Mike DeMeo wrote:
Hello,
I was given the above amplifier. I replaced the power cord as it looked bad, and I replace the selinium recifier with the 1N4007 diode. The amp powers up with no hum and I can hear the audio from the turntable.
Now the question. What else do I need? I noticed there is no volume control. What kind of pre amp is needed ?
The ST-70 is a classic power amp, they do not have volume control or anything else. Any conventional hi-fi preamp will work, tube power amps are very easy loads to drive, so tube or solid-state is fine.
If I understand you correctly, you hooked up a conventional phono cartridge directly to the ST-70. That was fortunate, as the very low output of the cartridge kept it from blasting you out of the room. Hook of a conventional line source like a tuner and it will come on at absolute max volume and probably blow something in the speaker from clipping in very short order. To properly amplify a phono cartridge (presumably MM (moving magnet) 5 mV, as most of the common cartridges are) you need a phono input on a preamplifier with an RIAA compensation curve followed by a line stage. A conventional preamp of the day like the Dynaco PAS2/3/3X has that. If it is a MC (moving coil), cartridge, it needs a separate MC phono stage to get it up to either 5 mV MM phono output, or all the way to a line input. The latter is frequently powered by batteries since it takes very little power and is *extremely prone* to hum pickup.
Assuming you get your preamp and other items, you almost certainly have to refurbish the ST-70 to have safe operation, as noted above, at the very least, replace the electrolytic capacitors, both on the B+ side and the bias supply side. Realistically, and if it was *me*, I would get one of the replacement driver boards, stuff it carefully with all new components, then replace the entire circuit and all associated components. It's relatively cheap, the boards in the original are the worst of the worst and almost certainly one stray bump from a problem (check the color change around the 7199 tubes - they are almost certainly turned to elemental carbon at this point!). All the parts live in a little furnace, although it is not at all the worst case, tube amps work primarily by heat-driven processes. 200 watts goes into it, 2 watts come out the speakers under normal listening volume, so the other 198 is used to heat up the tubes, the transformers, the cover, the room it resides in, etc.
If you changed the bias supply from a selenium rectifier to a silicon diode, make sure you also reset the bias as per the instructions (1.56 volts between the chassis and the test point on the front panel sockets, because it will likely be FAR too high, since the diode raised the bias supply voltage between 9 and about 20 volts.
Brett
p.s. After reading more carefully you seemed a little hesitant. If you are ever going to do a electronics refurbishment job, this is a pretty good place to start. It was a kit originally, and has very good instructions for assembling it, easy to work on, and replacing the driver board is a very "modular" oepration, you can build it entirely on the bench with the amp elsewhere, and the removal of the old one and connection of the new is very straightfoward. Every single part to rebuild it is easily available( or build it from scratch with all new parts, or from a kit) along with an endless stream of information (much of which is bovine scatology, but not all).
