RE: Fisher 200 tube amp fuse blowing issue

[movingcoilguy]

RE: Fisher 200 tube amp fuse blowing issue

I guys, I know there are many extremely knowledgeable electronics wizards on this site so I am looking for help or confirmation of a fuse blowing problem. I just had my Fisher Mono block amps serviced (200's) shortly after pluging one in a fuse blew, it happened several months ago as well , thats when I decided to take them to a tech, originally I thought maybe it was my cheap power bar causing issues, they were all checked by my tech and he said it appears a choke was arcing on the bottom plate and looked after that issue, I removed all the tubes but the rectifiers and tried the amp again, it blew again, again I tried with no tubes and the fuse did not blow, from these results can I be releaved in thinking my transformer is fine? also can this confirm that one or both of the rectifier tubes may have a short? or could there possibly be another issue? My tech biased them as well to the specs on the schematics which is very high 125MA which from what I have read is driving the EL34 tubes very hard, should I adjust this as well once I get the fuse issue sorted, I would great appreciate any suggestions or confirmation of what the issue might be, I have no way to test the rectifier tubes other then replacement, so should I take a chance and switch them with the other amp? I was concerned for damaging the other 34's. many thanks in advance D

 

[movingcoilguy]

Hello again, if the above post would be better suited to a different forum my apologize and please redirect it to the proper place, many thanks

 

[movingcoilguy]

I am quite surprised I haven't had one reply as usually there are many helpful people on many of these sites, by chance I do get a very nice member that wants to try and help sort out this issue heres an update, if you read the above posts? OK could there be an issue on the DC side still as I guess with the fuse not blowing with no tubes in it confirms the AC side and the transformer is fine but there could still be an issue on the DC side that could blow a fuse with the rectifiers in???? how would you proceed should I chance switching the rectifiers out and try again, as at this time My tube tester is in the shop, I could also try the 2 rectifier tubes in the other amp to confirm if one or 2 is bad? suggestions

 

[Tom Bavis]

Had to look up the circuit to refresh my memory - since it's intermittent, it may be a tube. Yes, swap the rectifiers, if the problem doesn't move, back into the power supply... electrolytic capacitors DO sometimes have intermittent shorts.

 

[movingcoilguy]

RE: Fisher 200 fuse blowing

Hi Tom, and thanks for the reply. Well I did swap out the rectifiers with some nos which was a bit of a risk as they are mullards but branded 5AR4, the fuse didn't blow but a small can that my tech added into the circuit to the number 8 terminal on the output trans which is speaker ground and the 2 which is going to the 80uf can I believe popped it was rated for 450volt but I guess not enough, I was searching on another site as well and it was suggested that maybe the 80 can is shot or has an entermitten short and probably the added small 450 rated was too low for the initial shock at start up? not sure but it was smokey for sure and I unpluged it straight away. what are your thoughts???????? thanks D

 

[movingcoilguy]

Oh I should mention my tech thought that maybe the other fairly new JJ rectifiers for maybe not that strong and when I switched the rectifiers to the NOS ones that the cap couldn't handle the extra volts???????? I am going to remove it as per his suggestion as he says the amp should work fine once it is removed but I am skeptical, I think it maybe be the 80 can as well???