First a little prologue…
A little while back, I decided to get back into my favorite obsession…2 channel audio, more specifically, analog and T-O-O-O-B-S . The first step was finding some info online ( a 20 year hiatus left me wanting on the current state of things) to help get me up to speed. I found this group of forums , and quickly learned how little I know, and a good deal of what I thought I knew was wrong. The bench of knowledge here is a very deep one.
I was originally planning to build the uber ultra-fi system right out of the gate. A month plus into my turntable project, I realized at this pace it would be a l-o-o-n-g time before I ever spun my first record (phono-pre, line stage, power amp, and speakers still in the thought stage), so I decided on an interim step…a massaged vintage unit.
This brings me to the point of this post. I decided on the Fisher 800C receiver. I always liked the vintage Fishers I had listened to in the past, and the 800C has the added bonus of AM (I get my news in the morning while I’m getting ready for the day). I would also note that I bought my unit BEFORE I read the Stereophoole review on the 500C ( same unit minus AM).
It is my hope that this group can help develop THE definitive set of mods for this already great piece. First the ground rules:
*Anything done must be 100% reversible-no cutting-no separate supply ect. (it is a classic after all)
*With the exception of the phono stage (total caa-caa being the universal opinion of those who have reviewed it) I want it to still be a Fisher (more or less). Avery Fisher was no fool, and in lew of the test equipment available today, he engineered a very nice piece of gear through voicing and intuition (much like the group here). If you suggest something that is a radical departure from the original topology, please back it up as to why.
* All modified circuits must draw no more current than the originals. 40 year old iron aint so easy to come by, and tends to be a little on the frail side. If it’s the greatest sound in the world, it does me little good if it melts the trannies.
The full pdf schematic of the 500C is to large to attach, so I have cropped snippets to aid in the discussion.
Big problem first..the phono section…
… I’m sure with the turntable/cartridges of the day, it sounded ok. But everything I’ve read suggests that it will not due today. I intend to completely re-due this circuit. It is limited to two 9 pin sockets, and the voltages that can be derived from the stock supply. Currently, I think the evil Kuei’s circuit..
http://www.diyaudio.com/forums/attachment.php?s=&postid=335624
.. would fill the bill nicely, but I am open to further suggestions.
Next up..the control section…
…hoo-boy, what a convoluted mess. I won’t need the high/low filters, but I would like to be able to switch the tone/loudness circuits in and out, I can re-purpose one of the filter switches for this. I also want to reconfigure the “reverb” loop as a true pre-out (after volume) main-in ( to PA voltage amp) loop. I am uncertain how much the 12ax7 in this circuit contributes to gain versus compensating for insertion loss of the RC circuits. Can I simply bypass from the grid of V15, and go straight to the balance control ?
On to the power amp proper…
…nice and simple. Some things I’m considering are, converting to cathode bias, regulating grid bias, getting rid of the cathode capacitor ,and reducing overall feedback to compensate for the new local feedback. It would be a big departure from original but I also thought about a small Jensen transformer for the phase splitting duties. This is the area I’m least certain about changing. The 500C/800C is famous for its low end extension and “slam”..I don’t want to lose that. I am all ears to the pros and cons of these mods, as well as any others you might suggest.
Finally the power supply…
…Basic stuff. I am most interested in diode recommendations here. The RC filters will be determined after phono section is finalized.
Well gentlemen, there it is..let the ideas/recommendations run forth.
Casey
A little while back, I decided to get back into my favorite obsession…2 channel audio, more specifically, analog and T-O-O-O-B-S . The first step was finding some info online ( a 20 year hiatus left me wanting on the current state of things) to help get me up to speed. I found this group of forums , and quickly learned how little I know, and a good deal of what I thought I knew was wrong. The bench of knowledge here is a very deep one.
I was originally planning to build the uber ultra-fi system right out of the gate. A month plus into my turntable project, I realized at this pace it would be a l-o-o-n-g time before I ever spun my first record (phono-pre, line stage, power amp, and speakers still in the thought stage), so I decided on an interim step…a massaged vintage unit.
This brings me to the point of this post. I decided on the Fisher 800C receiver. I always liked the vintage Fishers I had listened to in the past, and the 800C has the added bonus of AM (I get my news in the morning while I’m getting ready for the day). I would also note that I bought my unit BEFORE I read the Stereophoole review on the 500C ( same unit minus AM).
It is my hope that this group can help develop THE definitive set of mods for this already great piece. First the ground rules:
*Anything done must be 100% reversible-no cutting-no separate supply ect. (it is a classic after all)
*With the exception of the phono stage (total caa-caa being the universal opinion of those who have reviewed it) I want it to still be a Fisher (more or less). Avery Fisher was no fool, and in lew of the test equipment available today, he engineered a very nice piece of gear through voicing and intuition (much like the group here). If you suggest something that is a radical departure from the original topology, please back it up as to why.
