Tubes!

I mentioned in a previous post that the amplifier in my M3 didn't have any tubes in it. The amplifier used in this organ, the model AO-29, takes 11 vacuum tubes (or "valves" for our British friends), including two 6V6 power tubes that are used in the power amplifier section to deliver 10-12W of output power.

I'm not too interested at this point in restoring the power amplifier section (I will do so at a later date, when everything else is working), so I'm concentrating on getting the pre-amplifier section working. The preamp will boost the tiny signals coming from the tone generator, vibrato, percussion, or bass pedals into a signal large enough to drive a power amplifier. In my case, I'm going to tap into this signal somewhere near the end of the preamp chain and bring that signal out to the outside world.

I started looking into purchasing some tubes from online suppliers like Antique Electronic Supply. I wanted to purchase enough tubes to get the power supply, vibrato, and manuals working. Unfortunately some of the tubes, like the 12AU7, can be relatively expensive - running into the $15 to $20 per tube range. I also really just wanted something to get the amp working; I don't need high grade tubes at this point of the restoration.

In my ham radio hobby, I used to have a friend that bought and sold tubes on the side, but he got out of that business several years ago. Then I remembered my friend Ed, who is a Collins radio enthusiast (Collins was a high-end US radio manufacturer from the 1950's to the early 1980's). I suspected Ed might have a few tubes, or know someone locally that did. Well, bless his heart, not only did he have the tubes I was looking for, but he tested them for me to make sure they were OK, and didn't ask anything for them!

So a big THANK-YOU goes out to Ed W1OKH!! Here's the cache of tubes:


Left to right: (3) 6AU6, (2) 12AU7, (2) 12AX7, (1) 5U4

The 5U4 is the rectifier tube for the power supply. The rest are used in the preamplifier section. I needed 2 6AU6's and one each of the 12AU7 and 12AX7, so I have some spares now, too.

BUT, before I put any tubes in this amplifier, I need to pull the chassis off the organ and get a look at the physical condition of the components inside the amp. I have a strong suspicion I will need to replace the electrolytic capacitors before I do anything else... in fact, I may just do a preemptive replacement of the electrolytics. Better safe than sorry!