Converting A Fisher 660A to 6L6 tubes !

[Gary Mattis]

Why do you call the RCA bp 6l6gc "Holy Grail"? The holy grail, if you believe in that stuff, was a single cup which was used in the last supper. If you had the last RCA bp 6l6gc then it would be the HG. Otherwise, it is just an rca tube. btw, there are lots of them for sale on Epay and elsewhere. Also i find them to be excellent sounding tubes but Mullard KT66 sound better to me, by only just, in my single ended amp.

I'm not the guy who named them that , it is what people call them ! . I like RCA 6L6 GC output tubes as well as the RCA large envelope ST 6L6 G and GA's. To my ears they sound good . I like American made RCA 6L6 Output tubes. Not sure why they got the name but a good select RCA quad is hard to beat !. They are a reasonable price for what you get . They don't all make the grade at 500 volts but at 400 they usually perform quite well. The RCA 6L6gc is a very good tube !. It is the best 6L6 tube as far as I know and it has earned that name over the year's ! . I have used RCA Output tubes sence the 1960's with my first Heathkit 30 watt 6V6 PP stereo tube amp , My fender champ used 1 6V6 , I eventuality wore the output transformer out in that amp because I played it so much ! . Not one output tube failure with RCA in that little amp ! . Jesus would most likely recommend using RCA 6L6 GC tubes if you asked him ! , might want to ask how to be saved from sin while your at it ? GE and Sylvania 6L6 black plate tubes are not bad either, I just like the RCA sound better. Never used KT88 , is that a foreign tube number like EL34 ?

 

[Tube Radio]

I've never really noticed much difference in the sound of tubes.

Then again I mostly use tubes for antique radios and other LO-FI applications so maybe the differences aren't as noticeable.

 

[Gary Mattis]

I've never really noticed much difference in the sound of tubes.

Then again I mostly use tubes for antique radios and other LO-FI applications so maybe the differences aren't as noticeable.

Your right most old radios sound good but are not really hi Fi quality, but the old 6l6 through 50L6 output tubes in those old radios sound pretty good especially the ones with octal pre amp tubes. Even some of the minature 5 tube radios with 7 pin tubes have a pretty good sound. Sometimes they just got it right and some of the cheapest desktops sound great !. I roll tubes for people sometimes on a repair and they are amazed at the difference in sound sometimes, you don't need the most expensive tubes but some just sound much better than others.

 

[Tube Radio]

Agreed.

I often prefer big band & swing and doo-wop played through one of my antique console radios. Even though they aren't HI-FI the music just sounds right coming from the consoles.

 

[Gary Mattis]

Agreed.

I often prefer big band & swing and doo-wop played through one of my antique console radios. Even though they aren't HI-FI the music just sounds right coming from the consoles.

I have a 1963 Magnavox Class A EL84 stereo console that has a cheap original ceramic cartridge that you would think was a very expensive magnetic cartridge if you listen to it, I still don't know why it sounds so good ?

 

[Tube Radio]

Because it's Magnavox.

One of my bucket list items to hear is a three channel Motorola console.

 

[Gary Mattis]

Because it's Magnavox.

One of my bucket list items to hear is a three channel Motorola console.

That is what I have a 3 channel console, it has a sub in the middle. I Don't know much about it. I just replaced the caps in it and got a new needle for it.

 

[Gary Mattis]

I just got back to working on the 660 Ambassador and put the Original 7591 tubes in it , I listened to it all night last night with the 6L6 RCA tubes and I will do the same with the 7591 tubes. I have had it running around a hour now and the tubes we're kind of slow at first but seem to be getting better the longer they run. All the iron seems to be running cool now and did all last night with the 6L6 tubes. The 7591 are different from 6L6 kind of like Telefunken vs. Mullard . More sterile sounding but I don't think that they sound any better than The RCA 6L6 just sounds different from them. There's a sweetness The RCA6L6 have that the 7591 don't have , but I do like the 7591 tubes very clear mid rang and sharp highs, tight bass. I have 12" alnico's and two parallel 5" mids and Jensen horns Magnavox crossovers and Sansui air suspension cab's. I was not happy with the good Fisher OT because the bass was Very flabbie. What I ended up using was two EL84 OT's , they sounded the best in it to my ears. The Fisher OT ran very hot also . I'm wondering if both Fisher OT's we're bad even if one was still working because it was heating up the PT more than I was comfortable with. Anyway I can say that the Original 7591 tubes sound good to me from what I hear so far , the temperature is good on the tubes so far, they tested good at 100 percent.