* All modified circuits must draw no more current than the originals. 40 year old iron aint so easy to come by, and tends to be a little on the frail side. If it’s the greatest sound in the world, it does me little good if it melts the trannies.
The full pdf schematic of the 500C is to large to attach, so I have cropped snippets to aid in the discussion.
Big problem first..the phono section…
… I’m sure with the turntable/cartridges of the day, it sounded ok. But everything I’ve read suggests that it will not due today. I intend to completely re-due this circuit. It is limited to two 9 pin sockets, and the voltages that can be derived from the stock supply. Currently, I think the evil Kuei’s circuit..
http://www.diyaudio.com/forums/attachment.php?s=&postid=335624
.. would fill the bill nicely, but I am open to further suggestions.
Next up..the control section…
…hoo-boy, what a convoluted mess. I won’t need the high/low filters, but I would like to be able to switch the tone/loudness circuits in and out, I can re-purpose one of the filter switches for this. I also want to reconfigure the “reverb” loop as a true pre-out (after volume) main-in ( to PA voltage amp) loop. I am uncertain how much the 12ax7 in this circuit contributes to gain versus compensating for insertion loss of the RC circuits. Can I simply bypass from the grid of V15, and go straight to the balance control ?
On to the power amp proper…
…nice and simple. Some things I’m considering are, converting to cathode bias, regulating grid bias, getting rid of the cathode capacitor ,and reducing overall feedback to compensate for the new local feedback. It would be a big departure from original but I also thought about a small Jensen transformer for the phase splitting duties. This is the area I’m least certain about changing. The 500C/800C is famous for its low end extension and “slam”..I don’t want to lose that. I am all ears to the pros and cons of these mods, as well as any others you might suggest.
Finally the power supply…
…Basic stuff. I am most interested in diode recommendations here. The RC filters will be determined after phono section is finalized.
Well gentlemen, there it is..let the ideas/recommendations run forth.
Casey
Were it mine, I'd have less respect for 50 year old ideas of production engineering. And sensible engineering to meet the requirements of typical users 50 years ago.
What don't we need?
- Phono equalization other than RIAA
- Tape monitoring
- Tone controls
So... were this piece mine, I'd wire up a permanent RIAA network, and change the tone control section over to a simple buffer for a preamp out. And I'd get a good volume/balance control. If I would need tone controls, I'd build in a version of Quad's tilt control, the best consumer tone controls I've ever used.
Now the power amp. There are better ways of doing a one tube input stage in 2005. A voltage amp direct coupled to a split load inverter is one of them. A small input transformer coupled to a diff amp is another. Either way, the output is an easy-to-drive pentode stage, so don't muck with cathode bias.
Regulation of the grid circuit is an OK idea; more importantly, you want to regulate the screen supply. If that's not regulated, the idle current will drift with changes in line voltage no matter how well the grid supply is regulated. Plate regulation is much less important here.
What don't we need?
- Phono equalization other than RIAA
- Tape monitoring
- Tone controls
So... were this piece mine, I'd wire up a permanent RIAA network, and change the tone control section over to a simple buffer for a preamp out. And I'd get a good volume/balance control. If I would need tone controls, I'd build in a version of Quad's tilt control, the best consumer tone controls I've ever used.
Now the power amp. There are better ways of doing a one tube input stage in 2005. A voltage amp direct coupled to a split load inverter is one of them. A small input transformer coupled to a diff amp is another. Either way, the output is an easy-to-drive pentode stage, so don't muck with cathode bias.
Regulation of the grid circuit is an OK idea; more importantly, you want to regulate the screen supply. If that's not regulated, the idle current will drift with changes in line voltage no matter how well the grid supply is regulated. Plate regulation is much less important here.
Hello Casey,
I've been told that the phono preamp you've linked has error in its schematics: in fact it doesn't simulate well the RIAA curve.
http://www.videohifi.com/forum/topic.asp?TOPIC_ID=27017&whichpage=1&ARCHIVE=
In this thread on an italian forum you can see that RIAA network simulated on Spice, and the graphs of the response. Not good.
Lower on the first page you'll find a better phono preamp: also on my site you can find a tested design that I've just built: just 2 12AX7 for both channels.
I've been told that the phono preamp you've linked has error in its schematics: in fact it doesn't simulate well the RIAA curve.
http://www.videohifi.com/forum/topic.asp?TOPIC_ID=27017&whichpage=1&ARCHIVE=
In this thread on an italian forum you can see that RIAA network simulated on Spice, and the graphs of the response. Not good.
