PT current requirement

[loverocker]

Hey guys, I just did a search, and I found some postings where some of you were wondering about whether you could use an 18Wer's PT for the 36Wer. Did any of you guys try it?

From Richie's chart, the 11.6V across the 125r cathode resistor is 93mA - at idle. The 18W PT is rated at 120mA. So I'm wondering how much more current is needed (i.e. when not at idle), and if the 18W PT delivered it. Or went up in a puff of smoke?

Or did you ever work out a realistic current rating for HV winding from 'first principles'?

[loverocker]

<polite cough>

[rjgtr]

Since a pair of EL84s pulls between 72-92ma with 300V on the plates (according to the spec sheet), the 120ma has a nice safety margin. Don't forget that on startup with the caps not charged the amp will pull a lot more. That's why you see some Marshalls and Fenders with the first cap before the standby switch, to lessen this effect.

With the spec sheet saying 72 a pair that means 140ma for 4, so I think you'd toast an 18 PT pretty easily. Maybe not the first time you play it, but eventually.

[loverocker]

Thanks RJ. So I guess 36W builders specced out a new PT. Or maybe used an AC30 PT?

It was intriguing, because it reminded me that Wannatone (I think)managed to run an 18Wer off something like a 60mA PT from a donor PT... Guess that was marginal, too

[Wannatone]

LR,

that was an experimental amp.. wouldn't want to sell something like that. AC30 iron would work fine IMO.

Btw. Marshall started with a 3" PT in their 100 watters (not to mention the RS Deluxe boat anchor), then went down to 1 3/4 " step by step.... same size as JTM45 PT... go figure.

There is no straight way to settle for the specs. It depends on the tradeoff between reliability against cost... ICAS (intermittant commercial and amateur service) considerations come into play.

You CAN spec a PT for max actual current draw, but it might be on the heavy and costly side. Check the max current draw of fully dimed EL84's... add a few mA's for the preamp.. add the heaters and pilot lights and you are there. Then you may consider a factor of safety... remember to spec max ambient temperature... for certain applications the max working altitude... screen... surge/overvoltage... etc.
Then you are on the safe side. That's the way to spec for commercial or ministry applications...

For the cost saving solution you may take temperature measurements (winding, core, surface) with the amp in a temperature controled chamber at max ambient temp, amp fully dimed into a dummy load. Then check against max enamel wire temp... Much easier than trying to calculate the crap outta ya.

Adrian

[Plexi]

The Heyboer is a 200ma rated PT..290-290 in a stand up mount. I believe there big AC30 clone is a 250ma rated PT.

Richie