Some Comments & Modifying the Bias Circuit - Fisher 500

[@elinbenson]

Don't sell your self short. I have learned a lot from watching you. Ignore the pedants.

[@Bhamgurl1]

You shouldn't really cut yourself short. You're a great teacher And your knowledge on theory is amazing.

[@preiter20]

Unless any critique comes from Paul Carlson, Terry/D-Lab, Shango006, Manuel (electronics old and new), Uncle Doug, or the like I would let it pass like the wind. Thanks Tony for sharing your knowledge. I have learned a great deal from you!

[@zulumax1]

All great folks

[@georgejobin1744]

Tony your the best teacher I ever had. The things I do now because of you are unbelievable

[@peterfowler8377]

Watching you work through the circuit design to come up with a way to reset the bias was both educational and entertaining - please keep 'em coming!

[@codezero6023]

Enjoy your channel and Mr Carlson’s Lab

[@xray111xxx]

With you on Mr. Carlson too. I love his channel. Both Tony and Mr. Carlson are my favorites and go too. Other techs I cannot stand.

[@georgejobin1744]

@@xray111xxx I agree

[@jdmccorful]

Don't forget 12voltvids, he is also good.

[@R.AudioElectronics]

Another great video Tony. There’s a lot going on with thinking about the troubleshooting plus presenting it for video ie.. lights, focus, angle, shot etc. all the while explaining what your thinking and not confusing it with what you’re doing. So you do a damn good job no apologies needed. In your next video if you are going to be checking for THD and using the Keithley please consider showing hookup and meter functions. Thanks again Tony we appreciate you. Mike G

[@davidwinokur2131]

Excellent dynamic range on that amp!

[@_-Skeptic-_]

watching many tube amp. videos, I thought I've seen enough, but no. This is very interesting design. Thanks for sharing.

[@xray111xxx]

Awesome. I wanted to see Tony how you bias this amplifier. I love your videos Tony, don't let anyone give you any static. I am real happy you are here. Keep the good videos coming. I always look for your videos. Still my favorite of all time would be the Pioneer Receiver videos and the Sansui Integrated Amplifier you did. Those are some fine work. I can watch them over and over and still learn something new every time I watch them.

[@goodun2974]

Some of the Scott tube amps (with adjustable fixed bias) ran 3 or 4 preamp tubes in series off of the cathode voltage avaliable from the output tubes. I seem to recall the Scot 299 integrated amp did this. Another way to look at it is that the series resistance of the preamp tube filaments is what lifts the output tube cathodes above ground and develops the necessary voltage to heat the preamp tubes, even in a fixed bias amp with negative bias supply to the grid, that would normally be expected to have the output tube cathodes at ground and with zero voltage potential. I also have a Danelectro guitar amp with two topmounted 12Ax7s filament-wired in series and heated by the cathode voltage from the output tubes on the bottom-mounted amp chassis. The tube filaments actually *are* the cathode resistor! That amp gave me a heck of a shock when I unplugged the shielded RCA carrying audio from the top preamp to the bottom amp in order to track down some noise issues; the audio-cable shield was the only ground between the two chassis, and when I unplugged it, the 6L6s lost their cathode ground, and therefore had nearly full *plate voltage on the cathodes*, which went searching for ground through the now-cold, and therefore low resistance, 12AX7 filaments, through the shield of the audio cable that I had just unplugged and was holding in one hand, with the other hand touching the amp chassis. Zap! I hardwired a ground between the top preamp and bottom amp chassis, and lifted the shield ground, for safety and lower hum. Note that any amplifier or receiver with series-wired tube heaters needs for all tube pins and socket connections to be clean and tight, and if one tube doesn't make a good connection or the filament burns out, all the tubes will go stone dead and refused to heat. Plus, you will now have full plate voltage on the cathodes of the output tubes, if it is an amp design where the only cathode biasing for the amplifier consists of the series resistance of those tube filaments.

[@avro66]

Hi Tony, I've been watching your videos from across the pond for about six months now I've noticed on occasion you sometimes slip up with your commentary, I know what you meant to say though, we all do it sometimes, anyway keep up the great work and videos I look forward to seeing lots more projects in the future.

[@russellhltn1396]

Very interesting bias arrangement. A cathode bias with a different grounding point - with the tube filaments thrown in for added measure.

