6l6GC vs 5881

[cap217]

I have an amp that is designed for 5881s (from the builder) but he said it was ok to use a set of 6l6GC I had at home. The old bias was super crazy. There is a bias pot on the back and it read 0.088 and 0.093 the builder told me that this was crazy and they should be biased in the 0.035 range. So I took the tubes out and put in 6l6gc and biased to 0.035.

The issue is that I am getting a weird crackle or sound that is like a blown speaker on certain notes. I dont think its ghost notes but it almost sounds like a blown speaker and choked sustain. Its very unpleasant.

After some emails back and forth and a video clip he thinks that it may be an oversaturated transformer due to using 6l6gc.


My question is should I just get some 5881s and try them? What is the difference in the 6l6gc and 5881? And could the fact that the old tubes were biased so hot, it might have done something internally? The amp actually sounds good most of the time so I dont think its an amp issue.

Besides me shipping the amp back, your help is my best option.


Here is a quick clip. Doesnt sound great but its the best I can do.

https://www.youtube.com/watch?v=NNU278J ... e=youtu.be

[M Fowler]

Sounds like poor solder joint to me.

[jhaas]

I don't think this issue is related to 6L6 vs. 5881. The tweed bassman 5F6A is a common example of an amp "designed for 5881s" that folks routinely run 6L6s in.

You should be able to bias a little hotter than 35mA w/ 6L6s, but I don't think that has anything to do with the issue you're describing. Something around 46mA would get you closer to 70%max for 6L6 (30W).

Are these known good 6L6s?

[pdf64]

There is a bias pot on the back and it read 0.88 and 0.93 the builder told me that this was crazy and they should be biased in the 0.35 range. So I took the tubes out and put in 6l6gc and biased to 0.35.

None of this makes any real sense, eg 0.88 of what?
I can kinda guess what you mean etc but until there's more solid info and less assumption, I can't see how this can be moved forward.
1 Schematic
2 Test point where measurements were made
3 Meter setting

[cap217]

pics of the settings

[cap217]

I don't think this issue is related to 6L6 vs. 5881. The tweed bassman 5F6A is a common example of an amp "designed for 5881s" that folks routinely run 6L6s in.

You should be able to bias a little hotter than 35mA w/ 6L6s, but I don't think that has anything to do with the issue you're describing. Something around 46mA would get you closer to 70%max for 6L6 (30W).

Are these known good 6L6s?

They are brand new. I dont have spare 6l6 tubes so I might have to go buy another set. I did swap out the 12ax7s though.

[Teleguy61]

Sounds like poor solder joint to me.

I agree with Mark.
Doubtful that it is a tube issue.
For almost all intents and purposes, except very high voltage apps, 5881s and 6L6s are interchangeable.

[ampgeek]

Assuming those test points are measuring across a 1 ohm resistor and you have typical 6L6 type plate voltages, you appear to be running the tubes way hot!

Lower the bias adjustment until you get into the 0.035V to 0.040V range and see how it sounds. That would be about 35 ma to 40 ma total current flow through each power tube.

Cheers,
Dave O.

[cap217]

Assuming those test points are measuring across a 1 ohm resistor and you have typical 6L6 type plate voltages, you appear to be running the tubes way hot!

Lower the bias adjustment until you get into the 0.035V to 0.040V range and see how it sounds. That would be about 35 ma to 40 ma total current flow through each power tube.

Cheers,
Dave O.

These were the old tube bias. I swapped them out and they are now at .035. I was just showing this to see if maybe something happened to the amp because the old owner ran the tubes too hot.

[cap217]

If its a bad solder joint, wouldnt the noise be noticeable more than just a few notes?

[cbass]

A bad solder joint can unmask its self at any time and be very difficult to replicate.If those old tubes were in there for much amount of time they could have damged other components.I have know idea what kinda amp you have so that s about as much as I can come up with

[iknowjohnny]

Don't wanna hijack, but can anyone describe what the difference in tone is with a 5881 over a 6L6?

[cbass]

Just to be clear modern 5881's are the same tubes as 6l6gc
If you are talking about american made 5881's need to be biased cooler
23 watts vs 30

[vibratoking]

Many times a bad solder joint only manifests itself on ONE note. I'm with Mark too.

[Phil_S]

At 88mA they are off the charts. Easy to find out if it's the tubes. Does it go away when tubes are properly biased (~35mA)? You've got to adjust the bias anyway!