Hammond Amplifier Modification Issue - diyAudio
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Old 7th September 2014, 12:10 AM   #1
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Join Date: Jul 2014
Default Hammond Amplifier Modification Issue

I recently did some stuff to a Hammond L100 Amplifier so that I could use it as either a reverb unit or a Leslie preamp. I've just finished it and I noticed that there was a lot less volume than there should be.

On the stock amplifier there are three RCA sockets. input, reverb send and reverb return. There is also a connecter with three wires. dry signal, wet signal and ground. On the organ this connecter was used to connect the reverb mix switches (and a volume soft switch). This seemed to be the best point to not only mix the reverb with the dry signal but also to get a line out (for when I want to use it as a reverb unit).

The way that that reverb was mixed on the organ was to take the wet signal and use two resistors each attached to a switch to introduce two levels of reverb into the dry signal wire. I've attached this part of the schematic.

I've also attached a drawing of my current design. What I was hoping to do was to use a dual gang pot as a balance pot so that I could have complete control of the amount of reverb. This works well for the line out but not so well when it's used with a Leslie. The dry signal wire is the only one that carries on through the rest of the amplifier so when the pot is set for just reverb there is nothing going back into the amp. It's a compromise I expected though so that's alright.

The whole thing sounded a bit quiet when going through the Leslie so I tried unplugging the connecter (no reverb control) and the signal did get a lot louder (increased to a more expected volume).

I may be wrong but by using a potentiometer am I reducing the volume by effectively putting the signal through a resistor to the ground?

If this is the case, I've thought of another way of getting what I want but any feedback would be great because I'm certainly no expert.

What I'm thinking is to use a DPDT switch. Position A would connect the dry signal to the line out and position B would connect the wet signal to the line out. Then if I wanted a mix I'd use a potentiometer with an on switch (1 meg?) as a variable resistor. Hopefully this would allow me to mix the wet and dry signal and as there's no ground there's no danger of volume reduction. Does this sound about right?

Another thing that's on my mind was that if this were to work, would I have a floating ground? I'm no good at reading schematics but is R408 (4.7M) a pull down resistor. If so, could I replicate this by putting a resistor of similar value between the ground and positive of the output jack?

Sorry for writing so much, if anyone has any insight into whether this idea of mine will work and not reduce the overall volume please let me know.
Thank you for your time
All the best
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Old 7th September 2014, 05:12 PM   #2
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A quick addition. There is a rotary switch that comes just before the output jack so that I can switch between using the line out and the amplifier as normal.
Not including the rotary switch in the diagrams just made them simpler but I realised that someone might think that the volume loss would be due to something I'd have connected to the line out.
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Old 9th September 2014, 03:57 PM   #3
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Location: Jeffersonville, Indiana USA
Some of us prefer our Hammonds with the tonewheels installed. I was trying out the intro to Free Bird last night on mine, organ track on the left hand, electric piano in the right.
Hammonds have floating ground. The power transformer isolates the internals from the wall plug neutral, but then the secondary of the transformer is connected to the metal chassis for convenience of tube circuits. The is convenient in using a Hammond in a band where ground loops with the PA amp are such a problem everybody uses DI units.
Lots of people fool around with putting an effect instead of the reverb. also people put record players etc into the reverb input. Just remember the spring reverb expects ~ 7VAC out and about 25 mvac back.
The most common cause of low volume in a Hammond at this age is low power supply voltage due to leaky electrolytic capacitors. This can be detected by measuring the voltages on the caps versus the voltages shown on the print. Read the safety sticky about high voltage safety, use one hand measuring and a clip lead, you don't wan't to stop your heart electrically.
If you do hook your amp to a grounded external circuit, do the safety check for leakage on your power transformer that Hammond specifies. The setup is shown on p 2 of the service manual for A,A100,B,BC,BV, B2 etc you can download from archive.org. Requires a DVM, a resistor, and a capacitors, 3 clip leads, and a screw wall plug to access both the neutral and the safety ground. If your power transformer has leaky cotton insulation, it can burn up if it works into a grounded circuit. Lots of Hammonds in tiny churches were exposed to the rain over the years, to deteriorate the power transformer and AC switch wiring.
For more circuit details, read the L100 threads of the Hammond forum of organforum.com. Lots of people use their hammonds as the PA amp of their home band, most while leaving the tonewheels as one of the inputs.
If your pot load is less than the tube will drive (see the tube datasheet) then it shouldn't cut volume. A mixer is a couple of resistors to variably cut two signals from different inputs without the sources seeing the AC voltage from the other source, then an amp to boost the signal back up to a usable level. Mixers are better done in low current areas, instead of down in the power section.
Remember when fooling with Hammonds, they did not run their tubes into overdrive for the guitar crunch distortion effect. Leslies did allow this, one reason Mr Hammond detested Mr. Leslie.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 9th September 2014 at 04:03 PM.
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Old 11th September 2014, 06:56 PM   #4
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Here's an update on the Hammond Modification if anyone is interested.
My suspicions were correct about the potentiometer acting as a resistor between the signal and the ground.
I went ahead and bought the 1meg switched pot and the dpdt switch. Soldered it all like my drawing and it seems to work fine now. It's the correct volume on both the line out and through the Leslie.
I haven't put in a pull-down resistor but it all seems to work without it. If anyone has any opinions on this resistor in this circuit please let me know.
I know that the 'nothing is sacred' approach isn't for everyone but I've got a fantastic reverb unit, A valve preamp for DI'ing instruments and a Leslie preamp I can put anything in. It's a good tool for recording.
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