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The Fisher X-100 Rectifiertube problem?


Chakaranda
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Hi

I have recently got a Fisher X-100 in perfect shape and all parts original

It has been playing absolutely perfect for the few days I have owned it

Today I decided to open it up, clean the jacks and tube-pins and check the bias: The voltage on pin 2 of the output tubes was around 25v and on pin three 48v !! (should be 30v and 40 v = a ratio of 3/4 according to the schematic )

I then checked the voltages around the rectifier tube (5AR4/GZ34) : 357vac comming in (fine) and 427vdc out !

According to the schematic the DC voltage leaving the Rectifier should be 410 ?

I'm new to this: could it be that the internal resistance of the rectifier has dropped over the years and many many hours of use?

(So that it now puts out more voltage than it should?)

next I checked the voltage after the two parrallel resistors (connected between the DC outlet of the rectifier and the first cap) it is 417 ! ! it should be 390 according to the schematic

Also these two resistors is of a lower value than stated in the schematic: the schematic says 250ohm but the value printet on the resistors says 125ohm

I'm pretty sure they are the original resistors mounted by the factory, but why this change ?

any help is very much appreciated

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Well, firstly, be extremely careful when working in there as the voltages can be lethal! The small variations in voltages that you are noting are normal. These amps were designed to operate over a fairly wide range of line voltages. I don't have the schematic handy, but it will probably state the line voltage at which the specified DC voltages were measured. Also, take note that they may have been measured with a VOM having a fairly low input resistance (as compared with modern meters) which can result in a lower DC voltage reading. In addition, if they measured everything at 110 VAC, and your line voltage is 120, of course the DC voltages you get will be higher as well. Tubes of the same type can draw varying amounts of current which will affect the voltages which you measure. Has this amp been restored? If not, at the very least, the electrolytic caps should be replaced as they are often prone to failure after a long period of time. If you decide to do this yourself, be sure to put your meter across them before you stick your fingers in there. These caps may not be fully discharged and can store quite a wallop if you get your fingers across them. Regarding the 125 ohm resistors, they are no doubt wired in series to get the needed 250 ohms and also to double the wattage.

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Thanks for your answer, but please read my first post more carefully (since the AC output from the PT is at the specified 360 then the following DC voltage readings _after the rectifier_ should likewise be within the specified range ...but they are around 20vdc higher than they should be, as you can see first post)

The resistors are connected in parallel...I do know the difference ;)

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The schematic shows two 250 ohm resistors in parallel to give a net value of 125 ohms. So, if you're absolutely sure about your 125 ohm resistors being connected in parallel, giving you a net 62.5 ohms, that would account for an approximate 10 volt higher reading at C1, the input filter cap for the power supply, based on the assumed current draw of around 160 ma. Since the efficiency of tubes is so variable, measuring 20 volts high is very typical. Sometimes it's impossible to tell if parts are "factory," or just well installed by someone else. You can't always tell by looking at the solder joints..............BTW, where do you live?

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I live in Copenhagen Denmark (northern part of Europe)

The resistors are for sure original installed parts, and I'm sure they are installed in parallel: it's not "rocket science" to see that: the amp is point-to-point wired ;) the measuring confirms the value: i get 65ohm measured across ..close enough____

I'm sorry can't add a picture of the parts in question: if you could see what i see, you would agree ______

anyway maybe this is a possible explanation why i get somewhat higher measures than stated in the Schematics: just read this note in the schematics:

"DC voltages measured with vaccum tube voltmeter

AC voltages is measured using a 1000ohm per volt voltmeter" !!____ That would explain a lot, since I'm using a modern multimeter! and further on: Nominal tolerances in components makes possible a +- 15% variation voltage and resistor readings" ______ I'm starting to suspect that this particular X-100 is a variant from the one in the schematic I have: the schematic operates with EL84 output tubes: but this amp might well be born with the 7189a type tubes, and therefor adjusted accordingly to the higher ratings of a 7189 ! that would explain the difference in the value of parallel resistors installed compared to the values in the schematic....anyway since the schematic says that +-15% is normal tolerance this amp is more or less within specs.

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Fisher always manufactured two versions (I think) for export, 120volt or 220 volt power. So their power supply design might have a few built in "adjustments". We used to puchase them by as many as would fit on a military standard 40" x 43" pallet. And thats a whole lot of stuff. The packaging was superior with full styrofoam surrounding them and wood crating over that. Even then you would see a forklift hole occasionally. Keep in mind that Fisher was considered a "mid level" performance equipment manufacturer.

JJK

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Thanks...wow that's was a lot of Fishers ;)

anyway: Today I've got hold of the correct original schematic and service manual: The value of the two resistors in question are the correct values and it seems that there's no major problems: my readings of voltages are more or less exact on the spot and any case no more than 10% above or below what the schematic says.____ the heater supply is a little low (5.8vac) and the AC on pin 2 and 8 on the rectifier is 4.5v (should be 5vac) But I don't think that's any major problem ...or ?____ Bias is also fine (after adjustment!) +44vdc on pin nr. 3 on the output tubes.______I've read somewhere that back then they measured using tube-equipment that had some resistance per volt ..i.ex. 1Kohm per volt...if that's true ? (can anybody confirm this?) well if so, then that might be the reason why I get a little higher readings on the high voltages.....