[@souta95]

In regards to the first part of this video... As AvE says, the way to get an answer online is not to ask a question, but to post the wrong answer. Thanks for the videos! I have really been learning a lot of little trucks here and there that are new to me.

[@michaelmacdonald3408]

The one thing i hate about digital multi meters is having to watch the numbers rise analog is much better.

[@geirendre]

That's why you should have both. Digital MM when you need accurate readings, and analog when you just want to se variations.

[@vintageaudio7518]

Nice video. I connected the cathodes of 6L6GC's to a 75Ω resistor, the other end of the resistor to ground. Cathodes are at 9VDC but the negative rail dropped to -28V. Increase the resistance of R85 (400Ω) to get -36V, but -30V works fine on my copy. With 405V on the plates, 6L6GC's are idling at 23W each, max rating for Sylvania 6L6GC is 26W. Your amp might be slightly different than mine.

[@berniken6511]

Well done Tony another brilliant video . Thank you ......................................................Berni

[@brianfletcher9774]

FM IF is almost always 10.7 MHz. AM IF is usually 455KHz. The exception being pre-WWII radios, gotta check the schematics. Manufacturers used different IF frequencies of varying frequency. XrayTonyb....you just keep being you. The mistakes, they show you are human. Referencing them in your next video and admitting you made a mistake, means your humble. Keep on rockin ! You are awesome.

[@russellhltn1396]

What would be cool is to find a way to post "errata" - so the mistakes you find that aren't worth re-editing can be acknowledged. It might help dealing with the "pile on". Some tend to do it in the description, others will pin a comment.

[@solidstate90]

Please keep it to yourself Keyboard warriors

[@zulumax1]

Seems quite pleasant here

[@TheDefpom]

We all say dumb things when recording video, it is hard to focus on the camera, shot, focusing, and do diagnostics! I say dumb stuff all the time in my videos.

[@dl7majstefan753]

Very good explanation of this unusual bias and filament circuit. Maybe Einstein knew, that decades later a manufacturer will do this when he said: "Everything is relative"!

[@jonka1]

Your addition of the 6.3V zener was inspired. Have you considered bypassing it with the 100uF cap to take out its negative feedback effect? Or was that part of your plan?

[@zulumax1]

29:27 What you call conventional flow I have always called circuit flow for troubleshooting purposes. I think this is the leftover method of really old radio guys who learned the way they perceived electricity traveled from positive charge, that was too many electrons, to a negative charge, which is a depletion of electrons. It is much easier to follow a schematic through in this manner even though we know now that electrons flow from negative to positive, electron flow. Shango066 mentioned this too. My grandfather also used this method as he had been working in radio since the 1920's. If it is less confusing, I will use the easy path. Thank you for mentioning this Tony.

[@johnsampson1096]

A quick one, Tony, while I was taking my first basic electronics course our instructor from the military was trying to explain the math to Kirchoff's law. He made a small error with 30 students watching his every move on the blackboard. When he was called on it, his reply was "Well, excuse Me." This teacher really knew his art. It was Sylvania Technical School, Waltham, Ma. Great Fisher 500 series!

[@m.9243]

Most unusual filament design..Never seen an amplifier with the driving tubes filaments connected in series! I understand they form a part of biasing but still, it's a practice out of the ordinary. We live and always learn! Thanks Tony for your methodical explanation.

[@apollorobb]

Your not Perfect ?!?!?!? lol Im so shocked .I love your Videos Tony . As an EE i am a bit of a Stickler for Terminology . I never correct anyone to be a Arsehole . Only so there is no misunderstanding or Misinformation .

[@goodun2974]

