GZ34 failure modes and reasons

pre65 · Post by **pre65** » Tue Mar 06, 2012 11:08 am

If you still have the 75R resistor after the rectifier, the voltage drop across that should give you the current the amp is drawing.

IDM · Post by **IDM** » Tue Mar 06, 2012 1:09 pm

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

Mike H · Post by **Mike H** » Tue Mar 06, 2012 1:41 pm

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.

Mike H · Post by **Mike H** » Tue Mar 06, 2012 1:47 pm

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

IDM · Post by **IDM** » Tue Mar 06, 2012 1:57 pm

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

IDM · Post by **IDM** » Tue Mar 06, 2012 8:11 pm

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

Ali Tait · Post by **Ali Tait** » Tue Mar 06, 2012 9:09 pm

The EH Gold Grid are good for the money.

pre65 · Post by **pre65** » Tue Mar 06, 2012 10:28 pm

Ali Tait wrote:The EH Gold Grid are good for the money.

I'd second that.

Greg · Post by **Greg** » Tue Mar 06, 2012 10:54 pm

Ali Tait wrote:The EH Gold Grid are good for the money.

I'll third that! Anyone know the best place to buy at the moment? Is Chinaarts still selling?

pre65 · Post by **pre65** » Tue Mar 06, 2012 11:01 pm

Greg wrote:
Ali Tait wrote:The EH Gold Grid are good for the money.
I'll third that! Anyone know the best place to buy at the moment? Is Chinaarts still selling?

Can't find them on E-bay.

These are not too expensive.

http://www.hotroxuk.com/matched-valves- ... 0b-eh.html

Greg · Post by **Greg** » Tue Mar 06, 2012 11:28 pm

That Hotrox link doesn't state they are 'Gold Grids'. EH also made a standard option that was cheaper but much duller in sound. I'm not convinced these are the ones to go for. More research required.

pre65 · Post by **pre65** » Tue Mar 06, 2012 11:32 pm

Aha, sorry folks I did not realise.

richardcooper2k · Post by **richardcooper2k** » Tue Mar 06, 2012 11:35 pm

gold grids

http://www.hotroxuk.com/matched-valves- ... -grid.html

pre65 · Post by **pre65** » Tue Mar 06, 2012 11:37 pm

And here is Chinaarts on e-bay

http://www.ebay.co.uk/itm/NEW-Electro-H ... 519df1d6a2

Greg · Post by **Greg** » Wed Mar 07, 2012 12:01 am

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

audio-talk

GZ34 failure modes and reasons

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