Hi One of my monoblocks went off this evening as the fuse had blown. I replaced the fuse and cautiously fired it up. Within about 60 seconds I could see the GZ34 rectifier arcing (flashover?).
The tube is about 4 years old, so is that just end of life failure or is there likely to be something more sinister at play?
The amps are LD2+ and just recently I did modify the PSU to make the first cap 10uf not 100uf as the stock amplifier had. I did this as I thought this would be better for my rectifier.
Cheers
Ian
GZ34 failure modes and reasons
-
pre65
- Amstrad Tower of Power
- Posts: 21400
- Joined: Wed Aug 22, 2007 11:13 pm
- Location: North Essex/Suffolk border.
#2
Have you got any resistors before the GZ34 ?
In the date sheet it specifies the resistance needed, but that includes the transformer resistance.
In the date sheet it specifies the resistance needed, but that includes the transformer resistance.
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
- Amstrad Tower of Power
- Posts: 21400
- Joined: Wed Aug 22, 2007 11:13 pm
- Location: North Essex/Suffolk border.
#4
The limiting resistors go between the transformer and rectifier, one to each anode 
75R is a common value, depends what voltage you are getting.
See this data sheet for Rt
http://crude.mine.nu/sgz34.html
75R is a common value, depends what voltage you are getting.
See this data sheet for Rt
http://crude.mine.nu/sgz34.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)
-
pre65
- Amstrad Tower of Power
- Posts: 21400
- Joined: Wed Aug 22, 2007 11:13 pm
- Location: North Essex/Suffolk border.
#6
Never assume. 
Perhaps the amp "designer" misunderstood ?
If one half of the transformer secondary winding is 75R or over you won't need them.
According to the data sheet they are only needed with cap input.
Perhaps the amp "designer" misunderstood ?
If one half of the transformer secondary winding is 75R or over you won't need them.
According to the data sheet they are only needed with cap input.
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
- Amstrad Tower of Power
- Posts: 21400
- Joined: Wed Aug 22, 2007 11:13 pm
- Location: North Essex/Suffolk border.
#8
Maybe the resistor wattage was underspecified ?
Easiest thing is to measure the total resistance of the transformer secondary (HT bit) and report back.
Easiest thing is to measure the total resistance of the transformer secondary (HT bit) and report back.
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)
#9
just musing..doesn't LD have adjustable cathode bias????
not sure how it's wired but maybe an idea to check the cathode pot
not sure how it's wired but maybe an idea to check the cathode pot
There's nowhere you can be that isn't where you're meant to be
#10
No I dont think there is adjustable cathode bias. There is a 1K cathode resistor on the 300B which I have bypassed with a 90uf capacitor.
I just measured the DCR of the mains transformer secondary and I get 193R for the amp that failed and 192R for the amplifier that is ok. From which I assume the transformer is proably ok.
I just measured the DCR of the mains transformer secondary and I get 193R for the amp that failed and 192R for the amplifier that is ok. From which I assume the transformer is proably ok.
#11
I also measured the resistance from the rectifier cathode to Earth looking for some form of short, but it measures as 128.7K which is almost exactly right when I calculated all the bleeder resistors and potential dividers in parallel. So I am not sure where to look next. 
#12
OK I have now gone over everything and I cannot find any fault. None of the Black Gate PSU caps appear swollen or distorted and nothing else seems out of spec. Yet the burnt resistor tells me that the amplifier did see a sudden and massive current.
The driver circuit is an E180F triode strapped with a 10M45 CCS. I assume that if this had failed then the 10M45 would have fried. Does that seem like a reasonable assumption?
Is it possible or likely that the 300B failed in such a way as to cause a huge current surge?
Cheers
Ian
The driver circuit is an E180F triode strapped with a 10M45 CCS. I assume that if this had failed then the 10M45 would have fried. Does that seem like a reasonable assumption?
Is it possible or likely that the 300B failed in such a way as to cause a huge current surge?
Cheers
Ian
#13
ok, pot or not, it may be useful to measure resistance between cathode and ground, just in caseIDM wrote:No I dont think there is adjustable cathode bias. There is a 1K cathode resistor on the 300B which I have bypassed with a 90uf capacitor.
I just measured the DCR of the mains transformer secondary and I get 193R for the amp that failed and 192R for the amplifier that is ok. From which I assume the transformer is proably ok.
There's nowhere you can be that isn't where you're meant to be
-
Mike H
- Amstrad Tower of Power
- Posts: 20189
- Joined: Sat Oct 04, 2008 5:38 pm
- Location: The Fens
- Contact:
#15
Commonest causes of V rectifier arcing ~
1. reservoir cap too big a value (I've blown up 2 x 6X5's like that)
2. like Phil says not enough limiting resistance in series with the anodes (although that includes the transformer winding resistance, which is often enough by itself)
3. HT short-circuit downstream, includes a supply capacitor that only breaks down at high Voltage else to all other intents and purposes will look perfectly OK.
The burnt out resistor does sugest #3.....
I've also had in the past a shorted cathode bypass cap on an o/p stage that could blow a supply fuse
1. reservoir cap too big a value (I've blown up 2 x 6X5's like that)
2. like Phil says not enough limiting resistance in series with the anodes (although that includes the transformer winding resistance, which is often enough by itself)
3. HT short-circuit downstream, includes a supply capacitor that only breaks down at high Voltage else to all other intents and purposes will look perfectly OK.
The burnt out resistor does sugest #3.....
I've also had in the past a shorted cathode bypass cap on an o/p stage that could blow a supply fuse
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."