How big is too big?

[thermion]

As some of you know I build valve amps now & then and am scheming out a
First for me, a stereo amp. Think of a leak stereo 20 on steroids, using el 34’s or kt88’s. Something like a leak stereo 60 indeed. Doing a mock-up layout on the bench it’s going to be massive, about 400m/m square and weigh about 40kg. I thought about splitting it into two with a separate power supply, but I may as well make two smaller mono blocks instead.so I’m thinking at what point do homebrew valve amps become absurd? Have others made monstrous valve amps too? I’m getting the impression this one is getting out of hand should I build it to the trial layout on my bench

[Sergeauckland]

Back in the 1970s, I had to spend several weeks in East Africa. Knowing there would be a lot of hanging about in hotels waiting for something to happen, I took my valve data books and decided to design a valve amplifier from first principles.

It was a 100 watts P C Stereo amp, using KT88 outputs. When I got back, it took me a year or so to build it. Single Chassis, but with the compromise of a shared power supply so only one large mains transformer. Nevertheless, huge and heavy.

I used it for several years but endless modifications eventually made it unreliable and too cumberson, so sold the valves and Sowter ironwork to another enthusiast.

Still have the design notes and have thought about building another, but the cost of just the transformers is now frightening.

S

[G6Tanuki]

My approach would be to build a "power shelf" to provide the HT/LT/bias supplies. This would sit at the bottom of the rack, where its weight would be less of an issue.

Then the two amps built on separate chassis, to sit one above the other in the rack.

You can mount the chassis on shelf runners so you can slide the units out for easy access.

Putting the power supply on its own separate chassis will remove one source of induced hum. Splitting things into three separate units also helps with the whole manageability-of-weight thing, important for those of us with bad backs and where you don't always have an assistant on hand to help with the 'two man lift' thing.

[GrimJosef]

By the time you get to this capability commercial valve amplifiers tend to be split into two monoblocks, not least so that the owners can lift them. There are exceptions though. I've worked on a VTL ST-150 for example (a single stereo unit with four 6550 power valves in each channel, 150W/ch output, weighing in at 39kg https://www.vtl.com/products/power-amplifiers/stereo-amplifiers/st-150/). Lifting it safely up the steps to the workbench was a challenge.

They don't have to be that heavy though. Back in the 1960's Radford's STA100 (2 x KT88 in each channel, 100W/ch, details here https://www.radfordrevival.co.uk/resources/sta100/) came in at less than 29kg.

Stereo valve amps much bigger than this are rare, as far as I know. I guess the reason is that doubling from 100W/ch to 200W/ch adds a great deal of weight and cost but gets you just 3dB more sound. The natural nonlinearity of our ears means that the extra 3dB doesn't sound significantly louder, especially if you're sitting anywhere near the speakers.

Monoblocks can go huge, of course. The 10 x KT88 400 watter described at the end of M-OV's KT88 datasheet https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf is an example. I believe the Maplin magazine (1980's ?) might have had an article detailing how to build a pair (presumably) of these. They could also sell you the parts. I've never seen a pair though.

EDIT: G6Tanuki's already mentioned hum. This can be a serious issue in very high power valve audio amps. It's often quantified as being xx dB below full output and this number tends to be rather independent of the amp size. 80dB is often considered OK. 90dB is good but not so easy to achieve. Much better than that can become a real struggle. But 90dB below 100W is still 0.1uW which is audible if your ear is close to an efficient speaker.

Cheers,

GJ

[thermion]

I was thinking of making the power supply separate, but the voltage drop on the heaters, nigh on 10 amps at 6.3v is putting me off the idea. It’d need a hefty cable & decent plugs & sockets too. I don’t know what octal sockets are rated at, I’d look at jones plugs if I was going down that route. Either that or a separate heater transformer on the main amp chassis, but that is kind of defeating the object.

[knobtwiddler]

The Fairchild 641 amplifier (designed for cutting heads in mastering studios, but should be equally adept at driving speakers with right transformer taps) boasts 200W in pure class A. The 4CX250B valves are quite affordable if you know where to look.

