• New Sponsor: ShipNerd, Ship Your Gear with Us... for less! Click Here.

non matched tubes

jojogun1957

Member
Messages
704
Well I decided to put a pair of RCA blackplates I have had for years into my ProReverb.They are off by almost 6 milliamps.
These tubes have sounded good in the past 3 amps I used them in,but I never checked the bias before.
If they sound good is it safe to run them like this?:hide2
I'am getting 34.2 on one and 28.1 on the other.
Thanks for any input
 

Tuberoast

Gold Supporting Member
Messages
3,356
I have BP's that mismatch by that much and I don't really care because my ears are more than happy. It is safe
 

Raybob

Member
Messages
175
It's ok to run mismatched power tubes. Some people actually like that sound. The downside is the added hum. The farther off the match the more 120hz hum you will have. Set the bias to the highest mA tube.
 
Messages
1,871
Not a problem. Back in the day, a tube got swapped or replaced without ever thinking about matching. Certainly matching is an ideal case scenario, but drifting will almost certainly occur, even with matched tubes. Matching is a good idea, but not a necessity, at least within a reasonable parameter. I wouldn't suggest more than 10.
 

scottl

Member
Messages
17,057
Try this..... Swap the tubes. Listen. One way will be much more pleasing than the other way. The mismatch is altering the "balance" of the PI. So, one way will imbalance things in a more pleasing way than the other way.

Try it.....
 

Blue Strat

Member
Messages
30,415
Anything is worth trying if it takes a few seconds...but I have no idea what you're talking about regarding "altering the balance of the PI". The PI balance is independent of power tube balance.
 

Ronsonic

Member
Messages
3,302
I'm thinking the idea there is that since the power tubes are imbalanced and the PI is imbalanced, one setup will add imbalances while the other counteracts the imbalance.

Might be worth fooling with if one doesn't have big expectations and plenty of time. I doubt that tubes with an audible dynamic imbalance would make a good set in either case.
 

scottl

Member
Messages
17,057
Ron's response below. The imbalanced power tubes are either putting the inherent PI imbalance further out or more in, as it relates the net result of the entire system. Obviously the tubes are independent, but directly relating to the amps output stage balance. The factors include the PI, the power tubes, and the tranny windings.

Swapping absolutely is noticeable and I do it all the time. Try it sometime.

Anything is worth trying if it takes a few seconds...but I have no idea what you're talking about regarding "altering the balance of the PI". The PI balance is independent of power tube balance.
I'm thinking the idea there is that since the power tubes are imbalanced and the PI is imbalanced, one setup will add imbalances while the other counteracts the imbalance.

Might be worth fooling with if one doesn't have big expectations and plenty of time. I doubt that tubes with an audible dynamic imbalance would make a good set in either case.
 

Blue Strat

Member
Messages
30,415
It depends on what, exactly, is going on in a particular amp and particular PI circuit.

PI balance usually refers to DC, not AC, balance and affects which half of the wave will clip first and how much earlier. Nothing you do with power tubes will affect this but mismatched power tubes can add their own coloration.

6mA isn't enough of a mismatch to be audibly detected in repeated (9 out of 10) BLIND tests.
 

scottl

Member
Messages
17,057
Mike, please check your post. The output of a Fender style amp is an AC system. The PI balance, tranny balance (windings), and output tube transconductance all effect the harmonic distortion of the output stage. There is a direct relationship of the PI and the output tube matching. Clearly visable on a scope and distortion analyzer. This is why Dumble and Ampeg, for example, used a PI trimmer. The end result is effected by the trimmer setting (effecting the PI balance) but also by the balance of the output tubes and the tranny windings. All those items effect the wave on each half. Any one that is out of balance can be compensated for either changing the PI balance, swapping trannies (not recommended just making point), or using specific imbalanced power tubes

And imbalances of much less than 6ma can easily be heard/felt. Just try it..... Especially on cranked amps that are overdriven. The bloom and touch sensativity change.

It depends on what, exactly, is going on in a particular amp and particular PI circuit.

PI balance usually refers to DC, not AC, balance and affects which half of the wave will clip first and how much earlier. Nothing you do with power tubes will affect this but mismatched power tubes can add their own coloration.

6mA isn't enough of a mismatch to be audibly detected in repeated (9 out of 10) BLIND tests.
 

Blue Strat

Member
Messages
30,415
Mike, please check your post. The output of a Fender style amp is an AC system. The PI balance, tranny balance (windings), and output tube transconductance all effect the harmonic distortion of the output stage. There is a direct relationship of the PI and the output tube matching. Clearly visable on a scope and distortion analyzer. This is why Dumble and Ampeg, for example, used a PI trimmer. The end result is effected by the trimmer setting (effecting the PI balance) but also by the balance of the output tubes and the tranny windings. All those items effect the wave on each half. Any one that is out of balance can be compensated for either changing the PI balance, swapping trannies (not recommended just making point), or using specific imbalanced power tubes

And imbalances of much less than 6ma can easily be heard/felt. Just try it..... Especially on cranked amps that are overdriven. The bloom and touch sensativity change.

LOL. Dude, I've walked the walk for close to 30 years. I don't disagree with anything you're saying (except the perception in BLIND TESTS regarding 6mA matching). You may have missed my point though;)
 

scottl

Member
Messages
17,057
I know... I get a bit anal! But, I do have dogears! I can hear and feel small changes to the PI balance for instance. I have shown this to people who come over when we adjust their amps. Interestingly, they all notice it too.... There is a spot in the PI balance where the bass has a pillowy feel, not thuddy. Also, the sound is a little more dimensional.

Please note the Ampeg SVT schematic for instructions on balancing the PI. This is what Dumble based his method on. Notes in lower right. The window of being in adjustment is small fwiw.

LOL. Dude, I've walked the walk for close to 30 years. I don't disagree with anything you're saying (except the perception in BLIND TESTS regarding 6mA matching). You may have missed my point though;)
 

scottl

Member
Messages
17,057
Even better, come by and experience it for yourself. Nobody can pass any judgement on this unless they are present since my experience is limited to my particular type of amp.

It is real. Do you think there is a balance trimmer on the PI for fun and show or because it does matter?

:hide2


Let me know what happens in a double blind test. All else is hearsay. ;)
 

VacuumVoodoo

Member
Messages
1,545
Do you think there is a balance trimmer on the PI for fun and show or because it does matter?

:hide2
Only 1 trimmer? There should be 2. Seriously. One for DC balance and one for AC. Using only one trimmer that varies plate loads in the LTPI affects both AC and DC balance which means the two are never in balance at the same time.
Guess which amp has two balancing trimpots :D:crazyguy:D
 

Blue Strat

Member
Messages
30,415
Even better, come by and experience it for yourself. Nobody can pass any judgement on this unless they are present since my experience is limited to my particular type of amp.

It is real. Do you think there is a balance trimmer on the PI for fun and show or because it does matter?

:hide2
I don't doubt your experience.

I'm amazed at the number of evangelistic proclamations and the refusal to perform a double blind test. It takes the same amount of time and actually PROVES something. :)
 

Blue Strat

Member
Messages
30,415
Only 1 trimmer? There should be 2. Seriously. One for DC balance and one for AC. Using only one trimmer that varies plate loads in the LTPI affects both AC and DC balance which means the two are never in balance at the same time.
Guess which amp has two balancing trimpots :D:crazyguy:D
+1. You realize that the balance function on the VTV tester is for DC balance, right Scott? ;)
 




Top Bottom