Lower on the first page you'll find a better phono preamp: also on my site you can find a tested design that I've just built: just 2 12AX7 for both channels.
Hello Sy,
What I have respect for, is the ability to get good sound in spite of the constraints of 50 year old production engineering. No easy feat. I agree that ther are a multitude of better ways, which is why I started this thread.
Very true
Maybe..Tape-in could be used as another input, and tape-out could feed my pc for CD burning (can't spin the wax when I'm on the road)
90+% of the time I couldn't agree more, just one more thing to muck up the signal, BUT that other 10-% of the time it is most useful. This is why I want to keep it, but be able to switch it out.
Ok, I'm listening..could you point me to a schematic, and maybe elaborate on what makes it the shiz-nitty of tone controls ?
One of my weak points is associating circuitry with its name, could you point me to an example of a "split load inverter" ? I mentioned above that I was thinking of the input transformer approach, any implementation you could link to ?
Very good info. What solid state reg. scheme would you recommend for the screen?
Hello Giaime,
Hmm..I haven't simulated it myself, but I know more than a few people have built it and speak highly of it. I couldn't say one way or the other, maybe Kuei can step in.
Can't speak Italian, and the pictures came up X's here, bummer.
Looks like that would work nicely in this application..thank you.
Keep em comin' guys.
Casey
What I have respect for, is the ability to get good sound in spite of the constraints of 50 year old production engineering. No easy feat. I agree that ther are a multitude of better ways, which is why I started this thread.
Very true
Maybe..Tape-in could be used as another input, and tape-out could feed my pc for CD burning (can't spin the wax when I'm on the road)
90+% of the time I couldn't agree more, just one more thing to muck up the signal, BUT that other 10-% of the time it is most useful. This is why I want to keep it, but be able to switch it out.
Ok, I'm listening..could you point me to a schematic, and maybe elaborate on what makes it the shiz-nitty of tone controls ?
One of my weak points is associating circuitry with its name, could you point me to an example of a "split load inverter" ? I mentioned above that I was thinking of the input transformer approach, any implementation you could link to ?
Very good info. What solid state reg. scheme would you recommend for the screen?
Hello Giaime,
Hmm..I haven't simulated it myself, but I know more than a few people have built it and speak highly of it. I couldn't say one way or the other, maybe Kuei can step in.
Can't speak Italian, and the pictures came up X's here, bummer.
Looks like that would work nicely in this application..thank you.
Keep em comin' guys.
Casey
Those odd output tube would be gone if that were mine and the sockets and circuit parameters if necessary rewired to accept a tube that is commonly available.
Having to rely on only one remaining manufacturer (Sovtek) in a brand called Harmonx Distortion or something (I forget, I've got a really bad headache right now) which has shown to be a totally poor quality, short lived tube in all the NIB examples I've bought so far that I won't use them again doesn't sound like an upgrade to ME. Oh, ElectroHarmonix. Well I was close.
Plunk in a set of real NOS 6CA7/EL34's from Mullard and enjoy the same set of valves for 15+ years. Tight center-to-center spacing of the sockets probably prevent being really adventurous as in perhaps KT66's or KT88's.
Install a real choke in that existing CRC HV power supply. At least Fisher didn't use a stupid voltage doubler like others did here. Sherwood comes first to mind.
Install a separate HV fuse to each o/p xfmer to save it in case someone puts EH tubes in the amp and isn't standing there watching when the tubes go into hot plate runaway.
Put a big knob on the tuner control shaft so you hand will intuitively be able to find it. Ditto for the volume knob.
Merry Christmas
Having to rely on only one remaining manufacturer (Sovtek) in a brand called Harmonx Distortion or something (I forget, I've got a really bad headache right now) which has shown to be a totally poor quality, short lived tube in all the NIB examples I've bought so far that I won't use them again doesn't sound like an upgrade to ME. Oh, ElectroHarmonix. Well I was close.
Plunk in a set of real NOS 6CA7/EL34's from Mullard and enjoy the same set of valves for 15+ years. Tight center-to-center spacing of the sockets probably prevent being really adventurous as in perhaps KT66's or KT88's.
Install a real choke in that existing CRC HV power supply. At least Fisher didn't use a stupid voltage doubler like others did here. Sherwood comes first to mind.
Install a separate HV fuse to each o/p xfmer to save it in case someone puts EH tubes in the amp and isn't standing there watching when the tubes go into hot plate runaway.
Put a big knob on the tuner control shaft so you hand will intuitively be able to find it. Ditto for the volume knob.
