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Restoring a Sherwood S-5000



 
 
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  #21  
Old February 4th 14, 07:42 PM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

While I wait for JRC to update us on that, I thought I would post on some progress I've made. I've re-done the bias supply and finished the HV+ filter sections, for starters... But I am too lazy to write that up right now! Let's pause that part of the thing and go over the electrical safety for this amp.

Lots of people think that automatically putting a 3-prong plug on a vintage amp is a good idea. There is partial truth to that. It is certainly a lot more important on a guitar amp than it is a hi-fi amp, however. On a guitar amp, you can be holding an instrument that is at whatever potential the chassis is at, and if you touch a grounded microphone with your mouth it can be enough to rattle your fillings! That is a very potentially dangerous situation, especially considering the wide array of generally decrepit electrical situations one typically finds on a stage. So folks who automatically put a three-prong cord on a guitar amp will find no quarrel with me.

Putting a three-prong outlet on an integrated tube receiver/amp however is a different animal. For starters, chassis ground is signal ground on these amps. If you ground the chassis to your house ground, you have introduced a level of safety perhaps, but you've also put a potentially very noisy and dirty ground on your signal ground. This raises the probability of ground loop related issues, where AC travels your signal ground, and just about guarantees that your phono stage will be a complete pain in the ass to get quiet. Modern amps that are designed to have 3-prong plugs are a different beast, though not immune to this problem either. It's not uncommon to see "ground" floating a few volts above actual ground, and in lots of areas in the US, ground is merely bonded to the neutral bus in the breaker panel. So essentially, you've just connected your amp chassis to neutral - the same neutral that connects to every appliance in your house, and gets dirtied by any number of motors, compressors, dimmers, etc.

I think that converting to 3-prong on a the S-5000 is a mistake and not at all necessary for safety. If somebody feels otherwise, no reasonable argument will be ignored, so speak up. Safety is more important than tone.

Does that mean you should leave the stock power cord on your amp? No. For starters, the original cord is not polarized. Putting a polarized plug on your amp will do more for safety than just about anything else. First off, it guarantees that the hot leg of the AC goes through the fuse and not the neutral leg. That's very important. There's a twin capacitor in the S-5000 located near the convenience outlets. It's a ceramic and not at all likely to fail. But, if it DID fail by shorting, and you just happened to have the un-polarized plug still, then there's a 50/50 chance that the chassis now has full 120AC on the it, and that same 120AC is also bypassing the fuse now too!

On top of that we have the convenience outlets themselves, which while not terribly unsafe, are also unpolarized, but moreover using them tends to dirty the noise floor of the amp. So we need to deal with the electrics. Something has to be brought up to date. Here's what I do:

1) Replace the original cord with a polarized two-prong lamp cord.
2) Disconnect the courtesy outlets and all extra wiring for them
3) Wire the mains cord to bypass all that original extra stuff
4) Replace the dual .002 cap with a .01 cap, and move it behind the power transformer where the center-tap grounds.
5) Move the vintage Sherwood tag to the new cord!
6) CHECK THE VALUE OF THE FUSE!! SOME DIMWIT STUCK A 10A FUSE IN THIS UNIT!!

Here's what the unit looks like gutted of all the unnecessary AC cordage and with a new power cord:

https://www.dropbox.com/s/j6si8yqs91...%20%282%29.jpg


See how nice that cleans up? All of the wires to the receptacles is gone, the chassis noise cap is now behind the power transformer (and is now ALWAYS on neutral!) So by forcing the chassis cap to be on neutral, and the fuse to be on hot, we have improved the safety of this amp significantly without messing with ground or screwing up noise levels. All while maintaining a fairly stock look:

https://www.dropbox.com/s/u5dfec453z...%20%283%29.jpg


So that leaves the question of the AC receptacles... There are a couple choices here. For starters, remember how I mentioned that I've added individual cathode resistors? Well, we now have 4 external terminals unused! These AC receptacles can be re-used as bias current measurement points! If you're fearful about what would happen if somebody plugged a tuner into the socket (not much) you can solve that by stuffing the sockets with cut-off AC prongs, allowing probe insertion, but blocking an actual plug!

