Here is one I haven't run into before. I have always admired tube amps with ceramic tube sockets. They look like an upgrade from the original plastic or phenolic sockets. I have seen them used in high RF power and high voltage applications so there must be some advantage right?
Well, I scored on a box of ceramic octal and 9 pin tube sockets & decided to switch out the flaky tube sockets on a nice little Bogan mono tube amp I have. Before the tube socket replacement I had restored it, I replaced all the filter caps and replaced any bad fixed caps and resistors. I put good tubes in it and it worked fine. No hum. However, the tube sockets were crappy so I decided to replace them with ceramic tube sockets. Once I was finished I found that the phono stage now hums at 60Hz (not 120hz). By wiggling wires I can make the hum louder, but not quieter. The hum is not related to volume.
Second experience: I have a nice old McIntosh MC-60 power amp that has a very rusty chassis. I have restored it and it worked fine. I bought one of those aftermarket chrome plated chassis for the MC-60 off ebay. I just finished switching everything over to the new chassis yesterday. I figured while I was at it I may as well replace all the tube sockets too. Well it sure looks pretty. However, I have that 60Hz hum again. This time it is related to volume. I can make the hum louder but not quieter by wiggling wires around V1 (12AX7).
Have any of you experienced increased hum using ceramic tube sockets? Is it a mistake to replace the originals?
Well, I scored on a box of ceramic octal and 9 pin tube sockets & decided to switch out the flaky tube sockets on a nice little Bogan mono tube amp I have. Before the tube socket replacement I had restored it, I replaced all the filter caps and replaced any bad fixed caps and resistors. I put good tubes in it and it worked fine. No hum. However, the tube sockets were crappy so I decided to replace them with ceramic tube sockets. Once I was finished I found that the phono stage now hums at 60Hz (not 120hz). By wiggling wires I can make the hum louder, but not quieter. The hum is not related to volume.
Second experience: I have a nice old McIntosh MC-60 power amp that has a very rusty chassis. I have restored it and it worked fine. I bought one of those aftermarket chrome plated chassis for the MC-60 off ebay. I just finished switching everything over to the new chassis yesterday. I figured while I was at it I may as well replace all the tube sockets too. Well it sure looks pretty. However, I have that 60Hz hum again. This time it is related to volume. I can make the hum louder but not quieter by wiggling wires around V1 (12AX7).
Have any of you experienced increased hum using ceramic tube sockets? Is it a mistake to replace the originals?
If the sockets are new and unoxidized, the contacts are likely to be clean, tight, and not a problem.
The tubes could have oxidation on their pins, which can be removed. Wiring dress is often rather critical
in low level, high gain circuits. Pairs of wires should be tightly twisted.
Thanks for the reply, Bogan phono input wires were tightly twisted from selector switch to 12AX7 phono stage preamp tube. Didn't hum before socket upgrade, but did so after even with tightly twisted wire. Tried replacing with high quality teflon coated coaxial & no change.
The first stage in the MC-60 is a 12AX7 but begins at .5v line level. Leads were not originally twisted & since ground wire follows different route it is difficult to twist. Perhaps I should add separate ground wire to twist? or coax it?
The first stage in the MC-60 is a 12AX7 but begins at .5v line level. Leads were not originally twisted & since ground wire follows different route it is difficult to twist. Perhaps I should add separate ground wire to twist? or coax it?
OK,
I re-touched all the solder & no joy. I traded out the offending 12AX7 tube. no change. I started removing parts from the input into pin 2 of the first AF amplification stage which is V1 the 12AX7 tube. Hum reduced slightly but still there. I have 2 more MC-60's to compare with and they are pretty much identical as far as I can tell.
This is beginning to PMO!
I re-touched all the solder & no joy. I traded out the offending 12AX7 tube. no change. I started removing parts from the input into pin 2 of the first AF amplification stage which is V1 the 12AX7 tube. Hum reduced slightly but still there. I have 2 more MC-60's to compare with and they are pretty much identical as far as I can tell.
This is beginning to PMO!
While I actually prefer mica filled plastic sockets in most applications where capacitance matters I sometimes use ceramic sockets with 9 pin tubes, and very frequently use ceramic octal sockets.
I've never encountered the problem you describe.
Do you have an oscillscope? It is possible that lead dress and socket capacitance contributed to maintaining stability in the stages where you now have a problem.
You have not mentioned whether or not these are new or NOS sockets and what sort of condition they were in when you got them. If they were easily soldered they're probably not oxidized.
I've never encountered the problem you describe.
Do you have an oscillscope? It is possible that lead dress and socket capacitance contributed to maintaining stability in the stages where you now have a problem.
You have not mentioned whether or not these are new or NOS sockets and what sort of condition they were in when you got them. If they were easily soldered they're probably not oxidized.
Just had a thought: unlikely but just possible that the ceramic sockets are allowing RF parasitic oscillation to happen; the old sockets may have been sufficiently lossy to damp this. RF parasitics can show themselves as hum or distortion which seems not to obey 'the usual rules'.
As I say, this is unlikely but a marginal layout could just be held in check by RF lossy sockets and just pushed over the edge by RF lowloss sockets.
As I say, this is unlikely but a marginal layout could just be held in check by RF lossy sockets and just pushed over the edge by RF lowloss sockets.
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