Merry Christmas
Sorry Casey for the pictures, on my pc they look good. Anyway they show that circuit under a Spice simulation (with all the voltages, currents etc etc) with the graphs of the response: and obviously the RIAA curve looks bad 
Let's see if I can report them here:
Pic of the circuit (note: the second stage biasing has been changed from grid leak to LED bias)
The response of the circuit
Real RIAA response
As you can see there's something wrong...
Let's see if I can report them here:
An externally hosted image should be here but it was not working when we last tested it.
Pic of the circuit (note: the second stage biasing has been changed from grid leak to LED bias)
An externally hosted image should be here but it was not working when we last tested it.
The response of the circuit
An externally hosted image should be here but it was not working when we last tested it.
Real RIAA response
As you can see there's something wrong...
Hello rcavictam,
Clearly you have had som R-E-A-L-L-Y bad experience with EH. I can't comment one way or the other, as I have no experience (yet) with them. I can say though, that most everything I've read about the 7591 from them in the the Fisher has been very positive. In fact the Fisher "experts" I've read on the net claim the EH's sound better than nos!! Again, I have no personal experience, but even a blind pig can find a truffle once in a while
, maybe thats the case with the EH 7591. The fusing of the OPT's is a stellar idea though.
Nothing says I can't rewire for a different tube later if I have problems with reliability and availability. For the time being I want to keep to the tubes the OPT's were wound for.
Don't think I'm dismissing you out of hand, I'm not. I will be keeping a very close eye on the outputs.
Thanx for the heads up.
Hello again Giaime,
Don't tell anyone, but I'm posting from work
. The local network filter is blocking the pictures. I'll check them out when I get home.
Thanx again,
Casey
Clearly you have had som R-E-A-L-L-Y bad experience with EH. I can't comment one way or the other, as I have no experience (yet) with them. I can say though, that most everything I've read about the 7591 from them in the the Fisher has been very positive. In fact the Fisher "experts" I've read on the net claim the EH's sound better than nos!! Again, I have no personal experience, but even a blind pig can find a truffle once in a while
, maybe thats the case with the EH 7591. The fusing of the OPT's is a stellar idea though.Nothing says I can't rewire for a different tube later if I have problems with reliability and availability. For the time being I want to keep to the tubes the OPT's were wound for.
Don't think I'm dismissing you out of hand, I'm not. I will be keeping a very close eye on the outputs.
Thanx for the heads up.
Hello again Giaime,
Don't tell anyone, but I'm posting from work
. The local network filter is blocking the pictures. I'll check them out when I get home.Thanx again,
Casey
Just a few quick comments (I'm buried in a prototype at the moment):
Changing from 7591 to EL34 is not trivial. More filament current, higher drive requirement, not to mention form factor. I'd try the new 7591s.
You can find examples of the sort of input stage I'm talking about in Morgan Jones's "Valve Amplifiers." Check out the Bevois Valley project- that input stage could work very well in this application. While you're perusing that book, look up the Maida regulator circuit- that would be perfect for the screen supply.
There was a nice article about the Quad tilt control in Audio Amateur, then later in AudioXpress. The former was an article by Reg Williamson and Alan Watling; the latter was a tube adaption. I don't have the references handy, but if you can't find them, I'll dig them up.
Changing from 7591 to EL34 is not trivial. More filament current, higher drive requirement, not to mention form factor. I'd try the new 7591s.
You can find examples of the sort of input stage I'm talking about in Morgan Jones's "Valve Amplifiers." Check out the Bevois Valley project- that input stage could work very well in this application. While you're perusing that book, look up the Maida regulator circuit- that would be perfect for the screen supply.
There was a nice article about the Quad tilt control in Audio Amateur, then later in AudioXpress. The former was an article by Reg Williamson and Alan Watling; the latter was a tube adaption. I don't have the references handy, but if you can't find them, I'll dig them up.
JJ has a new 7591, same size as original. The EH are happier with a lower grid resistor (Fisher used 110% of the data sheet maximum...), and that may be true with the JJ as well. Wth 150-200K, grid drive is reduced further, but it should still be enough. Really should be a 7247 instead of a 12AX7, but those are hard to find...
Add 10 Ohm resistors as cathode fuses - gives you a point to check idle current too. As for regulating bias - DON'T unless you regulate the screen. As-is, the idle current driops as line voltage increases, since the control grid has more gain than the screen. Adjustable bias and balance is a good idea, if you can find space for the pots.
Install an inrush limiter (Keystone CL-70 or similar) in the primary and reduce the line fuse to 2.5A slo-blo. (too late in mine - line switch is already gone...) Add a 3-wire cord for safety, and delete the line-chassis caps.
