Substituting power supply for B+

[alchemy]

I have a project I'm working on that I need to lower the B+ to the plates. I have not decided exactly where I want it yet, and would like to unhook the B+ taps from the PT, and hook up a supply from a DC power supply.
What I'm wondering is, if I disconnect the B+ from the PT, but continue to use the heater taps, will this A) cause any damage to the PT from not having a load on the B+ windings?
and B) Cause the heater winding voltage or the bias tap to go up since a large part of the load has been removed? There is a 6.3v (4A) tap on the power supply so I could use this if I need to, but since I'd still be using the PT for the bias tap....

Seems to me like it should be fine, but I wanted to be sure since I've never done it before.

Also, any reason I should not use an old tube power supply to feed the B+? (I mean, that is what they are for, correct?!!)

Thanks as always for answering my stupid questions as I learn...

[jpfamps]

Unloading the B+ will be fine for the transformer, but will need to pay attention to the bias supply if it derived from the plate supply, ie you need to make sure that you still have a bias supply when you disconnect the B+!

As you as interferring with the B+ you will need to be fairly caution.

I have an old Heathkit valve power supply which I use sometimes, which will run a 30W amp fine. It was obviously designed for this purpose as it not only has a vaiable regulated B+ supply, it also has an adjustable C- (bias!), and 6.3 VAC and 12.6 VAC filament taps.

[alchemy]

The bias is derived from a separate tap on the PT. If I recall, these are usually a portion of the B+ and I would need to leave the center tap hooked to ground.
The power supply is an old Heathkit IP 32. Same one? Mine also has a C-, so I could get the bias from there. Really I just want to unhook the B+ and hook up the power supply to play with a few different plate voltages so I can decide what transformer to order. I didn't want to bother unhooking and rehooking up the other wires for a few minutes of testing.

[jpfamps]

Mine's an IP-17!

[alchemy]

Nice. I have not used mine for anything but forming caps since I got it, so I am glad to hear it's suitable as an auxiliary power supply. I wish the current rating on the B+ was a bit higher( says 125ma max) to go with the 4A heater taps. Anyway, at least it's good for smaller projects.

[imaradiostar]

A person could easily connect a bridge rectifier to a variac and a toroidal PT...poor man's adjustable high current B+ supply. Smaller variacs can be had cheaply on ebay, toroid form antek inc.

jamie

[Enzo]

I have several variable B+ bench supplies. That is exactly how they are made, a small variac driving a power transformer. The resiuts then rectified and filtered, with current and voltage meters. Actually I am thinking of selling them.

[alchemy]

I have several variable B+ bench supplies. That is exactly how they are made, a small variac driving a power transformer. The resiuts then rectified and filtered, with current and voltage meters. Actually I am thinking of selling them.

How come? Seems like an incredibly useful tool for designing/tweaking. Is it because you mostly do repairs and don't use them often?

[Steve Conner]

I had one of those valve regulated power supplies, but I don't use it any more since I got this.
http://scopeboy.com/xantrex-xkw-600-17/IMG_2217.JPG

[Enzo]

800v 1.7A??? Good Lord.


Alchemy, how many such things does a guy need? And yes, I do mostly repair work, I did all my designing amps and building them stuff 50 years ago.

[Wilder Amplification]

I have a project I'm working on that I need to lower the B+ to the plates. I have not decided exactly where I want it yet, and would like to unhook the B+ taps from the PT, and hook up a supply from a DC power supply.
What I'm wondering is, if I disconnect the B+ from the PT, but continue to use the heater taps, will this A) cause any damage to the PT from not having a load on the B+ windings?
and B) Cause the heater winding voltage or the bias tap to go up since a large part of the load has been removed? There is a 6.3v (4A) tap on the power supply so I could use this if I need to, but since I'd still be using the PT for the bias tap....

Seems to me like it should be fine, but I wanted to be sure since I've never done it before.

Also, any reason I should not use an old tube power supply to feed the B+? (I mean, that is what they are for, correct?!!)

Thanks as always for answering my stupid questions as I learn...

Out of curiosity, for what purpose do you need to drop the B+?

[alchemy]

This old amp came to me completely hacked up. No schematic exists anywhere, at least that I can track down. Both transformers had been replaced with incorrect units. PT was a center tapped 310-0-310, full wave rectified. 2 7591's driving a 15 watt champII transformer, plates at something like 440v (or so, can't quite remember). No matter what I did, the screens were getting too hot, until I changed the OT. Got the voltage down with in reason with zeners and dropping resistors on CT of power transformer , but ultimately it was creating so much heat I figured I'd just go with a new PT. Used the power supply to get it dialed in. Basically I ended up redesigning the whole amp and wanted to play with the plate voltage a bit before buying a new PT.

Worked like a charm by the way (though I was pushing the current limit on the power supply!). Thanks for all the advice folks!

[Glasgow]

That looks awesome! How much did that set you back?

[Steve Conner]

About 170 bucks, 2nd hand. I bought it from a friend who is a serious test equipment collector, he had two.

I also do work with power electronics, SMPS and the like. My old tube power supply used to struggle to fire some of that stuff up.