Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AmberB on July 21, 2023, 01:31:52 pm
-
Hello all!
I'm working on an amp chassis that I think is an AO-43 or a clone of it. There are no markings on the chassis to identify it, but it's almost identical to the Hammond AO-43. I didn't know quite what the chassis was when I gutted the preamp to rewire for guitar amp use. At the time I gutted it, I didn't realize that the choke on the end of the chassis was the load for the reverb driver tube, so all that got removed.
I put in a preamp that is basically a clone of the Ampeg Gemini 2. I want to add a reverb circuit to this amp, but I don't want to use the Hammond choke load type circuit, I have no idea what reverb pan would be used with that circuit.
I did a search for reverb circuits here and came up with a fair amount of information, but I would like to confirm some info.
I want to use a 6BM8 as the driver and recovery tube for the reverb because I only have 1 tube socket available and I'd rather not add another socket to the chassis if I don't have to. I plan on using a transformer coupled reverb driver. The question I have is about which transformer would be best for the circuit. I read in one of the posts that I found in the search that the standard Fender reverb transformer would work for the purpose. The person who posted the statement said that the 6BM8 pentode handles about the same current as both plates of a 12AT7, and that the high impedance primary of the transformer for Fender circuit should work with the 6BM8 to push the reverb tank enough for decent reverb.
So, my question is, Is the 25k primary impedance for the Fender reverb transformer too high for the 6BM8 to provide enough drive to the reverb tank, and if so, what impedance would be better for this purpose?
I have a Fender type reverb transformer right now...the specs for it say 25k primary impedance, 8 ohm secondary.
-
There is some useful information on one tube reverb in ARCHIVES that includes info on using a 6BM8 tube. Maybe you will find your answer there?
Adding 1-tube reverb (el34world.com) (https://el34world.com/Forum/index.php?topic=7957.0)
With respect, Tubenit
-
Thanks for the link, that makes it easier to find the info.
-
fwiw: tubenit published a schematic for a 9W Marshall-like circuit with reverb. He used a 12DW7 tube for the driver/recovery tubes. I'm building it with the Fender transformer. Tubenit's circuit is attached with that 12DW7 reverb.
-
I finally found some information about the original reverb tank for the AO-43 amp as used by Hammond in their organ. According to their manual for the L100 type organ, the spring reverb pan is a type 4L pan, and the part number they give in the manual is 121-000002.
there was a discussion on a forum that I found that said that the proper replacement for the 4L pan is a 4FB1B1C reverb pan.
For the purpose of using the original choke loaded, cap bypassed reverb circuit in the AO-43, the 4F is the important part, for the input impedance. The output impedance would depend on what recovery circuit design you would use.
To use the original reverb drive circuit with the choke and the 12BH7 driver tube, you would need to add another tube socket to the chassis for the recovery circuit. You could probably use a 6AV6 single triode 7 pin tube for that, unless you wanted an extra triode in the amp for something else. If so, a 12AX7 would be the way to go.
-
Is this line of thought really worth pursuing? If so I have a couple reverb tanks from AO-43 amps. Sure sounds like a lot of headaches to adapt that circuit to a guitar amp. However, I suggest to just use the choke in the power supply and build another more sensible reverb using a Fender reverb transformer.
-
The thought was based on the fact that the reverb circuit is already on the AO-43 chassis. It would just need a bit of modification to work for guitar. It may be a better reverb circuit than the resistor loaded circuit in most of the older Ampegs that had reverb. Of course, that's just my thoughts on it...