Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alange5 on July 07, 2018, 01:17:57 pm

Title: Brownface PT question
Post by: alange5 on July 07, 2018, 01:17:57 pm

I'd like to build something based on some of the Fender brown circuits - a cross between the 6G11 Vibrolux and the 6G5 Pro.


I'd like to incorporate the bias-vary tremolo of the 6G11 with the diode rectification and higher voltages of the 6G5.


The 6G5 calls for about 450V on the 5881 plates.  I have two PT options in mind:


Classictone 40-18005
http://www.classictone.net/40-18005.pdf (http://www.classictone.net/40-18005.pdf)
330-0-330 @ 240ma w/ 50V bias


Classictone 40-18041
http://www.classictone.net/40-18041.pdf (http://www.classictone.net/40-18041.pdf)
325-0-325 @ 180ma w/ 50V bias and 5v windings




If I opt for the 325V PT, will I be ok with the 180ma current (2x6L6 or 2x5881), and could I simply tie off the unused 5v windings?

Title: Re: Brownface PT question
Post by: sluckey on July 07, 2018, 01:59:00 pm

yes, yes

Title: Re: Brownface PT question
Post by: alange5 on July 07, 2018, 03:45:56 pm

Thanks.


Here's the schematic I'll be working with:


(https://preview.ibb.co/j6DMXT/brown_amp.jpg) (https://ibb.co/kW6x6o)


Any glaring problems?  Does the coupling before the PI look ok? (plate > 100k > .05 > grid)

Title: Re: Brownface PT question
Post by: sluckey on July 07, 2018, 04:47:25 pm

Quote from: alange5 on July 07, 2018, 03:45:56 pm

Any glaring problems?  Does the coupling before the PI look ok? (plate > 100k > .05 > grid)

Each preamp triode needs a 1.5K||25µF between the cathode and ground. Also, connect the .05µF coupling cap directly to the plate of the second preamp triode and remove the 6.8K, connecting point A directly to the 100K.

Title: Re: Brownface PT question
Post by: alange5 on July 08, 2018, 01:32:31 pm

Thanks for the reply.


Here's the updated schematic with the "A" version of the tone stack (used in both the 6G11-A and the 6G5-A):


(https://preview.ibb.co/kfgQcT/brown_amp_copy.jpg) (https://ibb.co/cC9YWo)



I have an extra cutout for a pot on my chassis.  I'm considering adding a master volume.  If I use the "type 2" master volume where a dual gang pot replaces the power tube grid leak resistors, will the tremolo still work properly?


Here's the Robinette example from a 5F6A:
(https://preview.ibb.co/hr9vcT/type_2_mmv.png) (https://ibb.co/fJYMHT)

Title: Re: Brownface PT question
Post by: sluckey on July 08, 2018, 03:24:59 pm

Trem should work with that MV. Before you commit to that tonestack you may want to find a source for that odd treble pot. Hoffman used to carry it, but not now.

Title: Re: Brownface PT question
Post by: alange5 on July 08, 2018, 03:55:43 pm

Thanks for the help sluckey.


Looks like amprepairparts.com carries those tapped pots.  They have a whole section dedicated to "brown" parts. 

Title: Re: Brownface PT question
Post by: sluckey on July 08, 2018, 04:15:28 pm

Thanks for the link.