Need Help With Sherwood S 5500 Amp.

[racfan9]

Hey Everyone...I just picked up a series 3 S-5500 Sherwood last week. The unit is extremely clean and had been in storage for a couple of years. When I went over to look at it the gentleman had it playing in mono thru a single speaker. Anyway I bought it and brought it home and first thing I did was pull the 7868 outputs and take some voltage reading at the sockets. I haven't been able to find a schematic for the series iii to see what the voltages should be so I'm hoping some of you Sherwood experts will chime in. Instead of typing all the numbers in I took a couple pics to upload. Also as I said, when I went over to look at the unit he had it on and playing and I noticed the 7868's in one channel was running quiet a bit hotter than the other channel. Anyway if you'll take a look at the figures and let me know what I'm up against I'd appreciate it.

Thanks,
racfan9

 

[cademan]

I don't have schematic nor am I familiar with Sherwood However those numbers look fine unless a tube was red plating, throwing sparks, or not glowing period.

Is this the stereo amp which uses four 7868 tubes?

If it has different branded output tubes and one or more of them have the heater protruding from either the top, bottom, or both ends, then it probably will look brighter than the other tubes but this does not present a danger of any kind.

If it was working with no smoke or sparks or weird things going on then it probably most likely needs a general safety recap and nothing more.

 

[RWood]

Terry -
The only voltages that you should be reading in AC are pins 4 and 5, the heaters. And they should be measured from one pin to the other. With no tubes installed, I would expect 7 VAC between the two and with tubes installed that will drop to close to 6.3 VAC if all is well.

All the other voltages are in DC, with respect to ground. When the power tubes are pulled, the one to focus on is pin 2, the control grid. This is internally connected to pin 6 so voltages there should be the same as pin 2. In a grid-biased amplifier like this you will find a negative voltage there, something akin to the -21 vdc you posted above. Check for the presence of negative voltage on pins 2 & 6 on all four power tubes. This is best done with the tubes pulled (as you are doing), lest you damage any that do not have negative voltage on their grids.

Here is the pinout for a 7868 Note that when looking at the socket side, we go counter-clockwise from 1 to 9.

If you can re-do your chart we these points in mind it will help us guide you. Be very careful, as the plate and screens have full, unloaded voltages such as the 500+ volts you saw. These, of course, are lethal. Keep one hand in your pocket. Better yet, use meter leads the have clip on the end and keep both hands out of the amp when measuring live voltage.

 

[PakProtector]

There is one more bit to keep in mind whilst testing. The energy storage in the caps is considerable, and an arc created due to a slip and short circuit can blast molten metal into your eyes...wear good safety glasses as well.

Also, it is possible for un-loaded voltages to be significantly higher than when loaded by conducting tubes. Check the cap's voltage ratings and don't exceed them.
cheers,
Douglas

 

[racfan9]

Hey guys thanks for the input. Dick, I measured the heaters with the following results. R channel 7.58 & 7.89vac. L channel 6.10 & 6.25vac and as far as Pins 2&6 on the outputs I'm coming up with zero's on all four sockets. Also I should mention my line voltage is 122 volts. And also in my original post I mentioned that the tubes in one channel were running quiet a bit hotter than the ones on the other side, so I'm assuming that must be the right channel?

Terry

 

[gadget73]

Pin 2 numbers being different are why one side is much hotter than the other. I'd expect about -17, you have about -7. Thats the bias feed. Pretty sure the Series 3 has left and right bias adjustments. I would set those for max negative and see what you get. Its not going to be right of course, but it should calm down the heat considerably. Mostly we need to verify that you can get reasonable bias adjustment range out of it.

also note the polarity. Those should be negative voltage. If its positive, thats a different problem. Probably means the coupling caps are done.

Series III service data
http://akdatabase.com/AKview/displayimage.php?album=101&pos=8


I'd break the connection from pin 3 to ground and insert a 10 ohm resistor on each tube. Use the voltage across that to set the bias controls properly. About 0.3 volts is reasonable. Shuffle tubes around as needed to get the tubes in each channel as closely matched as you can.

 

[racfan9]

Gadget...Ok I took a few more voltage readings except this time I took them from the bottom side of the amp instead of top side thru tube sockets. Anyway I adjusted the bias pots to -19.22 left ch. and 19.41 right ch. and they are pretty much maxed out on the neg. side. I also took readings on the heaters and for some reason I'm only measuring 3.71 and 3.65 vac on the right channel but on the left channel I'm getting 6.26 and 6.19 vac. Any ideas? There is one issue I may have found. In the pic that I'm uploading the bluish 6800 7w resistor coming off one of the cans will not register anything on my meter, I de-soldered one end of it and still get no reading. I've got some on order and will replace.

Terry

 

[cademan]

Sounds like you are doing a lot of un-necessary troubleshooting. The unit worked and probably needed just a simple tune up but it seems that you might be complicating things by taking readings that aren't really necessary. That's just my take.

 

[cademan]

I de-soldered one end of it and still get no reading. I've got some on order and will replace.

You're guessing and it won't do you any good to order these parts.

You are probably over thinking this. Your voltages are close enough that I just don't see any problems except maybe aside of some possible dirty pots and switches.

 

[cademan]

I mentioned that the tubes in one channel were running quiet a bit hotter than the ones on the other side, so I'm assuming that must be the right channel?

Terry

Hotter like how? Red plating or extra heater glow. You're not quite explaining this.

 

[racfan9]

Hey cademan, I probably am over thinking on this thing, but I pin a lot of that on the fact I just don't have much of a background in electronics and that's where all of you @ AK come in with all your expertise. It is appreciated.

