EL84 triode mode
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rock soderstrom
Tour de France
In essence, it's very simple, you put the two additional grids (G2+G3) of the pentode on anode potential, which essentially turns it into a triode. Sometimes this doesn't work because G3 is connected directly to the cathode internally (e.g EL84), so connect only G2 to the anode and you have a quasi triode.
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rock soderstrom
Tour de France
check this, good example for the EL84. The resistor between the switch and the valve is the so called screen grid resistor for pentode mode. It doesn't bother your tube in triode mode, you do not need it for an only triode mode pentode.
I have an EL84 based amp I modified with triode/pentode switching
I used a dual pole switch to connect g2 to the anode and cut in a different value of cathode bias resistance in parrallel with the existing value , to better suit triode mode .
Using a transformer designed for an EL84 single ended pentode wont get the best out of a triode , so its always going to be a compromise
maybe a tapped primary winding could allow better triode /pentode switching ,
On another EL84 based SE triode amp I have G2 connected directly , I found its happy with 220v at the plate but not much more.
I used a dual pole switch to connect g2 to the anode and cut in a different value of cathode bias resistance in parrallel with the existing value , to better suit triode mode .
Using a transformer designed for an EL84 single ended pentode wont get the best out of a triode , so its always going to be a compromise
maybe a tapped primary winding could allow better triode /pentode switching ,
On another EL84 based SE triode amp I have G2 connected directly , I found its happy with 220v at the plate but not much more.
thanks for the info. your explantion is crystal clear and helpful
thanks for the info. im currently working on a KLD amp designed for the triode / pentode switching thing. not being an electrical engineer i could not figure all that was going on. now i have more of an idea.
thanks for the schematic. its a big help
If you do away with the G2-current-limiting resistors, your EL 84's will soon be destroyed
rock soderstrom
Tour de France
I have seen many circuits with pentodes in triode mode, where G3 is naturally at anode potential. Many tube microphones and microphone preamps do this.(e.g. Neumann U47, Gates SA70 etc...).
It works perfectly and without any problems (and yes over decades of time). Triode operation of pentodes does not necessarily mean power tubes, this is also often done to reduce noise from the first gain stages in amplifier circuits. Let's stick to reality.
A look at the data sheets of the EL84 shows that it does not necessarily need a screen resistor in triode mode, as long as the limit values are adhered to.
Stop yelling, if you have something to say, then say it in a normal tone. That's how it's done here, if you have problems with that, then go to Gearslutz.
BTW, learn the quote function here in the forum, it's not that hard!
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I run triode connected EL84's with no current limiting resistor in G2 ,
220v at the anode , a pair of them have run very nicely for over a decade without a breakdown,
Typical 5W tube amps with an EL84 pentode run at between 250 and 270v , it is a bit high for direct G2 connection , it might be worth sacrificing a small amount of power in pentode mode so that in triode mode its still happy ,
I use additional LC and RC networks to drop back the HT voltage and noise .
220v at the anode , a pair of them have run very nicely for over a decade without a breakdown,
Typical 5W tube amps with an EL84 pentode run at between 250 and 270v , it is a bit high for direct G2 connection , it might be worth sacrificing a small amount of power in pentode mode so that in triode mode its still happy ,
I use additional LC and RC networks to drop back the HT voltage and noise .
Seems to me a special output transformer would be needed to properly match both triode and pentode mode to the speaker .
Switching on the primary involves high voltages and invariably noise unless the amp has a standby switch ,maybe a dual secondary with series/parralel connection to account for the different impedence requirements of triode and pentode mode
Maybe CJ might know more,
Switching on the primary involves high voltages and invariably noise unless the amp has a standby switch ,maybe a dual secondary with series/parralel connection to account for the different impedence requirements of triode and pentode mode
Maybe CJ might know more,
rock soderstrom
Tour de France
My attempts were not particularly desirable, I did not find the EL84 in triode operation to sound particularly good when simply switching in a guitar amplifier.
To do this really well I think there are a few things to consider, most of which you have already mentioned. The cathode resistor/ biasing of the tube is certainly an issue and the mismatching of the transformer should also be considered.
From my point of view, it's not worth it in terms of sound. When it comes to power reduction, there are better approaches.
I have a Peavey nanovalve with the triode pentode switch mod I described with a limiting resistor on G2 connection , theres a lack of clean power in triode mode because of the mismatch but the basic clean tone of the amp also changes and theres a different harmonic sparkle when you drive it . It all amounts to a useable variation in tones from an amp which originally only had volume only although I added passive treble cut
Real tube distortion without lifting the roof off the place thats what Im more after these days ,
50 and 100 watt tube amps cranked upto 11 , I dont do that anymore .
Loud amps create spill problems so why bother when 1.5, 5 or 15 watts will do .
Real tube distortion without lifting the roof off the place thats what Im more after these days ,
50 and 100 watt tube amps cranked upto 11 , I dont do that anymore .
Loud amps create spill problems so why bother when 1.5, 5 or 15 watts will do .
plate resistance, or impedance for the propeller heads, is lower in triode mode,
so if using the same output xfmr, you should see a bit more bass as your primary inductance stays the same while the plate r drops,
i thought i saw an amp that uses taps to compensate for this , ie they switch the xfmr to lower z while in triode
triode might be good for jazz but most folks keep the switch on pentode.
it is an easy addition for amp makers to sell more amps,
so if using the same output xfmr, you should see a bit more bass as your primary inductance stays the same while the plate r drops,
i thought i saw an amp that uses taps to compensate for this , ie they switch the xfmr to lower z while in triode
triode might be good for jazz but most folks keep the switch on pentode.
it is an easy addition for amp makers to sell more amps,
Thank you for your kind response - and for pointing out that i should use the cursive function, if i wish to stress a specific point in my text. My apologies -
: I was not not aware of the fine text-editor functions in this fora. I almost never yell at people - but on many platforms there's no cursive function, so i have acquired the habit of using KAPITAL LETTERS as a substitute for cursive. I will look into the qoute-function, you mention - but bare with me, if i don't get it right this time.To the "Tube-grids and experimentation" :
After a happy youth with lots of experiments - i got the chance to actually work on Marshall's and Fenders and other brands at a repair-shop. There, i understood why most repairsmen, after months of smelly, burned-out power-transformers and twisted/distorted tube-plates, develop a liking towards the "tested and proven" ! ( most disasters were due to owner experimentation and flawed/unstable factory designs, but there was also the occasional thermal-run-away caused by cheap early-production china-tubes ( i think they have learned to use the right alloys/materials by now ? )). The current-limiting resistors in good/stable output-designs are not for fun. All the common power/output tubes out there, will have their life-span drastically reduced if either Plate- or G2-currents get exceeded. In small signal-amplifiers, like microphone, line-level, RIAA-circuits and even the input-stages of power-ampliifers, we find all sorts of Triodes, pentodes, hexodes etc, in all sorts of kinky and weird configurations, with or without grid-resistors ( but, for small signal circuitry, these are often introduced to tame oscillations or simply to reduce bandwith ) - as these tubes are not dealing with currents anywhere near their operating limits. Don't get me wrong - i come from "a place of unlimited experimentation", but you know how sad people can become ( yes, even angry ! ) if they loose money - or irreplaceable equipment - like VINTAGE GEAR !!! .
Thanks - once again - ( looking forward to more sharing of interesting stories, opinions and experience ! / Wilhelm
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