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Radford STA25 Mk III -- a lot of questions.

There's a lot of helpful posts here which I want to respond to. But first let me say thank you for your interest, time, and advice.

First, it did not run for 6 hours continuously. It maybe ran for two periods of 30 minutes and one of an hour max.

Second, given that Emporium are in the loop and are willing to act responsibly, it feels right to give him one chance. I expect him to open it up and diagnose the problem. If he can repair it and he wants to, all well and good. If he can't, or if it's too expensive a repair for him to take on, that's fine too. We can talk about what to do. What isn't good is if he says he's repaired it but hasn't done it well, and so it fails again, as in crimsondonkey's case, and I'll try to stop that happening.

I take on board the thought that this is an old amp and needs a thorough overhaul, and I won't hesitate to get that done. But first, given that I bought it "in good working order" I would like to have it in good working order!

As far as opening up, it's a fabulous idea. What I'll do is ask Emporium to let me see photos.
 
if you've still got it, take the bottom panel off, its held on with a few little screws. Top covers the same. Unplug it !! dont touch the parts as caps can pack a punch for weeks.
If hes got it, you'll just have to trust him to give the work to someone competent. Why should he send you photos without charging you for dicking about.
 
mandryka said:
There's a lot of helpful posts here which I want to respond to. But first let me say thank you for your interest, time, and advice.
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:)

mandryka said:
First, it did not run for 6 hours continuously. It maybe ran for two periods of 30 minutes and one of an hour max.
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Ah, that changes the diagnosis.

Was the pop a loud bang with smell and a lot of smoke or a little puff and some smoke?
 
a thought - if he says repair is free - replacing all the other old parts with new is £50 while its here - take him up on it. You wont get new transformers. All the old caps are dried out and well past their best (surprising how they can reform and sound improves even after 50 years), resistors are pennies.
 
Darmok said:
:)



Ah, that changes the diagnosis.

Was the pop a loud bang with smell and a lot of smoke or a little puff and some smoke?
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Loud bang with a lot of smoke and a smell (it was wonderful!, really dramatic!)

when it was first plugged in I thought there was a strange smell, but it seemed to go away pretty quickly.

15 minutes before the bang and smoke I thought I heard a pop from the amp. And I thought I heard a little bit of distortion briefly, but I can't be sure of that. By this time any noise at all -- the wind, a dog barking -- was being attributed to the amp!
 
Radford Revival said:
Ideally but people like to fit 5A or 10A fuses to these unfortunately (not accusing anyone, this could have happened at any point during the amp's past)
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Understatement, imv. ;)

Tragically is a better term, a fuse wrapped in tin foil is sheer stupidity. I have seen this many times on my workbench. :(
 
Darmok said:
My wager is on bias capacitor fail.

;)

PS: Unless the mains reservoir capacitors had been replaced and not properly being reformed after a standing long shelf life.
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There isn't actually such a thing as a "bias capacitor".... I was guessing you meant the smoothing cap for the bias supply... I'll bet you £100 without even looking at it that it's not that! A virtually unheard of fault and completely different symptoms!
 
mandryka said:
Loud bang with a lot of smoke (it was wonderful!, really dramatic!)

15 minutes before the bang and smoke I thought I heard a pop from the amp. And I thought I heard a little bit of distortion briefly, but I can't be sure of that. By this time any noise at all -- the wind, a dog barking -- was being attributed to the amp!
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Well then, main reservoir capacitor fail, when you remove the base plate you will see a lot of cotton wool looking mess all over the place and a plastic capacitor solder tag hanging off the reserve cap.

The electrolyte has boiled and the pressure can has exploded.
 
Darmok said:
Well then, main reservoir capacitor fail, when you remove the base plate you will see a lot of cotton wool looking mess all over the place and a plastic capacitor solder tag hanging off the reserve cap.

The electrolyte has boiled and the pressure can has exploded.
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And why would that have happened?
 
Arkless Electronics said:
There isn't actually such a thing as a "bias capacitor".... I was guessing you meant the smoothing cap for the bias supply... I'll bet you £100 without even looking at it that it's not that! A virtually unheard of fault and completely different symptoms!
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No, in the Radford STA25/3 the 2 volt bias supply is bypassed by an electrolytic capacitor for each EL34, I will consult the circuit diagram for the value in microfarads,

STA25.jpeg


See component C11 / C12. 250 Mfd, 6 volt DC working.
 
