Re-tube Mark IV or wait?

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anomaly

anomaly

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I got some new Mesa labelled 6l6's in. A pair of str445's and str-448's (the 448's in the middle slots and 445's for the outer two). They cost me a damn fortune! I can't believe the price of these things now!?

The tubes currently in the amp are 10+ years old so I'd say they are due but I rarely play at band volumes these days. Mostly just low to moderate volumes, so I'm wondering how much life these old str-440's have in them?

Should I just put the new ones in to be on the safe side?
 
Yeah, you could throw them in and see if you notice an improvement. If the old ones are still working, keep them just in case. That's what I used to do. New tubes in, old tubes backup. But I haven't really changed tubes power tubes in any amps in a while haha.
 
I have a Mark IVa from the early 90s. Still using the original "coke bottle" tubes. I tried a new set of 6L6s just to hear. I preferred the older power tubes.
 
Thanks for all the replies. Think I'm just gonna leave the old ones in till I notice an issue, like one going microphonic or something.??. That has happened to me before.
 
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I had a mRak IV "B" version. As with most Mesa's its cathode bias w/o a trimmer. I did try other tubes and one set started redplating.
Ive had good luck with Sovtek 6L6 WXT+'s
 
Mesas are fixed bias. You need to try to find tubes that work within their bias range, which is why they always recommend using Mesa tubes. But most companies rate their tubes, so it shouldn't be too hard to get ones that will work from other places, too.

I would ditch those little rubber band things, though. They can put unnecessary pressure in those spots on the tubes as they heat up.
 
NinjaRaf said:
Mesas are fixed bias. You need to try to find tubes that work within their bias range, which is why they always recommend using Mesa tubes. But most companies rate their tubes, so it shouldn't be too hard to get ones that will work from other places, too.

I would ditch those little rubber band things, though. They can put unnecessary pressure in those spots on the tubes as they heat up.
Click to expand...
"Fixed bias" means it has a trim pot. Cathode bias means it does not. It sounds confusing i know.
Most rectifiers have "pads" on the PCB bias circuit traces where a trmmer can be installed. Not really needed tho.

People also ask

What is the difference between fixed bias and cathode bias?


In a fixed bias amp, a constant negative voltage is injected to the grids of the output tubes while the cathode is grounded to the chassis. In a cathode-biased amp, the grids are grounded through a grid return resistor and a resistor is placed between the cathodes and ground.Jul 9, 2022
 
Mesa's are NOT cathode biased.
They are fixed bias, but they omit the trim pot and just use a resistor in place of the trim pot, so that the bias voltage is non-adjustable.
 
fusedbrain said:
Mesa's are NOT cathode biased.
They are fixed bias, but they omit the trim pot and just use a resistor in place of the trim pot, so that the bias voltage is non-adjustable.
Click to expand...
Thats exactly what i posted did you read it ?
 
The OG Peavey 5150's are the same deal....
Fixed bias, but not adjustable, and typically very cold.
 


"non-adjustable" fixed bias. And you're stuck with vacuum tubes in a certain range for life of the amp.
Unless you know John Fields.
 
Yes electronics can be confusing. Especially since were talking 1920's technology from 105 yrs ago.
 
anomaly said:
Thanks for all the replies. Think I'm just gonna leave the old ones in till I notice an issue, like one going microphonic or something.??. That has happened to me before.
Click to expand...
With power tubes the amp may start losing its top end a little and get mushier in the bass. There is also the risk of one eventually red-plating if it starts leaking gas.
 
I would just stick with what you have.

Mark IVs are known to be biased really cold on the inner set of power tubes and less cold on the outer set.
When I needed power tubes, I ended up replacing the bad one and put a couple of the those in the inner slots and put new ones in the outer slots. I mostly run in half power mode anyway.
 
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