critique this PT

[Hardtailed]

I know the spec sheet states 120VAC, however, based on the voltages you're seeing, assuming you have 120VAC right at the wall socket, I'm willing to bet that that 290GX is rated for 115VAC and here's why -

690VCT / 115V = 6

6 x 120VAC = 720VCT

720VCT / 2 = 360-0-360VAC

360 x 1.414 = 509VAC unloaded

Take about 25 volts off of that 509 and you end up with your 484 loaded voltage.

The math supports this...unless you've got higher than 120VAC comin' out of your wall, but since EL34s almost always load the supply down by about 20-25 volts at an idle, you have to have an unloaded voltage of 510VDC in order to be getting 485VDC loaded.

Just checked for fun, the preciste turn-ratio is 1:2.94 (it's in the spec sheet). The primary is 0-120-220-240.
So in all logic, the turn ratio when using in 0-120 mode is 5.88, which gives something like 353-0-353. The unloaded voltage I get is actually 501V (measured with the amp on standby, first filter cap is still in the circuit), which is even higher than what 353-0-353 would translate to.

So yes, it sags under load, but only by 15V, and then it hits the stated voltage dead-on!
Is the Hammond using massive wire with less resistance than others? I don't know. It is pretty massive for a 50W unit (bigger than what my 100W JCM2000 had).

I just noticed though, they've changed the current rating to 150mA, like the MagCo unit... Did they change something, or did they just use a different rating system (unrectified current)... I don't know. The dimensions haven't changed.

So maybe the MagCo will behave exactly the same after all... that is: effin loud

[daz]

Daz, can you confirm that?

It gets 425. by the way, the magnetic comp PT is stupid heavy. The amp chassis with the 272JX hammond PT seems not to weight much more than the MC PT by itself ! Ok, slight exaggeration, but you get the pic.....it's a freaking boat anchor. Wht the current rating is so much lower i dunno. But it's marshall spec'd so i figured it would be the better way to go. I'm not into clones, but i DO want the basic marshall tone so i can take it from there with the way my circuit is different.

[Hardtailed]

It gets 425. by the way, the magnetic comp PT is stupid heavy. The amp chassis with the 272JX hammond PT seems not to weight much more than the MC PT by itself ! Ok, slight exaggeration, but you get the pic.....it's a freaking boat anchor. Wht the current rating is so much lower i dunno. But it's marshall spec'd so i figured it would be the better way to go. I'm not into clones, but i DO want the basic marshall tone so i can take it from there with the way my circuit is different.

So that confirms that the Hammond is rated loaded.

As for the current rating, as I mentionned, the 290GX has been re-rated to 150mA too. So they're both probably very similar in the end.
Let us know what B+ you get.

I'll be curious to know also what sound difference you get from a higher voltage.

[daz]

I'll be curious to know also what sound difference you get from a higher voltage.

You and me both ! I've always felt that the lower voltage was responsible for the fact that i have always wanted more articulation/clarity/dynamics out of it and figured higher V would get me closer. There always been a sort of battle with the high end because with a lack of the aforementioned i alway needed more highs but then nwhen i get them I'd find it slightly harsh at stage volumes. But w/o them theres not enough clarity and dynamics. Don't get me wrong....i don't mean to say it was a problem in a big way, but enough so that it was always a nagging issue. When i went from cathode biased to fixed, i got a nice boost in articulation, so that told me the issue was likely able to be rectified in the back end one way or another. Hope this is the magic bullet. I've also found i'm someone who likes the way higher voltages sound and feel in the pre stages, so i'm pretty sure i'm going to like this.

[Hardtailed]

You and me both ! I've always felt that the lower voltage was responsible for the fact that i have always wanted more articulation/clarity/dynamics out of it and figured higher V would get me closer.

I can confirm that with the ridiculously high voltages on mine, clarity is not a problem! Even the bar owner where we were playing once came to me to comment on how well he could hear what I was playing.
I even removed the cathode bypass cap on the stage that drives the cathode follower (V2a on a classic Marshall topology) to make it a bit fatter.

Also, I'm using a really low-DC resistance choke (barely over 100ohms) so my screens are pretty high too. And I'm only using 1.5K grid-stoppers on the power tubes.

[daz]

I see very few bypass caps on v2a in circuits like this, but i use that too. But every time i take it off i end up going back to it because it just gives a feel that i always miss w/o it. Adds some springiness to the feel.

[Hardtailed]

I see very few bypass caps on v2a in circuits like this, but i use that too. But every time i take it off i end up going back to it because it just gives a feel that i always miss w/o it. Adds some springiness to the feel.

