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Restoring a Sherwood S-5000



 
 
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  #11  
Old January 19th 14, 02:53 AM posted to rec.audio.tubes
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Default Restoring a Sherwood S-5000

OK, here are the stats. The main filter cap has been replaced with another can. Others have been replaced underneath.
Link to pics: https://www.dropbox.com/sh/zoqcrtzbmn1saha/JxXYbkaIIc

C1/a 472VDC
C1/b 470VDC
C1/c 388VDC
C2/a 301VDC
C2/b 217VDC
A lot higher than what you had, and what specs show! No wonder the 6BQ5's I originally tried for power amps were arcing.

There is an additional cap (see it in the C1 photo, to the right of the can) that has 46VDC on it.

Bias caps; These have been replaced, so my measurements are left to right as viewed in the pic:
-26.6vdc, -34.5vdc, -26.1vdc

As for the DC filiment voltages, the only one I found was 13.3VDC on the first two 12AX7's where is the 24V supposed to be?

Selineium rectumfryers have been replaced with diodes, see pic.

Tubes are as follows from the top, Left to right nearest the front: three Telefunken 12ax7, two Tesla ecc803s. Adjacent the filter cap are two RCA 7199.

I will note that the phono hum seemed to be less on my workbench than in the workshop that I am using the amp, not sure if that is because of smaller speakers, or lack of 8-ft flourescent lights above, or just power situations. But, it is still there. The amp is quite silent on FM input (used for my iPod)

--JRC


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  #12  
Old January 19th 14, 07:07 PM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

Good god, those voltages are HIGH! Quick question - did you per chance take those measurements with the power tubes removed? That would explain why they are so high. If so, please take the measurements again with the power tubes installed. If not, what is your AC line voltage? Can you measure the AC HV coming off the transformer?


Thanks,

-forkinthesocket

On Saturday, January 18, 2014 9:53:18 PM UTC-5, wrote:
> OK, here are the stats. The main filter cap has been replaced with another can. Others have been replaced underneath.
>
> Link to pics: https://www.dropbox.com/sh/zoqcrtzbmn1saha/JxXYbkaIIc
>
>
>
> C1/a 472VDC
>
> C1/b 470VDC
>
> C1/c 388VDC
>
> C2/a 301VDC
>
> C2/b 217VDC
>
> A lot higher than what you had, and what specs show! No wonder the 6BQ5's I originally tried for power amps were arcing.
>
>
>
> There is an additional cap (see it in the C1 photo, to the right of the can) that has 46VDC on it.
>
>
>
> Bias caps; These have been replaced, so my measurements are left to right as viewed in the pic:
>
> -26.6vdc, -34.5vdc, -26.1vdc
>
>
>
> As for the DC filiment voltages, the only one I found was 13.3VDC on the first two 12AX7's where is the 24V supposed to be?
>
>
>
> Selineium rectumfryers have been replaced with diodes, see pic.
>
>
>
> Tubes are as follows from the top, Left to right nearest the front: three Telefunken 12ax7, two Tesla ecc803s. Adjacent the filter cap are two RCA 7199.
>
>
>
> I will note that the phono hum seemed to be less on my workbench than in the workshop that I am using the amp, not sure if that is because of smaller speakers, or lack of 8-ft flourescent lights above, or just power situations. But, it is still there. The amp is quite silent on FM input (used for my iPod)
>
>
>
> --JRC


  #13  
Old January 20th 14, 12:45 PM posted to rec.audio.tubes
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Default Restoring a Sherwood S-5000

On Sunday, January 19, 2014 2:07:28 PM UTC-5, hugeshows wrote:
> Good god, those voltages are HIGH! >


yea, that's what I thought.. I did measure my AC line, but don't remember the exact voltage, but it was not out of the norm.

No tubes were removed, the amp was at idle with about 10 minutes warm-up time.
I'll take a look at the AC HV tonight.

Oh, and I don't suppose there is a line voltage selection of any time is there? If it was tapped for 110VAC that might explain it. I get something close to 120-125 VAC here.

