diytube.com

I got the board assembled, high quality as usual. Very nice fit.

Thinking about going with solid state rectification since I could use the extra voltage for the 300B tubes. If I have enough money, I may even get the JS-6123HS output transformers instead of the JS-6113HS.

Question for you guys is the position of the two 1N4007 rectifiers, that is if I can use them. Or I have a couple spare 1N5821 that are somewhere else in this amp. Anything else that I have to do other than what is pictured in this simple sketch...

And check the pics of my humble progress so far.

Flip the diodes around the other way. The banded ends connect together. It would be a good idea to add some a low value resistor in series, maybe 10 ohm 5 watt to lessen the start up shock.

TomMcNally wrote:Flip the diodes around the other way. The banded ends connect together.

x2 this.

The resultant V- the way you have them will make for dangerously explosive caps

(I'm experienced and use goggles on the smoke test - accidents happen!)

Thanks for catching that Tom! I do have some 1.5 ohm 5 watts resistors, will that work? If not, I can use a thermistor as an inrush current limiter as on the ST35. Some pins are not in use on this tube socket, so I can install it there. For the 5W resistor, it can go off the board right before the choke me thinks, on a solder connector block.

Updated scheme highlighting changes.

1N4007 or 1N5408 (UF4007 or UF5408) should be used, the 1N5821s are for low voltage < 50V usage only, and would fail rather catastrophically,
if used for B+ rectification.

The UF series of rectifiers are Ultra-Fast and generate less transient noise. The 5408s are rated for 3A and 1KV.

The 1.5 ohm resistor in series with the choke won't do very much, it would be better to connect a CL-90 Inrush Current Limiter in the AC primary side of the power transformer.
A good place to connect it would be between the fuse and the power switch, This will allow the PSU electrolytic caps to charge more "gently" and eliminates the dreaded "power-on slam" surge. This is the same ICL as used on the ST35 board.

On my more recent builds, I replaced R17A & R17B with another 1.5H choke. Gives me a CLCLCRC PSU filtering "string".



HTH

/ed B

Thanks for that info on the rectifiers. I have plenty of those 1n4007s.

Hope to purchase the output and power transformers tomorrow. Aiming at the Transcendar TT-311-OT and the PA774 Triode Electronics power transformer.

Got a 3/16" thick aluminum plate coming for the top that I will drill. One less hole with the SS rectification!

Wood skirt with 12x12" bottom plate. Solid oak. Coming along...

Here is the solid state rectification, two 1N4007 and a 1.5 ohm 5 watt following them as recommended to slow down the inrush current. Let me know if these look OK pin wise and otherwise. Check out this plank of a top plate.

I'd just yank that 1.5 ohm resistor out altogether. Currently it is *before* the cap and hurts its low impedance path to ground for AC. You've already got in series resistances from the primary windings and it's just a puny, tube-rectifier-friendly 22uF cap anyhow. Otherwise, looks great.

Shannon

OK, that change is done. See pic. Also, the Electro Harmonix 300B Gold grid arrived, so just the iron in due in still.

This is where things are tonight. Smoke test maybe Wednesday.

I am unclear as to where the two 6.3 VAC secondaries go, can you guys clue me in? Pics attached, feel free to mark up.

Thanks.

One set at J3-1 and J3-2, and the other set at J4-1 and J4-2. See the small "1" identifying pin 1. Good luck!

Shannon

Thanks, that makes sense now.

I got everything wired, no smoke, but the 300B filaments are totally cold. No voltage at all at J3 & J4, pins 4 & 5. There is 6.8 VAC present at pins 1 & 2 of J3 & J4.

BTW, high voltage B+ OK at 461 VDC.

Seems like bad Schotky diodes, but all four? One side AC, on the other side nothing.

Advise?

I might have an answer to my own question, is there supposed to be four more diodes topside of the board? I only installed four in total, think there is supposed to be eight.

Let me know.

Here is the outcome, the amp played beautifully once the missing Schotky diodes were installed. No hum, buzz, or anything abnormal.

I ended up using silver solder on the board, all silver in Teflon wiring (except for the power transformer's own wires), Jantzen silver Z-caps, Transcendar output transformers with silver wiring, etc.

Hats off to you Shannon, what a smashing performer this is! Bass is truly thunderous! it is silky in tone, has lots of gain, whisper quiet when idle, and much air and separation for instruments. This is certainly at the top of the heap of performers. Has played for 12 hours, not a glitch.

I am also surprised at how good these Transcendar outputs are.

Thanks for all the help.