6L6GC AB2 Amp

[kenpeter]

Bridge the cathodes of those 5965 with a cap???
Would be more or less differential above DC.

[chrish]

Once again, thanks for the input George.

I am particularly interested in your comments regarding the 6SL7/6SN7. I have been wanting to do an all octal amp and having monoblocks should give me the chassis space to do this. Also like the idea of not having to fiddle around soldering on those small 9 pin sockets! The thought of a glowing VR tube on the top is still tempting too! Can I fit 6 tubes on each chassis......

[tubelab.com]

I have been wanting to do an all octal amp...I am particularly interested in your comments regarding the 6SL7/6SN7.

OK, crack open the Book of Morgan (third edition), turn to page 478 and study! The Crystal Palace amplifier is an excellent starting point. If you look at my schematic, and at Morgan's schematic you will find some differences, and some similarities.

The concepts are similar an LTP feeding another LTP feeding a pair of followers. The cascaded LTP's insure good balance even at the highest frequencies, and the followers can stuff all the grid current into the output tubes that they can eat. I don't know why, but to me this combination just sounds good too.

It is not practical to direct couple all three stages without some really complex stacked power supplies (except in some screen drive applications) so there must be at least one set of coupling caps in the audio path. Morgan chooses to place the caps between the first and second stages, I usually place them between the second stage and the followers. I do this to prevent any thermal or aging related drift in the second stage from affecting the DC bias in the output stage. This is more important with sensitive tubes like the 7591. In applications (screen drive again) requiring maximum output voltage swing I put the caps in both places.

I prefer to use a mosfet for my followers, Morgan uses 6J5's. For tubes in the 6L6 familly either should work well. Morgan uses an LM334 for the first CCS and a discrete transistor circuit for the second CCS. I use 10M45's for both. The LM334 works well with 2 or 3 volts but is only good to 10 mA and 30 volts. The 10M45 needs 20 volts or so to work but can eat 450 volts. I use it because it works well, and is a "one size fits all" chip. Use whatever you like, or can get in Australia.

Remember a 6J5 is one half of a 6SN7. 6SN7's should give more than enough gain here so that 6SL7's are not needed. If you go to 9 pin tubes a 6CG7 / 6FQ7 is very similar to a 6SN7.

Now, it's Saturday afternoon and for the first time in months I am not at work and have nothing better to do, so its time to "make em GLOW".

[tubelab.com]

I have put together an amplifier that should be capable of finding the upper limits of a pair of 6L6GC's in AB2. It is not quite ready for power up yet, but there should be some real glow real soon.

[chrish]

You the man!

I hope that you have enjoyed your latest experiments! I have been following with interest and have "The Book of Jones" out on my desk.

As my job is a 'chauffeur de bus français' I will be in Singapore in a few days time and will be able to scrounge for parts. Two questions. First, Tubelab suggested (I think) a +/- 100 volt transformer supply for the mosfet source followers and for the bias. I am guessing that to set the bias, negligible current is required. Looking at 7591 data sheet, it is showing max signal screen current of 26.4mA for AB1 ultralinear. The 6L6 data sheet I have is showing about 20mA for AB1 tetrode, and 35mA AB2 (at 47 watts, way over my transformer spec). From memory, the small transformers I see up there are specified in VA. So, for around 26mA, I should conservatively have about 50mA? A 100-0-100 transformer would have to then be about 10VA?

Second question, as I will be shooting for a fully regulated B+ with maida style regulator, can I get away without a choke in the power supply? If not, I will scrounge for chokes as well...

Thanks everyone!

Chris

[GordonW]

Originally posted by Eli Duttman
Over on AA, Doug Piccard (Bandersnatch) asked for info. about the AcroSound TO-350 O/P trafo. IMO, that "iron" would stone cold KILL, when mated to 807s.

