Shopping for a power transformer.

johnblue

johnblue

Active Member
Okay, I've never built Amps from scratch and I have my heart set on monoblocks.

I posted before about my stancor 3311 transformers the PP resistance is 10k... but now I need some power supplies.

I'm going 807 or 1625 tubes in push pull.

I'm reading that max DC plate current for these tube types is 120mah. So do I just double that? Things add up quick if that's the case... It's going to be a pretty big traffo, but if I'll never need that maximum, I'm wondering where I stand.

How do I determine the rating for the HT voltage capability?

I'm going to shoot for about 450v to start with, but how much current is needed for a PP design with these tubes?

The heater current is straightforward, but I'm unsure of the HT current, and how it varies per application.

I know I have the front end to consider as well, and I am looking at the 807 data sheets, but I'm not exactly sure how big of a transformer I need with these tubes, and I hope to get some clarification before commiting any $.

What's the amp draw of two 807s sucking on 425v? How much extra current capability should I leave for good measure?


Thanks. :)
 
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1. You need to figure out the operating conditions of the output stage, and then add the requirements for filaments, and driver stages.i suggest starting with the RCA tube manual

2. Try downloading PSUD2 and fooling around with it for a while, it very nice freeware for power supply design
 
If you built per the datasheet example operating point it appears you'd need ~160mA max per pair of 807s, plus whatever the driver(s) take.

How much cushion one needs is an interesting topic. I will be interested to hear additional comment.

If you spec for max (class A operation excepted) shouldn't there already be cushion unless you are constantly running the amp WFO?
 
There are several things you need to know to estimate max current draw of the output stage with some accuracy:
1. The B+ voltage applied to the center tap of the output transformers
2. Plate-to-Plate primary impedance of output transformers
3. Class A or AB1 operation?
4. Cathode biased or fixed biased?
5. Triode, UL, or pentode connected output stage? If Pentode, what is the screen voltage that will be used?

That being said, you can sorta get into the ball park by looking at the data sheet and interpolating (aka guessing). For example, if you use 10K output transformers with 450V CT, class AB1, pentode connected with 300V screens, fixed biased, I'd estimate full power output would draw between 150 mA and 200 mA.
 
You could also work backward from the secondary. For example if you knew you wanted a given output power on the 8 ohm tap under an 8 ohm load, and knowing the turns ratio of the transformer, you could calculate the current needed at the primary to deliver that power at the secondary.
 
johnblue said:
What's the amp draw of two 807s sucking on 425v? How much extra current capability should I leave for good measure?
Click to expand...
Maximum 807 anode dissipation is 25W, so your answer is 25W / 425V = 59mA per tube for the upper limit. I would add perhaps 40~50% to the total for screen current, driver stages and design margin, then multiply by rectifier form factor to get required secondary current rating.
 
There has been a guy at the local flea market who does clean outs. I have been picking up some cool stuff from him and last Sunday I picked up a DC power supply. The transformer on it is made by Acme and it has both 5V @ 2A and a 6.3V @ 6A CT winding along with a 680V @ 125mA DC CT winding.
It's a big potted transformer with solder lugs on the bottom and a nice steel case that measures 3.75 x 3.75 by 5.25 tall.
Looks mil spec, he has a lot of old military radio stuff and this looks like a ham radio guys clean out. Actually has a Navy spec number on it, N17-T-75938-8778 plus it says EDO Corp 10320 and Acme Electric Corp T-14422.
Did a search for it but nothing comes up.
Anyway, what could I build with a transformer like this? Filament specs look good but I think the high voltage amperage is pretty light.
Thanks

BillWojo
 
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BillWojo said:
... 680V @ 125mA DC CT winding.
Click to expand...
Can you confirm that the marking on the transformer actually says 125mA DC, not AC? If that's true, then we would need to know what the convention was for this specification. How was the original rectifier & filter set up? Choke input or Cap?
 
There is a choke on the chassis, specs unknown along with another transformer. I guess that I will have to trace out the wiring and see if the choke is in circuit with the transformer in question.
My friend that is a lot smarter than me said that the 125mA DC rating on the transformer is what I should expect after rectification and filtering. I must admit I have never seen a DC rating on a transformer before.
I'll try and look at that tonight.
Thanks

BillWojo
 
Ok, that didn't take long, the choke is a Philco 32-8399, 352905 and is coming off of the rectifier tube, I believe it's a 5U4, that the big transformer is feeding. There is also two 0A3 tubes, I think they are a regulator tube? Not sure where they are in the circuit.
This PS has an octal socket that was used to power something else and is home made.
Any other info that you need?
Thanks

BillWojo
 
You can often run those old military transformers at a higher current than they're rated for. A class AB amplifier only pulls full current when it's under load, and load is far from continuous. I used an oil filled ex Canadian military Hammond transformer which has a secondary current rating of 200mA to build a 50W x 2 amp and it has been working reliably for 18 years
 
Thanks guys, that is alot of good Information. I've never had to dig deep Into tube data sheets. I'll make a rough plan and report back. Seems like 250mah would be more than plenty, but even that is a pretty big and expensive traffo. The parts pile is already starting to stack up and the power traffos will be the last "expensive" components..(famous last words)
 
maxhifi, you made me look at this transformer a bit closer, I sure do believe this one is oil filled as well, the outer steel case is soldered at the seams and the terminals on the bottom are on insulators like oil filled caps. It sounds like I should look for any more that he may have.
I have already grabbed any oil filled caps and all the loose chokes that he had.
I just looked in the box and pulled out a 5H, 130mA DC, choke rated at 1780 R.M.S. with a max DC resistance of 104 ohms, oil filled as well. Made by Raytheon Mfg Co and it's brand new.
Another oil filled transformer, 1000V CT @ 0.067A. No idea what that could be used for. Raytheon as well.
The best things I found were 3 oil filled caps made by Western Electric, I figured being made by WE they would have a bit of value but was floored when I saw recent Ebay sales on one of them (untested but with the WE logo) at over 400 bucks! All 3 of mine have the logo and test within 5%. Some days your just lucky.

BillWojo
 
Bill, I suggest you start a new thread on this issue. There are some old hands here who should be able to reach consensus about the meaning of that DC current rating. I suspect it was for the most common FWCT rectifier setup with cap-input filter, but I'm not confident.
 
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