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Any suggestion for cheapest SET amplifier project

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I picked up a couple of Sovtek 6B4 (octal 2a3) for $US50 on eBay... They are pretty cheap and there are lots of schematics available. I am currently running mine at about 25W dissipation with 125ESE for about 5W of audio power. (which is surprisingly loud)

I used 3 separate power transformers I had lying around, built the power supply on a separate chassis.

Sounds really good, a little negative feedback helped though.

I have probably spent $US150 on this project total...
 
6B4

I hear that late model 6B4s from Sylvania were rebased 6AV5s with screen tied internally to plate. 6AV5s are about as common as dirt and almost as cheap....

But the 6EM7/6EA7/6DN7/6GL7 or miniature equivalents 6DE7/6DR7/6EW7/6FD7 will make about the cheapest triode amp possible. Only about a watt, though, in class A1.
 
829-B maybe?

In recent ponderings (and having access to four of these) I've done a search and it seems that with the 2 halves of an 829 paralleled up and run in triode mode you can get around 4 or 5 watts of really nice SE from this tube.

Haven't tried it but there was a very short thread on Audioasylum tubediy (do a search on 829) where someone was running one at about 300v with a 2.5k tranny (IIRC)

NOS RCA of these are about 9 dollars.

**Special base required can be got from tubestore.com for around 7 dollars ea.

just a thought

drew
 
Hi arnoldc;

Don't forget the lowly 6BM8 SET! Run in pentode mode you can get just over 3wrms from this little $8-$10 valve (complete with it's own triode input section!). While it's not as nice sounding as my 2A3 (with pricey iron and some "boutique" parts), it's quite good sounding.

Just do use more substantial PS iron than I did on my little 6BM8 - that 269JX is going to get replaced with a 270DX as soon as it arrives. Even that may be cutting it a little close, so I'll update everyone on my experiences there (hopefully it will run a LOT cooler than the current PS trafo...). The next step up is a 270FX, but I hope that's not necessary.

Pix to follow once a friend emails 'em to me.....

All the best,
Morse
 
Hi Arnoldc;

>>>...is your OPT also 125ESE?...<<<

Nope. It's one of Ned's $10 cheapies from Triode Electronics. I may upgun to something like a 125CSE when funds allow (one consideration is that I am keeping the OPT's under the hood of this one - and that's a 17.8cm by 17.8cm by 7cm space for EVERYTHING but the valves and the PS trafo). FWIW there's a very real difference in sound quality between those junkers and the 1627SE's I used on my 2A3 - but then the little guys cost next to nothing and actually sound surprisingly good through the testbed RS 40-1354's I'm planning to mate with this one....

Anyway, if you can afford 125ESE's on this I think you will be very pleased - they are WAY better iron than what I used on my 6BM8.

Please do keep us all posted on your progress!
Good luck on your project and all the best.

Morse
 
Arnold,

Yes, I am refering to the schematic which U can use. i have built an 8 watts per channel using a Tamura 2007 output tranny.

By the way, I am also a Filipino. I reside in Pasay City. Maybe I can be of assistance to your El Cheapo SET amp.

John
 
hi,

pls see my laterest version kits
Any suggeestion was welcome.

thanks

thomas
 

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Hi Arnoldc;

>>>...no more than USD100...<<<

Is that for the whole amp, or just a part, like the OPT's? Sorry, but I'm not sure of your meaning from context.

If $100USD is the cost of the whole amp, including valves, you're down in 6BM8 turf - only 2 valves (since each contains both an input section and a power section) for stereo and a cheap set of iron, like the $10 OPT's I got from Ned.

One thing to definitely do right though is to NOT SKIMP on the PS trafo - I found that out to my regret with my 6BM8 (that 269JX was running HOT and it's successor is only 9 degrees C cooler - at least it's got plenty of overhead on the primary winding though). In Hammonds, you should not go below a 270DX, and a 270FX would frankly be a better choice.

If that $100USD is for a pair of OPT's I think that the 125ESE is your best bet - it's got lots of people raving about it these days and it's very adaptable.

If that $100USD is for all the iron together, I'd spend most of that money on the PS trafo and get the cheapest OPT's I could find to start with. OPT's are easy to upgrade, but a PS trafo is intimately interconnected with the rest of the PS. All your voltages will change when you change it and that can be a blessing or a curse, depending on circumstances. Best to get the heftiest PS trafo you can and plan to upgrade the OPT's later.

Just my 2 pence worth! Others here may well disagree on this approach.

Good luck on it!
Morse
 
Hi Arnoldc;

Hmmm, if $100USD is for the whole amp (including valves?) then if it were me I'd definitely go with the 125ESE's at the very most expensive; you might consider one of Hammond's slightly less expensive options like the 125CSE or 125DSC as well.

