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Old 16th April 2013, 07:09 PM   #1
scrimpu is offline scrimpu  United States
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Default Hammond Type G Amp

I got this power amp and I would like to convert it for guitar use. I plan to dump the 6J5 tubes and use the sockets for a preamp and PI instead. I haven't been able to find any useable schematics for a 4x 6V6 PPP design. I have some 6SN7 and 6SJ7s I can use. Any thoughts on what I can do with what I have here?
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Old 16th April 2013, 08:24 PM   #2
JMFahey is offline JMFahey  Argentina
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If it's in working condition, with tubes, etc., I'd use it basically as what it is, a *good* power amp, with the only mod of turning those 6J5 into an LTP inverter , or this one which leaves everything as is, needs only an extra resistor:
Click the image to open in full size.
And then would mount the Hammond G in the bottom of the cabinet, and build a new, modern preamp (modern means from 1955 up ) on a subchassis on top (similar to split chassis Gibson/Magnatone/etc.), fed through an umbilical which carries power up and signal down.
Best of both worlds: you mount the clunky heavy hot Power amp + PSU below, and a nice preamp upstairs.
Even a single 12AX7 will give you a classic Blackface preamp.
A killer combination with those 6V6 .

Plus there's no practical way to mount a preamp, specially preamp controls, jacks, etc. in that chassis.
You are comitted to get a neew chassis anyway, simplest is to worry only about a light practical preamp.
Good luck.
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Old 16th April 2013, 09:05 PM   #3
scrimpu is offline scrimpu  United States
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It is in working order and I do have the preamp from the organ. Perhaps it could be modified to a useable guitar preamp?
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Last edited by scrimpu; 16th April 2013 at 09:08 PM.
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Old 16th April 2013, 10:52 PM   #4
Enzo is offline Enzo  United States
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And there, you see? The preamp already has a phase inverter at its output. The 6J5s just improve the drive on the power amp.
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Old 16th April 2013, 11:54 PM   #5
JMFahey is offline JMFahey  Argentina
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True enough.
I sort of expected something like this because the power amp has 2 inputs, presumably already out of phase, and straight to the grids, which didn't even had ground return resistors (they trust the driver transformer in the preamp chassis).
The preamp is very interesting and certainly most advanced, but I see it *too* Hammond organ especific, maybe you'd have to strip at least part of it, but leaving V5 which is the phase inverter.
It has what must have been the best tremolo in the World, but it's too complex and parts of it seem to be "somewhere else", probably at some control panel by the organ keyboard.

*Maybe* your best bet is to (slightly) mod the power amp so it can work on its own and it can accept "any" guitar preamp you build.
And use the preamp as a "donor chassis".
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Old 17th April 2013, 12:06 AM   #6
scrimpu is offline scrimpu  United States
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Well the output transformer is used as a sort of phase inverter. Any ideas on how I can adapt this for guitar use?
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Old 17th April 2013, 12:30 AM   #7
JMFahey is offline JMFahey  Argentina
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Just to know where you're standing, do the next:

1) ground the NET C10/R24/R25 to put that triode out of the scene.

2) lift "in the air" the left leg of C11.
"left" means "as seen on schematic" meaning "the one that's connected to R21".
On the actual chassis it may be pointing *anywhere*, what matters is what it's connected to.
Now that's your input.

Mount a jack in the chassis, ground to ground, hot to C11 .
Plug a guitar, use its volume and tone controls because you have none in the chassis, play.

What do you hear?
Do you find it acceptable?


Note: a NET is "some part leg and everything that's connected to it" , so all points "are the same" meaning all have same voltage, signal, whatever, and if you ground any point of that NET, all of it gets grounded.
Of course, "ground" is the largest net of them all and is everywhere.
Speaking "Nets" is actually the simplest way of making connections clear, and it's not necessary to mention all of it, usually naming 2 parts of it is enough to identify it; for example I didn't mention R18 but it's clear that one of its legs is also part of the NET mentioned earlier.
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Old 17th April 2013, 01:53 AM   #8
scrimpu is offline scrimpu  United States
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Quote:
Originally Posted by JMFahey View Post
Just to know where you're standing, do the next:

1) ground the NET C10/R24/R25 to put that triode out of the scene.

2) lift "in the air" the left leg of C11.
"left" means "as seen on schematic" meaning "the one that's connected to R21".
On the actual chassis it may be pointing *anywhere*, what matters is what it's connected to.
Now that's your input.

Mount a jack in the chassis, ground to ground, hot to C11 .
Plug a guitar, use its volume and tone controls because you have none in the chassis, play.

What do you hear?
Do you find it acceptable?


Note: a NET is "some part leg and everything that's connected to it" , so all points "are the same" meaning all have same voltage, signal, whatever, and if you ground any point of that NET, all of it gets grounded.
Of course, "ground" is the largest net of them all and is everywhere.
Speaking "Nets" is actually the simplest way of making connections clear, and it's not necessary to mention all of it, usually naming 2 parts of it is enough to identify it; for example I didn't mention R18 but it's clear that one of its legs is also part of the NET mentioned earlier.
Tried this and output was very low, I will need much more gain than that.
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Old 17th April 2013, 02:50 AM   #9
JMFahey is offline JMFahey  Argentina
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Ok, testing beats everything else.
Consider that point your Power Amp Input .
Feel free to pull everything to its left and build whatever you want.
That preamp will have its own filament transformer but receives +V from the main chassis.
I think a 5E3 preamp or an earlier, octal tube version, is a good choice, specially because front panel controls will be simple , yet good sounding.
3 pots, 4 jacks, killer sound
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Old 20th April 2013, 05:03 AM   #10
cancon is offline cancon  Canada
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This amp was designed to be used with Electro-magnetic speakers...are you using them in your design? If not, are you using a safely designed dummy load?
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