BEST VOX TRANSFORMERS?

[ViperDoc]

I've been looking at the AC30. What transformers would you recommend? I see the 290NX et al from Hammond. Thx.

[Colossal]

I've attached the specification sheets for the 290MX and NX.

Voltages are very important to get right with these amps and the devil is in the details. I saved this quote from Pete PFD64 (speaking about the [Woden] power transformer):

"The Woden in mine puts out 586VAC with the HT unloaded, 560VAC at idle, 560VAC at max clean 25W, 554VAC at fully overdriven square wave 38W. The HT idles at 317VDC. 243VAC Mains feeding 245V PT PRIMARY, HEATERS 6.41VAC at the EL84s. Measurements taken with a calibration checked Fluke 189"

586VAC unloaded = 293-0-293VAC
560VAC loaded = 280-0-280VAC
Pete's mains voltage 243VAC = 121.5VAC USA.

Running 121.5VAC into a "vintage wound" power transformer (e.g. wound for 110VAC, 117VAC mains of yesteryear) will result in a higher B+ than the original, vintage amp ran at. You will get higher B+, higher plate and screen voltage, and even greater dissipation. I have a 117VAC clone PT in my Blonde Bassman. That makes the B+ run 30VDC hot unless padded down with a variac or other trickery. AC30s run and sound best with the EL84s at 120% of max diss. That is how God and Dick Denney intended it to be. But you cannot run them at 350V plates and screens and expect the tubes to live long biased to 50R shared. They should be running at 47mA and that's 118% of max.

The Hammond 290MX is shown as 275-0-275 and accounting for loading with a GZ34, you might expect 275 * 1.20-ish = 330VDC or so, probably less (more if your mains is higher than the primary at 120VAC). The original, early AC30s used Haddon power transformer, which was 285-0-285VAC at 117VAC. The Hammond MX is adjusted for today's higher mains at 120VAC primary. So that might be a smart bet.

The 290NX looks like the stand-up replacement for the AC30 CC2 Custom Classic and is not the right voltage.

Choose wisely...

image.jpeg

[ViperDoc]

I've attached the specification sheets for the 290MX and NX.

Voltages are very important to get right with these amps and the devil is in the details. I saved this quote from Pete PFD64 (speaking about the [Woden] power transformer):

"The Woden in mine puts out 586VAC with the HT unloaded, 560VAC at idle, 560VAC at max clean 25W, 554VAC at fully overdriven square wave 38W. The HT idles at 317VDC. 243VAC Mains feeding 245V PT PRIMARY, HEATERS 6.41VAC at the EL84s. Measurements taken with a calibration checked Fluke 189"

586VAC unloaded = 293-0-293VAC
560VAC loaded = 280-0-280VAC
Pete's mains voltage 243VAC = 121.5VAC USA.

Running 121.5VAC into a "vintage wound" power transformer (e.g. wound for 110VAC, 117VAC mains of yesteryear) will result in a higher B+ than the original, vintage amp ran at. You will get higher B+, higher plate and screen voltage, and even greater dissipation. I have a 117VAC clone PT in my Blonde Bassman. That makes the B+ run 30VDC hot unless padded down with a variac or other trickery. AC30s run and sound best with the EL84s at 120% of max diss. That is how God and Dick Denney intended it to be. But you cannot run them at 350V plates and screens and expect the tubes to live long biased to 50R shared. They should be running at 47mA and that's 118% of max.

The Hammond 290MX is shown as 275-0-275 and accounting for loading with a GZ34, you might expect 275 * 1.20-ish = 330VDC or so, probably less (more if your mains is higher than the primary at 120VAC). The original, early AC30s used Haddon power transformer, which was 285-0-285VAC at 117VAC. The Hammond MX is adjusted for today's higher mains at 120VAC primary. So that might be a smart bet.

The 290NX looks like the stand-up replacement for the AC30 CC2 Custom Classic and is not the right voltage.

Choose wisely...

image.jpeg

As always, you are the man. Thank you.

[pdf64]

Other useful PT info -
HT winding end to end resistance 118ohms
PT primary resistance 8.2 ohms (245 tap)

Neither of those Hammonds look to be a good fit with that, unfortunately

[ViperDoc]

Much appreciated.

[ViperDoc]

Just out of curiosity, I ran across these. Anyone used these, or know how they would compare to a Hammond? Seems like the secondaries on Vox-type TFOs vary in output from 270-315ish, and either have a 5V tap for tube rectification or not. I like that this appears to have unwired leads.

https://www.tubeampdoctor.com/en/mains- ... -gz34?c=73

[pdf64]

Sorry, I was too negative about the 290MX, a couple of 22ohm resistors would get the HT resistance into line.
Yes, that TAD one could be probably be similarly tweaked into range.

[Oddvar R]

https://www.tube-town.net/ttstore/tt-po ... -ac30.html

[ViperDoc]

I wonder what transformers Sluckey would recommend for a Sluckey AC30/4?...

[pompeiisneaks]

I wonder what transformers Sluckey would recommend for a Sluckey AC30/4?...

When I built mine I used Heyboer, and I think he recommended them, but not 100% sure. It's been a while.

~Phil

[ViperDoc]

Thanks, Phil! I've had trouble finding Heyboer transformers for anything Vox-related. I used to be able to call them, but they're obviously too busy these days.

[pompeiisneaks]

Yeah that seems to be the new state of things, hope it changes soon, too much demand, not enough supply.

~Phil

[ViperDoc]

Well, I decided to give a Hammond trio a go. I just pushed the button on a 290MX, a 159M choke (15H, 100mA--1960 SCH recommends "10-20H") and a 1750V OT.
Guaranteed I'll need some voltage massage later. I really can't decide which schematic to go with. I really like the Sluckey AC30/4 with the EF86, that looks like a fun design, but I also can get my head around a Hoffman AC30 with the tremolo delete--just Normal and Top Boost channels. I could easily fit that into an 18-watt chassis. Anyone have a favorite?

[Blueboozer]

TubeDepot has a Classictone AC30 PT in stock and a Mojo AC30 OT and Choke

[ViperDoc]

Full admission, I was studying the TW Rocket over the weekend and it looks very interesting. The circuits have some similarities, like the PT grid and screen resistors, the PI except for a cap value difference, I believe. The Rocket appears to have a much stiffer power supply and a cathode follower-driven tone stack among other differences that you masters will spot. Videos with decent playing show the Rocket has chime for miles. I wandered the interwebs into dangerous territory, watched a demo video of Chris Bryant playing a Suhr Scott Henderson Classic through a Dr. Z Stangray ( https://www.youtube.com/watch?v=sglsF8bpPIM )and impulse-bought a used Stangray combo in superb condition on Reverb.com before the video ended. I think I'm in trouble.