Power transformers

[diode breath]

I want to build a 3-12AX7, 2-6V6, 1-5U4 amp. I know there are a lot of schematics out there using the 3-12AX7 preamps (5E3 Deluxe, 5F6 Bassman, etc,) but I want to use a B+ value of about 350VDC. Most of the schematics I've seen show even Princetons with 400VDC B+.

My question is this -- what would be a good power transformer to use to get ~350VDC on the plates of the 6V6s? Would it be better to use a tweed Deluxe power transformer and install a zener diode on the secondary center-tap, and make adjustments as needed for the bias supply?

Any help'd be appreciated. Thanks in advance,
diode Breath

[booj]

https://taweber.powweb.com/store/025130sch.jpg

Here's one that might work. Unlooaded it will put out about 380 volts. With everything running it will probably drop down to about 350.

[Chuck H]

Maybe the Hammond 270ex (550ct @125mA, 115v primary). The 115V primary means you have to figure for that. With 125 wall volts (like I have) the 550ct secondary becomes a 600ct. I'm using a 270ex with a pair of el84's and three 12ax7's. I'm using a 150 ohm sag resistor (a 5u4 will probably drop the same or a little less at your current) and I'm getting about 355Vp. It also has a 5V@3A and 6.3V@4A secondary. A 5u4 takes 3A of heater current but IMHE the Hammond unit will take this all day long. So if you can tolerate 355 to 365Vp this model will work for you. RadioDaze sells them for US $58.00. Antique Electronic Supply doesn't have the 270ex, but they have the 270fx. It's the same as the 270ex but with 25mA more capacity on the HV secondary and 1A more capacity on the 6.3V secondary. It's also physically larger and just overall more PT than you need. But AES sells it for US $55.00. I didn't check shipping for either distributor.

Chuck

[tubeswell]

With a 5U4, 3 pre-amp tubes and 2 x 6V6 aiming at 350 B+ , I'd go for 290-0-290@120-150mA, 6.3V@3-4A, 5V@3A, (and a 50V bias tap if you want fixed bias).

[Chuck H]

Tubeswell, remember that the Hammond has a 115V primary. So with 120V from the wall it's actually a 288-0-288. At 125V from the wall it's a 300-0-300. So I'd say the Hammond 270ex (secondaries: HV 125mA, 6.3@4A, 5V@3A) would be perfect.

Chuck

[tubeswell]

Yup - good point Chuck

If the OP finds it a bit too high because of that, they can always zener down the B+ I guess. (Or put in a zener with a shorting switch, for carefree voltage adjustment - not that 10 or 20V here or there probably makes a lot of difference).

[Chuck H]

New info... Hammond started adding a 125V primary tap to all their 2XX series PT's. So now you can choose the primary that's right if you like. What I like best about this is that I can use the primary to +/- the B+ by 45 or 50 volts if I need to without dramatically effecting the 6.3V tap. All production since Aug 2009 has this feature.

Diode breath has 126 volts from the wall. So with a Hammond 270ex using the 115V tap would be 302-0-302. That will probably net about 365 to 370Vp. Using the 125V tap on the same PT would give 277-0-277 and probably net about 320 to 325Vp. For 6V6's I'd use the 115V primary. Diode breath, if you plan to cathode bias this build you should keep in mind that the plate to cathode voltage is essentially your plate voltage. So... If you have, say, 25 volts on the cathode and 370 volts on the plate your effective plate voltage is 345. If you have 20volts on the cathode and 320 volts on the plate your effective plate volts are 300. In this case I'd definitely use the 115V primary for this PT. JMHO

Chuck

[Diablo]

You folks have this pretty well covered, but I'll throw another PT out there for your review. This one from Weber looks to be very flexible for voltage tweaking and only $35. They call it the WPTGP.

https://taweber.powweb.com/store/PTGPsch.jpg

[diode breath]

Weber transformer


That transformer definitely looks good for experimentation, but I'd prefer a center-tapped unit. It would be difficult to convince me that you'd get true full-wave rectification out of a non-center-tapped secondary.

Thanks, Diablo!

[Diablo]

That transformer definitely looks good for experimentation, but I'd prefer a center-tapped unit. It would be difficult to convince me that you'd get true full-wave rectification out of a non-center-tapped secondary.

Thanks, Diablo!

I don't understand. I believe you just use a bridge rectifier with a ground connection to one side of the bridge. If you wanted tube rectifier sag, you could connect the DC output of the bridge rectifier to your tube rectifier - there's a 5V filament supply for the tube rectifier.

[Chuck H]

You could also elevate the bridge rectifier ground with a big resistor for sag. The rectifier tube would act as the foreward two diodes in the bridge.

That looks like a smart transformer for tweaks. What's up with the price? There has to be a catch?!?

Chuck

[olddawg]

I would consider using a 300-0-300 and put a 10v 5w zener (or 2) in the B+ line after the rectifier that could be switched in or out.

[black_labb]

You could also elevate the bridge rectifier ground with a big resistor for sag. The rectifier tube would act as the foreward two diodes in the bridge.

That looks like a smart transformer for tweaks. What's up with the price? There has to be a catch?!?

Chuck

i've used 2 of them. they run well, but i dont know that the taps are correctly marked for voltage. running a 5e3 circuit i have a blue/red wire and a red/white wire. unloaded the output of the SS bridge rectifier into the first filter cap is 420vdc. that would suggest 300vac, as opposed to 340vac or 210vac when connecting a blue/red and white/red. i think it runs at 410vdc with tubes in at idle.
i seem to remember having the same value from a different set of taps, so it may infact be centretapped and the datasheet has it wrong. think 300vac was what i found on other taps as well.

it is a great transformer for a 2xel84/6v6 build. cant remember much about the transformer from the other build, but i was a bit confused with the voltages there too, but i dont remember what i ended up doing. that amp will be rebuilt soon anyway, i'll see what the transformer outputs are then.