Stereo EL33 PP amp

[Sean Williams]

Been promising myself a "Tube amp" for years, but baulk at the sheer cost of the usual suspects, so, time to build my own.....

It will be a fairly simple design using EL33s as output valves (because I have a fair few...) with 6SL7 phase splitter/preamp stages

Ive done the chassis layout - A local company is going to machine this on their CNC mill for me (I have real trouble getting valve bases fitting nice and square!)

Will keep the thread updated as and when each stage is sorted

For now heres the CAD drawing for the chassis layout

[Peter.N.]

That's the way to do it.

[tri-comp]

Been there, done that:

https://www.vintage-radio.net/forum/...ad.php?t=67511

Absolutely the way to go. Good luck, it will look nice I'm sure.

rgds,

/tri-comp

Care to share the schematics. I'd like to know how you intend to run the EL's

[Sean Williams]

Still working on Schematic - but will be fairly standard topology - I'm not looking to break any new ground here.

Transformers are also from VVT well, Output trannys anyway, the rest of the iron is ex Racal RA17.

Hope to get the chassis machined this weekend - I can then start mounting the components - this will to some extent affect the final schematic - although I am not short of space, I want to keep everything used on the underside to a minimum.

Will probably end up using a commercial preamp as a seperate unit to this amplifier.

The outer casing of the amplifier will be some kind of hardwood - Onewatt has offered some red oak - this sounds nice - just have to sort out my somewhat basic woodworking skills!

More to do!

Sean

[arjoll]

Nice. I picked up a whole pile of 6V6's a couple of years ago, have been meaning to look into building a valve amp for myself and/or a wee gat amp for my 14 year old (who is a legend - and inseparable from his Les Paul Jr), the problem here is finding transformers!

Which reminds me Sean - I hope you watched the video I linked to on Facebook earlier in the week

[tri-comp]

Err... I could be wrong; it's kind of difficult to see on the layout, but aren't your iron-cores lined up in parallel?
I would choose a 90-degree angle of the powertransformer core towards the output-cores.

Best,

/tri-comp

[yestertech]

I'll follow this with interest, as I've recently acquired 2 separate transformers sets so I will have to go down the MONOBLOCKS route.
The transformers are for EL84s, but I'm hoping I can use EL33, EL37 or 6V6G as I'm not a fan of EL84. Depends on loading/DC capability I guess..
Sean, I'd be interested to know what the costs is of getting your chassis made ?

Hammond manufacturing do some great chassis kits c/w with a covering cage, but they're a tad pricey.
Still, that would be the only real expense.

Andy

[Sean Williams]

The OPTs are oriented at 90degrees to the power transformer's windings - the transformer is also fully enclosed in its own can, so I am hoping that coupling will be minimal

Andy,

Tough to quote costs - the machine shop is owned by a good mate of mine - Im going there tomorrow - hopefully he will do my chassis, while I weld up a Mini for him.....

So, if we are going on comparitive costs about £300 if you include my time and travelling!

Seriously though, I will enquire what the likely cost would be - most of it will be in machine setting time - the drawing is a DXF cad file, so that will import directly to his machine - we just need to load the correct tooling, and program the feed rates and speed, clamp the material down, then away we go.

Sean

[Sean Williams]

Chassis done, result.... Need more devopment time, and need to make sure I have enough of the same valve bases......

So, ok for a prototype, but NFG for the final thing....

A piccy of the mock up for the time being...

Will do some tweaks to the CAD model then get another plate cut

Sean

[tri-comp]

Nice and I see you have the right fluid to fill the bottles

Keep the pictures comming.

rgds,

/tri-comp

[yestertech]

Lookin' good !


Andy

[Andy Doz]

Nice !

I've built dozens of valve amps .. here's the latest :

https://www.vintage-radio.net/forum/...ad.php?t=68288

There's a link there to some pictures during construction. The link to the pictures is on facebook, but it's a public library, anyone can look without an account.