Darmok said:
No, in the Radford STA25/3 the 2 volt bias supply is bypassed by an electrolytic capacitor for each EL34, I will consult the circuit diagram for the value in microfarads,

See component C11 / C12. 250 Mfd, 6 volt DC working.
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Those are the cathode bypass capacitors, the supply capacitors are the 8uF / 150V caps which are supplied from a half wave rectified dedicated bias winding on the mains transformer (which has irritated me for years so in the ones I build or refurb I now use a PCB designed to use a bridge!)

The 250uF caps are to remove the small degeneration effect of the 39R cathode resistors at audio frequencies, the resistors of which are approaching a value high enough for this to be worth doing
 
Darmok said:
No, in the Radford STA25/3 the 2 volt bias supply is bypassed by an electrolytic capacitor for each EL34, I will consult the circuit diagram for the value in microfarads,

STA25.jpeg


See component C11 / C12. 250 Mfd, 6 volt DC working.
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Cathode bypass caps... no such thing as "bias caps".... and they are very unlikely indeed to fail. They also wouldn't blow up if they did....
 
Radford Revival said:
Those are the cathode bypass capacitors, the supply capacitors are the 8uF / 150V caps which are supplied from a half wave rectified dedicated bias winding on the mains transformer (which has irritated me for years so in the ones I build or refurb I now use a PCB designed to use a bridge!)

The 250uF caps are to remove the small degeneration effect of the 39R cathode resistors at audio frequencies, the resistors of which are approaching a value high enough for this to be worth doing
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Yes of course, my bad, my STA25 marches on without any problems, I forget the circuit terminology.

It is my pleasure to converse with you over my favourite Radford Classic.

Thank you for the reminder.

Cathode Bypass, mutter mutter, my defecting grey. :)
 
Radford Revival said:
Those are the cathode bypass capacitors, the supply capacitors are the 8uF / 150V caps which are supplied from a half wave rectified dedicated bias winding on the mains transformer (which has irritated me for years so in the ones I build or refurb I now use a PCB designed to use a bridge!)

The 250uF caps are to remove the small degeneration effect of the 39R cathode resistors at audio frequencies, the resistors of which are approaching a value high enough for this to be worth doing
Click to expand...

To add to that.... it is of course a fixed bias amp rather than cathode bias and the resistors are only there to allow easy measurement of the quiescent current.
 
Arkless Electronics said:
Cathode bypass caps... no such thing as "bias caps".... and they are very unlikely indeed to fail. They also wouldn't blow up if they did....
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Sorry Arkless but I have to correct you, on 2 occasions my brother in law returned his STA25 that I procured for him with the same fault, he hadn't adjusted the bias voltage and it had caused the fail. cathode bypass capacitor blown, EL34 had to be replaced.
 
Neither is wrong - a loss of bias or runaway valve will kill a cathode bypass capacitor, but a dead bypass capacitor (in the case of a fixed bias amp) won't cause runaway :)

A cathode biased amp will tolerate the cap failing open, but not short, of course.
 
Radford Revival said:
Neither is wrong - a loss of bias or runaway valve will kill a cathode bypass capacitor, but a dead bypass capacitor (in the case of a fixed bias amp) won't cause runaway :)

A cathode biased amp will tolerate the cap failing open, but not short, of course.
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I suspect in my bro in laws STA25, he didn't adjust the bias potentiometer which set in motion the EL34 runaway, causing the cathode bypass cap stressed by over voltage.

I should have got him an STA15 cathode bias amp, no ongoing maintenance required.
 
Darmok said:
I suspect in my bro in laws STA25, he didn't adjust the bias potentiometer which set in motion the EL34 runaway, causing the cathode bypass cap stressed by over voltage.
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Indeed - it doesn't help that on the originals the caps were only rated for 6V! You can approach that on the cathodes with the amplifier delivering full power, without a fault condition. Admittedly rare in real world use, but still.
 
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