800C Schematic on my page at http://www.audiophool.cjb.net/Audio.html
Add 10 Ohm resistors as cathode fuses - gives you a point to check idle current too. As for regulating bias - DON'T unless you regulate the screen. As-is, the idle current driops as line voltage increases, since the control grid has more gain than the screen. Adjustable bias and balance is a good idea, if you can find space for the pots.
Install an inrush limiter (Keystone CL-70 or similar) in the primary and reduce the line fuse to 2.5A slo-blo. (too late in mine - line switch is already gone...) Add a 3-wire cord for safety, and delete the line-chassis caps.
800C Schematic on my page at http://www.audiophool.cjb.net/Audio.html
Casey,
SY made an IMPORTANT point about regulating screen grid B+. Stable g2 B+ is THE way to maximize open loop linearity in pentode mode
If you regulate the C- supply, you MUST also regulate the B+ too.
Replace the doubler diodes with a pair of Cree 1200 PIV Schottky parts. They have zero switching noise; so, the 10 nF. snubber on the rectifier winding can be disposed of. Acquire a pair of GE/Thermometrics NTC inrush current limiters from Jim McShane and install them in the lines from the rectifier winding to the rest of the B+ PSU. Both the power trafo and the O/P tubes are handled much more gently when the turn on surge is damped. Increase the size of the caps. in the doubler stack to 330 muF. That will stiffen the supply a little without increasing I^2R heating a lot.
Thorsten's phono circuit is out because it uses the 6922. Where is the 6.3 VDC for the 6922 heater going to come from? A DECENT and simple phono stage is the "classic" RCA passive equalization circuit. The weaknesses of the RCA design are poor drive capability and mediocre deep bass extension. With the phono section in the same "box" as the downstream circuitry, drive is not much of a concern. The deep bass extension can be improved by changing the 2nd gain block's biasing method. Ground the cathode and use a 20 MOhm Caddock part as the grid leak resistor.
Buffer the recording O/Ps with JFETs.
A possibility for the phase splitter/driver is a 12AT7 wired in a Schmitt circuit. 3X red LEDs would provide the necessary grid bias and a 6 mA. CCS is the path to ground. 47 KOhm load resistors would be used on the 'T7 anodes. The non-inverting I/P of the splitter provides a convenient point for the application of loop NFB.
Another possibility for the tone controls is the Baxendahl circuit.
SY made an IMPORTANT point about regulating screen grid B+. Stable g2 B+ is THE way to maximize open loop linearity in pentode mode
If you regulate the C- supply, you MUST also regulate the B+ too.
Replace the doubler diodes with a pair of Cree 1200 PIV Schottky parts. They have zero switching noise; so, the 10 nF. snubber on the rectifier winding can be disposed of. Acquire a pair of GE/Thermometrics NTC inrush current limiters from Jim McShane and install them in the lines from the rectifier winding to the rest of the B+ PSU. Both the power trafo and the O/P tubes are handled much more gently when the turn on surge is damped. Increase the size of the caps. in the doubler stack to 330 muF. That will stiffen the supply a little without increasing I^2R heating a lot.
Thorsten's phono circuit is out because it uses the 6922. Where is the 6.3 VDC for the 6922 heater going to come from? A DECENT and simple phono stage is the "classic" RCA passive equalization circuit. The weaknesses of the RCA design are poor drive capability and mediocre deep bass extension. With the phono section in the same "box" as the downstream circuitry, drive is not much of a concern. The deep bass extension can be improved by changing the 2nd gain block's biasing method. Ground the cathode and use a 20 MOhm Caddock part as the grid leak resistor.
Buffer the recording O/Ps with JFETs.
A possibility for the phase splitter/driver is a 12AT7 wired in a Schmitt circuit. 3X red LEDs would provide the necessary grid bias and a 6 mA. CCS is the path to ground. 47 KOhm load resistors would be used on the 'T7 anodes. The non-inverting I/P of the splitter provides a convenient point for the application of loop NFB.
Another possibility for the tone controls is the Baxendahl circuit.
Attachments
valveitude said:
Would exactly 50% failure rate consisting of right out of the box to within 30 hours gentle use qualify as R-E-A-L-L-Y bad experience with EH? If so then YES.
I bought two NIB matched pair EH golden grid 300B's. I never spent so much money on a pair of audio tubes in my entire life and that includes NOS RCA and Westinghouse 805's! One EH 300B was NFG right out of the box and so I cannot use either as it was for a stereo amp. Makes a really spectacular light show every time it is turned on from a grid to filament partial short.
Half of a quartet of Fat Bottle 6CA7's had incandescent grid segments so I plan to return those duds. Then there are the 6L6WXT's whose filament connections went open circuit from poor pin soldering after 20 hours or so and once fixed they decided that they were still determined to commit suicide so they did the thermal runaway thing from gas or contamination/insufficient bakeout
.