How do you find the VVT iron? I've always been happy with Hammond, until I heard Edcor, which this amp uses. I've used Sowter iron and Danbury before.

[Sean Williams]

Hmm, been a bit lax on this, but found some more valve bases that were of the right type. so have spent today wiring the beast up (well, some of last night too!)

Need to do a bit of tidying with the wiring now (a few cable ties needed)

Copy of the circuit below - Thanks to the numerous other places it has appeared on the web.

I still need to install the NFB stuff, and play withthe cathode resistors on the EL33s (I opted for seperate cathode resistors of 150R - currently running with around 46ma cathode current - this is too much according to the datasheet!)


Will redraw the schematic correctly when I have finalised the design.

A question...

NFB - what benefit does it bring on a simple design like this?

Cheers
Sean

[Diabolical Artificer]

In your schematic is the heater connected to the cathode in the GZ34 or is it a mistake in the drawing? It looks a simple circuit,no intermediat stage.Are you making two,for stereo? Keep us posted,Andy.

[MALC SCOTT]

The cathode is connected to one side of the heater.

[Sean Williams]

Hi Andy,

yes, there are a pair of amps on the same chassis - I have used EL33s instead of 6V6, and used a seperate cathode resistor/cap combo on each valve.

Indeed, it is a very simple design - I have taken some liberties with the published power supply design - I used cap input with a choke instead of resistive smoothing - my HT is about 260v when all is said and done, so the EL33s are a little overrun.

I'm going to add the NFB today, i dont think it really needs it, however, this is sort of a prototype, so I can try other ideas out if needed.

As it stands, I am driving it with my Ipad - I dont think there is enough drive for full output, but certainly loud enough to get complaints from SWMBO!

There is quite a lot of gain as far as I can tell with the circuit as is - the EL33 is a sensitive valve anyway, the input stage needs some screening adding to the signal path, as there is some hum present that disappears when you short the input to ground.

As Malc mentions, the GZ34 heater has the cathode connected to one side, I admit it does look odd!

I can see how amp building gets addictive! - I think I might just become a regular at VVT for transformers!

Cheers
Sean

[Sean Williams]

NFB now installed as per the schematic, the amp seems a lot more "balanced?" sorry, decending into audiophoolia.....

There is certainly better definition, and the higher frequency notes sound less muddled.

I've also increased the cathode resistors on the output stage to 220r from 150r - this has reduced the anode current to a more sensible level - this means I have enough spare HT now to allow a seperate preamp stage if I decide to go down that route.

Just the wire tidying, and a cabinet to make now....

Sean

[glowinganode]

The phase splitter in the above schematic looks a bit strange.
I'd have thought the two cathodes should be strapped together and connected to earth with the 100r resistor // 100uF capacitor?
Actually 100r sounds far too low, maybe nearer 1K.
If you've plenty of gain, lose the 100uF capacitor and feed the NFB to the junction of a 180r and 820r in series, with the 180r to earth and the 820r to the common cathode.
If you do leave the bypass cap, it should be across the 820r.
Rob.
Rob.

[PJL]

It's a minimal component design which runs the DC for the phase splitter through the secondary but does have the dissadvantage that the feedback is not around the entire amplifier.

Hifi designs like the Leak control the gain of the phase splitter but that generally needs an additional stage to make up for the loss and loads more bits.

[Sean Williams]

Funny you should mention that Rob...

I too thought that the circuit was "odd" - it works suprisingly well, once the NFB is connected, but did take me a while to figure out why it didnt before I installed it....

Before I installed the NFB i did shunt the cathodes to ground with 100r, to be honest i didnt see much difference either way.

I think i will try as you suggest, there is enough gain, not an excess - I can squirt an Ipad into it at full chat without clipping - it is painfully loud at that point!

I might hook up a load across the o/p later and see what the distortion looks like, along with the power output - Im only aiming for 6-7 watts

Sean