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

[Matthews Guitars]

I've acquired the monster, the beast, the highest output production bass/guitar tube amp of all time, the mighty 435 watt Fender 400PS.

Who here has any experience with these? I may have some questions and it's efficient to ask experienced people who can shorten my path
to learning.

First off, I'm an experienced amp tech and quite careful around plate voltages. The 400 PS offers a healthy 700 volts of technician killing plate voltage,
so just be aware, I'm not some inexperienced noob with no business in a tube guitar amp!

The amp in question was rebuilt but not taken to completion by my now dead friend and local amp guru. He almost got finished with it, and then abandoned it,
and it's been quite a few years since then.

Courtesy of his work, the wiring job in the amp is WAY neater than they ever were when they came off the Fender production line.

I've safely powered it up on my Powerstat variac, and while I don't yet have a working audio signal path, it didn't blow fuses, throw sparks, catch fire, or
do anything untoward except that it did try to roast the 680 ohm 3 watt resistor on the 6L6 tube that functions as the driver for the six 6550 output tubes,
so maybe that's a 6550 that has a short condition that doesn't show up on my tube tester. No problem, I'll just swap out that pair of tubes and try a different pair.

The way this is constructed, it runs as three pairs of 6550 output tubes, each of which is in standby UNLESS their appropriate dedicated output jack has a speaker jack
plugged into it. So it's actually safe to run it with no speakers connected as the switching jacks will keep the unloaded tubes in standby.

But, it means you can only use the full 435 watts output power by connecting three speaker cabinets. It's 145 watts per circuit.

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[pdf64]

No experience of them thankfully, but a possible cause of that 6L6 cathode resistor overheating is the grid being open circuit, ie with no DC path back to the cathode.
Did the 330ohm 2W resistor in the HT feed to its OT also take a beating?
As a bit of background, have you seen this page? http://www.timeelect.com/400-histy.htm

[Stevem]

That driver 6L6 is wired as a triode no?

I can’t see how that grid possibly being open can be a issue since it Has the master feeding it.

Repair wise I would unpower the the primary of that driver transformer and see if your two preamp sections then are working by looking for sign at the input to the master volume.

After confirming that you can move on to finding out what’s going on with that driver/ phase splitter section.

A 3 watt ( up rated from 1 watt)!cathode resistor as on that 6L6 should not be getting hot.

[pdf64]

That driver 6L6 is wired as a triode no?

I can’t see how that grid possibly being open can be a issue since it Has the master feeding it.

Indeed.
A grid leak path via a pot is fundamentally less reliable than a fixed resistor.
Have you never encountered a bad pot, eg with a degraded track / wiper contact, such that the wiper is, at some point of its rotation, open circuit?
If not, I suggest you consider the possibility of that occurring. Despite my limited experience, I can assure you that occasionally, it happens.

[Matthews Guitars]

Closer investigation reveals that the circuit is less complete than I originally thought. Note that the cables at the master volume control are cut short. There IS no master volume circuit,
at the moment, and yes, some of the missing connections do involve that 6L6. I'm going to have to rebuild from V6 to the power tube feeds. And if you notice, this doesn't use a phase inverter tube, it uses the 6L6 to drive a driver transformer. So at least that is understood.

There has also been some reorganization in the unit as well. Some components, like the large dropping resistors, have been relocated. I THINK those circuits are still the same, just where they have been located is different.

The reverb transformer is literally not in the amplifier. I've got a new one coming. I'll have to rebuild the reverb circuit, and I'm sure the vibrato circuit is not complete, either, as evidenced by the fact that the footswitch jacks for reverb and vibrato weren't even installed in the chassis.

This is fun stuff and it's making me THINK. Which is great. It's even greater when I figure out the solution, implement it, and get it right.

[Stevem]

Yes, any class A/B2 tube amp like this must use a transformer as the combined driver and phase splitter.

The first thing I would do is make a resistance check to confirm that the primary side of that transformer is not shorted to the secondary side, and also that the two secondary side windings are not shorted to each other.
If the amp where mine I know that would help me to feel better about putting added repair time and money into the project!

Also if you do not put that master pot back in, at least for now I would add a 47K resistor from that grid to ground.

[Matthews Guitars]

Master volume circuit restored. I now have a running amp. It's not totally right yet, the volume level is really low, but I know that there's more to be done, and now that I have a complete signal path, I can find and correct that and other issues.

[Stevem]

Well that’s good to hear!
How does the output wave form look that you have now?

Are both halves producing pretty much the same voltage?

[Matthews Guitars]

I spent most of the day on this beast today, to good results. It's now running more or less as it's supposed to. Yes it gets VERY loud. I've got three cabinets hooked up to it and it's not lacking for power, that's for sure!

That's not to say it's complete yet but I was able to identify the parts of the audio path that were not complete, and completed them.

I still have a short list of things to do to it.

Install the replacement reverb transformer.
Complete the reverb circuit.
Connect the tremolo circuit to the back panel control jack.
Connect the reverb circuit to the back panel control jack.
Clean every pot, every switch, every jack, and every tube socket.
Hand select and install a good set of preamp tubes.

Check the bias, check all relevant voltages, to be sure it's operating in spec.

Then button up the bottom shield, stick it in its cabinet, and prepare for this weekend's guitar show.

I should have all that taken care of around noon tomorrrow.

[martin manning]

Good to hear it's being restored back to it full stadium-shaking glory ;^) Here's another write up on this one: https://reverb.com/news/the-most-powerf ... der-400-ps

[Matthews Guitars]

Getting worthy tubes for these things can be an issue, as they were built around the Tung-Sol 6550A and were the incentive for the development of the GE 6550A. Neither tube is impossible to get today in NOS condition, but they're not exactly cheap. No, they can be stunningly expensive.

However I've heard that KT-120s are solid, reliable tubes that run well in this amp and they are pretty reasonably priced. From Tube Depot six would cost $335.70.

[martin manning]

You could try KT88. Might be slightly less expensive.

[Matthews Guitars]

I've heard reports that recently made EH KT88s are junk that fails quickly.

KT120s are an option. I have six Sovtek 6550WEs to try in it, if there are no horror stories about that tube in this amp type.

[Stevem]

In the last 400 I did that needed output tubes I use KT88s from JJ.

Do you have a scope to check wave forms and a hefty size load resistor?

[pdf64]

My very limited experience of KT120 was trying a pair in a KT88 100W bass amp, idle HT around 580V, 5k OT.
Despite conservative bias, one of the pair lost all emission after maybe 10 hour’s use.
Whereas the pair of JJ KT88 that replaced them are still fine several years later.