Bias Procedure needed; Sherwood S5500IV 7868s

[Wharfcreek]

Hey AK,

Working on an old, VERY neglected, Sherwood S 5500 IV w/ 7868 output tubes. This amp has 2 bias pots, but no bias test point. I can't find a schematic or bias 'set' procedure anywhere. Just spent 2 hours searching the AK data base and going through 25 pages of threads. No specific help found, but I did see a post indicating that the center of the control pot (wiper) might be a possible reference point....and when properly adjusted I should see between 17 and 23 volts. However, this reference was NOT specific to the 5500 IV amp.....so I'm not sure if it's applicable. Thus I'm still looking. Any help with either the bias set procedure, a Schematic, or both, would be GREATLY appreciated. Thanks much!! Tom D
Wharf_Creek @ Hotmail . Com


.

 

[bhamham]

Negative 17-23v sounds right. I'd just measure it on pins 2/6 of the 7868s.

Another way is to do 10R resistors from pin 3 of the 7868s to ground. Take the voltage across the 10Rs and divide by 10 to convert to mA. Shoot for 30-34mA depending on what your plate voltage is and how hot you want to run your outputs. Try to match your pairs as best you can.

 

[jaymanaa]

Last couple I did ran good at -19, but I don't remember what tubes i was using.

 

[Wharfcreek]

Bruce and Jay....thanks for responding. This old amp has definitely seen better days, but remarkably, it's functional at the moment! One of the 7868 sockets had pulled lose from the retainer.....a function of tight pins I suppose. Anyway.....it looked as if when the tube was pulled out, the whole socket pulled out of the retainer and the OT connection on pin 7 I believe shorted against the chassis. The socket was burned in that area. I removed the socket by unsoldering all the connections and the retainer by drilling the rivets. Since the socket itself was already out of the retainer, I mixed up some quick-setting JB weld, and re-assembled the socket and crimped it back together as best I could. I think this socket retainer may have been defective originally as the crimp area was pathetically minimal. So, hopefully the JB Weld and what little crimping I could apply will keep it together. I also had to repeat this process on the AC Accessory jack at the back where it too had been bashed in. Reassembly was done w/ 4-40 screws and nuts. In both cases....I 'think' I prevailed successfully! Time will tell. Anyway, I cleaned the sockets, installed some good 12AX7s and some 'marginal' 7868s.....and kind of short-cutted my usual start-up routine by a few hours. I simply brought this amp up to about 60 volts for 10 minutes...then 70 for another 10 minutes, and at that point I started hearing music coming through my test speakers.....scratchy...but there. Clearly, the attaching point at the RCA connectors was dirty, as was the Volume control and Balance control. But, at 80 VAC and with some flipping on the controls, I got it sounding pretty good. I listened to a moody blues CD and just let it cook overnight. This am it was still running well....no overheating...no humming, hissing, buzzing, or other unwanted noises while at idle. So, I'll be turning it off around noon, spraying out the controls and switches, and hopefully doing that bias adjustment. From that point.....I'll give it a try on some 'real' speakers!! I'd like to find some better 7868s....but at the moment, my budget will not permit ANY expense on audio stuff. Some time this week I've got to perform the dreaded task of doing my taxes, which involves payment....which required the sale of a bunch of stuff in the past month to accommodate! Who's idea was this 'tax' thing anyway??? Ah well..... one of life's necessities!! Again, thanks for the help on the Bias question. I'll go for -19 on the wiper of both those pots and see how she runs!! Perhaps I'll post a pic or two of the beast!! Later, Tom D.

 

[Tom Bavis]

Bias voltage doesn't tell you much - current is what you want to see, so add the 10 Ohm resistors if you haven't already. About 25-30 mA per tube, and swap tubes if necessary get each pair close.

