critique this PT

[daz]

The link to the PDF spec file is below. I'm building a clean new version of my jcm style 50 watter with 3 ax7's and SS recto. Would this PT be too small (current wise) if i decided to add one or 2 ax7's for verb and/or loop? Any other thoughts about it? I've heard good things about this line as far as the OT's, tho the PT's i have heard run hot tho no problems that i've read of. If thats a worry, should i go with another hammond, and if so which would you suggest? I have the 272 JX in my current amp but i don't need the rectifier winding.

http://www.classictone.net/40-18023.pdf

[Hardtailed]

The link to the PDF spec file is below. I'm building a clean new version of my jcm style 50 watter with 3 ax7's and SS recto. Would this PT be too small (current wise) if i decided to add one or 2 ax7's for verb and/or loop? Any other thoughts about it? I've heard good things about this line as far as the OT's, tho the PT's i have heard run hot tho no problems that i've read of. If thats a worry, should i go with another hammond, and if so which would you suggest? I have the 272 JX in my current amp but i don't need the rectifier winding.

http://www.classictone.net/40-18023.pdf

Have you considered the Hammond 290GX? (the 50W Marshall replacement part)
No 5V tap, and has better current handling than this one (213mA if memory serves right)
Both are higher voltage than the 272JX (it's 300-0-300 right?)

With the 290GX and basis SS rectifying, I get 485V on the plates of 2 EL34s. (fixed bias). It doesn't get hot and sound the same all night long when I'm playing out (it doesn't lose breath like those JCM2000 sometimes do).

If you add extra tubes, it's mostly the 6.3V current handling you must watch out for. 2 EL34s + 3 12AX7 = 4.9A, and I believe both are rated at 5A.
I know the generic Hammond (like your 272JX) are notoriously conservative in their rating, and adding a tube or two might be acceptable. But I can't say for the 290GX or the Classic Tone.
I do know the 290GX doesn't get hot with 3 12AX7.
I'd be confident in adding one 12AX7... maybe not 2.

I went with an SS loop in mine, I want my loop to be transparent, and tube loops will never really be.
I subjected a few people to a blind test of my loop (it has a hardwired switch that takes it completely out of the circuit), and nobody could notice when I went from looped to unlooped, unless an effect was plugged in.

[daz]

Thanks HT. I'll look at that hammond. By the way, got a schematic of that loop?

[tedmich]

Thanks HT. I'll look at that hammond. By the way, got a schematic of that loop?

+1

I copied the XXX/JSX board.

[Hardtailed]

Thanks HT. I'll look at that hammond. By the way, got a schematic of that loop?

Actually, I "cheated" (or call it reverse cheating) and bought the "Zero Loss FX Loop" kit from Metro amps.
It's kind of expensive at 90$ (it's a simple design with 2 MOSFETs, one for buffering, the other for gain recovery), but the packaging is really neat (small pcb held by the 2 jacks, fit over the preamp tube sockets), comes with the switch, shielded wire and a variety of decoupling resistors so you can choose the best for your amp's B+ (it creates its own node off the PI node).
But mostly... it just works perfectly out of the box with a Marshall style tone stack (goes between the treble wiper and the volume top lug).

I tried to reverse engineer it and started tracing the schematic, but I stopped cause I have a feeling there may be intentionally mis-labelled parts. Mostly, I can't see why there would be a 2M series resistance on the input side. But maybe that's really what it is...
I also couldn't make out the value of the small "kemet" type capacitor. So anyway... I stopped.

But it's really just a basic MOSFET based loop, running off 290-320V. There has to be some schematic of those floating around the web.

The magic is probably in the proper selection of resistor and cap values, and I felt that the guy who designed it deserves the credit so that's also why I abandoned the idea of cloning it.

[fyl]

I'm building a clean new version of my jcm style 50 watter with 3 ax7's and SS recto

Get the 100-watt 40-18024 version.

[daz]

With the 290GX and basis SS rectifying, I get 485V on the plates of 2 EL34s

I forgot to ask...right now i have 430 on the plates, what tonal differences would i hear/feel with over 50v more?

[Steve Conner]

What about the 290KX? That's Hammond's replacement for the JCM900. It's a single 330V HT winding meant for a bridge rectifier.

Strange but true, I've got a pair of them powering my stereo, and I think they give about 440V under load. Each one runs two 12a*7s and two EL34s biased at about 40mA per tube.

Downside is that it's a drop-through type with a plastic bobbin that not all end bells will fit.

[daz]

Against my better judgment i bought the magnetic components one. thier stuff like marstran and metro are also copied after vintage marshall trannys so i just felt like that was the way to go. If it sucks i'll throw the hammond from the old amp in. That is, unless someone can tell me whether 50v more on the plates is going to be a tonal improvment.

[Gaz]

Higher B+ will give you more power before clipping - more headroom - so it should work well for a cleaner amp. It depends on the OT though, as far as power is concerned, as a higher primary impedance along with the higher voltage will keep power the same or lower it.

[daz]

Actually the OT i'm using will be lower than the one in my current amp. So i'll be going from 300-0-300 and 4k on the OT primary to 345-0-345 with a OT that is 3.2k. Cleaner headroom would be good.

