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Post subject: Frying tonight?
Posted: Wed Apr 08, 2015 9:07 am
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Quick preamble - I have a (sort of ) early Princeton clone - 5W, volume and tone, hand wired, 6V6GT , 12AX7 and a 6Z4 rectifiier.

I have replaced all of tubes (but put the 6V6 back because I didn't like the tone of the new tube). The new tube did nothing to change the problem.

The 12AX7 has been replaced with a JJ gold pin. The 6v6GT is the original (Ruby tubes perhaps). The 6Z4 is a Chinese ebay purchase - I can't seem to get them any where else and I'm not best pleased with it as a result and will perhaps replace it ultimately with something more traditional.

Any road up it makes a sound like frying bacon and more noticeably so when the guitar is on less than full volume. I also get a sound like a bad pot when changing volume on the guitar even though I don't have a problem on other amps with the guitars.

Is it possible to give me any indication where my problem may lie based on the above?

Many thanks in anticipation.

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Post subject: Re: Frying tonight?
Posted: Wed Apr 08, 2015 9:46 am
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Your 6Z4 is a Chinese designed tube. There are no US or Euro equivalent. The 6Z4 is known for poor long-term reliability. Noisy and poorly built. I would not put this tube in a combo guitar amp.

The closest US type would be the 6X4. BUT, rewiring of the pins would be necessary. I would go this route. As there are a lot of very good US and Euro-Philips made 6X4 (EZ90).

My favorite is the black plate Tungsol (USA) 6X4W. But, there are MANY very good 6X4 (EZ90) types available. Mullard EZ90 are also most excellent! I've used both in DIY CD DAC and headphone amps. These last forever. Very tough, little tubes. The 6X4W are low noise, mil spec rugged rectifiers.

Here's the 6X4 pinout/data sheet:

http://www.mif.pg.gda.pl/homepages/fran ... /6/6X4.pdf


Here's the Chinese made 6Z4 (sorry, no English versions):

http://frank.pocnet.net/sheets/095/6/6Z4.pdf


6X4 (EZ90) on EBAY:

http://www.ebay.com/sch/i.html?_from=R4 ... 0&_sacat=0


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Post subject: Re: Frying tonight?
Posted: Wed Apr 08, 2015 10:05 am
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The crackling on a guitar volume pot is usually from DC voltage on the pot.
Where the DC is coming from is the big question. A bad 12AX7 would be the first thing to check.
Then look at the load resistor on the input. Fender mounted these on the jack and a loose jack nut can cause problems because that's where the 1meg load resistor gets it's ground.
The DC leakage is probably small because a large leakage will mute the tube completely. This could come from a flux on the board. Paste flux is slightly conductive and can cause this type of problem. Clean the board and socket with solvent to remove all flux.


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Post subject: Re: Frying tonight?
Posted: Wed Apr 08, 2015 1:07 pm
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Thanks for your thoughts guys.

I had pretty much already decided the 6Z4 had to go. I'm not at all happy that the only source is China and that it isn't as per the original Fender schematics. Confirming this may be a problem is sufficient to seal its fate.

My understanding is the amp is a Chinese supplied kit for building re badging by local amp builders. I have seen several versions for sale with different names.

Having purchased the amp on ebay for less than a chassis and cabinet I have no issues on ripping the board out and starting again if necessary, as this was my original intention. Obviously if I can avoid this I'd rather play than solder....although I quite like soldering.

So, my plan or attack - rewire the socket and replace the rectifier tube. If this fails to solve the problem and I cant find any obvious issue with the board then I shall swap the board out for a new one and upgrade the components while I'm about it.

Other than the "Frying tonight" noise (which is spasmodic) I am really pleased with the amp and it remains a great buy at £100.

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Post subject: Re: Frying tonight?
Posted: Wed Apr 08, 2015 5:38 pm
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One other thing to try, re-tension your tube sockets. Loose connections on tube sockets can result in intermittent crackling/sizzling noises.

http://billmaudio.com/wp/?page_id=171

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