Transformer set for Hoffman AB763 one channel

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Hi all,

I'm looking to build the Hoffman AB763 one channel circuit. I'll be using two 6L6s. I've looked at the Hoffman parts store and think I've settled on the Mojo (Heyboer) 762 power transformer and Mojo 774 (Heyboer) output transformer. Also, for the choke should I use the Hoffman 125C1A or 125C3A choke? Thanks! Looking forward to trying one of the Hoffman layouts.

Ted

[sluckey]

big choke

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Got it, thank you. Looking forward to getting those nice Heyboer transformers :)
And I am very interested to try a Hoffman eyelet board. I have never used one before from his layouts, but I bet it’s going to be pretty good.

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Question about electrolytics for this AB763 I'm about to build. I'm building this amp with 6L6s and am wondering about the rectifier>filtering>dropping resistor arrangement.
I was thinking about going with the super reverb lineup which is a GZ34 to 35uf/700V>choke>22uf/525V>1K>22uf/525V>4.7K>22uf/525.
http://schematicheaven.net/fenderamps/super_reverb_ab763_schem.pdf
Question on this is I haven't seen many (or any?) 525v caps around as specified in the schematic. Can I go with 500V seeing as B+ after choke is 460 on the super reverb? Or should I run some larger caps in series to bring that voltage rating up? Like two 40uf@300v  to give me 20@600V?

Thanks!

[sluckey]

Assuming 120VAC line voltage, that PT has a 354-0-354 secondary. The theoretical maximum B+ it can make (using diodes) is 500VDC.

But... if your line voltage is 10% higher, the secondary would be 389-0-389. Now the theoretical maximum B+ it can make (using diodes) is 550VDC.

Now, that choke ain't gonna drop much voltage. IOW, if I was gonna use a series pair for the first cap, then I'd use an identical series pair for the second cap.

But, if space is an issue (maybe you're gonna put the caps under a doghouse) then go with the 500V caps and hope the B+ doesn't exceed that. Use a GZ34 because it takes about 20 seconds to put out any B+, thus giving the tubes enough time to be hot enough to conduct before B+ comes up.

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I should have plenty of space. Using Hoffman short board, single channel on a super Reverb chassis. There should be plenty of room for caps over by the transformer. Plus I’m going to use radial Nichicons in terminal strips.

So 35/700v before and after choke? Gotcha, thanks.

[sluckey]

That'll work. Don't forget to use 220K balancing resistors for both stacks. (4 resistors total)

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Thanks!

Another minor question: I'm going to run 1N4007 diodes in series with the HV secondary to the rectifier. These are 1A diodes; I'd be wise to run two in series for each half of the secondary, right?

Thanks!

[sluckey]

Another minor question: I'm going to run 1N4007 diodes in series with the HV secondary to the rectifier. These are 1A diodes; I'd be wise to run two in series for each half of the secondary, right?

No need for that. One diode per side is all you need.

[Willabe]

No need for that. One diode per side is all you need.

Because the SS diode and the tube diode are effectively in series?
 

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Thanks, I'll stick with one per leg.

And while I'm asking simple questions, here's a few more. This is my first build on a Hoffman board (AB763 single channel) and I'm wondering about the spacing between the lugs. I was thinking about using Sprague orange drops on the output tubes' coupling caps (.1uf), and Mallory 150s elsewhere. Am I going to have issues with fit using those big orange drops? Should I buy some smaller compact Panasonic radials instead? Thanks!

[sluckey]

Am I going to have issues with fit using those big orange drops?

Yes. Use the xicons instead.

[Willabe]

No need for that. One diode per side is all you need.

Because the SS diode and the tube diode are effectively in series?

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Thanks for the heads-up on the caps. Interested to try one of these boards!

[shooter]

 

IIRC a 2n4007 is like 1000piv @ 1A, unless he's fine tuning the B+ .6vdc at a time, they're cheap insurance

[Willabe]

IIRC a 2n4007 is like 1000piv @ 1A, unless he's fine tuning the B+ .6vdc at a time, they're cheap insurance

Yes, but why only 1? Sluckey said only 1 needed, that's only 1Kpiv. I know he's correct, just want to understand why.   

So? 

[sluckey]

No need for that. One diode per side is all you need.

Because the SS diode and the tube diode are effectively in series?

Haha. I just read your post again and realized you were asking a question. I thought you were just adding to my short answer. Yes, that's correct.

[Willabe]

Yeah, I should have wrote; Is it because........

That might have been a little clearer? 

Thanks Sluckey. 

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Started putting the amp together. Tube sockets, AC cord, pilot, transformers etc etc. I'm using the Hoffman short board ... plenty of room to put in the bias board, pot and the first two caps (34/700v and 34/700V) on either side of the choke. I'll run the later two filter caps between the board and pots.
Apart from wiring the bias pot in reverse  how's it look?

One thing I'm not sure with is the grounding scheme. I've always just grounded the HV CT, filter caps and assorted power stuff at a power transformer lug, and connected all other grounds to a point on the other side of the chassis via a 16g run of solid copper bus wire. What's preferred on Hoffman layouts?

Thanks!