Other tube socket and tube compatibility weirdness: a 6550 with a metal collar around the tube base typically has an internal connection to pin 1 and so pin 1 on the socket is supposed to be connected to ground. If pin 1 of the socket has been wired as a tie point then you risk having live voltage on the metal collar of the tube! And of course, tube amps designed for beam power tybes such as 6L6, 6V6 etc often have the sockets' pin 6, unused and unnecessary for those particular tube types, repurposed as a tie point for screen resistors or grid stoppers or whatever; which is why you cannot simply plug an EL34 into a socket wired for a 6L6 etc without checking it first. Always check socket wiring carefully before making tube substitutions; aside from the chance of blowing a tube there might even be personal safety considerations.. I have worked on some Chinesemade Hurricane amplifiers. Mono blocks with 8) 6550s, that had adjustable fixed bias but used 10 ohm cathode resistors to drive a built in display for measuring bias, so that the customer could easily bias each tube individually. Unfortunately, the cathode resisters were connected from ground to pin 1, not Pin 8 as you would expect, and there was a jumper on the socket connecting pin 1 and pin 8. The cathode resisters were known for and perhaps designed to burn "open", especially if you had a shorted output tube, and so if the customer replaced the presumably bad tube himself (because it wouldn't show any bias current in the meter) without realizing that there was a blown cathode resistor, you now basically have full plate voltage ( no ground connection, therefore no voltage drop) on the cathode, pin 8, of the tube, which was also jumpered to pin 1 on the socket, and pin 1 of the 6550 tube has an internal connection to the metal base collar. This poses a large potential for electrical shock if the owner, or a tech, fiddled with or changed the tube that wasn't registering any current on the amps' built-in digital bias meter. Does anyone know why those metal collar bases on a 6550 would be internally tied to pin 1, and expected to be grounded? Perhaps some kind of shielding if the tube is being used as a transmitter? I do know that the original metal shelled 6L6's had a ground connection at pin one for the shell, and those were frequently used as an early home brew ham radio transmitter tube. Some hams would reportedly suspend the metal 6L6 tubes upside down in a bath of oil to keep them cool!

[@davidgriffin79]

@5:32 I think it was implicitly understood, by those who know SI, what you did was correct (just for fun I did the calc in a spreadsheet whilst watching the video ["playing along at home" in Dave Jones' parlance] and automatically used the correct units).

[@P51ride]

I'm sure this bias arrangement is used in more amps. I've experienced this in Voice of Music Reel to Reel tape recorders. I had a 300 ohm 10 watt resistor open up and threw the bias off on the 6V6 output tube

[@electronicengineer]

Thank you Tony for once again sharing your electronics adventures with us. I love hearing your take (explanation) on each of the circuits that you work on. It is very educational (at least to me it is) to hear your verbalized thought process as you walk us through the various challenges that, in this case, a vintage Fisher 500 receiver can present to a technician that may also attempt to undertake the same level of quality focused analysis and associated repair(s) like you do. Your logical approach to the various technical issues are excellent and, in my opinion, are outstanding. You continuously offer up a wealth of extremely useful tactics in understanding the problems and also how best to overcome them as well in each and every video you create and share with us. So, to close this novel of a comment Tony, you have blessed us with yet another great learning experience in this KZbin video. Many, many thanks to you sir. Great video Tony!

[@stevehead365]

The current through the 12ax7 filament string is about right, the problem is that the plate dissipations are too high. Maybe you could find a way to drop the plate voltages, zeners in the B+ line, different rectifier tube? The plate currents shouldn't change much. Good luck with the rebuild, took me a while to get my head around the unconventional bias arrangement.

[@danishnative9555]

Love your work Tony. Would like you to talk sometime about liquid cooling and beryllium windows of X-ray inserts. Rotating Tungsten anodes. Very cool and honest you are.

[@zulumax1]

Need to do a video of tracing circuit flow from positive to negative side of power supply. It sounds backwards to which way the electrons are flowing, but because of the common grounds, or ground bus (not always the chassis) being on the negative side of the circuit one can loose track of where the path goes. Block diagrams are another tool for troubleshooting which can condense a seemingly complex circuit. Reading the theory of operation in the service manual is helpful too. Just tools in the tool box. Use what works for you, and don't fear learning from others something new.

[@ralphmadera4366]

It is amazing your knowledge and the ingenuity you apply to these circuits to make it work and make it better . Thanks 🙏

[@respecaudio8276]

Hey Tony your videos are awesome! I have a Yamaha RXA aventage 1030 that needs a repair to the input board. Are you interested?

[@erin19030]

Those crummy yellow caps won't last as long as the original paper caps did.

[@boxingday11]

Thanks so much for your upload 👍🇬🇧❤️

[@michaelwolak3098]

Hi tony another fine video wish you could have been my tutor when i was learning electronics Mike

[@rolandosalgado1552]

very good tips for restore another equipments, thanks for share 👍

[@P51ride]

Thank you very much for these videos. 73's

[@georgelittle5741]

is the output transformer about 4,5k or 5k gkl

[@hpelisr]

Awesome work Tony, great sound.

