Quickly comparing output transformers

[Mike Doolin]

Here's a way to quickly A-B different output transformers: install 6-pin Molex connectors on the transformer leads, and run wires from a mating Molex connector into the amp chassis to connect to the usual points. I had four different output transformers laying around, so I bolted them all to a plywood board that hooks onto the strap handle on top of the amp. It only takes about 10 seconds to standby the amp, swap connectors, and power back up. I even played the amp at a gig this way, actually changing transformers between tunes!

[Steve Conner]

Well, that's some hardcore amp geekery right there.
Can you hear a difference? Which one do you prefer?

[Mike Doolin]

I knew somebody would ask...

The four transformers were:

1) A brand new Mercury Magnetics JIMK-O for a Jim Kelly 4-6V6 amp
2) The original bf Bandmaster
3) A bf Bassman
4) A recent Mojotone Tremoluxe/Vibrolux #770

This particular bf Bandmaster chassis has holes for 4 power tube sockets although two of them were covered with sheet metal plugs (maybe they used the same chassis for Showman heads). I installed two more tube sockets so I could run four 6V6's or two 6L6's, reasoning that the output impedance of a 6L6 is roughly half that of a 6V6, so two 6V6's in parallel would be close to one 6L6. The Mercury OT is the Jim Kelly model, designed to drive four 6V6's, while the others were intended for two 6L6's. I managed to find a quad of 6V6's and a pair of 6L6's that happened to bias correctly at the same bias voltage, so I could simply swap them without resetting the bias.

I invited a couple of my guitarist buddies over to help test them out. One of us would play, another would swap transformers, the third would just listen. We rotated roles every now and then and tried a boatload of guitars, so we spent a good couple of hours at it. And then, I played my regular Thursday gig with the rig, swapping transformers between tunes and even swapping tube sets on breaks.

The verdict was surprizing, and unanimous among those who participated in the test. The Mercury was very clean, but uninteresting sounding; I wouldn't go so far as to say "cold", but not very warm either, and just not inspring. The Bandmaster was a little compressed and nasal, but still had that classic Fender character, a decent useable sound. The Bassman was much more open and big with a nice sparkle on top, quite a good sound. But the Mojo Tremoluxe was the real winner! Not quite as big in the bass range, but loads of character, complex overtones, sparkly shimmery highs, and a nice grindy midrange. And more surprizing yet, it sounded by far the best running off the 4 ohm tap into the 8 ohm speaker.

The verdict on the tubes was more mixed, because the tonal difference was more subtle. The 6L6's were more open sounding with more clarity, while the 6V6's had a bit of a midrange peak and a softer edge. Listening at home I thought I liked the 6v6's better, but live with a loud R&B band the 6L6's cut better with more punch and definition, so that's what I ended up with.

But those differences seemed to apply equally to all the transformers. In other words, the Mercury didn't sound noticeably better with the four 6V6's it was designed for, nor did the others sound noticeably better with the two 6L6's they were designed for. Each transformer retained its tonal signature, albeit mixed with that of the tubes being used.

I have to say, I'm a little reluctant to post this out of deference to Mercury Magnetics. They make a good product, and they were completely cool with me returning the transformer just on the basis of "I don't like the sound of it". On the other hand, who the hell am I, right? This is all totally subjective, and my opinions are just that, opinions.

But my purpose in posting this thread was more to encourage everyone to "listen with their ears", compare things whenever possible, and trust your own judgement and taste.

[Groover]

Sweet!

I believe I see an EVM12 in there.

The cab looks great too! Is that grille cloth black or dark brown?

[Mike Doolin]

Actually it's a Celestion C90 (Black Shadow) out of a mid-90's Mark I reissue Mesa Boogie amp. It didn't have a label on the back but the serial numbers identified it as a Celestion. Wonderful speaker that. The grille is "old style brown" and the tolex is "smooth blonde", both from Mojo.

[Steve Conner]

Excellent... it's nice to see someone on a forum actually doing practical tests, instead of the usual: "Mercury Magnetics are the most expensive, therefore they sound the best."

It's interesting that your favourite OT ended up being the smallest one.

[tedmich]

Foul Foul! Your conversations and the visual ques from the switches biased the experiment irrevocably! You should have only used mute/blind guitarists!

(kidding!)

good testing, were the opinions clear cut or did anyone require a little convincing? (thats were the trouble often is)

Guitarists tend to like "guitar" OTs i.e. small cores, limited frequency response and, not coincidentally, cheapest to make and ship. We LIKE distortion!

[Mike Doolin]

No, as I say, the opinions were unanimous, sometimes to the point where we all looked at each other wide eyed and said, "Wow!" If there was any prejudice on my part, I'd have to say that I really wanted to like the Mercury, just for the cool factor and the red end bells, and wanted to like the old bf's too, just for the cool factor and rusted end bells. The Mojo doesn't even _have_ end bells! and is cheap and readily available, so I wasn't expecting to prefer it. On the other hand, I can't say I'm disappointed that I ended up with the lightest weight OT.

As I think of it, the larger two (MM and bf Bassman) did sound more open and clean with more bass response, so maybe you could draw a conclusion about core size from that. But "open and clean with more bass" doesn't necessarily equate to "better", and even the cleanest guitar sounds are still quite distorted from an audio engineering point of view. Plug your guitar direct into a mixing board or your home stereo to find out how boring "clean" really is. We do like distortion!

[Groover]

Is the Mojotone Tremoluxe OT the one without metal "end bells"?

[Mike Doolin]

That's the one. Here it is on the Mojo site: Mojo Transformers Black Brown Vibrolux/Tremolux Output Transformer (4/8 OHM)

[ThermionicScott]

That's a really cool setup! I take it the order of the transformers in the first picture (front shot) is: MM Jim Kelly, Mojo Tremolux, BF Bassman, BF Bandmaster?

One of my current projects is a 2x6L6 amp using a Deluxe Reverb OT, so I hope that small-OT coloration works out for me, too.

- Scott

[Mike Doolin]

Yes, that's right.

6L6's with a Deluxe OT? So, are you going to run low B+ and a 4 ohm speaker?

[ThermionicScott]

Yes, that's right.

6L6's with a Deluxe OT? So, are you going to run low B+ and a 4 ohm speaker?

Low B+ and cathode bias. From what I've gathered, the early Fender cathode-biased 2x6L6 amps used a primary impedance of around 6.6K, so I think it'll work out as long as I don't blow the thing up!

Sorry for the hijack!

- Scott

[Mike Doolin]

Not at all, let me know how it turns out

[Old Tele man]

...a simple "right-hand" vs. "left-hand" weight comparison will tell you the heavier OT transformer will have the better low-frequency response, because the heavier it weighs the more primary core area/material there is and, thus, the higher the primary inductance, which yields increased low-end response.

...of course, that assumes everything else is comparable.