Sherwood S-5000 restoration

Discussion in 'Tube Audio' started by bberkom, Jun 22, 2017.

  1. gadget73

    gadget73 junk junkie Subscriber

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    yeah thats one of the things they can be touchy about. High impedance circuits are very prone to picking up noise.
     

     

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  2. FlaCharlie

    FlaCharlie Active Member

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    Thanks for the clarification. As you seem to understand, R69 only affects the bias supply, not the phono stage.

    As for R4, is there actually a 56k resistor in there? (Green/Blue/Orange marking?) That really makes no sense. The 20v drop shown in the schematic (270 to 250) over a 56k resistor would indicate that that stage should be drawing only 0.357 mA. That seems like way too little current draw for a pair of 12AX7s. Perhaps you misread the meter when you measured? It's obviously not 56 ohms as shown, as that would indicate a current of 357mA. So it's a typo for sure. If it's supposed to be 5.6k, that would make more sense, as it would indicate a current of 3.57mA for the two tubes, about 1.79mA each.

    EDIT: Well I'm totally puzzled now. :dunno: I have a couple of the vertical tube units too and I just pulled one off the shelf and took a look and I'll be damned - sure enough there is a 56k 1/2w resistor there. I would not have thought that those tubes would operate at such low current, but I'm not real familiar with phono stages. The slant tube version uses a 10k 1w which would indicate 2ma total or 1mA per tube.

    If you'll mostly be using the phono stage, I'd suggest you just go in and replace all the resistors and caps associated with those two 12AX7s. I'd probably use carbon films instead of carbon comps. I'm guessing your intermittent noise is coming from one of the old resistors. They can be noisy even if they're in spec. Or could be a solder joint, which would also be fixed if you replace them all.
     
  3. bberkom

    bberkom AK Subscriber Subscriber

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    I replaced R69 for the bias supply, and it gave me a little more negative bias voltage. I also installed a CL-80 which brought this back to around -23v, but it gets my output tubes pulling 26-28ma. The CL-80 also brought all of my PS voltages very close to the schematic running on mains voltage.

    I replaced the 10k 2w R36, which did bring the 275v supply back up a little where it is more in line with the other voltages. I then replaced the 56k R4, and the 250v supply is now around 209v. So it's up a little from 195v but still not close to 250v. Should I try decreasing the value of the 56k dropping resistor to get this voltage up, or is something else going on?

    This brings me to the rest of the phono section. I am listening to it right now, and it is surprisingly quiet and sounds pretty darn good. The extra noise I am getting is not just on phono but all inputs, so I don't really just want to go in and replace everything. However, the plate voltages are throwing me a little bit. The supply is reading 209v right after R4 like I said. For V1, there are two 270k plate resistors that should drop this supply to 130v at the plates. I actually had already ordered these replacement carbon films, and the originals read about 300k, so I put the new ones in. However, I am reading 120v at pin 6 for the left channel plate but only 102v at pin 1 for the right channel plate. Does this imbalance in the plate voltages for this phono tube tell me anything? How concerned do I need to be about these low and uneven voltages if the phono section works and sounds fine?

    For V2, the second phono section tube, the plate voltages are supposed to be 150v after 220k plate resistors that measured on spec. However, I am only reading 125v and 130v on the plates. I am thinking if I can sort out the 250v supply that these will come back up.

    The voltages at the other three line level preamp tubes were very close except for V4. The 275v supplies two plate resistors which are 150k (measured 165k so a little high and a candidate for replacement). The schematic says 190v at the plates for V4, but I am only getting around 150v. This is again pretty low. What would a low plate voltage like this affect?

    The only other symptom I am noticing now when listening is that the right channel is a good bit stronger than the left channel. I have to turn the balance pot to about +3db to the left to center the soundstage. I switched speaker cables to make sure it wasn't just placement related, but the right channel output is still stronger than the left. This is for all inputs, not just phono. I wonder if this could be related to the intermittent rustling I am getting in the left channel?