_______However I'm still a bit concerned that the DCvoltage on the plate of the output tubes is above 400vdc ..it is 418 ...seems a bit high doesn't it?

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You are worrying far too much!!! Having slightly low filament voltages is fine and will result in longer tube life. But, at the same time, the cathode emission may be slightly lower (from the reduced filament emission) resulting in higher plate/screen voltage readings (not a problem either). In addition, tube to tube variations are often large- if your 7189s happen to be drawing a bit less current, the voltages you measure can be higher. Back in the sixties voltages were measured either with a VOM (generally having a fairly low input resistance), or VTVM (vacuum tube voltmeter). The latter often had an input resistance, on some ranges, as high as 10 megohms which tended to load circuits far less than VOMs. Many modern multimeters maintain a constant input resistance on all ranges. So, the only way to have a "true" idea of your operating voltages is to use the same kind of equipment as the manufacturer did. The following site will give you a good understanding of how the typical 60's VOMs and VTVMs worked and will, I hope, set your mind at ease about the amplifier. Why not just sit back, put on your favorite music, and enjoy it? But, as mentioned in my previous post, if the electrolytic caps are original, they absolutely should be replaced. The most common cause of amplifier failure, in vintage equipment, is a bad electrolytic capacitor.

http://www.allaboutcircuits.com/vol_1/chpt_8/3.html
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Thank you very much for the link :)....very informative...

well yes maybe I worry to much about my old amp..but maybe it's understandable ? : I cannot afford to replace any of the difficult/impossible-to-get vital parts such as the output transformers, or the Telefunken tubes (all in all 10 telefunken plus one mullard) in this amp; so yes I am worried that the 418 volt on the plates of the output tubes is minute by minute frying the tubes plus the output transformers....I will do a recap, but I also really want to learn how test if they're OK or not, and get to know this amp in depth ___ btw: it's not that easy to find replacements for the big multicell electrolytic caps (cans) ....

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Well, as far as the tubes are concerned, you don't need to have Mullard or Telefunken tubes for the amp to sound great. In fact, if one were to replace the tubes with NOS Sylvania tubes for example, it may not be possible to tell the difference when listening. Often, the differences are extremely subtle (such as putting your ear near the tweeter and hearing a bit more hiss with one tube vs. another). Regarding the transformers, they are not overly delicate and were generally rated for much more current than the circuit uses. You have already checked and adjusted the bias which is great. Something you can do which can greatly increase the longevity of a tube amp is to keep it on an open shelf and place a small, quiet running muffin fan over the heat vents to draw heat out of the cabinet. If you want to develop a good understanding of amplifier circuitry go to: www.pmillett.com He has a vast library of books on the site, which you can download, covering just about every aspect of audio amplifier operation and design imaginable. As far as replacements for the electrolytic caps, you can either leave them in place and install modern, very small replacements under the chassis, or remove the inside of the cans and place the modern replacements inside. This site has links which can point you in the right direction for doing that:

http://www.dslreports.com/forum/r23472078-Rebuilding-Old-CanType-Electrolytic-Capacitors

So, I hope this will be of help to you. You can private message me any time if you have other questions which you don't wish to post on the site.

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Thank you very much again, this really is most helpful :)

Regarding the fan: we think alike :) I already planned to make such an installment in my tube amps :)_____ Regarding rebuilding caps: thanks again: I'll have a look at it, sounds like a very good and useable solution..__ btw do you know this site ? http://fragjanzuerst.de/eindex.htm he stocks new german produktion multicaps that (physically) fits these American amps.___......

Regarding tubes and sound: May well be you're right about the NOS Sylvania (I haven't heard them) but ... well being a musician (I'm a guitarist) i guess i have trained my ear for hearing even extremely minor nuances in sound: to my ear the differences in tube-performance (sound) are very clear and distinct. The Telefunken tubes I have are the ones i love the most (followed by Phillips miniwatts) So i want to preserve the Telefunken tubes this amp is born with as long as i can..also because NOS tubes of this (sound-)quality are getting real expensive and very hard to find.____I still have a lot to learn about tube-amps...three month ago i knew about nothing at all: one month ago i finished designing and building my first SE 5Watt Tube-amp (guitar amp) http://www.groupdiy.dk/index.php?topic=1591.0 ____ now one question is still lurking in my head: when a rectifier tube is about to wear out; could the first signs be that the DC output voltage raises? or would it lower ?

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I'm not familiar with the German replacement caps that you mentioned and, yes, Telefunken and Mullard tubes are definitely becoming more scarce and expensive. Congratulations on building your guitar amp! That's a wonderful way to learn, especially if you have access to an ocilloscope with which to evaluate the amp. By making small changes in bias and other voltages you can then look at the resulting waveform and try to correlate that with what you are hearing. Regarding wearout of rectifier tubes, generally you will see a decreased voltage. As tubes age the emission of electrons from the cathode (or filament, if a filament-type tube) decreases which will result in more of a voltage drop within the tube. You can always, if a replacement is not available or affordable, replace it with a solid state rectifier (keeping in mind that the B+ will greatly increase and necessitate changing the value of the filter resistors accordingly). Anyway, keep up the good work! It sounds like you are well on your way to some great listening experiences with the tube equipment..............

