Is the Groove Tubes rating system meaningful for a cathode biased amp, or does the amp adjust itself resulting in nearly the same sound for the entire GT range 1-10? Mark
....which is typically nullified if not reversed in a cathode bias amp. "Self biasing" amps "self bias" within limits. Some of them bias tubes to the point of destruction. It's always best to check the bias current just to be safe. A GT 5 rating is usually a safe bet.
Not necessarilly. The GT numbers assume that the tubes are going to be biased correctly. In cathode biased amps you never know whether they'll be biased correctly or not. Further, in any amp, tubes set for high idle current tend to break up sooner than those set for low current. Tubes with higher GT numbers draw more current than lower numbered tubes. Therefore, the higher numbered GT tubes (which supposedly run cleaner when biased correctly) will tend to break up sooner in non bias-able amps, including "self biased" amps, than lower numbered tubes.
The tube rating thing is kind of funny, IMO funny bad. Back in time tube manufacturers never offered rated tubes, the reason is simple; Any particular designation has operating specs. If the tube was "out of spec", it was rejected. Meaning, a 6L6GC that had an unusual bias draw, or lack of, wouldn't operate as intended(and is really not a 6L6GC anymore. Ever buy a set of tubes, and your amps bias control wouldn't adjust far enough, or took an unusual voltage?). A perfect example would a "10", or a "1" on bias rated tubes, I would avoid these. Mike's suggestion here of using a "5" as a safe bet is good advice, and what I would recomend. However, in my shop I only use tubes that meet the correct bias specs for their desginations. Tube data can be found all over the net if anyone wants to see how the tubes we're using in our guitar amps should be operating (most tube manuals give examples of output tubes in audio circuits, these show bias/voltage/current draw/output ect..). At the least, the manufacturers today need to let the public know this. It's never ending.
OK, Thanks all, especially Mike, for your replies! Now why am I asking the question? I have a cathode biased amp--its a Carr Rambler--that seems overwhelmed, even at LOW volumes (like 9'oclock on the amp vol), when I have amp settings low on the middle knob and the guitar rolled back just where treble kicks in. It's really noticeable with the rhythm pickup. Sounds like I have a (light) fuzz box anytime I play more than a single note. I play Chet style, nice and clean and lo volume--no thrash or anything like that. Primary guitar is a Gibson CG, but I have others with P-90s, etc. All behave similarly. After trying all kinds of different pre-amp tube substitutions and a different speaker, I can't seem to get rid of the problem. Maybe all Carr Ramblers just sound this way in these circumstances. I'm left, perhaps, with output tube options. The originals, in the amp when I recvd it used, were Ruby Svets. Just a couple weeks ago I bought new TAD6L6WGC's--everybody is raving about them, you know. Because I had not played the amp a lot before installing the TADs, it was then that I first noticed this distortion issue. Since then, I've A/B'd both sets of output tubes, noting that both produce the undesired distortion under the conditions I've described above. Since these tubes are rated for PC, I noted that one set has PC:39 (Svets) and the other PC:43 (TADs). [TC was dramaticallly different between the two sets, but I'm not concerned about that, yet . . . .] Anyway, I'm wondering if I should try a set of tubes with a PC of about 25 or something, just to see if that would make a difference. Yes? No? Maybe? Thanks, Mark
Oh, I should add, the problem occurs most noticeably in Pentode Mode and that there seems to be more of a hiss in Pentode than I would expect from a high quality amp like Carr. Various preamp tubes seem to have some effect on the hiss level--quietest with Telefunkens in V1 and V4, but still more noticeable than I would have thought.
I'm not sure you'll ever get the amp totally clean but a lower rated pair of power tubes could help. Have you tried a 5751 in the first gain stage? Does that amp use a tube rectifier? If so, which one? Switching to a higher rated rectifier could help as well.
Does anyone else find it odd/ironic/amusing that non-adjustable fixed-bias amps (like Mesas) are often condemned around here for being 'fixed' - despite mostly being set quite conservatively and sensibly for a correct-spec tube of the right designation - whereas cathode-biased amps, which in fact not only can't be adjusted any more easily but also negate the ability to even choose different tubes to adjust the bias, which is often too hot, are favored for not needing adjustment? Think about it. (Not aimed at anyone in particular, just me musing out loud... )
Yes, I did try a JG-5751-WA in V1 with little noticeable difference. The Carr has a fancy new technology solid state rectifier. You could read about it at carramps.com or in one of the reviews linked at his site. Thanks, Mark
I'm with you John. I'm no expert, but there are just so many contradicting views about bias, tube rating, matching and the like that it can be hard to make sense of it. I think most of this confusion stems from the companies that rebrand tubes and give them ratings. I like the sound/feel of most cathode biased amps I've owned, but I've found that these burn through power tubes real quick when compared to the fixed (adjustable or not) bias amps I've owned.
Did you try it in V2 as well? I forget how the preamp is laid out in that amp, but it may make more of a difference there. I had some noise in my Slant6V that drove me nuts, until I discovered that it was the power at my place that was the culprit. I plugged it in at my folks house and it was quiet. Have you tried plugging it in somewhere else?
I discovered that Myles Rose suggests a lower rated (GT Rating) tube might cool down a cathode biased amp a little. You nave to look under cathode bias in his section on biasing. See http://www.guitaramplifierblueprinting.com/Primers.html.
Well, V2 is all reverb stuff, and I think I ran the circuit with that tube out completely, making no difference in the problem.
One day my mains were low, about 108 volts, but on the next day, they were at 118-120 and I still had the problem. Taking the amp to another location is about the only thing I haven't actually tried, mostly because the mains tested normal when I was still having the problem. But that does not preclude other line issues, I suppose.
My voltage was fine it was the actual old wiring/bad grounding that was the issue in my case. I had forgotten that the Rambler only used 1 tube for the preamp.