If you don't mind altering the chassis, those holes line up perfectly with certain styles of slider switch. You could add any number of options, either with the bias or even put a standby switch if you wanted (not recommended).

In any event, the amp will be safer and possibly less noisy after you do this.

More to come!


-forkinthesocket
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  #22  
Old February 9th 14, 04:08 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

Hello all,


Well I've recently finished up the electrical portion of my S-5000 restoration. I've learned quite a few things along the way, and even though I've done several of these before, I made a couple mistakes this time that taught me a few things about how this amp operates.

First off, let me cover the mistakes I made in posts previous to this.

Mistake #1

Changing the chassis capacitor in the way I did it was a bad idea. I had a fundamental misunderstanding of the way that capacitor was designed to work in this amp, and it wasn't until I noticed increased hum that I realized my blunder. Simply using a single cap as is so common in radios and other types of amp doesn't pan out well with the S-5000. The reason is that the original cap is actually two caps connected in series and tied in the center to the chassis. One end of these caps are on hot, the other neutral. The result is a cancellation effect. A single cap simply dirties the chassis currents by connecting it to only one leg. One option would be to simply remove the cap for safety, but after thinking long and hard about it I began to ponder whether the cap was there to deal with inductive spikes on the power transformer when switched on or off. Without any way to answer that question, I simply punted and restored the original capacitor back into the circuit. The added hum was gone instantly. I'll write up that whole AC line redo in a little while...

Mistake #2 (JRC, I think this might be your answer to phono hum)

Believing my eyes and the schematic. The bias supply turns out to be a finicky thing on this amp. If you read the schematic, either Sherwood OR Sams, you see that there are three capacitor sections in the supply. In the original amp, these are all in a twistlock can and share a common positive, rather than a common negative like just about every other twistlock can. If you look at the pictures I posted earlier, you see that there are several things tied to the "ground" on this can, but that is incredibly misleading. You see, while this cap has a common positive, it is NOT on the can body!! There is a terminal that has the common positive, and it is NOT connected to the can. Whoever drew the schematics made the same assumption I did, that the common positive is connected to ground. It is not. It is connected only to the center-tap of bias winding, the DC filament return, and the ground bus that for the entire preamp section!! If you follow the schematic or your eyes when you rebuild the bias section, and you connect the common positive to ground, you will be instantly greeted by a NASTY hum as soon as you connect a magnetic phono cartridge! JRC, since your bias supply was rebuilt, and kinda sloppily, you may wish to redo that section and be sure that the common positive is not tied to the chassis!

I got everything dead quiet in this amp until I tried the phono section which was working beautifully before my rebuild. It now had a horrible hum, and so I knew it must have been something I had done. I poured over the amp with a fine-tooth comb, uncovering 3 mistakes from the factory in the process including two joints that were never soldered, one was the center-tap of the AC primary at its chassis junction!! The other was a tone control resistor that was connected but never soldered to the pot. Obviously, this amp had some pretty green hands making it. After hours of frustration, I started measuring things. I compared the bias circuit carefully to my restored version. After finally getting my meter out and measuring the original cap (still in place but disconnected) I discovered the truth! The schematics are wrong and that circuit connects to the center tap of the bias secondary ONLY, and is floated from the chassis. One wire snipped out and voila! A dead quiet phono pre!


I've got a lot more pictures to take and upload, and I'll probably get to that tomorrow. But I wanted to go ahead and correct my mistakes for the record now, lest anyone else go down my erroneous path with the bias and AC supplies.