I forgot to mention using Schottky diodes in the bias/heater supply bridge rectifier too. Eliminate SS diode switching noise in PSUs whenever possible.
The bias/heater supply can provide for 4 bottles. It's a given that 2 of those bottles will be in the phono section. FWIW, I'd use the remaining current in the splitter/driver heaters. With the tone control circuitry switched out most of the time, it makes sense to me to lower residual hum levels in the mode most listened to.
The 7247/12DW7 was previously mentioned. Recent production of the type is available from JJ and EI. If EIs made before the NATO bombing are available, they would be my choice. EI uses the TFK smoothplate design in their tubes. QC is the big question.
The bias/heater supply can provide for 4 bottles. It's a given that 2 of those bottles will be in the phono section. FWIW, I'd use the remaining current in the splitter/driver heaters. With the tone control circuitry switched out most of the time, it makes sense to me to lower residual hum levels in the mode most listened to.
The 7247/12DW7 was previously mentioned. Recent production of the type is available from JJ and EI. If EIs made before the NATO bombing are available, they would be my choice. EI uses the TFK smoothplate design in their tubes. QC is the big question.
Hi rcavictim,
Sorry you had problems with EH. So far I've only had 1 noisy 12AX7EH. I did get the wrong 6L6 in a 6L6EH box. (they replaced it) Of all the 6L6EH, 6CA7, 6SN7EH and 6922EH I got I've had nothing but very positive experiences. The Sovtek 6L6 whatevers seem to want to commit suicide in big amps and Fenders. But then again, many NOS tubes can't take a Fender Twin either.
I am dying to try some 7591EH's for my Eico ST-70 once I rebuild it.
Mind you, I buy mine directly from New Sensor.
-Chris
Sorry you had problems with EH. So far I've only had 1 noisy 12AX7EH. I did get the wrong 6L6 in a 6L6EH box. (they replaced it) Of all the 6L6EH, 6CA7, 6SN7EH and 6922EH I got I've had nothing but very positive experiences. The Sovtek 6L6 whatevers seem to want to commit suicide in big amps and Fenders. But then again, many NOS tubes can't take a Fender Twin either.
I am dying to try some 7591EH's for my Eico ST-70 once I rebuild it.
Mind you, I buy mine directly from New Sensor.
-Chris
Thanx for the input guys…
Hello Tom,
Thanx for the schematic link…proving once again why Photofacts are s-o-o much better than factory service prints…I like the fusable resistor idea, as well as an inrush limiter.
Hello Eli,
You crammed a lot of good stuff into a small space with your reply, I was all over the net expanding on what you wrote.
Couldn’t I rectify the output’s heater (6v) and isolate it with a 7806 reg. ? It would raise the current draw off of that winding slightly, but the VA would be offset by reducing the load from the 12.6 tap.
Again with my vernacular deficit, I hadn’t a clue what a “Schmitt” circuit was until I looked it up…I knew it as a “longtail”. What is your proposed advantage of this circuit over stock ? I think it would be one less stage, is this correct ? As I mentioned at the beginning of the thread, I am not opposed to a departure in topography in principle, but I do want to hear what the advantages over stock are. I’m a “even order” kind a guy, here is the distortion spectrum of a bone stock 500C using the aux input….
…not to shabby for a 1964 P-P circuit me thinks (for those with the fortitude to read through their piffle, here is the link to the Stereophoole review of the 500C I lifted the graph from…
http://stereophile.com/historical/605fisher/index.html
Once again, with my best “deer in the headlights” expression, I set out to answer “what is this Baxendahl of which you speak?”. I found out what it was, and then found it in a most unexpected place…
…the Photofact schematic of the Fisher 800C.
I see that it is fairly well agreed upon that a reg. bias and screen B+ is the way to go…could you ,or someone else, make a recommendation for the best (meaning I can implement it in the Fisher under-belly) RELIABLE circuit for the screen B+ ?
Sy,
I don’t have this book (though I admit I should), are there any examples on the net ?
Please do…I have no access to the info.
Giaime,
I can see the pictures now…that’s not Kuei’s circuit…its an adaptation. He has stated that if you change anything, you will have something else…I can’t say with any authority..I haven’t built/ simulated it.
Thanx all for the input…keep ‘em coming
Casey
Hello Tom,
Thanx for the schematic link…proving once again why Photofacts are s-o-o much better than factory service prints…I like the fusable resistor idea, as well as an inrush limiter.
Hello Eli,
You crammed a lot of good stuff into a small space with your reply, I was all over the net expanding on what you wrote.
