PT question

[tsl602000]

I have this Hammond 350-0-350 170mA PT . Can I use this with 4x12AX7's and two Tung Sol 6L6 tubes (solid state rect.)?

I looked at the tube data sheets, but I'm still confused.

thanks,
Coen

[jaysg]

Please give us the Hammond part number. 170mA is good for a 35W amp. The rest depends on having more information. B+ may be close to 500V.

[tsl602000]

type: 373BX

187VA

350-0-350

DC Current 175mA

5V CT @ 3A

6.3V CT @ 5A

[Phil_S]

The short answer is that maybe you can use it, but there are probably better choices. The long and possibly wrong answer follows.

Maybe someone will double check my thinking and math, as I am not very good at this and have no formal training in electronics. But no one else picked up the ball here, so I thought I'd give it a try.

A pair of 6L6GC are rated for max plate voltage of 450. With a solid state rectifier, plate voltage is likely to be 350 * sqrt(2) = 495V, so for starters you are running the plate voltage on the high side. I would guess this has been done before and it being about 10% over maximum design spec, probably not an operating issue, but possibly shortens tube life.

A pair of 6L6GC at max, including screen current will run up to 232mA. Given the higher plate voltage, you control this with bias voltage, which should probably be a bit more negative than spec. 232mA * 495V = 115VA plus maybe another 10VA for the preamp tubes, so let's round it out at 125VA on the HT secondary.

Filaments require 4 * 0.3A for the preamp tubes + 2 *0.9A for the power tubes. So, we've got 3.0A x 6.3V ~ 19VA; add this to the 125VA for the HT and you are still well below the 178VA rating.

At idle, these numbers suggest about a 60W amp, putting out 110W peak. This is probably pushing the envelope for 6L6 tubes. If you can run the bias voltage up into the -40 something range, you can pull down the plate voltage a bit and keep the wattage a bit lower.

I'd conclude this particular PT isn't really the best choice for the 2 * 6L6 project you describe, however, your description is rather limited. You could, however, add a tube rectifier. There is a 5V winding @ 3A, which will support a variety of choices. Depending on which one and whether you select cap input or choke input, you will probably be able to drop the B+ sufficiently to make this PT work. (You've got the VA capacity for the tube rectifier.) For example, a 5AR4/GZ34 is rated to drop 67V @ 250mA. In this particular setup, you won't get that kind of drop, but even a 35-40V drop would be very nice. Actual results will vary according to the circuit you build and the A/C line voltage, so you probably need to build it to see what happens.

I have not intentionally tried to mislead you here. I hope what I wrote is accurate. I'm sure someone will be along in a while to confirm it or correct it.

[Firestorm]

Phil, you shamed me into responding --

I'll say first that it depends a lot on whether these really are NOS Tung Sol 6L6s we are talking about, or NOS Tung Sol 6L6Gs, or NOS Tung Sol 6L6GBs, or NOS Tung Sol 6L6GCs, or something made in Russia that happens to have Tung Sol printed on it. Different ratings for all of those, BTW.

For just about anybody's 6L6GCs (despite what some data sheets say) 500V is pretty safe (but maybe not ideal). This transformer does have a 5V winding for a tube rectifier, so that might be a nice way to tame the B+, as Phil observed.

As to the 175ma rating -- well, 6L6s will have lot more "room to move" at 200+ma, but output tubes do not "draw" any more (B+) current than the power supply is willing to give them, so this might be a nice sounding Tweed Super sort of amp.

This PT also provides 5 amps of current at the 6.3 volt winding (that's rated separately from the 175ma B+ rating) so as Phil says, it can certainly support 2 6L6s and 4 12AX7s.

It just won't have output power much above 30 watts. But hell, I've got a '54 Twin that's only rated for 25 watts and its so loud I can't be in the same room with it.

Give it a whirl (and try Phil's GZ34 idea). If it doesn't blow your skirt up, get another Hammond tranny. It'll seem pretty inexpensive by the time you've bought everything else that goes into an amp.

[Andy Le Blanc]

that means along the lines of a vibrolux...... almost a super reverb
about 30 watt out of a pair of 6l6..... the load at max dissapation will
drop the anticipated B+ a bit..... and bias type will effect the out come too
5A on the fil. is nice....... you can do things with a smaller amp cranked
that you cant with higher watts...... and if you need more.... get an efficent
speaker.....

[tsl602000]

Thank you all so much for these replies. I expected to get a yes or no, but instead I learned alot. I really appreciate that.

I created an amp that is everything I always wanted and a few guitar playing friends seem to like it too, as I have to build a few more.
Basically it's a slightly modified Komet pre-amp with a slightly modified AB763 (Bassman) power amp with tube FX loop.
I built this amp also in an Express variation. That is the mentioned pre-amp and an Express power amp which has a much lower B+. I always loved that amp, but the AB763 version blows it completely out of the water.

In case you're interested: http://www.youtube.com/watch?v=T39mIraN-FY (BS warning though..) That's not me playing.

I think I'll leave that 373BX on the shelf and order an identical PT, which is a bassman replacement.

[Phil_S]

Firestorm and Andy: How is it that you come to the conclusion it is a 30W amp? Intuitively, I know you are right. The JTM45 I built with EL34's operates in this range. I just don't understand how to do the math to get from here to there. I'm sure it's hiding in plain sight and I'll go <d'oh headslap> when you tell me. Thanks.
Phil

[mlp-mx6]

Something along the lines of:

495V * 150ma (assuming some of the PT current goes to the rest of the amp or in PS losses) * 40% (approx. efficiency of a Class AB1 amp).

495 * 0.15 * 0.4 = 29.7VA, or about 30W

This is a VERY rough calculation/approximation.

[Phil_S]

* 40% (approx. efficiency of a Class AB1 amp)

Thanks! This is what I was missing. I understand these calculations are estimates, but it's still useful.

[jaysg]

For example, a 5AR4/GZ34 is rated to drop 67V @ 250mA.

5U4GB ?

[edit]GZ-34 drops closer to 20V.

[Phil_S]

For example, a 5AR4/GZ34 is rated to drop 67V @ 250mA.

5U4GB ?

According to my copy of GE Essential Charateristics, the 5U4GB will drop 50V @275mA, making the GZ34 a better choice for this situation. It would still be an OK choice, and especially good if it was on the bone pile.