Calculating Dropping Resistors

[Resonator Guy]

Recently finished my first completely scratch build, using recycled transformers etc. I even bent my own chassis. So I have a question about dropping resistors. How do I calculate the current draw of three tubes so that I can drop voltage to the rectifier. It is 330 Vac on each leg right now and I need it to be 260 Vac.
I find the Ma draws on tube datasheets somewhat confusing still. It has a 6X4, EL84, 12AX7 line-up.

[eschertron]

looking at the GE datasheet for the 6X4 here http://www.r-type.org/pdfs/6x4-1.pdf i see the rating for the AC input to the rectifier using a cap filter is 325VAC. i'd hazard a guess and say that 330VAC is close enough, as long as you heed the cap size guidelines.

330VAC RMS * SQRT(2) = ~ 466v peak.

the diode (from the datasheet) will drop about 22volts
466-22 = 444vdc unloaded.

Two questions:
1. what's the DC voltage target?
2. are you measuring 330VAC across the PT, or from each leg to CT?

[Resonator Guy]

Second one first, I measured 330 Vac from pin 1 of 6X4 to ground and from Pin 6 to ground.
I am comparing this to a schematic for a Kalamazoo one that doesn't give many voltages, however I get the following

V1 pin 6 schem says 110V I get 232V
pin 1 105V 225V
V2 pin7 250V 447V

I read the voltage from an actual Model One and got 266Vac from Pin 1 and from Pin 6, both to ground, so I figure that is about what I need.

[eschertron]

So the transformer puts out 330VAC from each leg to the center tap of the transformer, correct?
Is it possible that the transformer you have was intended for 220VAC (line) operation? Does the transformer have taped-off wires that may be for a different line voltage? Check the 6.3vac heater voltage. Is it in spec? That would establish that the PT is operating as designed.

Or are you using that transformer because that's what you came up with, and it's not part of the amp you are rebuilding?
Whatever the cause may be, your transformer is giving about twice the voltage you need. I'd hesitate to try to engineer a voltage-dropping solution. My suggestion is that you'd be better served to come up with a higher-voltage circuit to match the B+. Others will have more creative suggestions.

[Resonator Guy]

The transformer is intended for North American voltage (110-120) and the heater voltage is 6.3 V. I just wish to run this configuration with this transformer if possible. A higher voltage circuit won't fit in the space I have here, and I don't need or want loud. If voltage dropping won't work, and I calculate needing two 280 ohm 17.5 watt resistors to drop it then fine. But I was just trying to get a little more insight into the calculation as this is something I have never done before. I am not rebuilding an amp but building one with what I have on hand.

[eschertron]

so you came up with 280 Ohms * 2 based on a current consumption of ~180ma to drop about 100VAC? OK so far. I'd think rating the resistors at 100% dissipation is risky though. you'll want 40W or better to manage the heat.

[EDIT] OK, so 70V at 125ma, and you have 200% capacity. I'll buy that [/EDIT]

Not sure why you say "a higher voltage circuit won't fit". The electrons don't get any bigger you simply need to use a design that works for the higher volts. The 6X4 will be happy with the 447VAC if you use a choke filter, and simply subbing the EL84 for a valve that likes those voltages - 6V6 for instance, same ballpark power as an EL84 - and you've got a great little amp. Or should I say "champ". <insert 'sorry couldn't resist' smiley thing here>

There's nothing wrong with what you've got, and while you CAN dissipate half the PT's power as heat, I'm really trying to steer you away from that option. The engineering to change the voltage by THAT MUCH is weighed against getting the parts that the design calls for. A 330-0-330 PT is a high voltage PT. You want something more like 200-0-200 IMHO.

[eschertron]

...the thing I like about being a hobbyist, is the hands-on challenge of finding the elegant solution. I build an amp *starting from* the PT. If that's my limiting factor, then everything else follows from that.

Can I make an EL84 amp with 400+ volts B+? I think so, but I'd have to do some research to be sure. I think Chuck H. has mentioned that topic.

[Resonator Guy]

Yep, I am looking at squeezing a 6V6 in there, but I don't know if it will fit. It is an old Magnatone MOT cabinet that once had a 3 tube chassis with no PT, so there just isn't much space. That was why I was trying for three miniature tubes rather than octals, which I prefer. That said, I do want to make something out of it so I will see what might fit. I don't know where I could get resistors of that wattage that would be small enough anyway.
Back to the drawing board.

[eschertron]

Oh, well that is cool! Nice work on the covering, reminds me of a solid block of metal

What size speaker sits in there? Using the sofa for perspective, I'm guessing 10"? Even if it's an 8" speaker, I have faith that you can squeeze what you need into the cabinet.

On my last build (this year's winter project) I had to move all the tubes and the OT around to get away from the speaker cage. Not ideal placement, but it fits. There's always one more way to skin a cat.

[Resonator Guy]

The covering is original 50s Mother of Toilet Seat. Only the best. It takes an 8" speaker and a 10 won't fit. I measured. I just now desoldered and removed the EL84 socket and drilled a 6V6 socket. I will try wiring it up tomorrow and see what happens.

[eschertron]

The covering is original 50s Mother of Toilet Seat. Only the best. It takes an 8" speaker and a 10 won't fit. I measured. I just now desoldered and removed the EL84 socket and drilled a 6V6 socket. I will try wiring it up tomorrow and see what happens.

How'd it come out?

[Resonator Guy]

It hasn't come out yet. Spring finally arrived here, in a limited fashion, and I have been busy with must-do outdoor stuff. I hope to get at this before next winter, but I need a couple of rainy days.