help with a Hoffman AC30

[JCleghorn1]

Hi everyone, I'm new here and hoping someone can help me.

My friend's father recently passed away, leaving behind a nearly complete homebuilt amp that appears to be a Hoffman AC30 (much thanks to the guys over at The Amp Garage for the identification). To help carry the memory of his father, my friend turned to me to help him get the thing functioning again. The problem is... I'm a player with great interest in building amps, but haven't actually made the dive in yet. Can someone help me figure out what this amp needs to rock again?

The guy who built this was an electrician and an audiophile, so I'm hoping he wasn't clueless when he undertook this project. From what I've seen so far, it looks like I need 4 EL84's, 3 12AX7's and a GZ34 to get it running. Are there any other red flags that are keeping it from functioning? FYI, I see that he grouped one of the power tubes with the preamp tubes, however, I do think it's wired correctly. Below is a link to photobucket where I've uploaded the pictures I've taken. Please let me know if I need to take more.

http://s816.photobucket.com/user/jeffcleghorn/library/Amp?sort=3&page=1

Thanks for your help

Jeff

[JCleghorn1]

In addition to the tubes, I need to fabricate a bulb current limiter as well ...almost forgot that part

[TIMBO]

Welcome aboard, Not sure how they ID this as a Hoffman, maybe the transformers but the layout is not similar to a Hoffman.

DO NOT plug it in.

First get a schematic from the library and with a highlighter trace through the schem for any fopars (mistakes) and that those Ecaps are wired correctly.

Pay particular attention to the power supply, with noting that there is a ground connection from the mains cord directly to the chassis.
Safety First.

By comparing the amp to a schem this gets you familiar with how a amp is put together and what all those coloured bits are (if you don't already know)
Take your time on mapping the circuit as faults are hard to fix it we are barking up the wrong tree.

 

[tubenit]

Here is the layout and schematic.   Compare with photo which has some similarities.

I think it is certainly Hoffman layout influenced.

with respect, Tubenit

[tubenit]

1st time start up and light bulb limiter info are here:

http://www.el34world.com/Forum/index.php?topic=2376.0

with respect, Tubenit

[sluckey]

Definitely a Hoffman AC30 board.

[tubenit]

Look up "draining filter capacitors" using Google.

Here is a You Tube that offers some insight.

Vintage 16 filter cap drain

I personally use a 10 watt  220k  resistor with wires on each end going to insulated alligator clips.  I put one insulated alligator clip on the positive end of a preamp filter cap and then the other insulated alligator clip to a good solid chassis ground.

You can quickly drain a filter cap with a good working amp by playing your guitar thru the amp .......... while leaving the on/off switch to on ................. and then unplugging the AC plug out from the 110AC wall socket while still strumming.  This is supposed to lower the voltage to the filter caps to below lethal levels.

You should also look up some You Tube videos on "safely servicing tube amps".   There are lethal volts inside an amp even when it is unplugged.  IF you ever probe inside an amp use a non conductive tool and always have one hand behind your back.  I typically will use a bamboo chopstick to poke around a live amp.

Review the current suggestions and then post any additional questions that you have.  

With respect, Tubenit

[tubenit]

After I was familiar with amp safety and discharging filter caps etc............

Then my next step would be to look over the wiring and see if it looks OK and matches the layout and schematic.

Then I'd put in the tubes that are needed.

Hook up the light bulb current limiter and follow Sluckey's  "start up procedures" that I gave you a link to.

BE SAFE !!!!!

With respect, Tubenit

[Willabe]

I personally use a 10 watt  220k  resistor with wires on each end going to insulated alligator clips.  I put one insulated alligator clip on the positive end of a preamp filter cap and then the other insulated alligator clip to a good solid chassis ground.

Very good solid info Tubenit's telling you.

Just to make sure for clarity;

The amp has to plugged in (turned off) to have a drain path to ground. Edit; wrong, sorry.

You can quickly drain a filter cap with a good working amp by playing your guitar thru the amp .......... while leaving the on/off switch to on ................. and then unplugging the AC plug out from the 110AC wall socket while still strumming.  This is supposed to lower the voltage to the filter caps to below lethal levels.

Yes that will work if the tubes are hot. You can hear the amps sound fade as the filter caps drain off their B+ voltage.

I've done it before and it worked fine but I don't remember how low they drained the caps. If you do it that way just make sure you go in and take a voltage reading to be sure.


                  Brad    

[sluckey]

The amp has to plugged in (turned off) to have a drain path to ground.

Huh?

[Willabe]

Maybe I'm wrong on this but here's what I'm saying.

If you clip in a bleeder R to drain off the B+dcv from the filter caps it goes from B+ to ground. If the amp is not plugged in then all the B+dcv drain will be disapated through the bleeder R (heat) because the B+dcv has no other place to go except for the resistor limited short circuit.

But if the amps power cord is still plugged in then the B+dcv can be bleed off to ground through the chassis to the power cord safety ground wire then to earth through the mains fuse box. Less strain on the bleeder R? Now the bleeder R itself doesn't have to do all the work.

I'm thinking in terms of the bleeder R only limiting how fast the B+dcv drains off to ground instead of the bleeder R limiting the short circuit.

Now that I think about it maybe it's all the same to the bleeder R because all of the B+dcv is still dropped across the bleeder R?


               Brad            

[shoggoth]

No, never work with an amp plugged in if you don't have to (e.g. reading voltages, it necessarily has to be on)

Caps don't discharge to ground, they discharge from one terminal of the cap to the other.  The discharge resistor limits the current so you don't damage the cap.

Ground is typically connected to the negative ends of the caps, which is the only reason it's used for one end of the discharge path.  "Earth" ground through the power cord plays no role in discharging the caps.

[sluckey]

Now that I think about it maybe it's all the same to the bleeder R because all of the B+dcv is still dropped across the bleeder R?

   

[Willabe]

Ok, got it.

         
              Brad     

[JCleghorn1]

You guys are awesome. Thanks to everyone who has helped out so far; I'm learning a lot and having fun. I never guessed that my first step into amp work would be such a cool project and something so meaningful!

[tubenit]

Very glad to have you on the forum!   Thanks for your service to our country.

Please keep us posted on how this is turning out for you.

With respect, Tubenit