The Zenith amp project is wrapping up, so time to plot the next move.
I have here what looks like a pair of twin Hammond H-AO-43-1 organ reverb / power amp chassis. Each unit has power, output and reverb transformers plus six sockets - 1X octal rect. plus 5X 9-pin sockets. Two of the latter are for EL84 / 6BQ5, with the remainder taking 2X 12AX7 plus one 12BH7. I believe the 12BH7 is part of the reverb amp / driver.
The web says folks have made guitar amps out of these, reusing the extant reverb xfmr with a (generic?) spring tank. That might be of interest, if I could play anything other than the stereo.. so for my money, they should become audio monoblocks. But that's not one conversion I've found, at least not so far - but it cannot be uncharted territory.
So what should be done with them? Pull the xfmrs for other chassis, or build on the stock chassis? I'm all for the latter, naturally. What say you all?
I have here what looks like a pair of twin Hammond H-AO-43-1 organ reverb / power amp chassis. Each unit has power, output and reverb transformers plus six sockets - 1X octal rect. plus 5X 9-pin sockets. Two of the latter are for EL84 / 6BQ5, with the remainder taking 2X 12AX7 plus one 12BH7. I believe the 12BH7 is part of the reverb amp / driver.
The web says folks have made guitar amps out of these, reusing the extant reverb xfmr with a (generic?) spring tank. That might be of interest, if I could play anything other than the stereo.. so for my money, they should become audio monoblocks. But that's not one conversion I've found, at least not so far - but it cannot be uncharted territory.
So what should be done with them? Pull the xfmrs for other chassis, or build on the stock chassis? I'm all for the latter, naturally. What say you all?
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Do you have the PSU schematic? I'd like a look.
See my remarks in this thread. Much of what I said there can apply here. A pair of 6П15П-EB (6p15p-ev) "finals" with a 0A2 (0D3/VR150 if B+ is SS or 6CA4/EZ81 rectified) for g2 B+ regulation would work out nicely.
See my remarks in this thread. Much of what I said there can apply here. A pair of 6П15П-EB (6p15p-ev) "finals" with a 0A2 (0D3/VR150 if B+ is SS or 6CA4/EZ81 rectified) for g2 B+ regulation would work out nicely.
While I most definitely appreciate your sentiment - are there seriously multiple L-100 (or compatible) organ consoles out there, languishing in some dark & dusty corner for lack of a reverb / amp chassis?
I'm always willing to listen, but it just doesn't seem all that likely.. Meanwhile, +I for one+ would like to prevent these nice** old parts from ending up in a heap of scrap metal.
Btw. the part nos. on the transformers are as follows: output: A0 24158-0 / power A0 24157-1. There is also what might be a small choke on each chassis, with part nos. A0 24159-0. All have date codes from mid-1965 to mid-1966.
(** Why I am asking)
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All I have is what I've shown. The PSU schematic seems to be less common than the signal portion. If I turn one up, or decide to draw one, I'll be more than happy to post it for you. It must exist in some form, somewhere, though.
Again, I'm not looking for a slugfest, here - but if there's no established demand for these pieces, yet you say they ought not be torn up & re-purposed for hi-fi use, then what's left for them? It's like you're suggesting they should spend another decade or two in the attic until times & tastes change.
If that's not the case, then what +do+ you suggest be done with them at this late date? If there's a Hammond enthusiast out there who is just cringing at the thought of what might become of these pieces in my grubby hands, then they are encouraged to contact me. I'd be happy to work out an equitable trade / sale deal to save them from the shiny end of the iron.
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I don't know why it would be any more difficult than any other hi-fi conversion - as ever, it's always an option to strip out the works and rebuild fresh on the original chassis. But there's at least one 'extra' 9-pin tube socket, if the plan is to implement a typical old console amp circuit. The chassis has 5 total, 2 for the outputs and 3 for the driver.
So what's a clever 3-tube (per channel) drive circuit for the EL84 / 6BQ5? A single 12AX7 would do it all, but we can't leave those other sockets just sitting empty.. For starters, how about a proper (Williamson, balanced) phase splitter with a 12AU7 driven by a 12AX7 or AT7 AF amp? Use that remaining socket for a CCS for .. one of the stages? I'm also open to trying other types, such as the Euro pentodes (in triode mode) or one of the Russian types that seem to be in vogue.. but I've never worked with any of that stuff. Pretty much a triode-only guy, at least on the driver end.
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The problem with "classic" 2 tube driver setups, like Williamson and Mullard is excessive gain. Think in terms of a buffered, "unity" gain, control center and the 2 VRMS O/P of a "standard" CDP.
So, for a 2 Noval socket driver setup, consider an EF86 voltage amplifier DC coupled to 1/2 12BH7 "concertina" phase splitter, with 1 chassis using system 1 and the 2nd chassis using system 2. When the 12BH7s show signs of wear, criss-cross them between the 2 chassis, for doubled service life. A pentode voltage amplifier allows you to use 500 Kohm grid to ground resistors, without HF loss worries.
