Fender 400 PS - Parts or Project

[Wasnex]

It's not fair that the Normal channel gets eight controls but the Bass Instrument channel only four. We always seem to get shortchanged!

Is there some reason why you can't use the Normal channel? Also you could think of it as four additional controls that are added especially for bass .

 

[ajkula66]

As much as I love quirky vintage tube stuff, I'd pass on this once-mighty-amp unless it were given away for free AND the current owner agreed to cover a visit by mold removal services... that poor thing looks like a serious health hazard in its current condition.

 

[Jeff Scott]

Is there some reason why you can't use the Normal channel? Also you could think of it as four additional controls that are added especially for bass .

Of course! I do have a history of using guitar amps for bass (with the appropriate bass cabs). I would have preferred that Fender used Abnormal for that channel, though.

 

[agedhorse]

Specced tubes for the 400PS were GE6550A, an improved version of the standard GE6550.

More details here:

The GE-6550A, and The Fender 400 PS Amplifier. A most Amazing Amplifier!!!

I believe that some of this is lore and not completely accurate. The 400PS was shipped with Fender #010309 output tubes, Tungsol parts. Later on, it's possible that GE 6550A tubes were substituted (not confirmed), but there is no way a set of 6 are capable of 560 watts RMS unclipped, again I believe this is a mischaracterization of the truth.

I worked under Bill Hughes while we were under the Fender umbrella, and Bill was awfully familiar with this amp. He was pretty clear about the challenges, he had as much experience with the 6550 tube family as anyone in this industry, being the designer of record for the SVT.

Below was taken right off the drawing:

 

[spiritbass]

Specced tubes for the 400PS were GE6550A, an improved version of the standard GE6550.

More details here:

The GE-6550A, and The Fender 400 PS Amplifier. A most Amazing Amplifier!!!

Wow. An amp that can melt the glass on a tube is a monster. I've never seen that before! Since the "special design and selected" Tungsol tubes are surely unobtanium, this dinosaur should be left to rest in peace (or pieces)...

 

[J Wilson]

This is either a donor for parts or a well-heeled collector who can put STOOPID money in it to make it shake walls once more.

Then all you'd have to do is find the three Fender W-bins that were built for it.

 

[DJ Bebop]

 

[Short4since64]

It's a rare and awesome amp, but AFAIK not particularly coveted. It basically has three 145W tube output sections. So you need to run three 4 ohm cabs for full power. Not very practical for the majority of playing situations most of us would deal with.

From what I see this one is pretty rough. Potentially a good project for one person and piece of garbage for the next.

Not worth paying a tech to fix it (Most likely the cost will far exceed the value of the amp). If you don't have the skill, knowledge, and test equipment required to do the work, I would pass.

I would hate to see it parted out, and suspect turning a profit would be more work than it's worth.

The amp is often described as difficult to work on. The circuit is a bit unusual, but I don't perceive it as extraordinarily complex. The amp is said to be hard on tubes. Which could make it difficult to fix and keep running. Not sure why it would be hard on tubes as the working voltages shown on the schematic appear fairly similar to an SVT. My guess is problems are more likely to occur if you don't run all three output sections.

I wouldn't mind owning a 400PS, but not one in this condition. Also, I wouldn't be willing to pay big bucks.

PS400 is a perfect example of pre-WW2 brute force technology - output tubes run in full class AB2, transformer driven into substantial grid current by a smaller power amplifier, in this case a single ended 6L6 (about 6 watts) Used extensively in AM radio modulators using heavy transmitting tubes which are designed for this service. Very hard on the "consumer" grade tubes that are available to the musical market. And, if you're wanting to run a single load, it's looking for a load of 1.33 ohms (dummy plugs in the other 2 speaker connectors) Voltages are similar to an SVT, but the 12BH/12AU7s which drive the 6550s in the Ampeg are only capable of milliwatts of grid drive. This is an amp that a classic ham radio operator in his shack with the kilovolts, kilowatts, mercury rectifiers and the smell of ozone would understand, but most musical techs wouldn't.

 

[wagdog]

The closest I’ve ever come to one of these amps is owning the bent tube rolling stand, which I used with my Guild Thunderbass.

 

[Wasnex]

And, if you're wanting to run a single load, it's looking for a load of 1.33 ohms (dummy plugs in the other 2 speaker connectors)

This is over my head, but trying to pull the full 435W out of only one out of the three secondaries sounds like a really bad idea to me .

 

[definenredefine]

It's not fair that the Normal channel gets eight controls but the Bass Instrument channel only four. We always seem to get shortchanged!

Typical. Same thing on my Music Man head.

 

[Short4since64]

This is either a donor for parts or a well-heeled collector who can put STOOPID money in it to make it shake walls once more.

Then all you'd have to do is find the three Fender W-bins that were built for it.

Only actually saw one of these in use in my life, and the bassist had two Ampeg flatback fridges and some sort of 2-15 box. Loud. Very loud. Kept up with 2 Marshall-blasting guitars.

This is over my head, but trying to pull the full 435W out of only one out of the three secondaries sounds like a really bad idea to me .

There's only one secondary, actually 2 windings paralleled, as are all the output jacks. If running a single cabinet with three 4 ohm speakers, it would be a very good idea to replace one jack with a Speakon.

 

[agedhorse]

I’m pretty sure there are 3 secondaries on that OT. Each secondary feeds one jack. I’m pretty sure there is only one of the secondaries that the feedback signal is taken from, this would be the jack that must be used always. I don’t think the feedback is sampled from all 3 secondaries.

Just another quirk in an already quirky amp.

 

[micguy]

That’s a difficult and expensive amp to work on ime.

I think you forgot "dangerous" - a lot of those old things have been worked on by "not so professionals" before, and you never know what dangers lurk in one of those.

 

[agedhorse]

I think you forgot "dangerous" - a lot of those old things have been worked on by "not so professionals" before, and you never know what dangers lurk in one of those.

Totally agree

 

[rockdoc11]

DJ Bebop, thanks for posting those great ads! I love a couple of the lines in the first ad.

1. " . . . also sounds great in a small intimate room."

2. "Don't be fooled by its 210 pounds. It was designed to be very portable."

 

[dontvex]

If I'm not mistaken it's actually nicely thought out according to the schematic:

4 windings on the OT's secondary:

- 3x 4R output to switched speaker jacks where each jack switches on a pair of output tubes (via cathode to ground connection) when plugged in

- 1x dedicated NFB winding

So it shouldn't really matter which and how many outputs you're using as long as correct impedance of the connected cab(s) is observed.

 

[danfrank]

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!

 

[danfrank]

Ooops, I see it was sold. I wonder what it went for?

 

[Timothy Brosnan]

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!

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.