The rest of this is based on US availability and pricing. No offence, but I honestly have no idea of the availability and pricing of the following items in your country.

Valves - I really like the sound of the little 6BM8; now that I'm in my second 'mark' of this amp (improved PS, bypass caps, and cooling now provided with a 2.5cm x 2.5cm vent in the chassis directly next to the dropper resistor) I can see why the Japanese love this type of amp so much! It's dirt simple and it sounds way better than it should for the money spent. Also it's possible to make it VERY tiny - mine is only 21.6cm x 21.6cm x 8.9cm.

Caps - For coupling, go Sprague 716P 600V Orange Drop if you can get 'em, 715P 600V Orange drop if you can't get the 716P. They sound great and you can usually get a pair of 'em for under $4 here. For PS caps, Nichicon electrolytics are fine as are the new JJ's and the Sprague Atoms. However, the Atoms are a little more $$, and they're much larger than the other two types. The JJ's are the smallest if size is a consideration; the Nichicons are available here a little cheaper and they're only a little bigger than the JJ's. Whatever you use, I have found that bypassing them with .01uF 600V 715P OD's works terrific at removing grain from the sound. Also, don't forget to bypass your cathode bypass caps with something like a .1uF - .01uF polypropylene film and foil type - you'll be rewarded with a slightly smoother sound. The 715P OD works fine as do the "Dayton" caps that Parts Express sells here.

Rectification - The humble SS diode gets the nod for small size and cheap $$ outlay. I used the ubiquitous 1N4007 snubbed with 3kV ceramic discs I had laying around on my most recent amp. Figure that the cost of this approach will be less than a good socket for a rectifier valve, let alone the cost of the valve.... Also, you really should consider DC heating - it's much quieter and it allows for regulation of the voltage to the filaments. By all means, if $$ is a consideration, make sure that those filaments are not being given too high a voltage. Your valves will last much longer if you can keep the voltage within two or three percent of the nominal rating. If it's out by 10% or more, expect a shortened valve life.

Volume control - Nothing wrong with a carbon track potentiometer like the 100K Alps audio taper stereo pot that Radio Shack sells here for $3. I have a bunch of 'em on hand and I use 'em in all my budget projects.

Wire - ordinary 18-20AWG tinned solid core copper is easy to work with and sounds fine, regardless of what some of the further out 'high enders' might think. If you're in a real budget crisis, you can salvage wiring out of old computers and monitors that are being thrown out. Just make sure of continuity and the integrity of the insulating jacket before you use it.

Chassis - You can spend a lot of $$ on a prefab chassis, or you can make your own. If you make your own, wood is cheap but you have to make CERTAIN that it's properly dried, otherwise shrinkage, checking, and cupping are likely problems (particularly in view of the heat an amp can generate). If in doubt, a good sheet of 3.2mm thick aluminium alloy and a pine base are cheap and functional. If you've a sense of humour, you can use some of the darndest things for a chassis. I've actually seen a picture of a 6BM8 built onto a cookie tin (!!!) at a Japanese website. Be creative.

Valve sockets - I like ceramic, but there's nothing wrong with bakelite. The biggest thing is that I'd go with chassis mounts to help dissipate the heat via the top plate. I corresponded a while back with a guitar amp builder who swore that the valves lasted longer that way (and those guys should know - the guitar amp community tends to run 'em HOT for long hours); he also swears that the circuit boards (if any) last longer if the sockets are not mounted on 'em.

Jacks - I like the cheapie gold plated RCA's that Parts Express carries here in the states. Personally I do not like the Radio Shack ones; you may be able to find something locally that's cheap and good. Also you could consider BNC single ended inputs or balanced line inputs via XLR's if you are willing to build adaptors to standard RCA gear.

Switches - For power switches I just go to the local "Farm and Fleet" and buy a motor rated 250VAC 10A switch. Those suckers last forever and they're cheap ($2.5USD here).

Resistors - No need to break the bank here, with boutique brands. I like to mix and match carbon comps, carbon film, and metal films to get the right mix between warmth and detail. If you want to be stingy, just go with the cheapest carbon films of an appropriate wattage rating you can find (only place I steer clear of carbon comps is in the PS - they can be a fire hazard in the event of a big power draw through 'em). Then if you want to get some more detail, swap out the load resistors on the input valves with a metal film type. For a dropper resistor, you can go ahead and use a ceramic potted wirewound without guilt. Any extra inductance is a plus for removing high freq garbage. Just never NEVER use an inductive wirewound for a load resistor or a cathode resistor. You want something that's as purely resistive as possible for these functions.

Good luck on it - hope I've not put you to sleep with this!!
All the best,
Morse
 
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