Originally Posted by holland
I was taking a closer look at his site, he's got some cool stuff, including an amp that I was thinking of (no PCB though).
Thanks for the schematics, put it in the OP too (?). Are you able to list the 6DJ8 mod in detail?
A bit of a disclaimer.
I asked Tom how many 300Bs are working in the field (NPI). He said most folks don't let him know when they are working. I was the first.
Tom has two units, that makes mine serial #3. Add to that that this is the first HP implementation mine would now be serial #1. This is not a Bottlehead build, Tom has no intention of kiting it out. He does however provide a complete BOM that includes DigiKey part numbers. That the guesswork out of ordering the components.
Remember, I'm an idiot with a soldering iron… "OP" is that operating points?
If's so I don't have a clue but Tom talked about it them for the conversion to ECC88. ...
"Modifying the circuit to try out the two different operating points identified from the data sheet, selecting the best one, and taking a few quick THD measurements took a bit over an hour. At 1 kHz, the THD comes in at about 0.2~0.3 % for 1 W out. 1 % THD happens at 9.5 W. Max output power is just shy of 11 W."
ECC88 Driver Board BOM Modifications:
Switch the LEDs D1, D3, D6, D8 for high-efficiency Vishay TLHG6400.
R2 = 47 ohm
R16 = 47 ohm
D5 = Wire jumper (short circuit)
D10 = Wire jumper (short circuit)
TU1 = E88CC
TU3 = E88CC
Reworking the board was not too difficult. A Soldapullt is mandatory. I ordered the parts to have on hand if I wanted to take it back to 6N6P.
Also there is a set of Driver board schematics on Tom's site for the ECC99 / 12BH7A.
Originally Posted by randytsuch
In one of the early threads after it started working, you said there was "slight static", like vinyl.
It is still there? Is it like low level noise?
Otherwise, sounds like a winner, can't wait for the meet to hear it.
The snow is still there, It's a bit north of "slight", but fades into the music.
Tom is working on it. As you know anything shows up in a HP amp and this, as I said, is serial #1
Here is how the snow starts:
Turn the power switch on.
5 seconds of slight hum..builds a bit, then
A sound like a needle across vinyl.
A slight pop…then the snow. Like it passed gas.
It does not matter where the volume pot is set, the snow volume is constant.
I thought it was the 300B's. Tom's reply:
When the tube is cold, its cathode (= filament for a directly heated triode like the 300B) doesn't emit any electrons. So no current flows. The hum you hear is probably just some induction into the output transformers or something. The needle across vinyl, I'm guessing is the output tube starting up. Its filament/cathode is now warm enough that it starts emitting electrons. The slight pop (probably occurs about 20~30 seconds after power-on) is likely the input stage starting up. That brings the hiss.
I don't recall a needle across vinyl sound in mine. But if my hypothesis is correct, that sound would really depend on how exactly the filament warms up and how the voltages stabilize. That probably varies considerably from tube to tube.
Good news is that I've found a way to reduce the hiss by almost 25 %. You'll need a 10 uF electrolytic capacitor rated for at least 25 V. Connect the capacitor at the footprint for D2. The (+) pin goes to the square pad (what would have been the cathode of D2). Short out D4 with a wire. Connect a similar 10 uF cap at D7 and short out D9. These D2, D4, D7, D9 are the empty footprints by the LEDs. They were intended for zener diodes for use with the 12BH7A, but can be repurposed for these capacitors. I was able to lower the hiss on my amp from 375 uV to 300 uV.
It's certainly isn't COTs, it's a work in progress. (It also lack a UL approved sticker.
) Don't get me wrong, I like dead-bang quiet and I hope it can get there. All in all, I am enjoying my amp a lot.
I'm hoping a set of Balanced HD800's show up at the show so I can audition them.
Edited by sceleratus - 6/23/13 at 11:43am