 

[Gary Mattis]

I just got back to working on the 660 Ambassador and put the Original 7591 tubes in it , I listened to it all night last night with the 6L6 RCA tubes and I will do the same with the 7591 tubes. I have had it running around a hour now and the tubes we're kind of slow at first but seem to be getting better the longer they run. All the iron seems to be running cool now and did all last night with the 6L6 tubes. The 7591 are different from 6L6 kind of like Telefunken vs. Mullard . More sterile sounding but I don't think that they sound any better than The RCA 6L6 just sounds different from them. There's a sweetness The RCA6L6 have that the 7591 don't have , but I do like the 7591 tubes very clear mid rang and sharp highs, tight bass. I have 12" alnico's and two parallel 5" mids and Jensen horns Magnavox crossovers and Sansui air suspension cab's. I was not happy with the good Fisher OT because the bass was Very flabbie. What I ended up using was two EL84 OT's , they sounded the best in it to my ears. The Fisher OT ran very hot also . I'm wondering if both Fisher OT's we're bad even if one was still working because it was heating up the PT a lot more than I am comfortable with. Anyway I can say that the Original 7591 tubes sound good to me from what I hear so far , the temperature is good on the tubes so far, they tested good at 100 percent. I put a NOS Mullard in stage 1 of the tuner section, the PI was noisey with Telefunken tubes, put two new China tubes in and it is quiet now. Seems like one NOS Mullard and the rest China preamp tubes are fine. Made a cabinet for it.

 

[Gary Mattis]

The 7591 tubes seem good to me, one is glowing with the music a little bit ? , Sounds great at full volume, a little sterile down low compared to the RCA 6L6's. They basically sound just like Westinghouse tubes to me. The warmth of RCA is just not in any Westinghouse tubes unless they are rebrand RCA's..

 

[62caddy]

Glad to hear the 7591s ultimately brought satisfaction.

 

[Gary Mattis]

They sound really good at full volume. The guy who traded it to me broken for labor wants to buy it back from me now ! . He wants me to find someone to rewind the old Fisher OT's and put them back in , I told him I am not sure if I want to sell it or not ! , This is actually the best stereo I have ever had and I don't want to give it away for peanuts if I can enjoy it. I think it sounds great now even with the replacement OT's that I put in there. I can't get the right OT's (849-116) anywhere no one is making them for a 660 model. It is 8 ohm only now without the NFB , the only reason I would change the OT's back to Fisher is to hook back up the NFB. But it sounds good without it. I think I will keep it at this point , all the iron is running cool at full volume and other than that one output tube heater glowing a little funny nothing else seems to be a problem. I was wondering if that could be cathode stripping from start up or damage from the bad OT that I replaced. Anyone know what the wattage output is with this amp ? By cool on iron - I mean warm to the touch, not really hot. My homemade speakers take full volume on this thing and sound good. Going to try some top of the line Polks Thursday night on it and see how it sounds. I just wanted something to compare my speakers to. I'm guessing that I need Klipsch speakers for it ? Have not rolled any good pre amp tubes in it yet but I will later. Just stage one in tuner has a NOS Mullard. I have Ei, Telefunken, Mullard, Amperex and Black long plates to try. I'm guessing speakers dictate what tubes are best in there to some extent. My pre amp tube choice hands down is Vintage Mullards, But I have found a few China tubes quite good lately , I never did before but I can't argue with quiet idle and 104 output. Question is will they last or not ?

 

[Gary Mattis]

Yes, that's why I said it was too bad you didn't already have some so you could compare.

