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Fender 400 PS - Parts or Project

#41
Timothy Brosnan said:
This was probably why the designer, Ed Jahn’s, wouldn’t provide schematics, information on this amp, unless he was satisfied that the person working on it knew what they were doing.
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There were plenty of underqualified tech back then too. These techs just couldn’t get their head around something different. That’s why fender had some qualified gold level service centers to handle their more challenging products.
 
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#42
danfrank said:
To the OP:
Do you mind sharing a link if you aren’t interested in the amp? I am looking for one and I’m fine if it’s non-working.

As for the OT, there are four 4 ohm taps on the OT like stated above. There is also a hole in the chassis for a fourth output jack. I’m guessing here, but originally I think Fender had plans to use an extra set of output tubes (8 total) for a total of 600 watts using all four of the OT secondaries. I think they ran out of room on the chassis to execute that plan!
A hammond 1650W OT will work well for a replacement OT but it would be very hard to mount as the 1650W is even bigger than the original OT. But, the primary impedance of the 1650W is correct for the 400PS amp, if someone wants only one output jack for full power instead of 3.
The primary impedance of the original 400PS OT is 5600 ohms reflected per SINGLE output tap of the OT. So, if only one output jack is used, the pair of 6550s in circuit “sees” a 5600 ohm load. If all 3 output jacks are used, the six 6550s in circuit “see” a load of 1900 ohms.
The high voltage B+ on this amp is around 720 volts so it’s a very dangerous amp to work on, especially with the big filter capacitors in the power supply. They hold A LOT of charge!
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There wasn’t ever plans for 8 tubes, the secondary feedback winding was necessary to guarantee a feedback signal independent of the loading.

Because the reflected impedance to the primary varied with the number of connected speakers, the isolated feedback partially improves the operational consistency. It’s not ideal, but in practice it worked ok.
 
#43
agedhorse said:
There were plenty of underqualified tech back then too. These techs just couldn’t get their head around something different. That’s why fender had some qualified gold level service centers to handle their more challenging products.
Click to expand...
I remember getting a call ca. 1984 from someone very high up the food chain at Fender who was casting about for anyone who was qualified, but more importantly willing, to take on a Rhodes Chroma repair. The former lead tech who I replaced at that shop was a wiz with weird synth designs, and still had had a hard time previously, so my boss negotiated some kind of special deal and more or less begged me to take it on. And even though I was a former ARP factory tech and it was mostly their design, we had to go to over the phone troubleshooting with whoever was in charge of that project by that time. As so often happens, no big deal if you have the right information and tools but not something worth messing with otherwise!
 
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#44
Passinwind said:
I remember getting a call ca. 1984 from someone very high up the food chain at Fender who was casting about for anyone who was qualified, but more importantly willing, to take on a Rhodes Chroma repair. The former lead tech who I replaced at that shop was a wiz with weird synth designs, and still had had a hard time previously, so my boss negotiated some kind of special deal and more or less begged me to take it on. And even though I was a former ARP factory tech and it was mostly their design, we had to go to over the phone troubleshooting with whoever was in charge of that project by that time. As so often happens, no big deal if you have the right information and tools but not something worth messing with otherwise!
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When Fender shut down the Rhodes division, as well as some Fender legacy products, I visited the surplus components sale and saw a bunch of the Chroma keybords lined up for destruction (via a crawler tractor)... tens of thousands of dollars worth of essentially worthless technology. Broke my heart.
 
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#45
agedhorse said:
When Fender shut down the Rhodes division, as well as some Fender legacy products, I visited the surplus components sale and saw a bunch of the Chroma keyboards lined up for destruction (via a crawler tractor)... tens of thousands of dollars worth of essentially worthless technology. Broke my heart.
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Yeah, I heard that. I was at ARP during their peak period, when Close Encounters came out and the VP of Engineering doinked out the notes in the movie. Things went downhill there pretty fast after that!
 
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