That in turn, allows you to use (provided short, low capacitance, cabling is employed) an unbuffered 50 Kohm passive control center.A "hole shrinker" fitted with a 7 pin mini socket fills an otherwise unoccupied Noval position. That socket takes a 0A2 gas discharge regulator for O/P tube g2 B+. Please keep in mind the fact that sockets correctly wired for the 6Π15Π (6p15p) accept, without incident, 6BQ5/EL84s.
BTW, the EF86 small signal pentode is very linear. AAMOF, quite a few triodes don't measure up to the EF86, in the linearity dept.
Funny how this stuff can go, but both of these chassis are in working order..
All of the PSU caps measured out fine, so I ran them up on the variac and they just work. Listened to both for a few hours this eve, in the background while I was up to other stuff. Turns out that you can run a line-level input straight into the 'WH' jack and that's about all it takes. I did remove a bit of junk (like Molex connectors, and the 12BH7 plate chokes) prior to use, but otherwise they are totally stock.
Opt coupling caps are fine, too. The 12BH7 tubes are also not present, as they are part of the reverb loop.. not something we really want for hi-fi use. So they run as-is with a 5U4 / 2x EL84 / 2x 12AX7. Neat.
All of the PSU caps measured out fine, so I ran them up on the variac and they just work. Listened to both for a few hours this eve, in the background while I was up to other stuff. Turns out that you can run a line-level input straight into the 'WH' jack and that's about all it takes. I did remove a bit of junk (like Molex connectors, and the 12BH7 plate chokes) prior to use, but otherwise they are totally stock.
Opt coupling caps are fine, too. The 12BH7 tubes are also not present, as they are part of the reverb loop.. not something we really want for hi-fi use. So they run as-is with a 5U4 / 2x EL84 / 2x 12AX7. Neat.
Legendre,
be aware that the frequency range of the organ's tone generator ends at ~ 5 to 6 kHz. So that is what you could expect from the given output transformers. Not really high fidelity stuff, is it?
For a first try, I'd disconnect anything left from V8's grid (pin 7), put in a 1M grid bleeding resistor and connect a line level source via a 47nF capacitor. This would give you a first impression of it's sonical qualities. If you're satisfied, I'd go with the Mullard 5-10 circuitry, by adding a EF86, as yet suggested before. If you're not, what I really suspect, I'd convert the units to guitar amplifiers, or sell them to a guy who is eager to do so.
If I'd live in your country, this guy could be me.
Best regards!
be aware that the frequency range of the organ's tone generator ends at ~ 5 to 6 kHz. So that is what you could expect from the given output transformers. Not really high fidelity stuff, is it?
For a first try, I'd disconnect anything left from V8's grid (pin 7), put in a 1M grid bleeding resistor and connect a line level source via a 47nF capacitor. This would give you a first impression of it's sonical qualities. If you're satisfied, I'd go with the Mullard 5-10 circuitry, by adding a EF86, as yet suggested before. If you're not, what I really suspect, I'd convert the units to guitar amplifiers, or sell them to a guy who is eager to do so.
If I'd live in your country, this guy could be me.
Best regards!
Not sure what you mean by 'lead' compensation.. it's not a term I'm used to. As regards the action of C310 / R325, what I can see, is that at some (high) frequency, the effective plate load of V8 falls from ~470K to ~45K (470K || 47K). This would result in a loss of gain, that rolls on at... 3-6dB (?) per octave.
So the point of the network is to significantly reduce stage gain at some relatively high frequency, to reduce a tendency to oscillate / ring at high frequencies (perhaps above the audio band)? Or what else am I missing?
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Hi Mike,
Ah, ok.. you mean 'key click' as in noise from the keyboard contacts making & breaking?
Might well be the case, and it's easy enough to open the loop and see how it acts. Right now, I'm just running the pair, listening to them in the background. Later tonight I'd like to try and make some power measurements, and then take a look at bandwidth.
For an amp like this, what's the 'reference' power level for bandwidth? 100mW, 1W, 2W?
Full power? =)
ETA: What of C314 (0.001) + R335 4K7 across the pri. of the opt xfmr? This is another HF snubber, but why is it there? What it is compensating?
Ah, ok.. you mean 'key click' as in noise from the keyboard contacts making & breaking?
Might well be the case, and it's easy enough to open the loop and see how it acts. Right now, I'm just running the pair, listening to them in the background. Later tonight I'd like to try and make some power measurements, and then take a look at bandwidth.
For an amp like this, what's the 'reference' power level for bandwidth? 100mW, 1W, 2W?
Full power? =)
ETA: What of C314 (0.001) + R335 4K7 across the pri. of the opt xfmr? This is another HF snubber, but why is it there? What it is compensating?
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That makes good sense. Not only is there noise from the closing & opening of multiple switch contacts, but also due to the fact that the tone sources for any given key aren't connected simultaneously. But rather, randomly on a per-key basis - aka 'willy-nilly' - which also creates an artifact.
That's a Zobel network, which is frequently found in speaker xovers. Here, it's linearizing the HF impedance behavior of the O/P trafo.