Acrosound 7591A amp

[mr2racer]

Hello All,

Attached you will find a schematic for an Acrosound design that was updated by Keith Snook in England. I intend to build one using EH 7591A outputs instead of the 6L6. I have a pair of TO300's and the power transformer will come from a Fisher 800b. I'm going to use a SS rectifier instead of the tube. I want it to be an integrated amp so I am going to add an Aikido line amp using 6922's.

I know to do this I will need to change pin 8 to pin 5, pin 5 to pin 6, and if I'm not mistaken I need to connect pins 4 and 8 together?

The power supply will be easy. What I'm not sure of is the arrangement of the cathode resistors for the output. I found some EH7591A's from Jim McShane and they are reasonably well matched but I want to use individual fixed bias. Eli sent me a link to the Tung Sol tube data for the 7591A. Tung Sol specifies 400 volts on the plate with -20.5 volts for the bias for AB1 ultralinear connection.

If I substitute a 1k pot for R17 and then use a ten ohm resistor with the C6 capacitor as a bypass cap to ground for each output tube would that work?

[mr2racer]

Scratch that last sentence. Obviously the trim pot will go between the bias rectifier and grid 1#.

But I still need to know the values for R17 and C6 and how they would be grounded?

[Briney]

I've built this amp. Closer to the original schematic though. There is one important change to make and that is with the capacitors (c2,c3) and (c4,C5). I used a .047uf for c2 and c3. For C4 and c5 I used .47uf. It has something to do with Higgs Boson or something like that.
Sy explained it a while back and I forget the details.

Here's what mine looks like.

Sorry pic upload failed...

[Eli Duttman]

That Hafler/Keroes design has too much gain for use with 7591s. An important fact about 7591s is that small signal circuitry that works with EL84s and 6V6s also works with them. Fisher and Scott took full advantage of that fact and so can you.

I suggest an "El Cheapo Grande", but "lifting" a Fisher or Scott power section's small signal circuitry would be fine.

[mr2racer]

Thanks Eli,
I guess I'll match the 7591a's with either my Motorola 6V6 OT or the Sherwood S8000 OT. What do you think about Briney's suggestion to change the coupling caps to .47uf? Won't hurt the bass response?
Kevin