"2 Stage" PP 7591 Amp

[dhaen]

I was interested in Eli Duttman's suggestion in post 5 of the Looking to build a tube integrated thread, which he enlarged on in post10.

It got me thinking that I could replace a SS amp at home with such a thing; but I wanted more (what's new?)..:
25 watts
A cool running amp
Instant on from standby
Inactivity power-down to standby
These features are needed because of the expectations and bad habits engrained by using the SS amp for over 12 years.

Well, Eli has made some further suggestions and I thought I'd post my interpretation of them (errors accepted as mine). There are borrowings from others too.

I'm currently thinking about the PSU..
Here's the power amp bare bones:

[PB2]

The 7591 has some interesting characteristics, low heater requirements and higher than usual transconductance which reduces drive requirements. It is not widely known and therefore has a novelty factor, however it is very likely that it will become hard to get in the future.
The first consideration IMO is to select tubes and an OT that fit the requirements, preferably tubes that will be around for some time and at a reasonable cost.
Have people checked the going price for 7591s with all the talk about them?
The EL34 also has high transconductance and higher plate dissipation, it's only disadvantage is higher heater current which is a minor point. It is widely available and much less expensive.
I would not use the 7591 in new designs, the EL34 is a good choice because it will be produced for all those sockets already out there.

Pete B.

[alexistheo]

JJ is making the 7591. About $30/pair retail.

[anatech]

Hi Pete,
Check out the price for 7591EH. Not too bad.

While the EL34 is easy to drive, I'd rather use a 6L6GC or variant. It's heater is easier to run and this tube packs a wallop. I expect it to be around for a while too.

-Chris

[hacknet]

i was looking through the schematic and i was wondering if the mosfets provided any voltage gain. is it a solidstate equivilant of a cathode follower?

[dhaen]

The 7591's are in production again from 2 sources: JJ & EH They are even available in the UK for £10-16 each.

The source follower FET's do not have voltage gain, but act as current amplifiers to allow the driver stage 12AT7 to run with large value anode resistors; yes equvalent to cathode followers. Otherwise it would not be possible to drive the 7591 maximum grid leak of 300K in fixed bias without distortion.

[mr mojo]

dhaen,

Don't know if you've seen this before, but here's a link to my DIY PP 7591 amp:

First DIY amp pics-as promised!

I think the 7591 is a great tube-easy to drive, clean transparent sound-think SS with that great tube mid-range. I'm no expert by any stretch of the imagination, but if you've got a question about using this tube in a DIY amp that my feeble skills can answer, just give me a shout...

I use both the JJ and the EH tubes. Slightly different sound between each-JJ has the advantage of less adjustment of bias over time. Both have been reliable, good sounding tubes and run @ $60 US for a matched quad and are readily available.

Best,

mr mojo

[dhaen]

Mr Mojo,

I'm really impressed, as were all the viewers, and don't know how I managed to miss it first time round A splendid piece of workmanship

So you're saying the JJ's drift less... mmm noted, thanks

[PB2]

Originally posted by anatech
Hi Pete,
Check out the price for 7591EH. Not too bad.

While the EL34 is easy to drive, I'd rather use a 6L6GC or variant. It's heater is easier to run and this tube packs a wallop. I expect it to be around for a while too.

-Chris

Hi Chris,

Not to get into a debate over this but the first price I came up with was $17.80 each ($53 for 4) for EL34EH and $20.20 each for the 7591EH ($76 for 4). Just my opinion to go with the EL34s and also have more plate dissipation headroom. Could always build with enough heater power for other tubes.

I like the 6L6GC and 5881 also but they don't provide as much power in a ST70 but obvioulsy work well in the old VTL type arrangement which I notice is very similar to the Fender Twin Reverb. Fender is smart enough to use low value grid leak resistors on the outputs, VTLs were over 300K in some older models as I remember.

I notice 100K in the design in this thread, might be better at half that.

Pete B.

[PB2]

That's one nice amp Mr. Mojo, I'd want to run it upside down so I could see all the nice wiring you did!