7581A Power Question

[Jim]

Hi,

What is the max static dissipation for
the Phillips 7581A tube?

I've just put a pair in a '64 Bandmaster
head and they sound Killer !!!
Nowhere like in my '65 Vibrolux.

To get the nice fuller fat sound
I like, I had to bias them hot (23W static dissipation)

Is that ok? Or will they only last 2 or 3 days?

I hope it's OK because the sound is just !!! Awesome !!!
Better than with the RCA Black plate I had in it before.

Jim

[Jim]

Hi,

I've done some research on the net but can't seem to find
and straight forward answer.

Some say it will handle more than a regular 6L6GC. others say
it has to be biased the same as 6L6GC (under 20W idle)

What's the deal here?

All I know is that is does not sound right under 20W and
are absolute killer at 23W.

But are they going to be OK and last?
Or am I going to burn in a couple of hours or days?

That's the question.

Jim

Post by Jim
Hi,
What is the max static dissipation for
the Phillips 7581A tube?
I've just put a pair in a '64 Bandmaster
head and they sound Killer !!!
Nowhere like in my '65 Vibrolux.
To get the nice fuller fat sound
I like, I had to bias them hot (23W static dissipation)
Is that ok? Or will they only last 2 or 3 days?
I hope it's OK because the sound is just !!! Awesome !!!
Better than with the RCA Black plate I had in it before.
Jim

[RoccaforteAmps]

Real 7581's are a 30watt tube.

[Jim]

Hi Doug.

I've read that 7581A are 35W and 7581 are 30W.

Does that mean that I can bias them at
25W or more static dissipation?

Let's say, my Bandmaster has a 435V plate voltage.
As of now the Phillips 7581A are biased at 53ma.
This gives me 23.05W static dissipation.

Are you saying that I could go up to 30W or 35W???!

Let's say 435V DC x 65mA = 28.3W static dissipation?
And I would still be within a safe range?

Could you be more specific?

TIA

Jim

Post by RoccaforteAmps
Real 7581's are a 30watt tube.

[RoccaforteAmps]

<<Jim:
Does that mean that I can bias them at
25W or more static dissipation? >>


Absolutely, you can go up to 30watts, maybe even further. Remember, a 7581 is
just a more robust version of a 6L6GC, and 6L6GC's are underrated in the books.
Why I ignore tube ratings.

[Dave Curtis]

Post by Jim
Hi,
I've done some research on the net but can't seem to find
and straight forward answer.
Some say it will handle more than a regular 6L6GC. others say
it has to be biased the same as 6L6GC (under 20W idle)
What's the deal here?
All I know is that is does not sound right under 20W and
are absolute killer at 23W.
But are they going to be OK and last?
Or am I going to burn in a couple of hours or days?
That's the question.
Jim

Post by Jim
Hi,
What is the max static dissipation for
the Phillips 7581A tube?
I've just put a pair in a '64 Bandmaster
head and they sound Killer !!!
Nowhere like in my '65 Vibrolux.
To get the nice fuller fat sound
I like, I had to bias them hot (23W static dissipation)
Is that ok? Or will they only last 2 or 3 days?
I hope it's OK because the sound is just !!! Awesome !!!
Better than with the RCA Black plate I had in it before.
Jim

Here:

http://tdsl.duncanamps.com/show.php?des=7581A

-Dave

[RoccaforteAmps]

this thread is getting too anal, try the fucking tubes and see what happends.

[Fearless Freep]

Leave Moolie out of it, Doug.

Freep

Post by RoccaforteAmps
this thread is getting too anal, try the fucking tubes and see what happends.

[l***@aol.com]

Post by Jim
What is the max static dissipation for
the Phillips 7581A tube?

All ratings are identical to 6L6GC; 23w is hotter than the usual 70%
rule for AB1, they may be exceeding the 30w max a little @ full diss.
Assume U mean only plate diss; if total, then it's about right on.
All this assumes U trust the max manual ratings - which are typical &
vary from tube to tube.

