Suggestions on output tubes, please !

[davis419b]

What output tubes would you recommend for a Fisher 400. I just had it restored and the original Fishers were replaced with Electro-Harmonix 7868EH tubes. Had very bad luck on the purchase. First matched set had a cracked tube and the second set had one of 2 little plate things (dont know what they are called) on the top that had fallen off and laying beside the glass. Ended up having to use a mismatched tube for the weekend. Before the restoration it sounded the worst on the khorns and best on the heresy,s now it sounds best on the khorns and the worst on the heresy,s. I am confused and any help would be appreceiated

[tube fanatic]

I recommend going with NOS tubes if cost isn't a major issue. Mike Marx can set you up with two matched pairs: http://www.vacuumtubes.com/7868.html. It's hard to speculate as to why you experienced a reversal of the speakers which sound best with it. Are you able to verify correct bias adjustment yourself once new tubes are installed? If not, perhaps you can find a local technician who can assist you with that.

[davis419b]

I recommend going with NOS tubes if cost isn't a major issue. Mike Marx can set you up with two matched pairs: http://www.vacuumtubes.com/7868.html. It's hard to speculate as to why you experienced a reversal of the speakers which sound best with it. Are you able to verify correct bias adjustment yourself once new tubes are installed? If not, perhaps you can find a local technician who can assist you with that.

My tech is local and he re-biased it before I left with it. He wanted JJ,s but couldnt locate any. I am looking to see what brands others are using.

[NOSValves]

JJ does not make a 7868.... The EH is all that is available modern production. I have never had issues with them and have used many quads over the years. Not sure where you sourced them. I use Doug at www.dougstubes.com he does an exceptional job weeding out the bad ones before they land in your hands and will stand behind the tubes. The problem you will have going back to NOS types now is that the EH's have been forced down in the sockets so the NOS tubes will fit very loosely in the sockets and cause all kinds of issues if the socket contacts are not re-tensioned for them. The EH has much larger diameter tube pins that have to be forces into the original sockets the first time which is a nerve racking experience. I bet that is why you had one that with a cracked glass. I'd advise you to not give up and the EH 7868 just get a set from a reputable tube dealer that will stand behind them. Also make sure your local tech is biasing the tubes to draw no more than 28mA per tube. Every time I hear "local tech" I cringe, so many of them are total hacks (keep in mind I'm not saying your local tech is a hack)...

Craig

[davis419b]

JJ does not make a 7868.... The EH is all that is available modern production. I have never had issues with them and have used many quads over the years. Not sure where you sourced them. I use Doug at www.dougstubes.com he does an exceptional job weeding out the bad ones before they land in your hands and will stand behind the tubes. The problem you will have going back to NOS types now is that the EH's have been forced down in the sockets so the NOS tubes will fit very loosely in the sockets and cause all kinds of issues if the socket contacts are not re-tensioned for them. The EH has much larger diameter tube pins that have to be forces into the original sockets the first time which is a nerve racking experience. I bet that is why you had one that with a cracked glass. I'd advise you to not give up and the EH 7868 just get a set from a reputable tube dealer that will stand behind them. Also make sure your local tech is biasing the tubes to draw no more than 28mA per tube. Every time I hear "local tech" I cringe, so many of them are total hacks (keep in mind I'm not saying your local tech is a hack)...

Craig

Hey Craig, Thanks for the reply. The tubes came from Parts Express. He had the tubes biased at 28mA before I found the bad tube. When he put the mismatched tube in the bias on it was 24mA so he raised it to25mA which put the others to 29mA. What will this do to the sound ? Should I go back and try to get a third quad of matched tubes so they can be biased correctly. Your help is very much appreceiated !

[NOSValves]

Hey no problem I enjoy lending a hand when I have time. Well the standard spec is plus minus 15% on the output tubes so your within that spec. It won't hurt a thing to run it.. Technically the amp will make a small amount more distortion out of that channel but I'd be shocked if you could hear the difference. If you want them to match you should not have to buy another quad. If you have the boxes the 3 that bias at 28mA came in then you should be able to call Parts Express and get another matching tube from the factory match numbers on the tube boxes. By the way Parts Express has no burn in and testing setup for tubes they only use the factory matching. But since you already have 3 good ones I'd just buy another match to the 3 good ones if you have the numbers. Also Parts Express is pretty good about covering bad tubes you can send the bad one or even cracked one back and I bet they replace it for free.

Craig

[tube fanatic]

Keep in mind that only the tubes for each channel should be matched, so you don't need quads, just pairs.

[NOSValves]

Keep in mind that only the tubes for each channel should be matched, so you don't need quads, just pairs.

Ummm no you are incorrect. The Fisher receivers have no means to adjust the bias per channel in fact if not modified have no means to adjust bias at all! So you really should have reasonably matched quads. It would be fairly common if buying pairs to end up with a pair one channel pulling 15mA each and the other 30mA each... In fact for best sonic performance ALL TUBE AMPS SHOULD HAVE REASONABLY MATCHED QUADS. There is more to matching then just the bias current when it comes to Sonics... That is why the best tube sellers will indicate both current and GM matching.

Craig