Trust this transformer any more or less?

[nsubulysses]

I really appreciate all the insights.

Maybe this transformer would work better since you are recommending this Marshall style with full wave bridge -- Hammond 282X

If my math is right (500V x 1.4) this transformer with 1000V CT would give about 700V for B+ and then I can get my 350V from the center connection of my first filter cap. Like this Marshall but higher voltage and use two series caps instead of one? ?

Why is it necessary to connect the center tap to the middle junction of the two series filter caps. If the two caps have dropping resistors shouldn't it be about 350V anyway? Or is it because it makes the 350V supply stiffer rather than fluctuating freely with the B+?

[loudthud]

Maybe this transformer would work better since you are recommending this Marshall style with full wave bridge -- Hammond 282X

If my math is right (500V x 1.4) this transformer with 1000V CT would give about 700V for B+ and then I can get my 350V from the center connection of my first filter cap. Like this Marshall but higher voltage and use two series caps instead of one? ?

NO!

Why is it necessary to connect the center tap to the middle junction of the two series filter caps. If the two caps have dropping resistors shouldn't it be about 350V anyway? Or is it because it makes the 350V supply stiffer rather than fluctuating freely with the B+?

A simple Voltage divider would have a source resistance that is basically the two resistors in parallel. The capacitors don't change that much at DC. If you used 100K resistors, the source resistance would be 50K. Way too high for a screen supply.

Google Thévenin's theorem or look here: https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem

[nsubulysses]

I appreciate your big text LT. Sorry I was doign the math wrong. I was writing my post as fast as possible.

I will get back soon with next gameplan haha

[loudthud]

I apologize for the size 7 text, I edited it down to 3 bold. I hope you understand that power supplies are dangerous and expensive. I had a transformer custom wound for such a project. 420VCT @ 600mA. Still on the todo list. Kevin O'Connor published a schematic for such an amp in one of his less popular books on power amplifiers. I don't remember what he used for a PT, I'll see if I can find it tonight. The OT was a 100W Hammond 1650R with a 5K primary straight for the RCA 6550 data sheet.

[loudthud]

Guild made a couple of these 100W Two tube bass amps. Here's a schematic of one that uses a tube Voltage regulator. Note the separate 6.3V winding for the regulator tube.

http://www.prowessamplifiers.com/sch...s_Version.html

KOC published a two tube split rail schematic in his little known book, Principles of Power. He gives two possible power supplies. One uses a Hammond 278X with a solid state regulator for the screens. The other uses a Voltage doubler and a transformer he calls EG5BP and a separate heater transformer. I never heard of an EG5BP, but google found it. You can find it here: http://www.hammondsales.com/pdf/section1.pdf way down on page 72. I think Kevin chose the wrong transformer with too high a Voltage of 347VAC. I would try the EG5EP with only a 208VAC winding. There are many isolation type transformers that would work with secondaries of 220 to 240V. KOC gives plenty of examples of this in his SVT clones that you can find in TUT 5 (I think).

[Enzo]

SG made some 100 watt amps with single pairs of 8417 tubes for outputs. Awesome tube. The B+ was 600v and 300v.

[Lauri]

I really appreciate all the insights.

Maybe this transformer would work better since you are recommending this Marshall style with full wave bridge -- Hammond 282X

If my math is right (500V x 1.4) this transformer with 1000V CT would give about 700V for B+ and then I can get my 350V from the center connection of my first filter cap. Like this Marshall but higher voltage and use two series caps instead of one? ?

Why is it necessary to connect the center tap to the middle junction of the two series filter caps. If the two caps have dropping resistors shouldn't it be about 350V anyway? Or is it because it makes the 350V supply stiffer rather than fluctuating freely with the B+?

[ATTACH=CONFIG]50382[/ATTACH]

This is probably the kind of PSU circuit you're thinking about but 1000V center tapped winding is way too much voltage for anything sane. You can find an example of an amplifier using this kind of a PSU in GEC KT88 datasheet http://frank.yueksel.org/sheets/084/k/KT88_GEC.pdf

[Zozobra]

Good transformers from good manufacturers are good for many applications.
HOWEVER, every manufacturer makes various grades of products so there is really not sufficient information supplied by the seller on eBay. Specifications are given but they are just written in the description by the seller so I would consider those specs unverified. If the stated specs are correct we still don't know if the transformer would achieve that performance at a 100% duty cycle within a reasonable temperature rise. For $70 plus ~$40 S&H I would buy a known new and guaranteed product if I was going to put the effort into an amp build.

