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In Reply to: Re: Knight ka-25 posted by Ron Oehlert on March 2, 2007 at 16:46:35:
Yes, thank you. The problem is it is a kit that back in the day some guy slapped together with poor soldering skills and I think my problems are due to mistakes made when built. I have already found 3 mistakes. Some of the parts in the power supply may not be the right values etc. Plus this thing has a strange circuit with all the output cathodes tied together and connected in series with the 12ax7 filaments and all the screens tied together too. My main problem is I am measuring about 90 ma screen current for the 4 6bm8's, too much. I have more screen current than plate current! Plus the 400 ohm 10 watt resistor in the power supply is dissapating like 9 watts, too much total current in power supply. Anyway, thanks.
Follow Ups:
I can send you a scan of the Calrad SA-30 which has 4 6BM8s and the same arrangement in the heater string - tbavis(at)rochester(dot)rr(dot)com
Can you see the optical illusion when looking at this postings first 4 subject lines? It looks like a line through the 4 25's. Maybe it extends thhrough this posting now.
Certainly does extend.
..."this thing has a strange circuit with all the output cathodes tied together and connected in series with the 12ax7 filaments and all the screens tied together too."That cathode treatment is not uncommon for an enrty level amp. Rather than generate a DC voltage to power the filaments for the 12AX7's, the amp uses cathode biasing for the output tubes and uses the filaments of the 12AX7's as 'a big dumb power resistor' to set the standing current through all the output tubes. So, a 12AX7 has a heater voltage and current of 12.6V/0.15A if connected from the 4 and 5 pins. Two of those in series should be about 25V/0.15A. Now the 400ohm resistor is probably in parallel with the cathodes of the output tubes and at 25V should add approximately 0.0625A to the total current or .15+.0625= .2125A or 212.5mA. The 212.5mA will divide 'equally' between the four output tubes giving each a total current draw of 53.125mA. Anyway, that's probably the way it's supposed to work. Also, having all the screens of the output tubes connected together and driven by a single RC network from the V+ supply is also standard pentode operation.
You should measure the voltage at the cathodes of the output tubes and see if the DC voltage is in the 25-20V range. In addition to the 12AX7 filaments and the 400ohm resistor, there is going to be a sneaky 25-50uf/25-35V electrolytic capacitor in parallel. If that capacitor leaks, the whole circuit is going to start drawing too much current. Replace it with a modern cap in the 35V range. Let us know what you measure.
Yep, I've been surprised that desite the clear & complete instructions included with Heathkits, so many assembly errors also exist. However, all screens connected together is not unusual, nor is a common cathode resistor or tied to filaments. You could always use the sockets & iron & wire it however you see fit if you do not find it in Sams. Email me if you desire a copy via USPS of the Heathkit AA-32 integrated stereo amplifier schematic which uses 6eu7(a 6.3v low-noise version of 12ax7) & 6gw8/ecl86(an eclxx-family cousin to the 6bm8); might aid in deciperhing, has all 4 screens together but separate cathode R per channel & no tie-in with filaments. Obviously, tube pin-outs are diff.
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