2A3 Spud

[CLS]

OK I'm cheating

Everyone knows one 2A3 alone can never produce sufficient voltage gain for any source/speaker combo. So I'm cheating here with a very high gain pre amp - 417A/5842 with 1.125:1 OPT (reconfigured from 4.5:1 Lundahl LL1660). On paper, this pre (now driver) gives 135Vp-p (@ +/-2.1V input) and generous current drive for 2A3.

It's not a well-planned 'new' project. The major parts (irons and tubes) have been lying around for quite a long time, I just put them together now. And the aforementioned high gain pre amp is another major motivation. (I'd like to have as few stages as possible, now it's still two stages from DAC to speaker)



I'm lazy and hate metal work for amps, so I built it with (leftover) plywood. Other than those around tube sockets, I also drilled many holes around power and output transformers underneath for ventilation. These holes alone took me more than an hour. (However I was very happy it's not metal.)

The layout might be a little unusual. I put the tubes and OPTs at the rear for short signal paths. While the heaviest chokes and power transformer line up in the front makes it easier for me to lift/move. No looker, not to mention pretty, but I'm OK with that.

I own a 300B amp with a lot of irons which are in the rear half of the chasis (like most amps). With the load at the far side, it hurt my back several times. Ouch!

For a single stage triode, what can I do? It's all text book setup.



Maybe only one thing is worth mentioning - I planned to used LCLC filtering. Simulating by (not ideal) PSUD2 and got pretty much what I need. But in reality I just can not get the voltage as expected. So I add another small RC (33R+4uF) in front to get it there.

Similar situation happened in the heater power supplies. Voltage was far lower then planned. So I add 10uF here and reduce the inductance and DCR of the last choke to get 2.4V.

Oh, one more thing. I didn't use the cathode bypass cap in the beginning, but it buzz, big time. I planned to just rely on the WE connection (cap across OPT pri. and cathode), but interferences somewhere are too much. Putting the cathode bypass caps back on quiets it down immediately. Now there's a pretty pure 120Hz hum and it's quiet enough to ignore.

[Bigun]

Looks nice ! - you could paint the plywood perhaps to make it even nicer.

[TheGimp]

Or stain it and finish it.

I like wood.

[danFrank]

If you have sheilding / hum problems you can line the inside of the wood chassis with copper tape made by 3M. It is thin copper foil that has adhesive on one side and sticks to pretty much any type of surface.
Daniel

[CLS]

Thanks for all your inputs

Sigh~ I'm just too lazy to take anymore effort in the finishing I've applied 2 coats of bee wax (is that you call it?) on the outer surface, nothing was done inside. I'm just so eager to hear it, so built it in a hurry.

As to the possible buzz picking/interference, I'm still wondering. Last time I built a naked 6H30 spud amp on a piece of pine and got no problem. For many, that kind of build is bread board, but I just took it as it was, using it for about a year. It's dead quiet. (but that's not DHT, oh well)

I have some copper tape, but unfortunately it'd be too difficult to use it now, or I have to rebuild the whole circuit! Oh no!

The Lundahls should be very low in magnetic stray. Power chokes are sealed in their own metal case and should be OK, too. Maybe it's the choke(s) for heater supplies or they are not filtered well enough....

I'll take photo(s) of the inside later.

[boywonder]

Originally Posted by CLS

Oh, one more thing. I didn't use the cathode bypass cap in the beginning, but it buzz, big time.

I've applied 2 coats of bee wax (is that you call it?).......

Probably the bee's wax..........

[CLS]

LOL !!

Here is the internal view:

[CLS]

The 4 EI chokes are for heater supplies. Two on the top were originally secured on the side wall with the cores pointing horizontally. Later I tried turning them as they are now, but no effect on the hum.

Other than the circuit diagram above, I added a small cap in front of the first RC as snubber. 0.47u as in the photo is too much, I swapped it to 0.22 later.

The 5R4GY has a bad behavior at the startup - huge voltage surge. It rushes to almost 400V on the OPT at its max and then fall down to 309V. Worrying about the 20u PIO bypass cap with only 300V rating, I tried IDH rectifier GZ37 (CV378). Now the startup behavior is much gentler but with a much less voltage drop and give me 20V more on the plate!

So now the plate voltage and dissipation exceeds the max 250V / 15W rating in the datasheet, but it seems fine and play well all day.

Or maybe I should turn the voltage down a little.

[tinitus]

Originally Posted by CLS

As to the possible buzz picking/interference, I'm still wondering.

Im no expert, but your trafos seem to form a "perfect" ring around your whole amp layout
might not be "optimal"
thats the first I noticed
but I dont know

what I usually notice is supply to one side, and amp section on the opposite

[CLS]

It might look that way, but actually not as simple

2 heatsinks besides the power trafo are for bridge diodes of heater supplies. And along either side wall you see 2 (EI) chokes per side. Each 'path' of these 2 heater supplies is 'enclosed' itself by twisted wires. So they are very thin and twsited loops by themselves and form 2 large curves in an overall look.

And the HT supplies are more or less 'star' shape - thin blue/white twisted wires. Begin with the rectifier tube at the center, go to the 1st choke(s) at top coners, then back to filter caps in the middle, then go to the second choks(s), finally go down and feed the bypass cap and OPT. Each 'path' are twisted as far as possible. So I think the inner area of the 'loops' should be very small.

With the cathode bypass caps in place, now the hum is very low. I have to stuff my head into the mid horn to hear it. (this amp serves mid-high in active xover system only).