Impact of a 6.6k primary

[hipfan]

So, I'm stumbling around the Internet looking at random guitar sites the other night, and I found the following tip relating to JTM45's:

Output Impedance. The standard JTM reissue output transformer has a primary of about 6.5k, which is about 65% higher than your typical two-tube Marshall iron in an EL34 amp (e.g. 50w plexi). If you want more of an aggressive sound, set the selector one notch higher than the load, i.e. set it at 16-ohms for an 8-ohm cabinet, which tends to make the tone much more Marshally. It has been said that using the impedance selector in this manner will make the amp louder and it will sound less "constricted".

This sparked my interest because, while I don't have a JTM, I do have an EL34-based Mojotone Tone Machine amp, which also has a primary of 6.6k, like a JTM45, according to Greg Germino (he built the amp). The Tone Machine is currently functioning as a back up to my Demeter. While the Tone Machine has always been a solid, interesting amp, I came to have one issue with its performance - at "band" volume, the sound would get kind of squashed and mushy, especially if any kind of drive pedal were engaged. Hence, it got replaced as No. 1 by the Demeter.

I decided to "mismatch" the impedance as suggested by the excerpt above. I connected my 8 ohm cab to the 16 ohm tap on the amp. Lo and behold, the amp sounds much crisper, louder, aggressive, and, indeed, less "constricted" than it does with the "matched' load!!! Drive pedals all of a sudden are useful at high volume. Nice. I hate to use cliches, but it's like a whole new amp.

I'm kind of a neophyte on the more technical stuff involved with guitar amps, so I don't know the "why" of all of this. Does the story make any sense? Any observations? Thanks!

 

[Jackie Treehorn]

Generally, if you have a higher then average primary impedance, you get less power, it works the tubes less, and you get greater 2nd harmonic content. The primary impedance that gives the most output power, also gives a lot of third harmonic and little second, which gives you crunch.

 

[hipfan]

Interesting! Thanks for the reply, Jackie. So, with this in mind, it begs the question of why use a transformer with such a high primary impedance at all? Is it a design choice to emphasize the 2nd harmonic content, at the expense of the 3rd? Is there some other sonic characteristic achieved by such a design?

Or, on a related note, why wire up the amp so that a nominal "match" (i.e., and 8 ohm cabinet into the 8 ohm tap) fails to optimize the power potential of the amp? Wouldn't it make more sense to mark what is now the 16 ohm tap, as the 8 ohm tap - thus giving the most available power?

Thanks again.

 

[Jackie Treehorn]

Well, I think it's generally a tone, feel, and operating point choice. A lot of the even harmonics are cancelled out in the output transformer in a push-pull configuration. The odd harmonics sound great on power chords, but often make close voicings turn to mush. The 70's Marshalls used a lower primary impedance than the plexi era and JTM45 and many tend to prefer the earlier amps. I think a higher than "tube chart reccommended" primary impedance contributes to a sweeter sound and the primary impedance which is equal to, or slightly higher, than the point of maximum power output sounds meaner. There is a point slightly lower than maximum power output which has the least distortion overall which is where you'd want to be for a hifi type application.

In the case of your amp with EL-34's, a 3.3k primary sounds more in line with standard Marshall 50 watt values. You could also try KT-66's with the properly matched 6.6k tap a la JTM45 if you can get enough bias voltage.

 

[nek]

Why not ask Greg Germino, since he was the original source of information? While you're at it, ask him about the supply voltage. If its under 400v, does he think 6V6 output tubes would sound good in this amp? That might be another way to change the tone and response. Come back here and print his answer so we can all learn. That would be great.

 

[hipfan]

Would you like some fries with that order? How about a combo?



Edit: By the way, you're right that Greg should be a good resource for this information. However, I didn't ask him first because I asked him a bunch of questions right after I bought the thing. He graciously answered all of them, but I would understand if he wouldn't want to be pestered with too many questions about amps that he built for another manufacturer, but didn't design himself. He has his own amps to worry about.

Now, Andy Turner *did* design the amps, and he still runs Mojotone I think. Maybe I'll shoot him an e-mail. He also has answered some of my questions in the past. Very responsive fella. I'll post a reply if he gives me some info. Thanks.