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Any suggestion for cheapest SET amplifier project

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hey morse, thats a heap of good stuff you got there! thanks! i've been looking for 6BM8 designs on the net and most are japanese ones. man, these guys are even using tamura opts for the 6BM8 :eek: this tube must sound real good :confused:

thomas, you got me really interested in your suggestion about no more than 10H on chokes. can you please elaborate or point me to a direction where i can find explanations?
 
Hi Arnoldc;

>>>...this tube must sound real good....<<<

Yep, it's got a sound out of all proportion to it's cost; I can hardly wait to get a pair of Hammond 125CSE's (biggest 125xxx's that will fit in this chassis). My 2A3 definitely outshines it, but not by the margin I would like it to - and it was about 10 times the cost of this little 6BM8! (Gad, it hurts to type that!!!)

You can see pics of that and some of my other projects at http://home.earthlink.net/~shidenkai/index.html Sorry, but I still haven't got the pics of my 6BM8 (a friend of mine hasn't emailed 'em to me from his place - no digital cam here). Pics of the 6DJ8 headphone amp I built are here: http://www.diyaudio.com/forums/showthread.php?s=&threadid=14035&highlight=pics

By the way, what kind of speakers are you going to drive? I'd heartily recommend the Fostex line if you can afford 'em, and the Radio Shack 40-1354 if you can't. Another possibility would be the Radio Shack 40-1197 in a TQWT - it's a kissing cousin to the Fostex FE 103. Here in the states, both those RS speakers have been discontinued and you can often find them for as little as $3 each (!!!) when you can find them, that is. The 40-1197 is still available in Canada from what I understand albeit at around $12-$14 Canadian (sorry, can't recall the exact price).

Right now I'm driving a pair of RS 40-1354's in 10 litre cabs (salvaged from an "all in one" minisystem) with this 6BM8, and it's a sweeet combination. I can hardly wait to get some better cabs built for these speakers!

Before I forget, if you want to add amenities like multiple sources to your amp, you obviously are going to add to the cost in terms of a switch and some extra input jacks. Believe it or not, I used a simple DPDT toggle switch on this one (it was an afterthought) and there's none of the crosstalk problems I was told to expect by some of the "high endy" guys at another forum. YMMV, since I dug out an old industrial quality switch (gold plated solder tags and solid silver contacts) and I can't vouch for any other brand of switch in this app.

One thing I forgot in my other post was the importance of good signal wire from the input jack to either the selector switch or the volume control (if only a single source amp) - I usually use microphone wire with 2 conductors and a shield. It works fine to use each wire for a different channel and connect the shield to ground at only the source end.

Whatever you go with, you've got the right idea - start small, that way any mistakes will not be wallet killers (nor will they be as heart rending). And the moment you first turn it on and hear music will be just as magical with a $100 amp as it is with a $1000 amp..... :)

All the best,
Morse
 
hi morse,

my god, this may hurt too :scratch:

i have built a 2A3 integrated and started with 125ESE, then an opportunity came and i got MagneQuest DS-025 on this amp now.

then i made a 300B with Lundahl power tranny, and now the 125ESE is on this amp. May upgrade the 300B to Lundahl OPT so i can re-use the 125ESE.

after i made a Foreplay for about $69 u.s. dollars, i got the itch to make a cheap SET as well, thus this topic. ;)

right now, i have an infinity kappa 200 at 89dB sounds loud with the two amps i currently have (well, my third amp is gathering dust :( Anthem Amp 1 and Pre 1L). i've been trying out a pair of Coincident Tri Sigs recently and i heard it good driven by my friend's Monica (DC Darling). i may be able to try out a pair of Omega Grande 6 or Super 3, just maybe. :)
 
Hi ArnoldC;

Nope, it's not the Svetlana schematic. It's the one at http://www.diyaudio.com/forums/attachment.php?s=&postid=170718 with the following changes:
1. There should be another 100uF 450V filter cap after the 820r dropper.
2. The 820r dropper has now been replaced with a 1k2r dropper.
3. The PS trafo is now a Hammond 270DX.
4. The 22uF 450V filter cap after the 820r should be placed after the 4k7r dropper resistor to the screen grid.
5. .01uF 600V 715P OD's have been parallelled to all filter caps.
6. .01uF 400V Dayton film and foils have been parallelled with the cathode caps on the triode section.
7. The heaters are run on DC off a 4A 50V bridge from Radio Shack and filtered with a 3300uF 35V cap. Since the PS upgrade, I've parallelled a pair of .47r in series with the A+ to the heaters to bring the heater voltage down to 6.27V.

