300bSE journey under way

[cjkpkg]

Tre...that was a good eye...I was running a 1M grid resistor since that was the closest to 200K I had kicking around...

I have since been able to put in some 200K resistors. I seem to have lost some high end which is the opposite of what is supposed to happen.

HOWEVER, after investigating grid chokes I took a chance on the $50 ones on ebay right now. I think that I would rather risk $50 and maybe use them in another project than risk $167 for the Silk ones.

Tonight...B+ is 450, changed grid resistor to 200K, hooked up the James OPT to the 2.5K tap to plate. I must say that the bass is incredible now. I have made lots of little changes to this amp along the way and I am now an official fan of DHT's.

 

[Tre']

Those $50 grid chokes are pretty good.

They measure about 870 Hy instead of the 640 Hy.

They have a lot of DCR (20K ohms). It turns out that's a good thing. It damps out the low frequency resonance so you can use a small value coupling cap. (most of the time when an inductor is feed with a cap, like in a parafeed circuit, the cap has to be large (in the 2 to 8uf range) to keep the tank circuit resonance low. The lower the resonance frequency the lower the amplitude, but with the large DCR of these grid chokes there's no need for a big coupling cap.

I use them in my 300b SET amps.

Tre'

 

[kevinkr]

<snip>
Tonight...B+ is 450, changed grid resistor to 200K, hooked up the James OPT to the 2.5K tap to plate. I must say that the bass is incredible now. I have made lots of little changes to this amp along the way and I am now an official fan of DHT's.

Welcome to the club. I've been running 300B in SE for quite a long time, and am a big fan of DHTs in general. (01, 10, 12, 26, 45, 50, 2A3, 300B, GM70 are all types I like to work with..)

 

[cjkpkg]

Update...

Here is the latest...
-Red LED bias on the 6SN7's
-10M45 CCS on the plates of the 6SN7's
-300b grids choke loaded - $50 ebay chokes - pretty nice stuff

I had a bit of an issue with a bad PSU cap but I am back in business...New PSU caps and 5AR4/GZ34 rectifier and I am good to go...

Funny I ran the 6SN7's at 10mA and the new TungSol's ate it up and sounded great...the '60's GE6SN7's did not like the current that high - 2 pairs. I dialed the 10M45's back with a 560ohm resistor and they are now running at about 6.3mA and seem much happier.

I am still tracking down a hum but the overall sound, power, and bass is amazing. I want to CCS load and LED bias all my amps now...LOL...
B+ 400
B++ 337
6sn7 LED - 1.691
6sn7 LED - 1.686
6sn7 560ohm plate resistor - 3.58
6sn7 560ohm plate resistor - 3.48
300b cathode - 66.9
300b cathode - 68.3

 

[cjkpkg]

UPDATE

The journey continues...So to date I have made the following mods/changes:
1. 10M45 CCS plate loaded 6SN7s
2. Red LED's for 6SN7 Cathodes
3. Ebay Grid Chokes for the 300b's
4. CLCRC filtering where C's 2 and 3 are now 100uF and 50uF respectively
5. 300b Heaters are now running regulated DC from Broskie's H-PS1 boards.

I had intially set up the H-PS1 boards with the 5V xformers running a voltage doubler. The little ld1085 heat sinks literally were sizzling hot! I converted them to full wave center tapped with 12.6V xformers that I had laying around and the ld1085s seem much happier and running a lot cooler...

I must sat that I am at a point now where I am very happy with the amp and have learned a lot through the process. It's amazing how much power you can really get to the speakers with a single ended amp.

The only thing that I could not ever sort out was the "powerdrive" from George's website. I may have to revisit that someday and see what I was doing wrong...

 

[tomchr]

Pls check Pete's study High-Gm driver pentode test data here he also showed cascoded drivers behavior. In fact I also started with cascoded type drivers for 300b many many years back and moved into pentode driver, single 417, trioded 7788/d3a or no tube driver at all.

I started with two 6SN7's cascaded as well (actually I used two 6J5's (half of a 6SN7) but the circuit is the same). I tried two cascaded grounded cathode stages. I tried a grounded cathode stage followed by an anode follower providing a gain of about -4 V/V (for a total of 80 V/V in the 300B driver). I tried with and without a source follower.

I still conclude that a DC coupled circuit with a single high-mu driver sounds - and measures - the best. The catch with high-mu tubes is that the tube-to-tube variability can get quite high. So you need independent bias adjust on each tube if you want consistent performance.

~Tom

 

[tomchr]

I implemented the above schematic for the most part. I have abandoned powerdrive for now since I could not get it to work - must need to tweak a bunch of resistors to get it to work.

That's odd.... Why no work?

Cap couple into the source follower. Connect the source to the grid of the 300B (via 100R~1k resistor). Connect drain to B+ and source/300B grid to C- via a resistor. Figure on allowing 5 mA to flow in the source follower.

The 300B needs a swing of 180 Vpp (give or take) so C- supply of -180 V works well. This makes the source resistor about 22 kOhm. Set the bias for the 300B by manipulating the gate of the source follower (through a 100 kOhm or higher resistor).

For a visual, look at the .pdf schematic attached to the first post of this thread.

~Tom

 

[cjkpkg]

Im pretty sure I had a problem with the bias network. I had developed about -200V and connected it to the 22k source resistor and the bias network. I could never get the 300b's to bias up so there was a problem with the grid reference...

After the coupling cap I ran a 120k resistor to a 100k trimpot. One leg went to ground through a 22K resistor and the other leg went to B- through a 131k resistor.

I am thinking the trimpot hookup and values out of the trimpot were not right...I may have to dust that one off in the near future...

Was there no cathode resistor used in your application? It was unclear if necessary with "powerdrive"