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Re: Heater voltage in a JTM 60?

3/24/2004 11:15 AM
Le Basseur
Re: Heater voltage in a JTM 60?
You forgot saying that "in my oppinion" thing,assuming that a lack of courtesy could be considered an oppinion.;)  
Anyway,the Philips Electron Tube Handbook (1966) says:  
[QUOTE]Indirectly heated tubes,parallel supply  
It is reccomended to take measures to ensure that during operation the heater voltage differs as little as possible from the published value.  
When a tube,having nominal heater characteristics,is used at the highest or lowest nominal mains voltage of a voltage range as determined by the mains taps,the heater voltage,measured at the tube socket,must be within +/- 7% of the published value.Part of this deviation may be due to the tolerances of the heater voltage transformer.If this condition is fulfilled,then any tube of the type concerned may be used in the appropriate place and the circuit may be connected to a mains fluctuating by max.10%.[/QUOTE]  
The Siemens Special Tubes catalogue (1965/1966 edition) states a even lower tolerance of only 5% from the nominal specs.The same applies for Telefunken.  
The original Russian factory datasheets are giving a 10% tolerance.  
Anon,(or should I say "Trollie"?!?:D),you're right about your presumption.Indeed,every amp made in the tweed up to BF IS eating prematurely the tubes because the line voltage ratings are different today.You cannot see such a symptom on an average tube tester because this will measure only some basic parametres,not the filament's behaviour,so I don't expect you're gonna believe "such a crap",as you might say in your infinite gentleness:).  
And,again,you're right about the average (short) period between a retubing job.Most amp owners would throw out every few months the entire set and replace with fresh ones.  
Frankly,I don't see a problem if someone is slightly worried or intrigued about this issue.The older amps can be easily fixed about the filament over-voltage thing with the expense of a pair of ceramic resistors of a fraction of an Ohm and this doesn't imply a change of the amp's sound or a value loss of the same amp.  
What if a certain guy has some very good-sounding,dead-quiet and expensive ECC803S's in his amp?Should he pay some serious $$$ just because the amp has a (fixable) inherited flaw?  
...Oh,one more thing...a few weeks ago,I recall a "anonymus" arguing in a electrolitic max.voltage thread here on the BBS.How come that on that subject you were the strongest advocate of the "factory specs and reccomended tolerances",but in the present filament over-voltage theme,you turned yourself against the specs?  
(,don't get fooled,that was only a rethorical question....I really DON'T expect your answer!)  
3/24/2004 1:28 PM
Dave Rich
I had the same problem with my JTM 60. I think mine was something like 7.3 to 7.5 volts! My line voltage was 126 VAC. This was about a year ago so my memory is a bit fuzzy but I think I ended up with .165 ohms (two .33 ohm, 5W resistors in parallel) on the heater line.  
Also I would recommend installing a cooling fan directly underneath the EL34s. It's easy to mount one on that metal cage that encloses the tubes. Two of the filter caps are mounted directly above the EL34s and they live a short, unhappy life there.
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