Anyway I'm getting ready to play this thing so I can check the 7868's to make sure there not any red plating. Is using a infra-red temperature gun of any benefit? There is one thing I'm confused on. Your saying all the voltages look fine but why are the right channel heaters so much lower than the left channel?
3.71 and 3.65 vac on the right channel but on the left channel I'm getting 6.26 and 6.19 vac.

Terry

 

[cademan]

Hope I didn't come off as being too crass. I'm a happy guy who seems to do a lot of this stuff in his sleep. Lol. I (and a lot of us here) want you to get it working.

Most heater circuits are tapped from a single heater winding from the power transformer for all tubes so I can only deduce that maybe a tube or two has something up with them. A heater to cathode short could certainly bog things down.

B+ plate voltages can be a few volts different but are certainly within the design spec ratings for that tube. Look up tube data sheets and have them handy when doing tests. Any other out-of-spec voltages would be contributed to lousy resistors or capacitors.

Are your heater measurements being made without the tubes? All the sockets should read the same heater volt wise. Try switching the tubes that you seem to think are getting too hot to the other sockets and see what they do. If a tube is bad, it will usually follow the socket.

EDIT: As I said about not being familiar with Sherwood tube stuff, are there preamp tubes that are running as phono preamps acting as the bias for the output tubes? I'd have to see a schematic.

 

[gadget73]

Anyway I adjusted the bias pots to -19.22 left ch. and 19.41 right ch.

is that -19.41 on the right or +19.41 ?

Heater I wouldn't worry about too much. You can't measure that to ground anyway, it has to be measured across the pins. Basically if the tubes are lighting at approximately the right color, its probably fine. Not much to go wrong on an AC heat circuit, either it works or it doesn't. They have no real means of changing the voltage it operates at.

With no tubes, B+ will be very high. With too much negative bias it will be too high also, which is where the 10 ohm resistors come into play. That resistor is the screen supply. If its open, no screen voltage, no tube action. Its rare that those things go bad though, so double check the connections.

 

[racfan9]

gadget it's -19.41 on the bias. There wasn't much adjustment in either of the pots, about -14 up to -22. Was wondering if I could shoot some de-oxit into them?

I did fire it up this afternoon to check the output tubes, unfortunately one of the right channel 7868's started red plating about 4 minutes in. I shut it down and let it cool then switched the two right channel tubes and the same tube red plated again but in a different socket. I tested the tube and it was shorted so now I'm waiting on a couple replacements that I had already ordered. One thing I will say, it sure was sweet sounding during those 4 minutes.

As far as the low heater voltages go I was measuring across pins 4 & 5 and without the tube installed. Obviously I was doing something wrong being it sounded so good while it was playing.

Now one final question for this evening. I'm uploading a pic of the speaker terminals and how I hooked this thing up. Does it look correct, I'm using one grounding point which I believe is different from what the owner's manual showed. It's been awhile back since I seen the manual.
Also a pic of the tube beginning to redplate


Thanks,
racfan9

 

[RWood]

I believe that is incorrect. For the right speaker, you put the jumper on whatever your speakers are rated for (as it looks like you've done, 8 ohms) but then you put the two wires from the speaker where it says [SPKR]. Only your red lead is connected. Your LEFT speaker is correct. So just move that black wire for the right speaker off of Ground and connect it to terminal to the left of the red wire.

Hooking it up this way allows you to change the phasing of the Right speaker, with respect to the left. That is why the Right [SPKR] has no polarity markings. You can set the phase switch for whichever way sounds better to you.

 

[racfan9]

Thanks Dick.

 

[gadget73]

key-rekt. If you're using the stock switching arrangement, both speaker leads need to go to the SPKR terminals.

Or you can skip it, run both speaker - terminals to the single GND post, and just run the right speaker lead to whatever the impedance is you require. I'm not much a fan of phase switches, one more thing to give headaches.

Tubes look to be lit properly so I'm thinking your heater voltage is good.

Cleaning the bias pots is not a bad idea, but they are probably working approximately correct. Don't need a ton of adjustment range, and what you're getting seems reasonable. Still needs to be paired with a cathode resistor to actually get the current dialed in where it belongs.

 

[racfan9]

gadget, If I go the route of a 10ohm resistor from pin 3 to ground will that disable the use of the L & R bias adjustments pots? Is there any illustrations of this mod for us dummies out there.lol! What wattage should I use and is it one resistor per channel or per tube? I'll try to search AK for info on these mods.

Terry

 

[gadget73]

No, it will just give you something to measure voltage across so you can adjust the pots to get the tubes at the proper current level.

1/4 watt is fine, if you have 1/2w, that will also do it. One per tube.

I don't have any good pics of mine, but if you look at the bottom of the tube socket you will see a lead connecting pin 3 to ground. Remove that lead and replace it with the 10 ohm resistor. It really is that easy.

The touchy part is that you will need to measure from ground to pin 3 on each tube. You'll be next to some high voltages, so be careful. If you have minigrabbers, they make life easier for this kind of thing.

 

[racfan9]

Sorry for being uneducated in this stuff, so here's my questions. In the photo I've uploaded it looks like the second socket from the top is the only one with pin 3 going to a ground lug with the other three daisy chained to #2 correct? So I need to cut or remove all the yellow leads to pin 3 of the other outputs. As far as a grounding point for the new resistors can I just use the two spades that I've got circled in yellow in the picture?

Now as far as adjustments. Is that still done at the two adjusting pots that's already installed, or do I need to install two more? And one last thing, at least for now. Do the tubes need to be installed when adjusting the bias?

Thanks,
Terry