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7868 & 7591 Tubes

sony6060

sony6060

Super Member
Banned
I am unfamiliar until recently with the 7868 tube family. I understand the 7868 are same as 7591 tubes except the socket requirements. Anyways, I am getting just ridiculously good sonics out of these tubes. Like the 6BQ5, these 7868 are winners IMO. My favorites are RCA black plate & 1960s Sylvania. Anyone experience the same?
 
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I primarily use the Sylvania 7868 tubes in my 400 and I always thought they sounded great.
My dad had some nice RCA black plates in his 400 and he thought they were best sounding
output tube that he had tried.
I think most people who hear a good working Fisher 400 or some of the other amps that use
the 7868 tube find them to be winners.
 
I have 2 400s, one with Sylvanias and one with RCAs.

Both sound great, tho I've never done an A/B compare.

You can't lose with either 7868 tube IMO.

Cheers
 
tubeboob said:
I have 2 400s, one with Sylvanias and one with RCAs.

Both sound great, tho I've never done an A/B compare.

You can't lose with either 7868 tube IMO.

Cheers
Click to expand...

I'd be interested if you do A/B compare. I just purchase a pair of Sylvanias and a pair of RCAs. And they'll be going to my 400 that has very worn 7868's.
 
The 7868 is a great tube. I have a quad of new production ones in my sherwood receiver and it sounds excellent. The 7868 was one of the last designed power tubes before the switch over to transistors. Even though the 7591 is "electrically identical", many people seem to think they sound different, leaning towards the 7868 sound.

They also look way cool, like an overgrown 12AX7.


Have fun!!
 
What is the history of the 7868?

This place has a little data on it. http://www.radiomuseum.org/tubes/tube_7868.html The French says:

"comparaison entre 7868 et 7591
Si les deux tubes 7591 et 7868 sont proches, ils ne sont pas simplement identiques à part le culot : l'analyse détaillée des données techniques montrent que dans les mêmes configurations, le 7591 tient un courant de repos plus important, et surtout que ses réponses en distorsions sont meilleures que celle du 7868. "

"Comparison between 7868 and 7591
If the two tubes 7591 and 7868 are close, they are not simply identical except for the back/base: the analysis in detail of the technical data shows that they have similar configurations, but the 7591 has a rest current that is larger, and in the distortion response, it (7591) is better than the 7868"

Close enough it looks like, but you may want to push it less to keep distortion in check?

Our buddies at Tronola did a comparison and it looks interesting. I have a personal interest in this tube 'cause I own a x202b and the 7591's $$ are getting out of control, as noted. http://www.tronola.com/html/7591a_tubes.html

Would like to know from y'all who have done socket subs for other tubes, is the result worth it or does it ruin resale value of the piece? :confused:
 
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Just as Tung Sol was the father of the 6550 (1955) -- which spawned imitators such as the KT88 (1957) and all its siblings, Westinghouse was the father of the 7591 (born in 1958), which spawned its own line of imitators as well, being RCA's 7868 (1960), and Sylvania's 6GM5 (1961).

All three are electrically identical for all intents and purposes, but have different bases, because each manufacturer was trying to claim its own niche in the market for tubes of this power class. The 7591 was largely the winner is this regard with its standard octal base, while the 7868 did well in its own right with its Novar base. The miniature 9 pin based 6GM5 found very little market action however, being the last to show up at the dance, with production already well established for the other two tubes.

Dave
 
Prices for 7868 tubes are getting scary, I do like the prices and availability of the 6L6
tube much better, not sure on how much it would change the sound of an amp doing a
conversion.:scratch2:
 
I thought 6L6 types are harder to drive unlike 7591 types which are in the easy to drive EL84/6BQ5 dept.

Randy
 
Shadowdog said:
I thought 6L6 types are harder to drive unlike 7591 types which are in the easy to drive EL84/6BQ5 dept.

Randy
Click to expand...

