Philips 7581A selection

[andrewd]

My first post here, but long time lurker

I'm in dire need to feed my IIC+ with a new set of 7581A in the center pair position. As we all know, these tubes are going extinct; moreover given my geographical location in Japan, makes it harder to get these 7581A from the States. However, I manage to find some of these tubes in the local hi-end audio shop. The shop said that he can match pair the tubes with Hickok539C tube tester, but he admitted that he's not familiar with guitar amp applications. Now if I can tell him what kind of 7581A specs I require (plate current spec, etc), he will try to find it from the stock he has currently.

The problem is that the Hickok 539C will only go to 200 plate voltage while the IIC+ puts 480V out. At 70%MPD, 480V corresponds to 52mA current draw, I think.

So in order to meet the 70%MPD, is it OK if I told him to find tubes which draw about 122.5mA at 200V at the Hickok 539C? Will this give me the right 7581A for my amp w/o infant mortality problems at all?

Sorry for the long post and TIA, guys :wink:

 

[Boogiebabies]

The 7581A Phillips is a 35 Watt tube that can take up to at least 600V.
It's a brute !!!

 

[andrewd]

Does that mean that I can slam in ANY matched pair of 7581A in the amp and it won't hurt both the amp and tubes? Both are built like tanks, but even tanks (or even Airwolf) have weak point :lol:

 

[908ssp]

Can you get your hands on a Bias Right or other in amp tube biasing device? You could put the Bias Right extension sockets in your amp and just plug tubes in till you get the numbers you want.

Or you can buy a pair and have the amp biased for them. I had my MKI biased for some Black Plates. I haven't done it to my MKII's but putting bias pots in is one mod I would consider.

 

[Boogiebabies]

A matched pair or a bias mod would have to be the way to go.
It is not wise to just stick any old tubes in a great Boogie like that.
Even though it can handle a bunch, the tube may naturally draw a high plate current and have a high idle bias. A trim pot would be a simple mod as well, then you could run what you want.

 

[andrewd]

A matched pair or a bias mod would have to be the way to go.
It is not wise to just stick any old tubes in a great Boogie like that.
Even though it can handle a bunch, the tube may naturally draw a high plate current and have a high idle bias. A trim pot would be a simple mod as well, then you could run what you want.

Does it look like I'm pretty much stuck with bias mod? Wow, hauling a 50lbs amp in a train could be much a challenge :shock: (no cars for me here).

Will these tubes behave so differently inside the amp (@480V) and on the tester (@200V)? Or does the P = V.I law hold true here?

 

[Boogiebabies]

They may act perfectly at 200V, but at 480V you have no idea what a true natural plate current will be and what idle bias the tube will draw.
A minimum of 400V, like a maxi matcher needs to be used to test them in a real tube amp. Even then at 480+ volts there can be a plus or minus 5-15ma drift. Without a probe you will never know.


Where are you with no car, Manhattan? London?BFE?
I would not get on a subway or bus with a Mesa without a 9mm, mace and a tazer.

 

[andrewd]

Where are you with no car, Manhattan? London?BFE?
I would not get on a subway or bus with a Mesa without a 9mm, mace and a tazer.

Actually, Tokyo :wink: Here one oil change will cost you around $100. That's why I kinda miss my days going to Wal-Mart and picking up 1 gal of Mobil1 Synthetic for $19 :roll:

Anyway, back to the topic. I think I will search patiently for other tube sellers who can test tubes at least at 400V then; don't feel like frying the output tranny too soon, at least not when I'm still in my 3rd year honeymoon with the amp 8)

 

[RussB]

So in order to meet the 70%MPD, is it OK if I told him to find tubes which draw about 122.5mA at 200V at the Hickok 539C? Will this give me the right 7581A for my amp w/o infant mortality problems at all?

Whoa there!

I really don't where all this "bias at 70%" **** comes from, but it is internet mis-information at it's finest!

70% is MAXIMUM, and not to exceeded in a class AB1 amplifier, unless you like wear your tubes fast, and generate a lot of extra heat!

That said, get a pair of 7581A's that will draw around 30 to 40mA in the center position, and you'll be golden. 50 to 60% will yeild great tone, with longevity

...and infant mortality happens, regardless of setings.

Have yu tried Mike at KCA NOS Tubes? I believe he ships internationally.