Fender 400 PS, now on the bench. Know anything about this?

[Matthews Guitars]

Yes, I have Tektronix scopes on both my benches, and load resistors. I'm fairly well equipped.

I'm ironing out problems in this 400 PS and it's getting close to finished. I've had to replace a few pots,
and expect to replace one or two more. If Deoxit D5 won't bring a pot back, it's done. replace it.

[Stevem]

These type of full size 24 mm pots can be disassembled carefully and the wipers slightly bent to ride on the unworn part of the pots Carbon trace.

Sometimes the only way to fully flush out carbon trace grit is with hot soapy dish water while you have it opened up.

After I repair a pot this way and before I close it up I lube up the trace and the pots center terminal wiper with some clear silicone grease and then the pot will never ware , make carbon grit and get noisy again!

This may sound like a lot of work, but it’s how I have saved and kept a whole lot of original dated pots working when restoring amps for people.

[Matthews Guitars]

I mght do that for a valuable and all original vintage Marshall, but for this amp, if the pots are bad, I just replace them. It's so far from being original that there's no point polishing those turds!

[Matthews Guitars]

I've got the amp for sale at the Orlando FL guitar show this weekend. Nobody has wanted to buy it yet but it's something of a major attraction at the show, as very few people have ever seen one in person, or heard one. It's a running amp, and it sounds very good. Amazingly clear and transparent, with ridiculous, building-rattling volume capacity when hooked up to the right cabinets.

People are amazed when they see it. They start to fall in love when they play it. It's not exactly an easy amp to sell because honestly who needs a 435 watt amp? But somebody, sooner or later, is going to go for it.

If it doesn't sell at this show, that's fine. I'll just cosmetically restore it to as close to fresh-from-the-factory as I can make it, and offer it up wherever then.

The volume has been bumped up fairly high, a few times, into three speaker cabinets that can take the power, and it easily commands the full attention of the entire building without having to take the master volume above 5.

It's amazing that an amp so powerful also has such a nice detailed articulate sound to it. It can play warm and sweet or jangly or even overdriven and raunchy. I'm sure it would also be a heck of a pedal platform. And with so much power in reserve, too.

I replaced several bad pots, which helped a lot. And best of all, it's a QUIET amp. Very low noise floor. Totally cranked, you can hardly hear any hiss from the speakers.

[Stevem]

I would like to thank for putting the time and effort into keeping this item of guitar amp history alive and thumping for people to take notice off!

It’s so ironic that when I started to play out in clubs I was dragging around a 3-15” bass cab to play thru, for a long long time 5 night a week and never complained about it’s near 100 lb weight, unlike today where even young players doing 1 club jam a week complain about having to load and carry around there 45 lb Blues deluxe or whatever 1-12” driver amp to the jam, lol!

Be sure to sell that great amp to a good home if it does need to go!

[TUBEDUDE]

Most were well over 500W.

[Matthews Guitars]

The amp didn't sell at the Orlando guitar show, and honestly I didn't expect it to. But it became a fairly popular attraction. In fact, I think it got more interest than the vendor who had a real Dumble Overdrive Special at his table. In fact there were two Dumbles at the show, but only one 400 PS.

So I'll restore it. May even make a new front panel for it to make it look all pristine, along with recovering the cabinet and faceplate to original spec materials.

I think I can even get the steel grill material for the back screen.

[martin manning]

I think it got more interest than the vendor who had a real Dumble Overdrive Special at his table. In fact there were two Dumbles at the show, but only one 400 PS.

Sounds like it was a good show, and I can certainly understand the attraction to seeing and playing such a unique amp. It's also going to be a unique individual who wants to have one at home!

There is a little more info here: https://ampwares.com/amplifiers/fender-400-ps-bass/
Note it says there is an error on the schematic: V6 is incorrectly labeled 12AX7 where it should be a 12AU7.

I'm wondering how many were made? Looks like it was only in production from '70-'75.

[TUBEDUDE]

The first and last one I heard was at a high school battle of the bands in 1973. This kid had two giant 18" folded horn cabinets with it and a Jazz bass.
Sounded like the voice of God. Warm, powerful, enveloping. It immediately drew all the attention in the gym.
None of us knew then what it was. Or that his family had stupid money.
They didn't win, but they were the favorite of all us players in the crowd.

[Stevem]

Good catch Martin!!

A 12AX7 has no where near the current drive that’s needed!

Another example of this is when a Marshall major is fitted with a AX7 for the PI / driver tube and then your left wondering where the hell the other 100 watts of output went to!

[Matthews Guitars]

I tubed it up using the tube chart, not the schematic. So it should be good on gain levels.

Incidentally, it takes a Tube Screamer in the normal input VERY well!

Right now I'm finishing up the rebuild of a Bassman 135 to stock condition (to resell) and as soon as that's done, which may be tomorrow, then I'll be digging into the 400 with an eye toward solving its reverb and tremolo issues.

Once that's done, I rebuild my '63 Tremolux.

Once that's done, I'll retolex the 400's cabinet and redo the grill.

I've got a list of things to do. It keeps me as busy as I want to be.

[martin manning]

Looking at the PS400 output section a little more... That was some out of the box thinking to automatically switch pairs of power tubes in as more speakers are plugged in, and I'm wondering if Dumble's half power switch might have been inspired by that. I can't think of any other amps that lifted cathodes to reduce output power. The SVT survived where the PS400 did not, probably in no small part due to the cost and complexity of a system that required three speaker cabinets to be fully functional.

[professormudd]

This post just really made me want to find one of these amps for myself. That reverb article was great. Curious what kind of $$ you are looking for to sell this monster?

[Stevem]

The first one of these that I ever worked on the original owner bought it with the optional Gauss driver 4-12" big Ass cabinet which was a tad wider then the head itself.
If I recall right this cabinet split the pairs of speakers and ran on two of the 3 output circuits at the same time.

I liked the sound of that 4-12 cab much better for the average size club room back then since that dam folded horn cab did not come into its own until 25 to 30 feet out when they started to kick good, and in fact would blow away the bottom end of the average PA back in those days if it was not horn loaded also!

[TUBEDUDE]

That's true of folded horns. Likely why you see more SVT cabs, whose speakers pummel from go.