UNKNOWN TRANSFORMER

[Marcomel79]

So i got a hold of a set of transformers from an old tube organ, a Yamaha Electone B-6, from 1965. It ran two 7868 and two 6AU6. It has a 6.7VAC filament heater with CT, 13VAC and 192VAC HT winding with no CT. Plus a misterious black wire that has no continuity with any of the primary or secondary.

The HT seems very low and im wondering if any of you has any idea why..... the power transformer is massive though. The matching OT is 9K Ohms PP. And the filter choke measures only 8.7Ohms resistance.

I would like to use them to build an amp with the aforementioned tubes. Any ideas?
Here are some pics of the PT. I tried looking up the serial number but i got nowhere.....

 

[william vogel]

Solid state rectified, you’ll get about 260 volts HT for the power tubes. You can use the 13 volts for a bias supply to maximize the power output but it’s really not going to make more than about 10 watts rms. Using a doubler circuit would be disaster and way too much voltage. It’ll make a wonderful lead tone amplifier but won’t have much clean.

 

[Matthews Guitars]

7868 tubes are electrically equivalent to 7591s and drive requirements are the same as for 6BQ5/EL84 or 6V6 types.

So think of this as a transformer set for an EL84 or 6V6 push-pull amp.


Me, I'd be thinking "Trainwreck". Man I've GOT to get back to my Express project some time soon!!!

But you're going to have low plate voltage. As Willam said, you aren't going to have much clean headroom. But on the flip side, you might get some monstrous overdrive capacity out of it.

 

[Marcomel79]

Solid state rectified, you’ll get about 260 volts HT for the power tubes. You can use the 13 volts for a bias supply to maximize the power output but it’s really not going to make more than about 10 watts rms. Using a doubler circuit would be disaster and way too much voltage. It’ll make a wonderful lead tone amplifier but won’t have much clean.

Sorry William i forgot to specify that the HT is 192V across both leads, not 192-192... i guess that makes a difference?

 

[Pete Farrington]

i forgot to specify that the HT is 192V across both leads, not 192-192...

As you advised there’s no CT, that’s the only way it could be.

 

[Marcomel79]

As you advised there’s no CT, that’s the only way it could be.

.

 

[william vogel]

Sorry William i forgot to specify that the HT is 192V across both leads, not 192-192... i guess that makes a difference?

I got that. 192-0 no center tap. I multiply 192 x 1.35 = 259.2 volts. I use 1.35 vs 1.414 because they draw down a little and 1.35 is usually about right for 65% dissipation idling.

 

[Marcomel79]

I got that. 192-0 no center tap. I multiply 192 x 1.35 = 259.2 volts. I use 1.35 vs 1.414 because they draw down a little and 1.35 is usually about right for 65% dissipation idling.

What about that one wire with no continuity to any if the other wires? Have you ever see something like this? It was just grounded to the chassis....

 

[Pete Farrington]

It’s probably a screen, to reduce high frequency nasties.

 

[william vogel]

What about that one wire with no continuity to any if the other wires? Have you ever see something like this? It was just grounded to the chassis....

Like Pete said, it’s probably a shield/screen link that should be attached to the chassis. I would put it solidly to the chassis nearest the transformer and I wouldn’t ground anything else at that point.

 

[Marcomel79]

Thank you for all the help @william vogel and @Pete Farrington

 

[alpha al]

It was maybe used in a voltage doubler circuit.

 

[Marcomel79]

It was maybe used in a voltage doubler circuit.

I think so.....

 

[Marcomel79]

Hey @william vogel and @Pete Farrington, would this be a more suitable transformer to get a bit more power out of the 7868's? Will 100mA be enough for those tubes?

 

[thetragichero]

so i understand completely wanting to build with what you've got (I've turned a Hammond reverb amp run off 2x6gw8/ecl86 into a guitar combo), but if it were me I'd use a commonly-available power tube for whatever you decide to build

you might be able to use a voltage doubler with smaller capacitors (had a play around with a calculator found here, assuming 100ma current draw 22uf: https://web.archive.org/web/20111128050710/http://www.electronixandmore.com/articles/calc.html)

 

[Marcomel79]

so i understand completely wanting to build with what you've got (I've turned a Hammond reverb amp run off 2x6gw8/ecl86 into a guitar combo), but if it were me I'd use a commonly-available power tube for whatever you decide to build

you might be able to use a voltage doubler with smaller capacitors (had a play around with a calculator found here, assuming 100ma current draw 22uf: https://web.archive.org/web/20111128050710/http://www.electronixandmore.com/articles/calc.html)

I keep hearing how good 7868 sound, so i thought this would be a good opportunity to find out...

 

[Marcomel79]

@thetragichero, following your lead i would like to possibly use this PT

for another project, which brings me to my questions.

Looking at Rob Robinette's page, the 6V6 jcm800 caught my eye. Its supposd to put out about 22W with two 6V6.

#1: can i use this PT rated 100mA for two 6V6 in push pull fix bias?

#2: can i build a fixed bias circuit tapping the HT from a bridge rectifier since this PT doesnt have a center tap for the HT?

 

[Pete Farrington]

For a given DC HT current, the AC winding current is significantly higher with a FWB rectifier than a push-pull 2 phase rectifier whose winding CT is connected to circuit common.
The max DC HT current that a valve power amp draws is determined by several factors. Most significantly the OT primary impedance.

A fixed bias supply can be obtained from a FWB HT winding via a coupling cap.

There’s loads on good further info on both the above topics on the valve wizard site.

 

[Marcomel79]

For a given DC HT current, the AC winding current is significantly higher with a FWB rectifier than a push-pull 2 phase rectifier whose winding CT is connected to circuit common.
The max DC HT current that a valve power amp draws is determined by several factors. Most significantly the OT primary impedance.

A fixed bias supply can be obtained from a FWB HT winding via a coupling cap.

There’s loads on good further info on both the above topics on the valve wizard site.

According to valve wizard this is the current consumption for a pair of EL34s in fixed bias

Given that 6V6 are half the power rating of EL34 i should be well within the current limit with 100mA.... granted the voltage will be a bit lower, i still tend to like a cooler bias in all of my amps. Plus im a home player so i dont need to crank the amp when i play.... does it make sense?
By the way the OT has 9k Ohm primary..

Thank you for the tip Pete, i did find the schematic for the bias circuit.

 

[thetragichero]

your capacitor value for your bias circuit will determine how much voltage you can get so i would not put in tubes until you can verify appropriate bias range (this is a good rule of thumb anyway)
9k is an *interesting* primary impedance for fixed bias 6v6... deluxe reverb went 6k6. i mean it should still work