Cheers,

-forkinthesocket







  #23  
Old February 10th 14, 07:45 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

Hello all,


So I am finally getting around to writing up the place where I decided to call it quits, electrically speaking. Generally, I find the coupling caps to be good in the S-5000, and so when there are no indications of problems, I leave them alone. I think to do otherwise changes the character of the amp, and that is not something that I either encourage or discourage, because it's a personal choice. However, I think that for my tastes, once the power supply is up to snuff and everything checks out electrically and the amp sounds good, that's when I usually stop.

First off, the power supply rebuild... As you know, I went with the reproduction twistlocks from CE, which I purchased from AES. These capacitors seem to be of high quality, though I did have issues with the 1" diameter can and had to get a replacement. Regrettably, even the second one was a bit out of round at the base as if the crimping machine were having issues with that size, or the die was out of round. Anyway, I decided to install it and it worked out fine. There was nothing remarkable about this cap except that it was a 20/20 cap and the stock was a 30/20. So I added a 10uf cap in parallel to the underside and wound up with over 35uf measured for that section.

The bias supply is what turned out to be the most challenging, since it is sort of an odd circuit, the way its positive side is floated from the chassis. The main issue is that I decided to rebuild the voltage dropping resistors using the stock parts and obviously values as well. The result of this is that the increased voltage from the silicon diodes in the bias circuit along with resistor values that were derived for a selenium rectifier caused an elevated bias voltage which made the output tubes run rather cold. Keep in mind that the bias adjustment pots on this amplifier merely adjust the balance of bias voltage on each channel, they do not control overall voltage and there is no way to match each tube's current draw across channels.

After rebuilding the circuit with the stock resistors and new capacitors (underneath) I started taking measurements. I was seeing that the output tubes were drawing less than 15ma plate current at idle, and that's cold for any 7189 amp even at 440v plate. Bias voltage was in the 24v range, and the DC filament supply for the phono section was at about 26.5. Clearly, the stock circuit could not supply the right voltages with a silicon rectifier. Rather than attempting to re-engineer the bias and DC filament circuits just now, I decided to experiment with putting different value resistors in series between the diodes and the first capacitor, and I arrived with an interesting solution.

https://www.dropbox.com/s/gzmtpckjoy...ias_Supply.jpg

Keep in mind that those two green ground connections were later deleted in regards to the mistakes I mentioned in my last post. Anyway, as you can see, I've taken a very old Ohmite 10w wirewound resistor of the screw-mount type and mounted it inside the chassis, bypassed by a 40 ohm 2w precision resistor that Stephie sent me many years back. The resulting parallel resistance is 8 ohms, and that turns out to put the voltages right where I want them.

Why chose two resistors instead of one? Well, I wanted to be able to adjust the bias voltage a bit. I found that 10 ohms ran it just about as hot as I'd ever want the tubes to run, around 24ma a piece with some tubes! 8 ohms got my used Amperex 7189s to run at 21ma a piece, which is about where these Sherwoods run stock. Yes, that's a LOT for this type of tube, which is why you have to stick with ones that can handle the higher plate voltage. Anyway, the 40 ohm resistor can be removed by me at any point if I want to run things a but hotter, or have tubes that need a little less bias to get their current up. If I find a 2w rheostat to replace it with that I can mount internally without drilling, then I might use that as the shunt. Basically, by using a big overrated resistor like that 10w, I guarantee that my bias voltage won't fail due to a crappy pot. If I put a rheostat where that 40 ohm resistor is now and it faults open, the bias circuit fails hot, but safe because the 10 ohm resistor is in parallel.

So, with this arrangement I wind up with 23.6 volts at the top of the DC filament supply, and around 19v bias finally making it to the tubes after the balance pots. This gives me around 19-21 ma idle current per tube at 440v plate.

So, I've also modified that cathode resistors, and done something new - I reused the AC accessory sockets for bias measurement points, and it works brilliantly! You can either measure each tube individually, or measure the balance of a pair by connecting the meter to both points. Since the S-5000 is setup with a balance type bias circuit, this works out great and can be used in place of or along with the hum balance test described in the manual.. FYI- Hum balance in the S-5000 is not the classic hum balance circuit you find in other amps, it's really a fixed bias balance adjustment using injected and then cancelled hum to determine the center point.

