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garbage 10th April 2005 01:32 AM

c3g producing sparks
well... sometimes... but that means it is not stable anyway...
and this happens on one monoblock only. and i am quite sure both are wired the same. at other times, it just plays...

i've re-checked the wiring on the offending mono, and it is:
pin 3,4 = anode/plate
pin 2, 5, 7 = cathode
pin 6 = grid
pin 1, 8 = filaments

i've also checked continuity of the circuit, solder joints, etc. been at this for about 3 weeks, but seems the problem is intermitent.

the circuit is posted in one of my previous thread.

what happened this morning is that when playing a cd, there is nothing at all.
then i can see/hear sparks at the c3g, around the region of pins 2 to 4 (my chassis is taken apart for me to see fireworks...). the B+ is at around 575v instead of 634v, about 60v difference. i did not troubleshoot much as i am in a hurry to switch it off after seeing sparks.

question is, how reliable is a siemens c3g as a triode strapped pentode?

if i should i try other drivers like a 6c45 or something, any suggestions on candidates that would drop into my current driver anode voltage of abut 170v with filaments of 6.3v?

i'm very tempted to rebuild this drd in another chassis in an attempt to eliminate hum as well...

Morgan L 11th April 2005 01:03 AM

Hello garbage,

I have no idea why there are sparks, but I think I see a source for the hum problem: B+ ripple is transferred via your Ultrapath cap to the anode of the output valve, where it modulates the transformer primary. As the B+ will modulate the other end of the transformer in-phase, this is what we want to cancel hum - only, it's probably too much, so the hum via the Ultrapath cap dominates. What I'm trying to say is: try bypassing the 300B's cathode resistors (i.e. cap goes from cathode to ground, across both resistors), starting with about the same size you're using as the Ultrapath, then if the hum diminishes, increasing until quiet. I'd guess you'll need about 3 times as much capacitance to ground. The music signal will not pass through this cathode bypass cap, so the sound should not be negatively affected (AFAIK).

Morgan L

garbage 11th April 2005 03:03 AM

hi morgan

thanks for the suggestion. will try that out. ;)

interestingly, the approximately 60v drop in B+ coincides with the bias voltage of the 300b tube. i will re-check the soldering on the cathode portion to see if it is ok.

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