[Kruds]

I acquired a Knight KN1515 EL84 PP and have a question about its driver tubes.  World Radio Museum says it can use either a 12DF7 or a 6BH8 but these seem so dissimilar to me.  What am I missing?  To make matters worse the amp is marked 12DF7 and the service manual/schematic shows 6BH8. Anyone know anything about this model?   Thanks.

 

[FullRangeMan]

The amp have a 6V - 12V switch?

[Kruds]

Nope.  I starting looking into the 6BH8 and tried to compare it but hence my confusion.  Plus the 6BH8 is one of those dissimilar pentode/triode tubes and I was trying to understand how this could be a substitute.  I have noticed Fisher in their early 460 console amps use a 12DW7 (dissimilar triode, similar to a 1/2 12AX7 & 1/2 12AU7) which is interesting to me but I couldn't make the 6BH8 work in my head.  I am just trying to understand what black magic is being done?   

[FullRangeMan]

You could meter the filament voltage at the socket.

[Kruds]

Yeah, I figured I would reach out first and see if anyone knew.  I'll start actually digging into it next. 

I am just trying to understand how it's voiced and then see if I need to make any changes.  It came with one 12df7 and substitute 12AX7. So I went ahead and swapped in some 7025's until I can track down an affordable pair of DF7's. 

It sounds pretty good right now, just curious.....

[FullRangeMan]

Só you have a 12V supply.
7025 is a special version of 12ax7.

OBS> 12DF7 run 12V filament in Serie, in parallel its 6V.
https://frank.pocnet.net/sheets/127/1/12DF7.pdf

[Kruds]

Yes, I know.  I stuck with what the Amp was marked (ie 12AX7 derivative).  Here are the 6BH8 vs 12DF7 pinouts.  To me they look like they are different enough not to try the 6BH8 in the circuit.  So maybe they changed the circuit design and never updated the schematic?  I've read that is a frequent issue with some of these older designs.  Anyways the amp runs fine for now, but I was just trying to understand the discrepancy.  Thanks.

[FullRangeMan]

These documentations level you want exist only in IBM, Microsoft and military contractors. In commercial audio products the situation is very different, if you know what I mean.

[qdrone]

This has a Knight amplifier in it. I pulled all the tubes and there in a bag. I can look to to see what #'s I can see on tubes

 

[bentconvert]

I have this same amp and same questions. Mine is not working so went searching and found the same schematic as you and reached the same conclusion. On mine I noticed when looking carefully I see 6BH8 stamped into the metal near where 12DF7 is printed on the surface.

[Kruds]

I have this same amp and same questions. Mine is not working so went searching and found the same schematic as you and reached the same conclusion. On mine I noticed when looking carefully I see 6BH8 stamped into the metal near where 12DF7 is printed on the surface.

BentConvert

I really like the sound of it right now, but I think I may modernize a few things (like add an ON/Off switch, fix a pot or two).  Long term I'll probably recap, etc. 

Mine also came with a cage in pretty good condition, but I kind of like how it looks without.  Right now it is matched with some Klipsch KG3.2's.