AC30/JM AMP

[jeshin]

Hey guys, its been a while since I last posted and I think I've finally decided my next steps. I currently own a hayseed 15 (Vox variant) and I've always wanted to have both the dumble clean sound and top boost sound. The obvious solution is stereo amps with one of each but since cash is tight I can only afford one good amp.

Once my amp sells Im thinking of building an ac30tb with just the top boost channel and also run john mayer signature clean channel without the reverb.
The two channels run into the power section of an ac30 since ive read most of the magic with dumble amps happen in the preamp. Would you guys be able to tell me if this schematic in theory would work im especially wondering about the B+ voltage for the dumble preamp?

Thanks

credit to Aaron on this here for the john mayer schematic

[10thTx]

I love innovative ideas! I think it is a reasonable risk to attempt this.

IF I were giving this a try, I'd want a master volume on the top side & I would use smaller value coupling caps post LTPI. I would use .02 instead of .1.

The reason I would use a master volume on the top (Dumblish) channel is so that it would be easier to match the volume on both channels using an A/B box.

And I would not use the cross line master volume after the LTPI. I've tried quite a few master volumes and the cross line is my least favorite and sound too buzzy to me. I'd use the PPIMV (post phase invertor master volume instead).

With respect, 10thtx

[DonMoose]

The Dumble clean works largely because the power amp is nearly HiFi with NFB. The Vox power amp is open loop - no NFB.

It will be a cool experiment, but I wonder how clean you'll be able to stay at equivalent volumes. Only one way to find out.

[Mark]

I would have a long hard look at the output stage, the are quite a few posts in the Trainwreck section on the out put stage (I have my share on this topic).

The typical Dumble output stage as far as I can tell runs 60% to 70% plate dissipation, the Vox AC-30 has it's output stage running at a minimum of 100% plate dissipation and it is a class A/B amp. For the AC-30 output stage to run at 70% dissipation in class A/B you'd have to double the cathode resistor.

The 100 ohm resistors on the screens are an issue as they provide nearly no protection for the screens and if you increase them to 1K ohm which seems to provide the minimum protection the tone of the amp changes with less bass and mids (which translates to the amp not sounding as natural for lack of a better term.)

Sorry to rain on your parade but I think you are looking for two different amps, or one amp will be heavily compromised.

[jeshin]

Thanks for the replies,
10thTx what would changing the coupling caps to 0.02 do?

DonMoose & Mark,
Yeah going into this I was thinking that I would have to compromise. I use and play a lot more music that the top boost is more suited for so that side was my focus. Maybe if this turns out to be somewhat descent ill do the reverse adding a top boost channel to dumble amp.

Mostly I was thinking was that if with the schematic shown works ie doesnt combust etc.. Id give it a try and go from there

[10thTx]

Regarding the .02 cap ..................

Most EL84 amps have a .01 or .02 post LTPI coupling cap. The AC30 is different with the original .1 cap. However, the AC30 has a fairly trebly/thin tone stack, IMO.

My concern is the Dumblish tone stack with more bass and mids into the .1 caps will sound waaayy too muddy for the EL84 tubes. So, the .02 would be a consideration given some Dumble amps actually have .02 LPTI coupling caps. And would be more in keeping with what I usually find with EL84 amps.

So, the idea would be to keep the amp more transparent and not muddy.

Yes, the amp will be a compromise but could be a great sounding amp. I don't anticipate you will have strong "clean" tones with EL84 tubes which are easily overdrive.

I personally would be tempted to have a pair of cathode biased 6V6 tubes
(250R/10w & 47uf) and cathode biased EL84 (130R/10w and 250uf). I think maybe you could get a dab more clean headroom doing that?

BTW, the 100k screens on that partial schematic I posted showing the PPIMV should be 100 ohm not 100k.

With respect, 10thtx

[DonMoose]

Thanks for the replies,
10thTx what would changing the coupling caps to 0.02 do?

DonMoose & Mark,
Yeah going into this I was thinking that I would have to compromise. I use and play a lot more music that the top boost is more suited for so that side was my focus. Maybe if this turns out to be somewhat descent ill do the reverse adding a top boost channel to dumble amp.

Mostly I was thinking was that if with the schematic shown works ie doesnt combust etc.. Id give it a try and go from there

One idea - if you have access to both - is to try patching a Dumbloid FX Send into at Voxy FX return (no signal to Dumble power amp, but connect a speaker) - and see what that sounds like.

[10thTx]

One idea - if you have access to both - is to try patching a Dumbloid FX Send into at Voxy FX return (no signal to Dumble power amp, but connect a speaker) - and see what that sounds like.

Excellent idea! That alone may inform you whether you want to build this or not build this.

With respect, 10thtx

[jeshin]

Id love to try that haha the only thing is that I live in the middle of nowhere and I would assume the nearest dumble like amp is not in my province lol.
Thanks for the suggestions 10thTx i wonder what the power tube breakup is like with both 6v6 and el84. One this I really like is the ground between clecan and break up in the top boost.