The operating values are now:

Raw B+ = 265VDC with .157VAC ripple (measured at second 100uF filter)
Total current draw on HT secondary = 85mA
Total AC ripple as measured at the binding posts to the speakers = less than .001V (inaudible with 89dB speakers).

Pentode section
Ip = 33.8mA
Vp (relative to cathode) = 226.4V
IG2 = 7.2mA
VG2 (relative to cathode) = 215V

Triode section
Ip = .57mA
Vp (relative to cathode) = 83.1V (measured at plate)

With the 270DX, I measured 40.7VA (total from all secondary windings) and 55.2VA drawn (measured across the fuse on the primary); obviously the difference is the waste heat. For this reason I would honestly advise the use of at least a Hammond 270EX if not a 270FX.

Hope this is useful for anyone interested in a similar project!

Sorry for the delay in answering but for some reason the forum was not accepting replies to this post last evening. Ain't computers grand?! ;)

All the best,
Morse
 
Hi ArnoldC;

No problem; you might want to make a few revisions to better manage heat issues though.

I've not checked the watts of power out on an o-scope to be sure but it should be a little over 3 watts per channel rms at clipping; that fits with all the valve manuals, manufacturer's data, and direct comparison to my 2A3. Bass response is definitely hampered by my use of cheap iron, but with the 125ESE you'll have no problems there....

The biggest single thing to bear in mind in this design is the amount of waste heat generated by two sources - that 1k2r 25W ww dropper resistor dissipates an honest 9 watts of heat and the PS trafo itself is dissipating something like 14.5 watts of heat. My chassis design didn't really manage heat as well as I would have liked, so it's under revision (extra cooling ports, etc). For cooler operation, you could go to a CLC along with a lower voltage PS that has deeper current reserves, or you could just substitute a heftier PS trafo, currentwise. PSUD II is really helpful there!

One personal opinion is that I'd steer clear of Ei's 6BM8/ECL82. If you check the recent thread on the quality of Ei products, you'll see that my most recent Ei 6BM8's were substandard in quality. That's just my personal opinion though. Anyway, there are plenty of NOS RCA 6BM8/ECL82's out there, along with Svetlana's version and they seem fine!

Good luck - I just wish I had some pics of this one that I could post.....
Morse
 
frugal-phile™
Joined 2001
Paid Member
Morse said:

30 guage wire wrap wire is really good, a bit fiddly to work with thou

Chassis ... Be creative.

check out the aluminum pan section of the local 2nd hand store.

For power switches

Or yo can just use the pull the plug out of the wall method.

The 40-1197 is still available in Canada from what I understand albeit at around $12-$14 Canadian (sorry, can't recall the exact price).

$20 Canadian. 5% off if you use the appropriate CC, 10% off for 10 or more.

4. The 22uF 450V filter cap after the 820r should be placed after the 4k7r dropper resistor to the screen grid.

I was doing up an SEP 50C5 and was wondering about the way the screen was wired in the Mk 1. How does this map look for the Mk II (i'm going to crib off Morse -- just found another SE 6BM8 in my parts collection).

dave
 

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Hi Dave;

Thanks for reposting the revised schemo; since I'd missed the mistake with the screen grid in the posted Mark I schemo (it was always wired as it is in the revised schemo), I can tell that proofreading is not a career I should consider at this late stage in my life!! ;)

Yep, if you build the same circuit you'll be happy - I'm listening to mine through the 40-1354's in 10 litre junker cabs and it's gooood. Violins have a 'real' feel to 'em in classical music, and voices in pop music are 'right' - particularly oldies like Louis Armstrong, Frank Sinatra, Fats Domino and Sam Cooke. The sax accompaniment to Fats frankly outshines my 2A3's rendition of it (*sigh* - time to do some revisions on that amp now....maybe pull the cathode follower and go SRPP or to interstage trafos if I can scrounge up a pair cheap).

Also thanks for the corrections to the RS Canada pricing - I'd remembered it wrong! (early senility over here? who knows?).

One small revision (Mark IIb?) was the replacement of the 220kr 1/2w carbon film load resistors on the triode sections with 220kr 1w NTE metal films. It yielded a little more detail at the expense of a tiny bit of warmth. Overall I like the increase in detail, but that's a combination of personal taste and voicing with the other components used.

Hi ArnoldC;

$30 USD with most of the goodies, a good choke for the PS, 125ESE's and Mullards to boot (drool, drool)!! You're going to be mucho happy with that!!

