Ah..behold the power of caffeine
I was thinking in terms of scale, not absolutes. 20-30 ma one way or the other wouldn't be significant. And here I was about to be content for now.This is a real issue with this amp. Correcting this in the line stage reaped a huge return. I suspect addressing it here will do the same.
I don't see how replacing the PA input will help here much, the big 'ol volume/balance resistance is still in series with the grid. Replacing the line tube and reducing the values of the pots seems to be a better solution...the plan with the future 12B4 pre. The improved linearity with a ECC88 however has got me considering this...I'll have to revisit the earlier posts where some suggestions were made by Eli and others. The PA is going to be in my system for a while. The easiest approach would be to adapt the current design to the new tube, but if I'm pulling the knife out of the drawer, I might as well review my options.
Casey
Casey,
As SY indicated, you could retain Fisher's original topology and improve performance by using 12DW7s instead of 12AX7s in the power section gain/splitter positions. The 12DW7 has a mu 100 section and a mu 20 section ('X7/'U7). TFK & Mullard 12DW7s are PRICEY, but US made NOS is reasonably affordable and JJ offers current production. Triode Electronics charges $10 for a JJ 12DW7. RES charges $19 for NOS 12DW7s. RCA is ALWAYS nice.
As SY indicated, you could retain Fisher's original topology and improve performance by using 12DW7s instead of 12AX7s in the power section gain/splitter positions. The 12DW7 has a mu 100 section and a mu 20 section ('X7/'U7). TFK & Mullard 12DW7s are PRICEY, but US made NOS is reasonably affordable and JJ offers current production. Triode Electronics charges $10 for a JJ 12DW7. RES charges $19 for NOS 12DW7s. RCA is ALWAYS nice.
Hi Eli,
Ok..I assume this approach would give me comparable gain to stock (high mu of first section), but would it be as linear as using a ECC88 ? I actually need to scrub a fair amount of gain now with a pad at the input in order to use most of my volume control. I matched the gain of my line amp to the stock one for the tuner, but in hindsight, I should have gone for about 9dB less gain for the CD. So, losing some gain in the PA isn't a problem.
Casey
Ok..I assume this approach would give me comparable gain to stock (high mu of first section), but would it be as linear as using a ECC88 ? I actually need to scrub a fair amount of gain now with a pad at the input in order to use most of my volume control. I matched the gain of my line amp to the stock one for the tuner, but in hindsight, I should have gone for about 9dB less gain for the CD. So, losing some gain in the PA isn't a problem.
Casey
valveitude said:
I like the idea of matching up to the tuner's O/P level. Most CDPs have no problem driving a 10 KOhm load. So, a 10 K total resistive voltage divider across the CDP I/P jacks followed by a 100 KOhm volume control is OK.
I wouldn't be overly concerned with linearity. The loop NFB pentode mode NEEDS to yield a reasonable damping factor will also reduce distortion.
I have reservations about the 6DJ8's drive capability. A more capable triode would leave you positioned to try triode wired "finals" in the future. The 5965 is an obvious candidate, as its very linear, has a mu of 44, sounds good, and has good drive capability. A possible negative to the 5965 is its higher heater current draw than most 12A_7 types.
OK...were did I go wrong ?
I wanted to do some extensive listening before I spouted off again...something I need to do more often.
After I got the Fisher back together after my last modification of the line amp (different line amp really), I posted how impressed I was with the increased detail, and attack...well it's true that these got better, unfortunately it is equally true that you can no longer listen for any length of time without suffering listeners fatigue. The sound, for a lack of a better word, is strident. Gone is the "lush" quality of female vocals, recordings of sax that were "live" before are piercing now.
I made a 18dB L-Pad to remove the gain of the line amp (after the balance control) and put it inline with the CDP to see what my levels would be if I went straight to the volume control...perfect. It gets right on the edge of clipping at full volume. So, I will do this when I get off work tonight, but I still need to figure out why the simple 'AX7 stage sounds the way it does.
Casey
I think I can answer what went wrong..I'm still at a bit of a loss as to why.
I am barely profecient at LtSpice, and make mistakes often. This time it had to do with the FFT sims of my circuit. Knowing that it sounded way to "transistor" like, I re-ran the sims on a different PC.
Well wadaya know. The simmed FFT showed that it had around .1 or so THD, not bad. Unfortunately it appears as though the bulk of this is 5th, 7th, and 9th harmonics...EEEWWWW. Now I could have screwed up here as well, BUT, this fits with my (and the rest of the familys) listening impression...lots of "attack" and detail, but of the "etched" variety... unlistenable.
So, I've ran a input straight to the volume control. This is what I should have done all along, it sounds great. I still need to repair the tuner, so when I go back in I will restore the line/tone amp to its original topology. The only thing that will use it will be the tuner.
I won't be dinking with the power amp until after I finish my turntable/phono pre, and some better speaker.
