Avoiding damage to a Fender 400 PS tube amp

Lonesomedave · #1 06-16-2011, 04:14 AM

hey guys, I have a question,

below is a picture of the back of an old fender 400 PS bass amp. it is only a portion and shows the three speaker outputs.

although it is a 400 watt amp, it has divided it's speaker outputs into 3 equal parts of 145 watts each for a total of 435 watts.

my question is: do you have to have a speaker attached to each of the outputs when you turn it on, to avoid damaging the amp, or can you simply attach a speaker to one (as they seem to indicate in the picture) of the outputs and run the amp that way. I presume if you do that, you will only have 145 watts available, but I do not know. Please excuse my ignorance on this but I was just curious.

thanks,

/s/ Dave

AwkwardLoudness · #2 06-16-2011, 06:52 AM

Yes, you can attach one 4Ω cab to one of the output jacks and not use the other output jacks. As you suspect, only one third of the amp's power will be available as only two of the six 6550 power tubes will be utilized.

Rich K's website has lots of info on that unique glorious amp: http://www.timeelect.com/400-PS-IDX.htm

beans-on-toast · #3 06-16-2011, 07:06 AM

When using a single cabinet, you should alternate which output you use to prolong tube life by running the tubes down evenly.

There is a switch on each speaker output jack that enables two of the six output tubes when a cable is plugged in.

The user's manual is available here Fender Amp Field Guide Manuals

AwkwardLoudness · #4 06-16-2011, 09:35 AM

Be sure to use proper speaker cable(s) to connect the amp and cab(s). One of the images shows what appears to be a thin instrument cable.

gnjpowell · #5 06-16-2011, 09:53 AM

No, you do not need a speaker connected to each output, but you do need at least one. "Switching between inputs to extend tube life" is totally 100% FALSE, it does NOTHING. You do need all 3 outputs connected to 3x cabs to get full power from this amp. (3 x 145W) This amp was designed this way to minimize warranty claims on blown-up speakers....

Lonesomedave · #6 06-16-2011, 11:02 AM

thanks for the help, fellows,

I have a follow up to beans-on-toast (and by the way, thanks for the link to the 400PS manual !):

what do you mean "There is a switch on each speaker output jack that enables two of the six output tubes when a cable is plugged in."... I do not see a switch; where is it located?

/s/ Dave

throbbinnut · #7 06-16-2011, 12:31 PM

Actually, using different output jacks does turn on different pairs of tubes. See those points labelled A, B, and C on the schematic? Those are the cathodes of the power tubes, and they only get connected to ground if there's a speaker cable plugged in, connecting them to ground 1 pair of tubes at a time.

Chris

beans-on-toast · #8 06-16-2011, 12:39 PM

Quote:

Originally Posted by throbbinnut

Actually, using different output jacks does turn on different pairs of tubes. See those points labelled A, B, and C on the schematic? Those are the cathodes of the power tubes, and they only get connected to ground if there's a speaker cable plugged in, connecting them to ground 1 pair of tubes at a time.

Chris

That is what I see as well.

The tubes are in standby (heaters lit) but the cathodes are disconnected until you plug in to the speaker output jack. This connects the cathodes, tied together in pairs of tubes to ground which enables current flow through the tube. This is why the manual states that you can rotate which output jack you use to spread out usage of each pair if only one cab is normally connected. It is true that to attain full power output you need to connect three cabs, one to each output jack.

beans-on-toast · #9 06-16-2011, 01:36 PM

Quote:

Originally Posted by Lonesomedave

I have a follow up to beans-on-toast (and by the way, thanks for the link to the 400PS manual !):

.. I do not see a switch; where is it located?

The switch is built into the output jack. It is transparent to the user. It switches automatically when you insert the 1/4" plug into the speaker output jack.

The problem with this sort of design, is that you can run into trouble if the switch contacts get dirty. If you have the amp serviced, it would be a good idea to have your tech inspect these contacts and clean them with a product like DeOxit. Ampeg uses a similar type of switch in the SVT to switch transformer taps.

NecroticImbecil · #10 06-16-2011, 04:43 PM

From the pic, besides the potentially..exciting... use of a cheap Radio Shack patch cord as a speaker cord, the interesting thing is the back panel. Never seen one of those, usually thrown out by some previous owner as with most old tube heads, since it prevents fiddling with tubes. Is the grey cord coiled around the thing on the back of the panel the footswitch cord, or has the power cord been replaced? Stock power cords on those are thicker than my thumb.