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Hi there Tube techs ..well , I have to say that after replacing my radiotron & RCA mixture of 6sn7's for my first amp , ive now been hit with a dose of hummmmmm . This happened because I replaced all the 6sn7s with Eh 6sn7s ..as i didnt have a matching set of four for the 2nd monoblock . What was immediately apparent , was the new mid bass that I was missing before . However , with that , ive now become aware at a quiet listening level , a Humm that was just NOT what it used to be . Ive tracked this to V2 ...which is most significant in a swapout . If I change V2 with a Radiotron Australia for example , the Humm is gone , and I mean nill - zip - ear upto the speaker , and zippo - none . So thats with 3 Eh 6sn7 & 1 oddball 6sn7 in V2 , the hum is not hearable at 1ft from speaker. I guess that this is all about Heater ripple transfer internaly happening in the tubes . This is quite a first hand experience with NEW russian tubes , and the effects that they can have . At this stage , im siding to having an odd ball V2 in place , as my listening levels are normally on the quieter side of the m60 abilities. Speaking of heater / filament connections - What are the normal connections of the Output & Driver windings . I have it as follows ....Driver Transformer is wound with a 6.3 @ 3A fro the 4 6sn7's . The Transformer for the output stage's is All filaments on one bank ( x4 ) is supplies from a totally isolated winding from the other bank . So in a sense , the 6AS7g's are on their own bipolar supplies , and heater supplies also . I read on the forums that its possible to wire the driver stage in parallel with the Output stage , anotherwords , feeding them from the 30amp 6.3v toroid , instead of the driver txfrm . If this is so , this makes me feel that the output stage filament transformer secondaries must be common . Do I have this correct ?? I have tried to study many pictures , and cannot make out , if at all the filaments sections should be isolated from each other .
Currently I have wired them to be totally isolated from each other . Thats how I asked for the toroids to be wound . I asked for a 2 x 6.3vac @ 15 amp . - this allowed me to have truely isolated output stages within the chassis itself . HELP - have I gone over the top ?/\Stu
Follow Ups:
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!
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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