Casey
After all this..
what direction should someone (like me) take, that is wanting to improve his 500C go?
I'd like to bring mine into the present without changing the listening enjoyment this old design provides.
I've noted the upgrade kit(s) and cap replace board from triode electronics (which i tend to shy away from) on fleabay and the suggestion to lower the bias a touch to prolong tube life.
Any other suggestions without major modification?
Thanks,
pc
what direction should someone (like me) take, that is wanting to improve his 500C go?
I'd like to bring mine into the present without changing the listening enjoyment this old design provides.
I've noted the upgrade kit(s) and cap replace board from triode electronics (which i tend to shy away from) on fleabay and the suggestion to lower the bias a touch to prolong tube life.
Any other suggestions without major modification?
Thanks,
pc
You must deal with the reality that old stock 7591s are "unobtainium". Thankfully, current production ElectroHarmonix 7591s fit. The "gotcha" is that the current production tubes are unforgiving of the liberties Fisher took with the max. allowable grid leak resistor value. Search this page from Jim McShane's web site for 7591. You will find what you need to know. Also, peruse the 7591 data sheet.
The changes Jim McShane outlined are the MINIMUM needed to ensure long term reliable operation. IMO, you should go further. Fisher's decision to use a 12AX7 as the driver/splitter was "counting beans". A 'X7 has POOR drive capability. The 7591 grid leak resistor value "cheat" is directly connected to the wimpy nature of the 'X7. Changing the splitter/driver twin triode to a 12DW7 permanently resolves the drive issue problem. Only the value of some resistors change. The topology is unaltered. An added benefit of the 12DW7 mod. is improved resistance to slew limiting. "Concertina" phase splitters should be built around high gm types. The 'X7 is low gm.
The changes Jim McShane outlined are the MINIMUM needed to ensure long term reliable operation. IMO, you should go further. Fisher's decision to use a 12AX7 as the driver/splitter was "counting beans". A 'X7 has POOR drive capability. The 7591 grid leak resistor value "cheat" is directly connected to the wimpy nature of the 'X7. Changing the splitter/driver twin triode to a 12DW7 permanently resolves the drive issue problem. Only the value of some resistors change. The topology is unaltered. An added benefit of the 12DW7 mod. is improved resistance to slew limiting. "Concertina" phase splitters should be built around high gm types. The 'X7 is low gm.
Hi all, new guy to the thread here...I actually just picked up a 500C today from someone that I work with (he's moving to Florida) and upon a quick search online, I wasn't too surprised to find a thread about the Fisher on diyAudio. I was wondering, aside from replacing various caps and tubes, are there any relatively simple improvements I could make to it?
This is my first tube piece (I'm only 25...I figure I have time), it's hooked up to my Dynaudio Contour S1.4's and it sounds amazing. While I work on some other DIY stuff, this might be a perfect stopgap receiver before I get my separates together. I'm quite glad to have stumbled upon this thing.
This is my first tube piece (I'm only 25...I figure I have time
), it's hooked up to my Dynaudio Contour S1.4's and it sounds amazing. While I work on some other DIY stuff, this might be a perfect stopgap receiver before I get my separates together. I'm quite glad to have stumbled upon this thing. 
Hi EvilYoda,
Some metal film type resistors in the input stages. The carbon comps can be noisy. Measure your resistors as those old ones may drift off value. Replace all the coupling caps if it hasn't been done. Replacing the electrolytic types would be prudent as well if they are original.
Component type (general construction) is more important than the name. Avoid cheap parts, normal grade parts are more than good enough.
-Chris
Some metal film type resistors in the input stages. The carbon comps can be noisy. Measure your resistors as those old ones may drift off value. Replace all the coupling caps if it hasn't been done. Replacing the electrolytic types would be prudent as well if they are original.
Component type (general construction) is more important than the name. Avoid cheap parts, normal grade parts are more than good enough.
-Chris
Well...that was quick I don't suppose there's a list anywhere of resistor/cap values and suggested replacement parts? The entire piece is original...he was even the original owner, since '64. As for the tubes, I've read that people are replacing not only the 7591A's, but the 12AX7's as well...I saw that EH has a 7591A tube, but I haven't heard much about the JJ one that's also listed on tubedepot. Aside from NOS tubes, have most people gone with one manufacturer or another?
Thanks again for the help, this sound has me smiling nonstop right now.
I don't suppose there's a list anywhere of resistor/cap values and suggested replacement parts? The entire piece is original...he was even the original owner, since '64. As for the tubes, I've read that people are replacing not only the 7591A's, but the 12AX7's as well...I saw that EH has a 7591A tube, but I haven't heard much about the JJ one that's also listed on tubedepot. Aside from NOS tubes, have most people gone with one manufacturer or another?Thanks again for the help, this sound has me smiling nonstop right now.
