Altec 438 power transformer replacement question

[bobschwenkler]

I've bought a pair of 438As that need some upkeep. Both are missing their PTs. The circuit uses a voltage doubler on the 117V secondary. I'm wondering if there's any particular reason I couldn't use something like a 240V secondary and axe the doubler circuit?

 

[emrr]

I don't see a reason to avoid redesigning the PSU, if it seems the easiest path. 

 

[bobschwenkler]

The only reason the thought was crossing my mind was based on looking through Hammond's offerings and not finding an offering that was a great match. However, I'm not too familiar with manufacturers of tube PTs so I'm sure I could stand to do some more research and look for something that would work well.

Any quick directions anyone would point me in? The schem shows a 1:1 for the B+ and 6V CT secondary.

 

[emrr]

Look at Edcor, might be a stock, custom would be pretty cheap. 

 

[bobschwenkler]

I'll check them. Thanks.

 

[lassoharp]

The only reason the thought was crossing my mind was based on looking through Hammond's offerings and not finding an offering that was a great match. However, I'm not too familiar with manufacturers of tube PTs so I'm sure I could stand to do some more research and look for something that would work well.

Any quick directions anyone would point me in? The schem shows a 1:1 for the B+ and 6V CT secondary.

I built my 436 clone onto an existing preamp chassis using the stock PT which was a 300-0-300 - a very common value.  My final voltages were a bit higher than the stock values but could have been brought down by using higher dropping resistor values.  Mine also used a tube rectifier.

Since your unit uses silicon rectifiers you probably want to stay in a lower range - say 200-0-200 to 220-0-220.  Then just accept the slightly higher voltage or increase the value of the dropping resistors.  You can always replace the single 5W 7K with two lower wattage resistors.  Pretty sure both Hammond & Edcor carry these range values.

 

[bobschwenkler]

So you'd have ditched one half of the B+ secondary when wiring, right?

 

[lassoharp]

I should have mentioned this, but I was assuming using the two existing diodes and rewiring as a full wave rectifier instead of a voltage doubler.  So you would be using the entire sec, CT to ground.

Then just rewire the filtering caps as needed.  It looks like c12 could possibly be re purposed for some extra heavy duty 1st stage pre amp filtering by rewiring to where C11A is, or even in // with it.  Or rewire C12 in place of or // with C11B.

 

[bobschwenkler]

Oh, of course. I'll keep it in consideration, and it doesn't seem like it'd be too big a deal to implement, but I'll see what I can dig up that's closer to the original

 

[lassoharp]

For just keeping the voltage doubler part intact this looks pretty close but is a bit hefty on current so may be larger than original.  And for this, as you said, just wire half the secondary.


http://www.edcorusa.com/products/701-xpwr104_120.aspx