 

[Wharfcreek]

OK, pulled the plug, let it cool down, and checked to make sure PS caps had discharged. In looking at the bottom of my amp, I see that all 4 of my pin 3s on the 4 output tubes are tied together..... and grounded!! So, if I'm understanding this correctly, I should disconnect the jumpers that tie all 4 tubes together AND the ground connection, and then add 10 ohm resistors from pin 3 on each tube to ground. Then, measure current across the resistor, and shoot for 'pairs' as closely matched to 25 to 32 ma each as I can get them. I going to have to run out to go grab the 10 ohm resistors......I think I exhausted my supply of those. It would be REALLY nice to be able to adjust bias AND bias-balance on this Sherwood similar to what Scott did, or adjust each tube bias individually. I liked being able to adjust the tube pairs to proper levels. Perhaps you've got a suggestion for making a mod that would permit that? Thanks guys! Tom D.

 

[bhamham]

I had one of these two bias pots on a 5000-II that I did a while back. I tore it out and put a 4-pot setup so I could bias individually. Anyway, had a look at the schematic and even though it was 7591s should be similar.

Sherwood ran 6.8R resistors on each pair to ground and wanted you to see .7v at pin 3 (7868 cathode). Now correct me if I'm wrong but that's about 50mA bias on each output. .7/13.6=.051

Wharfecreek, you could leave it as is and shoot for .4v at pin 3 (.4/13.6 = .030 or 30mA) or tear those 6.8Rs out and put 10Rs in.

My brain's a bit fried so please check my math! I may have this all wrong.

 

[Wharfcreek]

Bruce,

I really appreciate the sugggestions, but I'm not quite sure that the two amps are that similar. I don't yet have a schematic located for my S5500IV, but in just looking at what I see underneath I think I see all 4 cathodes tied together and going straight to ground (assuming that pin 3 IS the cathode). On your schematic, I believe pin 5 is the cathode....and I do see the 6.8R to ground.....but I don't have that in my circuit. Further, I'm not sure what I've got for a negative voltage supply in this amp. I haven't probed around....but it looks like I've got a separate winding off the PT for negative bias......but I'm not yet sure. Having a schematic would be a HUGE help here. I see your amp already had a 'balance' pot.....and I think my amp simply has a 'voltage' pot...controlling negative bias to the pairs of tubes. What did you do to your amp to 'convert' it to bias adjust over each tube individually? Did you do a voltage/balance system like Scott, or go with just individual voltage supply to each tube? I'm heading up to the parts store later to buy 4 more test-point sockets ($4 each...wish I could find a better deal on those!!!)...and some 10 ohm resistors. From there I guess I can at least establish what each tube is running at and how the 'pairs' look. Then, like Mr. Bavis suggested, I can swap tubes around until I get the best pairs together. One question I have is this: Can I run one channel at a time in order to 'pair-up' the tubes? By that, I'm asking if I can just stick one tube in one socket of one channel, then run the other 3 tubes into the remaining socket and measure current draw that way.....without 'activating' the other channel at all. Seems to me that otherwise I'd be including the adjustment factor of the other bias pot...which would most likely be different causing an imbalance from channel to channel , not to mention tube to tube. OK.......thanks again.....and hopefully I can get this sorted out. Regards, Tom D.

 

[bhamham]

Tom, sorry didn't mean to confuse the issue with the 7591 schematic. There were many variations for these Sherwoods. If you're cathodes all go to ground then you're good to go - just put in your 10Rs. Yep, pin 3 is the cathode on 7868s.

 

[Tubologic]

I had one of these two bias pots on a 5000-II that I did a while back. I tore it out and put a 4-pot setup so I could bias individually. Anyway, had a look at the schematic and even though it was 7591s should be similar.

Sherwood ran 6.8R resistors on each pair to ground and wanted you to see .7v at pin 3 (cathode). Now correct me if I'm wrong but that's about 50mA bias on each output. .7/13.6=.051

Wharfecreek, you could leave it as is and shoot for .4v at pin 3 (.4/13.6 = .030 or 30mA) or tear those 6.8Rs out and put 10Rs in.

