Hammond AO-32 PT

[Meat&Beer]

Hi Guys.

Looking for a little info on this PT, I can't seem to find anything and I know a few of you have some source of old Hammond iron info stored away. This is the AO-32, taken out of a PR-40 tone cabinet. It was sitting on a shelf in the chassis it lived its life up to now in, and I had labeled its unloaded voltage as 100VAC before taking it out.

Since then, I'm not positive which wires I tested on the primary. There are three. Can someone inform me as to what I'm looking at here? I may be reading into it too much, but where is says 110 on the primary isn't quite in the center, and then there's a 120, and the third at the top is not labeled on the schematic. Is it a dual primary of sorts?

I'm looking to use this for a fixed bias KT-88 PP build for a friend. Any info is greatly appreciated.

Follow up question, this used two 5U4GB's in parallel. Would that make the max reservoir capacitance value double, or stay the same? They had it set up for choke input, but was wondering what the cap could be for a CLC type with this rectifier configuration if we choose to go that route.

Thanks guys! Double posting this over at the newly discovered diyguitaramps site.

[Phil_S]

I'd assume the choke input is used to tame the B+ and squeeze out more current. The rule of thumb (not so accurate, but OK for discussion) is 45% of 830V or 373.5 B+ at the first cap and current on-tap is 1.54% of the PT's rating. Naturally, CLC input will give much higher voltage. I'd wonder just how many big bottle tubes CLC input will support before the PT runs out of current.

As for the schematic, I think you can assume the unlabled is the "0" wire. There is a tap near the end of the winding for 110V wall supply and the whole winding is for 120V. Nothing dual here. Your ohm meter should tell you what's what. 0-120 will give you the highest reading, but 0-110 will only be about 10% less. Of course, you'll get the "10%" reading if you meter between 110-120, so that will be obvious.

Here is the PR40 schematic: http://www.captain-foldback.com/Hammond ... QR40_1.gif It looks to me like the PR40 supported 6x 6BQ5 (EL84) at 325V. This surely explains the choke input. I'd guesstimate 80mA per tube pair x 3 = 240mA, suggesting the PT is rated at about 155mA. OTOH, it looks like you have lots of 6.3V current on tap -- by the looks of it, over 5A and the double rectifier means 5V @ 6A.

I'm going to guess this PT will run a pair of KT88's with CLC input. I guess you know you'll probably looking to totem pole pairs of 350V caps to be sure they withstand the inrush current.

I'm not sure if you can double the input caps with a dual rectifier setup. Someone else will need to answer on that one!

[Meat&Beer]

Thanks for checking in, Phil! I thought it was 1.7 for choke input? I may have been mislead.

So actually, this is for a pre-existing amp that the PT is getting far too hot over the coarse of a couple hours, and has mysteriously shed about 30 volts at idle since being built ~4 months ago. Has 6550's in it now, planned on bumping B+ as well as tubes (and choke now that I think about it more.). I'll post the schem and more info tonight if I have time.

[Phil_S]

This choke input is from Hammond. That's solid state, so I forgot about that. I think you should deduct about 10% for a 5U4GB. With CLC input, I'd guess B+ around 525VDC. I'm wondering if you'll have enough current. You can try to estimate it with Duncan's PSU simulation.