Power Transformer sizing..

[dehughes]

So, I ordered the following power transformer from Heyboer today:


Primary: 120 Volts
Hi Volt: 270-0-270 Volts @ 120 mAmp
5 Volt rated @ 3 Amps
6.3v: Center tapped (3.15-0-3.15), rated @ 5 Amps


So, is 120ma going to be enough for my amp? I'm having second thoughts on the PT build.... The 6V6s are cathode biased and drawing about 74ma or so for the pair, and I figure the two 12AX7s are drawing maybe a couple ma for the both of them (PI and input). As well, the 6SN7 is biased pretty cold (preamp channel, 117v plates and 4.5v across a 2k7 Rk), so it shouldn't be drawing too much. I just don't want to build the PT too small and end up regretting it later...as I don't know how much "overbuilding" is required to have a properly functioning PT.

I wager I could still have them build it a bit bigger (doubt they've started on it yet), but then, I find that I like "right" sized transformers (r.e., not too big) all things being equal in an amp. My goal for this amp isn't to get it big fat and clean so much as clear but squishy under the right circumstances...without frying anything.

Thoughts?

[Old Tele man]

...two 6V6's sitting at idle is 144mA which already exceeds that PT's rated output...and the preamp tubes haven't even been added in yet (typically about 1MA per active triode section).

...then, there's the question of "how much" output power are you expecting to extract from those two 6V6's--10W, 12W, 14W or what?

...sounds to me like you need at least 150mA, assuming you're not expecting maximum possible power output.

[Tom Phillips]

...two 6V6's sitting at idle is 144mA ....

144ma at idle sounds high to me for two 6V6s. How did you come up with that?
V/R
Tom

[tubeswell]

Yeah a single 6V6 biased in Class A with a plate voltage of about 350 draws about 38-40mA tops - and that is Class A. In Class AB1 PP its more like each 6V6 would be idling at 22-28mA. So 120mA ought to be plenty. For example, the stock 5E3 HT winding is only rated for 70mA, and that is for 2 x 6V6 and 2 x pre-amp tubes.

[dehughes]

144ma at idle sounds high to me for two 6V6s. How did you come up with that?
V/R
Tom

Probably didn't realize I said 74-ish ma for the PAIR of 6V6s.

Good deal. Thanks guys. I was having transformer anxiety thinking I just designed a puny transformer and would have to give it another go. I'm already at transformer #4 for this project (first three were "stock" ones...now I'm going to design my own from here on out!).

[MWJB]

Current rating is fine for 2x6V6 in cathode bias plus 2x12AX7, plus 1x 6SN7.

270-0-270VAC seems a bit low for a typical 6V6 amp though, what B+ are you shooting for?

[dehughes]

Current rating is fine for 2x6V6 in cathode bias plus 2x12AX7, plus 1x 6SN7.

270-0-270VAC seems a bit low for a typical 6V6 amp though, what B+ are you shooting for?

Shooting for 350v, as I'm running them cathode biased with a 300 ohm cathode resistor, and with 350-ish volts on the plates I end up with just about 12w dissipation. I had a transformer that gave me these voltages (purely by accident) and really liked the way the 6V6s felt when "run soft". Contrary to most Deluxes, I know, but then this isn't a normal Deluxe.

[Old Tele man]

Probably didn't realize I said 74-ish ma for the PAIR of 6V6s.

...exactly, my bad (gotta get my glasses cleaned before reading).

[dehughes]

...exactly, my bad (gotta get my glasses cleaned before reading).

No problem. Thanks for at least checking out my post!

The consensus seems to be that 120ma should be fine for my uses. Common "Deluxe" power transformers from the Brown era seem to be in the 70-80ma range, so I should be safe, even with changing out a 12AX7 for a 6SN7.

[Old Tele man]

...ideally (cost not considered), you want the VA-rating (watts) of the PT to be ~1.5-times the "design" load (ie: 150%), however, this is obviously aimed toward robustness/service life. Most commercial musical amps "sqweek" by with as little as 100-110%.

...with 150% PT rating, you don't have to worry about heat buildup accumulation so much, nor saturation under long periods of heavy load. And, that means that *IF* you purposely want "sag" it would have to be designed into the rectifier/filters that you'd use.

...again, I appologize for not fully / correctly RTFQ (read the friggin question)! Oh well, getting old is hell!

[dehughes]

Good info, Old Tele Man. Thanks. That's good to know.

No need whatsoever to apologize.

As well, excellent point about designing *sag* into the circuit at the rectifier and filter sections....that's where I want it to happen, as I'm not a fan of under-rated transformers getting hot and dropping voltage because the current draw is over spec. That's the kind of sag I don't want in my amp.

[mike_mccue]

layman question;

Does the current draw being spoken of on the power transformer (e.g. 74ma for a pair of 6V6 tubes) coincide with the bias current measure when checking bias?

Is it that straight forward?

thanks,
mike

[Old Tele man]

Does the current draw being spoken of on the power transformer (e.g. 74ma for a pair of 6V6 tubes) coincide with the bias current measure when checking bias?

...yes.

...although not always true, a combined idle current of 74mA would indicate 37mA for each tube, but that's only true when both tubes are prefectly matched, which is seldom true. And, this possible imbalance condition can be a problem with cathode-biasing when only a single cathode resistor is shared between the two tubes.

[mike_mccue]

Thanks, I am slowly piecing the whole picture together.

best regards,
mike

[mike_mccue]

I have a follow up question.

I know how to replace tubes and check bias... but that's an understanding that I have only from doing that specific task.

I'm trying to learn how to predict the power transformer needs.

What is the name of the spec that you look for on a data sheet when designing a power supply? Is it the "Maximum-Signal Plate Current" or is a sum of more than one factor?

In the past I've used a copy cat approach to choose a transformer. I'd like to learn how to think this thru.

thanks again,
mike