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Some years ago I had built this 8W OTL for my brother.
Some weeks ago, while listening some loud music, my brother heard a sharp crack coming from the left channel, he saw a sudden blue glow inside one of the 6080 tubes and then the left channel was dead.
During the internal inspection, apart from a blown 2A fuse (of the negative HT rail) there was no other apparent damage.
I replaced the fuse but the bias pots were still not working.
After further investigation I have found out that the small power resistors on the cathodes of the 6080 tubes had changed their resistance from a few ohms to over 20K.
To me this is a mistery!
How could several power resistors possibly undergo such a huge simultaneous electrical change without any apparent physical damage?
I have replaced the faulty resistors (all) with new 10W ceramic resistors, re-adjusted the bias and so far everything seems to be working fine.
Best Regards
Luca
ecc230
Follow Ups:
You certainly were not using wire-wound resistors! So With heating their value drifted without the part otherwise showing signs of damage.
A 5 watt minimum wire-wound is recommended to survive arc-over events.
I would expect more than just 8 watts from that many tubes! Are you having saturation problems in the driver circuit?
I don't know where Ralph get that theory that 5 watts is not the same as 5 watts,these are factory spec.
Which resistors are you referring to?
I should dissect the faulty resistors to give an answer to your conjecture.
By the way, by replacing them with 10W ceramic ones I think that this problem will likely never happen again.
The 6SN7 driver is biased to have enough room before saturation, so I do not think it might be the culprit.
Thank you for your comments.
Best Regards
Luca
ecc230
Yes, 10 watt devices are likely indestructible :)
Now with respect to the 6080s. Everything I've seen about them indicates that they need to be preconditioned before use! So if the tubes are NOS, they should be run with filaments only for at least 72 hours. If they have already been run with B+ then its too late, but its something to keep in mind in the future.
BTW nice job on the construction!
Ralph can this..preconditioned before use....be done an then used later...or dose it have to be done right befor the b+ is added....thanks
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WOW...nice diy OTls....thanks for the post...The M60s that i have did not have any cathodes res..thay had a 1ohm on the plat..but i pull them an put in a 5ohm 6w on the on the cathodes.....i have found that the US made 6080..an the 6AS7...like GE..RCA...Dont work as well as the sovtek Tubes...the US tube go out from time to time.. till i just pulled all the US tubes ....an have had no prob...but if well all look at the cathodes res.... thay would be some out of 5ohm rang...if thay had went from 5ohm to 2ohm you could see this on the res...but move up from 5ohm too 10kohm...not so much.....Ralph has seen it all so well see what he says.....goodluck
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