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In Reply to: RE: bias points posted by Medium Jim on January 25, 2012 at 18:29:01
When I flipped the switches to put it in triode mode, I checked the bias (at 10 minutes, 30 minutes and two hours) and it had shifted slightly to the cooler side. It's funny; not since my first time turning it on has the bias meter given a different reading after 10 minutes - started cold and leveled off 'on the mark' for each tube by the one hour point.
I can't give exact bias voltage data because this amp doesn't have accessible test points. It just has a bias meter. Flip the mute switch, then hold each of the eight output tube metering switches while viewing the meter - adjust the adjacent potentiometer as necessary. It's rather ergonomic, if a little less romantic.
Follow Ups:
Waz:
Nice, on my model 9's, I warm them up for about 30 minutes with the gains turned all the way down. They also don't have metering that shows the exact bias, just turn the knob to the selected tube to be biased and bias. Before I do bias the output tubes, I make sure that the AC balance has been adjusted.
Even though you're biasing each tube individually, there're still push/pull and you might need to go through each tube a couple of times to really pinpoint them
I agree not to romantic, but very practical and one advantage is that if one tube does go bad or doesn't hold bias, you don't need to drop in a full set or pair when you have the ability to individually bias.
If you can find one or can get access to a TV-7 tube tester, you will be able to check your tubes. There are other less expensive tube testers, and it is a good thing to have if you do plan on buying vintage tubes. Don't be afraid to buy used vintage tubes as you can really save enough to more than pay for a good tube tester and verify the tubes you are buying.
It appears to me that you have a real good ear and you should be able to hear the difference from the 6SN7's that currently reside in there to some from the 1950's. The 6SN7 sort of went by the wayside around 1954 when the newer twin triode miniature 12AX7, and the rest of the "12" family of preamp tubes came into being. This doesn't mean that they totally disappeared and some of the best were made well into the early 1960's.
I put in NOS Mullard EL34's (xf2 production codes) in my amps and it did make a nice upgrade from what was in there. However, replacing the 6DJ7's with 6922 PQ's ( PQ= Premium Quality) , was even a bigger improvement in the soundstage. I eventually moved in some 7308's that are the cat's meow.
My point is, upgrade the 6SN7's first as they will will provide the most improvement.
You can mix or match your 6SN7's as they don't need to be biased and only need to be within normal limits for the tube itself. I mention this to let you know if you see a couple of nice or even an individual one, go for it/them until you build a full set for your amp.
Sorry for the ramble and hope some of it was worthwhile.
Jim
Music is the best thing you can do legally that doesn't require a medical prescription!
I can remember tubes, like some Raytheons, that one could tap and see no disturbance on the scope. Hell, they were made for equipment that was operated in a moving jeep. They were built . There are lots of options. I should imagine that one must be very careful, however, when negotiating the NOS waters. There be pirates looking to hornswaggle you with their counterfeits - arr.
A proper tester is an absolute must (remember when they were in all Radio Shacks?), unless one limits himself to buying tubes which have been thoroughly qualified by a responsible party. VAC warrants 'their' tubes and the output tubes are available in matched octets ($780). Kevin does state that he designs his products for neutrality, and that neutrality is achieved with the tubes he chose. He doesn't discourage experimentation, but he does remind the tinkerer that he is redesigning the product - not that there's anything inherently wrong with that.
Modifying is just one of the services we inmates provide.
I have to admit that the Raytheons were at the top of my list for my preamp. I got a nice high grade pair that are, well, upscale 6SN7WGTs.
But even within a specific type from a specific manufacturer they are not all created equal. Having a dealer who is able to grade the tubes properly, that you trust, is priceless.
I've never heard anything but good about VAC, I think you made a good choice.
BTW that price is fair for a matched octet of KT88s, I think. What 6SN7s is that amp using?
esande:
Granted, not all tubes are created equal even from the same mfg. Milspec tubes are also not a guarantee that they will be any better than for public consumption. In fact, some of the primo tubes are not military.
There really isn't any per se grading, other than checking for mutual conductance (how much life is left in the tube). A used tube can often test well within the NOS (new old stock) range. Next, what is the ma (milliamperes). Next one tests for gas leaks and shorts. Finally tapping the tube to check for microphonics.
I've stashed away some 6SN7's for some of my vintage guitar amps. I might just go through my stash and see what I have. I would gladly send Wazoo a pair or a quad if I have that many to give him a try out.
They will be used tubes, but I only hoarded the good shit.
Jim
Music is the best thing you can do legally that doesn't require a medical prescription!
Turns out that I only had three in my stash...most guitar amps used the 6SL7. Anyway, I have a nice pair of mixed matched RCA's, one a Smoke Glass from the late 40's and the other from the mid 50's. Both are of the same construction and test within 5% (a tight match).
Waz, I'm not kidding, I will send them to you try out before you start spending money on a set for your amp.
Jim
Music is the best thing you can do legally that doesn't require a medical prescription!
Waz:
For the most part, JAN or USN tubes are more ruggedized and such and less microphonic. Kevin makes a good point in that he can voice his amps based on how current production tubes spec and sound. This why I had to go with tube's from the 60's to get that neutrality. However, with line stage or preamp tubes, there is less effect on the tone, but more on the spaciality.
Jim
Music is the best thing you can do legally that doesn't require a medical prescription!
I replaced the 12AX7's in my CD player with NOS JAN5751's. Can't say if they are counterfit, but they way outperform the 12AX7's that were in there. More of the rich "tube sound" (don't start...). Ditching the tube covers and adding dampers made a noticeable positive difference too.
Regards,
Steve
5751's are a lower gain version of the 12AX7 and will be a little warmer than a 12AX7 as a general rule.
Jim
Music is the best thing you can do legally that doesn't require a medical prescription!
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