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paulrmanning wrote:I was given this ST70 amp some time ago and am in the process of cleaning it up. They have replaced the rectifier tube with a diode bridge rectifier and a couple caps. I cannot find any data on the caps or this particular mod. Anyone else seen this? the caps have TVLU 2925 and TVLU 1930 of them
PeterCapo wrote:paulrmanning wrote:I was leaning towards pulling out the mod and reinstalling the socket and getting a solid state rectifier.
You already appear to have solid-state diodes substituting for the tube rectifier. You'd just have to figure out how to get appropriate dropping resistor(s) installed as well as a thermistor or time delay to ease startup. If you pull the diodes out, reinstall the socket and then go with a plug-in, note that the Weber copper cap that emulates the soft start and voltage drop of a 5AR4/GZ34 is not recommended for hi-fi tube amps https://www.tedweber.com/wz34
If you believe Weber's caution against using their WZ34 in a hi-fi amp, then the choice you're left with is a plug-in solid-state replacement that contains diodes only, like their WS1 or equivalents sold at other places. In this case, you'd still want to find a way to add dropping resistors and also a properly rated thermistor or time delay to ease startup.
Apart from all that, there's still the question of the resistors on the speaker terminals, the extra resistors on the power tube sockets and whatever else may have been changed for better or for worse. Lacking complete documentation on it, I would not assume that the amp is otherwise functioning properly.
PeterCapo wrote:paulrmanning wrote:I was leaning towards pulling out the mod and reinstalling the socket and getting a solid state rectifier.
You already appear to have solid-state diodes substituting for the tube rectifier. You'd just have to figure out how to get appropriate dropping resistor(s) installed as well as a thermistor or time delay to ease startup. If you pull the diodes out, reinstall the socket and then go with a plug-in, note that the Weber copper cap that emulates the soft start and voltage drop of a 5AR4/GZ34 is not recommended for hi-fi tube amps https://www.tedweber.com/wz34
If you believe Weber's caution against using their WZ34 in a hi-fi amp, then the choice you're left with is a plug-in solid-state replacement that contains diodes only, like their WS1 or equivalents sold at other places. In this case, you'd still want to find a way to add dropping resistors and also a properly rated thermistor or time delay to ease startup.
Apart from all that, there's still the question of the resistors on the speaker terminals, the extra resistors on the power tube sockets and whatever else may have been changed for better or for worse. Lacking complete documentation on it, I would not assume that the amp is otherwise functioning properly.
PeterCapo wrote:New2Tubez wrote:For what it's worth, the Weber WS-1 is available with a thermistor option (WS-1t). I have this on my ST120.
That could be very handy. But is there any information available as to exactly what kind of thermistor it uses? Its ratings for cold resistance, maximum steady-state current, and hot resistance at different % of max current? I suppose Weber might provide that info if someone were to inquire. It would be important to know.
PeterCapo wrote:The 5AR4/GZ34 has a voltage drop that puts the B+ voltages at the proper value. The 5AR4/GZ34 also has a slow-start characteristic that prevents excessive stress on the amplifier's parts. A simple solid-state diode replacement, whether diodes as you have right now or the kind that plugs into an octal socket, loses the slow-start, and the B+ values will rise.
Earl wrote:PeterCapo wrote:The 5AR4/GZ34 has a voltage drop that puts the B+ voltages at the proper value. The 5AR4/GZ34 also has a slow-start characteristic that prevents excessive stress on the amplifier's parts. A simple solid-state diode replacement, whether diodes as you have right now or the kind that plugs into an octal socket, loses the slow-start, and the B+ values will rise.
What is the typical voltage drop of this particular rectifier?
