Converting MBLE amplifier to guitar amp.

[BricoMan]

A week ago I aquired an old tube amplifier, this thing came on the market as a kit from a Belgian frim called MBLE.
I guess through the years it had a lot of different owners because none of the component values arethe same as in the schematic.(see attachement).
The basic setup; transformers, tubes, etc, is luckily the same.

As this is my first amp conversion some questions...

I was thinking a converting this amp to a guitar amp, renewing all the components except for the tranformers.
The input would be completely removed and replaced with two input jacks, first one through 68k and 1M to mass, the other through 68k, as in VOX AC30.
Any remarks or hints on this are certainly welcome...

For the tone stack I am somewhat confused, the existing one in the schematic looks like a James stack, the output however is slightly different ?
As this amp was used for radio, record or tape players, I assume the existing values need to be adjusted for guitar usage.
Also I'd like to have a bass, treble and mid adjustment, like Fender/Marshall stacks have.

Maybe someone might help me on this.

Thanks in advance !

[RWood]

That's an awesome donor!

Front end of a '59 AC-15 and the power plant of a JTM-50?????? Wow.

Sounds like you are on the right track changing the input to suit guitar. If you find that the EF86 is microphonic you can tame the gain characteristics without sacrificing the tone. Great chapter on these in Melin Blencowe's book, and he uses the EF86 as an example.

For the tone stack, why not try it as drawn first before you make any changes, and see how you like it? If you want to look at some different options, try out the Duncan Tone Stack Calculator.

Got any pictures of this beast?

[BricoMan]

I'm very glad someone agrees with me about this donor

Must take a look about the microphonics issue concerning the EF86, there are indeed some people struggleing with this.

I might indeed keep the stack as originally, but I'm afraid it won't do for guitar...

I mostly followed this site:
Converting Integrated/PA Tube Amps into Guitar Amps.

According to this there might be the need for some attenuation after stage 1.

Anyway, thanks for the reply, pictures in attachment.

Really messy ptp soldering....

[BricoMan]

Conversion started.

Finally started converting the amplifier.
First I took several pictures, then I started noting down connections on the transformers.

Afterwards started to remove all of the components.
The chassis was then sandblasted and spray painted black.

Last friday I put back the transformers, mains cable, fuse, swtiches, etc...

Before ordering new resistors, capacitors and pots, I'm still wondering what to do about the tone stack, same as the original or use different values / setup ?

I'm also thinking of putting in another ECC83 as input stage, which then offers the choice between EF86 or ECC83 input amplifier ...

Any thoughts / suggestions ?

Thanks in advance,

BricoMan.

[RWood]

Cool - did you take a pic of the sandblasted chassis?

Regarding the tone stack, how about starting at the end: what amp do you want this to sound like?

Do you have enough room to mount a turret or eyelet board? If so that would make it easier to tweak values than on a p-to-p layout. You could design a good grounding scheme, too, if you did a thoughtful layout, like placing the filter capacitors with the stages that they filter, and grounding them with the other components from those stages.

RWood

[BricoMan]

Ground eyelit.

I uploaded two pictures: the first is the sandblasted chassis.
On the second you can see I mounted a ground eyelit for the components.
I also connected the transformers mounting bolts (all bolts mounted with aluminium grounding washers on "paint free" zones) on this eyelit, so they are on the same potential.

Hopefully I did good on this, time will tell I guess. If I made a ground loop through the transformer lug, I can still easily disconnect them...

I guess having a whole rail on the same groundpotential will be handy when connecting the components as everything is connected to ground eventually.

At first I intended to put the components on a pcb, but htere will be to little space.
I cleaned the original pots succesfully, they were a lot younger than the rest of the pieces inside.
These could be used to reinstall the tone stack as originally designed, afterwards I could still readjust the values / setup?
I have to start somewhere haven't I ?

Hope you like the pictures, more to come...

BricoMan.