Fender 400 PS - Power Section Only?

Adlerburg · #1 10-07-2010, 11:36 AM

Question to the Amp gurus please....
I was going to purchase a tube power amp and found the ones I wanted were really expensive.. Aguilar DB728 $$$$ TRACE $$$$.. THen I realized I have an old 400 PS in good shape. I would like to use only the power section of it and put my BBE Bmax-T pre infront of it. Is there an easy way to accomplish this? Bypass the Fender pre? If I'm able to do this, I'll have a GREAT 435W tube amp with GE6650As that prolly is better than all the newer ones (comments please)

Thanks.
-Mick

rbbrchkn · #2 10-07-2010, 11:49 AM

I don't think there's an FX return or poweramp input jack on there, but it should be a simple mod for any decent tech. If you're not using the vibrato or reverb footswitch, you could rewire one of those jacks as an input straight to the poweramp. No drilling or cutting, easily reversible if you decide to put it back.

And holy **** that's a great problem to have- those things roar.

BassmanPaul · #3 10-07-2010, 12:06 PM

Why not just use the Fender front end. All the BBE unit is is a Showman tone stack driven with SS.

Adlerburg · #4 10-07-2010, 12:20 PM

Quote:

Originally Posted by BassmanPaul

Why not just use the Fender front end. All the BBE unit is is a Showman tone stack driven with SS.

It's a tube pre.. and has a really nice built in soft-knee comp, as well as a built in BBE processer. I have grown to love this pre. I was even thinking of relocating the power section of the 400PS to a rack enclosure.

BassmanPaul · #5 10-07-2010, 03:01 PM

Quote:

Originally Posted by Adlerburg

It's a tube pre.. and has a really nice built in soft-knee comp, as well as a built in BBE processer. I have grown to love this pre. I was even thinking of relocating the power section of the 400PS to a rack enclosure.

I came to hate mine so it went. I found the processor sucked tone and the compressor crushed the dynamics of my signal. I should also say that I never use compression so I could be a little bit biased.

wcriley · #6 10-07-2010, 03:50 PM

Unless your amp has been modified, the only way to get the entire 435 watt potential is to connect a 4 ohm load to each of the three 135 watt power amp outputs.

pgk · #7 10-07-2010, 05:38 PM

Quote:

Originally Posted by wcriley

Unless your amp has been modified, the only way to get the entire 435 watt potential is to connect a 4 ohm load to each of the three 135 watt power amp outputs.

bingo

Adlerburg · #8 10-07-2010, 06:04 PM

Quote:

Originally Posted by wcriley

Unless your amp has been modified, the only way to get the entire 435 watt potential is to connect a 4 ohm load to each of the three 135 watt power amp outputs.

I know. It's really odd that there'd be 3 outputs.. just an odd number. I have 2 cabs, so I guess I'd be getting 135w per cab.

wcriley · #9 10-07-2010, 08:03 PM

It's not so odd...6 tubes...2 tubes per output.
The probable reason for dividing the outputs was that the power handling of the speakers available back then was pretty low.

Lots of info on the 400 PS here:
http://timeelect.com/400-PS-IDX.htm

Adlerburg · #10 10-07-2010, 08:47 PM

Quote:

Originally Posted by wcriley

It's not so odd...6 tubes...2 tubes per output.
The probable reason for dividing the outputs was that the power handling of the speakers available back then was pretty low.

Lots of info on the 400 PS here:
http://timeelect.com/400-PS-IDX.htm

Thanks.. Rich has been a friend of mine since childhood. He's built and modded most of my amps.

wcriley · #11 10-08-2010, 05:35 AM

Cool that you know Rich!
I figured you knew everything I posted about the 400 PS.
The info and link was mainly for people who've never heard of the 400 PS.

I became interested in the beast when someone offered to sell me one of the matching cabs. I didn't know Fender had ever marketed a bin, so I did some research. I'd like to have a 400 PS. Not to use...just to have. I think it's a really great design.

tombowlus · #12 10-08-2010, 07:43 AM

Quote:

Originally Posted by Adlerburg

Thanks.. Rich has been a friend of mine since childhood. He's built and modded most of my amps.

Does Rich know that you are considering ripping apart your 400PS, putting the power section in a rack unit, and/or driving the output section with a BBE preamp instead of the internal pre? If so, I'd think that maybe he'd say something like, "You are dead to me!"

Rich is working on my 400PS right now, so I know that he's a bit of a die hard about these amps (and rightfully so, IMHO). My advice would be not to totally rip your 400PS apart, but if there's an easy mod to allow you to use your BBE preamp to drive it's output section, then that'd be fine, I guess. I think Rich mentioned a mod that he can do which would optimize the output section for driving two cabs (not sure about all the power into one cab, though).