[@dennistryon4650]

Tony, awesome video and great explanation of how the bias circuit works. I also loved your solution to the bias problem. My only concern is what happens in the event of a 12ax7 being removed from the circuit and or an open filament on one of the 12ax7's? Wouldn't that destroy the zener diodes? I know that is a bit unlikely but since this amp will probably end up in a non-technical persons hands, removing a tube for testing while powered up may not be far fetched. Would this not destroy the bias circuit modification? I'm not being critical just thought something might be incorporated in the circuit to prevent that. Once again, AWESOME video! Dennis

[@jonka1]

Very good point and it's disappointing that you had no reply.

[@geirendre]

So you basically have the output audio present on the filaments of the 3 12AX7's, (you can see the voltage variation on the Fluke when you play the music) and that's not a problem neighter for the filaments nor as a source of audio feedback from the output stage back into these 3 input tubes? It's an unusual design, I can agree with you on that for sure :-) Nice and good explanation of the circuit, thank you.

[@jonka1]

The 100uF capacitor bypasses the audio signal leaving just the DC plus a slight averaging voltage change which will be insignificant.

[@dogbucket]

Very good series, as usual. Good to learn about repairing the IF transformers; you never know what will come in handy. About the equation to calculate C: as it turns out, if you enter the frequency in MHz, as you did, you'll get the capacitance directly in pF, which is the most useful unit in this case. This only works if the inductance L value is entered as H, not uH. Exercise for the student to see how this works out.

[@jonka1]

Yes I thought that was what he was doing, but apparently not.

[@zulumax1]

I enjoy the walks through the schematic and the redraw is priceless. Great job Tony.

[@electronicengineer]

I totally agree with you zulumax1.

[@P51ride]

What is your opinion about using a filament transformer with the one side of the low voltage ac secondary connected to one side of the AC line in series bucking configuration to lower the input voltage for these older amps and receivers ?

[@zulumax1]

How interesting, have you done this?

[@Digital-Dan]

5:40 fastest recap I ever watched.

[@gregjohnson5194]

One of your best videos ever!

[@diabolicalartificer]

Re the 12AX7 heaters, they're in series, not parallel, it's cool, i make video's too and screw up all the time, it aint easy. Anyway, 3 x 12AX7 heaters in series should be 37.2v yes, as per the datasheet (3 x 12.6v) @ 300mA, so they're under running them by 0.6v per valve? The average heater DC resistance is about 6r for a ECC type valve, so that'd be 18r in total say, so why the 400r resistor? 400r and 18r in parallel = 17.22 ohms, which will allow plenty of current to the OP stage. If the 400r wasn't there then 18r would still allow plenty of current to the OP stage, IE 36/18 = 2A, doesn't make sense, or am I missing something? Also noticed the OP valves screen grids aren't de-coupled, curious.... Anyhoo, will watch the rest of the video. Thanks for uploading.....DA. Edit, could you have just not added a 6r or similar resistor in series with the 12AX7 heaters?

[@xraytonyb]

The 12AX7 uses 300 mA when the filaments are in parallel (i.e. 6.3v configuration). When connecting between pins 4 and 5, the filaments are in series and they run at 150 mA. The cold resistance of the filament is much different from the hot resistance. It might read 18r when cold, but using ohms law, 12V / .15 A = 80 Ohms hot. That would equal 240 ohms, with 3 in series. The parallel resistor to the filament string will form a current divider, as well as reduce the overall resistance of that node. As this is situated between the cathodes of the OP valves and the return path of the power supply, it acts as a type of cathode bias. In order for this to work, the OP valves need to transfer enough power to make the voltage drop 36 volts (give or take) to properly heat the 12AX7 filament string. Unfortunately, this configuration will not raise the cathode enough to allow the control grid to be negative. This is why they took the voltage drop across the filament string and applied it to the grids of the OP valves. Pretty ingenious! This only works, however, if the OP valves are of the correct specification. Adjusting the value of the parallel resistor will allow you to "tweak" the setup a bit, but if the bias point of the valves or the B+ voltages are too far out of line, you need to change the voltage drop without changing the voltage across the filament string. The zener diode approach seemed to be the simplest solution. The zeners will prevent the filaments from going above 37 volts, but if the voltage drops below 36 volts or so, the resistance of the filaments will go up as temperature goes down and the bias will go up, which will raise the filaments back up. In a way, it's somewhat self-regulating! Neat circuit! Thanks for the comment!

