tube amp re-building

[dd12939]

so I have this tube amp out of a Hammond M-series amp. I believe it's from 1951. I finally got around to finding a schematic of the amp online and figured out that the one input (of two) that does work is merely a power amp input. totally bypasses the preamp. (as far as I can tell anyway) My assumption is that the amp is mad about not having an organ hooked up to it (I think it's looking for a transformer that isn't there) so only the poweramp input is working.
My real question is: How hard would it be to identify the parts of the amp which are preamp/organ related and remove/disconnect them and put in a VERY simple guitar preamp circuit (i.e. Fender Champ type)???
Thanks

PS. I know what you're thinking: this guy's gonna fry himself because he is clueless. I am clueless and I am NOT digging in the amp until I have remedied the situation. I'm just looking for ideas at this point.:usa1:

 

[firebirdVII]

Re: tube amp re-building

Can you post a link or picture of the schematic?

 

[greco]

Re: tube amp re-building

Even with the amp NOT connected to AC , I would suggest discharge the large caps if you are going to be putting your hands in the chassis (assuming it has been connected sometime recently).

Possilbly I'm being overly cautious here, worse yet, you are very capable with this sort of stuff and my response is way too basic.
If so, sorry! (just noticed that you have 1265 posts !!)

Others with more electronics wisdom/experience will advise you.

Hope you can get the amp "converted"/repaired/functioning.

Dave

 

[dd12939]

Re: tube amp re-building

Thanks for the replies.
This is the schematic I am dealing with. And I know about discharging caps and etc. but like I said, I'm not doing anything more than looking until I get the whole thing mapped out anyway. It never hurts to make sure anyway! :banana:

On the schematic, below where it says "Manual Amplifier" you can see "Spec Input." This is what I think is the main input which I can't use. Just below the V4 to the left of it is the "Radio Phono" input. This is the one I use. Given this, the plan as it I see it is to remove/disconnect everything before that point in the signal chain and install an early-fender type preamp section (Champ/Princeton/etc) and hook it up to the power amp section.
I'll also have to add 1/4" inputs, a knob or two and a power switch.

 

[greco]

Re: tube amp re-building

What a schematic!!

Looks like wiring from a 747 jumbo !!

Best wishes for a safe landing.

Dave

 

[firebirdVII]

Re: tube amp re-building

Do you know what part of the amp you have?

I would also like to state now that I am in no way a technician or claim to know proper safety procedures when dealing with this sort of thing, so keep that in mind.

 

[dd12939]

Re: tube amp re-building

Do you know what part of the amp you have?

On the first schematic, I have "Manual Amplifier" "Intermediate Amplifier" "Output Amplifier" "Pedal Amplifier" and "Power Supply" - - Pretty much the whole thing aside from the tone generation and that pesky vibrato scanner.
I took a little gamble to see if my hunches were correct - I pulled all the tubes save for the 5U4, 6V6's and the 6SC7 (phase inverter?) to see if the power input still worked. It sounded exactly the same as before, so I would say hunch confirmed. I redrew the parts of the schematic which I am now concerned with: the Output Amp and the Power Supply. The rest of the amp is dead weight from now on.

As far as I can tell, the part marked "Expression Control" is where a preamp section would be attached. After I get the rest of it planned out, I think the hardest part will be getting the right power to the new tube that will need to be installed.

 

[greco]

Re: tube amp re-building

After looking at your drawing (schem) that you posted, I feel really stupid advising you to dischage caps.
Again, VERY sorry

I'm following this thread with much interest.
Unfortunately, I'll be unable to use my computer during the next week, so I'll have to catch up later.

Please keep thr thread going (with updates) if you have time.

Thanks

Dave

 

[Foeburner]

Re: tube amp re-building

Ye, it looks like its some kind of radio IMO

 

[Foeburner]

Re: tube amp re-building

Oh now It doesn't look radio anymore

 

[Foeburner]

Re: tube amp re-building

Remove all parts before the expression control+pedal amp, and then you got only the poweramp.

 

[Scott_F]

Re: tube amp re-building

My suggestion is to call Bruce at Mission Amps and pay him to fix it up for you. Or, alternatively, Jeff Seal at Speaker Surgeon. Lastwinj as well? I'm not sure if he tackles this stuff or not.