[Gaz]

According to my 'neophitic' calaculations, you should currently be getting about 45 watts before clipping. With the new PT/OT arrangement you should be getting about 73 watts - Definitely an audible difference!

[Wilder Amplification]

According to my 'neophitic' calaculations, you should currently be getting about 45 watts before clipping. With the new PT/OT arrangement you should be getting about 73 watts - Definitely an audible difference!

OK...first transformer -

300-0-300

300 x 1.414 = 424.2V

Now...new transformer

345 x 1.414 = 487.83V

Now...those voltages are NO LOAD voltages...i.e. WITHOUT tubes installed. Take about 20-25 volts off of both those figures for the loaded voltage -

Old PT - 400V
New PT - 460V

Now...one must remember...output transformers are not 100% efficient. As such, Primary power > Secondary power.

Furthermore...at max clean signal out, the voltage from plate - plate = B+ x 2 for your peak-peak voltage across the load. As such...

Peak - peak primary power = (B+ x 2)^2 / Zp-p

So -

460 x 2 = 920V

920^2 = 846,400

846,400 / 3200 = 264.5 watts peak-peak

264.5 / 2.828 = 93.5 watts RMS primary power

Now IF that new transformer is around 60% efficient, you'll end up with about 56 watts at the secondary (93.5 x 0.6 = 56 watts).

On top of that you will have power supply sag...let's take the plate-center tap load into perspective here, which is 1/4 Zplate-plate -

0.25 x 3200 = 800 ohms plate-center tap

460V B+ / 800 = 575mA peak current

575mA x 460V = 264.5 Watts Peak-Peak

264.5 / 2.828 = 93.5 Watts RMS

Now look at the peak current at the load...does the PT have the VA rating to source that kind of current? More than likely not, so there will be B+ sag at full clean output, and this will also change things bigtime.

Now let's look at the old config -

400V/4K Zp-p

400 x 2 = 800 Vp-p across the load

800/4,000 = 200mA

200mA x 800 = 160 watts peak-peak

160 / 2.828 = 56.5 watts primary power

Again assuming an OT efficiency of 60%, output power would've been close to 35 watts.

So that said...I'm betting on 50 watts out or MAYBE slightly over that. Of course this will be an improvement over the 400V B+ on a 4K load @ 35 watts.

[Hardtailed]

OK...first transformer -

300-0-300

300 x 1.414 = 424.2V

Now...new transformer

345 x 1.414 = 487.83V

Now...those voltages are NO LOAD voltages...i.e. WITHOUT tubes installed. Take about 20-25 volts off of both those figures for the loaded voltage -

I don't know about the Magnetic Component one, but I can confirm that the 300-0-300 on the Hammond he currently has is the loaded voltage!

I have the 290GX in my amp (Marshall 50W replacement, 345-0-345) and I get precisely 486V on the plates!

Also, the old one, being a 270 series is rated at 110V on the primary (the 290 series is rated at 120V). I wouldn't be surprised if he gets as much as 435V on the plates.

Daz, can you confirm that?

I can say one thing: my "50W" Marshall clone with a 345-0-345 PT and a 3.2K OT is the loudest amp I have ever heard, on par with the Mesa Mark IV I used to have (85W, but through V30 speakers which have 3dB better efficiency)
And this power is insanely clean until you get to stupid loud volumes.

I'm wondering though, the 290GX is rated for 213mA on the HV secondary, the Magnetic Component only 150mA...
Possible that it will sag a little more, limiting headroom.
Might just be their rating system though: Hammond rates their PTs by running them through a FW rectifier, which sources current through only one half the of the primary at a time, allowing more current to be sourced than if you were just pulling all the time accross the whole winding.
Note how the ratio between 150 and 213 is almost precisely or good old 1.404 factor.

[Wilder Amplification]

I don't know about the Magnetic Component one, but I can confirm that the 300-0-300 on the Hammond he currently has is the loaded voltage!

I have the 290GX in my amp (Marshall 50W replacement, 345-0-345) and I get precisely 486V on the plates!

Also, the old one, being a 270 series is rated at 110V on the primary (the 290 series is rated at 120V). I wouldn't be surprised if he gets as much as 435V on the plates.

Daz, can you confirm that?

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.

So having said that, the math changes SLIGHTLY -

485 x 2 = 970Vpk-pk

970^2 = 940,900

940,900 / 3.2K Zp-p = 294Wpk-pk

294/2.828 = 104Wrms primary power

Again assuming a 60% efficiency rating on the OT -

104Wrms primary power x 0.6 = 62 Watts

However, if the OT is only 50% efficient, you'll end up right where I originally stated at just over 50 watts.

Remember it's only a 3dB perceivable difference in volume when you double the power out so the 10 watt difference between 50 and 60 watts is hardly anything to sneeze at.

On top of this...running the grounded CT FW rectifier, you'll get about 300mA source current. 485V / 800 ohms plate-center tap load = 606mA peak current, so there WILL in fact be some sag simply because that load at that voltage is trying to pull current that the PT cannot source.

Of course even though the math follows the theory this is all still speculation until the trannys are in and measurements are taken. But should give a semi-rough idea as to where you can expect to end up.