At the typical 400 volts plate & 350 volts screen as used in a typical 7868 amplifier with fixed bias a 6L6 is biased about -27 volts & has a rather high plate current of 55ma per tube. Some amps may not have enough drive voltage and especially so if a cathodyne phase splitter is used.
 
I've always thought that for the most part, the 7868 jobs from the same makers, kill the 7591 rigs esp. in dynamics and hard solid woofer control. I know, it's the same tube, yadda yadda. :dunno:
 
sony6060 said:
Interesting.
Click to expand...

I know, it's weird, but not subtle at all. I especially notice it on Sherwoods that have used both types of tubes in different generations. I really never cared for the sound of the 7591 until I got a 7591 amp with Acro iron and regulated screens (I'm not big on UL taps). I remember when I got my first 7591 job, a Fisher KX200. I gotta say I was pretty disappointed, because I was expecting a big bump up from the 6BQ5 amps I was running just because the tube was so much bigger and the advertised power was bigger. I didn't run the 200 for a week, and it's still sitting on a shelf. Crazy, I know. I'm going to mod the crap out of it someday and get rid of all that center channel mess. The huge OPTs have to sound good if fed the right diet.:yes:
 
jaymanaa said:
I know, it's weird, but not subtle at all. I especially notice it on Sherwoods that have used both types of tubes in different generations. I really never cared for the sound of the 7591 until I got a 7591 amp with Acro iron and regulated screens (I'm not big on UL taps). I remember when I got my first 7591 job, a Fisher KX200. I gotta say I was pretty disappointed, because I was expecting a big bump up from the 6BQ5 amps I was running just because the tube was so much bigger and the advertised power was bigger. I didn't run the 200 for a week, and it's still sitting on a shelf. Crazy, I know. I'm going to mod the crap out of it someday and get rid of all that center channel mess. The huge OPTs have to sound good if fed the right diet.:yes:
Click to expand...

Your info is great. I picked the Fisher 400 due to I prefer fixed bias amps, NOS 7868 were more reasonable in cost vs NOS 7591 (bought 3-1/2 NOS quads plus one RCA black plate emergency back up) & the more simple Fisher 400 design or they say so. Also, the later 400s like I own (later 500s & 800s too) have the extra IF section- nice. :D
 
I do like the fixed bias myself and the 400 is a nice simple amp to work on and maintain, luckily I have a
couple extra NOS 7868 Sylvania tubes.
I should get some more backup tubes before the prices get even higher, Oh I guess I will have to wait until
that tax return money finds it’s way into my bank account.
 
BrokenHill said:
I do like the fixed bias myself and the 400 is a nice simple amp to work on and maintain, luckily I have a
couple extra NOS 7868 Sylvania tubes.
I should get some more backup tubes before the prices get even higher, Oh I guess I will have to wait until
that tax return money finds it’s way into my bank account.
Click to expand...

I will purchase two more Sylvania 7868s. Once bought, I will have two NOS quads of Sylvania, have one NOS quad RCA 1960s grey plate & using one NOS quad RCA black plate. If I can get 8000 hours use per quad, I will have 32000 hours worth of 7868 tubes. Obviously, kind of worried about future NOS stocks. I read a Telefunken 12AX7 can last up to 50000 hours.
 
I have only had one 7868 tube go bad on me in the last five years and the one 400 that I sold used to see
about six to eight hours a day of use, when rebuilt and in good working condition those Fisher 400 amps
do get good mileage out of the output tubes although I don’t push them hard I use them for lower volume
listening, if I need loud volume I switch to the Hafler DH-500 for that purpose.

If you watch the online auction site closely you can find the Sylvania 7868 tubes under other labels such
as Conn or Dumont and sometimes get them a little cheaper.

12ax7 tube are plentiful and still easy to find although some of them can get pricey, I have also heard the
Telefunken tubes can last up to and even more then 50000 hours.

I do use a thermister on the 400 for a soft start, I think that helps the tubes too.
 
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