• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Organ - Allen model 20a amp

BmWr75 said:
Yep, there is no bass with the 30uF cap in place. This must have been the output that drove the tweeter/midrange in the organ. Will have to remove the 30uF caps tomorrow and try it again.

Tried the 16 ohm taps, but there is no output on them from either amp. :scratch2:
Click to expand...

I'm just guessing here but from the looks of your pictures you do not have the plug in that completes the circuit to the speakers. This maybe your issue with no sound at the other taps...?

I bypassed that octal plug as I did not see a need for it any longer and figured it was less stuff in the signal path. You may want to give it a try. It's easy to do.
 
I rebuilt a pair of these and they did not have a 30uF cap on the outputs. They have very good bass response and the 16 ohm taps work. I added new input jacks and banana output jacks.

Congrats, you have some very good amps!
 

Attachments

  • Bottom.jpg
    Bottom.jpg
    84.3 KB · Views: 89
  • Front.jpg
    Front.jpg
    75.9 KB · Views: 84
  • Left side.jpg
    Left side.jpg
    82.1 KB · Views: 83
  • Right side.jpg
    Right side.jpg
    75.9 KB · Views: 77
30uF caps removed, the bass is back. :D

Jumpered the 16 ohm connections to make them operational too.
 
ab0ez said:
I rebuilt a pair of these and they did not have a 30uF cap on the outputs. They have very good bass response and the 16 ohm taps work. I added new input jacks and banana output jacks.

Congrats, you have some very good amps!
Click to expand...

I have this very pair on my bench now. The owner was starting to have noise problems with the volume pots and some hum issues and asked me to install the new pots and some different speaker jacks. I think the amp has one too many gain stages and that is the root of a lot of the background noise. Bypassing that first 12AY7 stage really reduced the noise level. Unfortunately while running one of the amps, the power transformer developed a short and blew the fuse. I have it completely out of the amp and it still blows a fuse once the variac gets to where I have about 100V on the secondary.

Don't suppose anyone has a junker Allen 20 that could be had reasonably so we don't have to unpot the can for a new transformer?

John
 
I've been listening to these amps for a little while now. They just didn't seem to have a very solid center image for voices. Got me to wondering is they might be wired 180 degrees out of phase from one another internally. So, I switched the speaker wires on one of them.

Eureka......strong center image now!! Of course the bass is now more authoritative too.

Guess checking phase is something you should do when using two mono amps that were never intended to be used as a stereo pair.
 
Very cool!

Did you bypass the first gain stage of the 12AY7 on the the input that does not go to the pot?

I'm still looking for a second......:thmbsp:
 
Yes, I did the first gain stage bypass......you and Kegger are like H&R Block to me. :yes:

Have some 220uF caps on order to increase the capacitance on the first cap after the rectifier and choke.
 
Last edited:
BmWr75 said:
Bottom side pictures

All I need now.......is another one. :scratch2:
Click to expand...

Which will likely turn up in either Jolly Old England, or Hawii.:D

I am faced with a similar dilema with another brand that will remain nameless as there is enough competition for them as it is.
 
I am thinking about trying 12BH7 tubes in the 12AU7 positions. They draw more heater current. Is the power transformer on these amps up to the task of providing the extra current?
 
I've used 6CG7 tubes in the place of 12AU7 tubes with great results. All you have to change is the filament wiring.
 
This post is about 10 years late, but I just bought a type 20A chassis to maybe make into a guitar amp. Mine is very quiet. I'm waiting on a schematic from Allen Organs before I start tinkering with it. I was wondering if anyone else in the forum had any thoughts on how loud they should be. I was imagining they would be pretty loud with a pair of 6L6's.
Thanks!
 
WindyJohn said:
This post is about 10 years late, but I just bought a type 20A chassis to maybe make into a guitar amp. Mine is very quiet. I'm waiting on a schematic from Allen Organs before I start tinkering with it. I was wondering if anyone else in the forum had any thoughts on how loud they should be. I was imagining they would be pretty loud with a pair of 6L6's.
Thanks!
Click to expand...
That will basically depend on the efficiency of the speakers you use, the difference could be 15dB or more. But yes, a pair of 6L6's will get quite a lot of volume from a reasonably efficient speaker.
 
I worked on a couple of these as you can see earlier in this thread. Here's one of the schematics I marked up with some voltages. These were measured voltages on a particular unit so may vary a bit from one to another. You might also notice I removed the first voltage amp section of the input tube as there was too much gain and it caused a fair bit of background noise. Without the extra gain they were very quiet with a good sensitivity for home audio use.
 

Attachments

  • Allen 20 SN 0656 TubeAmp Schematic.jpg
    Allen 20 SN 0656 TubeAmp Schematic.jpg
    72.2 KB · Views: 49
nerdorama said:
I worked on a couple of these as you can see earlier in this thread. Here's one of the schematics I marked up with some voltages. These were measured voltages on a particular unit so may vary a bit from one to another. You might also notice I removed the first voltage amp section of the input tube as there was too much gain and it caused a fair bit of background noise. Without the extra gain they were very quiet with a good sensitivity for home audio use.
Click to expand...
Thanks Selmerdave and nerdorama. I've poked around inside it and I'm not getting the 35V at pin 8 of the 6l6's. But the amp works, like said before, it's quiet. I might try a set of 6l6cg's, the ones I'm using aren't GC's. Might make a difference? Have to see. Again, thanks so much for the help.
 
You must log in or register to reply here.
Back
Top Bottom