John Suhr Provided Details Regarding the SL68 (JMP SL clone)

[julkke]

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Check that out, the mains filter caps look smaller than the screens.. Almost looks like it's 32uf for mains and 50 for screens, or maybe the caps are just reversed in the layout?

Also check the OT..

[bmf5150]

Looking at the chassis shots and the way the wires turn ,it looks like mains and screens are in there normal spots!

[vh junkie]

I can confirm that Dave drops the voltage to the whole amp. Thats why I had the Variac inside my Variplex removed and a regular power cord installed. That way I could plug directly in to a Variac and lower the WHOLE amp. Dave did stress that the heater voltage being dropped was part of the equation!

This is the way I think it works:
There are additional taps on the primary of the PT
120vac, 150vac, (if this can be wired for overseas use, more taps would be required)
For 120vac(North America)
Normal mode: The power switch connects the 120vac input to the 120vac primary tap, yielding the HIv DC (about 475v?)
Brown mode: The power switch connects the 120vac input to the 150vac primary tap, yielding the LOv DC (about 390v?)
This let the Brown mode deliver the same voltage as if there was a variac turned down to 90vac.
The heater voltage is lowered in brown mode naturally (no additional circuitry required)
The bias voltage also behaves naturally, the lower bias (less negative) voltage output by the PT makes the bias hotter.
All that remains is to install separate pots for adjusting the bias voltage in each mode.
This is accomplished by switching the bias ground(above where it says HI on the board) thru the third pole (common) of the power switch and looping the switched outputs back to each of the bias pots on the board. Pretty clever!

[julkke]

Hmm, I think it should change to a higher primary tap than 150v for that much of a voltage drop? No? I might try the zener trick to knock my b+ down a bit and experiment how it browns out the sound.

[vh junkie]

Hmm, I think it should change to a higher primary tap than 150v for that much of a voltage drop? No? I might try the zener trick to knock my b+ down a bit and experiment how it browns out the sound.

Yes, it would probably be more like a 160vac tap to simulate lowering it from 120vac to 90vac; but, I though it might be simpler for folks to follow if I kept both voltage delta at 30v.

Personally, I think it sound best if the HIv ends up in the 400v to 410v range.

[julkke]

Hmm, I think it should change to a higher primary tap than 150v for that much of a voltage drop? No? I might try the zener trick to knock my b+ down a bit and experiment how it browns out the sound.

Yes, it would probably be more like a 160vac tap to simulate lowering it from 120vac to 90vac; but, I though it might be simpler for folks to follow if I kept both voltage delta at 30v.

Personally, I think it sound best if the HIv ends up in the 400v to 410v range.

Thats the voltage I'd like to get to. I'm currently on 470v. Wonder if dropping the voltage 60-70v with zeners would be a problem?

[vh junkie]

That is probably a topic for another thread. This one is about the SL68. If you want the sound w/o a custom transformer, you would be better off just using a variac to get you in the neighborhood and tweak from there.

[bmf5150]

I can confirm that Dave drops the voltage to the whole amp. Thats why I had the Variac inside my Variplex removed and a regular power cord installed. That way I could plug directly in to a Variac and lower the WHOLE amp. Dave did stress that the heater voltage being dropped was part of the equation!

This is the way I think it works:
There are additional taps on the primary of the PT
120vac, 150vac, (if this can be wired for overseas use, more taps would be required)
For 120vac(North America)
Normal mode: The power switch connects the 120vac input to the 120vac primary tap, yielding the HIv DC (about 475v?)
Brown mode: The power switch connects the 120vac input to the 150vac primary tap, yielding the LOv DC (about 390v?)
This let the Brown mode deliver the same voltage as if there was a variac turned down to 90vac.
The heater voltage is lowered in brown mode naturally (no additional circuitry required)
The bias voltage also behaves naturally, the lower bias (less negative) voltage output by the PT makes the bias hotter.
All that remains is to install separate pots for adjusting the bias voltage in each mode.
This is accomplished by switching the bias ground(above where it says HI on the board) thru the third pole (common) of the power switch and looping the switched outputs back to each of the bias pots on the board. Pretty clever!

