Fisher 800c best tubes to improve FM sound.
- Thread starter Freddie73
- Start date
If it's undergone an alignment, and/or your reception is already good, leave the MPX tubes alone. The tube won't really affect the audio quality but changing them around could mess up your reception.
The tubes that have the greatest effect on sonics are 16/17 for the pre, and 18/19 for the phono. If you can swing it, NOS Telefunken's are probably the best as that's what probably came with it originally. The Tung-Sols will probably sound good too. Tons of information here and elsewhere for rolling tubes in Fisher receivers.
UNLESS you've done the Phase Inverter mod on the 800c, DON'T USE RUSSIAN TUBES IN Phase inverter. See "400 Service Bulletin" in the Stickies Sub-forum. This applies to the 800c but it's not as critical. I've tried EH's, Tung-Sol's, and Sovtek 12ax7LPS there and they all go into cutoff. Use a Domestic Tube there or do the mod, which is listed in the "400 phase inverter mod" thread in the SUB-FORUM STICKIES.
Also if it's undergone an alignment leave the V1 thru V6 (RF AMP/Mixer/Oscillator/ IF STRIP) alone, except in the case where a tube goes dead. Then replace with GE RCA or Sylvania Rf/IF tubes.(See below for 6aq8) No need for expensive foreign tubes in the RF/IF stages as they don't affect the SQ. As for the 12ax7's, I've seen Fisher Branded Sylvania 12ax7 tubes in some very late production 800c/500c/and 400's. Fisher went with Sylvania tubes in a fair amount of gear as their stocks of Telefunkens ran out and the replacement costs went up. Sylvania's were less expensive and didn't change the sound of the units appreciably if at all. As FISHER changed over to S.S. tubes became a loss leader, and they went with the lowest bidder.
The only units that came with TELE 12ax7's (that still had original tubes in them) were my consoles. I find that TELEFUNKEN's, just like other manufacturers tubes have sections that become unbalanced (one reading approx 1/2 to 1/3 of the other) and are about useless except in extreme cases. I have a stash of domestic tubes with closely balanced sections, even if some are below minimum gm or emissions that perform better than those with unbalanced sections. With the exception of the V2 tube, all of the RF/IF strip tubes are single section tubes.
The V2 tube is a 6AQ8 (Dual TRIODE) which is used as the OSCILLATOR/MIXER tube. A weak section can cause problems in that the oscillator or the mixer section is weak, causing poor performance in the tuner. In 9 of 10 units with the 6AQ8 in the Front end of the tuner the 6AQ8 has at least one section that is barely operating and in some cases so poorly that stations cannot be received or highly distorted. The 6AQ8 in most all FISHER tuners is run hard and put away wet. So it IS prudent to replace this tube with a new either "NOS" if you can find one for a highly inflated price, a used one for about $20.00 or a good Used 6DT8 for 1/5th the price of a used 6AQ8. If you can find EUROPEAN EI 6AQ8 (ECC85) get them. they are a good tube at a decent price. (about $6.-$8ea). Otherwise the 6DT8 is a good sub.
Also if it's undergone an alignment leave the V1 thru V6 (RF AMP/Mixer/Oscillator/ IF STRIP) alone, except in the case where a tube goes dead. Then replace with GE RCA or Sylvania Rf/IF tubes.(See below for 6aq8) No need for expensive foreign tubes in the RF/IF stages as they don't affect the SQ. As for the 12ax7's, I've seen Fisher Branded Sylvania 12ax7 tubes in some very late production 800c/500c/and 400's. Fisher went with Sylvania tubes in a fair amount of gear as their stocks of Telefunkens ran out and the replacement costs went up. Sylvania's were less expensive and didn't change the sound of the units appreciably if at all. As FISHER changed over to S.S. tubes became a loss leader, and they went with the lowest bidder.
The only units that came with TELE 12ax7's (that still had original tubes in them) were my consoles. I find that TELEFUNKEN's, just like other manufacturers tubes have sections that become unbalanced (one reading approx 1/2 to 1/3 of the other) and are about useless except in extreme cases. I have a stash of domestic tubes with closely balanced sections, even if some are below minimum gm or emissions that perform better than those with unbalanced sections. With the exception of the V2 tube, all of the RF/IF strip tubes are single section tubes.
The V2 tube is a 6AQ8 (Dual TRIODE) which is used as the OSCILLATOR/MIXER tube. A weak section can cause problems in that the oscillator or the mixer section is weak, causing poor performance in the tuner. In 9 of 10 units with the 6AQ8 in the Front end of the tuner the 6AQ8 has at least one section that is barely operating and in some cases so poorly that stations cannot be received or highly distorted. The 6AQ8 in most all FISHER tuners is run hard and put away wet. So it IS prudent to replace this tube with a new either "NOS" if you can find one for a highly inflated price, a used one for about $20.00 or a good Used 6DT8 for 1/5th the price of a used 6AQ8. If you can find EUROPEAN EI 6AQ8 (ECC85) get them. they are a good tube at a decent price. (about $6.-$8ea). Otherwise the 6DT8 is a good sub.
Freddie73
Active Member
I checked a few of my suppliers and this is the least expensive for the RF/IF TUBES I could come up with.
V1- 6gk5-$5.00
V2-6AQ8-$15.00......Sub 6DT8-$3.00
V3 6HR6 $15.00......Sub 6AH6-$3.00
V4 6AU6 $4.00
V5 6AU6 $4.00
V6 6AU6 $4.00
AM TUBES
V7 6BA6 $4.00
V8 6BE6 $3.00
Total FM & AM Tubes with 6AQ8/6HR6 $54.00
Total FM & AM Tubes with 6DT8/6AH6 $30.00
www.abcvacuumtubes.com
You can ask for specific brands. I would suggest Sylvania
or GE Tubes. I've had very good results with both in RF/IF Stacks.
If you're worried about the subs, go with the original numbers, but I use those subs with equivalent results as the originals but with less
squealing from my back pocket. 
Matter of fact the only 6AQ8 I use is in my Sansui 1000A MPX as it doesn't work with a 6DT8 there. Otherwise all my units that require a 6AQ8 use 6DT8's. I've changed all my worn 6HR6's with 6AH6's with NO adverse effects.
V1- 6gk5-$5.00
V2-6AQ8-$15.00......Sub 6DT8-$3.00
V3 6HR6 $15.00......Sub 6AH6-$3.00
V4 6AU6 $4.00
V5 6AU6 $4.00
V6 6AU6 $4.00
AM TUBES
V7 6BA6 $4.00
V8 6BE6 $3.00
Total FM & AM Tubes with 6AQ8/6HR6 $54.00
Total FM & AM Tubes with 6DT8/6AH6 $30.00
www.abcvacuumtubes.com
You can ask for specific brands. I would suggest Sylvania
or GE Tubes. I've had very good results with both in RF/IF Stacks.
If you're worried about the subs, go with the original numbers, but I use those subs with equivalent results as the originals but with less
squealing from my back pocket. Matter of fact the only 6AQ8 I use is in my Sansui 1000A MPX as it doesn't work with a 6DT8 there. Otherwise all my units that require a 6AQ8 use 6DT8's. I've changed all my worn 6HR6's with 6AH6's with NO adverse effects.
