Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mungo Park on March 18, 2021, 10:46:34 am

Title: Potential transformer with high voltage
Post by: Mungo Park on March 18, 2021, 10:46:34 am

I have a old transformer, I don’t know what it came from, it was assumed it came from an old radio.
I checked the leads and it has a centre tap, heaters for 5.5V and 6.5V. I am assuming the higher voltages are due to modern 120v wall outlets.
The real concern is the ht voltages are a whopping 386 0 386. Can anybody see a way to use this transformer in anything guitar amp wise. A quick calculation tells me under “normal” amp power supply this would give me something like 540V to my tubes.
Any opinions, even if they are to use it as a paper weight/ door stop, shot put practice for the kids, my papers don’t seem to blow around much.

Title: Re: Potential transformer with high voltage
Post by: thetragichero on March 18, 2021, 11:56:03 am

no identifying markings whatsoever on it? if it's from 110v or 117v days it's likely a nominal 370-0-370 or 375-0-375, but how much b+ current? without knowing that we won't know what our output stage would look like
but yes, most definitely could be used in a guitar amp, likely a big 50w or 100w

Title: Re: Potential transformer with high voltage
Post by: pdf64 on March 18, 2021, 12:06:51 pm

Transformer regulation characteristics mean it’s inevitable that unloaded windings will put out higher than intended voltages.

Title: Re: Potential transformer with high voltage
Post by: HotBluePlates on March 18, 2021, 04:52:05 pm

Quote from: Mungo Park on March 18, 2021, 10:46:34 am

... The real concern is the ht voltages are a whopping 386 0 386. ...

Quote from: pdf64 on March 18, 2021, 12:06:51 pm

Transformer regulation characteristics mean it’s inevitable that unloaded windings will put out higher than intended voltages.

In other words: what's the d.c. resistance of the high voltage winding?  Might it load down the voltage output when current is drawn?

Quote from: Mungo Park on March 18, 2021, 10:46:34 am

I have a old transformer, I don’t know what it came from, it was assumed it came from an old radio. ...

This has always been a mistake when I committed it:  The power transformer will do great at supply the kind of tubes it supplied, operating the way they did in the original item, to deliver the power supply voltages found in the original item.  So when salvaging a transformer from something, you can pretty well bet it will work best with the same power supply & output tubes in the original item.

When we go to repurpose it for something else, a lot of effort (and sometimes $$) is spent forcing the square peg into its new role.

After making the mistake a few times, I learned to measure everything from the donor chassis, and even copy most of what existed in the donor circuit.  Then I got smarter, and only picked donor circuits close to what I wanted in the end, and simply modified the preamp.

The last few years I haven't built anything, and just buy vintage amps.

(https://i.imgur.com/069rb4P.jpg)

Title: Re: Potential transformer with high voltage
Post by: Mungo Park on March 18, 2021, 10:11:00 pm

Thanks foer the wise advice, looks like I will have to leave it on the shelf for now.  I did not remove it from its habitat so to many unknowns. Yesterday with the original amp and connecting it to a variance for vintage voltage much could have been learned.  To many wildcards now. 

Title: Re: Potential transformer with high voltage
Post by: 66Strat on March 19, 2021, 10:39:46 am

FWIW, the no-load voltages of the transformer in question are comparable to those of the Hammond 274AX.
https://www.hammfg.com/files/parts/pdf/274AX.pdf (https://www.hammfg.com/files/parts/pdf/274AX.pdf)

As HBP suggested, you may want to measure the DC resistance of the secondary windings to get an idea of the current carrying capacity of the transformer. Those resistance measurements could also be compared to the Hammond data.

Title: Re: Potential transformer with high voltage
Post by: Mungo Park on March 19, 2021, 05:30:51 pm

Thanks I will do that since it will give me some more info and a learning experience
   

Title: Re: Potential transformer with high voltage
Post by: tubeswell on March 20, 2021, 07:28:18 pm

Quote from: Mungo Park on March 18, 2021, 10:46:34 am

I have a old transformer, I don’t know what it came from, it was assumed it came from an old radio.
I checked the leads and it has a centre tap, heaters for 5.5V and 6.5V. I am assuming the higher voltages are due to modern 120v wall outlets.
The real concern is the ht voltages are a whopping 386 0 386. Can anybody see a way to use this transformer in anything guitar amp wise. A quick calculation tells me under “normal” amp power supply this would give me something like 540V to my tubes.
Any opinions, even if they are to use it as a paper weight/ door stop, shot put practice for the kids, my papers don’t seem to blow around much.

What type of tube load do you have in mind?

Here's a thing you can do to guesstimate the load -

1) adopt a nominal load, Say you want to run 2 x 6L6 and 6 x 12A_7 and a 5AR4 rectifier

2) nominate a current load for each winding - it'll be 1.9A on the 5V winding and 4.1A on the heater winding (if you allow for a 500mA 6.3VAC lamp) and about 150mA on the HT winding

3) use Ohms law to work out the DC resistance across each winding that would give you the ballpark current and voltage that you might expect, and rig up a DC resistive load for each winding, starting with the heater and rectifier windings (just leave the High Tension winding unconnected/floating for the first trial). 5/1.9 = 2.6R (this will need to be a power resistor capable of handling (5 x 1.9) x 2 = 20W, or a bunch of power resistors in various combinations of series or parallel to get 2.6R with 20W dissipation*). 6.3/4.1 = 1.5R (this will need to handle (6.3 x 4.1) x 2 = 50W. You can calculate how many of each type of resistor you need in series and parallel to get 1.5R with 50W dissipation rating*)

*This is where its useful to keep a big bag of 5W and 10W cement resistors of various resistances in your stash somewhere

4) Rig up the dummy loads on the 5V and 6V windings and flip the switch. Measure the VAC across the heater winding. If you get 6.3VAC, you'll be sweet with this heater load. Then you can also measure the VAC across the HT winding. But if you get under 5.7VAC on the heaters like this, you'll know that the winding probably isn't built for a pair of 6L6s (and you'll probably feel the PT heating up).

5) repeat this step with a 2k5 resistance of 100W rating** across each side of the 386-0-386 (HT) winding CT (or a 5k6 resistance of 250W** rating between each end of the HT winding  - with no connection to the CT) , and see what VAC you get

**I'm assuming 150mA of current to start with. While ideally you want 200mA for a 2 x 6L6 push pull amp, you have no idea what the HT power rating is, and if you assume 200mA and try to do a resistance for that, there's a higher risk that you'll burn the winding if its not sufficiently rated for a pair of 6L6s (with a bevy of preamp tubes). If it runs okay sucking 150mA, then alter the resistances to get 180mA, and then 200mA etc, but only if you're happy that the PT isn't getting too hot too quickly. Keep an eye on the Heater and Rectifier windings VAC as you increase the load on the HT winding, and see it these start to dip too far.


Alternatively if you want to start off taking it easier on the PT at Step 1, figure the secondary winding loads you'll need for a pair of 6V6s etc