Stromberg AR411 question
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| Posted on April 12, 2014 at 18:31:34 | ||
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Just spent over 10 hours testing, checking and changing a few out of tolerance part on a pair of old late 50's Stromberg Carlson AR-411 Amplifiers I want to sell. I can get one of them to give me about 6 watts Sine Wave Power @ 1000 Hz into an 8 Ohm resistive load (200W), but I cannot coax more than about 2.5 watts out of the other amplifier. Traced all DC voltages and they are within about 5% of each other and I also traced the 1000 Hz signal throughout the amplification chain and also see very close Peak to Peak voltage on my scope up to the input of the Phase 6AV6 phase splitter. Swapped tubes and this did not change things. Tried NOS 6CM6 power tubes, same.... changed the 12AX7 and also the 12AT7 Phono Stage tube, same. As a last resort I pulled out the 6CM6 tubes and injected 1000 Hz into the primary of the OPT's and measured the AC voltage on the 8 Ohm tap and calculated the primary impedance of the OPT's. Much to my surprise I found that the one that gives me the 6 watts has 6800 Ohms (matches the Sams Photofact specification) but the other one is giving me around 8100 Ohms. Both OPT's have the same base part number printed but there is a second number that does not seem to match, the 8100 Ohm OPT numbers are fuzzy and incomplete. Question: Can the higher impedance on the 6CM6 tubes give me such a power reduction when sine wave power testing? I can't find anything else to account for the difference. Even the 250 Ohm Bias resistor voltage drop is within 3% of each other at 22V (6 Watt) and 26V (3 watt). To me it looks like the 6CM6 may not be able to provide the same power with a higher primary impedance OPT. Playing music through them sounds wonderful and I only have to turn the higher power one down one number to get them to play well. 1959 factory Transformer error? Thoughts? Thanks! Rich
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