Recent Push-Pull Amplifier Project

Hello everyone, new member here. Thought I would upload a few pics of an amplifier that I completed last month. This was also posted elsewhere recently (before I found this forum), so my apologies if you've seen it before.

This amplifier embodies most of my general design goals regarding amplifier construction. The chassis is 1/8" aluminum with a 1/2" 6061 front panel. Exterior surfaces are clear anodized with engraved markings. The circuit consists of a 6J32P (EF86) in triode feeding a Telefunken ECC91 long tail pair. Outputs are 6P3S-E. The transformer set was liberated from a Harman Kardon TA7000X receiver.

Jack

TA7000X main.jpgTA7000X rear.jpgTA7000X Bottom Cover.jpgTA7000X Open.jpg
 
That's a tidy build! I like your stealth bias adjustment...

How does your output impedance selector work?
 
That's a tidy build! I like your stealth bias adjustment...

How does your output impedance selector work?
Thanks! I spent a fair amount of time designing the selector. What you see from the rear of the amplifier is an octal plug and socket. The two wires routed through the stem of the plug connect to the two "+" speaker terminals inside the chassis. The output transformers are "standard," with just four secondary wires each. The ground wire from each one is connected directly to its respective "-" speaker terminal. The 4-8-16 ohm taps are connected to lugs of the octal socket. To change the setting, you just pull out the plug and reinsert it in a different position.

Jack
 
Thanks! I spent a fair amount of time designing the selector. What you see from the rear of the amplifier is an octal plug and socket. The two wires routed through the stem of the plug connect to the two "+" speaker terminals inside the chassis. The output transformers are "standard," with just four secondary wires each. The ground wire from each one is connected directly to its respective "-" speaker terminal. The 4-8-16 ohm taps are connected to lugs of the octal socket. To change the setting, you just pull out the plug and reinsert it in a different position.

Jack
I'd really like to see a schematic/drawing of that. I'm having a hard time visualizing what is going on.
 
Thanks! I spent a fair amount of time designing the selector. What you see from the rear of the amplifier is an octal plug and socket. The two wires routed through the stem of the plug connect to the two "+" speaker terminals inside the chassis. The output transformers are "standard," with just four secondary wires each. The ground wire from each one is connected directly to its respective "-" speaker terminal. The 4-8-16 ohm taps are connected to lugs of the octal socket. To change the setting, you just pull out the plug and reinsert it in a different position.

Jack
Very clever! I like that a lot.
 
I'd really like to see a schematic/drawing of that. I'm having a hard time visualizing what is going on.
Here are all the schematics. This is the only electrical representation of the selector that I've drawn. If this doesn't clarify it, I could probably sketch the physical wiring and scan it.

Jack

Kyoto 7000X Amp.jpgKyoto 7000X PS.jpg
 
Good question! The plug isn't modified. Instead, I used a mill to expand the keyway in the socket. It covers three positions only, and that prevents inserting the plug into an empty position. A small Swiss file could also be used for this.

Jack
 
Good question! The plug isn't modified. Instead, I used a mill to expand the keyway in the socket. It covers three positions only, and that prevents inserting the plug into an empty position. A small Swiss file could also be used for this.

Jack
Ooh. Smart thinking!
 
As you have a reasonably long tail in the 2nd stage and an AC balance control with asymmetric anode loads, how close was the total resistance in each anode after you adjusted it?
 
As you have a reasonably long tail in the 2nd stage and an AC balance control with asymmetric anode loads, how close was the total resistance in each anode after you adjusted it?
It can vary quite a bit depending on the inherent balance of the two LTP triodes and the output tubes. If the LTP is unbalanced in the same direction as the outputs, the balance pot may need to be set all the way to one end or the other. What surprised me most about all this is that output tubes I had previously graded for gm on a Hickok tester and idle current in a test amp didn't correlate well when AC balance was the goal.

Jack
 
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