My amp idea of Vox AC30 - need help

[Kagliostro]

Hi

I'm new in the forum

now I want to submit you my amp idea

it was a Vox AC30/6 (from this schematic)

https://taweber.powweb.com/store/6v30_schem.jpg

with reverb from this schematic

http://www.voxamps.com/downloads/circuits/ac301978.jpg

and some (many) mod from Vox AC30cc2 and other amps, the most from Mesa Boogie

in red the components where I have major concerns

any help - suggestion - comment in welcome

Kagliostro

[markr14850]

That style of DC heater will probably still hum. Although the heaters themselves do see DC internally, it is still AC with respect to the cathode - which is what causes the problem.

Ciao,
--Mark

[Kagliostro]

I'll try to modify the DC heaters design

the 6,3v DC in that way comes from an AC30 limit edition 1990 schematic, with the difference that in the original schematic the + was connected to the ground of the amp

(someone told me it was a mistake so I didn't connect the + to ground)

here a new version with revised DC heaters (this time from Marshall TSL-100) hoping this is a better approach

Waiting for other councils and considerations - Thanks

Kagliostro

[Andy Le Blanc]

There is an issue with your rectifiers and the center tap in the PS.

[Kagliostro]

Opps .......

in the original TSL there was a false CT with resistors .....

Councils as to solve that ??

Kagliostro

[Andy Le Blanc]

Use a modified bridge, no ct, a Graetz bridge.

[Jana]

This is quite an ambitious project. Are you an experienced amp builder with many successful builds under your belt? Not trying to rain on your parade but if you're not well experienced this project is fraught with many frustrations. I would stick with something simpler and geared towards what you really want instead of trying to design a swiss army knife with the hope that there is a setting on this amp that will give you the sound you want.

[Kagliostro]

No I'm not an experienced amp builder

I'll try to build first the original schematic and than I'll add the mod

However I have a friend on my side very experienced in tube amp, many friends who are amateur radio and some that are technician by profession

only the first is experienced with guitar tube amps, and I've submitted the schematic also to him

at the moment I'm looking to the best schematic (on the paper) I can get, and only than I'll start with the build

Kagliostro

[KT66]

I have to agree with Jana here, there is way too much going on in your design. You're going to end up with a disappointing mess and a troubleshooting nightmare.

Having a schematic is one thing, having a good layout is another. I think the best thing for you to do would be to do an AC30ish build with a known good layout like a Liverpool or a Rocket. The Twreck forum has all of the info you need to build a great sounding amp.

Also, there is a layout on the Ceriatone site for their DC30 clone, and Hoffman amps has a layout for the AC30 w/ the topboost and normal channel.

Building amps is supposed to be fun, if you keep it simple it will be.

[Structo]

I have to agree with the others.
I've built a few amps and I don't think I would try to build that, especially since it is not a proven design.

That reminds me of some Soldano schematics I've seen.
Very busy!

Sometimes borrowing from one schematic and adding other features work out but other times it makes a mess.
Then you have no idea where to look for the trouble spot.

If you want a good versatile amp, look in the Dumble section.
An Overdrive Special type amp is very versatile and you get both a great clean channel and one of the best overdrive channels in tube amps.

[M Fowler]

Kagliostro,

I think you have been inside the Rebel 20 no?

[markr14850]

The relationship between heaters and cathode is the same in the new scheme as in the original, and will suffer the same issues.

[Kagliostro]

Thank for the advices

sorry I answer only now (had a problem of Internet connection)

No, nothing about rebel 20, before I don't know it and look to that only now (moreover an amp with interesting features)

My el84 - 6v6 mix idea comes from Mesa Boogie Blue Angel

May be I'm wrong but I think the approach of the rebel to have a 1W to 20W variable and to engage the el84 or the 6v6 or both, is different

about power I think they use the separated cathode resistor circuit with capacitor grounded by a pot instead than directly

about variable insertion with one knob of the el84 or 6v6, may be they use a PPIMV like a balance circuit or they play around a balance pot grounding the cathode cap

I'm reading and asking councils about my amp idea during the last 3 years, I'm patient and as I told my intention is to build the normal AC30 ( The layout I decided to use is this https://taweber.powweb.com/store/6v30_layout.jpg with a little of this https://taweber.powweb.com/store/6v30_layout.jpg ) and only when all is OK go on with mod

yes I know it is an ambitious project but I hope that with the help of you and other friends I can bring it to fruition

is a dream, what would become of us if we don't try to achieve our dreams?

Say that I also thanks those ho give good councils like to start with a less difficult project

About the DC heaters I investigated and arrived at the conclusion that the grounded 3,15v-0-3,15v CT isn't a problem

Mesa Boogie do that in many amps, looking to other schematics I decided to mod the DC Heater supply in the way you can see here attached

Someone has advice to give?

The relationship between heaters and cathode is the same in the new scheme as in the original, and will suffer the same issues.

Pleas markr14850 can you give me councils about that problem ??

is to try to rise the ground for the DC heaters ?

THANKS to all for your patience

Kagliostro

p.s.: for Structo, looking to your avatar, I do the same when I was a kid, the knife was all in steel, but I touched both the conductors at the same time, so the knife fuses and I do nothing to myself and I'm still here

[markr14850]

For the dc heaters to be effective at reducing hum, the heater must be DC with respect to the cathode. Simply being DC with respect to itself has no value in this case. The relationship is what matters.

If using only one transformer secondary, you have to choose whether to directly ground the AC side, or the DC side. You must not do both or your rectifier will explode.

You could choose to not ground the AC side, and just ground the DC side. This is very quiet.

Or, you could directly ground the AC side, as you first showed, and then add a large (at least 10,000uF) cap from the DC side to ground. This is slightly less quiet. This might be what your latest picture shows, I'm not positive. I worry about those resistors.

In either of these schemes, you have to be very careful not to break the rule about directly grounding both sides. This can happen in strange ways. I blew up a rectifier once by hooking a scope up to the non-grounded side - the scope was ground referenced.

If you enjoy complicated schemes that produce the "ultimate" results, a better way would run the rectifier off a second transformer for just the heaters of the preamp tubes. Or, run the heaters off a SMPS - look for posts by user drz400.

Good luck,
--Mark

[Kagliostro]

Thanks for the councils and explanations

I'm confused

if the rectifier can explode why Mesa boogie do that ??

I'm missing something

look to the attached schematics

Kagliostro