Hello all
some time ago I built Joe Rasmussens Tube Gainclone ( http://www.customanalogue.com/diytubegainclone/index.htm )
The amp worked fine for some weeks, until I had the unfortunate idea of pushing the amp a little too hard (well I was listening to Keith Jarrett and you know.....)
It made a THUMP and THUMPTHUMPTHUMP and I thought OK I fried the IC's and have DC at the output.
I changed the IC's (I wrongly assumed they were gone), improved the heatsinking and..... nothing changed, the amp THUMPed.
I made some measurements with dummy loads (they get hot!!) and I have DC at the output that slowly (4-5 sec.) "oscillates" from +12VDC to -12VDC.
Fried Transformer? Coupling capacitors? Any ideas where I should start looking?
Thanks!
some time ago I built Joe Rasmussens Tube Gainclone ( http://www.customanalogue.com/diytubegainclone/index.htm )
The amp worked fine for some weeks, until I had the unfortunate idea of pushing the amp a little too hard (well I was listening to Keith Jarrett and you know.....)
It made a THUMP and THUMPTHUMPTHUMP and I thought OK I fried the IC's and have DC at the output.
I changed the IC's (I wrongly assumed they were gone), improved the heatsinking and..... nothing changed, the amp THUMPed.
I made some measurements with dummy loads (they get hot!!) and I have DC at the output that slowly (4-5 sec.) "oscillates" from +12VDC to -12VDC.
Fried Transformer? Coupling capacitors? Any ideas where I should start looking?
Thanks!
Hi Nuuk
yes... I might have been overreacting
I pulled the tube out and.....
no DC! Well I get a spike when I turn the amp on but then it quickly settles to 15mV, I assume that's like it should be.
I put the tube back in and measured if there is DC after the coupling capacitor but there's none.
I'm perplex
there is a lot I don't understand!
Is there a next step? Thanks!
yes... I might have been overreacting
I pulled the tube out and.....
no DC! Well I get a spike when I turn the amp on but then it quickly settles to 15mV, I assume that's like it should be.I put the tube back in and measured if there is DC after the coupling capacitor but there's none.
I'm perplex
there is a lot I don't understand!Is there a next step? Thanks!
I'll have to order one and test again.
I tried another transformer but the problem remains (at least I know it's not the transformer...)
the fastest solution would be to change all the components... but it could happen again
and that's not the goal....
it still puzzles me what component could have been "forced"... the tube seems odd (I used a new EH 6922), but I'll try as soon as I get a new one.
what do you think about using a pcc88? there's plenty around, NOS european and american brands and you can get them for less then an ecc88 or e88cc.... and the heater is easily adapted with a different resistor.
I tried another transformer but the problem remains (at least I know it's not the transformer...)
the fastest solution would be to change all the components... but it could happen again
and that's not the goal....it still puzzles me what component could have been "forced"... the tube seems odd (I used a new EH 6922), but I'll try as soon as I get a new one.
what do you think about using a pcc88? there's plenty around, NOS european and american brands and you can get them for less then an ecc88 or e88cc.... and the heater is easily adapted with a different resistor.
Haven't got the tube yet....
but I made some more measurements:
when I switch on without the tube I get this DC spike before it settles on 15mV. What I didn't notice before is that it behaves exactly like when the tube is in, DC goes up to 12V (and the dummy load gets hot) and decreases quickly. The only difference is that without the tube DC settles on 15mV instead of going up to -12VDC.
So I cut the PSU from the tube circuit and: No spike at switch on and DC ~3mV.
At this point I don't think it's the tube.
I have the feeling it must be something obvious but my knowledge is too limited
Any ideas?
but I made some more measurements:
when I switch on without the tube I get this DC spike before it settles on 15mV. What I didn't notice before is that it behaves exactly like when the tube is in, DC goes up to 12V (and the dummy load gets hot) and decreases quickly. The only difference is that without the tube DC settles on 15mV instead of going up to -12VDC.
So I cut the PSU from the tube circuit and: No spike at switch on and DC ~3mV.
At this point I don't think it's the tube.
I have the feeling it must be something obvious but my knowledge is too limited
Any ideas?
You will get a high DC offset while the tube buffer 'settles down'. That's why I used a speaker protection module that delayed connecting the speakers to the amp for a second or two. Another option is to use some sort of output delay on the output of the tube buffer.
Of course, that is probably nothing to do with your problem. Did you copy the Joe Rassmussen circuit exactly?
Of course, that is probably nothing to do with your problem. Did you copy the Joe Rassmussen circuit exactly?
Tubes go mad and do this when they are B+'ed underheated or heated without B+. Check connections and make sure that there isn't anything loose.
Sometimes playing loud makes vibrations that break weak contacts.
BTW i don't like that design. The tube should be ccs-loaded and powered from higher rails, even 60-80V would do better.
Sometimes playing loud makes vibrations that break weak contacts.
BTW i don't like that design. The tube should be ccs-loaded and powered from higher rails, even 60-80V would do better.
Hi everybody
The amp is an exact Joe Rasmussen copy, except for the speaker delay I have. I think I'll install a DC protection circuit next
The speaker delay has a 6R8 10W resistor for each channel.
Nuuk, your tube bufferd gainclone looks great!
I think I'll go for a dual mono too, the 2 channels should not fail at the same time...
Well, if it's my mad tube I'll know it when I get the new one, but I would still wonder what happened. I'll need a tubeshrink
The amp is an exact Joe Rasmussen copy, except for the speaker delay I have. I think I'll install a DC protection circuit next
The speaker delay has a 6R8 10W resistor for each channel.
Nuuk, your tube bufferd gainclone looks great!
I think I'll go for a dual mono too, the 2 channels should not fail at the same time...
Well, if it's my mad tube I'll know it when I get the new one, but I would still wonder what happened. I'll need a tubeshrink
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