So the bias has been left stock, except there is now 8 ohms between the diodes and the rest of the circuit. The dual 12 ohm 1w cathode resistors have now been replaced with 4 individual 1w precision 10 ohm resistors. From the cathodes, I ran a 4 conductor solid core ribbon wire over to the AC sockets which were reclaimed in the AC supply rebuild (more on that later). Here's how I did it:

https://www.dropbox.com/s/o20s9dxgon...sistor_Mod.jpg

The ribbon cable simply goes down between the output tubes, where it ties to each cathode. From there, it runs under the lip of the chassis towards the sockets. Just before the sockets, there's a screw hole. At that spot, I carefully bent a dip in the cable (solid core) and wrapped it with a bit of friction tape to protect it from future screws. And then each channel's pair was soldered to a socket, I laid it out logically.

Now I can measure each tube's bias, correlate "hum balance" to real-world figures, and see just how hot I'm running things without even opening the chassis. If you measure from each terminal to chassis, you see a number which is converted into ma with no math at all, save moving a decimal twice.

https://www.dropbox.com/s/jic4damuuo..._terminals.jpg

Here is the AC power input and the bias measurement points again. This is after I restored the stock chassis-line capacitor arrangement, but now that I had re-purposed the AC receptacles that the cap was mounted to, I had to add a terminal strip to the bolt on the transformer. From there, I re-wired the power supply to put the hot leg of the AC through first the fuse, then the power switch, then the transformer primary. Jumpers were made to connect hot to the new terminal strip and the cap, where neutral and the other primary leg was connected. The cap was grounded to the chassis at that spot, which seems to have the same hum level as its old position so far, fairly low.

I must say that I find having bias measuring points far more useful than an accessory outlet that dirties the noise floor when used, and despite a nostalgia for keeping things stock, this modification works well for me.

I'm getting sleepy, so that's all for now.


Cheers,

-forkinthesocket
  #24  
Old July 23rd 14, 03:57 PM posted to rec.audio.tubes
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Default Restoring a Sherwood S-5000

I have an opportunity to buy an S 5000 in beautiful shape. Would you be kind enough to call me to discuss restoring it.
Bill
630 745 8261
  #25  
Old July 24th 14, 04:49 AM posted to rec.audio.tubes
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Default Restoring a Sherwood S-5000

"C24/B - 250v 204v"

Hi, did you figure out why you measured 46v below what the schematic reads?

Sorry if I missed the explanation but I only see how you resolved the bias voltage supply.

Cheers!
  #26  
Old October 10th 14, 09:02 PM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

On Wednesday, July 23, 2014 11:49:00 PM UTC-4, wrote:
> "C24/B - 250v 204v"
>
>
>
> Hi, did you figure out why you measured 46v below what the schematic reads?
>
>
>
> Sorry if I missed the explanation but I only see how you resolved the bias voltage supply.
>
>
>
> Cheers!


Sorry folks, haven't checked in here in a while. I'll take another look this weekend and see if I can answer some of these questions.

Cheers!
  #27  
Old October 11th 14, 03:50 PM posted to rec.audio.tubes
Peter Wieck
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Default Restoring a Sherwood S-5000

On Sunday, January 12, 2014 5:11:25 PM UTC-5, hugeshows wrote:
> Ok, pics coming soon...
>
>
>
> First step was to take voltage measurements off the bias and HV+ rails. Here's what I got on my S-5000. By the way, there are two variants of the S-5000 that I will discuss later, basically the older, longer version and the newer, shorter version. The one I am working on here is one of the shorter types.
>

A few things out in the deep background he

a) Voltages at the wallplate: These days, with average household electrical use being anywhere between 50% and 200% greater than it was when that Sherwood was 'new', Utilities that have not significantly upgraded their final distribution systems (most) have increased the voltage in those systems. It is not uncommon for us to see up to 130V commonly at the wallplate at our house these days. And 5% over the 'nominal' 120V would account for most of your B+ variations.

