Another GE 7700 Question - Output tubes

[millenium911]

The GE 7700 seems to be working well.
But I do have a question. One output tube from each channel does not glow at the base like the other. In this case it is tubes V4 and V10 that do not glow at the base. Not at all. Is this normal?

 

[jon_s]

If they are visible glowing at the top, it just means the filament is configured a bit differently - in that case, nothing to worry about. If not glowing at all, or if the plates are turning red, then something is wrong

They could also be different production runs - check the date codes on the tubes, and if you have two different pairs, then swap them around so that one pair is in each channel.

 

[millenium911]

They are visibly glowing at the top and the plates are not turning red.

I will swap them around to see what happens.

Thanks!

 

[millenium911]

Jon - I see you have one of these too! Mine is currently connected to a very rare pair of sony ss-3300's from the mid-60s. Sounds great!
I've replaced caps C1-C3. Would you recommend replacing others? And I see you recommend changing the GE 6aw8a to RCA (6aw8a or 6ba8a). Since I will have to purchase these is one really better than the other?
Thanks again Jon!

 

[jon_s]

Actually I've discovered some complications in the driver tubes, so I wouldn't sweat it that much. I have GEs in now and they sound fine. Some RCA 6BA8As use suppressor-grid pentodes (extra grid inside) and others use beam pentodes (extra shiny metal plate inside), and both sound different. The amp sounds good regardless, of course. You can get any of these tubes for $1-4 each at eBay or any major tube seller - just look inside to see that they are built the same before picking a pair.

Caps C1-C3 are a good place to start - I'd also change at least the coupling caps C21-22 and C41-42 going to the output tubes. The selenium bridge rectifier should also be replaced with a radio shack equivalent, then add a wire wound resistor after it to bring the voltage down to spec. I actually omitted the resistor since I want to keep the 6973s running cool, my bias supply voltage ended up at -27 instead of -25, which is correct for the MS-4010 vs. the 7700.

 

[millenium911]

Thanks again. I've read about the need to replace coupling caps but did not know which ones these were. I see these are 0.1 mfd. Is there a preferred manufacturer/type to use here? Are these in the 500vdc range?

And yes, have replaced the selenium bridge with a radio shack unit and added the wire wound resistor so that the bias supply voltage is right at -25.

 

[jon_s]

I'd probably recommend CDE WMF 0.1uF 400V from Mouser. Or, standard 716P orange drop. 400V should be fine (what is there stock), but 630V wouldn't hurt. I have Russian K40Y9s installed, but you do need to wrap their metal cases in heat shrink so they don't short anything out.