Here is a link for the schematic and general theory: https://www.technicalaudio.com/pdf/Fairchild/Fairchild_641_Manual.pdf

Good luck with those transformers!

[GrimJosef]

Originally Posted by thermion

... It’d need a hefty cable & decent plugs & sockets too. I don’t know what octal sockets are rated at ...

Good quality ones are rated at 10A per connector https://uk.farnell.com/omron-industrial-automation/pl08/socket-chassis-8-pin/dp/1181272. If you needed, say, safety earth, HT1, HT2 and 0V then you'd have 4 pins left so you could split the heaters L-R and each pair of pins would only need to carry 5A or so. You could also double up the cable cores, obviously.

Ohms Law applies so voltage drops are predictable and if you could boost the voltage a little in the psu then you could still arrange to have 6.3V at the valve pins.

Cheers,

GJ

[G6Tanuki]

Nothing to stop you wiring the heaters in series and using a higher voltage transformer.

In the past I used this trick when needing to power four VHF tetrodes over a significant length of cable where voltage drops on the heaters would otherwise have been an issue.

The problem of sourcing modern high current high voltage connectors that have proper modern safety compliant protection for use in high voltage/current valve circuit power supply to amplifier interconnections is tricky. Jones plugs and Octals are very much a no-no, particularly because all the pins mate/de-mate at the same time, when to comply with the current safety zeitgeist you need to ensure that the earth is always the first one to be made when connecting and the last one to be broken when disconnecting.

I am building a power supply that does 6.3V at 5A and 1500V at 200mA at the moment... There are no sensibly priced power connectors for this.

[Diabolical Artificer]

I've built 3 big amps, two 120w 6 x EL34 monoblocks, a stereo two 4 x 807 amp & two 4 x 807 monoblocks fed by a separate PSU. All took a lot of fettling, protoyping etc & cost a lot in parts & time. None of them was ever really cranked all the way up all the time so never saw their full potential, however big amps do sound good even at normal listening levels EG a few watts.

DIY over engineered power supply's capable of supplying lots of current means the amps can reproduce peak transients in music like bass kick drums etc. You do need good OPT's capable of handling LF without saturating & also good up to at least 30khz.

I hand wound all the mains tfmr's myself using toroid cores with thick gauge wire so capable of 400v + at 1A, same with the heater wndg's. It was a lot of work but worth it as to buy custom wound tfmr's to the same spec would have cost more than I could afford.

What you get for all this work is well worth it, see attached pics & my Youtube channel for more info. Andy.

[thermion]

A man after my own heart! I’ve built a few big amps too. Machined the chassis up from aluminum I cadged at work, a few transformers from rallies etc etc. all good clean fun. I think I’ve built most published designs now, Mullard 5-10 & 20, Williamson, Gec 30w kt88 & a PX4 amp. Some of my creations..

[G6Tanuki]

As to actual amps, in the past I built quite a few, for paying customers.

Two or four 807 in push pull (parallel push pull for the big 240 Watters). 750V on the anodes, transformer driven zero bias Class B, typically using something like a 6N7 dual triode as driver.

Always solid state rectification, always fan cooled.

Power supply and amp always on separate chassis though installed in the same cab.

My amp chassis of preference was a U shape with the valves mounted so they stuck out horizontally each side of the U, with the electronics and driver transformer on the inside of the U.

I got out of the game when a couple of guitarists wanted even louder.

[knobtwiddler]

A good place to find inspiration for large valve amps is to check out what was being used by the Reggae sound systems in the 70s and 80s. A quick search threw up This one (24 x KT88s).

As an side: I did hear a story of a system in West London, where the owner was determined to run his MOSFET amps in Class A, but its size made heatsinks prohibitively expensive. No problem, he rigged up a water pump / radiator-based system to cool them.

[ScottBouch]

(Thermion... Your machined cases are stunning! They look like a work of art.)