"2 Stage" PP 7591 Amp

[Eli Duttman]

The Miller effect does not apply to voltage followers. The transfer capacitance of the FET does matter, but being low Rp/high gm the 'T7 is well suited to drive it. Look here, where sokke had problems using the HIGH Rp EF86. Sokke's Thread A tip of the hat to cogsncogs.

There's a nice exposition available on MOSFET use. Read MOSFET "Follies". MOSFET "Follies"

[dhaen]

With a pair of 7591 you ought to do 20 W without getting too far from a true Class A bias. You'll be AB1, but it will be closer than most that claim the 'A'

Actualy Douglas I aim to be close to class B for cool running.

i was looking through the datasheet and the gate capacitance of a mosfet caught my eye. is the gate capacitance going to be problematic for the 12at7 to drive? does miller effect apply? thanks!

Hacknet, I'm sure gate capacitance would be an issue with my original choice of anode resistors. However there are changes afoot. Miller shouldn't be an issue with a 'follower.

R5 and R6 are way too big. A 1.5 KV. B+ rail is needed for 470 KOhms. Ib is 3 mA. per 'T7 section. 68 KOhm INDUCTIVE wirewound parts seem right. The FET buffer allows "rational" values for the B+ rail and load resistor.

Yes I can see that now..

Speaking of FET buffers, I see you selected the topology that works for an AB2 amp.

Ah, hadn't considered the difference. Class B is a whole new experience!

I was thinking in terms of the simpler DC coupled gate and cap. coupled source. With a DC coupled gate and cap. coupled source,

Will change to that now I realise the difference... although I LIKE grid current

Make the 7591 grid leak resistors 33 KOhms.

Well, with enough current to drive it, I might as well. This circiut should even work with gassy valves... Well, at least the bias drift will be less.
Thanks Eli

Drawing revision coming later today.

[PB2]

Hi Chris,

I mainly go solid state, but I learned on tube amps and have always felt that there should be a better way to do the driver stages so I explore them every 5 or 10 years. I got back to tubes this time in thinking about building a guitar amp with my son. Also looking at rebuilding a few old stereo tube amps.
The tube prices came from Parts Express but I've not bought any.

Pete

Originally posted by anatech
Hi Pete,
Where do you get yours?

-Chris

[anatech]

Hi Pete,
I have a similar comfort level with solid state. Trying to get them sounding smooth is a challenge I enjoy. Solid state can be used to advantage in tube circuits as well.

With tube gear, I have been lucky to pick up some chassis' over the years. I experiment with them. The Eico I rebuilt took me completely by surprise. I don't want to mess with it any more.

Funny you mentioned a guitar amp. I have to build or rebuild one for my daughter. Trout has done a lot of work there.

Anyhow, give New Sensor a try at some point. I have to import them, which is a pain. Costs me a lot more than it does you. The EH line is just a good, reliable brand of tube. You may like them. The signal tubes are very quiet too.

-Chris

[dhaen]

Amp circuit update:

Changes to:
12AT7 Anode loads
FET configuration

Not sure if this is what Eli meant....
If it is, what is the suggestion for current through FET? By my normal rule of thumb of Rsource = 1/4Iload I'd need 25mA. That's a lot of power.

[Eli Duttman]

John,

I've looked at the latest signal drawing revision.

C1 is too small. The 56 nF. value was chosen to protect the questionable O/P "iron" in El Cheapo. With decent O/P "iron", the high pass pole should be at about 17 Hz.

There are compromises in every design. It seems FET buffer drain current and 7591 grid leak resistor value are intertwined. A lower Id equates to less waste heat. Is an Id of approx. 15 mA. and a 47 KOhm 7591 grid leak resistor acceptable to you? If so, R7 and R8 are 24.9 KOhm Mills MRA-12s in series with 1.5 KOhm/1 W. Carbon parts. The Carbon parts are connected to the FETs to isolate them thermally from the NIWWs' heat.

BTW, 680 nF. coupling caps are "perfect" for 47 KOhm grid leak resistors, but they are a tad small for 33 KOhm GLRs.

[dhaen]

Eli,

C1 is too small. The 56 nF. value was chosen to protect the questionable O/P "iron" in El Cheapo. With decent O/P "iron", the high pass pole should be at about 17 Hz.

Ok I'll revise it.

Is an Id of approx. 15 mA. and a 47 KOhm 7591 grid leak resistor acceptable to you?

So 15mA across 400V+150V *4= 33W. Ouch! That's a bit high for a "cool running" amp. How about a class AB buffer? Or perhaps I don't need the full rail voltage for the buffer.

What is your proposal for the best OPT a-a impedance?

[dhaen]

I've split off the power supply post to here

[Eli Duttman]

John,

I'm beginning to see how much heat generation is an issue with you.

You could revert to the cap. coupled gate/DC coupled source topology that you originally put in the CAD drawing. You'd have the annoyance of constructing a bipolar medium voltage PSU for the FETs, but I^2R losses in resistors will come down. I suppose that B2 operation for occasional peaks might be OK. 0.1 mA. of g1 current very occasionally is probably OK. I don't think a "steady diet" of g1 current is good for 7591 longevity.

A 2nd option is to forego FET buffers completely and use 68 KOhm 7591 grid leak resistors. The approach has some open questions. Will there be enough open loop gain? Is 68 KOhms too large for fixed bias 7591s? Someone more astute than I am will have to answer the questions.

[Bandersnatch]

Helllllloooooo, not enough open loop gain? for the faux-pentode cascode, it is gm*plate-load. Take a 5687 and run it to 10 mA/V gm, and put 20k plate loads on it. ten to -2 times ten to 4 is 100, or specifically in this case 200. Drive only one grid, and single plate gain is still going to be 100x. Drop this by a third from the grid circuit load....and you still have a single phase gain of ~65!

Look at Merlin v2, on ART's Group Build Projects folder for the hybrid cascode:
http://audioroundtable.com/GroupBuild/Projects/

cheers,
Douglas