Couldn’t I rectify the output’s heater (6v) and isolate it with a 7806 reg. ? It would raise the current draw off of that winding slightly, but the VA would be offset by reducing the load from the 12.6 tap.
Again with my vernacular deficit, I hadn’t a clue what a “Schmitt” circuit was until I looked it up…I knew it as a “longtail”. What is your proposed advantage of this circuit over stock ? I think it would be one less stage, is this correct ? As I mentioned at the beginning of the thread, I am not opposed to a departure in topography in principle, but I do want to hear what the advantages over stock are. I’m a “even order” kind a guy, here is the distortion spectrum of a bone stock 500C using the aux input….
…not to shabby for a 1964 P-P circuit me thinks (for those with the fortitude to read through their piffle, here is the link to the Stereophoole review of the 500C I lifted the graph from…
http://stereophile.com/historical/605fisher/index.html
Once again, with my best “deer in the headlights” expression, I set out to answer “what is this Baxendahl of which you speak?”. I found out what it was, and then found it in a most unexpected place…
…the Photofact schematic of the Fisher 800C.
I see that it is fairly well agreed upon that a reg. bias and screen B+ is the way to go…could you ,or someone else, make a recommendation for the best (meaning I can implement it in the Fisher under-belly) RELIABLE circuit for the screen B+ ?
Sy,
I don’t have this book (though I admit I should), are there any examples on the net ?
Please do…I have no access to the info.
Giaime,
I can see the pictures now…that’s not Kuei’s circuit…its an adaptation. He has stated that if you change anything, you will have something else…I can’t say with any authority..I haven’t built/ simulated it.
Thanx all for the input…keep ‘em coming
Casey
Casey,
The long tailed pair and the Schmitt phase splitters are variations on the differential amplifier theme. There are some subtle differences. Phase splitter write up here.
Differential phase splitters are linear and they provide gain. The split load, AKA concertina, AKA cathodyne, splitter is linear too, but it is near unity gain (slight insertion loss). Both of the above work better when high gm types are used. The 12AX7 is a poor tube for phase splitter duty. I favor the differential topology because loop NFB can be applied to a high impedance grid circuit instead of a low impedance cathode circuit.
The 6922 draws 300 mA. of heater current. Figure on 2X that much for the AC RMS current you would rectify. I don't see where you can get the current for a 6922 and stick to your requirement for no additional power trafos. JMO, use a 12AX7 based phono circuit, just not the Fisher OEM.
I thought there was a thread on this site about the tone control circuit, but my primitive search technique failed to turn it up. The schematic I recall buffered the "tone stack" at I/P and O/P.
Regulating screen grid B+ is clear cut. Look at dhaen's PP 7591 thread for hints on how to do it with Zener diodes.
I don't know if you can regulate the bias supply. Frankly, I don't think the Volts are there to use a series regulator. I wouldn't loose any sleep about unregulated bias. The anode B+ and the bias supplies are simultaneously affected by mains voltage fluctuation. So, the 2 supplies tend to "track" and the operating point stays reasonably stable.
The long tailed pair and the Schmitt phase splitters are variations on the differential amplifier theme. There are some subtle differences. Phase splitter write up here.
Differential phase splitters are linear and they provide gain. The split load, AKA concertina, AKA cathodyne, splitter is linear too, but it is near unity gain (slight insertion loss). Both of the above work better when high gm types are used. The 12AX7 is a poor tube for phase splitter duty. I favor the differential topology because loop NFB can be applied to a high impedance grid circuit instead of a low impedance cathode circuit.
The 6922 draws 300 mA. of heater current. Figure on 2X that much for the AC RMS current you would rectify. I don't see where you can get the current for a 6922 and stick to your requirement for no additional power trafos. JMO, use a 12AX7 based phono circuit, just not the Fisher OEM.
I thought there was a thread on this site about the tone control circuit, but my primitive search technique failed to turn it up. The schematic I recall buffered the "tone stack" at I/P and O/P.
Regulating screen grid B+ is clear cut. Look at dhaen's PP 7591 thread for hints on how to do it with Zener diodes.
I don't know if you can regulate the bias supply. Frankly, I don't think the Volts are there to use a series regulator. I wouldn't loose any sleep about unregulated bias. The anode B+ and the bias supplies are simultaneously affected by mains voltage fluctuation. So, the 2 supplies tend to "track" and the operating point stays reasonably stable.
Casey,
If you can source a UCC88, Thorsten's circuit becomes viable. The UCC88 has a 12.6 V./150 mA. heater that's perfect for 1 of the 4 DC heater "slots" the receiver has.
Since you would be using a section of each tube in both channels, perhaps a 7058, which pins out like a 'DJ8, is appropriate instead of a 12AX7.
