AKAI M7 M8 / Roberts 770 Mod updates

[Matt C.]

once you get the first channel working, +1 to doing a full tear down of the other channel and building it up from scratch. that's what I ended up doing and am learning a lot about tube circuits in the process. once you get everything taken out of there and cleaned up it seems like a decent platform for experimenting with all sorts of different circuits.

regarding transformers, originally I just cruised craigslist until I found a Shure M67 for cheap, but I eventually bought new transformers from Cinemag. I think it was around $60 for a mic input transformer and $25 for a steel core line output transformer. not super cheap but if you end up with a circuit you really like and want to mess around with other transformers, those are a fine option

[johnblue]

So what happens if I gut the amplifier part... I am leaning towards that, I really do not need it.

Plus if I get rid of it... I could probably see and understand the circuit I am working on better.

What do you think?

[Matt C.]

Plus if I get rid of it... I could probably see and understand the circuit I am working on better.

yes.

like I said it might be worth getting the first one working without tearing it all out, it'll give you some faster gratification and you'll be able to hear the thing in action before spending more time or money on it.

but taking everything out and building it from the ground up will give you a much better understanding of how the circuit works, how to build from a schematic, how to optimize the layout, etc.

also, safety first. get safety goggles, don't work on it with its power cord attached, always drain every capacitor before working on it (get a high power, low ohm resistor for draining the caps, and keep a multimeter handy for double checking that there's no stored voltage)

[The Scum]

For draining the caps, my preference would be a smallish, high value resistor (often 470K), permanently installed between B+ and ground. That way you never need to remember to do it otherwise.

And then check it with a multimeter before you go digging inside. Even if you've got a permanent resistor, it may have failed, come loose or cooked itself to death.

Before you start, go and Google for "tube amp drain resistor" and "dielectric absorption." You need to leave the drain path in place while you're working, because the caps can recharge themselves if it's not there.

Make sure the power rating of the resistor is high enough to handle the current it'll draw and power it'll dissipate while the amp is on. And if you don't know what this means, perhaps you're not quite ready to be working inside tube amps.

[johnblue]

i broke one of the 8ohm dummy resistors.

what can i replace it with?

20w 8ohm non inductive resistor? those retangle square things?

[Orpheus]

Yes, that will work. I only had 16R so I had to parallel them.

[johnblue]

Its been a few months... I never got my pre's working. I gave up. I was spending so much time and effort on this project, I stopped writing music...

I just don't understand things well enough to get it done, and looking for parts (caps and resistors was hell for me. I went to fry's once, and the aisle was hell.

A musician friend blessed my studio with a decent solid state pre, so I have that going now. I have spent the last month tweeking rock-wool and I built a vocal booth and got my room quiet.

A couple days ago, I was at the goodwill... pristine Roberts 770x $30...

I just can't shake this project... "just when I thought I was out...it keeps pulling me back in"

I have to have it... but this unit is so nice, I am ready to have someone else do it.

Can you guys recommend someone in los Angeles... preferably the San Fernando valley that hat can do this?

I could pay them, and I would also give them my old unit that I killed.