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Sherwood S1000 Mod Question


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I haven't really delved all that much into tube amplification but I have a pair of Sherwood S1000's that I've been wanting to use/try out in a stereo setup for some time... Ideally I would be looking to use them as power amps solely. The amps have a tape monitor switch that bypasses V1 (Phono Pre) & V2 (Pre/Cathode Follower) completely but leaves the tone controls and volume in place... I am curious what your thoughts on bypassing those functions are? 

 

These amps appear to have been changed/revised quite a bit over their production so it may not be exact but I have included what I feel is the closest version of the schematic I have.

 

Thanks

Sherwood-S1000-II 6BA8A 6L6 Schematic.pdf

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  • 3 weeks later...

I’m not really familiar with Audio Asylum and when I went to check it out I would say I found it easy to navigate... 

 

I do realize that the Klipsch forum prob isn’t the best place to ask for this advice but I figured it was worth a shot?

 

I am mostly interested from a general stand point, is it possible to jumper past the eq section? Like run a wire from one of the rca jacks straight to the loudness/volume pot? Could I just leave the other half of V3 unused? Or run a wire from the tape/monitor switch to pin 2 of V3 and throw like a 1M resistor to ground in there? Even better would be to remove C28, leave R40 and use that as the input... bypassing V3 altogether?

 

If this were SS I would have better idea of where I could jumper in to the power amp from but I am just not very knowledgeable when it comes to tube amp design.

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  • 3 weeks later...

Ok, so I used the tape input with its 100K to ground and placed a 0.01uF behind that with pigtail...

 

Amp A: Disconnecting C25 (.022) from V3 pin 7 (grid) and attaching the aforementioned pigtail everything seemed to work quite nicely. The volume pots on both amplifiers are trash so it was good to see that they could be bypassed. 

 

Amp B: Disconnecting C28 (.033) from V4 pin 7 (grid) and attaching the pigtail everything worked but with much less volume and missing bottom end. After reworking this amp B to match the wiring of amp A, I notice that while the volume was better there still seemed to be an issue? The very top end of the output is distorted and still lower than amp A by a little bit? Both amp have had all electrolytic and paper caps replaced (they more or less match). They are also both running NOS 5Y3GT's, NEW matched 6L6GC's, NOS 6BA8A's and NEW 12AX7's. Any ideas on what would cause the distortion/low output?

 

Also can some one tell me what function R54 (240ohm) and C1 (50uF) are performing? When going through the amps I notice one had a 270ohm and one had 300ohm (technically 270 + 30) in the R54 position... I have a schem for a similar Bogen MO30 that uses 200ohm and 50uF... I'v also seen schem at use nothing at all in this position... what does changing the value of the resistor or cap do?

 

Thanks

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  • 2 weeks later...

I'd like to thank everyone who's replied so far for trying to help... Work has been very busy and I've had and still have some family matters on my plate so this is realistically on the back burner for now. The other night I did tinker a bit just to clear my head of other things....

 

Currently both amps are now set to use C28 (0.033uF) going to V4 as the input. I have also completely disconnected the front end of the amp. It's hard to explain on the schematic but they actually made this rather easy on the chassis. This disconnected R57, removing C3, all the preamp heaters and the incandescent light on the front panel. I partially did this because it is near impossible to check the voltages or anything else in the power amp section without separating the 2 halves of the chassis. I also wanted to give this idea one more shot after realizing the amp I originally tested was not functioning properly. Well, on the good amp this setup sounds amazing, even on just an ok test speaker! Previous there was, what I would consider, a tremendous amount of noise. Currently the amp is so quite I didn't think it was working the first time I turned it on to test it. The malfunctioning amp makes some noise on power up but does quite down now and has about half of the output of the good one??? There are so few components left I am curious what would likely cause this? I did swap both the 6BA8A and the 5Y3GT since they were purchased as NOS but may not have actually been but the problem persisted. I guess I will check everything else when I eventually have time...

 

Side Note: since none of these Sherwoods seem to be very popular/well known I thought I would mention that I got this whole idea from another one of their models, the S4400 & S360. The S4400 was a stereo pre with a mono power amp and you could buy the accompanying S360 to make it fully stereo. I guess this wasn't a popular setup at the time because the S360 is exceedingly rare. The S4400/S360 uses a 7199 instead of a 6BA8A and 4x 7189 instead of 4x 6BQ5 or 2x 6L6 used in most S1000's but other wise seem pretty similar. I actually have a schem for the 4400 but haven't been able to get my hands on a 360. At times, 4400 owners have had an easier time getting a hold of a second 4400 or a 7189 driven S1060 to convert and use as an S360 which made me think maybe I could do that to the 1000's I had lying around and just use my own pre...

S1060_schem.pdf S4400_schem.pdf

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  • 1 month later...

Just so that this thread isn't left completely open ended... the issue with the one amp I was having was the Damping Factor Switch, which I ended up removing from both amps.

 

Everything appears to be operating great now!

 

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