Tube Choices in a fixed bias MKIVa

[Mongo1]

Hi all,

One question that I have been wondering about is related to bias, etc. when replacing tubes. With my 92 MKIVa being a fixed bias amp, what do folks do about maintaining proper voltages, currents, etc when using non-Mesa tubes?. I've always used Mesa tubes because my expectation was that the QC on Mesa tubes ensured that the setup would retain the proper power characteristics. I've been content with the tone (ecstatic would be more accurate), but then again, I have never ventured out of the box. Tone nirvana could be one experiment away...

When you swap out tubes for JJ's or GT's or whatever in a MKIV, do you have to get an adjustment or are you content to just hope for the best? I'm sure I could see if a tube was glowing too hot, but its the less extreme deviations from allowable limits that concern me. I figure pushing currents, etc could eventually damage the amp.

What are your thoughts?

I'm a newbie here, so please forgive me if this topic has been discussed to death.
I've been searching and reading threads/posts for hours and hours...
Awesome site!

Mongo

 

[stokes]

If,as you say,you are happy with the tone you get now,just leave well enough alone.When it comes time to change tubes when they wear out,the "proper voltage" is not the concern,it is the current draw you are concerned with.The amp has x amount of volts which will rise or fall according to how much current a particular tube draws.In a given amp,one pair of 6L6's may draw 35ma's at 430 plate volts,another pair may draw 40ma's,in the same amp the ones drawing 40ma's will cause the plate volts to drop to maybe 425 volts (just an example not rock solid numbers).When you buy pre tested tubes to "match" your amps bias setting,they are selected to be "safe" for that amp.At the voltages cited,for example, "safe" could be from 20ma's to 50ma's,some will tell you they cant hear the difference,and to them it makes no difference.I have let people A/B different settings,and some can hear a difference,some cant and some have a preference,but cant put their finger on why.It is a very subjective effect,and is not necessarilly sonic,but different settings will have a different "touch" or "response" or "feel".So using tubes from Mesa or another supplier that rates tubes for Mesa,there can be a fairly wide tolerance.If you want to use NOS tubes from an untested source you need to be able to change the bias or current draw.

 

[Mongo1]

MKIV owners,

If you user Non Mesa tubes, do you re-bias?
Do you have preferred tubes that work with the MKIV fixed bias?
Is there an easy way to verify that a given tube setup is within range?
- Weber Bias Rite, etc...

I would love to have a 'how to' on verifying a MKIV setup.
Something like:
* When using a Bias Rite, verify that your plate voltage is within '# to #' range.
* Verify the rating for maximum plate dissipation (in watts) for 'x' tube.
* Using the formula: Plate Volts * mA = watts
* Verify your idle current is within '# to #' range for 50% idle plate dissipation, or is within '# to #' range for 70% idle plate dissipation.
* If an adjustment is necessary, first drain your caps using this procedure....
* Change cathode resistor 'x' to the value determined by 'x' formula using the information obtained in steps 1,2,3,etc...

Granted, it would be need to be much more detailed and specific, but would be invaluable for an owner like me who would love to be able to 'know' my amp better. I have a great respect for the voltages and cap charges present inside an amp. Detailed information like this would allow me to be knowledgeable and work safely.

Please forgive my ignorance if this type of 'how to has been compiled before.

 

[LerxstLee]

MKIV owners,

If you user Non Mesa tubes, do you re-bias?
Do you have preferred tubes that work with the MKIV fixed bias?
Is there an easy way to verify that a given tube setup is within range?
- Weber Bias Rite, etc...

I would love to have a 'how to' on verifying a MKIV setup.
Something like:
* When using a Bias Rite, verify that your plate voltage is within '# to #' range.
* Verify the rating for maximum plate dissipation (in watts) for 'x' tube.
* Using the formula: Plate Volts * mA = watts
* Verify your idle current is within '# to #' range for 50% idle plate dissipation, or is within '# to #' range for 70% idle plate dissipation.
* If an adjustment is necessary, first drain your caps using this procedure....
* Change cathode resistor 'x' to the value determined by 'x' formula using the information obtained in steps 1,2,3,etc...

Granted, it would be need to be much more detailed and specific, but would be invaluable for an owner like me who would love to be able to 'know' my amp better. I have a great respect for the voltages and cap charges present inside an amp. Detailed information like this would allow me to be knowledgeable and work safely.

Please forgive my ignorance if this type of 'how to has been compiled before.

Most tube stores will help you get mesa "spec'd tubes .
Here is the GT to mesa spec chart .

The bottom line here is;

Lower numbers will start to distort at lower volume settings on your amp, and have more dynamic touch and feel.
These are the GT 1-3 range, and the Fender "blue" painted tubes. Many Jazz or Blues players like this range, and
they work well for recording where levels are going to be lower, or in smaller venues.

Medium numbers are the most versatile, will drop into most amps and be very close to most factories bias settings,
and are the best general choice for most players wanting a versatile well rounded amp. These are the GT 4-7
range, and the Fender "white" painted tubes. The range of 4,5,6 are able to be used in Mesa Boogie fixed bias
amps, and on the Mesa scale convert roughly to:

Mesa scale Groove Tubes scale
Red 4
Yellow 4
Green 5
Gray 5
Blue 6
White 6

Nice to keep handy

/cheers

 

[Mongo1]

That's a good chart to have.

Thanks,

 

[LerxstLee]

That's a good chart to have.

Thanks,

Anytime , even if you do not want to use groove tubes , you can still ask for " mesa Spec'd "
from most tube sellers , NOS or any of the new ones .

If anyone has used the groove tube 6L6gcGE , they bought the GE tooling and " meturology "
So they should be as close to a NOS as any older ge . If you have used them let me know .

Thanks

 

[stokes]

That's a good chart to have.

Thanks,

Anytime , even if you do not want to use groove tubes , you can still ask for " mesa Spec'd "
from most tube sellers , NOS or any of the new ones .

If anyone has used the groove tube 6L6gcGE , they bought the GE tooling and " meturology "
So they should be as close to a NOS as any older ge . If you have used them let me know .

Thanks

More marketing B.S. from GT,this time.Like New Sensor or whoever is "re-issuing" Tung-Sol and Mullard.Just exactly how do you "re-issue" what you didnt "issue" to begin with.And JJ supposedly made their tubes on the original Tesla machines.Funny,but they aint even close to real Tesla's.