Hi systux,
Very interesting experiments and conclusions. Thanks for sharing.
I would like to be sure I correctly understand your data point “6p15p (bias volatge around -11.6V at 320V HT)”. Did you run the 6P15P tube in UL (10k primary) with high voltage supplied at 320 V for extended time? No red plating with the screens at about 317 V after transformer losses and plates at around 314V, with about -11.6 V fixed bias? Did you perhaps take note of the output power and screen current under these conditions? This is an encouraging answer to the question posed by the thread title under these operating conditions, which are used in many 6BQ5/EL84/6P14P PP UL amplifiers.
Also, in prior post #25 you wrote: “I tried several fixed screen voltages, but tubes seem to perform best when screen and anode voltage are approx. the same.” I assume you mean 6P15P because that is not the case with all tubes. (There are knowledgeable people that believe that in general pentodes and tetrodes perform best when the screen voltage is about 75% of the plate voltage.) Could you elaborate on “perform best” in your observations, in what way(s)? Was this measured or just perceived by listening?
Yes , I've run them for several hours without any problem. Note that the max screen and anode voltage for the 6p15p-ev is 330V for both. With fixed bias current setting of around 60-70% of max tube dissipation there is no problem at all. With 320V HT , 7k6 OPT and 43% UL taps with 240Ohm screen resistors I got ca. 11.3V in 8 Ohm which is around 15Watt.
I'm not impressed with their sound though, nice delicate mids and highs but bass wasn't very spectacular, but that has probably much to do with the type of music I listen to mostly.
I found the 6p43-p much better soundwise , though they require some serious drive with 280V HT , 6.2K OPT with 43% UL taps. They required -35V (not -45V as I mentioned erroneously in a previous post) grid voltage for ca. 25mA bias current. This gave ca. 20Watt output , with the added benefit that el86 ( I got 10 of them for 2,40 euro a piece, new in box from EI Yugoslavia, very nice tubes) is a drop-in replacement for the 6p43p-e , where the el86 is a true pentode and the 6p43p-e is a beam tetrode. EL86 sounds more like the 6p15p-ev leaning towards a JJ el34 and the 6p43p-e sounds more like a JJ 5881.or brimar 6bw6.
Tubes are fun, as long it's affordable, OPT's with complicated seperate anode and g2 windings and corresponding high-end prices are not fun.
As for the g2 voltage, yes, in pentode mode optimal settings for beam tetrodes is g2 substantially lower voltage than the anode , but not for audio pentodes like el34 and el84 or 4654 when you use HT within reasonable limits. KT77 has 300V max. screen voltage in tetrode mode according to datasheet at anode HT of 400-450V , where the el34 has 425V G2 voltage with anode HT of 400V in the average 35Watt p/c pentode amp. KT77 in UL mode can have 600V on the other hand , same with 6550 , in pentode mode max. 300V on the G2 , but in UL mode 600V is no problem either, if you look at the tungsol 5881 datasheet you will see the same pattern , 270V G2 with 360V anode HT, but in UL mode 450V HT is defined. When I say G2 should be around anode for best performance I mean UL mode and an affordable price for the OPT. For guitar amps things are completely different , where the amp is part of the timbre palette of the instrument and pentode mode offers much more characteristic sounds.
O.c. there are a number of transmitting and vertical deflection tubes with electrode structures where it is not wise to use a too high G2 voltage allso not in UL mode, but other than for experimenting, why would you use those for HiFi amps when there are so many good audio pentodes available to fool around with.
