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In Reply to: RE: You check the bias every week? posted by Chip647 on April 07, 2016 at 17:48:16
Actually Chip, I run the amp without the top off so I check the bias just about whenever I use the amp. It is very easy to do with Rogue amps and only takes a minute.
Do you have anything constructive to add to this thread or do you just sit there and nitpick other people's posts?
Edits: 04/07/16Follow Ups:
When one claims massive differences in sound quality between two otherwise comparable tubes there is usually more going on. These are not standard KT88 tubes and each has its own electrical properties which may or may not be compatible with the voltages, amperage and loads presented in your amp. Usually when there is consistent failure of a tube, the problem is with how and amp is working with a tube rather than the tube itself. This is the problem that drives tube dealers mad as they keep sending customers good tubes only to have them destroyed by the customer and having to replace them. I am not saying there wasn't a problem with your chinese tubes before you installed them, but that is what my point was: don't assume that you can pass judgement on differences in tubes unless you are 100% certain of the operating conditions (IE using the tube the amp was designed for rather than something that is "compatible"). For such massive differences to be so readily discernible to you points to something other than you listening to the tubes being run optimally.
Here are the facts:
1) The OP JerryS asked for (subjective) opinions of the Tung Sol KT120 vs the Treasure KT120 for use in a Rogue Audio Stereo 100. I own and have used both tube types in a Rogue Audio Stereo 90 which is a very similar amp.
2) Before buying the Tung Sol KT120, I contacted Rogue Audio and was assured that KT120 were fine to use in the Stereo 90.
3) I clearly stated in my post that the Treasure KT120s were one of the best sounding tubes IN THE ROGUE AUDIO STEREO 90 and outperformed the Tung Sols IN MY OPINION IN THAT AMP.
4)I never said that any tube had failed, only that after 2 months the bias on the Treasures had become unstable in THAT AMP and I would check the tubes for gas or other problems. As stated above, I was assured the amp is fine to use with KT120 by Rogue, and have not had any biasing problems with the Tung Sol KT120.
5) I never used the term "massive" in my comparison, I said the Treasures were WAY better in my opinion in the Rogue Stereo 90. I much prefer the sound of the amp with the Treasures, which was what Jerry was asking (as he intends to use the tubes in a very similar amp). The difference was major to me but might be somewhat less to another listener.
6)Since both sets of tubes are KT120s being run in the same amp, my opinion of the sound is not affected by the tubes being run optimally or not, since both tubes are the same type and subjected to the same operating conditions in the amp (which again are fine for KT120 according to Rogue). Your last sentence makes no sense at all.
7) I was only trying to help out JerryS, since I have a very similar amp and have used both types of tubes he was asking about, and he was grateful for the help.
I don't know why my subjective opinions have to be scrutinized by you Chip647. Its my opinion, and if you have a different opinion that is fine. Your entitled to state that opinion (do you actually have an opinion on the tubes in question?). But you are not entitled to belittle my opinions based on incorrect facts.
Edits: 04/08/16
Whatever the so called facts, what IS a FACT is that the grid resistor should be less the 51k or else thermal runaway might occur.
And that seems to be what happened. The amp might not be suitable to use kt120, and that is the main cause of the problem.
I would think Rogue Audio would be aware of the value of the grid resistor and would take this into account when recommending tubes for use in their amps.
There was no thermal runaway, no red plating, etc. The bias was not holding at 35ma and was varying by plus or minus 15ma or less. This was after holding steady bias for 2 months.
Further, none of the above problems occurred with the Tung Sol KT120 (same tube type as the Treasures) which held vary stable bias in the amp. They just did not sound as good.
Unstable bias could indicate that the grid resistor is not small enough to keep the tube in a stable condition.
And a vendor that "don't have a clue" is not uncommon.
Why not open up and examine the value of the grid resistor ( or check the schematics) so we could end this (sub)discussion.
Hmm... if one cannot believe the manufacturer as to what tubes are compatible, then everyone is just screwed, ehH
You know, this is the second time in as many weeks that you've denigrated the use of "alternate" tubes in an amp. It seems to me that you have been wrong both times.
Is the problem what Chip647 complained about? Your moniker indicates you are a dealer; are you sensitive to the idea of "compatible" tubes because you've been burned?
Cerebrate!
I did not know the Stereo 90 could use the KT-120's. Rogue's site seems to imply KT-90s at most.
Indeed, could this be an amp compatibility issue? The Tungsols always settle in rather quickly in my M-180s. In any case, $700 for a re-tube is not compatible with my wallet.
kt120 , at least in "modern tung-sol" variant needs a grid resistor of less then 51k. If you have a larger one, as one typically have on a kt88 amp, thermal run-away might occur.
http://www.mif.pg.gda.pl/homepages/frank/sheets/084/k/KT120.pdf
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