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In Reply to: Humm , Mid bass , and Russian 6sn7s and filaments posted by Satelliteman on August 11, 2006 at 00:38:03:
We've seen a few EH tubes that were prone to leakage and thus causing hum. The newer tubes are supposed to have this fixed.We do not use the filament windings on the driver supply transformer anymore. In fact the newer amps use a driver supply transformer that lacks these windings. Instead we obtain the filament supply for the driver from the output section filament torrid. This works much better- try it!
Follow Ups:
Thankx Ralph ...can you tell me why the driver section had its own filament winding to begin with ? Whilst wiring the first chassis , I was very tempted to do just as youve spoken about above .re - feeding it from the 30amp toroid . Leakage - can you or anyone explain this a little more ( so I can learn whats going on inside these tubes ) .
Now - also regarding the newer wiring methods ( which ive only just become aware of ) does the Standby circuit still keep the HT B+ ( 300 vdc ) running ...as it used too ?? Because if I was to rewire the chassis's again , id prob have a switch for Filaments only ...and this also fed the secondary switch ( standard ) ..so that the Output Section ( 100 vdc bipolar supplies ) & the B+ ( +/- 300 vdc ) was together . At present I have it as original methods ...Heaters & B+ ....connecting to the second switch controlling the output 100vdc only .
Cheers for all the info so far - very informative .
Stu
The leakage RK mentioned is heater-cathode leakage, which is fairly typical in the Russian 6SN7 types. Since the filaments are AC, any leakage will superimpose a 60Hz AC signal onto the cathode, which results in increased hum.There is no need to join the windings of the large filament toroid to power the driver filaments - you should be able to run two 6SN7 off of each 6.3V secondary, without further issue. This is how it's done in all later M-60.
I would not suggest changing the wiring on the main power / standby switches. You want the driver B+ / B- supplies to be present before you apply power to the output section. The output section negative bias voltage is derived from the HV driver supply, by way of the V4 tube - and this voltage needs to be allowed to stabilize before power is applied to the output. The stock arrangement pretty well guarantees this.
Hope that helps, sounds like you're having fun!
Excellent ...Excellent & More ! - This is what the forum is all about - Great advice , and I fully understand you with all things said . Wiring each pair of 6sn7's to the Output Filament transfromer windngs will be very easy . This leaves me wondering what to do with the now ' so called spare ' 6.3vac winding in the HT toroid ...I might now use it purely to drive a lamp - I dont have any at all wired at present , but thats kinda how I like it - less is more .
YEP - having heaps of fun , and having a second monoblock nearly built im really getting excited to having OTL Stereo in the house.
Thanks guys for all youre support .
Stu - New Zealand
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