Hammond M2 Repair & Restoration--help!

[6267]

I have an early hammond m2, I think it's a 1952 model. I got it free because it sounded very terrible. The actual tone generator and vibrato scanner work fine (signal sounds great disconnected from power amp and run to a different amp). The power amp on the other hand, seems like it underwent a very poor attempt to recap, as well as other unknown modifications. I am trying to reverse engineer some of this and get it back to stock. Some of the distortion and bad sound were being caused by a failing field coil speaker, which of course also doubles as a 1,000 ohm DCR choke for the power supply. When I connected a different 8-ohm speaker (leaving the field coil in circuit) I got rid of most of the distortion, but power level is clearly below 1w. All tubes test fine, 5u4 rectifier was replaced with an IEC mullard I found in a parts bin. I measure 408 volts dc at the 6v6s plates. If the field coil choke was failing, is this something I might see? Stock it's supposed to be around 260. This is with tubes removed. It is a fixed bias amp and I measure -6 volts at the 6v6s' grids (I am surprised they did not let all their smoke out, so to speak). Would it be right to suspect that this is a messed up power supply?

[drewl]

Does it have a switch with internal power resistors connected to it?

I've worked on a few and some have a switch that can shunt the output signal across some power resistors, thus your low output...

[6267]

I am assuming this is the same as the "volume loud/soft" switch on the console. The organ is vaguely audible when set to loud, and it is not audible at all set to soft. The power amp has clearly been monkeyed with, and I've tried different tubes as well as testing the ones it has. I'm mainly wondering if a failing choke would cause a major increase in B+ voltages––also, is there a way to wire replacement chokes with lower current ratings together to get a higher current rating (series, parallel?). The replacement chokes I have are 32H 40ma 540ohmdc, the speaker coil was 1000 ohmdc, unknown henries, and probably more than just 40ma.

[Albert Kreuzer]

Hi 6267,

it's the Ohms, not the Henrys
Wire up a 700 Ohm / 20W resistor in place of the field coil and use a speaker with a permanent magnet.
The resistance is important to establish the voltages in the power supply.
I used two 330 Ohm/10W in series on the bench, which worked fine. I guess two 1500 Ohm/10W in parallel would also work.

If you can find a 10H inductor with 700 Ohms DC resistance, now that would be perfect

HTH
Cheers,
Albert (still working on my '57 M3)

[6267]

Ok so the current rating of the choke does not matter in this application, just the DCR? According to my schematic, later m2s had a 700 ohm dcr, earlies had 1000 ohm dcr on the field coil. Mine is an early m2, and I have two 32H 40ma 1.5kv 540 ohm dcr chokes. Can I just series them and use them in place of the field coil (I don't have too many 1k 20w resistors around )

[Albert Kreuzer]

Yes, the current DOES matter. In older M2s like yours you have a 1000 Ohm speaker coil with 100V DC across it (later models have 700 Ohms / 85V).
So you have a DC current of 100mA, be it a resistor or a coil.
The bonus with a coil is additional hum filtering, but a resistor is good enough
(You could use 40 half-Watt resistors ).

Cheers,
Albert

[6267]

Ok, so first off, my voltages now look pretty good, although some of the preamp stages are around 75-80 instead of 90v at the plate (probably drifted resistors). Apparently milspec chokes understate their current ratings a bit. It's not hot or humming or showing signs of doing anything except be a heavy ancient piece of metal. As per schematic, I have -17v at the grids, and around 300v at the 6v6's plates. When I attach a 8 ohm load to the voice coil terminals, the amp happily sits there. When I attach an 8 ohm speaker, the 6v6s starts showing lots of plasma or electrons or some other blue glowing substance. I get no power out, less than 1 watt. I can hear very slightly any output I put in. I've previously tested all the tubes, but I am starting to wonder if it's a bad tube (unfortunately I don't have a 6sc7 sitting around to swap in, but I've swapped all the others). Should I suspect that there is something wrong with the phase inverter? It's a cathodyne/split load 12sn7. It seems like most of the amp is properly biased (+1.5 on the preamp cathodes except the cathodyne PI and the cathode follower for the vibrato scanner). The strange thing is that the entire preamp uses 1 cathode resistor! should I try to seperate out the cathode resistors for the preamp tubes (2x 12sj7, 1 6sc7)? Maybe one tube is drawing too much current?

[Enzo]

Scope the output when this occurs. if the difference in load matters then perhaps the amp is unstable and is going into RF oscillation.

[6267]

Someday I will have a scope...until then it's seat of the pants. I replaced the cathode resistor for the preamp section and the bypass cap–volume increased but is still very quiet. I think I'm missing voltages on one side of the 12sc7...I am guessing drifted carbon comps b/c this organ is 60 years old. It very well could be oscillations, I'm getting a distorted output right now. At least I have a working B+ supply and I think I'm getting somewhere. It's either lead dress and oscillations or a 12 cent resistor. There are some nasty solders in this thing from whoever did the cap job, components were put in parallel to acheive resistances. I am fairly sure the cathode bypass cap for the preamp stages was literally burned by the previous solderer. Some components were simply cut out and not unsoldered, so oscillations could come from the random leads in some places. I expect (or hope) that it's just some crappy components, and if I keep replacing suspects the amp will work again.

[6267]

It's alive! There was a drifted resistor in the cathodyne PI, but I replaced it and suddenly, sound output. The cathodyne was direct-coupled to a voltage gain stage, so somehow the cathodyne being messed up was sucking the anode voltage off of the previous tube (the cathode was hotter than the anode, and there was a strangely connected resistor that did not match the schematic...from anode to grid 550k?!?). It was even further confusing because the voltage gain stage was set as grid leak. So, long story short, direct-coupled grid-leak voltage gain and cathodyne pairs are very strange indeed! I think I need to fix the vibrato drive section next, but I am getting loads of gain and classic sounds out of the clean channel now.