So the higher vac primary drops the the voltage , as well as the heaters???i wonder how the extra primary 150vac effects the nature of the power transformer such as sag ??vh junkie, you are the man..

[vh junkie]

So the higher vac primary drops the the voltage , as well as the heaters???i wonder how the extra primary 150vac effects the nature of the power transformer such as sag ??

Sag? Well that would depend... if these were designed to run overseas, the taps would have to run up to 300vac+(for a 240vac input)... there is also the issue of overseas current running at 50hz instead of 60hz... I am not a PT expert, but from what I gather the PT needs to be built sturdier to support the 240v/50hz scenario, so sag would be limited.

The lower voltage at the PT input could be achieved by other means:
1) A built in variac hooked of the power switch (big and heavy)
2) A smaller (3x3x4?) inch tapped autotransformer.

Both of these would be hard to mount on the chassis itself, so unless someone has seen one of these, I would go with the tapped primary PT.

[julkke]

What would happen if you guys running 120v would hook up say, 220v tap? Would the voltage go too low?

[rgorke]

So the higher vac primary drops the the voltage , as well as the heaters???i wonder how the extra primary 150vac effects the nature of the power transformer such as sag ??

Sag? Well that would depend... if these were designed to run overseas, the taps would have to run up to 300vac+(for a 240vac input)... there is also the issue of overseas current running at 50hz instead of 60hz... I am not a PT expert, but from what I gather the PT needs to be built sturdier to support the 240v/50hz scenario, so sag would be limited.

The lower voltage at the PT input could be achieved by other means:
1) A built in variac hooked of the power switch (big and heavy)
2) A smaller (3x3x4?) inch tapped autotransformer.

Both of these would be hard to mount on the chassis itself, so unless someone has seen one of these, I would go with the tapped primary PT.

I have often bee curious about the impact 50hz vs 60hz has on tone. Meaning when an amp is built for one, say 50hz, does running it with 60hz have any tonal impacts or vice versa?

[bmf5150]

Good read on John suhr and his amp
http://online-discussion.dhenderson.com ... php?t=1093" onclick="window.open(this.href);return false;

[julkke]

Good read on John suhr and his amp
http://online-discussion.dhenderson.com ... php?t=1093" onclick="window.open(this.href);return false;

Yep nice read indeed. I might do a recap with mallories now!

[EddyInChicago]

Gentlemen,

Something I noticed in the gut shot John Suhr provided. Also, a few mentioned the specs; in particular the mixer and tone stack capacitors. I believe, 470-470/470pF mixer and 500pF (or 560pF) tone stack caps is incorrect. Looking at the picture carefully, I noticed a few things regarding the mixer cap and tone stack caps; the caps are longer than the resistors around them. And, I'm sure we can agree that John is using 1/2 watt resistors (they look to be Xicon).

So, what is my point? Glad you asked I've provided a picture that compares a 1/2 watt Xicon resistor to the CDE mica caps that are used in John's amp. The top cap has a value of 200pF (1000v) and the bottom two caps are 500pF and 560pF (1000v). All three caps are of the same type. I have (obviously) the 200pF, 500pF and 560pF CDE's. So, I think, the mixer and tone stack caps are closer to 200pF than 500 to 560pF.

John has also mention that every SL68 amp he builds, is put on a piece of lab equipment (he did say, but I forgot the name. It's an analyzer of sorts) and the EQ is swept to match the "Holy grail amp". In Pete Thorn's demo video, it's my opinion, that the amp almost nails the EQ part of Ed's sound, however a bit too much gain and clutter to my ears. EVH's amp, to me, sounds less distorted and more open and articulate. However, since the EQ (or voicing) of the amp is nailed, we get the impression that everything "falls together".

So, this is my contribution, opinion and detective work to this topic, for what it's worth. Check out the pic and let me know what you think.

Thanks....Ed

P.S I do believe that, as someone mentioned here, that the power tranny is switched from it's input. For example, two sets of leads that tap different points of the primary of the power transformer.

[bmf5150]

If he is using 200 or 250pf he must be compensating for the brighter sound of the ppimv he is using!