[Lord Valve]

Post by Jim
Hi,
What is the max static dissipation for
the Phillips 7581A tube?
I've just put a pair in a '64 Bandmaster
head and they sound Killer !!!
Nowhere like in my '65 Vibrolux.
To get the nice fuller fat sound
I like, I had to bias them hot (23W static dissipation)
Is that ok? Or will they only last 2 or 3 days?
I hope it's OK because the sound is just !!! Awesome !!!
Better than with the RCA Black plate I had in it before.
Jim

Spec for class AB operation is 70%-80% of design max,
which is 30 watts. (Some specbooks will list design max
as 35 watts for this tube, but I prefer to treat it the same as
a 6L6GC.) 23 watts is on the hot side, but well within the
capability of a 7581A. I had a Fender come in for service
once that had a pair of 7581A installed in it, and they were
running at 31 watts static. (!) The dude who owned the
amp said he put 'em in "around ten years ago." (!!!) They
were still working fine.

Lord Valve
Expert

[Jim]

Ok.

If I get all this right, this is what comes out of all the answers I got:

A normal 6L6-GC has a 30W rating.
A 7581 has a 30W rating.
A 7581A has a 35W rating.

Given the fact that, for normal AB operation,
70% to 80% of the max rating is recommended:
If I had to bias a regular 6L6-GC I would bias
them at 21W static dissipation (which his 70% of 30W).
The same for a 7581.
It would be 24.5W static for a 7581A (70% of 35W)

Therefore, the 23W is just bellow the 70% and is
very well within a resonable range.

However, given the above data, I don't understand
why everyone says a 6L6-GC has to be biased
under 20W since 70% of 30W is 21W??!

TIA

Jim

Post by Lord Valve

Post by Jim
Hi,
What is the max static dissipation for
the Phillips 7581A tube?
I've just put a pair in a '64 Bandmaster
head and they sound Killer !!!
Nowhere like in my '65 Vibrolux.
To get the nice fuller fat sound
I like, I had to bias them hot (23W static dissipation)
Is that ok? Or will they only last 2 or 3 days?
I hope it's OK because the sound is just !!! Awesome !!!
Better than with the RCA Black plate I had in it before.
Jim

Spec for class AB operation is 70%-80% of design max,
which is 30 watts. (Some specbooks will list design max
as 35 watts for this tube, but I prefer to treat it the same as
a 6L6GC.) 23 watts is on the hot side, but well within the
capability of a 7581A. I had a Fender come in for service
once that had a pair of 7581A installed in it, and they were
running at 31 watts static. (!) The dude who owned the
amp said he put 'em in "around ten years ago." (!!!) They
were still working fine.
Lord Valve
Expert

[Ned Carlson]

Post by Jim
Ok.
A normal 6L6-GC has a 30W rating.
A 7581 has a 30W rating.
A 7581A has a 35W rating.

From what I can tell, the only reason GE put a 35W rating on
7591A was that they were pissed at RCA for declaring 7027A
a 35W tube. RCA had a bad habit of sticking abnormally high
ratings on tubes, even though they knew the tubes wouldn't
hold up well at the ratings they claimed.

RCA was claiming 30W for their 6L6-GC and 7027 despite
the fact that their tubes had the old black nickel-steel plates
instead of the grey 5 ply plate material that GE & Sylvania tubes
had. When RCA gave up and started using the grey-plate stuff,
(and quit trying to make 7027's that looked like Telefunken
metal base EL34's), they called 7027's 7027A and added
5 watts to the plate dissipation ratings.

Here's some links you can look up re the plate material:
Loading Image...
Loading Image...

I would consider 6L6-GC, 7581 and 7581A all the same thing
for practical purposes.

Post by Jim
Given the fact that, for normal AB operation, 70% to 80% of the max rating
is recommended: If I had to bias a regular 6L6-GC I would bias them at 21W
static dissipation (which his 70% of 30W). The same for a 7581.
It would be 24.5W static for a 7581A (70% of 35W)

Up to the point where you get pink spots on the plates or blow
the fuse or power transformer, under the US Constitution you
are legally allowed to run the tubes as hot as you'd like to
make 'em sound good, without black helicopters and the ATF
dropping in to arrest you for amp & tube abuse.

OTOH, the Law of Thermionics (I think God handed this down
on computer punch cards to Nikolai Tesla in 1897) dictate
that the hotter you run tubes, the faster the tubes wear
out. Some handle abuse better than others. Your choice.