Bias Supply Transformer: Huh???

[ChrisK]

Hi,

I would appreciate some help understanding some basic transformer stuff. Awhile ago, Tom Mcnally and Mesherm helped me a lot with the following idea:

The cheap and easy way that a lot of guys use is a reversed
filament transformer, feed 6.3 into the 12.6 winding and get
about 60 volts out ... the tiniest of transformers will work.

I have to do this because my power toroid doesn't have a 70v tap.

So, I have several small power transformers that I've tried on the bench, but the results don't make sense to me. I must be doing something wrong. The primary is rated at 117vac and the secondaries are 12.6-0-12.6. When I apply 6.3 volts (the toroid is rated at 10 amps) across the 12.6 volts taps, the 117 output reads 27vac (no load). When I apply 6.3 volts across the 12v CT and just one leg of the 12.6 tap, I measure 52.5 vac across the primaries.

I don't understand. A simple calculation (assuming no losses) tells me I should expect about 79vac across the primaries. Obviously, there's something I don't get here and could surely use some help. I don't understand this as well as I thought I did...

As always, thanks in advance. Once I get this straightened out, I'm ready to heat up the Mark III X2 Integrated Stereo Amp. Maybe even by tonight, with pictures to follow as soon as tomorrow.

Cheers,

Chris

[mesherm]

A 12.6-0-12.6 transformer is actually a 25 volt tranny with a centet tap so when you put 6.3 volts across the 25 volt secondary you get about 1/4 the primary voltage. 27 volts is about right. Now when you put 6.3 across the center tap and a 12.6 lead. you should get about 1/2 but your using only half the secondary windings so the efficiency is a lot less. The readings you are geting are about right.

One option would be to use a voltage doubler on the 12.6-0-12.6 with the 117 volt primary connected to where your PT primary is. This would give you 60-70 volts DC depending on the capacitors you use.

Another option is to tap off of one side of the HV PT secondary using a film capacitor and half-wave rectifier as you would with a 70 volt bias tap.
The film capacitor should be a 630 volt or higher and the value dictates what voltage you will get. I dont know the exact value but my best guess would be to start with a 0.05 to 0.1 mf cap.

[ChrisK]

The readings you are geting are about right.

Thanks, Mike.

Now it makes sense. Thanks for taking the time to help: It's back to the bench to try your ideas.

Regards,

Chris

[ChrisK]

One option would be to use a voltage doubler on the 12.6-0-12.6 with the 117 volt primary connected to where your PT primary is. This would give you 60-70 volts DC depending on the capacitors you use.

OK, so I bread-boarded a voltage doubler and trippler. Unloaded, I measure 72vdc output from the doubler and 108vdc from the trippler. Will 72vdc be sufficient once the circuit is loaded with TWO bias circuits (one for each Mark III channel)? I'm not sure because I don't know how many ma's are demanded by the bias circuit.

If 72vdc is insufficient, I could use the voltage trippler with 108vdc with a drop-down resistor. But, I would need to know I to calculate the R value.

Anyone know offhand how many milliamps are drawn by two Mark III bias circuits?

Thanks,

Chris

[mesherm]

Load the doubler output with a 10K resistor and that should simulate a couple of bias adjustment circuits. The bias to each tube is applied through at least a 100K resistor into the control grid so the actual bias current drawn by the tubes is negligable.

[ChrisK]

Load the doubler output with a 10K resistor and that should simulate a couple of bias adjustment circuits. The bias to each tube is applied through at least a 100K resistor into the control grid so the actual bias current drawn by the tubes is negligable.

The timing of your response is perfect. I was just starting to lay out the parts on a terminal strip for the permanent installation but needed some kind of a reasonable test. It's nice to know if I'm in the ballpark before the solder flows.

Probably anal-retentive, but I thoroughly dislike desoldering stuff. 8-(

Thanks, Mike. I'll post an update later.

Chris

[Hogweed75]

I've used this exact same method on a larger amp that I have. It uses large UTC transformers that didn't have a bias tap but did have an extra 6.3v tap.

I used a 6.3v transformer reversed which is attached to the UTC transformer. Now you have 115v out which is rectified with a full wave bridge. This is attached to a Hammond 158M 10 Hy choke. The choke input drops the voltage to the 1000uf/100v cap. Then I used a cheap KA7924TU 3-terminal -24v Linear Negative Voltage Regulator that is floating off ground with a NTE5142A 33v 5 watt Zener. You now have a negative 57 volt regulator. Then I used a 2200uf/63v cap. All caps are bypassed with a .01uf polystyrene rel cap’s that I had a bunch of lying around. At least bypass with polypropylene.

I was able to use this same regulation and 10 Hy choke in three Mark IV Poseidon mods that I did but didn't do choke input. Cap input and then the choke since we're working with a lower output voltage from the transformer. I mounted the choke in place of the rectifier socket on top of the chassis and used solid state high voltage rectification. So that I don’t zap the tubes until warmed up, for the B+ I used a 555 timer circuit board that I purchased off ebay but I don’t see anymore. The filaments and bias are on for about 60 seconds before the main B+ kicks on.

I also used a full wave bridge on the filament supply with a 10,000uf cap bypassed with a styrene. I also used two of the C-354 1.75 Hy chokes on the B+. Transformer, 80uf cap, choke, 100 uf cap, choke, 100uf cap, resistor down, 100uf cap, 100uf cap, resistor down, 150uf cap, 300v out. Works very well! Very quite and clean sounding with lots of tight bass.