RadioHead Tube Preamp - Revisited

[Paul Marossy]

I built Gringo's "Radiohead" circuit a about 2-1/2 years ago. It uses a couple of old radio tubes - a 12AT6 and a 12BA6. It had some problems, though, so I put "on the shelf" and kind of forgot about it. This weekend, I decided to revisit this project. I gave it a complete face lift. But the biggest improvement was dumping the back to back transformer arrangement. Due to all the losses involved, my B+ voltage (and filament) voltages were too low, so it didn't have much headroom and it distorted really badly. But with a dedicated filament transformer and the back to back transformers acting like a 1:1 isolation transformer now, this thing really sounds good. I also got my first good B+ shock today while working on it - my left hand still smarts a little bit! 

If you're interested in more, check it out here:
http://www.diyguitarist.com/DIYStompboxes/RadioHead.htm

BTW, is Gringo still around? I haven't seen him here for a long time... 

[Joecool85]

Any sound clips?  I'd love to hear this beast.  Also, how much would it cost in parts to build this thing...roughly.

[Steben]

Euhm... is it normal to have DC to the filaments?

[MartyMart]

That's a nice build Paul, I was wondering if this could be made, with relavent changes, using
a pair of either 12AX7A's or 12AT7's ( I have a few of both types )
I guess that it could end up being any of the "known" nice preamp stages, such as
Bassman/Sovtek  etc etc ..
I have two spare 240v/15v 2.3VA transformers ( PCB style ) so would have to get the 15v
down to 12.6 with a small ohmage 2 watt resistor ( 15 - 22ohm ? )
Otherwise the main schem should work fine, what if B+ were a little more than 165v ?
what are you expecting to see hiting the plates ?

Thanks for any "tips",
Marty

[Satch12879]

Euhm... is it normal to have DC to the filaments?

::furrows brow:: Um... yes.  That's how they're powered.

[Paul Marossy]

Any sound clips?  I'd love to hear this beast.  Also, how much would it cost in parts to build this thing...roughly.

No sound clips, but it does sound cool. Kind of "complex" like my Octal Fatness, thanks to the pentode on the input. I am not sure what it cost, probably less than $50 - but I had a lot of the parts lying around already. 

Euhm... is it normal to have DC to the filaments?

You can use either AC or DC for tube filaments. Tubes started out running of off batteries anyways. 

That's a nice build Paul, I was wondering if this could be made, with relavent changes, using
a pair of either 12AX7A's or 12AT7's ( I have a few of both types )
I guess that it could end up being any of the "known" nice preamp stages, such as
Bassman/Sovtek  etc etc ..
I have two spare 240v/15v 2.3VA transformers ( PCB style ) so would have to get the 15v
down to 12.6 with a small ohmage 2 watt resistor ( 15 - 22ohm ? )
Otherwise the main schem should work fine, what if B+ were a little more than 165v ?
what are you expecting to see hiting the plates ?

Thanks. You could use different tubes, but then it will become a different animal. The pentode on the input has a lot to do with how it sounds. 15V is no problem, just go with a DC heater supply and use a 12V voltage regulator. You'd have to look at the spec sheets for those tubes to determine the maximum plate voltage. It's been a while since I looked at those data sheets, but I seem to remember 200V being about max. But - 230mA is too low for a back to back configuration - you'll have too low of a B+ voltage when it's all said and done... 

[Gringo]

Now that's a tight fit if i ever saw one

Gotta give mine a reliable/usable home, but that will come when i stop making mods to it...

[Paul Marossy]

Ah, Gringo! You are still around... Say, what mods are you doing to yours?