Building a monster - 6 x 6550 200 watt amplifier

[Mikey R]

Evening all,

Ive not got much to say about the specs for what Ive got planned, but Im kind of excited so had to write about it. Since not all that much is decided at the moment, I wont be giving out any details, but heres the background.

Those who know me, know that Ive been thinking about putting together an old school lead / bass amplifier for a while, but Ive never really been able to kick myself into getting it started.

However, reading about all the thermionic goodness that a few of our Basschat membership endulge in has been the catalyst to get me started. Ive been staring at schematics, reading everything I can find on the net, as well as Morgans excelent book, for long enough and I think I just need to take the plunge and get something underway, even if its only a platform for experimenting on.

So, first up, I needed a pair of transformers. There are a few places that do them, a couple of guys wind transformers in the UK, and there are a few from overseas. The amp builders in the US tend to use Hammond transformers from Canada, surprisingly they are available in the UK for not too many beans. However, I wanted to use stuff locally made if possible, just for the mojo. Enter the infamous John Wood.

Now, John Wood is an absolute audio guru - solid state or valve, he knows it all. I heard on Basschat that John winds transformers, so I got in contact. I was expecting a short "Yeah, Im after something like x," and "Ok, that'll cost you y," but John doesnt work that way. An hour long telephone call later, and my brain was totally expanded with all the knowledge he had transferred. I was making notes so fast, I can barely decypher what I'd written. And then he asked me, who I planned to get to wind the transformers. "You, I hope!" I replied, "but Ive got a few things to sort out first." We agreed I would call him back in the new year, which I did, and we put together a spec for what I was going to build, what I would need, and agreed a price. He's winding them right now, so with any luck I should have them soon.

Ive got a few other projects on at the moment, so this is going to take a while, but I'll try and keep this build thread updated as I go along.

EDIT: Ive finalised the specs, so here they are:

Preamp: 2 x ECC83 with solid state EQ bass + treble, possibly a mid, possibly an effects loop
Driver: 1 x ECC83 phase splitter,1 x ECC81 long tailed pair driver, 1 x ECC82 cathode follower (6550 are quite a heavy load to push)
Power amp: 6 x 6550

Speaking with John, if I have 600V on the plates and 300V on the screens, then with three pairs of modern valves I should hit 200 watts bang on. I could hit 200 watts with two pairs, but with 400V on the screens, but this would be quite hard on the power valves and I think it would be less expensive in replacement and more prudent in the long term to be more gentle.

Edited April 17, 2013 by Mikey R

[KiOgon]

Subscribed looking forward to this

[3below]

Excellent, share the specs please.

[Mikey R]

[quote name='3below' timestamp='1359762395' post='1960182']
Excellent, share the specs please.
[/quote]

Youre just going to have to wait

Edited February 2, 2013 by Mikey R

[Roland Rock]

Sounds exciting! John Woods co-owns the transformer/coil winder machine with John Chambers, who made a transformer for my amp.
Looking forward to seeing what you have planned!

[Billy Apple]

I'm in! [size=4] . Always fancied building a valve amp.[/size]

[Mikey R]

Still waiting on the transformers, Johns custom winding them.

[Mikey R]

Friday night, and Im spending it on the sofa designing the bias circuit...

Im leaning towards a Hiwatt style power amp, but modified with the inidividual bias adjust for each power valve from Merlin Blencowes book.

Schematics of all the Hiwatt variants are at http://hiwatt.org/tech2.html - this is a really great resource.

[Mikey R]

The iron has arrived!

[JapanAxe]

Blimey that looks well chunky! Would I be right in thinking that the big boys are for mains and output, smaller ones chokes? Or for driving a reverb spring?

[Mikey R]

Theres going to be two HT supplies, stacked one on top of the other, so theres one choke for each. Youre right about the big ones - output on the left and power on the right.

This is the provisional layout, I'll know for sure if everything else is going to fit when the caps and valve bases arrive.

Edited April 11, 2013 by Mikey R

[3below]

Ok that is some 'big iron', what are you running 4 x KT88?

I cheat when getting transformers - peavey VT series - great iron s/h at low prices, not enough B+ to run 4xKT88s though.

[Mikey R]

Im going to be running 6 x 6550 at 600v on the plates and 300v on the screens, theres no way I could drop 300 volts at those currents accross a resistor so Ive gone for the (fairly usual) stacked 2 x 300v HT supply.

