6L6GC AB2 Amp

[rknize]

Originally posted by tubelab.com
I have a few 45's that I have collected over the years. Most are well used and not "grade A" quality.

Yes, mine are all well used as well. I have one pair that may be new and another that very strong...all Raytheon. The rest are various odds and ends from eBay in varying condition. The TSE seems to work fine with the weaker tubes, but they work in class A of course.

Could make a serious Dynaco-on-steroids with a circuit like this.

[JoshK]

My $.02 on power. If you are getting 75w out of a pair of KT88's, triode strapped in AB2, then there really isn't much point in going to pentode to extract more power unless you are driving B&W 801s or Apogees. 75w is plenty-o-power for more domestic situations even with inefficient 6.5" two ways.

[rknize]

The 35WPC that I get out of my ST-70 is PLENTY of power for my 97dB speakers. 75WPC would be pretty insane...but kinda fun too. Been looking for the right PP project to build on top of the crazy power supply I've partially built for an abandoned 100WPC tube amp....

[tubelab.com]

My $.02 on power. If you are getting 75w out of a pair of KT88's, triode strapped in AB2, then there really isn't much point in going to pentode to extract more power unless you are driving B&W 801s or Apogees.

That is my opinion also. I wanted to try pentode mode. It got me about 2 db more power which didn't sound that much louder (if at all) on my 87db Yamahas. Once that little 7 inch cone has reached its Xmax more power only makes more distortion and possibly dead speakers. The speakers in the living room have 15 inch woofers and are 96 db they start shaking the walls with a simple SE using this same pair of KT88's.

I have these Plitron OPT's that are rated for 400 watts at 20 Hz. I got them almost 2 years ago when someone mentioned them on this forum. I have visions of a multi kilowatt monster amp using a big bunch of sweep tubes, but a project like that will likely never get built or used. I have a perfectly good working 845SE amp (40 WPC) that I dont use because it is big, heavy, and puts out lots of heat. It seems like a waste to use 400 watt transformers to build a 75 watt amp, but it is a bigger waste to have them sitting in the box.

It has been about 2 years since I have built an amp, any amp. I am almost done with the Tubelab SE that I started 2 years ago. Once that is done I really want to build something, so this may be my chance. Once the Simple P-P boards get here I will be busy with them.

The original idea:

Eight 807's in Push Pull?

[SigloOne]

When do you think the Simple P-P boards will be in and available? I know that you mentioned that you still have to write a manual and such, but was just curious. Still jonesin to build one!

Also I was wondering if the Simple P-P boards could be used to build monoblocks. Just a thought.

-Steve

[tubelab.com]

When do you think the Simple P-P boards will be in and available?

The boards are scheduled to ship in about 2 weeks. Advanced has been known to ship early. I have a skeleton of the manual written but the time frame for completion is really up in the air.

The boards should arrive at about the same time we get our next pass IC chips back at work. The last chip led to 60+ hour work weeks for almost two months. If the boards arrive one weekend before the chips it could make a months difference in the time to get everything done.

I usually want to build at least one amp, and get a beginner to build one with the instruction manual before I start selling them. It really cuts down on the questions.

I already have two Simple P-P prototype boards up and running. I have not tried my usual mono block experiments with them yet. There are usually 3 ways to make a mono block out of a stereo amp. The eaisest way is simply to leave out one channel. I have done this with the Tubelab SE and the Simple SE, and I am sure that it would work on the Simple P-P. Another option is to parallel both channels for twice the power output. Some circuit changes could be used like wiring all 4 tubes up to one OPT. Another option is to use the amp as a two channel amp for bi-amping a single speaker.

[Salas]

Originally posted by tubelab.com
I think this is how I blasted the IRF820's. One of them blew when I switched ranged on the audio generator... I would try the 820's again except I inly have one left. I will get some more and some 10 or 12 volt zeners next time I order parts.

Maybe the addition of a diode also is even safer? Like with the classic Zener & diode protection combo for lateral Mosfet amplifiers?

[tubelab.com]

I think in the circuit that we are contemplating the addition of the 1N4148 diode may be a bad idea. The gate can go up to 10.7 volts positive with respect to the source before being clamped by the zener (assuming a 10 volt zener). The addition of the diode will allow the gate to be pulled negative far in excess of the gate source breakdown rating. This may not have been possible in the solid state circuit shown, but it is certainly possible here.

A single zener will clamp the gate to 10.7 volts in the positive direction and 0.7 volts in the negative direction. Since the fet should have been cut off once the gate has gone below 3 to 6 volts (depending on the fet), clamping the gate at -0.7 volts should be OK.

I have noticed that many mosfets have internal zeners. This is what I have been using. They use two zeners in series in opposing directions. This clamps the gate by the same amount in each direction. Since this is what the mosfet manufacturers do, this is probably the most correct solution. I would choose a zener voltage 3 or 4 volts below the gate source breakdown rating.

[SpreadSpectrum]

I use 15V zeners in opposing directions with FQPF2N90. I just copied what some of the MOSFET manufacturers do internally. Now that I think about it, a singe should work just fine since it really shouldn't go negative anyway.

[tubelab.com]

When do you think the Simple P-P boards will be in and available?.......The boards should arrive at about the same time we get our next pass IC chips back at work. The last chip led to 60+ hour work weeks for almost two months. If the boards arrive one weekend before the chips it could make a months difference in the time to get everything done.

Well. IBM won the race in the shipping early department. I was handed 900 IC's today and told to test them all. I think most of my Tubelab time is gone for the next few weeks. I have squeezed in a few experiments during the last few days. There was no major new discoveries, but here is the summary:

Attempts to drive several sweep tubes in conventional grid drive did not work near as well as the KT88's. Since I am out of power supplies I had to run the output tubes in triode mode. This puts the screen grid well above their ratings. Screen drive will require a board modification.

I have had no success with cathode feedback in push pull in previous experiments. CFB requires a well balanced secondary since uneven feedback that varies with frequency will increase the distortion, destroy frequency response or both. Granted most of my P-P OPT's are low priced bargains. Could these Plitrons be different? Yes, they are. Limited testing revealed a reduction in distortion at all frequencies and power levels in pentode mode. There is a considerable loss of power output too. Maximum power output was redused from 130 watts to 105 watts. I think the driver was running out of headroom. There was a similar reduction in power and distortion in triode mode too. No listening tests were performed.