    I am pretty happy with how this sounds at this point, but I am still concerned about the low voltages in the preamp sections and the channel imbalance. Maybe I just need to be happy with it and move on, but any suggestions are welcome.
     
  4. RWood

    RWood AK Subscriber Subscriber

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    Good info about the voltages and it sounds like you are getting really close. Re the phono tube voltages and mismatches I would focus on the cathode resistors, voltages on pins 3 and 8, and condition of bypass caps (if there are any). The voltages are low here compared to the plates so they are often overlooked but checking the cathodes is a great way to see how well a tube is biased. Also mismatched triodes can cause what you are seeing so trying different tubes there could be revealing.

    For the volume mismatch it could be the actual tracks on the volume pots themselves. You can measure them. If it really matters you could swap the input wires on the two pots to see if it changes (indicating an imbalance in the circuits leading there) or stays the same (mis-tracking pots).
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    Different voltages section to section on the tube probably means the two triodes perform a little differently. Try swapping it with one of the other 12ax7's to see if things change. Also possible the cathode resistors are off on those section(s), but either way I suspect its not a big deal.

    I honestly wouldn't go whole hog with replacing resistors unless you've got good reason to. I have done it in an amp before, but well over half of them were seriously off value. Eventually I quit testing and just changed them all. Neither of my Sherwoods have needed more than a couple replaced, and some of them were because I broke a leg while unsoldering something.
     
  6. bberkom

    bberkom AK Subscriber Subscriber

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    I will swap some tubes around and see if that affects the plate voltage balance between the two halves. I bet that's all it is. I'll also poke around and get the readings on the cathodes of the phono tubes and for V4 since its plate voltages were low. When I get the reading for the cathode voltages, which are spec'd at 1.5v and 1.4v for phono V1 and V2 respectively, what will that tell me about the tubes? What does it mean if that voltage is high or low and what affects that cathode voltage in something like a preamp tube? Would a high or low cathode voltage be affecting the low supply and plate voltages I am reading in the phono section? Still trying to learn what I'm doing in there and not just replacing parts...

    Here is a shot of one of the channel's phono circuits. The supply is the 4 in the triangle and should be 250v, where I am only getting around 209v. The bold line at the bottom of the image is ground.
    upload_2017-6-29_18-10-8.png

    I have a few other amps that have pretty unbalanced ganged volume pots, so I had that thought as well. I will probably investigate a little further by either measuring the resistances of each half of the pot or swapping the wiring like you suggest. I'm hoping that's all it is and not some imbalance with the circuits.
     

     

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  7. gadget73

    gadget73 junk junkie Subscriber

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    Higher voltage at the cathode assuming the cathode resistor is on value means more current through the tube. Usually you'll see lower plate voltage if the current is high. If its not a stupidly off spec number it likely just means the tube in yours is a little stronger than what was in the one they checked. Some back of the napkin math says the tube sections are drawing about 1ma and 1/2ma respectively.

    If the supply voltage is 40v under spec, then the other tube voltages are going to be screwy. It still may not be enough to truly bother it but it would be worth checking the other steps in the supply to confirm things are right. If the schematic calls out a specific switch position for voltage measurements, that would also be worth duplicating too. Sometimes it matters.

    What sort of meter are you using? Sometimes the schematics call out what the meter loading was. if the schematic voltages were done with a VTVM, a Simpson 260 will probably read low. A DVM will probably read the same as a VTVM, and higher than if the mfg meter was something like a 20k/volt impedance.

    Not sure if its useful at all, but my S-7000 phono stage is supplied from 170v and has 80v at the plates. The S-5500II feeds from a 200v source and calls for 95v at the plates.
     
    Last edited: Jun 29, 2017
  8. bberkom

    bberkom AK Subscriber Subscriber

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    I've been thinking about those voltages as well. I've been working on a Scott 299b recently and it uses a 170v supply and 100v at the plates. Those voltages do seem high on this schematic. As long as the cathode voltages look reasonable, I think I'll leave it alone. It sounds great as is.
     
