? about cathode biased amps....

[terrapin]

With a Cathode biased amp if you changed from 5881power tubes to 6L6 power tubes do you simply install and let the amp do the rest?

And, what would be the sonic differences between 5881's and 6L6's?

Thanks

[TweeDLX]

With a Cathode biased amp if you changed from 5881power tubes to 6L6 power tubes do you simply install and let the amp do the rest?

And, what would be the sonic differences between 5991's and 6L6's?

Loosely, yes. The amp doesn't "do" anything, it just sits there. It's not a bad idea to at least check the bias after changing tubes (Yes, even if it is cathode biased). Different tubes can run hotter or colder, and depending on where you have the bias set, you may run the risk of red-plating your tubes.
I'm assuming you meant the difference between 6L6GC and 5881's, not 5991's. Most 6L6GC tubes have more headroom, and sound a bit "bigger", and punchier.

[donnyjaguar]

I thought these tubes were basically the same thing?

[TweeDLX]

I thought these tubes were basically the same thing?

5881 = 25watts; 6L6GC = 30 watts; and plate voltage requirements are pretty different as well (but you already know that!). I'm talking "true" 5881's, and not the Sovtek relabeled 6P3E. I hear a marked difference in tone between the two types.

[frankie5fingers]

I switch between 5881 (NOS, well I guess they're ANOS now) and 6L6GC regularly with a cathode bias tweed. As TweeDLX mentions, I made certain that it could handle the voltage difference first, but once you're comfortable with that, it's just a matter of swapping one for the other.
FWIW, obviously there's a bit more headroom to be had by adding a few watts. My criteria are about holding the bass together, glassy highs, strong mids, and of course, breakup and when it occurs. I've found far more difference in tone by using NOS over new manufacture than by switching from the 23 watt 5881 to the higher wattage 6L6GC, though. I've used RCA and GE 6L6GCs, Tung Sol, RCA (Sylvania?), and Sylvania 5881s, as well as a bunch of new manufactures. Using the 6L6s let me put a 12AX7 in V1 (either a Raytheon Long Black or Mazda Long Silver), with a 5751 in V2. With 5881s I prefer a 12AY7 (like Raytheon) in V1.
The biggest and glassiest is what I expected, the RCA 6L6GC. I found the GE to be unexciting, not muddy but not sparkly either.
I have just about as much clean high end by using a nice NOS 5881, even with a 5751 or a brighter 12AX7 (Amperex et. al.) in V1.
The Sylvania is my favorite. I get all the headroom I need, plenty of glassy highs, strong mids, and enough breakup for anything I want to do.
For the new stuff, the New Sensor TS Reissues are OK. They have a pleasant, warm sound, they break up pretty quickly and the bass doesn't fall apart too badly; probably a good pick for higher gain playing. My current production favorites are Winged C 6L6GC or the GT 6L6GEs for a little earlier breakup. My .002

[terrapin]

I switch between 5881 (NOS, well I guess they're ANOS now) and 6L6GC regularly with a cathode bias tweed. As TweeDLX mentions, I made certain that it could handle the voltage difference first, but once you're comfortable with that, it's just a matter of swapping one for the other.
FWIW, obviously there's a bit more headroom to be had by adding a few watts. My criteria are about holding the bass together, glassy highs, strong mids, and of course, breakup and when it occurs. I've found far more difference in tone by using NOS over new manufacture than by switching from the 23 watt 5881 to the higher wattage 6L6GC, though. I've used RCA and GE 6L6GCs, Tung Sol, RCA (Sylvania?), and Sylvania 5881s, as well as a bunch of new manufactures. Using the 6L6s let me put a 12AX7 in V1 (either a Raytheon Long Black or Mazda Long Silver), with a 5751 in V2. With 5881s I prefer a 12AY7 (like Raytheon) in V1.
The biggest and glassiest is what I expected, the RCA 6L6GC. I found the GE to be unexciting, not muddy but not sparkly either.
I have just about as much clean high end by using a nice NOS 5881, even with a 5751 or a brighter 12AX7 (Amperex et. al.) in V1.
The Sylvania is my favorite. I get all the headroom I need, plenty of glassy highs, strong mids, and enough breakup for anything I want to do.
For the new stuff, the New Sensor TS Reissues are OK. They have a pleasant, warm sound, they break up pretty quickly and the bass doesn't fall apart too badly; probably a good pick for higher gain playing. My current production favorites are Winged C 6L6GC or the GT 6L6GEs for a little earlier breakup. My .002

Great info..................Thanks. I have a matched pair of Winged C 6L6GT's and a matched pair of Sylvania 6L6GT's I am gonna try in my Juke Coda. I prefer 7025 preamp tubes, so plan to go with those. What do you think??

[frankie5fingers]

I guess the 7025 is a "ruggedized" (is that a word?) version of 12AX7. I think they tend toward an old Mullard, right? I'm not a tube expert by any definition, but I really like the Amperex over the Mullard. My opinion clearly, but I find the Herleen(?) Holland manufactured Amperex to be quite glassy, with very musical presence, if that makes any sense. I'm a real fan of the Raytheon long black plates too. I think if you're looking for tight and glassy, that's a great tube to go with, relatively inexpensive too.

[rockon1]

I guess the 7025 is a "ruggedized" (is that a word?) version of 12AX7. .

The 7025 is a low noise version. The 6681 is a "ruggedized" version. Bob

[terrapin]

The 7025 is a low noise version. The 6681 is a "ruggedized" version. Bob

ABSOLUTELY.....and I love them!

[InyoTim]

It's not a bad idea to at least check the bias after changing tubes (Yes, even if it is cathode biased). Different tubes can run hotter or colder, and depending on where you have the bias set, you may run the risk of red-plating your tubes.

I was testing my collection of 7591's in a recently bought Ampeg that is cathode biased. One of the tubes red-plated after a few minutes of playing. I did the 1 ohm resistor mod to pin 5 of each power tube to see what was going on. One side was way beyond max watts at idle, and the other side was right at max watts. I went through my tubes and found two that were matched and had a cathode current of 80%. I'm going to have to change the 140 ohm cathode resistor to a higher value. It always pays to check. Assume nothing.

[frankie5fingers]

+1 TweeDlx. Cathode bias or not, checking bias is a (more than) fine idea. Bought a pair of ANOS TS 5881s for a Cathode bias tweed a few years ago, supposedly matched etc. One ran very hot, red plated and blew an HT fuse. Not a big deal but had I checked I could've at least gotten the dough back on the set.
ABC - Always Bias Check. OK, not exactly bumper sticker material, but it's still a good idea.