Design Maximum Values - Push-Pull Class AB1
6L6GC: Plate Voltage - 500, Screen - 450, Max Plate dissipation - 30 watts
EL34: Plate Voltage - 800, Screen - 500, Max Plate dissipation - 25 watts
7591: Plate Voltage - 550, Screen - 440, Max Plate dissipation - 19 watts

As you can see, the 6L6GC and 7591 are very close in voltage handling, although it is rated for lower wattage overall.

Anyway, was not trying to convince you to like one over another. I have no preference personally. Just thought it would be interesting to be able to hear all 3 and compare.

I , I got the original 7591 Fisher tubes and put them in but I don't think that they sound any better than the 6L6 or El 34 did in there except it sounds more sterile now , seems like the RCA 6L6 were richer and smoother sounding, the Fisher tubes are the square hole 7591 tubes. I would like to try some Mullard EL 34s for comparison against the 7591s and 6L6s , the difference is not really even noticeable between the 6L6 and 7591 as far as definition goes. It is more a matter of sterile vs. richness in my opinion. I think from past experience with tone quality that the Vintage Blackburn Mullard EL 34s cannot be beat and that the RCA 6L6 come in second with tone. I don't have the money for a quad of Vintage Blackburn Mullard El34s so I guess I will just leave the 7591 tubes in it but I am not very impressed with the sterile sound of the 7591 tubes. Sounds like very defined but sterile crappy sounding Westinghouse or Sylvania output tubes to me , I never liked the sound of any Sylvania or Westinghouse output tubes. The RCA 7868 tubes sound really good in the Fisher 400s. Those Seem much better sounding than the 7591s that are in my amplifier. The only real problem is that the 7591 tubes sound sterile to me. Definition is good but sterile is bad. There are times when the tubes try to shine a little bit in the mids and highs but it is not the normal sound for the 7591 tubes something about them makes them more sterile than sweet sounding. It is Almost like they are afraid to try to sing. Tubes with stage fright would be my best description of the 7591 tubes, I see why they decided not to use them anymore. It is not a terrible sound and you can get used to it and live with it. Just not a rich shiny sound, I tried it on 3 different Set's of good speakers for comparison, and my buddy thought that a 30 watt 6V6 amplifier he had sounded better than the Fisher with 5791 tubes , On the other hand He liked the amps sound with the 6L6 RCA's when he first heard it. The tubes are the only real difference between the two times he listened to it. I think the guy is right , we used a turntable and record for the test. I'm not sure what to do with it at this point, leave it or go back to 6L6 and EL34 wiring. Maybe if RCA made some 7591 tubes of their own brand it would have better tone quality than the Sylvania or Westinghouse tube's ? Not sure if RCA ever made any 7591 tubes or not ?

 

[kvflyer]

I... Not sure if RCA ever made any 7591 tubes or not ?

I don't think so. I have dozens (literally) and most (if not all) are Sylvania. Hitachi and Toshiba made them in Japan.

 

[Tube Radio]

In order to do a proper comparison, the amp would have to be rebiased for each tube type.

 

[Gary Mattis]

In order to do a proper comparison, the amp would have to be rebiased for each tube type.

