Advice on Fender style build

[markmalin]

Hi guys,

I've built up a Fender style chassis, AB763, with a Dumble style overdrive circuit (switchable). I'm attaching my hacked schematic - it's basically a single channel Vibrolux w/out vibrato but with reverb. I'm running KT66 power tubes. I don't have exact voltages but the pre-amp tubes are all within the 190-205'ish range. I don't have a value on the KT's just now, but can supply that later. What's not shown in the schematic is that I've wired up a master volume similar to Trainwreck. Ganged 1M pot in series with the output tube pin-5's (you can kind of see it in the chassis picture).

I've built this amp before, but it's been a while and I used a different master volume circuit (though I can't remember it and I've long sold that amp!). The amp sounds OK, but just isn't quite "there" yet. The previous one would get this great Fendery clean, but when you switched in the OD it would give a great D'style OD tone. This one, however....the low end is just kind of "flabby". The mid and high's are "ok", but maybe a bit shrill...however, I'm only using a temporary cab with a 200W EVM12L speaker. This will eventually use 2 10" Celestion greenbacks like I did before.

Anyhow, I would truly appreciate anyone's advice on how to make the low end less flabby (farty?). Or...if you see anything stupid that I've done in the circuit, please let me know!

Humbly,
mark.

[tubenit]

Try jumpering a larger A/C coupling cap on the 2nd triode in OD.   0.005 muf seems small considering you have 0.1 between the PI and the power tubes

The .0047 is standard in the D-style OD there.  That's exactly what I always use & I have tried larger values also.

Mark,

You always do such great builds, I am not sure this approach will be useful or not ?  I think you're a lot more knowledgeable about this stuff then I am.  Having said that ...............

I'd have the  V1-3 cathode cap be 10uf not 22uf and  V1-8 be 5uf not 22uf.   I'd change the coupling cap between the LTPI and power tubes from .1 to .047.

IF that didn't work, I'd then have  both the mid cap and bass cap be .047 instead of .047 & .1.  (see Duncan Tone Stack calculator)

Best regards, Jeff 

[markmalin]

Try jumpering a larger A/C coupling cap on the 2nd triode in OD.   0.005 muf seems small considering you have 0.1 between the PI and the power tubes

The .0047 is standard in the D-style OD there.  That's exactly what I always use & I have tried larger values also.

Mark,

You always do such great builds, I am not sure this approach will be useful or not ?  I think you're a lot more knowledgeable about this stuff then I am.  Having said that ...............

I'd have the  V1-3 cathode cap be 10uf not 22uf and  V1-8 be 5uf not 22uf.   I'd change the coupling cap between the LTPI and power tubes from .1 to .047.

IF that didn't work, I'd then have  both the mid cap and bass cap be .047 instead of .047 & .1.  (see Duncan Tone Stack calculator)

Best regards, Jeff

Thanks so much, guys.  Jeff - I'll try the 10uf and 5 uf caps on the V1 cathodes, as well as 0.047uf's between the PI and power tubes and see how that sounds :)  Unfortunately, I'll be traveling tomorrow so I won't be able to try until Sunday afternoon.  Looking forward to giving these values a try - I remember a couple weeks ago looking at pictures of the last build I did like this and noticing 0.047's after the PI and was scratching my head "...now why the heck had I done that?..."  lol


Mark.

[HotBluePlates]

IF that didn't work, I'd then have  both the mid cap and bass cap be .047 instead of .047 & .1.  (see Duncan Tone Stack calculator)

Or even 0.022uF for each. That seems to clear up bass but not leave things too thin. Disclaimer: I haven't tried those values in a Dumble-type circuit, where Tubenit may have.

[Mike_J]

I think tubenit's advice on lower cathode caps is very good.  I would go from 10uF on V1 and 5uF on the overdrive channel.  If that is not enough then lower the 10uFs to 5uF.  My guess is you should get what you are looking for by then.  The clean won't be quite as Fendery  with the lower cathode caps but you should be a lot happier with the overdrive.

Thanks
Mike

[Mike_J]

Another thing to try is to put a .02uf (in parallel with a 4.7M to remove clicks) before the input trimmer.  Should help with the bass problem.

Thanks
Mike