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Old 1st September 2020, 07:06 PM   #731
itsikhefez is offline itsikhefez  United States
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Thanks for info George.
I'm rebuilding a Fisher 400 and a Sherwood S-5000 and both use 12AX7's as phase inverters... I have a nice stash for them so wanted to reuse but can obviously just acquire 12AT7's
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Old 2nd September 2020, 05:24 PM   #732
itsikhefez is offline itsikhefez  United States
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For an EL84 build, I really like the look of the 4 power tubes with 2 input tubes in front.

The rectifier seems out of place for me visually so I'm I'm going to try a slightly different approach for B+.

SS diodes instead of the tube rectifier, with a Hammond 370HX (550VCT) PT and 193H (65ohm) choke.
Based on PSUD, I think this combo should get me in the same ballpark as the design B+ of 310-330 VDC.

Question, perhaps I can use 2A diodes in this case to fit better in the tube socket?
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Old 2nd September 2020, 05:57 PM   #733
Tubelab_com is offline Tubelab_com  United States
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The 1N5408 diodes are fine. That's what I used in my 6CW5 version of this amp since it eats more current than the 6BQ5 / EL84 version.

I just put the diodes on the bottom side of the board and did not make a hole in the top plate for the 5AR4.
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Old 3rd September 2020, 06:22 AM   #734
itsikhefez is offline itsikhefez  United States
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I'll be using Hammond 1650FA OPT's, wired for ultralinear mode.

How do I wire for multiple output taps (4ohm, 8ohm) and feedback?
Based on some of the related posts, my understanding would be to choose one (say 8ohm), connect that through the feedback terminals to the binding post.
Adjust the compensation cap with 10khz square wave method based on that.
Then wire the other secondary tap (4ohm) to the additional post directly?
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Old 4th September 2020, 02:52 AM   #735
itsikhefez is offline itsikhefez  United States
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Besides the wiring question above, how bad would it be if I connect the 12AT7 PCB pads with short wires to chassis mount sockets? Only reason I would do this is for symmetry on the top plate.
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Old 13th September 2020, 08:47 AM   #736
itsikhefez is offline itsikhefez  United States
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I've made progress with my build and need some help with final tweaks.
(You can ignore the question above regarding off-board sockets as I've went with all of them PCB mounted).

1. Feedback compenstation cap -- I bought 120pF, 270pF and 470pF caps to experiment. 10Khz squarewave was ugly without the cap. It got incrementally better up to 470pF.
Is this good enough or is it worth trying some higher values like 560pF ? (pics attached). OPT is Hammond 1650FA as FYI.

2. 270R cathode resistor -- I measured about 11.35V across which comes out to 42mA. With B+ around 320-325V comes out to about 13.5W dissipation, is that correct? Is it better to change this to 300R or 330R ?

3. With the 370HX PT (550VCT), SS diodes and 193H choke (5H, 65R) B+ came out too high at around 350V. I put a 150ohm resistor in series with the choke temporarily and that brought it down to 325V.
I have a 270HX on order which has a 125V primary, along with a CL90 so hoping that will drop B+ a bit so a lower value resistor can be used.
Any other suggestions to bring B+ down?

Thanks !!
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Last edited by itsikhefez; 13th September 2020 at 08:51 AM.
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Old 15th September 2020, 10:50 AM   #737
rock12 is offline rock12  United States
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Default Parts list

Bought boards a year or so ago getting around to building. Is there a parts list anywhere all I received were the boards
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Old 15th September 2020, 12:04 PM   #738
kerontt is offline kerontt
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Default Parts list

Yes, there is a parts list:

Parts List | Tubelab

Let me know how you get on.
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Old 15th September 2020, 01:29 PM   #739
rock12 is offline rock12  United States
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Thanks most of overlooked this
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Old 15th September 2020, 04:57 PM   #740
itsikhefez is offline itsikhefez  United States
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Attached is the BOM I used, maybe you'll find it useful.
PM me if you prefer it in .xlsx or .csv format
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