Hello all, today I switched on my ST-70 amp for about half hour, then I switched it off to change the Audio cable, in 20 seconds I switched it on again and saw the blue spark in 5AR4 for 2-3seconds, then there was no any sound from both speakers. All tubes are lit up.
Can anyone give me some advice what happened for this? 5AR4 or power transformer was damaged?
Thanks in advance
Aaron
Can anyone give me some advice what happened for this? 5AR4 or power transformer was damaged?
Thanks in advance
Aaron
Almost certainly you need a new 5AR4. There's no HV, so no sound. Is there white fuzz inside on the glass? Order one from a reliable source.
Avoid power cycling the amplifier. The rectifier tube has a built-in warm up delay, and the tube must be cold at the start for this to work properly.
Avoid power cycling the amplifier. The rectifier tube has a built-in warm up delay, and the tube must be cold at the start for this to work properly.
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Installing a single UF4007 in series with each plate lead (pins 4 and 6) will reduce the likelihood of 5AR4 rectifier failure. Banded end of diode faces socket. This increases PIV that the rectifier can withstand and reduces occurrence of arc over during a hot start or momentary interruption in power.
I recommend waiting 2 minutes before turning amp back on.
I recommend waiting 2 minutes before turning amp back on.
Not sure who first suggested this idea, but where the 5AR4 is concerned it has greatly reduced failures where I use them. Even NOS 5AR4 can be fried this way so it is not unique to current production tubes although they may be a bit more susceptible.
Where the 5AR4 is concerned it is always a good idea to have a spare or two. Really this is true for tubes in any amp you use on a daily basis.
Where the 5AR4 is concerned it is always a good idea to have a spare or two. Really this is true for tubes in any amp you use on a daily basis.
Make sure that the first cap in the B+ has not been replaced with a larger value uF, or paralleled with an additional cap. Tube rectifiers do not like to charge large value capacitors. In the ST70, the original values do stress the 5AR4 rectifier to near the limits. Changing to more uF in the 1st cap, and even the 2nd cap (right after the choke) will take out the rectifier. Hot starts are also hard on the amp (turning on when the output tube cathodes still have some heat in them).
Hello everyone, I bought one JJ 5AR4 tube and UF4007 today, just finished the yellow page mods, plugged in new tube, switched on the amp, it was working flawlessly, however, after 20 mins, I heard loud hum from left speaker then I saw the plates of one of left channel EL34 and the new 5AR4 were burned very red. Then I switched of the power immediately.
Is this caused by new rectifier tube or the mod? Do I need to adjust the bias after I replace new tube?
Thanks
Is this caused by new rectifier tube or the mod? Do I need to adjust the bias after I replace new tube?
Thanks
You have a bad EL34 tube, most likely. Less likely, a leaky coupling capacitor feeding it, or else a bad solder joint in its grid circuit. That is, the EL34 problem is stressing the 5AR4. The simplest way to verify is to replace and rebias the left pair of EL34.
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Always adjust bias after tube change ! As the "redplated" el34 is junk you have the opportunity to order a matched pair and do initial bias adjustment as per the dynaco manual. ( don't reuse the EL34 that has been overheated) Maybe you should instead buy a matched quad from mcshane to really get a fresh start.
Before you install a new matched pair of EL34s, check the coupling caps for leakage. The original "Black Beauty" caps become very leaky with age. Even a simple Ohmmeter can find that. If they are leaky, the EL34 grid bias will not be able to overcome the problem, and you will have one or more over conducting EL34s.
Before you reset the bias for the EL34s, do a little mod: Replace the 2 each 15.6 Ohm cathode resistors with 4 each 33 Ohm resistors, one for each cathode, and add 2 test points. That way, you can monitor all 4 EL34 cathode currents individually. Each one should be 1.65V, which is 50 mA.
Note: I finally got the resistor values right, and the calculation right.
Before you reset the bias for the EL34s, do a little mod: Replace the 2 each 15.6 Ohm cathode resistors with 4 each 33 Ohm resistors, one for each cathode, and add 2 test points. That way, you can monitor all 4 EL34 cathode currents individually. Each one should be 1.65V, which is 50 mA.
Note: I finally got the resistor values right, and the calculation right.
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I started adding additional diodes to Stereo 70's a long time ago. Probably about the time I came on this site and called them junk. I first added them to my Quicksilver mono block amps and I still have the same sets of 5AR4's in those Quicksilvers and they have a lot of years on them now.
The very best thing to do for your Stereo 70 is modify the bias circuit and add additional pots so that each tubes bias can be adjusted individually. Your problem with the El34 turning red was that pair of EL34 tubes and not the result of adding several 1N4007 diodes.
Another thing I do after I add the additional bias pots is to slightly lower the bias. You will hear absolutely no difference after doing this and the tubes will last a lot longer.
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