Connect Hammond organ to guitar amp - Page 1
Forum section: electronics / projects

author: JohnnyP
author_rank: Regular Contributor
timestamp: January 22, 2021, 05:38:50 pm
content: I’m spending a few days in Yuma with my 93 year old Mom. She has a Hammond M-100 organ with a tube amp. She’s pretty hard of hearing and is asking if there is a way to add an external speaker.The schematics are on line here:https://medias.audiofanzine.com/files/m100mm2m3svcman-text-1-470816.pdfPage 62 shows the signal goes through the loudness foot pedal control, then into the amplifier. An RCA jack is provided for input from an external source.I have used the jack before to inject a signal, to verify the amp was working. I connected a guitar to the “Mic” input on a Casio keyboard, then connected the Casio’s “Line Out” to the RCA jack. The guitar played through the organ’s speaker.It would seem that I could use the jack as an output, to drive a guitar amp. Anyone see a problem with that?I’m looking at using a 15 W Fender “Rumble Bass 3”. The spec says 1M ohm input impedance. It’s $100, and I don’t think the store will let me return it if it doesn’t work. I told the manager what I wanted to do and he thought it was sketchy.

author: Wolfram
author_rank: Frequent Contributor
timestamp: January 22, 2021, 06:49:51 pm
content: The Phono RCA connector on the loudness control housing hooks up to a virtual ground summing node, so there's not a lot of signal level here. It might work, but probably not well. V4B is a virtual ground mixer using capacitors rather than the more common implementation using resistors. This allows the swell control pedal to use a variable capacitor, eliminating the wear issues that potentiometers have.There are other ways to hook up a guitar amplifier to an AO-29. The easiest and least invasive is to use a resistive divider from the speaker output. A 22k resistor in series with a 1k resistor to across the speaker, with the 1k resistor connected to the grounded side, should work. Take your output signal across the 1k resistor, making sure that the ground of the guitar amplifier input is connected to the grounded end of the speaker output.Are you sure the organ is operating like it should? A correctly working M-100 will be pretty loud. You can use a normal multimeter in AC mode to measure the signal level at the speakers, playing a single low note with only the 8' drawbar out, to get an idea if it's performing as it should. There might be a procedure for this in the service manual.You can also increase the volume by adjusting the trimmer capacitor accessible through a hole in the loudness control housing.

author: bob91343
author_rank: Super Contributor
timestamp: January 22, 2021, 10:47:27 pm
content: You don't show enough to see what the output stage is. If it's pushpull 6V6s you can expect maybe 8 Watts maximum. As said above, that ought to be loud enough.

author: JohnnyP
author_rank: Regular Contributor
timestamp: January 23, 2021, 12:44:38 am
content: Click the link in my original post for the complete schematic and wiring diagram.Wolfram: Thanks for the circuit analysis. Someone posted your suggested method on the Hammond forum.I bought a 10 W Fender Frontman today, just to try it. It worked fine, as far as I could tell. I’ll use it with a Casio keyboard that’s on its way to me.Mom says the organ is plenty loud as is, especially if you flip up the “Volume” tab up. When she asked for an external speaker, she meant a Hammond Tone Cabinet, for a richer tone. I thought she was just looking for more volume.

author: JohnnyP
author_rank: Regular Contributor
timestamp: January 24, 2021, 09:58:52 pm
content: Ok, now she is looking for more volume. I think there is something flaky with the amp.Would you have a look at my thread on the Hammond forum? I have done some troubleshooting on it.https://organforum.com/forums/forum/electronic-organs-midi/hammond-organs/748676-hammond-m-100-no-sound?p=754410Scroll to post #22

author: JohnnyP
author_rank: Regular Contributor
timestamp: January 28, 2021, 05:19:41 pm
content: Bench testing the Hammond amplifier:The complaint was low volume. It started playing loud the instant I touched the probe of my DVM to measure the plate voltage on one of the tubes.It's not like I initiated a better contact by pressing on the solder joint, etc. It's like the probe zapped it into working, when I barely touched it.I shut it down and touched up a few solder joints, then turned it on and it was low volume again. But it came back when I touched the plate signal with the probe some time later.Have a look at the video. Ideas are welcome.https://www.youtube.com/watch?v=2hInES-GiDU&feature=youtu.be

author: The Soulman
author_rank: Super Contributor
timestamp: January 28, 2021, 06:49:52 pm
content: Have you tried reinserting the tubes?

author: JohnnyP
author_rank: Regular Contributor
timestamp: January 28, 2021, 07:53:45 pm
content: I bought a set of tubes for it, replaced V1, V2, V3, and V4.I'm touching up solder joints. I have a syringe with "no clean" flux gel.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: January 30, 2021, 07:50:13 pm
content: Sounds like she wants a Leslie.https://en.wikipedia.org/wiki/Leslie_speaker

author: JohnnyP
author_rank: Regular Contributor
timestamp: January 30, 2021, 08:18:49 pm
content: No Leslie. She decided she would be happy with a DR-20 Hammond external speaker. It's a tall tone cabinet, looks like furniture. She has one for her other organ up in Bremerton.