If you can source a UCC88, Thorsten's circuit becomes viable. The UCC88 has a 12.6 V./150 mA. heater that's perfect for 1 of the 4 DC heater "slots" the receiver has.
Since you would be using a section of each tube in both channels, perhaps a 7058, which pins out like a 'DJ8, is appropriate instead of a 12AX7.
Sy,
Well...after the "shopping for the season" my card has taken about all the abuse it can handle, for a little while anyway
Eli,
2X ? Why so much ?
Reviewing the power supply...
... you'll notice 3 light bulbs in the 6 volt string..I'll betcha if I replaced them with led's I would have more than enough reserve to fire up up a 6922, and end up drawing less current overall (not to mention the cool factor of having a blue dial face)
The other thing this print shows, is the "105-120 volts" range at the primary. I was reading a thread over at the asylum were a 500C owner was stressing over his higher than published B+. It turns out the design center is 110V, which puts the actual voltages on a 120V line around 9% higher than published on the print. The Lm337 regulators have a 3 volt dropout, so if the nominal 110 supplied bias supply is -22V, its -24 on 120, giving me a maximum of a regulated -21V bias.
My bigger problem is getting my AC filaments back down to 6.3V/12.6, there 6.9/13.7 on 120...any suggestions ?
I haven't recieved my unit yet, so I can't confirm this, but based on what I've read, it seems to be the case.
Thanx for the "phase splitter" link by the way. The Schmitt is looking better all the time. Looks like I'll be doing some reading.
Casey
Well...after the "shopping for the season" my card has taken about all the abuse it can handle, for a little while anyway
Eli,
2X ? Why so much ?
Reviewing the power supply...
... you'll notice 3 light bulbs in the 6 volt string..I'll betcha if I replaced them with led's I would have more than enough reserve to fire up up a 6922, and end up drawing less current overall (not to mention the cool factor of having a blue dial face)
The other thing this print shows, is the "105-120 volts" range at the primary. I was reading a thread over at the asylum were a 500C owner was stressing over his higher than published B+. It turns out the design center is 110V, which puts the actual voltages on a 120V line around 9% higher than published on the print. The Lm337 regulators have a 3 volt dropout, so if the nominal 110 supplied bias supply is -22V, its -24 on 120, giving me a maximum of a regulated -21V bias.
My bigger problem is getting my AC filaments back down to 6.3V/12.6, there 6.9/13.7 on 120
...any suggestions ? I haven't recieved my unit yet, so I can't confirm this, but based on what I've read, it seems to be the case.
Thanx for the "phase splitter" link by the way. The Schmitt is looking better all the time. Looks like I'll be doing some reading.
Casey
Casey,
When cap. I/P filtration is used, the rectifier conducts in brief, high current, pulses. The heating in the power trafo is I^2R averaged over time. A DC draw 1/2 the AC RMS capability is SAFE, from a heating perspective. Please observe that heating increases as the value of the filter cap. increases.
The current for a 6922's heater may be present if the incandescent pilot lamps are replaced by LEDs. However, the Volts are probably not present. You have to bridge rectify the AC and that means a forward voltage drop across 2 diodes. Schottky diodes are best and the total drop is about 1 V. You'll get approx. 7.5 VDC at the filter cap. That's marginal, at best, even if a low drop out regulator IC is employed. Stop giving yourself an ulcer and hunt a UCC88 or 2 down.
I wouldn't bother with regulating the bias. However, if you are determined to do so, regulate only the bias, not the DC heater supply. Also, use a low dropout regulator IC from Linear Technology, as a generic LM337 is very shaky.
Pairs of low value resistors in series with the filament windings will allow you to get 6.3 VAC +/- 5%.
When cap. I/P filtration is used, the rectifier conducts in brief, high current, pulses. The heating in the power trafo is I^2R averaged over time. A DC draw 1/2 the AC RMS capability is SAFE, from a heating perspective. Please observe that heating increases as the value of the filter cap. increases.
The current for a 6922's heater may be present if the incandescent pilot lamps are replaced by LEDs. However, the Volts are probably not present. You have to bridge rectify the AC and that means a forward voltage drop across 2 diodes. Schottky diodes are best and the total drop is about 1 V. You'll get approx. 7.5 VDC at the filter cap. That's marginal, at best, even if a low drop out regulator IC is employed. Stop giving yourself an ulcer and hunt a UCC88 or 2 down.
I wouldn't bother with regulating the bias. However, if you are determined to do so, regulate only the bias, not the DC heater supply. Also, use a low dropout regulator IC from Linear Technology, as a generic LM337 is very shaky.
Pairs of low value resistors in series with the filament windings will allow you to get 6.3 VAC +/- 5%.
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