I have a variac, and so I will put it inline and see what it takes to get them lower. I guess I should look at AC filament voltages too?

--John
  #14  
Old January 20th 14, 05:23 PM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

I'm trying to dig up my notes on some of the S-5000s I've done before. I am having a hard time remembering exactly what the volts came up to after re-capping, but I seem to think that your number is quite high.

It is likely that our differences are a result of line voltage, the fact that I've still got old caps in mine, or both. I will do some more measurement soon and see if I can come up with a reason. Our units are pretty close in serial numbers. I can't tell off hand if those repairs in there were done correctly, they look a tad sloppy... But I would expect other problems to arise very quickly at those voltages if something was really wrong in there.

But 470v is a LOT higher than any 7189 is rated for, by about 30v... So yeah, no wonder you arced over some tubes in there. Frankly, I'm a bit surprised the ones you have in there now can handle it.

Anyway, we both have some more measuring to do. I'm curious about your line voltage. Let's start there.. Maybe yours is abnormally high and mine abnormally low...

-forkinthesocket

> No tubes were removed, the amp was at idle with about 10 minutes warm-up time.
>
> I'll take a look at the AC HV tonight.
>
>
>
> Oh, and I don't suppose there is a line voltage selection of any time is there? If it was tapped for 110VAC that might explain it. I get something close to 120-125 VAC here.
>
>
>
> I have a variac, and so I will put it inline and see what it takes to get them lower. I guess I should look at AC filament voltages too?
>
>
>
> --John


  #15  
Old January 22nd 14, 07:32 PM posted to rec.audio.tubes
hugeshows
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Posts: 29
Default Restoring a Sherwood S-5000

It looks like I may be getting my parts today, if the tracking info is to be believed...

Meanwhile, here is the video I promised showing the LF driver 'sucking in' when high amplitude bass attack happens. I am hoping the rebuilt power supply will solve this issue as it usually does. This whole thing has prompted me to want to really investigate the phase of the output on this amp...

https://www.dropbox.com/sc/fq395aclt88zwyl/tyuwer-T-Z

Might wanna download it, the dropbox player kinda stinks. And please, these are my test speakers. Yes I know the cones are dirty thank you very much..

-forkinthesocket
  #16  
Old January 25th 14, 12:12 AM posted to rec.audio.tubes
hugeshows
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Posts: 29
Default Restoring a Sherwood S-5000

Hello all,


So I did in fact get my parts the other day, and I've made some nice progress. Sadly, one of the caps arrived with some damage, apparently from the factory, and I am in the process of returning it.

However, the large 40/40/20/20 filter capacitor is just fine, and it has now been installed. The results were nothing short of stunning, more on that later. All of the HV+ voltages pretty much stayed the same, actually losing a volt here and there. I suspect that either my line voltage was slightly different or perhaps even that missing volt was ripple that existed with the old cap and was fooling my DMM. Unfortunately, I'll never know because I have yet to buy a new scope since moving cross country. My old one was a TEK 535A, with Scopemobile, so it was a bit big for my current flat. So whether it was ripple or just a line fluctuation, I'll never know but overall the voltages did not change really from what I posted earlier.

The sonic situation got noticeably better immediately on first audition. There was no 'breaking in the cap' or anything like that, something I'm not really convinced of anyway... The first thing I noticed right away was that the bottom two octaves suddenly were really there. They had been there before, and I had gotten fairly used to the sound of the amp as it was. But replacing the main cap made the hum that was barely there to begin with disappear, and all of a sudden there is a quickness and depth to the lower octaves that just wasn't there before. In addition, the mids and highs seem to have really perked up as well, the highs have increased markedly, while the mids just seem clearer. The sound of the amp improved so much it was just obvious within the first 30 seconds of listening. So at least for now we can say that the cap I ordered works well in this amp. Here's how it was installed:

