Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Papa Jim on October 18, 2016, 08:20:33 pm

Title: Will these part values be good for situation?
Post by: Papa Jim on October 18, 2016, 08:20:33 pm
Sorry, guys I can build anything but am not skilled at parts values on designs. Attached is the section of the 2W Trainwreck style amp I built, where I want to add a Master Volume (PPIMV).  I need some reccomendations on a few resistor and cap values. The values shown in blue are original schematic values, and the values and parts in green are the new PPIVM parts and values. Anways my hope is that the mod will turn out as good as the amp did. So would someone skilled in amp circuits please take a look at it. Thanks to all.
Title: Re: Will these part values be good for situation?
Post by: 2deaf on October 18, 2016, 09:41:40 pm
Get rid of Rb, Rd, Ra, and Re and that -V Bias thing.

OT lug 2 is connected to lug 4 instead of to the 8 ohm lug on the selector switch.

NFB and presence will cease to function as you turn the PPIMV down.
Title: Re: Will these part values be good for situation?
Post by: PRR on October 18, 2016, 10:13:07 pm
> that -V Bias thing.

Some negative bias is needed because there is no cathode bias for the 6SN7.

So it should look a lot like a big fix-bias amp.

Hasn't that been done before?

http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/PPIMV's/Improved%20PPIMV.gif (http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/PPIMV's/Improved%20PPIMV.gif)
Title: Re: Will these part values be good for situation?
Post by: Papa Jim on October 18, 2016, 10:14:58 pm
Whoops, my bad. I must have messed that up just in the schematic last night. The amp is not wired like that schem though. It is exactly like you said to do. Attached is the corrected schematic. Now back to the PPIVM like I need badly sorry.
Title: Re: Will these part values be good for situation?
Post by: 2deaf on October 18, 2016, 10:29:00 pm
Quote from: PRR on October 18, 2016, 10:13:07 pm
> that -V Bias thing.

Some negative bias is needed because there is no cathode bias for the 6SN7.

The 4-4-0 already has V-1 @ -12.5 and V-2 @ -12.3 coming off of VR1 and VR2 in the bias circuit.  That -V bias thing is carried over from the diagram you referenced and is not appropriate for this amp.
Title: Re: Will these part values be good for situation?
Post by: Papa Jim on October 18, 2016, 10:39:44 pm
So I should go this route with the circuit you showed us. The new PPIVM parts are surrounded in red. The other amp parts and values are stock. What ya think about this new attachment.
Title: Re: Will these part values be good for situation?
Post by: 2deaf on October 19, 2016, 06:39:16 pm
Why did you change to a 100K linear dual-ganged pot.?
Title: Re: Will these part values be good for situation?
Post by: Papa Jim on October 19, 2016, 07:49:02 pm
Quote from: 2deaf on October 19, 2016, 06:39:16 pm
Why did you change to a 100K linear dual-ganged pot.?

Thanks so much for catching that. Dang, I don't know where my mind has gone. Too many irons in the fire.  It is indeed a 500K. I had that drawing also that PRR posted somewhere and grabbed it. It had been changed for something else I was playing with. I changed it to 500K and attached it here.
Title: Re: Will these part values be good for situation?
Post by: kagliostro on October 20, 2016, 01:12:19 am
Quote
Too many irons in the fire.

(http://i.imgur.com/7gkjlIg.jpg) (http://i.imgur.com/2NyOjJN.jpg)

 :laugh: :laugh:

Franco
Title: Re: Will these part values be good for situation?
Post by: tubeswell on October 20, 2016, 03:01:26 am
Suggest you try the NFB/presence control schematic from the 6G6B Bassman  as it won't have that characteristic scratchy sound that a tweed bassman presence control has.