Peavey Nashville 400 amplifier enhancements

[rossmoffett]

My father-in-law came visiting in the USA and bought this well-loved Peavey Nashville 400 for his steel guitar.  Understandably, country/western music is not very popular in Singapore so he has a hard time sourcing equipment like this locally.  This model of amplifier does not come with a 115VAC/230VAC switch, however, so I have to mod the power supply.  Peavey isn't stocking the international version of the transformer for this amp anymore, and I don't want to add weight with a downconverter.  He's getting rather old and I'd rather reduce the weight so it's easier for him to get around.

I figured I might be able to buy a SMPS from eBay and place an inductor in series to get rid of some HF before it gets into the audio amplifier.  The power amp needs +52/-52VDC supplies, while the pre-amp runs on +15/-15VDC.  Probably needs to be about 400W to be safe for the 50V section, and not more than 100W for the 15V section.

I see lots of supplies on eBay such as this one with negative taps, but I suspect this is just a 0V/ground tap.  Can anyone who purchases these confirm for me?

[wraper]

I see lots of supplies on eBay such as this one with negative taps, but I suspect this is just a 0V/ground tap.

Yes this is just 0V. It have only one 48V output, it does not provide +-48V with center tap.

[rossmoffett]

Bummer.  Can you give me some advice on good search terms to weed out these single-channel supplies?

[wraper]

You will have really hard time to find SMPS with such requirements. IMO easier to find the transformer that would fit.

[rossmoffett]

One secondary is 20VAC CT (big power resistors pull it down to 15VDC in the rectification/filter stage), the other 36 VAC CT.  Amperage unknown, but I presume that 2-4 amps should do it for the low voltage section and probably 10A+ for the HV side.  Back of chassis says it may pull 600 watts but that seems extreme even for this inefficient 210W amp.

Schematic here on my google drive

Toroidal trafos would be ideal, in order to reduce weight.  Know any good sources?  Hammond doesn't seem to have exactly what I need unless I pay a couple times more than the amp is worth.

[wraper]

Cannot suggest anything about places to source them as I'm not in the US.

[rossmoffett]

I had another thought.

If I purchase two ~50VDC SMPS and tie the 50VDC output of one to the 0VDC output of the other, will that cause some kind of short or is there sufficient isolation that this would give me a negative bus supply from the 0V output of the second supply?

[Marco]

The 15V DC lines can only deliver 86 mA before the resistors pull the voltage below 15V ... 100 Watt is ridiculously overdimensioning it, it delivers 5W into a short.

So buy a 2x36V toroid (plenty on ebay) and some dinky (2x)20V transformers or two 28VDC or 32VDC switching power supplies for the 15V DC lines (yes really, that's how much that resistor+zener regulator drops).

[dadler]

I have had good experience with the transformers I have purchased from here:

http://www.antekinc.com

They also sell on eBay, but you can get better pricing and better selection on their website.

[rossmoffett]

Thanks for analyzing the low voltage circuit for me - I wasn't sure what was going on there.  I think I'll get two of these for  hammond toroids my HV side and find a power supply in my parts stash for the +/-15V supply.  I haven't been successful finding a beefy enough toroid to handle the load all on its own, but these two together with their center taps taped off should make what I need.

Power Transformers Dual Secondary, Open 225VA, 36V CT@6.25A
Hammond Specs

While I was typing this I saw the link from dadler, thanks for that!  Those prices are great and it looks like they have some bigger ones for me.  This one in particular appears to have everything I need.

[dadler]

Thanks for analyzing the low voltage circuit for me - I wasn't sure what was going on there.  I think I'll get two of these for  hammond toroids my HV side and find a power supply in my parts stash for the +/-15V supply.  I haven't been successful finding a beefy enough toroid to handle the load all on its own, but these two together with their center taps taped off should make what I need.

Power Transformers Dual Secondary, Open 225VA, 36V CT@6.25A
Hammond Specs

While I was typing this I saw the link from dadler, thanks for that!  Those prices are great and it looks like they have some bigger ones for me.  This one in particular appears to have everything I need.

No problem!

A while back, I was speccing toroidal transformers for a couple of power supply projects. I was originally looking into Hammond as well, but happened upon AnTek. I guess they are used in higher-end audio equipment. Prices are good, quality is high. Made in USA in NJ.

Here are the few I picked up recently:




They are AN-5435, AS-1215, AS-0509, respectively.

[calexanian]

It would be much easier to get a auto transformer so you can run it on local voltage

[rossmoffett]

It would be easier, except in the long term an old man has to lug it around.  With a Toroidal transformer I'll drop a few kg instead of adding more, for the same cost.  I've already got the whole thing disassembled for a tone circuit mod, scratchy pot replacement and to replace the 30 year old electrolytic capacitors, so it's not actually that much effort.

[rossmoffett]

For the low voltage supply of +/- 15VDC the old trafo has an 18VAC CT winding.  The replacement I want to use has an 18VAC and 12VAC winding, both not center tapped.  If I place them in series, I get plenty of voltage to make the +/- 15VDC but without a center reference for the diode bridge.

Does 250 ohms, 5 watts seem like a suitable resistor to use to create a virtual center tap?  If the circuit load is only 86 mA that seems alright since each side of the winding will be sinking 120 mA to ground.  Or, without a virtual or real center tap, are the capacitors suitable on their own to create a center reference?  I've never really worked on solid state amplifiers before so I haven't ever had to worry about creating a negative bus, always just grounded the low side of the diode bridge.

I bought this transformer (specification)
Amp Schematic Again

[NiHaoMike]

Use switching PSUs for the main rails. He'll really appreciate the savings in weight. The low level supplies are low power and a simple transformer will do. Doesn't cost much to get the right one.

[rossmoffett]

Too late..

I wasn't having any luck finding good +/- voltage supplies at reasonable prices (they were priced like bench supplies) and wasn't sure if I could put two in series to get a dual supply.  Switching to a Toroid cut some transformer weight already.  I also ordered a replacement modern 15" speaker with a Neodymium magnet to cut a further 6 kg.

I was pretty concerned with the downsides of a switching PSU too.  They are tough to get the noise out of in amplifier applications.  For a while everyone hated new amps with SMPS, then after the manufacturers were able to get the noise out all is fine again.

Still not sure about this accessory supply operating with no center tap.

EDIT:
When I simulate without a load, even with an AC offset, it seems like it will center around ground with +/- 15VDC taps.  But when I add a load, then that channel goes to zero.  I'm not sure if this is a mechanism of the simulator that I'm getting wrong or what.  I guess I'm going to have to just clip the center tap on the old trafo and see what happens - then repair the aftermath if necessary.

[void_error]

Aren't those resistors (R1 & R2) a bit too large at 150k?

[rossmoffett]

Way too large - I don't know how I missed that.

It's operating fine now - guess I'll give it a go without a virtual center tap.

[kaindub]

http://www.altronics.com.au/p/mc5540-powertran-40-40-300va-toroidal-transformer-with-aux-windings/

This transformer has almost the correct voltages and separate windings for the amp and preamp supply