and a pair of mosfets. I have connected it up to a bunch of different tubes just to see what would happen. I have extracted over 100 watts from a pair of 6L6GC without any glow using AB2. I have not tested to see if that board will put out enough drive voltage for screen drive. I belive that it will be OK.

I was testing several tubes and I tried the Electro Harmonix KT88 in triode mode. This sounded real nice and produced about 75 watts. I found some old WWII surplus tubes that looked cool. I managed to find 2 good ones, saw them crank out 100 watts in triode, and heard some magic coming out of my speakers. I am fixated on this design right now, so screen drive is on hold.

[whitelabrat]

Originally Posted by tubelab.com

...screen drive is on hold.

No hurries, no worries.

So I whooped up a schematic for entertainment sake. Half baked, but you get where I'm going with this. It's sort of a chimera of the Tubelab Universal and the PMillett 307a PP.

A photo of the current cast and crew is below too.

[vinylkid58]

Originally Posted by whitelabrat

So I whooped up a schematic for entertainment sake. Half baked, but you get where I'm going with this. It's sort of a chimera of the Tubelab Universal and the PMillett 307a PP.

I'll be watching your project with interest. I have a small stash of 6JM6's here, which coincidentally appear to be virtually identical to your 6GV5's, except for some of the internal connections.

Jeff

[whitelabrat]

I have to admit I have an alternate motive for this amp. I'm looking resurrect my bass playing and it would be fantastic if I had 2 x 100w driving my SWR cabs. I've got a nicely insulated basement so I can get loud. The Alembic needs action folks.

I've got a DI box that can provide the gain needed. I've never cared for tone controls. I can control tone with playing style.

[whitelabrat]

It's time to build this puppy just for my own amusement. I'm going to do just one channel first. I'll keep it simple by trying to stay as close to the original schematic at first and then I'll tweak.

Tubelab if you're reading, what kind of 20k 5w resistor did you use on the screens? Wirewound (inductive/non-inductive), MOX, film?

For the pots, I would guess that 1/2w is sane?

[kenpeter]

So, you screen drive guys find optimum G1 at cathode potential?
Or do you prefer G1 with a sand diode's drop above or below the cathode?
How high can you take G1 without ruining the G2 protective effect of its shadow?

[tubelab.com]

Tubelab if you're reading, what kind of 20k 5w resistor did you use on the screens?

You mean the resistors that are missing from your schematic (SG to ground or negative supply). I have used whetever that I had laying around. My latest experiments have been using some 10K 6.5 watt WW resistors that I got for Tubelab SE amps. I have also used the white Xicons that I use in the Simple SE, but they are hard to find in 20K (although I have some in another brand) I have also used some 20K 3 watt MOX parts from Mouser. All work good.

For the pots, again I used whatever I could find, mostly leftovers from Tubelab SE amps. The issue with most pots are their maximum voltage rating. I usually use additional resistors on each end of the pot to lower the voltage across the pot. I have been running the mosfets on a lower supply voltage to keep their dissipation down. You need resistors from grid to ground on the 7W7's. I'm just guessing, maybe 470K. Put a 10 ohm in the cathode of each output tube so you can measure the cathode current.

So, you screen drive guys find optimum G1 at cathode potential?

That has been the accepted norm, but my findings dont exactly concur. I find that a slight negative bias can improve distortion especially at low currents. Increasing the negative bias improves the distortion but causes another problem. More negative bias means more positive screen voltage is required to get the same tube current. At high power output levels, on positive signal peaks, the screen grig voltage will be high (maybe 250 volts) while the plate voltage will be very low. This is the recipie for a melted screen grid (or worse). Actually this isn't much of a problem with normal music, but it led to a nuclear event inside a 6BQ6 as I was extracting 100+ continuous watts out of a pair. There is a happy medium here and it sems to vary with tube type, but I haven't had the time to experiment lately.

[whitelabrat]

This thread has become something more like a blog. Regardless:

I thought this was an interesting read. It kinda follows what I was thinking with the inter stage transformer, except it uses a chip amp to drive the output stage. I'm not going in that direction, but it's something to think about.

I also found another matching big Tek power transformer cheap. Monoblocks.

I've been trying to put the wraps on a couple outstanding projects before diving into this one. Along the way I've discovered the likes of the 6HV5's and friends. Very interesting tube, but it would need at least 900v. Could it be both driver and output? Some experimentation is needed.

[tubelab.com]

Along the way I've discovered the likes of the 6HV5's and friends.

The 6HV5 has a lot of friends including the 6JD5 which is on the $1 list at ESRC. I have used it in a very simple true spud amp. Just the tube, an OPT and a power supply. No it didn't work very well and yes it takes a lot of voltage to get it to work.

These tubes have a Mu of 300. This makes them a better 811A than the 811A is. The 811A has some well known drawbacks like a high plate resistance, and the need to run in A2 for any reasonable plate voltage. In spite of these drawbacks there have been some well regarded amplifiers made with them. I would assume that the same techniques used with the 811A could be used with the 6HV5 family.

The high plate resistance almost guarantees the use of some type of feedback to lower the output impedance. After spending some time with the red board I have a new respect for schade type feedback. It makes a pentode output stage work like a triode. Will it make a triode look like a better triode?

It is safe to assume that these tubes wont be on the dollar menu once someone figures it out.