Filament breakage failures (partial or complete) were very frequent in the case of Svetlana and older Sovtek DHTs, and some others as well. I'm not sure why they failed to achieve what Western Electric and Marconi-Osram seemed to achieve near faultlessly, for decades.
If you choose to run filaments from ac (not recommended for any DHT, but still..) - please take one precaution: don't run a 1.2A filament from a 6A rms winding. If you do, the filament will actually suffer a 6A+ startup pulse - the cold filament resistance is well below 1 ohm in a 300B. It takes little imagination to see that the breakages of the filament are most likely to happen during this violent cold start.
The latest version of the EML DHTs have altered their filament design, to give a very slow rise in current, which I suspect is aimed at avoiding this exact problem.
With proper current limited dc, filaments are protected from turn-ON surges.
If you choose to run filaments from ac (not recommended for any DHT, but still..) - please take one precaution: don't run a 1.2A filament from a 6A rms winding. If you do, the filament will actually suffer a 6A+ startup pulse - the cold filament resistance is well below 1 ohm in a 300B. It takes little imagination to see that the breakages of the filament are most likely to happen during this violent cold start.
The latest version of the EML DHTs have altered their filament design, to give a very slow rise in current, which I suspect is aimed at avoiding this exact problem.
With proper current limited dc, filaments are protected from turn-ON surges.
Sorry, but again not enough information. Too many unknown variables...
If he's had the amp running for 4 years (assuming reasonable on time) then there are some number of operating hours on the tubes. Cathode depletion would be a far stretch, but again, not enough detail. No idea what the operating point is for the 2A3, which affects overall useful life. No idea of the condition of the original 2A3 (NOS, used, etc.) 4 years ago. Did the hum just start suddenly or build up over time?
If AC filaments are being used, what is the topology for nulling the filament hum? Is there a balance pot? Was the replacement tube a simple replace or was there any adjustment required (aka, hum pot)?
If he's running some newer production 2A3 tubes (meaning not a real NOS unit from the 50's or earlier) then it's anybody's guess... could be gas, a cathode coating flake that's touching a grid wire, some other internal contamination, etc..
Regards, KM
the guy was supposed to send me his 2A3 set after i cured his 6SN7 line amp of hum, needless to say i haven't seen the amp nor the construction details.......then i got a call from him the other day telling me that he replaced the 2A3 tube with a new one and that the hum was gone, i did not press him for more details.....i will ask him again if and when he calls me...
I use both dh and idh valves, and I find very little difference,
The main valves I use are pre 1935 designed British triodes,
I have used many of the sub two watt IDH triodes as output valves and they give nothing to the px's except power, the likes of mesh 41mp and the 41mpx, ac/p1 etc, (not the later types) They were designed as output valves, but also make top notch ..possibly the best drivers I own,
As those same valves get newer production, they don't sound quite so well, but still very good compared to most of the modern stuff,
As an earlier comment stated is about the metals they are made from.
I have some 600 volt military transmitter versions of the px25 and they don't sound just quite as good as the audio versions, as they use different metals, they have the sound of transmitter valves.....as in missing the finer harmonics in the recording compared tho the audio version. I have both the Osram and Mazda versions of these.
My understanding is both the cathode coating and the quality of how it was applied was/is critical to the emissions of the valves.
I have it on good authority that some of the early pentode a where built on the same lines and provide exceptional detail. I have a box of early (pre 35)ac/pens that will be tested one day
My understanding is the original mono plate 2a3 /6a3 was a lesser copy of the original british px4
post war production is to be avoided.with many valves. Just hear the difference between the wartime vt75/kt66 vs the post war kt66 or a globe px25 and the later straight sided version. its not subtle with the right kit.
The main valves I use are pre 1935 designed British triodes,
I have used many of the sub two watt IDH triodes as output valves and they give nothing to the px's except power, the likes of mesh 41mp and the 41mpx, ac/p1 etc, (not the later types) They were designed as output valves, but also make top notch ..possibly the best drivers I own,
As those same valves get newer production, they don't sound quite so well, but still very good compared to most of the modern stuff,
As an earlier comment stated is about the metals they are made from.
I have some 600 volt military transmitter versions of the px25 and they don't sound just quite as good as the audio versions, as they use different metals, they have the sound of transmitter valves.....as in missing the finer harmonics in the recording compared tho the audio version. I have both the Osram and Mazda versions of these.
My understanding is both the cathode coating and the quality of how it was applied was/is critical to the emissions of the valves.
I have it on good authority that some of the early pentode a where built on the same lines and provide exceptional detail. I have a box of early (pre 35)ac/pens that will be tested one day
My understanding is the original mono plate 2a3 /6a3 was a lesser copy of the original british px4
post war production is to be avoided.with many valves. Just hear the difference between the wartime vt75/kt66 vs the post war kt66 or a globe px25 and the later straight sided version. its not subtle with the right kit.
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