You can get a mag comps PT for a little more that the asking of that ebay listing:
http://triodeelectronics.com/potrfor10maj.html

[nsubulysses]

This is probably the kind of PSU circuit you're thinking about but 1000V center tapped winding is way too much voltage for anything sane. You can find an example of an amplifier using this kind of a PSU in GEC KT88 datasheet http://frank.yueksel.org/sheets/084/k/KT88_GEC.pdf

Yes! thanks for posting that.

it seems like if I do it in this arrangement it would be easiest and also it seems like this arrangement would be even better (?) than the marshall style arrangement where center tap goes to the node between the series caps for B+ supply. Just guessing, but it seems like screen supply would be stiffer because it would be more isolated from B+?

It also seems like I have to do regulated screen supply or voltage doubler if I want to do an off the shelf transformer, and for the Marshall style using FWB and utilizes the center tap half voltage I would need to have a transformer made. I didn't really come across any that can do that.

LT, I have those KOC books and am checking them out now thanks

[nsubulysses]

You can get a mag comps PT for a little more that the asking of that ebay listing:
http://triodeelectronics.com/potrfor10maj.html

Zozobra I appreciate your help but this is not high enough voltage for my application which is 100W amp with only TWO power tubes.

It does show the Marshall arrangement though where the center tap is connected to the middle of the series power supply caps rather than just disconnected. I guess this must be an advantage to connect it, but what is it? less voltage sag when amp is turned up?

[nsubulysses]

I had a transformer custom wound for such a project. 420VCT @ 600mA. Still on the todo list. Kevin O'Connor published a schematic for such an amp in one of his less popular books on power amplifiers. I don't remember what he used for a PT, I'll see if I can find it tonight. The OT was a 100W Hammond 1650R with a 5K primary straight for the RCA 6550 data sheet.

Just curious, why 600mA? is it in case you ever wanted to add two power tubes and do 200W amp?

http://www.jacmusic.com/KT88/kt88_spec_sheet.pdf

Genalex KT88 datasheet says with 560V on anodes and 300V on screens you can do a 100W amp and max anode current is 2x145mA, so 290mA

[Bloomfield]

Hammond has other lines of plate transformers outside of the usual guitar amp stuff which get up into some higher voltages. You'd have to use a separate filament transformer but they are fairly cheap.

Here are some examples (with Canadian prices) http://www.a1parts.ca/transformers/H...700_Series.htm

That A-1 site is very good for finding Hammond transformers; easier to navigate than Hammond's own site.

Andy

[loudthud]

Just curious, why 600mA? is it in case you ever wanted to add two power tubes and do 200W amp?

http://www.jacmusic.com/KT88/kt88_spec_sheet.pdf

Genalex KT88 datasheet says with 560V on anodes and 300V on screens you can do a 100W amp and max anode current is 2x145mA, so 290mA

First of all, the numbers you see for current on a tube data sheet are the DC current taken from a bench power supply when the amp is making max sine wave power. (560V x 290mA =~ 160VA) When the amp is banging the rails making a square wave, it will draw more. For the KT88 example, probably on the range 360 to 400 mA. That's not the RMS current in the secondary of the power transformer. It will be higher still. When you see transformers rated for DC current output, the manufacturer is really making a Wild Ass Guess (the WAG method) what your circuit will be and what your application is. It's not the same number your are going to use when you go to get a custom transformer wound. If you tell the custom transformer guy you want XXX DC output current, he'll know you're an amateur smuck who doesn't know what he's doing. When the transformer burns up, he'll say, "Oh you want DC current?". Then you'll get a transformer that's twice as big and cost twice as much.

This is all well and good except that guitar amps don't have to make square waves continuously. KOC uses a 100VA transformer for a 100W guitar amp. Way undersized IMHO. See the example above for the EG5BP and EG5EP. In my experience, you can get away with this but the power transformer is going to run hot and might need a fan. Those Hammond control transformers are tough as nails and can probably stand it.

The 600mA is an AC RMS number that came from taking a 300VA core and subtracting heater and bias windings. It was a conservative design meant to not get very hot and last forever.