My brain's a bit fried so please check my math! I may have this all wrong.

Your math is correct: with the values indicated on the diagram each tube is biased at 51 mA plate current and dissipates 21.6W which is exceeding the max.ratings for a 7591 (19W max. plate dissipation). Either the values shown are wrong (SAMS P-F are know to be plagued with such errors) or SHERWOOD decided to push their 7591's very hard which will result in shortened tube life and reliability issues. I would stay with 30-35mA per tube as recommended and install separate cathode resistors. Use 0.5W non-flammable film resistors which will also serve as fuses and protect your output transformers if anything goes wrong.

 

[Wharfcreek]

Roger on the .5W non-flamable 10 ohm resistors. Heading out shortly to the parts store. Just finishing up cycling through some tubes on the tester. After that, it's off for a ride. Bruce, I'd still like to know how you did your 4-pot set-up. Thanks all, Tom

 

[bhamham]

Bruce, I'd still like to know how you did your 4-pot set-up. Thanks all, Tom

Tom, it's been a few months back but here's the gist of it - I tore out the A/C jacks and put a perf board with test points and 4-10K 1-turn trimmers (might go multi-turns next time). Used Drew Bolce's design for the Fisher 400/500:

I used 22uF/50v caps instead of 33uF and 150K instead of the 200K resistors, but the 12K resistors worked well.

I was able to fit some tag strips above the output tubes to mount the caps and resistors. I also did 100R screen stabilizers. It's pretty tight working on those Sherwoods.

 

[Wharfcreek]

Nice Work Bruce!! Looks like you shoe-horned in the pots and parts all in good order! The sacrificial Accessory plugs don't amount to too much in my book! I've never cared much for the idea of daisy-chaining AC off the back of other units; particularly when I see things like 150 ma max current draw on the plug....and 300 ma current on the device. Seems like it would be a no-brainer NOT to plug the one into the other, but people do it all the time.....and houses do burn down!! Alas......just no gittin' around it. Anyway....nice work on your amp....and hopefully I can figure out something equally suitable. But, first things first....and I've got to make sure the thing works properly....meaning finding some good tubes, getting the controls all working quietly, and hopefully get some kind of temporarily acceptable bias adjustment on the amp. Once I'm sure it's worth the effort, I'll go for a 4-tube bias adjustment mod! I think the amp as a 'model' is certainly worth the effort, I'm just not sure that this particular amp is worth the trouble. It's pretty rough!! Anyway, just a couple more tubes to run through the tester...then I'm off for some 10 ohm resistors!! CIAO...and thanks!! Tom D.

 

[Kegger]

Nice clean looking work there Bruce!

And Dam Tom another amp in the stable aye?

 

[Wharfcreek]

Hello Kegger my friend!! Yes, yet another mouth to feed!! Hopefully this one won't take too much to get running. My 'collection' of integrated amps is growing far more than I thought it would a few years back. I remember just 3 years ago I parted with one of my only two integrated amps.....an old Stromberg Carlson unit. Now, after some soul-searching and some puttin' out of the proverbial feelers.....I came up with a replacement from, of all people, Mr. Bavis! However, it only added to the dozen or so I now have adorning some shelves....all of which I've had a hand in resurrecting to some degree. Not sure what I'd call my 'pride and joy' yet......but I guess we'll see in time. (That time being when I go so broke I have to sell 'em off one by one to pay the bills....and the last one to go will, I guess, be my favorite!!). Anyway....looking forward to seeing you in about 4 weeks. Keep a chair for me, and I'll bring some beverages! BTW.....see if you can ferret me out a few 7868s will ya???