First off, while most of us have recapped a few amps in our day, I strongly recommend doing this the way I did, because I have done this particular amp several times and there are annoying and time consuming mistakes that are easily made and I will show you how to avoid them here. First, snap a picture of the underside of the main cap (C53 in Sherwood Scehmatics, C1 in Sams)

https://www.dropbox.com/s/u8ze7xcu7f...onnections.jpg

So once you have a record of what went where, you are good to start. First of all, you will NEED at least a 140W soldering GUN to get through this. Don't even try this with a pencil iron, there is too much metal down there and it will heat sink your little iron into uselessness. Don't use a butane iron. Those things have exhaust ports that will melt nearby wires, and you'll feel really stupid if you do that. Use a big Weller gun of at least 140W and you'll make it through. Make sure you have some desoldering wick on hand. A sucker will frustrate you on this cap.

Here's the general idea... All of the skinny rubber coated wires should just be snipped right by the terminal. Don't bother trying to desolder these, as the heat needed will make the vinyl jacket melt back even further than if you had just snipped and stripped. All of the resistor leads and cloth transformer wires should be de-soldered and removed intact. The power resistors do not have any extra length on them, so you are forced to desolder their terminals completely and unwrap and extract the leads. Use skinny needle nose pliers for that. Here is what is left after you've snipped all the little wires (except for two grounds underneath the resistor)

https://www.dropbox.com/s/ur200ncddd...53_snipped.jpg

Now you're ready to begin desoldering. Using the solder wick, carefully reclaim the 4.7K resistor, remove the 33 ohm resistor coming from the rectifier, and then all of the cloth covered transformer wires. Use desolder wick to remove as much solder from those old terminals as possible, and then with the pliers, carefully peel the lead off. The factory typically wrapped the lead around the terminal a couple times and you'll be surprised how much you can reclaim. Going slow and desoldering thoroughly is the key. Once you've gotten all those off, you can either desolder or snip the two black grounds underneath the power resistor depending on how much spare slack is available in your amp.

Once you've finished that, all that is left is the ground tab that is soldered to the chassis. Don't even attempt to desolder that. The chassis and can are such good heatsinks, you'll need 250W to do it, and as you are about to see there is utility in doing it another way. Flip the amp over, tilt the capacitor out of the clamp, and simply bend it back and forth until the tab snaps off. Trust me. Set the old cap aside. Now flip the amp over and using larger pliers, grab the tab sticking out of the solder and twist and pull it like a sadistic dentist. If you do it right, you should be left with a blob of solder and a nice clean slot in the chassis. Hmm, thought I had a pic of that but guess not.

Ok, so now you've got the old cap out and everything is just peachy. Time to take a break. Now we need to carefully compare the cap we pulled out to the new one. Here are some pics of that. Remember I told you to trust me and just rip the old cap out? Well here's where it comes in handy... We now have a perfect guide as to the orientation of the cap as it was in the chassis, and that is very critical for both alignment of the connections, but also making sure that the rectifiers first cap is 20uf and no more or less. We must align at least that section of the cap to its original position in the amp. Otherwise, we'll be making jumpers and splices to high voltage connections exposed to heat. That's not ideal.

So here are some pics comparing the caps. Sorry they're not so hot.

https://www.dropbox.com/s/0uhqs4dpen...p_Newcap_1.jpg

https://www.dropbox.com/s/efyhagz1ru...p_Newcap_2.jpg

https://www.dropbox.com/s/raq55pq9jr...p_Newcap_3.jpg

As you can see, other than the shoulder at the base, the dimensions are identical. So here's the important bit, triple check it before moving on: Stand the caps up next to each other as in the third picture. The missing tab is our key that lets us know how the cap was in the chassis in the first place. The 20uf section for the rectifier is the one to the right of the missing tab, where the red wire went. So grab the new can, read the legend on the side, and rotate the cap until the 20uf section in positioned in the same place relative to you, and mark the tab that will be going through the chassis with a sharpie. as in pic #3. Triple check this! You don't want to solder this can into the chassis and be like "oh ****" later on. That's one critical comparison out of the way.

