Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Effects => Topic started by: luthierwnc on May 28, 2023, 01:31:11 am

Title: bright/harsh scale for 12A&7 tube variants
Post by: luthierwnc on May 28, 2023, 01:31:11 am

Hi All.  I built a tube pre-amp gizmo that sits on top of an amp with a relay activated footswitch on the pedalboard.  To start it's got an ANOS 1968 Mullard.  Despite adding a few tweaks to get the brightness down it is still a bit ice-picky.  It might work as a treble booster but an always-on.  I have some old-stock 12AY7s and a single 5751 to sub but it made me wonder if there is a scale or chart that compares the output characteristics of the 12A variants across the frequencies.  That's in full understanding it will vary a lot by run and manufacturer.  Still, a guide would help.  Extra credit for someone who already compared a boatload of tubes.  Thanks, Skip

Title: Re: bright/harsh scale for 12A&7 tube variants
Post by: Williamblake on May 28, 2023, 05:13:21 am

Are you sure you want C3 that way if you want less highs?

Title: Re: bright/harsh scale for 12A&7 tube variants
Post by: luthierwnc on May 28, 2023, 06:12:37 am

I'm not married to it.  IIRC the plate resistor and cathode networks came from a Kingsley Page but there are other pedals involved including here at Hoffman.  I did change the K cap on the other triode to 10uf since that's where the wave-form really distorted.  Maybe 4.7uf on C3 is in the cards.  The smoother cap between pins 6 and 8 is new.  Not mentioned; I also have to tame the gain and volume too.  Putting a resistor across the gain pot made the sweep more usable but both it and the volume pot are raw loud past about 12 or 1 o'clock.  Right now the low cut is disabled until I sort the highs.  Might increase the tone cap size again to .01-ish.
Thanks, Skip 

PS; the device does sound pretty good already if you like rawk.  I need to make it a little more ladylike.  sh

Title: Re: bright/harsh scale for 12A&7 tube variants
Post by: pdf64 on May 28, 2023, 06:22:34 am

The input stage 10M grid leak seems rather high? Even a smidge of grid current might mess up the operating point.

Title: Re: bright/harsh scale for 12A&7 tube variants
Post by: luthierwnc on May 28, 2023, 08:57:54 am

Thanks PDF64.  I slipped a 1m in there.  Will test when the household's had their coffee.  sh

Title: Re: bright/harsh scale for 12A&7 tube variants
Post by: pdf64 on May 28, 2023, 09:26:32 am

Regarding your original query, have a read https://www.thegearpage.net/board/index.php?threads/updated-tube-comparison-table.2279173/#post-33039795

Title: Re: bright/harsh scale for 12A&7 tube variants
Post by: luthierwnc on May 28, 2023, 10:59:40 am

Perfect!  I'll keep this.  Skip

Title: Re: bright/harsh scale for 12A&7 tube variants
Post by: luthierwnc on May 28, 2023, 09:41:16 pm

I made the changes and was able to bring the highs into manageable range.  The sound is still a bit harsh so I'm back to the woodshed to smooth the response.  It might also be that for just a little kick I use it just past unity.  Thanks for looking and good luck with your projects.  sh