Cheap project competition

[Paul Barker]

I have in mind throwing together a cheap 6em7 amplifier incorporating the phono correction within it, so if you like a complete unit requiring only an mc step up.

The power output attainable from the 6em7 is limited, but should make reasonable levels on some of the more efficient speakers we have at our gatherings in the smaller rooms.

I just thought it might be nice to throw it open, if you want make it a competition with two weightings, one for cost of parts (what shall we price them at, mine will come from the scrap bin?) other on sound quality. Give a rank for each not an absolute figure.

So say there are 4 contributors one would be ranked 1 for cheapness, but 3rd in the sound stakes, result is 4. An amp that costs a bit more 2, but sounds better, 2, would be equally scored. Lowest final number is winner.

Onloy way to value parts is that the builder has to either scower completed sales on ebay or RS catalogue or wherever and do print offs to prove what it could have been built for if he's had to actually buy the parts and not just root around at home like me.

Any takers?

6em7 not obligatory, but it's a tip, hard to beat for sound versus cost.

[Paul Barker]

Caveat,

should we allow full diy efforts to the extent of transformer winding? Might not be fair. Any thoughts?

TIP again, sources of cheap output transformers for project:

old tape players, if you can find one for a fiver, and you got a power supply too for a monoblock that's fine.

I have used torroidal filament transformers as parafeed output transformers with excellent results, you just have to realise they are wound for their power rating at 50hz, and there isn't sufficient inductance to provide anything below that, but there are two ways to deal with this. 1/ Series connect a pair, and / or 2/ fiddle it with a small parafeed cap to boost bass, sounds fine, don't get all hung up about it, this is a cheap project.

If you have a used stock of Maplin chokes no reason not to use them,. just value at the last known selling price.

[James D]

Great idea!!!

bet I can beat the 6EM7 with an ECL82 - umm well maybe....

How shall we define the price catorgories?

1) £100 or less
2) £101 to £150
3) £151 to £225
4) £226 to £300

And shall we have a thread to publish designs before the shoot out? Should we have power catagories? What context the rest of the system - I guess GL75 or 78, homemade unipivot Denon 103 or my $30 Discman as front end and what speakers? Or just any speaker under £300ish ?

I say you can wind your own but the time has to be costed at £30 an hour

This could be fun!

James

[Paul Barker]

Sounds good to me James.

Well I have a winding machine that can produce a rough transformer in half an hour, but if I have to value my time at 30mins then so do you guys, how long to hook up your amps etc? I'll use clip leeds so save my time there. Or shall we not include time in the equation to make it fair?

[James D]

I think £30 to make a pair of OPTs is fair. I can use clip leads too so I don't care if you include making time. Really I just think if you wind your own OPTs you should include the the time cost or a default cost... Otherwise why bother costing your spare parts bin - just call it all scrap and see how it does...

I guess the big saving we all make here is by not having a proper chassis. We'll all use a sheet of mdf or an old box instead

I've got four different ideas rough out already - not sure which is best yet. I might make a series of amps at different price points adding upgrades as we go along but the same basic design i.e. adding buffers and shunt regs, etc as the price increases. That would be interesting as it would show what you gain as you increse the spend.

James

James

[Paul Barker]

yes, I xpect if he still has it Nick will put up his 6em7 amp, what I may do is order boutique resistors but for the basic 6em7 amp, on the basis there are so few resistors anway in such projects, and it would be interesting to evaluate. I agree that 30 is fair for a pair of home jumble wound output transformers, though I would probably go parafeed torroid to keep the costs down, with maplin choke load and I'll have to find a cheap old leaky ex radio rally paper in oil parafeed cap. For such parts we'll have to be honest, everyone knows the sortof thing.

It did occur to me I have a cheap high voltage transformer that Stuart Dean only charged me something like £10 for that would make the gm70 sing. Are we allowed to use stuff another bb'er sold us cheap?

I also wandered, what if we auction off some or all of the amps afterwards for some good cause or to finance meets? Builder gets his published costs back remainder goes to the fund. That should keep it honest.

trouble beeing I don't think there would be many wanting to buy.

[Darren D]

Right, done mine ...LOL

OK I did it a couple of years ago but it's still together...??

How does that count?

OK, maybe I'll rebuild it, been thinking about it for a while.

[NickG]

yes, I xpect if he still has it Nick will put up his 6em7 amp

I was thinking of something else entirly...

Nice idea about the action, but I doubt anyone would want one of our bits of wood, and if we did that we might have to think about appearance, which would remove any advantage to using clipleads :-)

[Paul Barker]

actually not wanting to hijack my own thread, but what fun it would be to come together with a 6em7 amp, only rule is no other valve type allowed, use as many as you like, no solid state, just 6em7's inductive resistive and capacitive components. Maybe that will form, a subgroup?

Now what is Nick up to?

It has occured to me that the gm70 is well cheap and that a 6em7 would both volt amp and drive it, Stuart Dean's cheap power transformer, only costly bit is the output transformer. Now I won't cheat and say I can make those for 15 quid they cost 50 each in cheap m6 lams and copper, 30 quid labour each would be more fair for that one. So that makes parafeed cheaper with torroidal power transformers, don't worry I've done it before worn the t shirt, I know it sounds great, cheap opt's are nowhere near as good.

[James D]

I was thinking of using a nice sweet little dht with choke load and a choke load power pentode CF hanging off that with parafeed OPT But I'm also tempted by JRBs pentode power buffer too. Too much choice.

I found a really nice little rf DHT that needs using in cathode drive but then I need two OPT per channel.... It's a lot easier with a 6EM7, 6DN7, ECL 82/86!

James