Another alternative would be to just use the 400PS with the compressor pedal of your choice and one of these:

or one of these:

Adlerburg · #13 10-08-2010, 07:45 AM

Quote:

Originally Posted by tombowlus

Does Rich know that you are considering ripping apart your 400PS, putting the power section in a rack unit, and/or driving the output section with a BBE preamp instead of the internal pre? If so, I'd think that maybe he'd say something like, "You are dead to me!"

Rich is working on my 400PS right now, so I know that he's a bit of a die hard about these amps (and rightfully so, IMHO). My advice would be not to totally rip your 400PS apart, but if there's an easy mod to allow you to use your BBE preamp to drive it's output section, then that'd be fine, I guess. I think Rich mentioned a mod that he can do which would optimize the output section for driving two cabs (not sure about all the power into one cab, though).

Another alternative would be to just use the 400PS with the compressor pedal of your choice and one of these:

or one of these:

Yes he does... And I was lectured for 3 hours for even considering it!

tombowlus · #14 10-08-2010, 07:46 AM

Quote:

Originally Posted by Adlerburg

Yes he does... And I was lectured for 3 hours for even considering it!

LOL! Gotta love Rich!

NecroticImbecil · #15 10-08-2010, 09:17 AM

Would it be illegal to just plug your preamp into the lower gain input on one channel of the PS, and just watch your volume on the pre, starting at "0" and slowly turning it up until you achieve a workable volume?

In the tradition of replies written by people who are really just guessing, I offer the following pseudo mathematical gibberish:

Bass guitar output = "Instrument level" volume, actually ranging from 0 (volume all the way down) to X

Preamp output = "line level" volume, actually ranging from 0 (volume all the way down) to Y

Y > X

Using lower gain input reduces Y and X by amount Z, should help reduce possibility of Y exceeding X by so much that unwanted distortion is generated. Probably won't prevent it.

"Instrument level" (0-X) is then a subset of the values in (0-Y).

Couldn't you just set the output volume to somewhere in (0-X) on the preamp? If it's too high, won't there be distortion from the input section of the tube amp to tell you you need to turn it down?

Research what "flat" settings on the EQ on the Fender if need be and set it accordingly. You could try this, at least, without spending any money. If it works, super. If not, perhaps you could trade the PS for a power amp or newer head with an effects in or preamp in allowing you to bypass its pre.

BassmanPaul · #16 10-08-2010, 10:20 AM

Quote:

Originally Posted by wcriley

Unless your amp has been modified, the only way to get the entire 435 watt potential is to connect a 4 ohm load to each of the three 135 watt power amp outputs.

I don't see it that way. Indeed you do have three outputs but all six 6550s are in push pull parallel. I would expect each output to have the full power available. That is of course making the assumption that the schematic is correct. I simply don't see the point of doing something so complicated. It only makes sense if all three outs are loaded constantly. There again the CBS years were not Fender's finest.

BassmanPaul · #17 10-08-2010, 10:24 AM

Quote:

Originally Posted by wcriley

It's not so odd...6 tubes...2 tubes per output.

That would require three different identical output transformers. As drawn the schematic shows just one with three output windings.

wcriley · #18 10-08-2010, 02:36 PM

I stand corrected. Was just going on what I'd been told. (I'm not very good at reading schematics)

I'm sure I read somewhere that one of the supposed advantages to having the 3 outputs was that you could alternate between them, using a different pair of tubes time, making the output bottles last longer. I'll look around and see if I can find where I read that. Or maybe I'm imagining things again.

BassmanPaul · #19 10-08-2010, 02:53 PM

There have been many amps that used a pair of push pull output sections with separate OPTs. Montgomery Ward was one who used the topology often. The output stages were fed from the same phase splitter/driver stage and each was connected to a separate speaker. I believe this was a way to get more power from lesser rated components like the OPTs.

Why CBS would use this particular scheme I have no idea. All I can see it doing is playing havoc {(and let loose the dogs of war!) Oops sorry - classical education or Star Trek

} with the plate loading of the 6550s. Not having worked on one I can't offer any other insights.

KramerBassFan · #20 10-08-2010, 08:22 PM

Quote:

Originally Posted by BassmanPaul

There have been many amps that used a pair of push pull output sections with separate OPTs. Montgomery Ward was one who used the topology often. The output stages were fed from the same phase splitter/driver stage and each was connected to a separate speaker. I believe this was a way to get more power from lesser rated components like the OPTs.

Why CBS would use this particular scheme I have no idea. All I can see it doing is playing havoc {(and let loose the dogs of war!) Oops sorry - classical education or Star Trek

} with the plate loading of the 6550s. Not having worked on one I can't offer any other insights.

Gretsch also had a series of "dual output amps" in the late 60's, but as far as i know, they were Valco products.