GZ34 failure modes and reasons
- pre65
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#31
If you still have the 75R resistor after the rectifier, the voltage drop across that should give you the current the amp is drawing.
The only thing necessary for the triumph of evil is for good men to do nothing.
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
#32
Well doing that it measures as 2.26 volts dropped across 83 ohms giving 27.2mA. But that is with an effective 300B anode voltage of 300V (as the HT is way lower than it should be).
Without the 300B the HT comes upto 451v with a current draw of 13mA. The CCS on the driver is set for 10mA so not so far out.
Is it right that an increase from 13mA to 27.2mA (when the 300B is inserted) would lead to the drop of the HT from 431 to 300V?
I also tried swapping the 300b. The first gives total draw of 27.2mA (as measured above). The other measures as 34.2mA but the HT drops back to 270V. The bulb in this instance was brighter, so presumably less volts to the amplifier?
I think I am getting stuck. Any thoughts on these observed results?
Cheers
Ian
Without the 300B the HT comes upto 451v with a current draw of 13mA. The CCS on the driver is set for 10mA so not so far out.
Is it right that an increase from 13mA to 27.2mA (when the 300B is inserted) would lead to the drop of the HT from 431 to 300V?
I also tried swapping the 300b. The first gives total draw of 27.2mA (as measured above). The other measures as 34.2mA but the HT drops back to 270V. The bulb in this instance was brighter, so presumably less volts to the amplifier?
I think I am getting stuck. Any thoughts on these observed results?
Cheers
Ian
- Mike H
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#33
Could be just that that 300B is dying.
Either gas inside is making it conduct more, or poisoned grid? Emitting electrons so is getting a positive bias from anode?
I had a cheap Chinese one once that would start off at correct current as biased, then after a minute or so the anode current went up and up and up and up and up and up ......
Had another one more recently that suddenly decided to turn into like a neon lamp, pulled down the HT until it extinguished, HT restores, it restrikes, pulls down HT again, speaker was going BANG! BANG! BANG! BANG! just as well I was still in the room at the moment it happened so could turn it off quick
I'm assuming your 300B grid is cap coupled from driver, if so another possibility to bear in mind is leaky coupling cap pulling up the grid bias. Can be simply tested by trying it with coupling cap disconnected.
Either gas inside is making it conduct more, or poisoned grid? Emitting electrons so is getting a positive bias from anode?
I had a cheap Chinese one once that would start off at correct current as biased, then after a minute or so the anode current went up and up and up and up and up and up ......
Had another one more recently that suddenly decided to turn into like a neon lamp, pulled down the HT until it extinguished, HT restores, it restrikes, pulls down HT again, speaker was going BANG! BANG! BANG! BANG! just as well I was still in the room at the moment it happened so could turn it off quick
I'm assuming your 300B grid is cap coupled from driver, if so another possibility to bear in mind is leaky coupling cap pulling up the grid bias. Can be simply tested by trying it with coupling cap disconnected.
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
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#34
Appendix ~ but if it "seems better" with one 300B plugged in (low current more HT) and "seems worse" with the other 300B plugged in (higher current, low HT) then suggests to me the latter 300B maybe knacked, and not actually a circuit fault at all
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
#35
Mike,
I was begining to wonder if it might be a simple failure of the 300B that led to the whole problem.
I have replaced the coupling cap between the driver and the output stage so I don't think this should be an issue at the moment.
Do you think I should remove the bulb so as to know where the voltage should be then replace the 300B's (I have some spares) and carefully switch on while measuring current through the amp?
Cheers
Ian
I was begining to wonder if it might be a simple failure of the 300B that led to the whole problem.
I have replaced the coupling cap between the driver and the output stage so I don't think this should be an issue at the moment.
Do you think I should remove the bulb so as to know where the voltage should be then replace the 300B's (I have some spares) and carefully switch on while measuring current through the amp?
Cheers
Ian
#36
Finally resolved the problem it was the 300B that failed. The failure pulled a massive current through the amplifier leading to the frying of the resistor after the rectifier and also destroying the rectifier.
The bulb trick is very useful for testing suspect gear, however, the way the voltages are pulled down within the amplifier is deceptive when yopu are fault finding.
So I am sorted other than needing some new 300B's. My spare pair are mesh plate and cannot dissipate enough power. Any thoughts on the best reasonable price 300B's?
Cheers
Ian
The bulb trick is very useful for testing suspect gear, however, the way the voltages are pulled down within the amplifier is deceptive when yopu are fault finding.
So I am sorted other than needing some new 300B's. My spare pair are mesh plate and cannot dissipate enough power. Any thoughts on the best reasonable price 300B's?
Cheers
Ian
- pre65
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#38
I'd second that.Ali Tait wrote:The EH Gold Grid are good for the money.
The only thing necessary for the triumph of evil is for good men to do nothing.
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
- pre65
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#40
Can't find them on E-bay.Greg wrote:I'll third that! Anyone know the best place to buy at the moment? Is Chinaarts still selling?Ali Tait wrote:The EH Gold Grid are good for the money.
These are not too expensive.
http://www.hotroxuk.com/matched-valves- ... 0b-eh.html
The only thing necessary for the triumph of evil is for good men to do nothing.
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
Edmund Burke
G-Popz THE easy listening connoisseur. (Philip)
-
- Old Hand
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#43
Thoughts are just little stories that our brains tell us !
#45
Thanks guys, Chinaarts seems to remain the best option. I've had two 300B quads from him in the past and his service is reliable. His quad price is £220 vs Hot Rox at £276.
When I originally bought from Chinaaarts a few years back I paid £140 per quad. Times are a changing
When I originally bought from Chinaaarts a few years back I paid £140 per quad. Times are a changing