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

Tone Slinger · Post by **Tone Slinger** » Thu Nov 07, 2013 1:51 pm

I think your right, both Michael (JMP amp clip above) and Suhr are on the 47k/8 ohm tap. I think Michaels sounds better than Suhr's Sl-68 because of his transformers. I'm thinking the Suhr is a bit too smooth and 'soft' because of the TOO low voltages it runs at, via those custom Heyboer trannies. I definately prefer Michaels clip to the Suhr's considering he is using a Lar/Mar ppimv (as Estrada obviously used in one of the Sl-68 clips as well). Suhr has that too (Lar/Mar ppimv) as we all know, but most of the SL-68 clips is attenuated (like Thorn's clps). Having said all that, I still think that the Suhr Sl-68 is one of the best 'marketed' amps out there for early Ed type tone. The 27k/16 0hm tap is to be considered. Ralle likes it. I'm under the impression that original 12xxx tops had one of these 3 NFB circuits, either 47k/8 ohm , 47k/16 ohm or 27k/16 ohm ?

mightymike · Post by **mightymike** » Thu Nov 07, 2013 3:31 pm

I liked Michael's Major clips

rgorke · Post by **rgorke** » Thu Nov 07, 2013 6:18 pm

stef wrote:I think that Suhr and Michael both have 47k/8ohm. However, using the PPIMV (Michael and Estrada) affects the NFB (decreases it) and the amp sounds more distorted (due to lower NFB) and brighter and looser. My amp sounded slightly hotter and brighter even with the PPIMV (L/M) on 10.
For the first time I'm experimenting with a 27K NFB resistor

I have 27k/16 ohm....

EJSLPlexi · Post by **EJSLPlexi** » Wed Nov 13, 2013 2:11 pm

Tone Slinger wrote:I think your right, both Michael (JMP amp clip above) and Suhr are on the 47k/8 ohm tap. I think Michaels sounds better than Suhr's Sl-68 because of his transformers. I'm thinking the Suhr is a bit too smooth and 'soft' because of the TOO low voltages it runs at, via those custom Heyboer trannies. I definately prefer Michaels clip to the Suhr's considering he is using a Lar/Mar ppimv (as Estrada obviously used in one of the Sl-68 clips as well). Suhr has that too (Lar/Mar ppimv) as we all know, but most of the SL-68 clips is attenuated (like Thorn's clps). Having said all that, I still think that the Suhr Sl-68 is one of the best 'marketed' amps out there for early Ed type tone. The 27k/16 0hm tap is to be considered. Ralle likes it. I'm under the impression that original 12xxx tops had one of these 3 NFB circuits, either 47k/8 ohm , 47k/16 ohm or 27k/16 ohm ?

Clips and video's do NOT do the SL68 justice. The amp is a beast in person and i was shocked at how much the clips/vids degraded how great the 68 sounded. Another thing is the amp feels amazing while you play. very responsive and singing.

Stephand · Post by **Stephand** » Sun Feb 16, 2020 11:32 am

I know I'm 7 years late on this one. I really like the info and informations about the very SL68

I have read all 10 pages of discussions and did not find anything about the missing Resistors on the filter caps (56K)… see picture provided by John Surh. Normally on a SL 68 we will find two resistors 56k on the dual caps.

Is it possible that he do not use any?

johnnybgoood · Post by **johnnybgoood** » Sun Feb 16, 2020 3:21 pm

Stephand wrote: ↑
Sun Feb 16, 2020 11:32 am
Normally on a SL 68 we will find two resistors 56k on the dual caps. Is it possible that he do not use any

Those 56K resistors usually found on the Screen Filter Caps are NOT necessary. They are there to send voltage to ground from the B+ source. When the amp is off, they also drain the caps to ground so that they do not hold a charge. Think of them like any grid leak resistor that drains signal to ground.
Here's the chassis of the SL68

SDM drawing here showing the 56K resistors on the filter caps.

wdelaney72 · Post by **wdelaney72** » Mon Feb 17, 2020 10:57 am

If anything, removing the 56K resistors reduces ghosting.

Their purpose was to ensure dual screen caps evened out the DC shared on the 2 halves of the cap.

I never use the 56K resistors.

Stephand · Post by **Stephand** » Mon Feb 17, 2020 5:45 pm

Make sense
Is someone have define the 2 resistors value in series with the 2 bias pots?
I believe the one in between the pots is 8.2K... not sure and have a hard time with the one above.

here is a second picture that can help

Stephand · Post by **Stephand** » Fri Feb 21, 2020 6:01 pm

here is the picture sorry

mean_street · Post by **mean_street** » Thu Mar 19, 2020 11:04 am

5,6k im between the bias pots and 33k before. I’ve had two of these, one with and one older one without the PPIMV, both were pretty good sounding.
The amps use Mallorys throughout and CDE in some key positions (Cathode of V1b and V2a, cap on Presence pot). There is also a fat cap on the cathode of V2a. Both of mine had a 470pf bright cap. Filtering is 50uF throughout. The rest is pretty much a stock plexi.

Edit: My Sl68 had the 56k resistors on the filter caps, I checked the gutshots I took before selling it . Mine also had a red fiber turret board.

frost822 · Post by **frost822** » Sat Mar 21, 2020 10:00 am

The filtering on the sl68 is all 50/50 uf cans? I have not been able to find much info on what exactly the sl68 uses for filtering. Could you provide what is used?

mean_street · Post by **mean_street** » Sat Mar 21, 2020 11:20 am

Mains and Screens - 50uF (all 50/50 uF caps),
PI Filter Cap - 100uF (50/50 uF parallel),
Preamp - 50uF/50uF

I think these were ARS filter caps. With the fat cap you really need the added filtering to keep the bottom end somewhat together, the amp has enough gain that you won’t miss the lower filtering much. Hope that answers your question.

frost822 · Post by **frost822** » Sat Mar 21, 2020 11:56 am

Thanks, I tried the sl68 specs in my old Marshall and used lower filtering in the pre and screens. The low end was flubby and loose. Was going to try without the fat cap but this stiffer filtering makes sense.

mean_street · Post by **mean_street** » Sat Mar 21, 2020 2:17 pm

No problem! But I think the tone of the Suhr amp depends really on the choice of materials (like every other amp).
You should come close to the tone with the mentioned filtering, Mallory coupling caps and CDE caps in the key spots. It’s a cool spec for sure.

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 4:09 pm

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

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