Building a new amp from Hammond AO-39 amp! Schematic posted.

V2Racing

Member
Messages
126
I've been kicking around what I'm going to build out of the 4 organ amps I have. I found a schematic online drawn by Steve Luckey of a AC15 normal channel he built from an AO-39 organ amp.

I wanted something different and wanted to play with some different ways of tone shaping, so since the AC15 has a treble cut between the PI and power tubes and a volume bright cap, that was almost different enough from the amps I have.

I decided to use the schematic as written from the PI on through the power section, but do my own preamp with a 6U8A or one of it's close cousins with a triode and a pentode in it. I went triode first into the pentode. I am going to have DPDT switches on the cathode resistors to change gain and voicing along with what some call a squish control off grid 2 on the pentode to ground.

I may change some of the resisters in the power section for C and D to up the voltage to the 6U8A and the PI to push the EL84's harder.

The amp will have no NFB and will be cathode biased.

What think you all?

Mike

 
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Jeff Gehring

Silver Supporting Member
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5,583
There is no ground reference for the pentode's grid. Also, you might want to add large value resistors between each of the selectable cathode bypass caps and their respective switch common connection, to allow them to pre-charge and reduce switching pop.
 

Rockinrob86

Member
Messages
3,650
I built an 18 wattish amp out of one of these that is awesome.

And then I fixed the hammond and kind of wished I still had the reverb! But the AO39 amp is a go to around here, so it could be worse
 

V2Racing

Member
Messages
126
There is no ground reference for the pentode's grid. Also, you might want to add large value resistors between each of the selectable cathode bypass caps and their respective switch common connection, to allow them to pre-charge and reduce switching pop.
Thanks Jeff! A 1M ground reference should work shouldn't it? The high value resistor on bypass caps is a good idea. I have a reverb pedal that needs a cap on it's switch to charge the cap. It sounds like a gunshot when you switch it on.

Mike
 
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V2Racing

Member
Messages
126
I have made changes to the schematic that Jeff suggested and went through my parts and made some changes to use what I have and also to up the voltages to the 6U8A. Also added a switch to the bright cap.

Mike
 

V2Racing

Member
Messages
126
I have an off topic question? How could this thread have gotten over 22,000 visits since Monday when the stickeys don't have that many hits???

Mike
 

HotBluePlates

Member
Messages
8,896
... How could this thread have gotten over 22,000 visits since Monday when the stickeys don't have that many hits??? ...
Someone has a bot that keeps reloading the pages that have the text "AO-39."

I bet they have a warehouse of 300 AO-39 amps bought for $50 each, and they're just waiting for the forum-craze to hit so they can charge $400 for them.
 


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