No bang, no flash, just music!

Hi guys;

Just a quick preliminary announcement - my 6BM8 lives!

So far so good after about 45 minutes of listening - no bangs or flashs, no background hum, and the sound's amazingly good through the garbage speakers I use for initially testing all my amps. Later I'll dig out my EPI 100's and wring it out a little better.

And all this for under $75. You can't beat DIY!!

Pics to follow ASAP.

All the best,
Morse

Oops, didn't mean to overstate it....

Hi John;

Hmmm, maybe my post was a little deceptive on costs to new DIY'ers.

A lot of the small parts were recycles (scrap aluminium stock, wood leftover from furniture projects, parts stripped off old computer PS's, knobs and switches out of my junk box, etc), but the main costs were:

1. PS $26 for a Hammond 269JX
2. $20 for OPT's
3. $14 for capacitors
4. $6 for sockets (overpaid, darn it!)
5. $8 for gold plated RCA's (okay, I should've used recycles but I couldn't resist)

Maybe I should've counted the $12 for a pair of cheapie 6BM8's, though I personally regard valves as fungibles rather than a permanent part of the amp.

If someone had to source all the parts new, it's going to break $100, but not by a whole lot - I just totalled it up and my estimate's maybe $125USD for all new (counting a new set of valves and a $13 plank of first grade cherry).

Still can't beat DIY!

All the best,
Morse
PS to any new DIY'ers reading this, save those parts off everything you throw out - including computers!! You'll never know when you need an RCA jack, an IEC socket, or a knob off some junker you were going to just toss out. Heck, I even strip out bundles of wire from old computers and use it on my projects - usually I can get 5 metres or more of useable wire from an old 386 or 486, in nice 25cm lengths.....

Hi Lee;

>>>...Just curious, where do you get two OPT'S for $20...<<<

There are actually a number of OPT's to choose from in that price range. The first option would be a line matching trafo - this was discussed in a previous thread. The other option is an old 'table radio' OPT. Triode Electronics has a couple that are in this price range (these are the ones I used), and you can check the other parts houses too - Angela Instruments, Antique Electronics Supply, etc. FWIW, I'll probably order up a couple more for the parts bin soon just to have 'em on hand. I never know when the "DIY bug" will hit me!!

Mind you, none of these hyper low cost options can compete with one of the better Hammonds - my 2A3 (with a pair of 5 kilo 1627SE's!) really does sound rather better than this little 6BM8 - as well it should!! Still, it's amazing how good this little guy sounds.....

All the best,
Morse

How hot is too hot??

Hi guys;

The only issue I'm having so far is that the PS is running hotter than I'd expected. While it's nominally rated for 500VCT @60mA (or 30 VA), I'm drawing 308V @ 84mA (or 25.9 VA). I usually allow an honest 100% overdesign, so this is new for me.

For the record, I just measured the PS at 60 degrees C (140 degrees Fahrenheit) after several hours of continuous operation (it's stable at this temp). Is this too hot? Usually I run my PS's so that I can pick up the device by them without getting scalded. If it's too much for this little PS, I may go up to a larger one like the 270DX and drop the excess voltage with a higher value dropper resistor (I'm using a 25 watt dropper that's dissipating 5.7 watts right now so there's a big margin power dissipation wise there).

FWIW the PS's primary is rated at 50VA, and I'm only drawing a measured 35.7VA.

What do you all think? Thanks for any suggestions.

Hi Dave;

$40 for a 6BM8? You've got me beat!! FWIW, my OPT's look like clones of the ones on yours.

Hi Frank;

The schematic's a really simple one - it's your basic cascade with the following specifics:

Triode:
Rk = 2k4r
Ip = .5mA (measured)
bypass cap of 220uF/35V on Rk
Rl = 220kr
Rgrid leak = 1M
Voltage as measured at the top of the load resistor = 189V (there's a 30kr dropper with a 22uF 450V decoupling cap after it)

.22uF Orange Drop for coupling

Pentode:
Rk = 390r
Ip = 34.8mA (measured)
no bypass cap
5kr->8r OPT (550r series resistance, so I get 19V drop across it - yes, it's a cheapie)
Rgrid leak = 470kr
Rg2 = 4700r
Ig2 = 6.7mA (measured)
decoupling cap = 22uF 450V

B+ = 239V

If I can get pics at a friend's house tomorrow, I'll see if I can scan in the schematics for anyone who's interested.

All the best,
Morse

Thanks Frank;

>>>...Looking at the data you provided, I wouldn't worry about the 60 degrees Celsius, chalk it up to copper losses in the Xformer and Class A...<<<

That's reassuring to know! I just remeasured, and 60C is at the hottest point (nearest the valves); on the opposite side it's 5 degrees C cooler. Okay, so I'll keep this trafo on there, but maybe I'll remount it (it's mounted to the wood chassis now) onto an aluminium plinth so that there's an extra thermal mass and radiative surface for it. Back to the tools....

All the best,
Morse

Almost - my bad....

Hi Dave;

Yes, it's just about right - only problem is that I didn't mention that the 30kr dropper and 22uF/450V decoupling cap for the triode section is shared between the two channels. That was my fault though. Everything else is spot on - thanks! That's a much neater schematic than my chicken scratch!!!

By the way, the PS is straightforward - a 269JX wired with the HV CT grounded and a pair of 1N4007's (snubbed with .01uF 3kV ceramic discs) going into a 100uF/450V Nichicon axial. Next up is a 820r 25w wirewound and finally another 100uF Nichicon. I'm also using DC on the filaments, resulting in one of the quietest amps I've built! :0

Thanks again,
Morse

Ack! One other thing....

Hi Dave - one other omission on my part - there's a 110 ohm gridstopper (carbon comp) on each pentode section. I had a bunch of 'em lying around and figured 'better safe than sorry'....

Sorry for the slovenly posts!!
Morse