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Post by crimsonqueen on Apr 19, 2017 14:28:11 GMT -7
Hi, One of the screen resistors is burned (colors are gone and it's cracked). Should I change both? The other reads at 465k, but it should be 470k. Thanks
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Post by Jaguarguy (Mike) on Apr 19, 2017 15:04:25 GMT -7
Welcome to the forum! A lot of good people and information here. You might want to send an email to Don at repairs@drzamps.com - he'll know how to do it for sure.
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Post by DRZ on Apr 19, 2017 15:11:47 GMT -7
Hi, One of the screen resistors is burned (colors are gone and it's cracked). Should I change both? The other reads at 465k, but it should be 470k. Thanks If you're talking about the 6V6's screen resistor it shout be 470 Ohm, not 470K Ohm. I would replace the both as well as the 6V6 set. Z
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Post by crimsonqueen on Apr 19, 2017 15:23:43 GMT -7
Hi, One of the screen resistors is burned (colors are gone and it's cracked). Should I change both? The other reads at 465k, but it should be 470k. Thanks If you're talking about the 6V6's screen resistor it shout be 470 Ohm, not 470K Ohm. I would replace the both as well as the 6V6 set. Z Thank you Mr. Z. Yes, I meant 470 Ohm, not 470k. It burned after I tried another set of 6V6 tubes (JJs) from another amp (Traynor Darkhorse). The tubes I was using with the Z28 (Tung Sol) were not in the amp when it happened. Do I still need to change them or simply recheck the bias?
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Post by DRZ on Apr 19, 2017 16:49:02 GMT -7
Well a shorted 6V6 will cause the Screen resistor to pull too much currant, then burn. The tube caused the resistor to fail. So don't use the set there in when the failure happened. Yes you need to check bias EVERY time you swap output tubes in a fixed bias amp. Set the bias to 28 mA per tube.
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Post by crimsonqueen on Apr 21, 2017 16:02:21 GMT -7
I am having a hard time finding the resistors locally, and may have to order online. The ones in the amp like NTE's, but would other brands do?
They look like 3 watt flame proof metal oxide film 470 ohm. Is that correct?
Thank you
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Post by DRZ on Apr 21, 2017 16:19:10 GMT -7
Yes
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Post by Ridgeback on Apr 21, 2017 16:55:45 GMT -7
...and aside from designing and building classic amps like the Z-28 (his finest IMO), you get this kind of backup from the man himself.
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Post by crimsonqueen on Apr 21, 2017 17:46:40 GMT -7
Yes. Thank you, Doctor.
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Post by crimsonqueen on May 10, 2017 11:24:36 GMT -7
I gave up on finding the resistor around town and ordered some from Digikey. They arrived the next day! I took me a while to get to the repair, but I changed them and replaced the power tubes with JJs. I also rebiased the amp. I don't know if the resistor had been faulty for a while, or if I had not biased the amp properly when I got it last year, but it now sounds amazing!
I played an improvised blues gig yesterday and used the new and improved Z28. I was blown away!
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