Ampex 375 problem.

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mg73

mg73

Well-known member
Joined
Dec 9, 2005
Messages
163
Location
sweden
I'm trying to bring this dinosaur back to life.
It's an Ampex amplifier for driving AC motors. 300/350 Transport.
It's capable of putting out up to 130V but now its  21v :'(
I've replaced all el.caps and checked resistors and filmcaps
everything seems fine and the DC voltages looks like the ones on the schematic
so I suspect the output tubes but before I start ordering these rare beasts
do the signal voltages looks normal until the output(the red ones)?
375%20SCHEMATIC%201_zpsrwhwmqw9_1.jpg.html
 

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  • 375 SCHEMATIC 1.jpg
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807 is not a "rare beast"; it is 6L6 with a top-hat. With re-wiring, 6L6GC will work. But true 807 are $20 each at thetubeStore, cheaper than good 6L6GC. €15-€40 in Germany EUR.

The AC voltage gains look reasonable. V2305 (6F6) looks low, but it has NFB into it.

How did you pick 0.7V drive? I suspect it needs a lot more. Look at the oscillator. V2303 limits signal to +/-1.5V. V2303 is a gain-stage on that. I do not have data on 6F6 as RC-amp, but I would bet on a gain of ~~~10: +/-15V or 10V output. If pot 2311 is set to half, that's 5V instead of 0.7V and 150V instead of 21V. Considering all the unknowns, that's good agreement.

If "0.7V" is a limit of your equipment, use a 6.3VAC heater transformer. It should be 60Hz not 50Hz, but that is probably not critical.
 
PRR said:
807 is not a "rare beast"; it is 6L6 with a top-hat. With re-wiring, 6L6GC will work. But true 807 are $20 each at thetubeStore, cheaper than good 6L6GC. €15-€40 in Germany EUR.

The AC voltage gains look reasonable. V2305 (6F6) looks low, but it has NFB into it.

How did you pick 0.7V drive? I suspect it needs a lot more. Look at the oscillator. V2303 limits signal to +/-1.5V. V2303 is a gain-stage on that. I do not have data on 6F6 as RC-amp, but I would bet on a gain of ~~~10: +/-15V or 10V output. If pot 2311 is set to half, that's 5V instead of 0.7V and 150V instead of 21V. Considering all the unknowns, that's good agreement.

If "0.7V" is a limit of your equipment, use a 6.3VAC heater transformer. It should be 60Hz not 50Hz, but that is probably not critical.
Click to expand...

Thanks PRR excellent info as always!

I tried switching out the 6F6 with the other (V2302) but same result ,if not both is bad.
0.7v what was I had  "laying around" at the moment you right I should definitely try something more powerful.
I haven't tried the internal osc yet not sure if it's working.
I try to boost up the input and if that's not enough I resolder for 6L6 have loads of them.
Thanks
 

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