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Fisher 800c tube rolling.

Freddie73

Freddie73

Active Member
First time tube rolling 12ax7’s. Sovtek WB’s, LPS, Doge (Select Shuguang), Tung-Sol Gold pin, Genelex Gold lion Gold pin.
Ended up with SovtekLPS in Phono V18-19. Gold Lions in Phase Splitters V14-15. Believe it or not I like the Doge Chinese tubes in V16-17 Filter Amps. It’s just more articulate in high kids and highs over Tung Sol. Gold Lions have more punch in phase splitters. Didn’t try in phono stage but LPS have about the same gain as the Doge but more quite. I know everyone has different results, but as of today it sounds great.
Thinking about rolling V100-102 one tube at a time in case alignment messes up. All tubes in those positions are Sovtek WB’s. Is that a bad idea?
 
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Tube rolling the mpx adapter won't change the way anything sounds, only affects fm stereo reception. Got any oem fisher ax7's to compare all the aftermarket ones to? Just curious. ..:)
 
I've done some tube exchanges. Sovtek 12AX7LPS and Tung-Sol 12AX7 sound wonderful in the left and right preamp stages. Reissue Tung-Sol 7591A's sound great in the amp stage, but require backing off the bias.
 
Dave G. has tested new release Tung-sols in the phase invertor section and found them to be lacking at high output levels. The noose needs to be removed from the circuit, especially in the Fisher 400. I had Dave restore one of my 400's which I supplied all new Tung-sols with, I have compared it to a completely stock unmolested 400 that has all original tubes still, the unmolested unit does sound better. Hard to beat the old originals if you can find them, here is the link to the thread on the subject. http://audiokarma.org/forums/index.php?threads/fisher-400-service-bulletin.671281/
 
I read that thread and think I remember a post that said the noose is not as tight on Fishers other than the 400. But I don't play music at high output levels (what is the definition of "high"?) so no worries.
 
High is defined as anything over normal background levels, where it drives you out of the room.

To the OP, If you run Russian or Chinese tubes, remove the noose. I've got 2 800c's. Even tho it's not as bad as the 400 the 800c does respond to the noose removal and NON-US/EURO tubes. Or just put Tele's, Sylvania, GE, or other domestic tubes in Phase inverter.
 
Hmmm. Drive me out of the room levels? I'm already half deaf from 45 years of playing Rock n Roll in bands....LOL. I've sort of learned my lesson with volume. Anyway, I probably have a link in my notes to the "loosen the noose" post, but if you can find it easily, please post a link where the specific instructions are. I remember reading that post and my amateur, electronic brain had problems translating for my 500-C. I think because of my SDS Labs board, the noose instructions become more complicated, meaning I need some help with altering the instructions for my situation. I've attached the schematic if that helps. THANKS !
 

Attachments

Thorne;(After typing in the below, I seem to remember you did this already on you 500c. It's in your thread!)

Find the lead from the PI Pot that goes back to the SDS Board(Probably the wiper or center). With the unit OFF and UNPLUGGED!!!, Ground that lead at the pot. Then measure the resistance across the resistors at pins 1 and 3. Twitch the pot with the leads on resistor 1 or 3. the resistance will change on one of them. If pin 1 is static match pin 3 with pin 1 via the pot. If 3 is static,match pin 1 to pin 3. There is a resistor to be removed IIRC for the noose. It's in the "QUICK & EASY PHASE INVERTOR ADJUSTMENT for the 400" thread in the FORUM STICKIES, in the 1st page on the STICKIES.
 
Ah yes, IF adjustments. See, I didn't get that at all from the original directions. It seems there are so many names in these threads for the same part(s). I'll read the thread again, but I don't remember doing it that way. Dave G posted some directions somewhere in response to my post where he suggested starting backwards from IF5 to IF1, watching the strength meter until the adjustments reached the highest possible strength. I don't understand what IF adjustments has to do with loosening the noose and measuring resistance. See, I get confused quite easily.

UPDATE: This is what I don't know I can do: "Ground C91C(350V B+) and follow directions outlined in POST #1". C91C is replaced with my SDS Labs board, no? If so, I don't know what to do. If someone can point out C91C to me, I assume I know what the phase inverter tubes are - V13 and V12, right? Then I can follow the procedure in that "quick and easy" post.

Larry even made this thread edit just for me ! "12MAR2018 EDIT: NOTE FOR THOSE with 500c or 800c USING AN SDS BOARD! Make the connection to ground the B+ at the center connection tab of the Phase inverter POTS. Then adjust as instructed above for your individual units. " But what is a "center connection tab"? I assume a terminal block? All these terms ! I see "B+" mentioned all the time and I still don't know what that is. Sigh, I feel so uneducated.

OH ! "phase inverter pots". Those are the 2 pots where the adjustment is made on top of the 500-C? I see - THAT center tab on the pot ! Finally I get it ! So to ground that tab I put a wire from the tab to the chassis? That is C91C??? I think the center tab resistors are R101 and R104.
 
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The MPX tubes are Sovtek 12ax7 WXT’s. I will leave alone. My 800C has been serviced and upgraded with a in rush limiter, protection fuses(resistors) New orange drop coupling capacitors. I’m running new Tung Sol 7591’s. It plays pretty loud and extremely clear, but my hearing is from years of moderately loud music. My kids in their 20’s complain I’m playing at “Live,” volume at times. So I guess the next step is to bite the bullet and buy NOS RCA black plates or Telefunken 12ax7’s. Building a new cabinet now. I appreciate all the threads on that. Can you post short videos of stereo playing on this website, it really does sound great now?
 
