Baked Spuds

[i_should_coco]

I've been loaned something very cool, so figure I should put it to good use. What is this cool I speak of? Well, here they are.

'Transformers :roll: ' I hear you say.... Yeah, yeah, so they are. However, as you have probably figured, they must be more interesting than that and so it goes. They are rather diminutive (about the size of preamp outputs) but they're designed for power amp use. They have 80% nickel cores and would probably not be good for more than a watt or so full range. That's not the plan, though. Even though they have a nominal impedance ratio of 2k7:8, they don't have enough inductance to support that at LF. Which brings me nicely to my plan for these: My system has multi-way active horns, with an amp on each way. Currently I'm using a borrowed Audio Innovations Series 300 for lower-mid duties and and 801A amp I built and which is also back on loan for upper mid/treble duty (1kHz up). The plan for these is therefore to be used to build a really nice HF amp for my S2s and RAAL ribbons and the 801A will be re-purposed for lower-mid duties.

My first thought was to build a spud amp (one tuber, gettit?) using something like the 6C45-PE. It's pretty popular for such use, but I have my doubts. the next thought was to use a DHT, say a 45, but that means a driver stage and complex filament supplies, making it a lot of effort for something experimental.

So, I have decided to use a 6EM7 tube. Now this pushes the definition of one tube a bit as it's actually two devices in one bottle. Ah well, sometimes you just have to cheat a little. Anyway, the 6EM7 is a really nice tube (the 801A amp uses one as input/driver). As mentioned, the bottle contains two devices, a high mu valve, a bit like a 6SL7 (sort of similar to a 5751 which is often used to replace.12AX7s) and a low-mu, high power section a bit like a baby 2A3. It's indirectly heated so no faffing with complex multiple heater supplies and should allow a nice simple two stage amp.

I spent the day re-stacking one of them as the owner had decided to dismantle it and stack it as 1x1 for parafeed use. It's now back to being butt-stacked (chortle) and matches the other one for inductance.

Now I need to scour the garage for a suitable mains transformer, chokes and caps.

 

[HoopsOnToast]

Nice, I was planning on making a Hawthone Audio Spud amp kit before they stopped selling them.

Will follow this with interest. :^

 

[pure sound]

Is CV finally having a clear out?

 

[i_should_coco]

Yeah, I'm hoping he'll simply forget that he ever lent me them!

 

[Tel]

So is that 4 amps for the system?

 

[i_should_coco]

I need 5 really, but the crossover only supports 4 currently.

 

[speedysteve]

Nice. Hope they sound super sweet.

 

[awkwardbydesign]

"a spud amp (one tuber, gettit?)"

For ages I (vaguely) wondered what that meant. Thanks for clearing that up.

 

[i_should_coco]

I've done a circuit design and started on the chassis.

Circuit is simple enough - direct coupled 6EM7 with tube-rectified choke-input supply (might as well do it properly). The design was dictated by pragmatism - i.e. the mains transformer I had handy which happened to have a winding for a tube rectifier and the voltage was too high for AC coupling. The other parts of the design will be dictated by what I can find in the garage / scrounge as the budget for this is approximately £0.00 because I'm skint. Luckily, I'm such a hoarder, I should be ok.

With appropriate output transformers, it would make a nice little 3W amp for full range, assuming you have super-sensitive speakers.

Also started making up a chassis, nothing special, just an old mule that I can sacrifice.

 

[RobsterD]

Sweet ,watching with interest Pete .

 

[i_should_coco]

Good progress today. As I said, just a lash up really, to test the concept, so don't expect tidy wiring!

Just mains wiring and the output transformers to do.

 

[John (big)]

I am in awe!!!

 

[BD Audio]

I must say the lack of horrifically high voltages and spaghetti of crocodile clip leads we usually see in your projects disappoints me. ;-)

 

[i_should_coco]

Bit of a setback, what I thought was a mains transformer doesn't seem to be. :?

So I need to dig something different out. Looks like clip leads will be involved after all.

 

[pure sound]

A minor setback. Clip leads FTW!

Presumably this is always going to be run high passed from the dig xover so no need for any high pass within? The bypass cap on the o/p stage cathode seemed quite a small value. What effect does changing that have in the sim? What's the standing current in the OPT going to be?

 

[i_should_coco]

Yes, no internal high pass, the transformers won't pass much LF anyway.

Current should be about 38mA, CV tells me that they should be able to handle 150mA or so.

The bypass is deliberately small as it doesn't have to do low frequencies.

 

[Tel]

I recognise those caps!

 

[awkwardbydesign]

Re-reading this thread, you say "borrowed". Does that mean they will have go back at some point? After doing all the work on them?

You must have more energy than me! Plus more skill. :notworthy:

 

[i_should_coco]

It's more of a bodge up than a build, just to see how well it works. I've already suggested the extra space is his flat is payment enough to which he mumbled something like "f*&% £ou". I may have to exchange something other than a couple of rolls of bog-paper if I want to keep them, though.