Hammond AO39 conversion - 5879 preamp & Supro 6973

[bmccowan]

Hello,
Some of you helped me with an AO39 to Supro 24 Lite conversion a few months ago. It sounded great at higher volumes, rather boring at lower volumes. I think others have commented on this too. I just play at home, so I was underwhelmed. One of my favorite amps is a Les Paul GA-40 I rebuilt years ago. So I figured, why not? So I married the 5879 channel 1 of the GA-40 to the PI, tone stack, and power amp of the Supro, boosting the filter caps from 20/10/10 to 32/32/16. Wow! what a difference! The amp now sounds great at lower volumes. Lots of grit, tight bass. A definite keeper. I didn't sketch this out as its pretty much a matter of replacing the standard Supro preamp with the GA-40 preamp. But if others are interested I can patch together a schematic. I built it point to point. Have others done a similar amp - any of the HoSos use 6973 power tubes? I know there are many 5879 fans here. I have previously built a Z-28 style amp with two channels one EF86 and one 5879. I prefer the 5879; more grit, less chime. Anyways, I thought I'd just share my experience, as a successful experiment is a good thing.
Cheers

[tubenit]

I'd sure like to see a schematic.  Thanks for offering to share one.

With respect, Tubenit

[bmccowan]

Hi Tubenit,
I cannot refuse someone who has posted so many schematics! As an added bonus for me, it forced me to learn how to use JSchem this morning. I'm sure I have a lot more to learn with it.
I replaced one of the AO29 filter cap cans with an F&T 32/32; disconnected the other can, and installed a 16uf cap near the preamp grounds. It's very quiet.
If you see something that looks wrong; it probably is, so please give a shout out.

[sluckey]

Can you save as another format? Image, pdf, etc. THX

[bmccowan]

Sure

[tubenit]

I like that!  Cool design.  THANKS for sharing it! 

With respect, Tubenit

[bmccowan]

Hi,
I'm happy to do so. Like you, I like that 5879 tube. I built a Spitfire a while back that I never liked that much, even after cutting out the bright cap. I may try working a 5879 into that build and see what happens.
And Sluckey, I may even build it into a toaster oven!

[silverfox]

It seems to me the resistor capacitor network between v1, input and v2 phase inverter, could be reduced to a simpler form. What if a a Marshall tone stack was inserted instead? Also the existing said network, seems as though it has cut too much bass.
I don't have the knowledge to perform the analysis. Speculative only.

silverfox.

[bmccowan]

Hi,
Thanks for the comments. The circuit could easily be simplified by leaving out the "air switch" (I wonder who came up with that term) and replacing two 005 capacitors with one larger one. I think the only reason for two is to keep the air switch from making a popping sound. Other than that, it seems a pretty darn simple circuit to me. The way I sketched it may make it look more complex? Bass seems right to me, but I like a midrange tone, and this amp excels at that, as do the Valco 6973 amps when they are cranked more than my ears can take. On the Marshall TS - I also do not know how the sound would change. I make guesses but I pretty much work trial and error (sometimes many.) I have a few AO39 chassis, and may experiment with an 18W type circuit.