Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 67polara on September 04, 2011, 03:33:16 pm

Title: Doubled triode?
Post by: 67polara on September 04, 2011, 03:33:16 pm

In a doubled triode preamp circuit how can you turn one 1/2 off and on without a pop or to much change of tone?  Can I just put a switch in between pins 2 and 7 or do I need to do something else also?

Title: Re: Doubled triode?
Post by: HotBluePlates on September 04, 2011, 08:04:15 pm

You can put a switch between pins 2 and 7, to turn off the signal to one grid. Place a large-value resistor across the switch, such as a 10M.

Title: Re: Doubled triode?
Post by: 67polara on September 04, 2011, 08:46:06 pm

Thank you that is what I thought I had to do.

Tony

Title: Re: Doubled triode?
Post by: sluckey on September 04, 2011, 09:08:52 pm

Make sure you have a 1meg from each grid to ground.

Title: Re: Doubled triode?
Post by: PRR on September 05, 2011, 12:21:04 am

> off and on without a pop

Title: Re: Doubled triode?
Post by: tubenit on September 05, 2011, 06:33:04 am

Another option to consider is Geezer's "Fatness" Control.  You can use a mini-pot if you want as I did.

with respect, Tubenit

Title: Re: Doubled triode?
Post by: kagliostro on September 05, 2011, 06:42:23 am

Hi Tubenit

Quote

Another option to consider is Geezer's "Fatness" Control.  You can use a mini-pot if you want as I did.

please can you share your experience with the use of that pot, I'd like to know if the use is quiet or using the pot a bit of scratch noise is to be expected

Many Thanks

Kagliostro

Title: Re: Doubled triode?
Post by: tubenit on September 05, 2011, 06:48:22 am

It worked fine for me with no issues. And the description "fatness" is applicable. The advantage of it, from my perspective, over a switch is that I found a "sweet spot" that was less than "10". 

IIRC, Geezer made a similar comment about using the fatness control and finding a sweet spot.


With respect, Tubenit

Title: Re: Doubled triode?
Post by: 67polara on September 05, 2011, 10:30:35 am

I love the fatness control on my TOS except I only use it in two positions.  Off and at 9 or so.  I was thinking since it sounds like a mild form of OD only "FAT" I would try two gain stages in a row doubled with a foot switch to cut them off just for fun. I would leave the controls as trimmers so I can tweak it a bit.

Tony

Title: Re: Doubled triode?
Post by: kagliostro on September 05, 2011, 01:38:18 pm

Thanks for the answer

Kagliostro

Title: Re: Doubled triode?
Post by: Fresh_Start on September 05, 2011, 02:39:48 pm

A question about the "Fatness control" circuit:  Is the 250K pot in parallel with the 1 Meg "Volume" pot in terms of serving as a grid return resistor?  Doesn't that reduce the gain of the primary triode versus not having the 2nd triode and 250K pot?

I don't doubt that the circuit works nicely - just want to understand how the circuit works as a whole.  Reducing the combined gain probably is a net benefit anyway.

Cheers,

Chip

Title: Re: Doubled triode?
Post by: tubeswell on September 05, 2011, 03:12:08 pm

Quote from: Fresh_Start on September 05, 2011, 02:39:48 pm

A question about the "Fatness control" circuit:  Is the 250K pot in parallel with the 1 Meg "Volume" pot in terms of serving as a grid return resistor?  Doesn't that reduce the gain of the primary triode versus not having the 2nd triode and 250K pot?

The 1st stage's plate resistor, the 1M vol, and the tone control, and the 250k fatness control, are all in parallel as far as the AC load on the 1st stage is concerned.