Are these 7581's ?

[Sigs]

Are these 7581's ? can they be used as a replacement for KT66 ?

 

[StingRay85]

Hard to judge based on this picture only

 

[StuC]

They look like Sovtek 5881s.

 

[Pete Farrington]

7581, like KT66 and 5881, are part of the 6L6 valve type family.
So there’s a fair degree of compatibility / interchangeability between them all.
The main constraints being the voltage and dissipation that the application will subject the valve in use to.
I suppose the valves in the photo could be 7581 but given the resolution etc, it seems somewhat optimistic that a positive id might be made.
They seem to have a button base.
I’m not aware of 7581 ever being made in that format, but I’m no expert.
Are they even all the same, eg the left one seems to have a rounded anode structure, whereas on the right it seems more squared off

 

[Pete Farrington]

They look like Sovtek 5881s.

Yes, the apparent button base was leading me to think 6P3Sx were a likely suspect, but the valves in the photo look a bit taller than I’d expect those to.

 

[playloud]

7581, like KT66 and 5881, are part of the 6L6 valve type family.
So there’s a fair degree of compatibility / interchangeability between them all.

Do you have any thoughts on which variants and/or makes (if any) might be able to withstand the high plate/screen voltage demands of the JTM 45/100 circuit*?

As has been discussed, the only KT66s which are reliably able to do so are the GECs.

I know the 7581As are rated for higher plate dissipation than regular 7581s (35W v. 30W), so you can bias them hotter, but unsure if that translates to higher plate/screen voltage handling (the data sheets I could find - unsure how accurate - cite a max of 500V/450V resp).

Using stock standard 6L6s would clearly be "cruising for a bruising" in this instance.

* Assuming the PT @Sigs is using is an accurate 1204-43 repro, he should expect 560/559V, or even slightly higher.

 

[Pete Farrington]

I’ve no direct knowledge, sorry.
I’m not sure there’s an inherent reason that GEC KT66 should be so much more capable than other, later vintage production 6L6 types.
eg later Musicman amps used 6L6GC (probably Sylvania STR387) in a split rail 700V anode - 350V g2 HT design.
And the late 70s early 80s era Fenders run them up above 500V (though their partial UL OT takes a bit of stress off the g2s).
I’ve seen reliable reports that Soviet era 6P3SE are a suitable and less spendy substitute for such higher voltage amps. In that they’ll cope, but won’t achieve the same power output.

Realistically, doesn’t the 45/100 HT sag a lot under load?
560V at idle probably isn’t that big a deal; notice how valves don’t tend to fail when idling? Regular SLs tend to sag maybe 100V at full power, I suspect the 45/100 may be similar. 560V at full power would be much harder on the output valves.

 

[Matthews Guitars]

Those tubes are Sovtek coin base 5881s. They have a rated max plate voltage of just 250 volts (That's what the docs say!) so no, they'd be a TERRIBLE choice for replacing KT66s.

Even the original tung-sol 5881s only handled 400 volts on the plate.

 

[Matthews Guitars]

If high plate voltages are a problem, the Tung-Sol KT-120 tube may be the best answer. They are 6L6 family tubes and can handle 850 volts on the plate in tetrode mode, 650 in triode. But...they draw a lot of filament current. By adjusting a few component values you should be able to run a KT120 in any amp that takes anything in the 6L6 or EL34 families. At most minor mods would be required.

 

[Amadeus91]

For comparison @Sigs here are a couple of pair that I have.

 

[Pete Farrington]

Sovtek coin base 5881s. They have a rated max plate voltage of just 250 volts (That's what the docs say!) so no, they'd be a TERRIBLE choice for replacing KT66s.

Just to note that the rating system used for Soviet military valves there needs to be considered.
Does anyone have any info regarding that?
Our experience with the non equivalence of the design centre, design max and absolute max systems that we are familiar with should surely make us suspicious of taking info from totally unknown rating systems at face value?
eg perhaps they apply to extreme service conditions for aerospace / deep sea servo controllers, and can be significantly increased in less arduous, non critical applications such as guitar amps?

Whatever, 6P3SE have been variously marketed in the west for audio / guitar amp applications with HTs far above 250V, and I’m not aware of them being reported as having a high failure rate; quite the contrary.