E/Y,
JJ has been inconsistent of late in the quality of Octal based tubes they produce. OTOH, the Saratov, Russia plant, where EH tubes are made, has been doing an excellent job across multiple types. IF it's properly made, the JJ 7591 is good.
The TFK ECC83s Fisher used are outstanding and should not be replaced until they are worn out. You could replace the phono section 'X7s with the quiet Sovtek 12AX7LPS and store the TFKs from those "holes" for future use elsewhere in the "box".
JJ has been inconsistent of late in the quality of Octal based tubes they produce.
OTOH, the Saratov, Russia plant, where EH tubes are made, has been doing an excellent job across multiple types. IF it's properly made, the JJ 7591 is good.The TFK ECC83s Fisher used are outstanding and should not be replaced until they are worn out. You could replace the phono section 'X7s with the quiet Sovtek 12AX7LPS and store the TFKs from those "holes" for future use elsewhere in the "box".
Ah, thanks for the advice...I won't be using the phono stage as one of my projects is a Pass Pearl phono, so no worries there. Guess it'll be the EH tubes for now!
Now I'm trying to figure out the best and safest way to clean the body, due to all the obstructions. Eventually I want to take the glass out to clean it...and I know one of my bulbs is out.
Now I'm trying to figure out the best and safest way to clean the body, due to all the obstructions. Eventually I want to take the glass out to clean it...and I know one of my bulbs is out.
rcavictim 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.
Slightly off topic, but I feel I have to respond...
1. Your experience is not the norm. I rarely if ever have trouble with EH power tubes, especially in the last few years. I have NEVER had a 6CA7 EH act up.
2. If you got a bad 300B out of the box I blame whoever sold it to you. Didn't they test it? After being shipped 1/2 way across the world I think they should be tested.
3. Right now, the pin soldering on EH stuff is the best out there by far - WAY better than JJ, that's for sure.
4. The one EH/Sovtek power tube I don't carry is the 6L6 though, so I cannot comment on those.
5. JJ 7591s don't like 330K grid resistors either.
If you have one good 300B I'll be happy to match one up to it - just pay me for the tube, the matching is on me. Drop me an email if you like.
Hi EvilYoda,
Please, discontinue using it if it is original! If you experience a capacitor failure or rectifier failure you may lose some rather expensive parts. Never mind the tubes becoming damaged possibly. Losing a transformer is a very real risk here.
The best thing to do is get ahold of the service manual or a clean (legible) schematic. There are some floating around if you don't have it yet. Study each section carefully. Go through and discharge each capacitor through a resistance.
Read the safety section in this forum!!!
After this, take a number of digital pictures from many angles at high resolution. You will be very happy you did this. Take a meter (decent digital, Fluke preferred or the new Agilent) and measure all the resistors. Note which are out of tolerance. Replace these with metal oxide, 1 W minimum (for the voltage rating). Low level stages may benefit from metal film resistors. Do everything, even the phono stage.
The glass is easy to spoil. You will need patience and nothing more than water and soft cloth or paper towel. Never rub! You will lose the lettering for sure if you do. You must pat very carefully.
Electroharmonix is my personal choice. I have not tried the 7591A's yet (I do need a set) but the 6L6EH have been wonderful. They sound great, just like my NOS tubes (I don't believe in NOS tubes). They are also very reliable. Any brand may have the odd defect, so don't buy them until you are ready to try them. Expect the odd noisy signal tube. Buy an extra or two. I've had 1 noisy 12AX7EH in over 5 years using these. Not bad really.
-Chris
Please, discontinue using it if it is original! If you experience a capacitor failure or rectifier failure you may lose some rather expensive parts. Never mind the tubes becoming damaged possibly. Losing a transformer is a very real risk here.
The best thing to do is get ahold of the service manual or a clean (legible) schematic. There are some floating around if you don't have it yet. Study each section carefully. Go through and discharge each capacitor through a resistance.
Read the safety section in this forum!!!
After this, take a number of digital pictures from many angles at high resolution. You will be very happy you did this. Take a meter (decent digital, Fluke preferred or the new Agilent) and measure all the resistors. Note which are out of tolerance. Replace these with metal oxide, 1 W minimum (for the voltage rating). Low level stages may benefit from metal film resistors. Do everything, even the phono stage.
The glass is easy to spoil. You will need patience and nothing more than water and soft cloth or paper towel. Never rub! You will lose the lettering for sure if you do. You must pat very carefully.
Electroharmonix is my personal choice. I have not tried the 7591A's yet (I do need a set) but the 6L6EH have been wonderful. They sound great, just like my NOS tubes (I don't believe in NOS tubes). They are also very reliable. Any brand may have the odd defect, so don't buy them until you are ready to try them. Expect the odd noisy signal tube. Buy an extra or two. I've had 1 noisy 12AX7EH in over 5 years using these. Not bad really.
-Chris
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