  9. FlaCharlie

    FlaCharlie Active Member

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    Yeah, according to his schematic the two 12AX7s should be drawing a total of 3 mA. That's based on the cathode voltages and value of cathode resistors.

    This is why I'm so puzzled - as I mentioned earlier, the schematic shows a 20v drop across a 56k resistor between the last two sections of the PS. That final cap supplies the phono section and the math would indicate that the total current drawn is only 0.357 mA (20 / 56000 = .000357 A) That's the total current for two 12AX7s, four tridode sections!

    Does that make sense?? Is there a flaw in my calculations (wouldn't be the first time) or am I thinking about this wrong or what? If anyone could set me straight I'd appreciate it. That's why I suggested earlier that the correct value of R4 might be 5.6k, not 56k. That would indicate a total current of 3.57 mA, which jives with the rest of the schematic.

    First, the schematic you posted shows the last two supply voltages should be 270 (not 275) and 250. If you replaced R4 and it's now 56k, and if you're dropping 61v across it (270 - 209) then that would indicate a total current of just a hair over 1 mA for both the 12AX7s. So, again, this confuses me.

    That's why I suggested that you measure all the parts connected to those two tubes. If you do that first, and make sure they are closely matched, you can eliminate them as a potential issue. You did this for the plate resistors but you haven't mentioned your measurements for the cathode resistors. Get the cathode resistors very closely matched to the schematic values and then see how the PS voltage is affected. If the voltage on the last cap is still way under spec, I would then try replacing R4 with a lower value resistor. (5.6K? I use a cheap resistor substitution box for such adjustments. It comes in very handy. Trying different values with clip leads works fine too.)

    I must have misunderstood, I thought the noise was only in phono mode. That's why I suggested replacing all the parts in the phono stage. Again, the imbalance in the plate voltages could be caused by incorrect cathode resistor values. Eliminate that possibility first. Any imbalance after that would just be differences between the tube sections. Another tube might be better balanced. The tubes may have tested OK but could still be unbalanced in terms of current draw.

    Just measure all the parts on V4, just as you did on V1 and V2, and replace as necessary. As for the balance issue, that's why there is a balance pot. It might improve after you go through the tubes that are in front of it. As long as the imbalance is not extreme, just turn the pot to balance it out.

    You should also test the 7199s on a tester and check their voltages just as you have with the 12AX7s.
     
    Last edited: Jun 30, 2017
  10. bberkom

    bberkom AK Subscriber Subscriber

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    Thanks for all of the good advice. I believe the channel imbalance is due to the volume pot tracks after doing a little more listening. It behaves similarly to the old wonky control on my PAS2. I won't worry too much about that anymore but may still measure the resistances to confirm. I am going to measure all of the cathode voltages and resistors today once I get back into it and see how they look for V1, V2, and V4.

    I was looking at the Sam's schematic for the angled-preamp-tube version and cross referencing it when I misstated those voltages. I did order a 10K resistor for the R4 position that thought may get that last power supply voltage in line after looking at the cathode resistors and voltages. The Sam's schematic uses a 10k resistor there, so I am thinking that it might translate well to this version. Everything else in the PS and phono preamp sections is nearly identical.
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    yeah something is mixed up. 3ma seem reasonable based on cathode currents, 0.3ma according to the B+ dropper doesn't make sense though. I think you may be right about the resistor needing to be 5.6K. Possible somewhere between engineering and drafting a decimal got dropped, and assembly went off the draft.
     

     

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  12. bberkom

    bberkom AK Subscriber Subscriber

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    Well, what do you know? I dropped in that 10k 1W resistor in place of the 56k, and my PS voltages are back in line! All of my supplies are a little high running on mains voltage, but I am now seeing a 257v supply for the phono tubes. The plate voltages went up accordingly, as did the cathode voltages.