[@xraytonyb]

I almost forgot, the screen grids have a 68r resistor on them, but I may not have drawn them in.

[@markanderson350]

@@xraytonyb Damn this is new to me too. They did not have Zener diodes or even silicon diodes in the early days. I never thought of the self regulating thing either as a reason for using filament strings as a resistor. I have seen videos where a small battery was used for bias, it was not a conventional battery and it would often still have charge on it even today. . They looked like a thick diode maybe, Oh the hz and farads, henrys and ohms. I am no math whiz even though my dad was an actuary, but the whole thing cancels out if you use Khz or Mhz you are basically multiplying by 1000 or a million the henrys and farads will then be millihenry or Mfd. It can be simplified. Its like going to the smallest common denominator I guess. You get rid of all the zeros. Yes you have a habit of saying the wrong word when you are in a hurry. You brain knows the right word but your mouth does not listen. I really appreciate the videos. I do not do any of this anymore. Its in my past and you wake it up.

[@williamsquires3070]

Hi Tony. I’ve found it’s more useful (and less confusing) to use electron current when dealing with tube equipment, simply due to the physics of how thermionic emission works. For those reading along, here’s the low-down: You have a hot filament (tungsten wire that gets white hot, due to current flowing through it), and a “cold” (unheated) plate, or anode, all enclosed in an evacuated glass bulb that’s pretty close to a vacuum (or the filament would burn out.) Because of the hot filament, there’s a sh*tload of electrons (negatively charged particles) hanging around, but they have nowhere to go (yet.) So they’re sort of like teenagers near the stage at a heavy metal concert. 😏 Now, if we connect a high voltage between the filament and the plate such that the plate is positively charged, now the electrons are attracted to it, return through the B+ line and the full-wave rectifier, and they return to the hot filament (of a tube, such as a 5Y3, or a 5U4). Everyone’s happy. But, if we reverse the polarity and hook the - terminal to the plate, and the B+ line to the filament, then the electrons coming out of the high voltage supply all jam up at the “cold” plate like rush hour traffic that’s stopped due to an overturned semi. This is because the plate isn’t hot enough to release a cloud of charged particles, AND - furthermore - they’re the wrong polarity. Even if the electrons COULD leave the plate, they’d be repelled by the cloud of electrons already hanging around the hot filament! Like charges repel, and all that jazz. So, essentially, electrons are not only the majority charge carrier, they’re the only charge carrier. I hope this explanation is both educational AND entertaining! 😁

[@zulumax1]

Think of the B+ supply being a vacuum cleaner which is powering the circuit. It takes practice to follow an energy path that seems backwards. It is a difference of potential that powers the circuit. Maybe Tony can explain it better, but it is easier to follow the energy path in a schematic for me.

[@williamsquires3070]

zulumax1 - one could also think of the B- as a leaf-blower that powers the circuit. 😏

[@jonka1]

There you go again trying to sound superior.

[@zulumax1]

I hate over edited videos! I would rather have the mistakes left in and just go for it unscripted. It shows us there is a human on the other end. Shango066 leaves his mistakes in and I love his stuff better because of it. I have put a few amateur video attempts up myself and I refuse to edit. Improvements coming with better microphone will be the next step.

[@ianide2480]

Can you recap a discussion about farads? ;)

[@ianide2480]

I would like to think we can can have a discussion about tubes but I think you might be biased. I'll just stop there, heh

[@diabolicalartificer]

The formula is this - 1/(2 * pi * R * C) or sub F for either R or C { * = times by/multiply and / = divide by}. So to find the 3dB point of an RC network of 2K5 and 220u we would write it like this 1/(6.28 * 2500 * 0.000220) = 0.28hz. Some examples of capacitance in farads..... 10n = 0.000,000,010, 100p = 0.000,000,000,100, lastly 47u would be 0.000,047. hope that helps.....DA

[@xraytonyb]

I must resist the temptation to offer my ohm opinion at this time, as my mental capacitance is impeding my ability to properly resonate with the subject at hand. ;)

[@Madness832]

@@ianide2480 Stuck in a vacuum, huh?

[@electronicengineer]

@@xraytonyb Nice!!!

[@FixItStupid]

The Fisher 500C So Fine

[@jdmccorful]

Another slam bang good learning session has happen. Thanks!