Building a monster - 6 x 6550 200 watt amplifier

[Mr. Foxen]

Gas soldering iron exhaust is what I generally used. I've got a fitting for one of my gas irons for the purpose too.

[Mikey R]

[quote name='Mr. Foxen' timestamp='1366292150' post='2050602']
Gas soldering iron exhaust is what I generally used. I've got a fitting for one of my gas irons for the purpose too.
[/quote]

Cheers Oli! Any suggetions on where to get turrets?

[Mr. Foxen]

No idea on press in ones, I just get premade strips from Hong Kong.

[6v6]

[quote name='Mikey R' timestamp='1366293358' post='2050628']
Cheers Oli! Any suggetions on where to get turrets?
[/quote]

There are a few options, none cheap unfortunately, so plan your board carefully to avoid expensive mistakes (/me has made a few of those over the years..)

In the UK:
http://www.ampmaker.com/store/Turrets-tools/
http://www.bluebellaudio.com/
http://vyseamps.com/miscellaneous.htm

RS also do several different types of turrets but check the dimensions carefully as I'm not certain which will be most suited to amp boards, e.g:
http://uk.rs-online.com/web/p/crimp-tool-positioners-turrets-locators/2508828424/

In the US there are loads of other suppliers, may possibly work out cheaper than UK suppliers, not sure, e.g:
http://www.turretboards.com/
http://www.hoffmanamps.com/

If you actually care about cheap then eyelets work out a lot cheaper, can be a bit easier to work with, and look better in Fender style amps IMO.

[Beer of the Bass]

In my smaller builds like preamps, I like using those small pre-made tagboards like Burman used (ampmaker sell them), but some larger caps don't fit the spacing too well. Personally I found eyelets to be a bit of a pain in the bum on the one amp I built using them, and another one I've repaired. They're easy enough to assemble and work with, but if you need to re-work them, getting under the board to remove excess solder from the underside of the holes is a hassle. I'd love to do a big bass amp like this myself some day, but the cost of iron is putting me off!

Edited April 22, 2013 by Beer of the Bass

[Mikey R]

Cheers guys, I reckon its going to be the ones from Bluebell. I need to nail down the circuit first though, and I really seem to be procrastinating over that.

[Mikey R]

I decided to stop procrastinating and start on the metalwork for the chasis. First will be to build the power supply and verify that works, so I started cutting the holes for the transformers, chokes and the big caps.

Marking out, the old school way using paper templates. I really should have drawn these out on sqaured paper to ensure I got everything square and in perfect alignment, but I got pretty damn close using a lower fidelity method:



Last minute check to make sure everything fits:



And with the bulk of the holes cut:



I still need to deburr these holes and cut the holes for the bridge rectifiers and circuit boards mounted inside the chasis. Still, its a good start.

Checking that the holes are in the right place for the capacitor mounting clips:



And the inside of the chasis:



Its a little tight, and theres not much space between the terminals and the chasis. I can either file out a little around the terminals, or fill the void with hot melt glue. I'll probably do a bit of both, I dont want sparking here.

Edited April 28, 2013 by Mikey R

[Merton]

Looking good dude
When do I get to gig it......?

[Mikey R]

cheeky git - I hope youre looking after the brown bass!

[JapanAxe]

[quote name='Mikey R' timestamp='1367167369' post='2062023']
And the inside of the chasis:



Its a little tight, and theres not much space between the terminals and the chasis. I can either file out a little around the terminals, or fill the void with hot melt glue. I'll probably do a bit of both, I dont want sparking here.
[/quote]
That's very tight! Maybe MDP will lend you his big triangular 'zero fret' file to create some more clearance?

Edited May 6, 2013 by JapanAxe

[VTypeV4]

Can't believe I've missed this till now..!

Looks like a great project, Beavis on steriods eh!? Great project, keep the pix and info rolling!

[Mikey R]

[quote name='JapanAxe' timestamp='1367829523' post='2069776']
That's very tight! Maybe MDP will lend you his big triangular 'zero fret' file to create some more clearance?
[/quote]

Yeah, it was too tight for comfort, I bought a bigger punch and widened out those holes, theres plenty of room now.

[quote name='VTypeV4' timestamp='1367855404' post='2070262']
Can't believe I've missed this till now..!

Looks like a great project, Beavis on steriods eh!? Great project, keep the pix and info rolling!
[/quote]

Cheers dude, its going to be a bit of a slow project, Im feeling my way dead slowly and making sure Ive got it right.



The big Sovtek 6550 output vavles have arrived in the post Bigger than an EL34 but a little smaller than a KT88, I was worried they wernt going to fit in the small space behind the transformers, but its ok theres loads of room.

EDIT: Just took a quick shot to show the scale of these mahoosize components, thought I'd share it here too:



Thats a 50 pence piece for scale

Edited May 8, 2013 by Mikey R

[umph]

Looks good, just wondering why you went for a choke on both the power supplys? Resistors would've been fine and saved a lot of space.

[Mikey R]

[quote name='umph' timestamp='1368987165' post='2083731']
Looks good, just wondering why you went for a choke on both the power supplys? Resistors would've been fine and saved a lot of space.
[/quote]

There would be quite a bit of current flowing through those resistors, so I wouldnt be happy with the dessipation or the voltage drop.

Next one I build probably wont have dual chokes, but this is an experiment in overengineering.

Edited May 21, 2013 by Mikey R

[umph]

[quote name='Mikey R' timestamp='1369155853' post='2085588']
There would be quite a bit of current flowing through those resistors, so I wouldnt be happy with the dessipation or the voltage drop.

Next one I build probably wont have dual chokes, but this is an experiment in overengineering.
[/quote]

Chokes don't work right unless they're having the right amount of current being pulled through them. There wont be that much current being pulled from the top rail either. The layout worries me with the transformers like that though

[umph]

Also you have seem to have two phase splitters and a driver in your schematic, any chance I could see your schematic?

