I have a Sherwood S-5500 III integrated tube amplifier that has not been used in decades. My plan is to restore it and have on order new capacitors for the power supply. It has the original tubes which I will remove and test in a tube tester. The tubes are (5) 12AX7 (Telefunken) and (4) 7868 (Sherwood). It is solid state rectified. Before I power it up with a variac and dimb-bulb tester, I will clean all of the controls with contact cleaner because I'm sure they are crunchy.
To avoid damaging the hard to find tubes, can I power it up and measure the power supply voltages without the tubes installed?
To avoid damaging the hard to find tubes, can I power it up and measure the power supply voltages without the tubes installed?
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It is common and prefered practice to remove the valves before powering up equipment that has stood for a very long time or has no history.
If those are genuine Telefunken 12AX7 tubes, you could sell them on ebay (each currently $80 and up for used)
and buy new JJ replacements for around $20 each. If you do sell them, don't clean off the silk screen logos.
and buy new JJ replacements for around $20 each. If you do sell them, don't clean off the silk screen logos.
Thanks for the quick replies. That's good to know I can remove the tubes before powering it up.
I don't plan on selling the Telefunken tubes any time soon, I'd like to hear them for a while and compare them to JJ's
I don't plan on selling the Telefunken tubes any time soon, I'd like to hear them for a while and compare them to JJ's
Before removing tubes or any other tampering ; connect a variable DC source top the B+ and (slowly)
increase voltage while observing current. 100V per hour, stop if current does not go down to zero and figure
out what is leaking.
4-5 hours later your electrolytic caps has passed ( or failed) Then connect speakers and power on. Connect
a source and check sound. If anything is missing or wrong then fix it.
Tubes are replaceable, your transformers are not.
My bench power supply goes to 50 volts DC.
What are the advantages of the method you suggest over using a variac ?
What are the advantages of the method you suggest over using a variac ?
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You have absolute control of the B+ current and does not need to have any AC in the initial testing.
When B+ is at the anticipated voltage AND no current is drawn you know that B+ and caps are without shorts.
Testing one item at a time is usually beneficial.
Next step is to figure out if any bias supply works ( power on with removed B+ or removed power tubes)
Finally power on with signal and listen for good or bad sound.
When B+ is at the anticipated voltage AND no current is drawn you know that B+ and caps are without shorts.
Testing one item at a time is usually beneficial.
Next step is to figure out if any bias supply works ( power on with removed B+ or removed power tubes)
Finally power on with signal and listen for good or bad sound.
Use of a Variac can cause overvoltage of some of the filament supply capacitors, due to no filament loading,
so watch them carefully.
Right, it depends on what voltage rating the capacitors have, especially the final one.
The series resistor(s) and the tube filaments form a DC voltage divider.
When the tubes are removed, there's no more dividing down.
The series resistor(s) and the tube filaments form a DC voltage divider.
When the tubes are removed, there's no more dividing down.
One example : Old oltronix psu
I am not aware of new production to reasonable price but threads in this fora has discussed building such devices.
Simplest is to connect a variac to a tube powersupply ( Si rectified) and with Voltage and current meters.
Sometimes that is worth a 2nd look. After you are satisfied with the tests with no tubes,
cut the B+ lead from the rectifier to the first cap at the half way point.
And insert something like 5-10K, a few watts at the break
Hookup a VM to watch B+, whatever you have & stuff the rectifier back in.
Apply power, the voltage should slowly increase. It may be only a short time or could take hours,
the caps may very well reform to working condition.
Unhook the power occasionally, check that the first cap is not getting hot. 🙂
Then keep things simple. Just use a fused Variac, connect 10 ohm 2W resistors to the output,
and slowly turn the Variac up, starting at 0, to around 60VAC.
Then gradually increase the Variac over 30 minutes or so, until stopping at 120VAC.
Let cook for an hour or two while observing, in case something happens.
and slowly turn the Variac up, starting at 0, to around 60VAC.
Then gradually increase the Variac over 30 minutes or so, until stopping at 120VAC.
Let cook for an hour or two while observing, in case something happens.