I have been busy with a new lab, hopefully one that I will not outgrow for many years. Had to buy a new house to build it and there was much to do.
This weekend I serviced an RM-9 which I am happy to say is in excellent health for an amp that is over 30 years old. All the caps and everything else is original and fully up to spec..... except the tubes. The KT-88 output tubes sourced from thetubestore.com ranged in bias voltage from 48.3 to 56.3 and transconductance from 2750 to 4500. RAM TUBES are always +/- 0.5 volts and +/- 5% Gm. Here's the link to our matching technology.
http://tubeaudiostore.com/tubin1.htmlThe other problem the amplifier had was counterfeit Mullard E88CC driver tubes. These tubes would not hold balance in a 15 minute period within 50 volts. They literally wandered all over the place and were very sensitive to a few percent line voltage variation. My selected drivers will center the pot and hold within 5 volts for hours and with line variations of 10%.
The Mullards appear to be Mullards but not E88CC. They are likely either PCC88's (the 7 volt tube) or 7ES8's the variable gain tube. If the customer cares to leave them behind I'll study them when time permits. If you haven't read my article on this from 30 years ago, here's the link.
http://tubeaudiostore.com/suitof6dfora.htmlBecause the 7DJ8, 6ES8 and 7ES8 were made in quantity for color television there is a great surplus of them on the European market at very low prices. Disreputable people are labeling them into the 6DJ8/6922/ECC88 family where they do not belong. One thing I do that others seem not to is to spot these when they show up. Be advised!
I also serviced at RM-1 (circa 1979) which is in fine condition other than the tubes which were triggering the voltage detecting automatic mute. Unlike most mute circuits which are simple timers, the RM-1 has a DC detecting mute that monitors DC or low frequency noise on the output. This is because the RM-1 is DC not capacitor coupled internally all the way to the output. I believe this is the only tube preamp that has ever been made this way.
The mute was being triggered by low frequency 1/f noise which is excessive in the 6H23PI tubes that were substituted for the specified 6DJ8/6922 in the phono stage. Although these tubes had only 6 db more audible noise they had 30 dB more 1/f noise causing several volts of drift at the output. Although some like the 6H23PI, I cannot recommended it for this reason. Even in an AC coupled preamp they will cause a lot of woofer motion which results in Doppler and Inter Modulation distortion in both the speaker and amplifier.
I am happy to report that this year I have found an excellent batch of 6922 that are the lowest noise I have ever seen in my 32 years of using this tube. They are currently available at tubeaudiostore.com. If you have been plagued by phono noise, they will give you the lowest possible noise in your preamp. They have renewed my interest in making a full function preamp with moving coil input.
