Groove Tubes ratings meaningful if Cathode Biased?

GT's ratings are an indicater of how quickly the tubes distort (1 being the quickest).

 

....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.

 

...which still holds true with cathode biased amps.

 

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.

 

Very interesting info!

Thanks Mike!

 

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.

 

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... )

 

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?

 

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.