Power transformer question

[fopoman]

Been wanting to build an amp using 4 6l6's and a Pair of Proreverb trannies. My question is...what's the lowest ma PT I can use? I'm trying to not use a twin reverb transformer. It's basically a single channel deluxe no reverb or tremolo with a quad of 6L6's.

thanks folks

[Structo]

First add up the heater current demands.
That many times dictates what PT you will have to use.
You say four 6L6 but how many preamp tubes and what types?

6L6GC tubes draw .9 each or figure 1 amp each.

There is 4 amps right there on the heater circuit.

12ax7's draw .3 amps each, so if you have three of those there is another amp.
So we're at 5 amps on the heaters.

My latest build which is a Dumble clone has 4 x 6L6 and 3 x 12ax7.
I used Fender Twin transformers.
The main fuse is a slo blo 5 amp fuse.

Hope this helps.
Tom

[guitarmike2107]

also bare in mind going low on the secondary current rating will cause the voltage to sag and the PT to work harder/hotter. you may want allot of voltage sag but if you decide you dont want it after you have build its expensive to replace.

[Phil_S]

This is sort of a trick question, because we don't know the exact operating parameters of the amp. So, we have to assume design center maximums in figuring the requirement.

Look up the 6L6GC in the RCA manual or get the data sheet at http://tubedata.itchurch.org/index.html . Assuming push pull class AB1 with plate voltage = 450, that's 55W per pair or 110W for the quad. Zero signal plate current is 116mA * 2 or 232mA. Maximum signal plate current is 210mA per pair or 420mA per quad. You've got to add something for G2 current, and preamp tubes; 44mA max for a quad of power tube screens and 3-5mA per preamp tube, I'll guess 4 of those for 20mA (and if it were 2 or 3 the answer wouldn't differ that much). Note: these numbers are taken from the RCA manual, where the numbers are for "typical operation."

Let's see here. 420+44+20=484mA for the max. 232+11+12 = 255mA for zero signal. Do you need that sore of rating for your amp? You'll have to be the judge. Hint: in a PP amp, the power tubes only conduct roughly half the time. If it were a perfect 180 degrees on and 180 degrees off, you'd have them at idle half the time and somewhere between idle and max when in conduction. The correct answer is probably somewhere in the middle.

I think, the bottom line is that you will still need a pretty hefty PT. You've got to allow for efficiency losses (heat dissipation) and whatnot. Don't try to shave this to the bone as it will not give you a good result.

If your problem is the cost of such a transformer, though others will scoff at what I say, haunt eBait for a while looking for an appropriate PT. I am habitually addicted to watching the auctions (tho' I rarely buy and only at what I consider true bargain prices). I often see what I think of as "whopping big" PT's from organs and whatnot, that will meet your voltage and power supply requirements quite well and you'll pay much less than if you buy something new from Hammond, Mercury, Heyboer, etc, all of which are good quality products where I find the only real deterrent to buying is price.

Here are examples that might or might not be good:
http://cgi.ebay.com/Heavy-duty-power-tr ... 45f566e4cd
http://cgi.ebay.com/Large-Tube-Power-Tr ... 2eb12b1dd6

Remember, you also need 6.3V @ ~4.2A to 4.8A depending on the number of preamp tubes.

For $50 plus shipping, this one will probably safely do the job at a very good price. You hook up the primaries and high voltage secondaries in parallel for the full capacity and it works great. At 550mA on the secondary, I think it will be more than enough. Plate voltage through a solid state rectifier should be ~500V more or less. If you can figure out how to power a tube rectifier (maybe with a separate 5V supply), you can drop some voltage. But, then again, your goal here is unclear.
http://www.antekinc.com/details.php?p=88

Good luck.

[Structo]

Here is a good article talking about current demands and filtering capacitors.
Basically it tells you how to design a power supply for a push pull amp.

http://www.freewebs.com/valvewizard1/smoothing.html

[fopoman]

thanks for the help folks

Nathan

[fopoman]

what about the mojotone highpower tweed twin tranny or the marshall jtm100?

[Structo]

Mojo transformers are just relabeled transformers from other manufacturers.

Check out Magnetic Component trannys for good quality and friendly priced units.

http://store.triodestore.com/macointr.html

[ToneMerc]

what about the mojotone highpower tweed twin tranny or the marshall jtm100?

U stated you didn't want to use a Twin tranny, that would be my easy choice. My reason not to use a Twin PT would be because of it's footprint. If thats the case you definitely don't want to use a jtm100 as it weighs a couple more pounds and has a larger laydown footprint.

Is the reason for the "smallest" question because of size? One option is to get a larger tranny stuffed into a smaller footprint. In the past I've had the specs below built into a Vibrolux (022723) footprint by Heyboer, basically a Twin PT in Vibrolux clothes. This lil beast could power a quad of El34's

120 Volts
Secondary: 335 - 0 - 50 - 335 Volts @ 450mA
6.3 Volts @ 2 Amps
6.3 Volts CT @ 6.5 Amps (3.15V-0-3.15V)


TM