So I spotted a reasonably priced old ST-70 amp on craigslist for $400. The amp worked but had some issues. 90% original wiring and someone kinda replaced the output terminals. Lots of heat sweat around the old 7199 input tubes. It did work but had a bit of hum and noise, but had a new full set of output tubes and rectifier. He shaved a bit off the price and I took it home. I'm not totally sure what and manufacture date is, but it doesn't have super old looking wire from the transformers. I'm guessing early 70's but I could be wrong.
I had come across this amp in the passed and was aware of the modification that were popular. I was just lucky that I had the cash and was willing to give this whole tube thing a shot. After digging into the amp I decided to just replace everything.
New quad cap, octal driver board, sockets, input terminals and so on. It took two nights and got everything right the first power up. Its now singing sweet music.
I knew tubes get hot but was a little weary of just how hot these thing get. At full warm up I'm measuring near 400 degrees F on the power tubes. Wow that's hot.
The amp with old board and parts had lots of noise, but after the rebuild there is none. I mean almost zero noise from the amp. I have a few other solid state amps and they all have some noise but this has none that I can detect with my ear. Of course this is very dependent on the source.
The amp currently has JJ EL34's. They are brand new and will give this amp some time before I settle on whether it sounds pleasing or not. I'll definitely give the amp some one-on-one time. Here are some pics the enjoy.
I purchased some JJ KT-77's from a local dealer and i'm now running them. I let them warm up good them re-biased them. The EL-34's were dead quiet but the left channel now has a very quiet hum that has faded over the last few days. Is this a normal thing for brand new tubes? Like I mentioned before the EL-34's don't sound unpleasant. The amp overall had a very heavy mid-range at first impression. Big difference from my SS amps. My ears are still very new to the tube sound. The KT-77's out of the box have a slightly subdued mid-range compared to the 34's. I pulled out my testing mic and sure enough there was a very slight reduction in the 1k to 3k range. Both treble and bass seemed to preform the same.
I've read about tube rolling and have seem discussions about which tubes will make a bigger difference on the overall sound profile. Do you guys think replacing the 6SN7's will change more than replacing the power tubes? What i've read is that some of the older tubes may not run at the B+ voltage that is on my new board. What more do I need to know before plugging in random preamp tubes? Also my power transformer runs rather hot. I've read on the forum that the original was a bit small for the amp. Do you guys think its a "must replace" item? Whats the down side of just letting it run a little hot. It hovers around the 140's F but takes about 40 minutes to get that hot.
Assembling the octal board was a snap and the documentation was very useful. However I did receive a small print out of a different wire diagram for the 2.2k resistor that differs from the schematics that I can find. As you can see in the picture below I wired it as the supplement showed. The B+ hovers near 380v DC.
Assuming I did a good job I hope this amp will provide plenty of listening enjoyment. Haven't made my mind yet on whether it sounds better but it definitely has a "different but not unpleasant" sound.