I took some stuff from the 1987 and the 2204.
The 1987 had a 0.68uF cap there.
However, I still have a hot-biased 2nd stage, like a cascaded 1987 really.
Have yet to try a 10K cathode resistor there, I should some day (this one is also currently bypassed by a 220uF)

[daz]

I've tried that. In fact, i even tried making the entire pre like a 800 with that 10k cathode, but it sounded pathetic compared to how i had it. I don't know why but i even took it down to 6k and it seems the colder the worse. So 6k was better but still not good. I don't get why they did that even tho i've heard others explain it. The question still arises, why if it sounds like that compared to what it does biased normally with a bypass cap?

[Hardtailed]

I've tried that. In fact, i even tried making the entire pre like a 800 with that 10k cathode, but it sounded pathetic compared to how i had it. I don't know why but i even took it down to 6k and it seems the colder the worse. So 6k was better but still not good. I don't get why they did that even tho i've heard others explain it. The question still arises, why if it sounds like that compared to what it does biased normally with a bypass cap?

I've always figured that the people at Marshall thought it was too much gain with a regular bypass stage
That explanation works for me!
We have to remember that they were trying to do a clean amp when they released the good old Plexis

Now, on an amp with 3 cascaded stage before the cathode-follower driver stage (Recto, Slo, etc), I can see why they would do that for the 3rd stage.

[Bruce / Mission Amps]

My 2 cents

I've always been under the impression that most all transformer's ratings, ie.: 300v@150ma means just that... it will push 300vac while delivering 150ma.
Hammond power trannys always seem to be off by about +105%.
So a 300v PT might actually make over 315vac while delivering 150ma.
A decent test is to use solid state diodes on the secondary with NO filter cap and a few high wattage resistors tied from the cathodes to the center tap to determine simulate a big load.

Since this is AC, keep the RMS figures in mind here!

300v/.15A = 2000 ohms.
300v x .15A = 45 watts
So make those resistors really high wattage and don't get branded by one while it's cooking.

I have at test jig here that does just this and can run either with an octal rectifier, 9 pin rectifier or solid state block and with a bunch of chassis mounted terminals for primary wires, Hi-V, bias, 6v, 5v and anything else I can think of.
And it has an On-Off switch, fuse and a pilot light so I don't forget it's on!

[Wilder Amplification]

Just checked for fun, the preciste turn-ratio is 1:2.94 (it's in the spec sheet). The primary is 0-120-220-240.
So in all logic, the turn ratio when using in 0-120 mode is 5.88, which gives something like 353-0-353. The unloaded voltage I get is actually 501V (measured with the amp on standby, first filter cap is still in the circuit), which is even higher than what 353-0-353 would translate to.

So yes, it sags under load, but only by 15V, and then it hits the stated voltage dead-on!
Is the Hammond using massive wire with less resistance than others? I don't know. It is pretty massive for a 50W unit (bigger than what my 100W JCM2000 had).

I just noticed though, they've changed the current rating to 150mA, like the MagCo unit... Did they change something, or did they just use a different rating system (unrectified current)... I don't know. The dimensions haven't changed.

So maybe the MagCo will behave exactly the same after all... that is: effin loud

Gotta love the accuracy of specs.

My main point with my post was to illustrate what's actually going on. Gaz basically took the B+ voltage, squared it then divided it by the primary plate-plate load when that only gives you an approximation of OT PRIMARY POWER, yet we all know that transformers are nowhere near 100% efficient and as such you will not end up with all of the primary power at the secondary. So I figured I'd clarify with some more realistic values.

Now if the transformer was 70% efficient with those calculations the OP would end up with roughly 50 watts out at the secondary. But without knowing the exact efficiency spec of the OT there's no way to tell how much of the primary power will end up at the secondary.

[Gaz]

My only experience with transformers has been Hammond and Heyboer, both of whose specs are apparently under load. That simplified equation has given me very close approximations. It's good to know all transformer manufactuers may not rate their components under load.

[Wilder Amplification]

My only experience with transformers has been Hammond and Heyboer, both of whose specs are apparently under load. That simplified equation has given me very close approximations. It's good to know all transformer manufactuers may not rate their components under load.

Yeah even if you were to plot this stuff on a load line (which you'd see that Marshall's load selection for a 485 plate voltage is in fact on the low side of things) this would only approximate primary power without taking into account power supply sag at full output, which depends on what kind of current the PT can source as well as the spec'ed transformer regulation percentage under load. On top of this, at peak signal the voltage across the full plate-plate winding of the OT primary is actually double the B+ voltage, in which you'd have to know exactly what the B+ is at full output in order to know what to square and divide by the primary load. This gives you peak power, in which you would divide that by 2.828 to get RMS power.

Of course this is primary power. OT efficiency comes into play and is what will tell you how much of that primary power will end up at the secondary.

On a single rail supply with only a 3.2k plate-plate load, the amount of primary current that would be required to produce 75 watts at the secondary is VERY optimistic for 2xEL34s without redplating mid-swing.