Fender 400PS

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nacho459

Well-known member
Joined
Jul 20, 2004
Messages
339
Location
Pasadena CA
So my friend was at GC the other day and noticed this odd Fender bass amp buried in the back for $300. He noticed it had 6 6550's and he was in need of a bass amp so he picked it up. He went home to do some research on it and found out it is a Fender 400PS.

http://www.timeelect.com/400-histy.htm

This amp is insane! apparently you can hook it up to 3 4ohm cabinets for a total output of 435 watts! :shock:

Schematic

According to the schematic this is done by using a single 6L6 output section to drive the six 6550's. I've never herd of anything like this before, it seams like a good way to destroy things. And apparently you can't use regular 6550's or they will literally melt!

http://www.timeelect.com/6550a-ex.htm

You have to use 6550A's!

This seems like an insane way to build an amp! trying to squeeze 70 watts out of tubes intended to put out 35.

If any of you guys understand the physics of an amp like this and more importantly an explanation of how it doesn't just blow up like that amp in Back To The Future clue me in.
 
A really lame attempt to catch up with Ampeg. It's not "you can hook up three speaker cabinets", it's "you have to hook up three speaker cabinets" because one cabinet only gave you 1/3 of the wattage.

Edit: Had to stare at the schematic a bit - it's the first time I'd actually seen one. The legend is that there are really 3 power sections, and the schematic does seem to bear that out. Each pair of tubes has a connection to an output jack, I'd guess to turn that pair on or off. Clearly you'd need to be using a cabinet on each output jack to get the full output power, and the literature suggests that if you only use one or two cabinets you regularly switch speaker jacks to evenly wear the tubes.
 
the 6550s will be loaded oddly with only one speaker attached. what a stupid amp. 435W from 6x bottles is bonkers. a sixtet of 845s and we're talking biz, but who would trust a bassplayer with 1250VDC?
 
Sure is a lot to go flamingly wrong there.

> there are really 3 power sections

I would say one power section, but made of three pairs. If you don't put anything in the output jacks, all the tubes are biased near-Off by the A B C returns to the 4K7 resistors at the output jacks. But there is only one transformer and its four secondaries are all paralleled together: this is a 1.33Ω amp with options to work at 2Ω or 4Ω at 2/3rd or 1/3rd of full power.

Most lesser tube amps are very un-fussy about load. 4, 8 or 16 ohms changes the way the bass resonance blatts, but the change in loudness is small and the tubes hardly care. This amp is worked so close to or over the limits that the load has to be RIGHT or things go wrong. Going a little higher Z is OK, but any lower or a lot higher will melt tubes or punch-through the transformer.

Note that G2 is fed 1/2 of full B+, typical for the huge power pentodes.

Note that, while oddly drawn, the B+ is a voltage doubler. This may dispel notions that doublers are always wimpy. Note that it has 315uFd of 700V filter cap....

The 6L6 is running 11 Watts idle, can deliver 4 or 5 watts of power to the grids. No, wait, it runs Triode, so only a couple Watts. While 6550 likes a low grid resistance and six of them gets very low, this does not make sense in Class AB1. They must be pushing significant grid current, grids swung up to +15 or +20V, grid current near 60mA peak, to justify the Watts of drive power being made. Driver transformer is possibly 2:1+1 ratio.

> I've never herd of anything like this before, it seems like a good way to destroy things.

AB2 and even full B2 drive used to be common when we strained to get a whole Watt, and also very common in kilowatt amps such as the 25KW amp that used to come with every big AM transmitter. But if you cheap-out on the drive, distortion can be obnoxious. And since the arrival of 6L6, it just has not been necessary to go Class 2 for big home or small gig work.

> And apparently you can't use regular 6550's or they will literally melt!

Actually, I would be leery of using any tube except the ones made FOR that amp. We are outside 6550/6550A ratings in so many ways, I would want tubes maufactured with the express condition that they would survive a Fender 400PS. A tube could meet or exceed all 6550A specs and still fail in this amp. I think only TungSol had the attitude and materials to make tubes for this amp.

OTOH, properly biased EL34 loaded to only 2.66 ohms (200 Watts) might live a long time. 1960s RCA 6L6GC might go as low as 2 ohms. It is the very low plate load impedance (and the unusual biasing) that will melt tubes.

> trying to squeeze 70 watts out of tubes intended to put out 35.

Properly run 6550 and 8417 will make 100 watts per pair reliably. It's in the book for 8417. The main problem is rail-sag, crossover distortion, and ignorant techs tying to bias-out the crossover distortion.

I used to run 450W-550W of tube amp, modded Bogens. Same 700V B+, and 350V on G2, but six pairs of 8417, not three pair 6550. Pure AB1, no trace of grid current (Bogen's lame driver would die before it delivered current). Which is cheaper: 6 extra ordinary-spec tubes, or that honking big driver, special protection schemes, and six special order tubes? Doubling the output tubes does mean more weight to heat the cathodes, but even with the hyper-hot 8417 I had no reliability trouble, and sure no melted tubes. You have to be careful around such amps, things go wrong in a hurry and very expensively, but this Fender is really pushing its luck.

Note that bias is specified as 35mA per tube, far below where most folks would set 6550 bias. Peak current per cathode is close to 500mA, and a 500/35= 14:1 range of current and over 3:1 variation in Gm ensures crossover distortion. If you do "the right thing" and bias for minimum crossover distortion, you will probably melt and implode tubes before you get there.

> he was in need of a bass amp

Then he is still in need of a bass amp. Without a current supply of the special-made 010309 tubes, and great understanding of its special needs, it is going to be a headache. I don't see a clean way to make it less insane. Re-labeling the output jacks as 8 ohms each would help, but who wants all this weight for just a lousy 217 watts? Sell it to the guys with the web page.
 
Fixed one of those once when I worked for Cape Cod Audio like 10 years ago. Power supply had gone boom. Tungsol 6550's I believe.

It's suprising on little Cape Cod to have work on this Fender beast, a half dozen SVT's and a Marshall Major 200 watt head. Now almost everybody uses small combo amps.

A buddy of mine thought his Hartke 350 watt head was a loud as an SVT. Then one day he tried an SVT... :twisted:
 
Interesting stuff, hadn't seen things like this before :shock:

6550-1.jpg


http://www.timeelect.com/6550a-ex.htm
 
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