[discreet]

That looks much more interesting than the iron I get to use.

[Mikey R]

The big caps and valve bases have arrived, so Ive been figuring out the best layout. Any opinions here would be greatfully received, its a bit of a squeeze.

Unusual. This one maximises the space between the power valves and the big caps, whilst keeping the power supply components to one end. Plenty of space for the tag boards also:



Classic. This ones a bit like a Hiwatt, with the caps in a line. Im not so keen on this one, as the caps are rather close to the power valves without leaving much space for a heat shield. Also, the wiring for the power supply will need to go right accross the width of the chasis, which will be both untidy and possibly a cause for hum:



Or probably my favourite. This one should leave just enough space for the tagboards, but not much space if I want to add anything later. Theres plenty of space for a short heat shield. Also, the power supply is fairly well kept in one place, but the 600V HT supply will be right next to the center tap of the output transformer, which is really handy, and the first valve of the pre-amp is as far from the mains transformer as possible:



There are compromises in all of the possible layouts, but thats always the case.

Edited April 17, 2013 by Mikey R

[Mikey R]

OP updated with the specs

[RIM Basses]

Looking cool bud, been looking into building my own valve amp with four kt120s gonna get some books off amazon and do my homework ha

[Mikey R]

Cheers dude! KT120s are a little weird, theres no typical operation on the data sheets so you have to work it out yourself from the graphs. I believe the new big Matamps are driven by KT120s so you definately can get a great tone from them.

Personally, I decided to go with the tried and tested 6550 configuration from the data sheet, I just found it easier. Whatever you go for, good luck!

[Mr. Foxen]

Have you had the transformers made so they heater winding can supply slightly more current to swap in KT88 if you fancy?

[Dad3353]

[quote name='Mr. Foxen' timestamp='1366243961' post='2050168']
Have you had the transformers made so they heater winding can supply slightly more current to swap in KT88 if you fancy?
[/quote]

Nice thought, but a bit late now, anyway, if it's not the case..!

[Mikey R]

[quote name='Dad3353' timestamp='1366273020' post='2050278']
Nice thought, but a bit late now, anyway, if it's not the case..!
[/quote]

Dont worry, the power transformer is specced so that I can use 6550s or KT88s, I could probably also use KT90s or KT120s. Unfortunately, with six power valves, tube rolling will be rather expensive!

Next question, probably for Mr. Foxen and umph - Im thinking of staking my own turret boards. Ive got some phenolic board on order, does anyone know a cheap place to get turrets from?

Ampmaker does screwmount ones: http://www.ampmaker.com/store/Screwmount-turrets.html

Bluebell is probably the cheapest for classic turrets, when bought in bulk: http://www.bluebellaudio.com/connectorsturret.htm

Any others?

Edited April 18, 2013 by Mikey R

[Mikey R]

This project is really an exercise in totally overbuilding everything. The heater wiring for the power valves will carry just shy of 10A, so everything needs to be able to handle that.

This is a trial run of the heater wiring. (The single strand wire will be replaced in the final implementation with PTFE coated multi strand.) Ive tried using some super chunky heat shrink to keep the wires together, however whilst shirnking it down, I melted both the wire and the cigarette lighter.

Rethink needed!

[Billy Apple]

[quote name='Mikey R' timestamp='1366286336' post='2050489']
This project is really an exercise in totally overbuilding everything. The heater wiring for the power valves will carry just shy of 10A, so everything needs to be able to handle that.

This is a trial run of the heater wiring. (The single strand wire will be replaced in the final implementation with PTFE coated multi strand.) Ive tried using some super chunky heat shrink to keep the wires together, however whilst shirnking it down, I melted both the wire and the cigarette lighter.

Rethink needed!


[/quote] Would a hot-air paintstripper gun not do you better for the shrinking?

[bartelby]

for small lengths I just use the side of a hot soldering iron, as it's usually on when I need heat shrink

[Mikey R]

Cheers for the suggestions gents. Yep, hot air gun is the orthodox way to shrink your tubes (!) but I didnt have one handy, I dont think the hair dryer would be hot enough. But I didnt think of using a soldering iron - good suggestion bartelby!

So ignoring the heat shrink for the moment (which might not even be necessary) I had a play with how the wiring would look on the actual bases:



This single strand wire holds its shape niecely, I dont think the PTFE stuff will be quite so well behaved.

Edited April 18, 2013 by Mikey R