Do you have access to a variac such that you might reduce the input voltage to 115V or so (much closer to what would have been the case in the 1960s throughout most of the US)?

b) Selenium diodes decay over time (and when they fail it can be quite spectacular and leave you gasping in amazement). As they decay their resistance increases.

My guess is that your selenium diodes are 'out' about 10% - so the higher line voltage overcomes some of that. NOTE and WARNING: if I am correct in that assumption, those diodes are closing in on the failure point. Get that block (or individual units) replaced ASAP.

I am not above installing the occasional dropping resistor in some of my equipment that was designed against a typical wallplate voltage of 110V or so - most 'stuff' made in the 1930s and 1940s. Post WW-II, nominal was about 115V, into the 1980s, mostly around 120V. And from about 2000, commonly well over that voltage. Filaments do not like voltage in excess of their rating. 5% high typically will translate to a 40% drop in life. AND, despite anecdotal and cult beliefs to the contrary, nor do they like low voltage very much either. Look up 'lazy cathode' and other phenomenon along those lines. Generally, there is nothing much wrong with 5% low. Beyond that and the consequences are worse than the benefits. Interesting aside - clearing 'lazy cathode' (AKA 'rejuvenation' in some circles) involves running filaments at a high voltage under controlled conditions. So be aware.

c) Drifted resistors - resistor technology has changed (mostly for the better) since the 1960s. These days, even 'cheap' resistors are typically well within 5% however they are marked. But older compositions often drifted - usually higher. It is always a good idea to check all the resistors.

Best of luck with this - it is a worthwhile endeavor.

Hum in the pre-amp: Given the extra amplification of the phono-stage, it will exaggerate any hum in the system that may not show up at from the line-level inputs. But, as it happens, most sources of hum in the phono section will be related to bad shielding, and/or a shield that is grounded at both ends (or the one end that should be connected is not). Start with the simple stuff such as physical damage or a failed connection. Then, look for the obvious - caps.

Never mind Andre - he is neither of consequence nor any particular use. Note also that the pictures (of himself) he allows are over 30 years old... There is far more ego than substance going on there.

Peter Wieck
Melrose Park, PA
  #28  
Old October 26th 14, 09:49 PM posted to rec.audio.tubes
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Default Restoring a Sherwood S-5000

I notice things have died down a bit.

Anyway, I looked and foud that the S-5000 used the Williaamsin configuration but the S-5000A did not. I had an S-5000A for a while. Damn thing got disconnected from one speaker at some point and had an arced over tube socket. Not happy.

But you got the real McCoy I guess.

So, do anything lately on it ?
  #29  
Old October 30th 14, 04:04 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

On Sunday, October 26, 2014 5:49:24 PM UTC-4, wrote:
> I notice things have died down a bit.
>
> Anyway, I looked and foud that the S-5000 used the Williaamsin configuration but the S-5000A did not. I had an S-5000A for a while. Damn thing got disconnected from one speaker at some point and had an arced over tube socket. Not happy.
>
> But you got the real McCoy I guess.
>
> So, do anything lately on it ?


A bit, actually. Of late I've been on audioasylum more than here, but I certainly don't mind coming to some sort of conclusion with this thread, and I realize there is more work to do. I recently acquired an S-360, a rare monoblock Sherwood meant to turn the S-4400 unit into a complete stereo amp/preamp, and I will be diving into that soon.

Are there any particular questions still outstanding that you had?


Cheers.
  #30  
Old October 31st 14, 06:58 PM posted to rec.audio.tubes
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Default Restoring a Sherwood S-5000

It's really just curiousity.
 




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