GZ34 failure modes and reasons

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Mike H
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#46

Post by Mike H »

Yep still got them in my watch list. Might be import duty as well though! :shock:
 
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DavidC
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#47 Testing of Tube Amp without driver and output tubes.

Post by DavidC »

Hi,

A newbie question.
Is it safe to test a tube Amp by powering it on just with rectifier tube , without the driver and output plugged in? Will it cause the rectifier tube to arch?

I'm asking because I built a DIY power amp and tested as such to do some measurements with just the rectifier GZ34. The rectifier arch and fuse blew.

Thanks
David
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Nick
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#48

Post by Nick »

Normally the only danger is if the caps are not rated to take the off-load voltage of the power supply. If they are not, its a broken design (IMHO). if you are getting arcing, then its either a problem, too large first cap, or least likely a faulty rectifier.
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#49

Post by DavidC »

The B+ design is like this:

380 VAC CT, GZ34 -- 5uF/630 VDC -- 10H Choke -- 100uF/500VDC -- 100uF/500VDC --bleeder Resistor. The 5uF/630 VDC is an oil cap and the
100uF/500VDC is the electrolytic. I powered on without the rectifier and the HT measure at 385 V. If the rectifier arch without the driver and output tube,what is the possible cause? I was asked to measure the resistance from the B+ connection to chassis to check. If the resistance is below 50K, there is some kind of shorts, valid?

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David
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pre65
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#50

Post by pre65 »

Some valve rectifiers specify a minimum resistance of each transformer winding, so you may need extra resistance in each one.

I seem to remember adding 75R to one of mine.
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#51

Post by simon »

Which brand of GZ 34 is it David? I've had problems with new production ones arcing, and had a particularly impressive display at Nick's once a few years ago. It would be unlikely to this though I'd think.
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Dave the bass
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#52

Post by Dave the bass »

Yup, I had a JJ GZ34 that made lightning from new.

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#53

Post by pre65 »

Have a look at parameter Rt on this data sheet. The value varies with transformer voltage and only seems necessary on cap input. Ignoring this can cause flashovers. :wink:

http://www.mif.pg.gda.pl/homepages/fran ... g/GZ34.pdf
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#54

Post by steve s »

If the fuse that blew is between the rectifier and the 1st cap it indicates something wrong after that point, I'm not sure how your 100 uf caps are connected, but without a load they should have received around 540 volts +... Depending how the caps are wired of course ,the caps are lightly to short internally, just as nick pointed out above, also check the bleed resistor is still functioning and showing the correct resistance.
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#55

Post by DavidC »

I did some measurement from the B+ to chassis ground and it came up to 1Kohm. It seems there is a short to ground. I then proceed to disconnect the wires that connected the 300B's socket filament pins from the 5V AC tabs from the power transformer. I measured again from the B+ point to chassis ground, now it's in Mohm. I powered up the Amp with just the GZ34 , now with no arching. The GZ34 is an Amperex but I think it's a rebrand from new production, not NOS. Now I'm confused how the wire connection for the filament supply rfom the power transformer to the 300B socket can cause the short.

I'm using a 100ohm pot with cathode biased with 100uF/100V across a 1Kohm resistor to ground.

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Mike H
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#56

Post by Mike H »

Wiring mistake?

Or your filament supply is grounded somewhere, but it mustn't be as 300B filament is also its cathode.
 
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#57

Post by DavidC »

There is only wire connection from the 5 VAC tabs of the power transformer to the 300B socket pin 1 & 4 with the 100ohm pot.
The cathode biased resistor 1Kohm and 100uF/100V connected with the center pin of the pot, while the other end is connected to ground.
There are no other connections.
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#58

Post by Nick »

This is a topical post. make sure you have the 300b in the right way (or wired up right). if one of the heater pins is in the hole for the anode, then the heater will connect b+ to the cathode. Bet you have a 1k cathode resistor and I bet its connected to the centre tap of the heater supply?
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#59

Post by DavidC »

Yeah, the connection of the pins are correct. 1K cathode resistor indeed connected to the center tab of heater. The thing is I disconnected the B+ connection to the plate while I did the test with just the rectifier power up and the arching occurred. Any possibility of power transformer problem?
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#60

Post by pre65 »

David, have you checked Rt on each secondary winding on the mains transformer ? :wink:
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