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Hi :) Thanks a lot (for compliments and all) :) Regarding the Rectifier Tube: Ok I consider it fine then :)) Regarding my guitar-amp: It was a learning by doing process (that's the only way I know)___ I don't have access to an oscilloscope: The amp is built using the cheapest DVM plus my ears as the only tools for measuring performance :))) (I am a bit proud of the result; I think it sounds great (here's a sound clip http://enlightened-being.net/kingloyal.mp3 ) And thanks to AC-filtering, multiple star-grounding and shielded signal path the amp is virtually hum-free :))......Regarding The Fisher and the caps: I've decided to give it the cap-job now: in total there are only 11 electrolytic caps plus a few Mylar-caps (which I'm not sure will need replacement??) so it will not be that expensive if I in the first place use a little lower quality than "handmade boutique quality" caps. Today I have measured a few caps leakages (shorts) here and there, so The Fisher is now on the workbench awaiting some fresh caps :))) Once those are installed, it'll be interessting to see if the voltage readings change for the better :)) ....Really looking forward to listen to music again through tubes; My old NAD 7240 are OK, but those transistors doesn't really sound _musical_ as the (telefunken) tubes does :)))

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My computer speakers aren't exactly "hifi," but your sound clip sounds very silky (if you have access to a digital camera, please post pictures of your amp). Ultimately, regardless of what test instruments show, the ears are the final arbiter of good sound (some well respected amps look positively awful on the 'scope). You can often find decent oscilloscopes very inexpensively, so it might be worth picking one up. With that, and an inexpensive sine/square wave generator, you can really learn a great deal about amplifier performance. As you discovered, star grounding can make a huge difference in eliminating hum. If you're going to the trouble to re-cap, I'd replace the mylar caps as well. Over time they can have leakage problems. It seems like you have become a definite convert to "tube sound," something which I agree with totally!!! If you can find a dealer, or someone who has Klipsch speakers, listen to them with tube electronics and you will have a whole new area in which to spend money! The combination is truly unbeatable (and, no, I do not work for the company!)..................

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Pics of the amp can be seen here: >http://www.groupdiy.dk/index.php?topic=1591.0< (please let me know if you for some reason can't get to the page)...Yes it does sound "silky" and warm :) ___ Actually I have always been a tube-kinda-guy: I've always been using Guitar tube-amps (transistors simply don't get it right when it comes to musicality), but I never thought I could afford a real HiFi stereo tube amp...But sometimes one gets lucky: I got this X-100 for less than $100 through a friend, and the next day I was offered a The Fisher K10 Dynamic SpaceExpander for the same amount :) Those K10 units are extremely rare over here (Denmark, Europe) so how lucky is that ?..he he.. ;) ...and both units in perfect shape and working condition ...So yes It's a true pleasure to step into this world of Stereo HiFi Tube gear ..Finally: Now it sounds RIGHT ;) ...I Dont know if there are any Klipsch dealers over here ? (I'll check it out). anyway loudspeaker investments is a bit down the line: I have some amazing homebrew "studio-monitors", built by a guy who whatever he do, only do the absolute best: ..i mean; the cables are pure silver....yes pure silver! i don't know what speaker-units is in there (they're from early eighties) but they sound amazingly natural ;)....an oscilloscope I'll get in a future not too far away from here: but I have to prioritize and get the Fishers going first 'cause I need them urgently in the home-studio for my recording-sessions ;)

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Pics of the amp can be seen here: "http://www.groupdiy.dk/index.php?topic=1591.0" (please let me know if you for some reason can't get to the page)...Yes it does sound "silky" and warm :) ___ Actually I have always been a tube-kinda-guy: I've always been using Guitar tube-amps (transistors simply don't get it right when it comes to musicality), but I never thought I could afford a real HiFi stereo tube amp...But sometimes one gets lucky: I got this X-100 for less than $100 through a friend, and the next day I was offered a The Fisher K10 Dynamic SpaceExpander for the same amount :) Those K10 units are extremely rare over here (Denmark, Europe) so how lucky is that ?..he he.. ;) ...and both units in perfect shape and working condition ...So yes It's a true pleasure to step into this world of Stereo HiFi Tube gear ..Finally: Now it sounds RIGHT ;) ...I Dont know if there are any Klipsch dealers over here ? (I'll check it out). anyway loudspeaker investments is a bit down the line: I have some amazing homebrew "studio-monitors", built by a guy who whatever he do, only do the absolute best: ..i mean; the cables are pure silver....yes pure silver! i don't know what speaker-units is in there (they're from early eighties) but they sound amazingly natural ;)....an oscilloscope I'll get in a future not too far away from here: but I have to prioritize and get the Fishers going first 'cause I need them urgently in the home-studio for my recording-sessions ;)

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