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Old 28th March 2007, 09:53 PM   #21
Cobra2 is offline Cobra2  Norway
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Quote:
Originally posted by Brian Beck

You could just turn the volume control down. That's an input attentuator already provided!

That's the problem, into my ST70 or SS amps, I have "full pull" at 3-4 clicks...

Arne K
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Old 29th March 2007, 12:45 AM   #22
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Quote:
Originally posted by Cobra2
A total redesign is not interesting for this unit, and they have too high value in "unmodified" form.
I wish to keep the "classic/oldschool" circuit fairly intact.

Would it be possible to eliminate / jumper one gainstage?
Linestage has 26dB gain, half would be more than enough...

Arne K
You can try swapping 12AY7s for the 12AX7s in the line stage gain "slots". Like the 'X7, the Y7 is low gm/high Rp, but mu is in the 40s. I have no idea as to how the NFB circuitry will behave. Swapping a 'Y7 for a 'X7 is common among guitar guys to reduce gain.
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Old 10th May 2007, 06:48 AM   #23
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>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
Click the image to open in full size.

TIA!!!
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Old 10th May 2007, 11:28 AM   #24
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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.
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Old 10th May 2007, 11:33 AM   #25
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Thanks again Brian!
I'll have my friend measure the voltages.
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