Real TrainWreck mojo!

[Gregg]

Until I see and/or hear such thing happening I'll try to stay away from myths and legends no matter where they come from.

[Chuck H]

Well... I am right in the middle of just such a thing happening. I have two amps here right now. One I built as a prototype and the other as a pre production model. They both have the same circuit, tubes, power transformer and OT impedance. They are different in layout and OT brand. The prototype has the mojo thing and the pre production unit just sounds like a good amp. Even when I alter the circuit on the pre production unit for more gain it's the proto that still has more gain. The proto has a very appealing crunchy but saturated modulation to the distortion. The pre production has this too, but not as much and not as good. I'm not in a huge hurry to figure it out, but soon I'll get them both opened up at the same time and do a lot of bench testing and comparison.

[mikepukmel]

Well... I am right in the middle of just such a thing happening. I have two amps here right now. One I built as a prototype and the other as a pre production model. They both have the same circuit, tubes, power transformer and OT impedance. They are different in layout and OT brand. The prototype has the mojo thing and the pre production unit just sounds like a good amp. Even when I alter the circuit on the pre production unit for more gain it's the proto that still has more gain. The proto has a very appealing crunchy but saturated modulation to the distortion. The pre production has this too, but not as much and not as good. I'm not in a huge hurry to figure it out, but soon I'll get them both opened up at the same time and do a lot of bench testing and comparison.

Very interesting! Any chance we can get a comparison sound clip?

[Chuck H]

Very interesting! Any chance we can get a comparison sound clip?

AAaargh! I wish I could participate in the sound clip thing like other's do here, but I'm not rigged up with any recording equipment. I know everyone uses computer interface now, but I don't know anything about the software or how to get good recordings like that. My local guy with an actual recording studio is pretty much out of the game now and converted his studio into a bar. I never wanted to trouble with learning the new recording media, but maybe I should bite the bullet and do it.

[Helmholtz]

Well... I am right in the middle of just such a thing happening. I have two amps here right now. One I built as a prototype and the other as a pre production model. They both have the same circuit, tubes, power transformer and OT impedance. They are different in layout and OT brand. The prototype has the mojo thing and the pre production unit just sounds like a good amp. Even when I alter the circuit on the pre production unit for more gain it's the proto that still has more gain. The proto has a very appealing crunchy but saturated modulation to the distortion. The pre production has this too, but not as much and not as good. I'm not in a huge hurry to figure it out, but soon I'll get them both opened up at the same time and do a lot of bench testing and comparison.

I would start with interchanging the tubes and the OT. Tubes may have a variation of 20% or more for specified parameters, i.e. for linear operation. The variation in non-linear operation is much greater.
OTs may differ in primary (magnetizing) inductance, influencing bass response (and transformer distortion), as well as leakage inductance and distributed primary capacitance. In result some OTs show a pronounced leakage resonance peak in the audio range while others don't. In amps with NFB, OT differences show most when the power stage is overdriven.

[Chuck H]

I would start with interchanging the tubes and the OT. Tubes may have a variation of 20% or more for specified parameters, i.e. for linear operation. The variation in non-linear operation is much greater.
OTs may differ in primary (magnetizing) inductance, influencing bass response (and transformer distortion), as well as leakage inductance and distributed primary capacitance. In result some OTs show a pronounced leakage resonance peak in the audio range while others don't. In amps with NFB, OT differences show most when the power stage is overdriven.

Thanks Helmholtz. Good coverage of probabilities. I have actually done one more ear test experiment short of opening up the amps. I'll try to be clear about it. Here's what I did:

These amps have an effects loop right behind the PI. So to narrow down whether the "mojo" was in the preamp or power amp I simply plugged the amps into each other. That is, I would plug the effects sent from one amp into the return of the other. Then I would dummy load the power amp not in use and plug whichever one I was using into the same speaker cabinet for testing. The mojo follows the prototype preamp in all tests. Running either preamp into the either power amp gave the same results. So then I swapped the preamp tubes AND the results were the same. So...

Doing this eliminated the PI and the entire power amp from the possible reasons for the difference, eliminated the possibility of layout coupling between the preamp and power amp and eliminated any difference between the preamp tubes. So the difference is in the preamps alone. Invisible couplings within the preamp itself due to layout and lead dress and/or minor variance in component values.

The prototype having that special edge over the pre production amps has been the case since I built them. I have made several circuit refinements (over the course of eight years). If I alter one of the amps and I like it I would make the same alteration to the other amp and then compare. No matter what I change, the proto amp always keeps the magic.

I suppose the next step would be to build another amp with the same layout as the prototype and see if it sounds the same. Alas, no time budget for a build right now.

I'm still going to get both amps opened up at the same time and test/scope for finite details. I may yet have an AHA! moment.

[Dave H]

I'd plot the frequency response of the preamps to see if there's any difference. It only takes a few seconds to plot 20Hz to 20kHz with my PicoScope (see below) but it's a pain to do manually.

[Gregg]

AAaargh! I wish I could participate in the sound clip thing like other's do here, but I'm not rigged up with any recording equipment.

You don't need any special equipment to record sound clips for comparison purposes. Just run your mike to your PC sound card mic input and use free software like Audacity for example.

[Chuck H]

I have no sort of spectrum analyzer. What I'll do is a frequency sweep on the oscope for each stage. Hopefully something jumps out that'll give me a clue.

EDIT: And another to Dave because I hadn't considered doing this before thinking of how to reconcile not having a PicoScope. The test wouldn't exactly be spectrum analyzer results, but it would be a gauge of how each stage is responding to input signals. Which may even be more informative.

[nosaj]

I have no sort of spectrum analyzer. What I'll do is a frequency sweep on the oscope for each stage. Hopefully something jumps out that'll give me a clue.

EDIT: And another to Dave because I hadn't considered doing this before thinking of how to reconcile not having a PicoScope. The test wouldn't exactly be spectrum analyzer results, but it would be a gauge of how each stage is responding to input signals. Which may even be more informative.

Chuck, can you explain what you mean by doing a frequency sweep with the oscope?

Thanks,
nosaj

[Chuck H]

Sure. I just sweep the useful frequency range with the signal generator and observe the waveform on the scope, which will rise and fall demonstrating characteristics relative to input frequency and how the circuit is tuned. Because I have an old signal generator I'll need to double check the source voltage @ frequency to be sure of what's happening if something pops out that I want to check on. Otherwise it's pretty simple.

[Raybob]

Interesting thread here. Something I found interesting, that no one has mentioned is the chassis material. Are the clones made from steel or aluminum?

Someone at Reverb.com is claiming, "Fischer would use only aluminum chassis because he believed that steel would interfere with the magnetic field of the output transformer and negatively affect the amp’s overall tone. "

[nosaj]

Interesting thread here. Something I found interesting, that no one has mentioned is the chassis material. Are the clones made from steel or aluminum?

Someone at Reverb.com is claiming, "Fischer would use only aluminum chassis because he believed that steel would interfere with the magnetic field of the output transformer and negatively affect the amp’s overall tone. "

It does have an effect not sure where I read that might have been Dave Funk's book.
nosaj

[Chuck H]

Ken also said that aluminum is "faster" than steel. I have no comment on that.

[The Dude]

I suppose that depends on if you're building race cars or amps.