#1
hello guys, i own a randall rd50c (the old diavlo series) and had it for three years....originally the amp came with EL34s but recently i purchased some new tubes for it, 2x ElectroHarmonix KT88s and 4x JJ ecc83s (12ax7)...i installed the tubes myself and after one hour of playing on it it was fine...however, after a day it started getting this weird burning-type smell from it and there was this popping or crackling noise so i was wondering what the problem could be.

is it from the power tubes? because the amp is supposed to be self-biasing...but i'm guessign the Kt88s have more power than el34s.

or could it be from faulty preamp tubes? i don't think this is the case as i even used the old preamp tubes and still there was the smell and the noises.

please help!

yours sincerely,
dr. metul

ps. even this doctor (me) can't fix this situation.
#2
Musician's friend and the manual says that the amp is compatible with multiple tube types "6CA7, 6L6, 5881, 6550, KT88, KT77, and 6V6 tubes" but I don't know how that would work. 6CA7's are El34's I think. It says no set up is required including bias. Just switch back to the old EL34's if you still have them and see if the symptoms still occur. There might be some other stuff you have to mess with outside of bias that they don't explicitly say.

Alternatively, EHX could have just sent you a shoddy pair of KT88's and they could have blown within a day :/ Also, try making sure all your tubes are properly seated and try spraying some contact cleaner in there as well. And I presume you are not mismatching the power tubes, both would be the KT88.
Last edited by Will Lane at May 4, 2015,
#3
both power tubes are kt88 and i'm positive nothing is wrong with them, something else must be the problem....i switched back to the old EL34s, the smell went away but there still is a crackling noise

also, there is a really noticeable drop in volume...probably half of what it was before
Last edited by Leon72 at May 4, 2015,
#4
I don't know anything about your particular amp but always use contact cleaner (DeOxit) on the pins of the new tubes. Insert and remove the new tubes until they seat smoothly.
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#5
Quote by Leon72
both power tubes are kt88 and i'm positive nothing is wrong with them, something else must be the problem....i switched back to the old EL34s, the smell went away but there still is a crackling noise

also, there is a really noticeable drop in volume...probably half of what it was before
That very much sounds like the power tubes (KT88's in this case) have blown for whatever reason. EHX is not known for their build quality. Is the volume drop there with the EL's in as well?
#6
KT88 tubes run at twice the power of EL34's. Heater current is the same but apart from that they pull more current. You risk blowing the power supply up. Did you check on the power supply specs before you tried such a risky experiment?
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#7
Quote by Will Lane
That very much sounds like the power tubes (KT88's in this case) have blown for whatever reason. EHX is not known for their build quality. Is the volume drop there with the EL's in as well?


Even when i switched back to the ELs, there still was a volume drop
#8
Quote by Cathbard
KT88 tubes run at twice the power of EL34's. Heater current is the same but apart from that they pull more current. You risk blowing the power supply up. Did you check on the power supply specs before you tried such a risky experiment?



No i did not check the specs, my friends just told me to go for it and t should be safe, just purely because the amp is self biasing

In this case, did i blow up my power supply?
#9
Well, a burning smell accompanied by popping sounds can never be good.

You may have blown a screen resistor too.
Bottom line, it has to be opened up and inspected.
Gilchrist custom
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#10
Quote by Cathbard
KT88 tubes run at twice the power of EL34's. Heater current is the same but apart from that they pull more current. You risk blowing the power supply up. Did you check on the power supply specs before you tried such a risky experiment?


It wasn't a risky experiment, the amp is specifically designed for a host of tube, KT88's among them.

From the corporate blurb "The RD50C combo comes stock with 2 JJ El34 power amp tubes, but is compatible with 6CA7, 6L6, 5881, 6550, KT88, KT77, and 6V6 tubes. Best of all, it's fixed bias, so no setup is required when switching tube types. "

The fact that there's apparently been a failure is unfortunate, but not expected.
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#11
Possibly, the KT88's tried to run at the 2x power, but weren't able to take it (ehx tubes eh?) and blew, causing something else to blow?

Take the amp to a tech and see what they say. Try only putting other EL34's in next.
#12
I agree take it to a tech. The crackling sound can be from old resistors, bad preamp tube, or flaky connection somewhere. The bad smell is not a good sign though, last time I smelled it I was buying a transformer not long afterward...fortunately it was a Fender Champ, only around $35 or $40. The previous one I bought was for a Peavey MX, $125...eventually found out the Champ had also fried a capacitor, which was causing it to arc across the power and rectifier tube sockets. That $2 cap was what fried the transformer...
Hmmm...I wonder what this button does...
#13
Quote by Arby911
It wasn't a risky experiment, the amp is specifically designed for a host of tube, KT88's among them.

From the corporate blurb "The RD50C combo comes stock with 2 JJ El34 power amp tubes, but is compatible with 6CA7, 6L6, 5881, 6550, KT88, KT77, and 6V6 tubes. Best of all, it's fixed bias, so no setup is required when switching tube types. "

The fact that there's apparently been a failure is unfortunate, but not expected.

Oh. Fair enough. Still, KT88's will be at high end of its range.

Regardless. It needs to be opened up by somebody that knows what they are doing. Burning smell, bad.
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#15
Quote by Cathbard
Oh. Fair enough. Still, KT88's will be at high end of its range.

Regardless. It needs to be opened up by somebody that knows what they are doing. Burning smell, bad.


Agreed. Probably something simple, but it needs to be checked out.

Schematic can be found here, and it looks to me like they used one side of 2 different tubes for the PI, which would be annoying?

http://bmamps.com/Schematics/Randall/Randall_RD50_FORTIN_+_ECN_0222312.pdf
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Last edited by Arby911 at May 6, 2015,