Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on October 23, 2015, 09:32:15 pm
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The clean channel on the D'Mars is excellent to my ears and has a nice tight punchy bass. While the OD channel has sounded good, it has sounded quite a bit thinner then the clean channel in the bass response.
The problem was increasing coupling caps or cathode caps in the OD section only added distortion/mud and the clarity of the OD suffered.
So, today I thought that maybe if I bypassed the tone stack going into the OD that this would help the bass response be tighter and have more punch without the mud/distortion.
I tried using an .02, then .068 and settled on a .1. I originally tried a 150k after the .02 but the drop in volume was too great. I then tried a 56k but it did not have a "musical sweet spot" with that value. Ultimately, I settled on a .1 into a 10k resistor. I did try it with NO resistor but thought the 10k actually added some smoothness.
What surprised me was how high a value coupling cap was needed for the bypass to get a similar bass response as the TMB tone stack?? The .1 value was much higher then I expected. Having this .1 between the clean and the OD has allowed the OD to still have the clarity in the overdrive without turning to distortion or mush, IMO.
Wished I had stumbled on this mod yrs ago!
I should add the TMB tone stack on the clean is pretty much a "set and forget" thing for me. And the OD channel still has a hi-frequency cut that allows some tone shaping as well as the pickup switch and tone pot on the guitar. So, no loss for me there.
With respect, Tubenit
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I am really loving this mod! The OD sounds very transparent and MUCH thicker doing this! This is definitely a "keeper" mod for me.
However, I noticed that the mid-boost feature in the tone stack doesn't work on OD now. (actually really isn't needed with this mod)
So, I pondered some different options.
1) bypass tone stack mod
2) switchable between bypass tone stack mod ................... or using tone stack
3) using a pot to mix between bypass of tone stack or using tone stack
4) using a relay switch to use the tone stack bypass as another boost feature for both clean or OD
With respect, Tubenit
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Here is yet another option.
This allows the THICK OD tone and the normal clean using the TMB tone stack.
OR .........................
A THICK OD and TMB tone stack combined on the OD ............................... and then just the TMB tone stack on the clean channel.
with respect, Tubenit
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Hi Tubenit thanks for sharing :thumbsup:
it will be nice if you add schematics and explanations to the original D'Mars ODS tread
Franco
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There is an option to make a small daughter board
with trimpots and caps to create a tone circuit for just the OD.
Worked for Howard on the HRM...
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Here is an example of one...
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IIRC, the original HRM was off the plate of the second gain stage of the OD of the D-HRM. So, I am presuming on the D'Mars that you're thinking of using this off the cathode follower behind the 5879 pentode?
IF so, that sounds like a good idea. Thanks!
With respect, Tubenit
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Tubenit, I'm considering building one. After a bit of searching, I can't seem to find what size the chassis is. Any insight?
Thanks for the concepts!
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20" X 8" X 2.5" with outward edges to attach.
There is a current chassis photo, updated information, schematic and layout in SCH form here in reply #1:
http://el34world.com/Forum/index.php?topic=17305.0 (http://el34world.com/Forum/index.php?topic=17305.0)
Not a particularly neat wiring job but the amp is very very quiet at idle even when the pot settings have the volume cranked up
With respect, Tubenit
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Looks neater than my work... :worthy1:
Thanks for all your help!
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Ahem...... I believe it's time for another recording session?
Jim :icon_biggrin: