7591 PP Amp Project

[SupLar]

Hello!

Greetings from (mostly) a EL34 lurker (I have posted a few times). I am a frequent reader and VERY appreciative for this great site.

Anyway, been working on a 7591 PP amp build using (mostly) salvaged parts (the transformers came out of an old Hammond organ, of course).

Quick question:

Screens:
I know there has been past posts regarding 7591s but I'm just verifying... data sheet shows screen connects at pins 4 and 8 so my understanding is this means both pins are externally connected, correct? 

Screen Resistors:
7591 amp schematics that I've looked at do not show screen resistors (I've read past posts on this subject too) and I'm not sure what to do here.  Seems like I should have them?  I will be around 375VDC B+... any suggestions for screen values and wattage?

I'm sure I will have other questions as I move forward...

Thanks!
Laurance

[sluckey]

I know there has been past posts regarding 7591s but I'm just verifying... data sheet shows screen connects at pins 4 and 8 so my understanding is this means both pins are externally connected, correct? 

My AO-63 amp has a jumper between pins 4 and 8 on both sockets. See pic...

     http://sluckeyamps.com/hammond_2/big_guts.jpg

7591 amp schematics that I've looked at do not show screen resistors (I've read past posts on this subject too) and I'm not sure what to do here.  Seems like I should have them?  I will be around 375VDC B+... any suggestions for screen values and wattage?

My AO-63 has no screen resistors. My screen voltage is 388V.

Ampeg did not use screen resistors on their amps either.

What are you building? Which Hammond is the donor? Here's my AO-63 conversion from last December...

     http://sluckeyamps.com/hammond_2/hammond2.htm

[SupLar]

Thank you for the reply!

I'm building a ~30 to 45W amp for a friend using the AX84 50W PP as a template - but substituting 7591s. I've made some minor circuit mods based on the parts I have on hand.  I will post my working schematic.

Not sure what the donor Hammond was, but it was not an AO-63. BTW, I love what you did on your conversion. Very nice indeed.

[eleventeen]

Any particular reason you want to use 7591's? They are reasonably harder to source than 6L6s and more expensive.

[SupLar]

Mostly because 7591s are something different (and I want to hear how they sound) and I've got 2 laying around. But your point about using 6L6s is very well taken and I might end up going that route for practicality - the OT I have is 6K/8R

The JJ, EH, and Tung Sol 7591 reissues seem to be priced fairly reasonable and the reviews I've read have been promising...

[SupLar]

Below is a link to my working schematic.

I finished wiring the sockets and installed the 7591s and fired up the amp. I'm not getting sound except for some parasitic oscillation when the amp is dimed.  The good news is no red plating or crazy voltage readings.

B+1 352VDC
B+2 350VDC
B+3 331 VDC
B+4 319 VDC
B+5 314 VDC

Right side 7591:
Plate 352 VDC
Screen 354 VDC

Left side 7591:
Plate 352 VDC
Screen 355 VDC

Grid 1 voltages are -16 VDC

As you will notice, the screen voltages are slightly higher than plates. I'm coming up with plate current of 28.57mA and dissipation at 10.19 watts on the right tube and 25.87mA/9.23 watts on the left (assuming I measured the values correctly).  It would be nice to be around 14 watts plate dissipation once I get the the amp working, but of course I have bigger fish to fry at the moment, namely figuring out why the amp isn't working.   

Link

{EDIT-- unsnarled PhotoBucket's ugly link syntax -PRR}

[Willabe]

Link's not working.   


 

[SupLar]

Maybe this link will work... if not I'll have to research how to do links again on the forum.


<a href="http://s1126.photobucket.com/user/SupLar/media/image.jpg1_zpspb259msk.jpg.html" target="_blank"><img src="http://i1126.photobucket.com/albums/l604/SupLar/image.jpg1_zpspb259msk.jpg" border="0" alt="7591 power section photo image.jpg1_zpspb259msk.jpg"/>[/url]

[galtjunk]

This looks interesting.
I have an amp pulled from a Lowrey organ.
It has 4 7951's and 2 output transformers so it looks like I could build two amps from the parts.

[kagliostro]

... if not I'll have to research how to do links again on the forum

Upload (for free) your image on http://imgur.com/

copy the image link, select the small icon under the Bold option icon (may be Mona Lisa)

and paste your link when asked

et voilà



Franco