There is one more thing you must check before installation. Look at picture #1 above. Notice anything? The ground tab on this side of the can has no hole! And, if you look at the original can, you see that this position had wires soldered to the tab. So, seeing that, the time to drill a hole is obviously before installation. The correct size in this case was 9/64". With that out of the way, it's time to stuff the cap in... But wait... If you have rust on your amp top that you plan on removing at some point, the time to remove the rust under the cap is NOW, before you stuff in the new one. So grab some scotch-brite and some CLR and you don't have to do the whole thing, but at least to the bits you won't be able to get at later. Here's the cap stuffed in place after cleaning the rust underneath:

https://www.dropbox.com/s/a4svoftuaz...Clean_Rust.jpg

As you can see, it look stock with the exception of the sticker. If you want to remove the sticker for appearance, the time to do it was before you soldered it in!

Rather than trying to remove the blob of solder on the chassis, it's better to just fold the tab over and press it into the blob as far as it will go (cold) and then use your big solder gun to melt more solder into it. This will work, you have enough heat with a gun.

https://www.dropbox.com/s/4sbo4o1lz9...%20%281%29.jpg

There's the new cap installed. As you can see, there was plenty of heat to bury the tab in that solder blob, and it made an excellent connection. So re-soldering the wires is in the reverse order of removal, and it's pretty important to do it that way. First reconnect all the grounds. After the grounds, do the all the red and orange wires but don't solder the skinny red wires, just connect them up. After that, connect the 33 ohm resistor, the two OPT leads, the 4.7K resistor, and solder it up. That terminal has a lot going on, so go carefully if you want it to be neat. After that, connect the 4.7k and the remaining wires and solder them up. The result should look stock like this:

https://www.dropbox.com/s/vi2i11spzl...%20%282%29.jpg

That's all there is to C53! The result as I said before has been night and day and this amp sounds fantastic already, and we've not finished the entire power supply yet.

Now I'd recommend stopping for a bit, coming back, double checking all the connections, and firing it up with the bottom off so you can measure all thje voltages again. Be certain before you turn it on. A good 5AR4 is not cheap to replace.

Now that we've got the old cap out, we can inspect it a bit. Bend back the tabs as I have done he

https://www.dropbox.com/s/1bd9r7wmac...53_removed.jpg

If you look carefully, you'll see that one section in particular has started to ooze the white goo from around its terminal. Interestingly, it's the [email protected] section, the smallest section in the can! Well, this new can now has that same section getting [email protected], so that is not likely to happen again anytime soon.

Okay, so until I either get the replacement cap for that second twistlock or decide to do the bias supply, I'm done for a few days.

Cheers,


-forkinthesocket









  #17  
Old January 25th 14, 12:30 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

errata from last post:

1) When orienting the cap as I described, it is not only important to align a 20uf section with the rectifier, it is also important to align the two 40uf sections with their original locations! So, the new cap has two 20uf sections, meaning there are two ways you can orient it and have a 20uf section for the rectifier. One of them will give you the two 40uf sections where you want and one way won't. Starting with the section to the right of the missing ground tab, and going counter-clockwise, orient the new cap so that you start with 20, then 40, and 40, then 20, again starting with the rectifier's section and going counter-clockwise. The other way is 20,20,40,40 and that isn't what you want.

2) I mentioned that the new cap was 500v, but it is actuall 525v as I had said in earlier posts.


-forkinthesocket
  #18  
Old January 26th 14, 12:17 AM posted to rec.audio.tubes
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Default Restoring a Sherwood S-5000

On Wednesday, January 22, 2014 2:32:18 PM UTC-5, hugeshows wrote:
> ... And please, these are my test speakers. Yes I know the cones are dirty thank you very much. -forkinthesocket


Finally had a chance to get back, it's been quite a week of keeping FM radio working in my area (my current day-job)

You will be pleased to know that I have the same Tannoy speakers as you (but mine are clean!) Funny story as to how I got them, a certain TV station I used to work for got a package deal for a monitoring station, and it included these Tannoy speakers. They would not fit in the space alloted, so I traded them for a set of Optimus bookshelf speakers (the smallest they made) and we were both lived happily ever after...

