Unusual output / bias configuration

[rathmann]

http://www.captain-foldback.com/Hamm...JR20_early.gif

I have one of these lying around that I got off of ebay a few years ago. It appears as though the entire amp is set up to amplify a balanced signal. Wacky. I had originally got it just for parts, but looking at it I wonder if it is unique and oddball enough to try to repurpose mostly as is for something useful.

That issue notwithstanding, looking at T2 and T4 and their pairs of 6v6s, can anyone briefly explain what's going on with the cathodes and output transformers? I've never seen this configuration before. Is it basically just fixed bias (cathode grounded through the special secondary winding) but with some feedback applied directly to the power tube cathodes?

I think the power transformer for this amp would be great for something roughly akin to an AC30 topology, but with 6BM8s. I'd like to try the open loop, high current thing, but I will probably have to dig up an OT or dismantle an older project that has a Matchless OT in it.

Any thoughts or comments regarding the ouput config on the Hammond AO-15 would be much appreciated!

Thanks

Steve

[trobbins]

As you deduce, the output stage is a typical fixed bias PP with fixed screen voltage - the additional transformer windings providing negative feedback to bring distortion down - organ amps are similar to hi-fi amps in the respect of aiming for low distortion. It's not to the level of feedback as per a McIntosh.

The way they generate their fixed bias is not typical - and would depend on operating current - I guess if the amp stayed in class A then it would be a pretty constant voltage.

Yes, the signal input appears to be balanced - I believe there were a few balanced hi-fi designs back in the early days.

The OT's are probably 10k PP, but it would be interesting to find out the difference between them - possibly the bass channel has less feedback to allow more output power rating.

It would take a bit to reconfigure the whole amp, if that was your aim. You may be able to push the output power level up by not using the cathode feedback windings, and the output voltage could be upped by doing a capacitor input power supply filter - but you may need to somewhat regulate the fixed bias from class AB variation. You could get a fixed bias from the typical capacitor input negative rectifier circuit, which would also raise your HT by another 19V, and reduce your parasitic loss.

Maggies used dual channels if you wanted to go that way.

Ciao, Tim

[TransLucid]

hammond amp

The input stage is balanced to implement the PP power amp that drives the reverb unit. The rest of the circuit looks to me to be single-ended. The second stage looks balanced, but it's really just two independent class A stages, one to drive the bass amp and the other to drive the treble amp. Apparently, the reverb unit has two outputs (or, maybe, one output drives both stages).

Those amps are built like trucks, so I'll bet the transformers are all excellent.

[Howlin' Mad Mac]

Yea, what TransLucid said is right. The input stage is balanced for use with a Hammond reverbration amplifier (I forget the reverb units' model #.) the other two sections are just PP Class A amps (around 20 watts a piece), 1 for the upper manual, 1 for the lower & yes old Hammond transformers are very, very heavy duty.

[J M Fahey]

So you have the unique opportunity of building a stereo guitar amplifier (with incredible reverb) to plug your stereo Gibson ES3x5 into.
Wow!!
Can't say I don't envy you.

[rathmann]

Cool. Thanks for the replies guys.

After studying the schem a bit more I'm going to have to stick with my initial observation that each of the output sections is amplifying a balanced signal. Inputs are 1 & 6 at P1 - go to the reverb but continue through the "mixing" resistor network joining the reverb return (from S1) and get split off and sent to V3 for the treble amp and V7/V8 for the bass amp. The outputs are indeed push-pull but if you look closely you'll notice that the stages prior the output tubes are not phase inverters. They're just equally configured gain stages that amplify the pair of already-out-of-phase signals.

Anyway, as cool as it is as a design concept it's just a little too oddball, I think, to restore or repurpose as a guitar amp. It's worth the $30 I paid just for the PT and chokes. The OTs might have a harder time to find new homes, but it's always good to have stuff lying around.

[trobbins]

In my view, if I had a chassis with valve bases and transformers all fitted and sized for each other, and with the output stage and power supply circuitry and distribution all in place, then I'd think I was at least 60-80% of the way to finished. Your amp is like a clean sheet for doing a special stereo effects amp - I know of one aussie amp that went the stereo path ( Goldentone4 ), and many Magnatone high end versions were 'stereo'.