Sceeta Posted October 25, 2018 Posted October 25, 2018 Hi All, So it looks like I've got a blown tube, although it doesn't seem to make any difference to the operation of the amp??? It definitely has lost it's nice glow in the dark though so I reckon it's fingered. It's a Tung-Sol KT120. As you can see in the glow in the dark photo, the left hand tube is the one in question. Question is can I just replace the one tube or do I need to replace a pair or all four. Amp is a Rogue Audio Atlas (not a Cronus as that is just the top plate to accommodate the bigger, non standard tubes.) that has been modded by Dallas Clarke. Fantastic amp. Anyone have any suggestions for replacement tubes as well. Thanks in advance for your advice.
VanArn Posted October 25, 2018 Posted October 25, 2018 You could interchange V4 and V3 to see if the fault is due to the valve or the amplifiers' filament supply circuit. Keep your finger on the on/off switch just in case ,as that grey patch on the KT120 (V4) looks suspicious. There is some risk involved with this procedure as it is possible that the output valve has suffered a filament to cathode short cct. It would be wise to have the amplifier inspected and tested by a technician and have a pair of new valves installed.
Sceeta Posted October 26, 2018 Author Posted October 26, 2018 Thanks for the advice. I suspect that the valve is stuffed. I might take it out and leave it out and put v3 in it's place and see what happens. There is no burnt smell from the amp so I'm hopeful that there is no problems there, just a failed tube.
Volunteer sir sanders zingmore Posted October 26, 2018 Volunteer Posted October 26, 2018 16 hours ago, Sceeta said: although it doesn't seem to make any difference to the operation of the amp??? This has me scratching my head. Anyone got any ideas as to why this might be?
BATMAQN Posted October 26, 2018 Posted October 26, 2018 39 minutes ago, Sir Sanders Zingmore said: This has me scratching my head. Anyone got any ideas as to why this might be? I’m scratching my head as to why the op is worried about it @Sceeta you’d know if it was buggered it would crackle pop fizz I had a 6ca7 go on the Topaz a while back and it did affect the operation of the amp not all of my tubes glow the same either it’s nothing to worry about but when they start to crackle that’s the time to do something. I replaced the tube and the amp is still singing away nicely today just sayin. 1
bob_m_54 Posted October 26, 2018 Posted October 26, 2018 It does appear like the filament is not heating, but there would definitely be a noticeable performance problem if it is. Have you checked the tube current, as per the biasing instructions?
SteveLuck Posted October 26, 2018 Posted October 26, 2018 Re replacements I'm loving the Sophia EL34 i bought silly price but they do KT88 as well would they work in your amp? https://www.sophiaelectric.com/
maxsimonjp Posted October 26, 2018 Posted October 26, 2018 I have checked this when i noticed one tube did not glow as much as the other on my phono.It appears it could happen but nothing to worry as long as the amp sounds normal.
Sceeta Posted October 26, 2018 Author Posted October 26, 2018 So I swapped out v3 and v4 and the tube in v4 now glows so it looks like the amp is ok. Checked the bias and the tube I put into v3 from v4 appears to be dead as nothing comes up on the bias meter. Strange that it doesn't appear to affect the operation. Would the fact that it is balanced output make a difference as in the other power tube carries the difference???? Any ho looks like I need a tube. I can't seem to find any one other than tung sol that make kt120 ??? 1
Sceeta Posted October 26, 2018 Author Posted October 26, 2018 1 hour ago, SteveLuck said: Re replacements I'm loving the Sophia EL34 i bought silly price but they do KT88 as well would they work in your amp? https://www.sophiaelectric.com/ Nice looking tubes. I think they'd work but would have less power than the kt120, not that the power is an issue as I am only driving some Krix KDX Mains with it. Might be worth trying something different.
BATMAQN Posted October 26, 2018 Posted October 26, 2018 1 hour ago, Sceeta said: So I swapped out v3 and v4 and the tube in v4 now glows so it looks like the amp is ok. Checked the bias and the tube I put into v3 from v4 appears to be dead as nothing comes up on the bias meter. Strange that it doesn't appear to affect the operation. Would the fact that it is balanced output make a difference as in the other power tube carries the difference???? Any ho looks like I need a tube. I can't seem to find any one other than tung sol that make kt120 ??? I was wrong sorry mate. 1
bob_m_54 Posted October 27, 2018 Posted October 27, 2018 17 hours ago, Sceeta said: So I swapped out v3 and v4 and the tube in v4 now glows so it looks like the amp is ok. Checked the bias and the tube I put into v3 from v4 appears to be dead as nothing comes up on the bias meter. Strange that it doesn't appear to affect the operation. Would the fact that it is balanced output make a difference as in the other power tube carries the difference???? Any ho looks like I need a tube. I can't seem to find any one other than tung sol that make kt120 ??? You are welcome . But I too, do not know why it seems to sound normal, for the rest of the amp.
Sceeta Posted October 27, 2018 Author Posted October 27, 2018 Thanks heaps for all your replys guys. I think the next step will be a call to Dallas to get a bit more info.
aussievintage Posted October 27, 2018 Posted October 27, 2018 On 26/10/2018 at 12:21 PM, Sir Sanders Zingmore said: This has me scratching my head. Anyone got any ideas as to why this might be? It's fairly normal. Unless you push the amp hard you probably won't notice much. You're probably working down in, or near, the class A region.
aussievintage Posted October 27, 2018 Posted October 27, 2018 On 26/10/2018 at 1:05 PM, BATMAQN said: I’m scratching my head as to why the op is worried about it @Sceeta you’d know if it was buggered it would crackle pop fizz I had a 6ca7 go on the Topaz a while back They don't all go in obvious ways like this. Some die silently.
aussievintage Posted October 27, 2018 Posted October 27, 2018 On 26/10/2018 at 6:13 AM, VanArn said: as that grey patch on the KT120 (V4) looks suspicious. It does. The getter flash certainly looks oxidised, indicating the valve may have developed a leak, and the heater finally going open circuit. 1
doogie44 Posted October 28, 2018 Posted October 28, 2018 I had a recurring problem with blown 6550 valves in a VTL 300 Monobloc amp (taking out a resistor on the circuit board each time), Turns out the culprit was over-voltage in my area. After investing in a voltage regenerator AND having Electricity Australia 'change my phase' the problem never recurred. But--I did need a valve tech to find some of this out 1
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