On many different message boards and such I see over and over again when the subject of 6F6 tubes comes up as a potential fill-in for 6V6 circuits people warn to make sure the plate voltage isn't too high or the 6F6s will fry. Then, looking at multiple data sheets for each tube I see repeatedly the max plate voltage for 6V6 shown is 315VDC and 6F6 shows 375VDC. What gives? I did see one person mention that in their opinion the 6F6 was overlooked because it was outshone by the 6L6, and the data sheets seem to be more in line with this statement. So why is the 6F6 over and over again seen as a weak, frail option in a 6V6 circuit?
Then, to delve further, there is a 6 pin predecessor of the 6F6 - Type 42. Other than it's base this tube is electrically identical to the octal 6F6. There are tons of Type 42 tubes listed on eBay, mostly at fairly respectable prices which would indicate these tubes do still enjoy widespread use, yet there is little to no chatter online from builders/hobbyists experiences with these tubes. Could it really be there are silent leagues of old timers out there who continue to buy these up on a semi-regular basis to keep old radio consoles running?
Doesn't take a genius to see that I'm asking because curiosity wants to see me try and build a guitar amp using Type 42 output tubes!
But wait, keep your fork! There's pie
Additionally, after having decent results building an all-octal amp using a 6SL7 as phase inverter (albeit a cathodyne style one) I'm interested in using two separate triodes wired to make a long tailed pair. The 6SF5 tube is a single octal triode that has the same characteristics as one half of a 12AX7, just with bigger pieces. The 6SL7 is great but is widely known to be a little tame next to a 12AX7 (ie amplification factor 70 compared to 100), while the 6SF5 ranks right up there w/12AX7 at 100.
At this point, if anyone is inclined to encourage me to keep things simple and just use a 12AX7 don't bother. I want to do this because I find it interesting, I know it may look like I'm just making more work for myself. Good! A friend recently enlightened me to the existence of a classic hi-fi amp known as the Quad II which uses two separate pentodes (EF86) in a long tailed pair configuration.
Then, last but certainly not least, has anyone heard of or tried making/using a circuit that utilizes two separate tubes in a long tailed pair configuration but where one half is a pentode and the other half a triode? For example an EF86 and a 6SF5? Or is this strictly a hi-fi trick to enhance balance which in guitar amps at least is known to sound not as good as imbalance?
Happy Monday!
