How much should I filter this power rail?

[bigsbybender]

Hey guys,

I'm making a rack mount 1 channel instrument preamp.. Somewhat Ampeg B15N based with just 1 6SL7 Octal.

I'll have 0-220v on the secondaries of the PT and I'm using a bridge recto as there is no CT. This should give me ~300-310v.  What's next is where I'm stuck... What should I use for Caps? I need to compromise between quiet and still having a little "touch sensitivity".  Should I copy a larger amp's "rail" with a network of R/C or just filter really big after the bridge recto?

As always your help us greatly appreciated...

J.

[DrNomis]

You could try using a 47uF/450V Electrolytic Cap for your first cap after the Bridge-Rectifier, then add a 22uF/450V Electrolytic with a 10k/1Watt resistor between it and the 47uF Cap, have a look at the power-supply section in the Matchless Hotbox Schematic, because you're using a solid-state rectifier, you can get away with using higher value supply filter caps, note that smaller values will probably increase the mains-hum level, so the what value you use is basically a balance/tradeoff between mains-hum level and general feel, hope that helps.....

[eleventeen]

The very low current demand of a single preamp tube means, other things being equal, you can use a smaller cap. That would not be my tendency, though, given that low noise is the design objective. For the most part, we do not install 4ufd and 8 ufd electrolytics any more.

Honest question: Is the touch sensitivity allegedly connected in the way you imply to the size of the filter cap? Source for that opinion?

[sluckey]

I would start with a three stage filter just like Ampeg did. 30µF-->1KΩ-->40µF-->22K-->40µF. Might need to juggle the dropping resistor values.

[bigsbybender]

Honest question: Is the touch sensitivity allegedly connected in the way you imply to the size of the filter cap? Source for that opinion?

I built a number of Champ and GA5 style amps a few years ago to sell relatively cheap and clear out parts. (Odd small radio transformers and Unmatched 6V6 for example) I always have a stock of 22uf caps so I used them in every location in the amp. They sounded and played very stiff. It was difficult to alter the tone with picking dynamics on the guitar... I looked and saw that's where I deviated from the schematic the most and went with either 8 or 10uF for the preamp section filter and the amps opened up and played very well dynamically. This is how I formed that opinion. I filter pretty softly now in preamp sections to get this "feel" unless I am building for bass or a direct clone.

[eleventeen]

Thanks, I appreciate that insight. Never woulda thought.

[John]

I think Tubenit likes the smaller caps for the preamp as well.
Bigsby, you probably already know this, but I do find that elevating the heaters and using a humdinger is useful for reducing  heater hum from the 6SL7s.

[bigsbybender]

I was thinking of the hum balance pot... but raising the heaters should be easy in this case since it's just one tube and it has to be cathode biased in this case. This is going to go into my recording studio rack so noise is troublesome. It will be transformer coupled on the output with a groundlift and phase switch, so hopefully I get clean signal to my Pro-Tools Rig.

j.

[bigsbybender]

Well not 100% clean guitar/bass tone...  The tube is to put a little 'hair' on the tone. I was meaning clean by keeping hum and buzz to reasonable levels since it will be plugged into a digital audio workstation. I just need to keep the operation noise low enough where a noise gate can kill any nasties without sounding choppy.

j.

[rzenc]

The attached schem is something I developed with help from Sluckey, PRR, HBP and others. Should be able to supply enough juice to power up to 4 preamp tubes running 12V@150mA each.

Use at least a 2"x3" heatsink plus thermal grease. Keep an eye at chip temperature.

Hope this helps

Best Regards

R.