Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Madison on September 24, 2010, 06:47:14 pm

Title: 18 watt volume/tone with 1M pots ?
Post by: Madison on September 24, 2010, 06:47:14 pm

Is it a problem using 1M pots for the volume and tone?
I always forget, does it shift the frequency range up?
If so can I compensate somehow?

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: LooseChange on September 24, 2010, 08:43:48 pm

You want the 500k pots? Just strap a 1M resistor across the outside lugs or the pot.

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: darryl on September 24, 2010, 09:54:19 pm

One of the addenda to the Marshall 18-watt circuit at Schematic Heaven (http://www.schematicheaven.com/marshallamps/marshall_18watt_schem.pdf) claims that some amplifiers left the factory with 1meg potentiometers installed.

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: Dave on September 25, 2010, 10:58:15 am

I like 1 meg pots because they can be used generically by doing what Loosechange said. You can strap a resistor in parallel with a 1 meg pot accross the outer legs and customize the resistance to anything you need less than 1 meg.

Dave

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: LooseChange on September 25, 2010, 03:56:30 pm

The smaller volume pot value = less gain.

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: jjasilli on September 25, 2010, 04:10:58 pm

Yes, but the grid leak resistor VOL POT  is also a shunt resistor = high pass filter.  The smaller the value of that resistor, the more hi's will leak to ground.  This can be compensated for by tone scaling -- doubling the value of any associated cap in an RC circuit.  Ultimately, the resulting tone is a matter of taste.

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: Madison on September 26, 2010, 05:39:06 am

Thanks gents.
The reason I was asking is because I have a "buzzy" 6SL7 only when I add a 22uF cathode bypass cap.Without the BP Cap the gain isn't enough for my taste.
This is a SE Champ style build.
So, I am thinking that lowering the tone pot down to 500k or maybe even lower will reduce the buzz?
Shot in the dark.
I didn't like the 5F2a tone/vol so I went with an 18 watt style.
I have no resistor between the vol and tone pot right now and it seems to works fine.

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: darkbluemurder on September 29, 2010, 03:23:48 am

Try a cathode bypass cap lower than 22uF. That would shift the corner frequency upwards and reduce the buzzing. The gain in the highs and mids would stay the same. I would try a 1uf - they can be obtained in a non-electrolytic variant as well.

Cheers Stephan

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: LooseChange on September 29, 2010, 04:33:08 am

Hey Madison,
Put the same bypass cap back and add a grid stop resistor to the grid of the next gain stage. Put the resistor on the tube socket as tight as you can against the pin.  Try something between a 10k and a 33k.

Title: Re: 18 watt volume/tone with 1M pots ?
Post by: Madison on September 30, 2010, 08:37:48 am

Hi guys.
Thanks for the ideas.
I went ahead and took out the 6SL7, besides being "buzzy" it ended up being microphonic as heck and I couldn't tolerate it.
Still having a tiny bit of buzzing trouble though much better.
Sound strange, but I swear buzz is coming through the plates/B+.
Thanks again.