Thorne; Phase Inverter (PI or more correctly P.I.) has nothing to do with tuner adjustment. What it does do is make sure each 1/2 of the push pull signal(correct me if I'm wrong Dave) is 180* out of phase(or as close as possible) so that you get an equal signal on each 1/2 of the complete signal (make a sinewave on paper with a line 1/2 way thru it) The upper 1/2 and the lower 1/2 are 180* out of phase (upper +) & (lower -) and distortion is nulled out(or as much as possible due to transformer winding). As you get away from 180* Distortion increases and volume can be affected. Which is why a distortion analyzer is the best way to do this. The voltage null that we do is a quick, easy way to accomplish basically the same thing, but a bit of distortion is still there. The difference is only seen on a distortion analyzer and isn't heard.


IF = Intermediate Frequency which is the frequency put out from the oscillator to add to the RF frequency so that you can eventually tune in the music, 455kz for AM and 10.7mHz for FM. If you take 2 FM receivers, put them next to each other, and put one on 90mHz and then tune the other to 100.7(rock it back and forth) you get a NULLED Signal. The Static in the background goes away and it sounds like a blank signal, like when the transmitter goes off line.

RF = RADIO FREQUENCY (Usually the Front End of the tuner (in the box)). This is the original frequency sent by the transmitter.

MPX= Multiplex. No explanation needed. These are ALL related to Tuner Function and have really nothing to do with any of the amplification sections of the unit involved.

PI = Phase Inverter. Line Stage is any Gain or Tone stage between the EQ STAGE (ie: Phono) and the AMP Stage.

Put THAT in your Funk and Wagnell's:naughty: and Smoke it. Guaranteed to make you give up smoking, and may or may not (YMMV)make you smarter. :bigok:
 
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LOL. Thanks Larry. IF and MPX I knew. Please read my post# 9 as I have re-edited it several times. I think if you can answer my questions in there, I might finally get it. As always I appreciate your able assistance and patience. I really want to learn. Thanks.

PS - Sorry if I'm steering the thread off-topic.
 
Freddie; Camera or phone mic's aren't going to pull in all the frequencies that the 800c will put out, plus they sound tinny at best. If you have a video camera with a stereo mic setup that's best, but even at that unless you record directly to the media from the 800c you won't get good fidelity, it'll still sound like a good pocket AM radio.

Thorne; for example. The signal comes in to the Antenna. It goes to the RF AMplifier (usually a 6GK5 in the 500c/800c) which just amplifies the signal, then (6aq8 used for the mixer/local oscillator) " In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequency of the local oscillator and frequency of the input signal"(taken from Google). The IF stages on a radio are tuned to an intermediate or IF frequency(Seepost #13). These take the IF FREQUENCY and actually tune them to the dial. If any are off slightly (which is why you peak them to highest voltage which is supposed to be 10.7mHz) you will get degraded FM at BEST or NOTHING at worst. "An FM radio receiver usually has at least one stage of amplification for this purpose. It provides a constant level of signal to the FM demodulator stage, reducing the effect of signal level changes in the output. If two or more signals are received at the same time, a high performance limiter stage can greatly reduce the effect of the weaker signals on the output. This is commonly referred to as the FM capture effect."(Again from Google) And then the discriminator strips an AM carrier wave out and the fm signal has de-emphasis added to attenuate the hiss. This signal then goes out as a FM MONO singal to the amplifier or to the multiplexer for conversion back to a FM STEREO signal and off to the amplifier.
 
With regard to adjusting your Phase Inverter ADJUSTMENTS. The B+ for the inverter comes from the SDS board to the Center Tab of the Phase Inverter POT! Attach a Alligator clip with a lead on it and attach the other end to chassis ground. This is done with the unit OFF and UNPLUGGED! Then follow the instructions on the Quick and Easy Phase inverter" Thread in the FORUM STICKIES Section for the 500c. BTW That was my edit.

Way back when Dave and I wore short pants there were still radios from the 1920's,1930's and some 1940's radios that ran on batteries. There was a "A"battery for tube filaments and a "B" Battery which was a high voltage battery (some went as high as 90v IIRC) for the amplification stages. These got changed out regularly or recharged. Anyway with the Advent of Wall Voltage radios the "A" battery was done away with and the term discarded. However the "B" battery became the Hi-Voltage circuits and that voltage was termed B+ or B- if in a Negative voltage circuit (some radios and phono radiographs were - voltage.) and the Term Stuck. So B+ is usually termed as any voltage source that provides a voltage that is higher than that needed for bias or tube heaters. In Solid State it's the Higher voltage needed to power the amplificatioin stages and is also known as Rail Voltage.
 
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Thanks so much Larry. So after grounding the center tab, to measure resistance the black lead of multimeter goes to ground and red lead goes to pin 1 of each phase inverter tube (V12 and V13 for me)? And then I turn the adjustment pots on top until pin 1 reads the same value as what? pin 3?
 
Yes pin 3. But make sure Pin 1 adjusts as some units use pin 3 as the dynamic resistor. Then pin 1 would be the static resistor.
 
Wow. Mine were WAY off. Pin 1 measured 36.9k while pin 3 measured 49.8k. About the same for the other pot and tube. OK, now back to our regularly scheduled program.
 
Put the noose resistor in and wait for Dave. He got married y'day and is out of town for a few days. The 500c and 800c isn't as critical as on the 400. It won't hurt it to keep the noose in on them.
 
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