The AM Forum

 A design uses an E80F (CV2729)pentode for a voltage amplifier. The datasheet with curves is attached. It's a simple audio amp application, but the tube has certain characteristics not usually seen in a tube with a 1.3W plate dissipation. I don't find a "USA" direct or electrical substitution.

Eb peak 600V
Eb 300V
Pb 1.3W
Eg2 200V
Pg2 0.4W
Vg3 -100V
Vg1 -100V
Ik 9mA
H-K V -120 /+60
Vf 6.3
Base 9 pin

This is quite a bit more than the usual 6AU6 and seems like more than the general TV set I.F. amp pentodes, but not near the common 6AQ5 or 6CL6.

The GIF shows it used where the parameter "S" is 1.85mA per volt. I'm not sure what "S" means, this being European data.

I have no real objection to using a 6AQ5 or 6CL6, or any other small power pentode but without experiments and without any E80F's, I don't know what to start with.  It's used as a voltage amplifier.

I don't want to use the European tubes for the simple reason that they are not so plentiful here in the states. Otherwise I have no objection. In replicating this design, I prefer to go with US types, the only direct sub is the EL34, which is one tube that is common enough.

Does anyone have a suggestion for a pentode about this size and characteristic?

The GIF shows it used where the parameter "S" is 1.85mA per volt. I'm not sure what "S" means, this being European data.

dimensionaly this is conductance (in Seimens) ... this would be .00185 Mho or 1850 uMho or uS

the tube specs are a bit unusual and will need a closer look ... its running at .75W plate diss ....if you are willing to rewire and resocket, I would try a 6au6 and maybe change cathode resistor .... bet it works

 In the US they use micromho ( Ohm in reverse so 1 divided by Ohm ) in microamps per Volt,   in Europe they use S from  Steepness , Slope,  Steilheid of the curve of the tube  all beginning  with a S in miliamps per volt .  Just a factor of thousand difference.  The E80F is a philips longlife tube with goldplated pins and >80000 Hrs of life span. Easy to obtain also cheap,  see for instance   http://www.electrontubes.nl/Voorraad/voorraadset.htm   this guy has plenty of them and  English spoken,  ships al over the world.  Anton

That's good news. If I have a couple sets of spares I'm happy. Thanks!

It has to be decided, no harm in exploring the 6AU6 for now.

It's interesting about the S parameter. OK, well 1.85mA per volt, it's borne out by the curves. The 6AU6 is after all very similar in voltage and current ratings despite its smaller size, well, 7 pin vs 9, but that's an envelope size which maybe deceived me, not the real tube size.

the actual operation best I can tell is:
Eplate 175V
Eb (supply) 230V
plate load 18K Ohms
Ik 3.7mA
Vk 2.5V (K bias through 608 Ohms)
Eg2 85V (from 230V supply through 120K)
Rg 1Meg from G1 to point x below.

There's a 4-resistor divider that makes up the cathode resistor. I guess that's minor enough, and no way to tell, or maybe to even discuss it further until it's tried. The project is a long way from being ready to try anything, esp. with any feedback signals.

cathode resistor segment Ohms: K-11-{fb1}-180-x-390-{fb2}-27-GND

caaaarazy! I might understand more or less how to keep the feedback gain the same while adjusting the Rk.  8-)

The 6AU6 and E80F curves cross at 5mA, so it's a little weird. The screen volts may need changed too as it has a huge effect on the plate current.

100V plate, 100v screen:
6au6 0V bias=10mA
E80F 0V bias=7.2mA

6AU6 -1V bias=5mA
E80F -1V bias=5mA

6AU6 -2V bias=1.8mA
E80F -2V bias=3mA

This is that amp 'project' that used the QB5/1750 tubes and was converted to 4-1000's, then stripped for parts. got to start somewhere, a new schematic is one place. I saw the ad for those big tubes but already have two 4-1000's and that stage of it is way off. for one I don't have 4750VDC, and with something like this, I think the effect of plate voltage decrease is squared for its effect on power out in an audio amp -because Rload is fixed - so 3500V instead of 4750 means 60% voltage, 60% current.. ugh, 48% power. Not a 2KW amp any more -the horror..

Hopefully I may receive the LV transformer and some chassis parts, control panel, final tubes chassis, etc. and it's not known if the 4-1000 and QB5/1750 use the same socket. Looks similar. The -1750 tube only needs a fan not a socket blower and chimney. better in some ways.

The power stages are a subject for another topic.

I have at least 20 pcs QB5/1750  here for spare, so if You need some...
The E80F E88CC etc are the  SQ series of tubes for professional use. Long life,  vibration resistent, better contact pins etc.
 for military and professinal use in scopes measuring equipment,  transmitters  etc.  In An power Amplifier with vibrating trafo's that could be a must to survive. Anton

EF86 (http://tubedata.tubes.se/sheets/010/e/EF86.pdf) is pin compatible, and the Transconductance (S) is close.
A high quality version of EF86 is EF806S (http://tubedata.tubes.se/sheets/128/e/EF806S.pdf) (=6267).
An equivalent to EF86 is 6CF8.