    Before changing the resistor, the cathode voltages (those resistors are in good spec by the way) were running around 1v instead of the 1.4-1.5v on the schematic. The cathode voltages are now up around 1.2-1.3v after changing to the 10k resistor.

    It definitely seems like some errors were made in this area of the schematic. The 10k resistor used in the other version of this amp brings everything in line in this version. So I would imagine it'll be best to leave that sucker in there and call this a wrap. There's a very slight buzz in both speakers, but I can only hear it a few feet from the tweeters on my sensitive horn speakers. Nothing I'm going to go chasing now.

    I have it hooked back up now, and it sounds excellent. My wife loves the low profile and look of this amp, so it'll be sticking around for a while. Needs a little more cleanup, some faderlube, and its covers/case, but other than that, it's time to stop obsessing and give it a listen.
    IMG_6439.jpg IMG_6438.jpg
     
  13. bberkom

    bberkom AK Subscriber Subscriber

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    Also a ton of thanks to @gadget73 and @FlaCharlie for helping me with this amp and explaining things to me so clearly. I learn a little more each time I tackle one of these. It will be getting lots of use, so thanks again.
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    Cool, glad you got it sorted. I dig Sherwood gear. I just wish they'd shipped more with lids.
     
  15. bberkom

    bberkom AK Subscriber Subscriber

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    I have the cool speckled painted case for mine. It's just such a pain to get off and on that I haven't been using it while working on it. There's not much to it. Pretty thin steel, a couple breaks, some mounting holes/slots, and that's about it. I want to get the case on it, but I can't stop listening to it.

    I've been listening to a variety of music for the past couple of hours, and I am really impressed by this amp. It's very detailed, and the imaging is nice. It does make a lot of heat being so compact and with the strange angled tubes, so it'll be even nicer in winter.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    I have one for mine too that I picked up. I have no bottom plate though so I can't exactly use it. I've sort of given up trying to find an original at this point, so my next thought is to buy or borrow a sheet metal brake and just make one. Its not a complicated piece, I just don't have the tooling to make it.
     

     

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  17. FlaCharlie

    FlaCharlie Active Member

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    Glad you got the PS voltages sorted out. Certainly looks like the stock 56k was a production error. The Sherwoods are great sounding amps.

    I'd still suggest you measure the voltages on the 7199s sometime. These tubes have a reputation for having quite a bit of variation in the voltages. I ignore the voltages on the schematic and follow the suggestion of Joe Rosen, who was a prolific poster over on AA many years back. He claimed the best sound from 7199s resulted when the plate voltage of the pentode section was 72v +/- 3v. The way to get this voltage is to play with the resistor value on G2. A resistor substitution box really comes in handy to accomplish this and you will likely end up with different G2 resistor values for the two tubes since there is a lot of variation between tubes. Supposedly, if the voltage is too high it can result in a H-K short in the triode section. You can do a search over at AA for his posts, which are quite entertaining, though often quite acerbic.

    By the way, is there any chance you could post the "vertical tube" schematic you have? Or if you know of one that's already online, could you post a link?
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    the G2 voltage is a little more critical on direct coupled ones since it will upset the balance of the triode section if its too far out of whack
     
  19. bberkom

    bberkom AK Subscriber Subscriber

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    I will measure the 7199's again at some point and look at what you are suggesting. They were close enough to the schematic that I didn't pay them much attention. I found the schematic on this thread that was linked to in another AK post I found. On the third post he links to dropbox here and includes a nice high-res photo of a factory schematic. The Bias supply +/- is reversed, which I understand is common on these schematics, plus the one 56k resistor that may or may not be an error, but this one matched the components and layout of my amp almost perfectly. If you can't access it, PM me and I can email it to you.
     
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  20. raycarr00

    raycarr00 New Member

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    Question: What voltage AC rating were the capacitors you used? The schematic states that 400v should be used but doesn't state whether it's AC or DC. It seems apparent that the 400 v recommendation is in DC, as it's a DC fed circuit.
     

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