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Hammond AO-39 rebuild/update

ab0ez

ab0ez

AB0EZ
Yeah, I know, I still have a couple of other big projects to finish up, but I picked up a pair of Hammond AO-39 6BQ5 mono amps for a crazy cheap price.

These amps originally have a balanced input. Couple that with the age of the components inside and, you guessed it: A total strip down and rebuild, complete with unbalanced RCA style jack input. Here's a few shots of the chassis as I unpacked them.
 

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Got the chassis all stripped down and the transformers stripped down and checked out. The HV measurements were within 2.3 VAC of each other. All other voltages and ohm readings were identical. This speaks extremely well of Hammond's quality control, especially all these years later. Next stop: The powder coat shop.
 

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Here's the original Hammond schematic. At least two chokes will be added to the power supply and the front end will be changed to unbalanced.
 

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I have a pair of these I'd like get working. I'd prefer a volume pot so I can go CD straight in... Looking forward to your schematic....
 
I have a pair also that I that I reporpoised (sic, aren't I? lol) with a couple of Dynaco Z565 OT and rewired them as per the ST35 circuit but mono blocks. Using 5V4's for rectifiers delivered to within 1 volt of the Dynaco schematic. They sound very nice indeed
 
guitbanger said:
I have a pair also that I that I reporpoised (sic, aren't I? lol) with a couple of Dynaco Z565 OT and rewired them as per the ST35 circuit but mono blocks. Using 5V4's for rectifiers delivered to within 1 volt of the Dynaco schematic. They sound very nice indeed
Click to expand...

Nice job! I'm either going to stick with the 6BQ5s or put in an 832A dual tetrode. That particular tube is a great match with the original Hammond OPT.
 
Any plans yet for the conversion to single ended input? The circuit as drawn requires balanced input since it doesn't have a phase splitter. That input stage isn't a long tailed pair and has feedback coming from the output tube plates. SO, it seems like you will have several changes to convert it from balanced input.

You could always get in input transformer and leave the circuit mostly as it is.

Keep up the good work.
John
 
Hi John, and thanks. The schematic attached is a very rough and preliminary version of what I did to the last pair of these and a picture of the finished product. I may try a SS adjustable G2 regulation circuit, vs the usual tube regulators I use.
 

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If you cared to consider the transformer input idea, Bud Purvine (O-netics) is local and sells a nice pair of input transformers for about $140/pr. I use them in my amp and they sound great IMO, give you passive phase splitting, and you could keep the original circuit with the parallel feedback from output plates to input cathodes. It gives the output tubes an effectively triode behavior without losing the gain of the pentodes. I think it would be pretty cool to give it a try. I can send you some info on the O-netics transformers if you are interested. Either way, should be a fun project.

John
 
That's an interesting idea John and since I haven't started to put anything together yet, why not? Do you still have my email address?
 
Email sent. I included the data sheet for the transformers and a schematic for my amp and how I used them.
 
Thanks John, I got it. In the absence of a CCS (or any negative voltage source) on this amp I can either go with a conventional cathode biased 12AX7 or CV4024, cathode bias the output tubes, or I could possibly squeeze in a small bias transformer and go with fixed bias and a CCS. Decisions, decisions!!!
 
You wouldn't want a CCS on the input stage since they get feedback from the output tube plates and have to work separately, assuming you keep the feedback scheme. I think the output plate to driver stage feedback is pretty cool. I have two pairs of O-netics 8k PP output transformers that I've been thinking about and this Hammond circuit would fit pretty well.
 
Good point on that. Just mulling over options about the overall topology of this project, such as sticking with 6BQ5 outputs or putting in the similar 6GK6 output tubes.
 
Taking Johns' suggestions above and after looking again at the schematic and rather unique form of plate to cathode feedback, I decided to keep the overall topology of the amp, beef up the power supply considerably, and add in Onetics 101499 input transformer. They aren't cheap, but the result should be worth it. I present Rev 1 of the new and improved Hammond AO-39 for HiFi use. Thanks John.
 

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Hi John,

Haven't done much with the chassis other than got them back from the powder coat shop. I've been out with left shoulder replacement surgery and just now up to typing with both hands!

Hope all is well with you and yours.

Jim, AB0EZ
 
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