Fender 400 PS

earwighoney

I came across an article about the Fender 400 PS. 

Reading the article my mind was struggling to comprehend how damn loud the amp must have been and whether it was an amp too heavy for even @ICBM to carry around. 

There's no way I'd even try out an amp like that without industrial grade ear protection and wearing an adult nappy. 

ICBM

Otherwise known as the "Four Oops"  

It's meant for bass, so not really that excessively loud.

I've never actually seen one in person - but even for me (especially these days…  I'm getting on a bit now), too heavy to carry around, that's why it comes with its own trolley! For the head .

Although according to the spec, it's a few pounds lighter than an Ampeg SVT - those are brutal, even just getting one onto its own cabinet is a two-man job, safely.

sm55onl

Everyday is a school day on this forum.....

https://reverb.com/fr/news/the-most-powerful-tube-amp-of-all-time-behind-the-fender-400-ps?utm_campaign=1612019_blogfrfennder400ps&utm_medium=FB&utm_source=FB

http://timeelect.com/400-faq.htm
http://timeelect.com/

jpfamps

I've only ever seen one of these.

It requires 3 speaker cabs to get full power, and I only had access to one, so I never tested it sonically at full power although it was doing over 400W into a dummy load.

It was heavy......

ecc83

I fixed a Marshall bass head a few years ago. 6 X EL34 IIRC but might have been 6550? The fault was due to an attempt to put about 10 amps through a 6A rated 6mm connector, PCB burned up didntit.

That was 'quite' heavy, had a fan in the back.

I see from the map that the Fender had V low Z drive to the OP valves so, pulled grid current?

VERY high fideliteee!

ICBM

ecc83 said:

I fixed a Marshall bass head a few years ago. 6 X EL34 IIRC but might have been 6550? The fault was due to an attempt to put about 10 amps through a 6A rated 6mm connector, PCB burned up didntit.

That was 'quite' heavy, had a fan in the back.

They did two really big heads with 6550s in the early 80s, the 2000 (6x6550) which is meant for guitar and the 2001 (8x6550) which is meant for bass. I'm not aware of a 6xEL34 model but I could be forgetting something. More recently they did a 400W 8xKT88 model.

I've never seen a 2000 personally but I've worked on, and borrowed for a while, a 2001 - a huge-sounding thing, a bit too complex for its own good really but brilliant with a Rickenbacker because you can use the two separate channels with the stereo output of the bass. Weighed an absolute ton.

I briefly had a Trace Elliot V8 as well, and worked on two or three more - 400W 8xKT88. Poorly-made and unreliable though.

The other real beast I've worked on is the Hiwatt 400 - only 6xKT88, but operated at extremely high voltages, and the two I've worked on would both produce roughly 445W at the point of clipping. I thought these were probably the most powerful valve audio amps (production, anyway), but the Fender apparently just beats it.

John Chambers built a prototype 1KW amp using transmitter valves, I think.

DJH83004

John did make some monsters in his time IC. I do get to occasionally work on some big stuff,  this is a 20 x KT88, reggae disco amp and a 12 x KT88 baby brother, late 60's amps just for driving 18" bass bins 

sm55onl

Yehaaa !....twenty of them bad-boy KT88s.

http://www.r-type.org/pdfs/kt88.pdf

How were they operated, taking into consideration the attached pdf KT88 data-sheet ?

olafgarten

Laney still sell the Nexus Tube which has 8 x KT88 and weighs 35kg, just a few kg under the SVT. 

DJH83004

SM55, funnily enough you have hit the nail on the head, they were built around vintage GEC KT88s which would stand a lot more over voltage and general abuse , particularly on the screen grids, I got involved when the owner was replacing them with modern valves and they died after a very short while, so my job was basically fitting larger screen grid resistors and reducing the anode volts (B+) to an acceptable level for modern KT88s. It was still turning out in excess of a kilowatt of audio,  as you can see my dummy load plus active load (including all the cabs I had connected not in the image) - is that not a pretty sight for all you valve lovers! 

   

ICBM

When I test really high-power amps there's a simple way of making an effective dummy load - an old electric heater or kettle in the 2KW range has about the right DC resistance to simulate a 16-ohm cab. It's not exact so power calculations will be a bit off, but effective enough to test the amp at full power without blowing anything.

sm55onl

'...in excess of a kilowatt of audio....'
Bonza !

DJH83004 (and others), is it the case that the valves were in push-pull configuration to get this crazy output level ?

i was doing some amp electronics 101 reading there:

http://www.valvewizard.co.uk/se.html
http://www.valvewizard.co.uk/pp.html
http://www.aikenamps.com/index.php/what-do-the-terms-push-pull-and-single-ended-mean

sm55onl

Olafgarten,

Laney Nexus tube bass - specification:
http://www.laney.co.uk/products/nexus-tube/

schematic (this has the 8 x KT-88)
http://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Nexus-Tube-Nxt-Bias-Schematic.pdf

DJH83004

SM, Merlin Blencowe's articles (Valve Wizard) are an excellent way to get to understand the operation of valve technology, regarding the reggae amps, they are a simple push-pull design, with multiple KT88s in parallel pairs and BIG transformers. Their secret is they run the valves at the absolute limits of B+ and screen grid volts. They also have a 1 ohm resistor in the cathode leg which basically acts as a fuse, so if a valve arcs over, the resistor goes open circuit with a puff of smoke and the gig continues     

sm55onl

ICBM said:

They did two really big heads with 6550s in the early 80s, the 2000 (6x6550) which is meant for guitar and the 2001 (8x6550) which is meant for bass. I'm not aware of a 6xEL34 model but I could be forgetting something. More recently they did a 400W 8xKT88 model.

I found a schematic on't t'nternet
http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-400PS-Schematic.pdf

It seems an early one but doesn't mention 6550s....010309s (6 off) ?

Also, see the linked discussion thread - refer to comments made by CoolBlueGlow
http://www.tdpri.com/threads/fender-400ps-on-a-diet.315591/
He notes a slightly different set of tubes in comparison to the schematic above - but i assume they're replacement valve types that can be used ?

ICBM

sm55onl said:

It seems an early one but doesn't mention 6550s....010309s (6 off) ?

Note at the bottom - "The 010309 tubes are special design and selected Tungsol 6550 tubes."

This does become a problem when servicing these super-high-power amps - the quality of modern-production valves is rarely as good as the original *standard* types, let alone the special order ones.

sm55onl

A visit for me to Specsavers or comprehension lessons