>V1 phono stage – first voltage gain stage
>V2 phono stage – second voltage gain stage
>V3 phono stage – cathode follower to drive feedback RIAA network, line stage and >tape output
>V4 line stage – first voltage gain stage
>V5 line stage – second voltage gain stage
>V6 line stage – cathode follower to drive feedback and output to power amp
Hi could someone explain this?
My friend has a pair of mullard 12ax7s that don't have any hum when used in other preamps.
He put these in V4 & V5. There was hum on the left channel.
Now he switched the tubes in V4 & V5 and the hum went to the right channel.
Any ideas why this happened?
BTW, just to verify, which tube is which in this pic?
My friend placed them on the 2 leftmost sockets
TIA!!!
>V2 phono stage – second voltage gain stage
>V3 phono stage – cathode follower to drive feedback RIAA network, line stage and >tape output
>V4 line stage – first voltage gain stage
>V5 line stage – second voltage gain stage
>V6 line stage – cathode follower to drive feedback and output to power amp
Hi could someone explain this?
My friend has a pair of mullard 12ax7s that don't have any hum when used in other preamps.
He put these in V4 & V5. There was hum on the left channel.
Now he switched the tubes in V4 & V5 and the hum went to the right channel.
Any ideas why this happened?
BTW, just to verify, which tube is which in this pic?
My friend placed them on the 2 leftmost sockets
An externally hosted image should be here but it was not working when we last tested it.
TIA!!!
I believe that V4, V5 and V6 are the three tubes in the lower left of your picture, going from left to right. On the right side, V1 is in the back, V2 is in the middle, and V3 is the closest to the front panel.
Regarding your hum, I can't say for sure, but the channel "assignments" for the two triode sections are reversed from V4 to V5, so a problem with a tube section would move from one channel to the other with tube swapping between V4 and V5. I would say you have at least one bad tube, possibly with leaky heater-to-cathode insulation. If you have access to a good tube analyzer, you could check the tubes for leakage resistances. In conjunction with a bad tube, it could also be that the SP6's heater supply capacitors have gotten old and there is too much ripple or reduced heater voltage on the heater supply. The SP6 puts the heaters for V4 and V5 in series across a 24.7VDC supply. The early schematic shows a ripple measurement of 50mV P-P on the 24.7 V supply. If you have a scope you should check that too. The raw heater supply (at the first cap) should give 28 volts prior to the RC filter, which drops it to 24.7 volts.
Regarding your hum, I can't say for sure, but the channel "assignments" for the two triode sections are reversed from V4 to V5, so a problem with a tube section would move from one channel to the other with tube swapping between V4 and V5. I would say you have at least one bad tube, possibly with leaky heater-to-cathode insulation. If you have access to a good tube analyzer, you could check the tubes for leakage resistances. In conjunction with a bad tube, it could also be that the SP6's heater supply capacitors have gotten old and there is too much ripple or reduced heater voltage on the heater supply. The SP6 puts the heaters for V4 and V5 in series across a 24.7VDC supply. The early schematic shows a ripple measurement of 50mV P-P on the 24.7 V supply. If you have a scope you should check that too. The raw heater supply (at the first cap) should give 28 volts prior to the RC filter, which drops it to 24.7 volts.
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