Just look at this set of conditions obtained from TDSL.
Class Va Vg2 Vg1 Ia Ig2 Ra S Rk Zout Pout THD
AB2 P/P 600 300 -30.0 30-100 2.5-10.5 6,400 80.0 3.5

Regulated 300 VDC applied to the tertiary winding allows for full bore, maxed out, performance in UL mode. It seems that not much in the way of GNFB would be needed, to obtain both low distortion and good damping.

I'm thinking of doing something pretty much identical to that, using Bogen DO-60 monoblocks... the OPTs in those have a pair of extra windings... taken together as a CT tertiary winding, it gives a good 25% UL winding. Not perfect UL ratio, but I should be able to get a pair of 6550/KT88s to make great power. About 620v on the plates, and maybe 350v on the screen windings. Even with the slightly mis-matched UL ratio, the inherent low distortion of the KT88 should out-perform the 807... at least in the IM distortion department, if nothing else.

I'm thinking about using the HK Citation II front end, with a 6SN7 cathode follower, direct-coupled to the grids of the output tubes... think of a Citation II front-end (driver/phase inverter) coupled to the Heathkit W7 back-end (cathode follower/output tubes/OPT). No sand here... but, I've got the example of the Heathkit W7, Ampeg SVT and other amps to give me good starting points on how to get the cathode follower working right...

Regards,
Gordon.

[tubelab.com]

This was too easy, and rather anti-climatic. I wired up the 6L6GC P-P class AB2 power amp using the driver board I built yesterday. I have adjustable power supplies on everything so I set the knobs to what I remembered to be the max specs for an RCA 6L6GC, but there is no way I am going to put my black plate RCA's into something that has never been tested, NOPE, CHINESE. I got out my old friends the Chinese Coke bottles that have been seen glowing brightly on this forum before. The tubes were wired in pentode mode. 400 volts on G2, 500 volts on the plate 6600 ohm load. I set the current to 50 mA per tube (25 watts).

I started cranking up the audio generator. The power flowed freely. I cranked the generator up until the distortion meter read 2%. At 2% distortion the output power was 75 watts. OK, run it into clipping. 90 watts. Clean clipping, no ringing or funny stuff. Backing the power down to the 5% distortion point yielded 85 watts with visible clipping present.

[tubelab.com]

This is the output signal at 75 watts.

[JoshK]

Which tubes did you use in the driver board in said experiment?

[tubelab.com]

I consulted the tube manual and found that the max screen grid spec in pentode mode is 450 volts, so I went there. It didn't help, in fact I think that I lost ground. The bias became touchy, power output didn't go up, so I shut it off.

I returned to 400 volts and played with the other knobs. I found that raising the bias current to 60mA per tube cleaned up the slight flatening that was visible on the bottom of the sine wave in the scope photo. Power at 2% improved to 80 watts, but the 5% level only went up to 87 watts. A full crank on the signal generator could make the analyzer read 104 watts but I am not sure how accurate it is when reading a square wave.

I also put the scope on the output tubes grid. The signal was swinging from -95 volts to +27 volts indicating the fact that the positive grid region was reached. We are well into AB2.

OK, I just had to take the obligatory picture with the lights turned off. We all want to see the glowing plates and melting grids, right?

Well, there aren't any. No GLOW, none, zip, nada, zilch! Maybe a little blue on the glass, but nothing in the red department. The red line seen in the picture is actually a reflection from the glowing heater. This thing ran at full power for at least 45 minutes, nothing got toasty except the load resistor.

I am sure that I could make red happen but the plate power supply I am using is capable of putting out 1.5 AMPS! It just eats tubes, and I would hate to kill these tubes they have served me well for 2 years, and I got them out of a Bandmaster that I retubed. I do have a box full of Chinese firecrackers (old KT88's) and some Penta EL34's, but that experiment will have to wait until I install a crummy OPT.

I have a self imposed rule of no high voltage experiments after 10PM especially when there is no one else at home, so all power is no shut off. I may have more time to tinker later in the week, or next weekend, but then I am away for 10 days, and I know that I will be behind again when I get back. I will try to draw up a schematic for what I am actually using, and then maybe some screen drive experiments.