[Mikey R]

Hey Umph, will post it when Ive got it to hand. The power amp is based losely on the bastard child of a Marshall Major (concertina phase splitter driving a long tailed pair) and a Hiwatt 405 (cathode followers driving the grids of the output valves). I'll add individual biasing for each output valve on the advice of John.

Ive recalculated the values using the curves from the Chelmer Valve ECC81/2/3 datasheet, and pretty much followed the recommended operation from the 6550 datasheet for class AB1.

If you look at the schematic for the Hiwatt STA preamp, its not so far to change the cathode follower of the ECC83 into a concertina, by splitting the load resistance (100k in the schematic below) between the cathode and anode. If you do this, then, since the ECC81 has such a high input impedance, each grid will see perfectly equal but opposite polarity siginal, therefore you dont need to use different load impedances (33k and 39k in the schematic below) on the anodes.





Regarding the layout, yes it was a compromise. I had a choice between orienting either the PT or the chokes at 90 degress to the OT, the only way to have all three orthoganal would be to use a lay down type power transformer and that would have come with its own difficulties. From what Ive read, chokes tend to be noisier than transformers, so they were the critical component to be 90 degrees to the OT.

Im not concerned, since the Hiwatts, as well as the big Orange and Matamp chasisse that Ive seen picture of have had their PT and OTs in the same orientation.

Edited May 23, 2013 by Mikey R

[umph]

[quote name='Mikey R' timestamp='1369340985' post='2088185'] Hey Umph, will post it when Ive got it to hand. The power amp is based losely on the bastard child of a Marshall Major (concertina phase splitter driving a long tailed pair) and a Hiwatt 405 (cathode followers driving the grids of the output valves). I'll add individual biasing for each output valve on the advice of John. Ive recalculated the values using the curves from the Chelmer Valve ECC81/2/3 datasheet, and pretty much followed the recommended operation from the 6550 datasheet for class AB1. If you look at the schematic for the Hiwatt STA preamp, its not so far to change the cathode follower of the ECC83 into a concertina, by splitting the load resistance (100k in the schematic below) between the cathode and anode. If you do this, then, since the ECC81 has such a high input impedance, each grid will see perfectly equal but opposite polarity siginal, therefore you dont need to use different load impedances (33k and 39k in the schematic below) on the anodes. Regarding the layout, yes it was a compromise. I had a choice between orienting either the PT or the chokes at 90 degress to the OT, the only way to have all three orthoganal would be to use a lay down type power transformer and that would have come with its own difficulties. From what Ive read, chokes tend to be noisier than transformers, so they were the critical component to be 90 degrees to the OT. Im not concerned, since the Hiwatts, as well as the big Orange and Matamp chasisse that Ive seen picture of have had their PT and OTs in the same orientation. [/quote]

Hmm interesting, I'd be tempted to run a cathode follower instead of the concertina, as I don't think it'll clip well or have enough headroom to drive up your splitter. As to the balance issue a pot will work better as you'll be able to offset the balance in the valve itself.

I just foresee issues that could be dealt with using resistors for your ht. With modern caps and proper layout it's very easy to get a quiet layout

[Mikey R]

I need to check my values, but Im pretty sure theres tons of headroom on the concertina so the output valves will be driven way into class AB2 by the time the concertina starts to clip. And at that point, its going to be LOUD!

If the load of the concertina is perfectly resistive, or close enough, then you get better balancing than you would with a long tailed pair. Adding another trim pot into the design is just something else to forget to tweek when youre servicing the amp over time, and is something I dont want to have to worry about - I would much rather it was well balanced by design.

Yeah, Im starting to have doubts about the power supply. However, John gave me the design, its pretty much the bass amp he's been building for clients for years, and Im willing to bow to his experience here.

Edited May 24, 2013 by Mikey R

[Mikey R]

Of course, the other option is to take a balanced out from the solid state preamp. This would also simplify the build, as the global negative feedback components would be mounted to veroboard instead of tagboard.

[VTypeV4]

What is the SS pre-amp design? An active type with both cut and boost on the EQ section a bit like the Sound City ones of yore or an altogether more modern slant?

Beavis runs the basic Marshall mod Fender type circuit (the one when all the controls are 'off', there's still signal unlike the Fender) which works OK although the Yam NEQ does the real work..

[Mikey R]

I was playing around with a design this morning, its actually going to be a hybrid design, with valve input stage, solid state EQ, and a valve overdrive section (the second valve section from the Bassman, with the directly coupled cathode follower).

I was unsure about whether the EQ should precede or follow the overdrive section, but Ive figured out a way to make it work how I want. The bass frequencies bypass the overdrive section, so you always get the low frequency thundering presence. The bass control just controls the gain on this path (+15dB, to - infinity). The treble control, when set to boost (up to 15db), boosts pre-overdrive to really drive the valve hard. When you wind it down to cut (down to - infinity), it cuts post-overdrive, so you can cut out all the fizziness if needed. Its a modified Baxandall, with one half of the pot forming part of a potential divider controling the gain directly, and the other half a part of a potential divider controling the negative feedback.

I still need to prototype it on breadboard to make sure it'll work, but you can do things with opamps that wouldnt be practical with valves.

EDIT: Heres the gain curve for the bass and treble pots, +15dB to -inifity:

Edited May 28, 2013 by Mikey R

[JapanAxe]

Any update on this Mikey?

[Mikey R]

Fraid not, Ive been stalled doing other things. Life getting in the way and all that

A good friend has suggested I prototype the preamp with a smaller power amp and power supply, to iron out the bugs. Im tempted to take his advice.

[JapanAxe]

Sounds like a plan.