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5AR4 questions

I will pull an 5AR4 from another amp and look at heater voltage and bias. Would not have gone on with heater voltage being off but it may need another look. Thanks to all who have responded on this issue. I will keep you posted with my findings. Thank you AK community!!!
 
You can check the heater voltage without the 5AR4 installed, and I'd probably do that first.

with all tubes except the rectifier installed, pin 2 to pin 8, measure in AC volts. Should be approx 5 volts.
 
chetgallery said:
Had a 5AR4 blow on a Pilot 240 after about 30 hours after a complete restoration. I normally knock the line voltage down due to the higher line voltages these days. On this one I used a CL90 thermistor and a 15 ohm 60 watt resistor. I would like to hear the voices of experience on my voltages. Pin 4 and pin 6 both have 235 volts ac and that produces 270 volts dc out. Is this voltage too small and has it expedited the 5AR4’s life? There are no minimum voltages that I can find. Hope to get a better insight on this. I have restored many tube amps with the voltage knocked way down without any issues.
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what do you have at the wall outlet 115,117,120,125. AC
 
At 120 out of a variac, no rectifier, pins 2-8 is 5.08 vac. Checked the rest of the filiment voltages and they were fine. 6bq5’s had 5.8 ac on the filaments and the 12ax7’s all had 6 vac between pin 9 and 4 and 5 which are tied together for 12 vac.
Checked all the tubes and none were any worse than before. Thank you 2 amp fuse.
 
I beginning to wonder if the 5AR4 was damaged during the fall from the bench. Very fine sounding amp. I’m the second owner and it’s a keeper. Thanks again to all who have posted. Would have never known to check voltages without a rectifier. Have read about swapping out the 5AR4 for a 5U4 or 5Y3 and rewire a few pins but not sure on that. I have several 5U4 and 5Y3 NOS and would like to hear opinions on different rectifiers.
 
5.8 on the 6bq5 is low, 5.02 with no rectifier load also is a little lower than I'd expect. That will drop with a tube in it. I think you have too much drop on the primary side. Might want to bypass that resistor and leave just the CL90

5U4 draws 3 amps vs the 1.9 of a 5AR4. 5Y3 has 125mA of current capacity vs the 250 of a 5AR4. Without knowing how much margin there is on that 5v winding I wouldn't suggest a 5U4 and a 5Y3 will be horribly overloaded.
 
Could that have led to the 5AR4 going south?
It would be easy to just use the CL90.
Maybe just enough resistance drop to get 6.3 vac on the 6BQ5’s? 5 ohms or 10. Could experiment.
 
chetgallery said:
Could that have led to the 5AR4 going south?

chetgallery said:
Most likely original to the amp in 61?
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Is this the same GZ34/5AR4 that has been in the unit since, maybe, 1961?

This is the same unit that you recapped 90 days ago, right? The new filter caps can stress an old, borderline rectifier.

Tubes wear out. Even Holy Mullards (edit: or Dutch Amperexes). ;)
 
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Mr Boochie said:
Is this the same GZ34/5AR4 that has been in the unit since, maybe, 1961?

This is the same unit that you recapped 90 days ago, right? The new filter caps can stress an old, borderline rectifier.

Tubes wear out. Even Holy Mullards. ;)
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Yes it appears to be the original Bugle Boy 5AR4. The fuse was also looking original. it is the same unit that I rebuilt about 90 days ago. Should I go ahead with another 5AR4 and watch closely? Idle voltage was 5.08 vac on the 5AR4 and the rest of the heaters were 1/2 volt shy of the 6.3 and 12.7 volts..
 
I have the VTA ST-120. It originally had the 5AR4 rectifier in it. I took it out and subbed in an NOS Mullard GZ37 with no modification. My amp has been running perfectly fine with the GZ37 for several years. It also has a quartet of KT-120 tubes in. Never had an issue. I would look into swapping out that 5AR4 for either a GZ33 or GZ37 and not worry about any more problems. They are a bit expensive now, but would you rather keep working and hoping that whatever you do fixes things or would you rather put something else in and not have anymore problems? I would check out and compare the GZ33/37 against the 5AR4 and see if it would work in your amp.
 
I may have one or both of those. Its an option.
First I want to check voltages without the dropping resistor like Gadjet73's recommended first. This is all new to me. I have converted several RR decks into stand alone amps without any issues. Albeit they were usually 6X4 rectifiers. What a learning experiment this is.
 
Its an EL84 amp, it won't need anywhere near what a set of KT88 making almost 4x the power will. A 5ar4 is plenty sufficient for an SA 232.

A single rectifier of any type is grossly undersized for an amp that claims 120 watts of output. It was undersized for the ST70 at half that much.
 
This amp outputs 30 watts max according to the owners manual. Let me get some readings without the dropping resistor and go from there.
Still don't know if its just an old tube that gave up? I did take a fall.
 
chetgallery said:
Yes it appears to be the original Bugle Boy 5AR4. The fuse was also looking original. it is the same unit that I rebuilt about 90 days ago. Should I go ahead with another 5AR4 and watch closely? Idle voltage was 5.08 vac on the 5AR4 and the rest of the heaters were 1/2 volt shy of the 6.3 and 12.7 volts..
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Yes. Use that other one you currently own. And good luck!

I'm pretty sure your unit is okay. You just need a new rectifier.

:thumbsup:
 
I will swap in a known good 5AR4 and check voltage between pins 2-8. Then remove dropping resistor and measure again to see how much load takes the voltage down.
Thanks for the heads up on the GE and Sylvania tubes. I shall return with numbers in a few days. Thanks again to all who have helped. I don't have the knowledge that any of you have. Just started this hobby about 8 years ago and haven't had any real troubleshooting issues.
 
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