Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 3choplex on February 02, 2011, 12:47:59 am

Title: New Schem, feedback requested
Post by: 3choplex on February 02, 2011, 12:47:59 am

I've been agonizing over this schem for a while now, and feel like I've hit the wall. I'm really unsure about my cathode follower, I think the other portions are reasonable, but I'm not experienced enough to really tell. Would anyone mind taking a look and letting me know if anything sticks out as problematic/would sound bad, etc.? I really appreciate the assistance.

(http://img140.imageshack.us/img140/5688/usethisoneschem.jpg)

Title: Re: New Schem, feedback requested
Post by: tubeswell on February 02, 2011, 02:44:19 am

What exactly is it about the CF that you are agonising over?

Title: Re: New Schem, feedback requested
Post by: VMS on February 02, 2011, 03:11:53 am

I think the place of the volume might be problematic. The way it is now there is DC-voltage on the pot and that could sound scratchy.

Here is the traditional way to wire a tone stack and volume control:

Title: Re: New Schem, feedback requested
Post by: kagliostro on February 02, 2011, 03:32:18 am

+ 1 for VMS

why did you put the Volume control before TS instead after it ?

Kagliostro

Title: Re: New Schem, feedback requested
Post by: 3choplex on February 02, 2011, 08:58:11 am

Quote from: kagliostro on February 02, 2011, 03:32:18 am

why did you put the Volume control before TS instead after it ?

Just a noob mistake. It comes before on the 5E3 schem and I didn't think through the fact that in the 5E3 it follows a common cathode stage, not a cathode follower.

What's troubling me about the CF is that I am not convinced I have biased it correctly. The plate should be around 200v, and I level shifted the grid to 83v. My intention was to bias it to about -1.4, and I can see I can get there on the load line with a 100k Rk, but I'm not sure that's actually what I would be getting.

Title: Re: New Schem, feedback requested
Post by: PRR on February 02, 2011, 11:04:31 pm

> in the 5E3 it follows a common cathode stage

_AND_ a DC blocking cap.

The 5E3 is strange. It works JUST the way it is, not if you apply mix/match changes.

Put the volume control where it normally goes when using THIS tone-stack.

This tone-stack also does the DC-block function.

The cathode follower biasing will work. Note that you generally do not need a cathode follower when using the 100K resistor in the tone-stack; Leo used the CF when using a 68K resistor here.

> My intention was to bias it to about -1.4

It will work. It may work with 83V on grid and 84V-85V on cathode, about 1.4V difference.

Title: Re: New Schem, feedback requested
Post by: 3choplex on February 02, 2011, 11:57:58 pm

Quote from: PRR on February 02, 2011, 11:04:31 pm

The cathode follower biasing will work. Note that you generally do not need a cathode follower when using the 100K resistor in the tone-stack; Leo used the CF when using a 68K resistor here.

Thanks. Would you recommend changing it to 68k?