All the best,
Morse
 
frugal-phile™
Joined 2001
Paid Member
Morse said:
through the 40-1354's in 10 litre junker cabs and it's gooood.

My 1354s are still sitting on the shelf... they will be going into a set of bipolar ML-TQWTs. (and i have some more on the way so i can try out a couple other option too)

An externally hosted image should be here but it was not working when we last tested it.


dave

PS i fixed the Mk 1 map.
 
Hi Dave;

My Gawd, those are beautiful speakers (drool, drool)! Great workmanship!! I've thought about building a set of T-lines for years (and admired your site - my compliments) but don't have the space for 'em at the moment.

For the future though, do you think that a 2 driver bi-polar TQWT using the RS40-1197 would have any potential? I've managed to track down 9 of 'em since RS USA closed 'em out and I've thought about building a set of 4 of 'em for a HT system I'd like to build; I'd shield the drivers for the centre channel and mount 'em in a midget BR for under the TV use. If necessary I could probably substitute an FE 103 for the front driver in the bi-polar centre channel (should I be unable to find another 1197).

I'd probably power it with a 5 channel 6BM8 SEP, now that I'm getting a feel for designing with this valve. It's cheap and I like the sound.

All the best,
Morse
PS - Thanks for revising that MK I schemo!
 
frugal-phile™
Joined 2001
Paid Member
Morse said:
My Gawd, those are beautiful speakers (drool, drool)! Great workmanship!! I've thought about building a set of T-lines for years (and admired your site - my compliments) but don't have the space for 'em at the moment.

That is just the 3D computer model. On some thread or othr i posted a pic of some cabs for some PHY 8s -- that is the sort of cosmetics -- we haven't finialized it yet (ie trying to avoid spending the big bucks on materials)

For the future though, do you think that a 2 driver bi-polar TQWT using the RS40-1197 would have any potential? .... If necessary I could probably substitute an FE 103 for the front driver in the bi-polar centre channel (should I be unable to find another 1197).

Chris & I have built quite a few of the BD-Pipes w 1197s -- they are fine (with suitable driver mods). We are now working on a design specifically for the 1197. The 1st iteration are centre channels using coincedent driver magnets for shielding.

The 1197s need a little help above 10k -- an ApexJr 14mm tweeter + 2 uF cap works well (we puesdo horn load them)

dave

PS i have lots of 1197s if you need a single.
 

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Hi Dave;

Wow, those are some terrific looking computer graphics you've generated! I feel really old, being from the pre-graphics 'mainframe computer' and slide rule era.... Anyway, they'll be terrific looking speakers when you build 'em.

Thanks for the data on the 1197 BD's. That's pretty much what I had in mind. Thanks also for the offer of selling an 1197 so I can complete my set for that one; I'll email you on that soon.

By the way, one late mod to the Mark II amp was a cooling fan under the chassis - it did a terrific job of moving heat out from under there, but it was way to noisy. Back to the drawing board on that one. Maybe some kind of baffle arrangement and an internal mounting to let the fan circulate air without being annoying? Hmmmm.

However, one mod that has worked well was the fitting of some .068uF 600V polypro caps I had laying 'round to bypass the decoupling caps to the screen grids. Sounds a tiny bit smoother now, and vocals sound a bit more natural (Less sibilance? I'm not sure - it's a subtle effect).

All the best,
Morse
 
frugal-phile™
Joined 2001
Paid Member
Morse said:
Wow, those are some terrific looking computer graphics you've generated! I feel really old, being from the pre-graphics 'mainframe computer' and slide rule era.... Anyway, they'll be terrific looking speakers when you build 'em.

Hey, i started on mainframes, with punchcards when graphics were carefully layed out arrays of characters. I did get an HP25c 1st year of university and slowly got to put away the slide-rule -- thanx for the complement thou (i was always a drawer and MacPaint & MacDraw got me started on serious 'puter graphics.

The plans and scematics are really quite simple... one of the favorite graphics i've done is this:

An externally hosted image should be here but it was not working when we last tested it.


(picture deep linked from HD3D Movies)

We have 2 variations of these built already. I just need to add drivers and play with em a bit.

dave
 
Arnold,


Yes, I know Mr. Mike Aw Young. 100 usd for a SET stereo amp? Well, it would boil down to DIY all the way from chassis to trannies. I myself wound my own trannies and make my own chassis. It is possible also on your part for a 100 usd budget. The hard part is to hone your skills and be a craftsman.

Comeby and visit me sir in case you have time,


John
 
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