I have Hammond monoblocks!

[GordonW]

Thanks to a fortuitous trade with Jcricket, I now have a pair of Hammond AO-44 reverb amps!

These amps use two 6GW8/ECL86 "combo" tubes... each tube has, in effect, a half 12AX7 and a 3/4 scale EL84 in one bottle. Rectifier is a 6CA4/EZ81.

Removed all the signal processing/compression circuitry (EEEEEWWW! A transistor! Yuck! Outta there! ), and rewired the inputs to basic power amp topology (220K to ground, .022uf cap, 470K to ground, input tube grid). Then, I recapped the amps, and "updated" the amp that Jcricket sent me to the same spec as the later-model one I already had (several cathode resistors, a triode plate resistor and the feedback resistor were different... it looks like Hammond reduced the gain and increased feedback at some point). During this, I upgraded the power supply (about doubling all the capacitances, and adding a 22 ohm 2w "buffer" resistor to the output of the 6CA4 to limit current spikes) and added a .01uf film bypass cap to the 220uf cathode cap on the output tubes (cathode bias).

This, realistically, is about the simplest push-pull topology I've ever seen. A grounded cathode stage feeding a split-load, feeding cathode-bias push-pull outputs. Monoblock amps, tube rectified, with only three tubes per amp! Both amps literally fit with the footprint of a sheet of notebook paper, together!

OK, cut to the chase- the sound? Magnificent. These amps have an extremely neutral, fast, character... they are very faithful to the original tonal balance ("color") of the original source material. If the music was recorded with a forward balance, you get a forward balance. If it was recorded with enhanced bass output, you get enhanced bass output. Pretty much nothing added, nothing taken away.

Of course, it's only like 10 watts per channel here... but if your speakers are reasonably efficient, that 10 watts goes a long way.

There's an old audiophile saying... "you have to get the first watt right". Well, these are DEFINITELY some amps that do THAT. Pretty amazing detail, for such a simple thing... that probably ain't coincidence, thinking about it...

I still have to put proper binding posts for outputs; once that's done, I'll see about getting some pictures. Looking forward to trying them with some big E-waves and some Tannoys at the shop!

Regards,
Gordon.

[Jcricket]

SOOPER COOL!!!!!!!!!!!!!!
Glad to see it went to such a good home. I would love to see them!
Also, I got first dibs if you ever want to let them go!

[GordonW]

Originally Posted by Jcricket

Also, I got first dibs if you ever want to let them go!

You may have to duke it out with BrassTeacher (more specifically, his wife) about that. They spent several hours listening to them last night (I took them over there, to see how they would tolerate running his KEF 104.2s... the answer was NO PROBLEM!)...



Regards,
Gordon.

[GordonW]

Whaddya know... a pic!



In that photo, they're hooked up to and quite competently running a pair of JBL L50s. PLENTY of power.

Regards,
Gordon.

[Jcricket]

Sahhweeet!

Thanks for the pics!!

[Stephen J]

Hmmmm...those look interesting. I'll have to keep my eyes open. I could have picked up a Hammond ''tuber'' a couple of weeks ago for $50.00 long drive and funds were low.

[spaceman]

Looks great, but kinda small. I'd try some Hammond, if I found some cheap & local.

[stanbp64]

Those look great! Are the OPT's original? Have you worked up a schematic you could post? Again, fine looking amps
Stan

[GordonW]

Everything on top of the deck, except the binding post plate, is original.

The outputs are Hammond AO24158-0... used on the AO-44 reverb amps, as well as the main amp in the L-series organs (where they are use with PP 6BQ5 output tubes).

Regards,
Gordon.

[GordonW]

Originally Posted by stanbp64

Have you worked up a schematic you could post?

Much of this is stock values for the later (1966 and later) AO44s. For earlier ones you have to change out some resistors and such...

(BTW: All caps that do not have a voltage rating listed, must be at least 400v. I used 630v.)

Regards,
Gordon.

[BrassTeacher]

Originally Posted by GordonW

You may have to duke it out with BrassTeacher (more specifically, his wife) about that. They spent several hours listening to them last night (I took them over there, to see how they would tolerate running his KEF 104.2s... the answer was NO PROBLEM!)...



Regards,
Gordon.

Yes, the little amps drove them well. Great detail, and SUPER quiet! And given my preamp, that little tube amp being super quiet is saying a LOT!

The little amps were more amazing the more I threw stuff at them that they shouldn't have been able to do. For instance, playing pipe organ stuff! In particular, E. Power Biggs' recording of Bach's Passacaglia and Fugue in C Minor. The amps did not shy away from any of the pedal notes at all, and had great clarity throughout their range. No, they didn't shake the walls, but they did play it well at a substantial volume!

[GordonW]

A few notes about the schematic...

- The input stage is all-new (up to the input of the first triode stage). The stock Hammond has some sort of weird transistor gain/compression/processing stuff... a bunch of yucky stuff like electrolytic caps, a transistor, some light bulb "compressors", and such. Great for "crunchy" organ sound, very bad for hi-fi. So, Occam's razor was applied... the simplest way to wire it was the better.

-The 22 ohm, 2 watt resistor in the power supply is new. That's so we can run bigger supply caps (a "buffer" for the rectifier tube, to prevent big current spikes).

- On mine, the values on the power supply caps aren't exactly as the schematic... I think the first two stages have a 30uf (original) in parallel with a 33uf(added), and the last two a 10uf (original) in parallel with 33uf (added). But, anything close to those values will work... it's not that critical, except for not going really any BIGGER on the first stage than 60-68uf, I'd think. OTOH, If you're just building the circuit from scratch, just use 50uf x 4 or so... four 47uf or 68uf caps would be fine, as long as they're at least 400v rated.

- As I mentioned, earlier model amps (like the one I got from Jcricket) have slightly different resistors... 220k on the plate and 2.7k on the cathode of the input stage (as opposed to 100k/3.3k), and an 82k feedback resistor (56K after revision)... they seem to have higher gain, and less feedback, with no compensation cap (68pf on the later models, across the input stage plate resistor). They also have a different bias resistor on the phase splitter- 2.7k as opposed to the 3.9k used on the later ones. I decided to standardize on the later-model design... still plenty of gain, with slightly more feedback and the compensation on the input, to insure resistance to oscillation. The amp is pretty much stable as a brick like this!

Regards,
Gordon.

[AudioSoul]

I am jealous, not just of the amps but of your ability to just tear into a tube amp like that and make it better. When I look at schematics like that my brain goes in the clouds.......

[clydeselsor]

Very, very nice!!

[Kegger]

Whats the voltage on the plate of the driver Gordon? And the overall B+?

You could probably do away with the cap between VA and Phase Splitter.
(direct couple those 2 stages)