It is and was biased properly, or I guess it was , I don't remember exactly because I used both the EL34's and 6L6's in it . I liked the 6L6's best but I don't have any Mullard Xf2 EL 34's to try in it either. Th 7591s are kind of dry, flat and sterile sounding tubes when compared to the EL34's and the 6L6's , I left the 7591's in there just because it came with them , not because I thought that they sounded any better than the 6L6 or EL34. I would not go out of my way to purchase the 7591 tubes, that is for sure. I was given the Fisher quad so I put them back in. I would not say that they suck because definition is good, but they are no Mullard's or RCAs tone wise. The 7591 is more of a Classical music tube is what I think about them. For that dry kind of music they would probably be good. But I don't listen to that stuff and I prefer Vintage Blackburn Mullard XF2 EL34's and Vintage Holy Grail RCA 6L6 GC. I don't like Telefunken 12AX7 tubes , I prefer Mullards or Amperex instead due to the dry and sterile sound. The Westinghouse, Sylvania and Telefunken tubes just don't sound very good to me in most situations. I even think that much cheaper GE output and 12AX7 tubes sound better than those 3 brand of tube's. But Vintage Blackburn Mullard EL34 and 12AX7 are my favorite tubes. In 6L6 Vintage RCA'S first. Vintage Tung Sol 5881 second and GE 6L6 GB or GC in third. The 5881, GB or GC is according to the plate voltages. Tubes are like tasting food with your ears. I have RCA 6L6 in one guitar amp and 5881 Tung Sol in another. I use the RCA tubes most of the time, almost always. I do like the 7868 variant of the 7591 used in Fisher 400's and in Bogen CBH 100's, it seems that in those two amplifiers they really sound good. Just show's that they can sound good and that their are some acceptions. In both cases the tubes seems to work very well. I am actually replacing the Fisher 660 Ambassador with two Bogen CBH 100's Mono's with the first tube changed to a 12AX7 , I like the attitude of two 100 watt 7868 Mono block's. I have always preferred 100 watt tube power amplifiers over 30 or 50 watts. I'm not knocking Fisher tube amplifiers at all and I think they sound great and are very good deals, I just want to take it up a step with two 100 watt Mono block tube amps. And I would like to try some different Speakers out also maybe a Pair of Klipsch speakers with horns . I don't know that much about audio speakers and I need to learn some more about them. I am still using a Pair of homemade Sansui 4 Way's that I made with Paper cone Alnico 12's, two 5" mids and Old Jensen horns with a Magnavox crossover. They don't sound bad but I am sure that they make much better speakers than what I made. They are just about perfect for the Fisher 660 A , it seems to really like them but with my new 100 watt amp project I will have to up the speaker game. It is kind of funny that I started with no tubes and a 6L6 conversation and I ended up being given the original 5791 tubes. I'm kind of over the Whole project now and I have learned a little bit more about tubes and I think it was a success and it worked well but I don't know if I would want to go through it over again. The preamp is clearly designed for the 5791 and it still works fine with EL34's and 6L6 tubes. It is a give and take thing because they make much better sounding 6L6 and EL34 tube's, and with a lot more choices. But because of definition it sounds OK with the 7591 tubes in it. I would say do it if you want to. I did not go to the extreme on the conversation and I compromised it some and I am sure that you can make it work even better than what I did with a few preamp change's. I wanted to be able to go back to 7591 so I limited my modifications. That was the right decision for me considering that I was later on given a free quad of Fisher 7591's. I liked the sound with all 3 tube type's in it and I can't say that the 7591 blew away either of the other tube's by definition either . Just 3 different sounding tubes.

 

[Tube Radio]

If you want good speakers and don't have an issue with running a powered sub look at the electrovoice FR-200 speaker.

I have a pair running on a 25WPC solid state amp and they sound amazing.

Probably would sound equally good with tubes.

 

[rl1856]

I acquired a 660a a long time ago, and had BuzzyBee (Paul Gryzbeck RIP) restore and covert it to run EL34 outputs. I did not intend to use the companion tuner-pre thus I knew there would be enough excess voltage available from the OT to safely accommodate EL34 tubes. The resulting amp sounds great, and runs cool. Frequency extremes are well defined. I recall Paul saying the amp delivered abt 22-23wpc with a clean square wave. I use original Sovtek (pre legal squabbles-new sensor) EL34 and Telefunken <> 12ax7. Good under rated amp, as are the 440 series of EL84 amps.

 

[tonyg88]

This thread is a great read

 

[GordonW]

Not sure if RCA ever made any 7591 tubes or not ?

All RCA labeled 7591s should be either relabeled Westinghouse or Sylvania tubes. They never made a 7591.

In fact, the 7868 was RCA's effort at circumventing the copyrights on the 7591. Same parameters, different base, different licensing.

Regards,
Gordon.