Later,

Tom

 

[Wharfcreek]

OK.......running into a bit of a snag here! I did as suggested; installed the 4 resistors to ground. Removed all the jumpers and disconnected the original ground. Installed 4 Test points and wired 2 of them to the pair of output tubes closest to the TPs, and wired the other 2 TPs to the wiper on the Rt and Lf Bias Control pot. I can get to about -19.2, and that's it. I can go down to about -15....which shoots the bias way up. But, at -19, I'm still looking at what I think is too high a number at the tubes. I'm measuring using the voltage setting....and reading about 350 to 450 mv. If that converts to 35 to 45 ma, then I'm still way too high, yes/no? And, if I can't apply any more negative voltage from the pot, then I got a problem!! Part of my snag here is I've got a bunch of pretty bad tubes! I've pulled the 4 best ones out of my X-100-B....and also the 4 that I had ear-marked for this amp. All 8 of these test just barely into the green on my Hickok 6000A. So, now I'm cycling them through on the two socket with the TPs....and hoping to find some 'pairs' to work with. But, this project is starting to get a bit frustrating.....and perhaps expensive if I can't find a way to bias each tube individually! Thanks guys!

Tom

 

[BuzzK]

I'd run the tubes about 25 mA each.

You really need to get matched pairs of good tubes. Using a quad of dissimilar worn tubes will not give satisfactory nor reliable results. You can get thermal runaway from gassy tubes, etc.

You can change the bias circuit to give you a better range for adjustment. Sherwood ran the outputs hot back when line voltages were lower, so you'll need to compensate for today's supplies.

Looking at my notes, I:

• Shorted out a 2.2K resistor (but I don't see one on the schematic I have);
• Changed the resistors in series with the pots from 22K to 18K

And that gave me the range I needed.

 

[Wharfcreek]

BuzzK, you mentioned '....the schematic I have.'......and I'm wondering if I could get a copy of that? I don't have any kind of schematic for this amp....and having one would be a huge help!!

I've been working on this thing in Marathon fashion now for about 40 hours straight. It's now 3 am and I've been at it pretty much all day.....with, of course, a run to the RatShack for resistors. However, as of about 10 minutes ago, I finally sat and just listened to it.....and honestly.....it's amazing! My Fisher units pale in comparison, and I'm not sure even my mighty 299D Scott would be able to keep up! The high frequency is clear, distinct, light and airy yet able to dominate when it should. The bass is almost indescribable; rich, smooth, powerful, commanding....yet not 'thuddy' or pounding.....just.......enveloping! And....everything in between....just articulate and supportive. In all.....I'm REALLY amazed at this amp.....and all this with OLD tubes and OLD caps!! This thing is even quiet at idle!! Too bad it's just been so neglected over the years! The chassis is nasty....the face plate is half worn off, the knobs look like they were stained with nicotine.....the bright trim ring is even pitted as if soda had been spilled on it and just left there. There's even a big mark across the front where it looks as if about 5" of old Dymo Lable had been applied!! What a shame!! But.....looks aside......this think CAN dance!! As to bias adjust.....I have now hooked up the 4 installed test points to all 4 tubes....and no longer have a connection to the bias pot wipers. I managed to find a set of tubes that, with -19.5 volts...the limit of my pot sweep at the moment.....I had one pair that ran about 33 and 35 ma....and another pair that ran 36 and 38 at the same 19.5 negative. I dropped the first pair to about -19 volts which bumped that pair up to 'even' with the other set....so now running all 4 tubes between 36 and 38 ma..... but I simply cannot go any lower unless I can increase the negative voltage at the pot. I think there's an available -27 or so.......don't know why I think that, but maybe I saw it on another schematic? So, I've got some hope that once I can get an actual schematic for this amp, I can tweak that circuit a bit more.....including, perhaps, finding a way to bias each tube individually. For now.....I'm delighted with the results of the past 2 days of work!! I plan to break out the car polish and see what I can do to spruce it up a bit. Thanks to all for the help!! Final note: Still looking for some good 7868s...... so any references there would be greatly appreciated. Regards, Tom D.

 

[BuzzK]

PM sent