So my line voltage is 120.6 VAC with the same meter.. I put the amp on a variac and brought the line voltage to 110VAC and got close to the same measurements as you did. Does the primary transformer have voltage taps?? I didn't really look hard.

I hope I am not destroying anything by continuing on such high voltages.. power tubes are disposable, they are not anything special, but I don't want to cook the amp or the preamp tubes.

--JRC
  #19  
Old January 26th 14, 01:27 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a Sherwood S-5000

Hi There,

So my memory must be failing me in my old age. First off, I've been saying this whole thread that the max capacitance of a 5AR4 is 20uf. Well, I knew better at one point and I am embarrassed to say that I've looked it up and it's 60uf. So, you could put more capacitance at the rectifier, but the result would be raised voltages. I can see that your cap has been replaced.. Can you check and see what the capacitance off that first section is? If not, please measure the AC output of the transformer secondary (the HV feeding the rectifier) and post your findings. It also seems that the big main cap has two sections tied together. Knowing all those values would be useful.

Your line voltage is right where mine is at. Unless we see a difference in the voltage coming off the power transformer, the elevated voltages are probably a result of the work that's been done. I really wonder what that section of the cap is coming off the rectifier.

On the other hand, if those output tubes can handle the voltage and it sounds good, we might want to go with it. The preamp tubes are not getting that much more voltage than they're used to, they're not cooking. Those Russian military versions of the EL84 you have are rated for 500v on the plates, so you're OK there. You'll be forced to keep looking for HV tubes but that thing is probably making 26-27W at those voltages! Have you measured the plate current draw yet? What's the BIAS voltage running at?

So, here's what I need to help:

AC voltage on HV secondary of power trans (loaded)
Capacitance of that first section
Bias voltage and plate current measurements

While those repairs seem a bit klunky, they might be OK. I'm a little worried about whatever splice that is shrinkwrapped near the transformer, though It might just be a ground wire.

We'll get back to the phono section soon, let's check and make sure everything else is ok first.

Cheers,

-forkinthesocket


  #20  
Old February 2nd 14, 11:48 PM posted to rec.audio.tubes
hugeshows
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Posts: 29
Default Restoring a Sherwood S-5000

Hello all, and particularly JRC,

I have done a LOT of work on the S-5000 this weekend, and it is going to take quite some time to write all of if up. However, while rebuilding the bias supply, I think I may have stumbled across a reason why JRC has such high plate voltage. After replacing all the caps and diodes in the bias supply, the bias voltage wound up being quite a bit higher than before. As a result, the tubes were biased pretty cold, and were only pulling around 7 watts each. However, during this time I noticed that my plate voltage climbed way up, almost to where JRC was seeing his.

I guess it seems pretty straightforward, drop the plate current and the tubes load down the HV+ less. I've never seen it quite so dramatic as this, but I think it may be a good starting point.

JRC - Can you please do the following and post results?

1) Measure the ACTUAL resistance of your 12 ohm 1W cathode resistors.
2) Measure the voltage drop across them
3) Measure your plate voltage again since we will be using it for math

Calculate your plate dissipation (per pair) of output tubes, and then the wattage. If you're not familiar with those calculations, go here and plug in the numbers:

http://www.webervst.com/tubes1/calcbias.htm

From this we can figure out just how many watts per side you are pulling and your idle current. I have a sneaky suspicion that while your plate voltage is quite high, your dissipation is going to be pretty low. This would explain why your tubes aren't glowing cherry red.

One of the things I did this weekend that I need time to write up is I replaced those 12 ohm cathode resistors with 4 x 10 ohm 1W 1% precision resistors. Now each tube has its own cathode resistor, and because they are 10 ohms each, we can very quickly determine the idle current in each tube rather than each pair, and because it's 10 ohm, no calculator is needed!

More to come.

JRC, please post what you find out. I think we're close here!

Cheers,

-forkinthesocket
 




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