Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on October 28, 2015, 02:19:26 am

Title: Stereo guitar preamp
Post by: Toxophilite on October 28, 2015, 02:19:26 am

So I've been ogling Hoffmans stereo guitar pre.
It uses more tubes than I would imagine seems to have 4 gain stages per channel with a bassman 5F6-a-like cathode follower tone control
I was curious as to the rational as even the bassman only seems to use 3 gain stages :dontknow:
Can someone explain it? Is it simply a matter of more gain?

Looks neat though
I might try something simpler though as I like to have a straight in guitar pre for recording late at night(in my apartment)


I've built several of these and used to use one all the time for quiet recording:
http://www.beyondsanityproductions.com/index.php?option=com_content&view=article&id=13:akai-roberts-preamp-mods&catid=18:diy-articles&Itemid=28 (http://www.beyondsanityproductions.com/index.php?option=com_content&view=article&id=13:akai-roberts-preamp-mods&catid=18:diy-articles&Itemid=28)


Which gives you two preamp channels , with one feeding into a SE amp
It was easy for me as there was essentially a cookbook like recipe for me to follow
Has anyone built one of these? they're useful. I sold my last one
Because of the time I spent here I think I could get the 12ax7 channel on one of these a little hotter which would be cool


However I think a Hoffman like standalone stereo pre would be even more useful and fun to build
Have to find or make an appropriate chassis

I have several PTs which might be appropriate I wonder how skookum they would have to be??

 I see my unit as basically running 2 12ax7 ( 2 Ab763 preamps)
Next I'd have to build a revibe!!..it never ends!! :icon_biggrin:

Title: Re: Stereo guitar preamp
Post by: EL34 on October 28, 2015, 06:20:15 am

You should link to the documents on my library page so the guys know what you are referring to
here's the link to the schematic
http://el34world.com/projects/images/stereopreampschematic.gif (http://el34world.com/projects/images/stereopreampschematic.gif)

My tube pre amp is just the pre amp out of a Marshall type amp and it has the same tone controls
(http://el34world.com/projects/images/stereopreampschematic.gif)

V1B and V3B are just there so you could try and get less clean sound in the pre amp if you want thatVolume pots up - Master down

Or not, Volume pots down -  Master up to get a cleaner tone

On the PT, I already stock the PT I used on that project and the Re-Vibe

It's the Mojo779 on this page

http://hoffmanamps.com/MyStore/catalog/TransformersMobile.htm (http://hoffmanamps.com/MyStore/catalog/TransformersMobile.htm)

(http://hoffmanamps.com/MyStore/catalog/images/IMG_8643.jpg)

Title: Re: Stereo guitar preamp
Post by: Willabe on October 28, 2015, 08:45:45 am

Your counting cathode followers (CF) as a gain stage. They are not.

Title: Re: Stereo guitar preamp
Post by: Toxophilite on October 28, 2015, 10:12:06 am

Right, sorry about that
Thanks for answering those questions
I'm completely unfamiliar with Marshall schematics, being a newbie, though I did know they came from the same source
I quite like the whole stereo pre and revibe combination you made!
THanks again!

Title: Re: Stereo guitar preamp
Post by: EL34 on October 28, 2015, 10:17:47 am

There's a bunch in the schematic library
http://el34world.com/charts/Schematics/files/marshall/Marshall_50w_2204s.pdf (http://el34world.com/charts/Schematics/files/marshall/Marshall_50w_2204s.pdf)

Title: Re: Stereo guitar preamp
Post by: Toxophilite on October 28, 2015, 10:31:46 am

Cool, and thanks
I'll check those out, i was looking at the 18 watt at one point but for my first few builds I've been pretty Fender-centric .