Overdrive only preamp design?

[Luthierwnc]

Hi All, Weather's cold so it is time to sketch. I'm contemplating a build with a dedicated HRM (or Ripper) OD channel with the PAB on. It doesn't seem worth the time or space to put in a true post V1a tonestack just to bypass it but there are important frequencies leaking to ground even with the PAB.

How would you design a low-parts-count circuit that approximates a high-plate bypassed tonestack? As a point-of-reference let's say if the tonestack was there, it would have the Treble, Mid and Bass at 12:00.

Thanks, as always, Skip

[Structo]

Interesting concept.

I have yet to build a HRM.

But I have read that when you switch to HRM overdrive, you should also turn the PAB on to disable the clean tonestack due to the lossy nature of the plate loaded tonestacks.

I'm sure there are a lot of combinations of switching that are possible.

You could have a relay switch the PAB on when going to OD.

You could also have a foot switched gain setting, like a choice between two overdrive, like a mild crunchy tone then step on the switch for more gain and overdrive.

I'll be watching what you do with this.

[LeftyStrat]

I have played around with something similar, though not in a Dumble. For example, a Fender channel and a Marshall channel with a fixed tone stack in each, followed by a global Baxandall tone sack.

I started out with fixed, but came to the conclusion that trimmers were cheap enough, and allowed some tweak-ability.

[Luthierwnc]

Trimmers are as complicated as it will be but if there is an easy pass filter with one or two caps and a couple resistors, that would be handy and also use less real estate on the board. Some of the PAB variations have caps to soften the transition on the N/O side of the relay. I'll revisit those to see if they would work as stand-alone circuits.

The larger story on this one would be putting a Clubman preamp on the the other side of the inverter and EL34s in cathode bias. I've looked at the Bad Cat but that is too gainy for my taste. It also needs another tube since the clean channel on that amp swipes half a triode from one of the hot 12AX7s.

sh

[John_P_WI]

How would you design a low-parts-count circuit that approximates a high-plate bypassed tonestack? As a point-of-reference let's say if the tonestack was there, it would have the Treble, Mid and Bass at 12:00.

Thanks, as always, Skip

Read this thread: https://tubeamparchive.com/viewtopic.ph ... sc&start=0

Schematic on the 2nd page.

Good luck,

John

[Luthierwnc]

Yup! That's what I was looking for. This project is in the doodling phase and I have two (with most of the parts bought) in front of it -- but part of the process is ignoring movies I've memorized on TV and drawing out ideas on clean sheets of paper. Thanks, sh

[BobW]

Skip,

Although its a pre based on the #124, check out the preamp layout for the Dynaco here:

https://tubeamparchive.com/viewtopic.ph ... 91&start=0

hth Bob

[10thTx]

I thought this was an interesting thread and idea.

Based on my tweaks with the Tweed Overdrive Special, I would be tempted to try one of these ideas.

I'd use a full size volume and master volume.

I'd try to use a mini-pot for the OD trim, OD drive and HRM tone pots. Or you could use a trim pot for the OD trim.

I like the idea of being able to switch from .0047 to .01 after V1a. I think this might be useful going from single coils to humbuckers ...... OR as a mid boost for solo's.

The .047 coupling cap after V1b has never worked out for me so I'd try a
.02 there (which has worked out).

I have used the PAB idea in a tone stack following a cathode follower and like it. I also prefer the .033 in the mid cap over the .022. I think the .033 gives a clearer mid tone.

The mid boost after V1a and the PAB in the tone stack would give two different boost options for solo's.

I like the mosfet CF and don't think it hinders tone at all.

Just throwing ideas out.

with respect, 10thtx

[10thTx]

contemplating a build with a dedicated HRM (or Ripper) OD channel with the PAB on.

OR ......... maybe you could try something to get both an HRM & Ripper tone?

With respect, 10thtx

[Luthierwnc]

plenty to chew on.

The problem with ripper or not is that my original thought (which is cooling off) was to use a sharp-cutoff pentode into one half of the inverter and a more conventional 12AX7 arrangement into the other. Anything that changes the phase on one side (including a Dumbleator) will make the other side squeal like a stepchild in the toy aisle.

I've drifted toward the idea of using a 5879 as the first gain stage of a 5F6-A type amp.

Cheers, Skip

[10thTx]

I've drifted toward the idea of using a 5879 as the first gain stage of a 5F6-A type amp.

You mean something like this except fixed bias?

http://www.el34world.com/Forum/index.php?topic=9049.0

With respect, 10thtx

[Luthierwnc]

Yeah, but I'd go cathode bias with EL34s. I don't have an EL34 amp right now and it looks like the kind of project that would either sound great all the time or not so great and get made into something else. Fixed/cathode is an easy thing to design in so I'd probably add it to cover my bases. The Clubman power section looks inviting. sh

[10thTx]

Yeah, but I'd go cathode bias with EL34s.

Maybe like this?

I'd probably use lower value cathode caps on the EL34's like maybe 22uf-47uf range?

OR ............. you could add the OD section I used on my Double C amp and have an even more versatile amp.

With respect, 10thtx

[Luthierwnc]

That helps narrow it down. I like your thoughts on using a lower B+. I might go with 300 R or 330R cathode resistors too just so it doesn't get so toasty inside. Thanks, sh

[LeftyStrat]

Hate to resurrect an old thread, but I'm kind of curious where you ended up on this. I've gotten the bug to do an EL34 cathode biased amp. I was thinking something along the lines of a Rocket preamp combined with a Dumble OD.