Hammond ao-44 reverb amp conversion

[jebbo]

So I've got this little Hammond amp from an organ and I'm converting it to a small guitar amp for my son. I ordered carbon film resistors from Parts Connexion. I installed a couple and remembered I should check their value before installing and all of them measured within spec except the 1megs( all 3) were in about 680,000 ohms and the 470K ( all 4) were about 380,000 ohms. I changed the battery in my tester and checked again with same results. I had my kids check the colors on the resistors and they appear to be the correct colors for the values I ordered. Could the colors be wrong fro m the factory or could I somehow be measuring wrong? Any ideas?

 

[gadget73]

Are you reading in circuit? If so, its probably just got something across it. If you're measuring the new ones out of circuit and reading 380K, and you aren't touching the probes I'd say the resistors are duds.

 

[Alan0354]

You cannot hold the lead of the resistor with your hands when you measure such high resistor value. leave the resistor on the table, press the probe on both leads to measure the resistance and see. I would not hold resistor over 100K by hand to measure, any moisture on your hand will create a resistance path through your body.

And if you measure in circuit, that does not work at all.

 

[jebbo]

Ok well I was holding them in my hand so perhaps that is the problem. I'll go check. Thanks guys!

 

[jebbo]

Ok measures a lot better when you don't hold them in your hand!

 

[Alan0354]

Ha ha, Been there. We worked with 100M to 5G, you breath on it, it change the reading. You don't look at it right, it will change.

So what are you doing on the amp? What modification are you doing? I designed guitar amps before, just curious.

 

[jebbo]

I found a schematic from a web site PeeWee Hammond ( AO-44 conversion ) it takes the front end off the amp and replaces it with a fender ab763 preamp circuit. My son has a 66 BFSR and a 66 bassman head, thought he might like a little 1-12 combo.

 

[Alan0354]

Wow, your son has some nice amps. Those worth a ton of money!!!!

I used to have a BF 67 Vibrolux Reverb and I sold it, had a Marshall Plexi 100 triple stack, then a 73 Twin Reverb before a quit in 79. I should have kept the Vibrolux. I bought a SF 73 Pro Reverb a few years back as reference to design my own amps. I still have it, it's original circuit, just recapped.

No other amp can beat the rich clean sound of the classic Fender and the Reverb is fantastic. I don't even like the ones with master volume as the signal goes through more electronics. It's never as good as the non master volume model. I bought the non master volume Pro Reverb.

Both amps I designed and built are channel switching, high gain with power scaling.....that is I can turn down the power to like 3W in 5 steps. So I can play like cranking the amp to full at low volume. I just switch to design audio hifi amp in the last two years.

I don't know that particular PeeVee conversion, but I am very familiar with Fender circuits. I learn electronics starting by modifying the Twin Reverb in the late 70s and went on to have a full career in EE.

 

[jebbo]

Alan0354, yes he does. I have been able to build a few amps, pedals and speaker boxes for him but up to this point I can't really fix something if it breaks. There's a lot to learn and I enjoy trying. I would have to say his 66 BFSR is his favorite. Might be mine too. He was plying a gig last night and the guitarist playing in the opening band's amp was messing up and he let him use the BFSR. It was good!!!!

 

[Alan0354]

Post question here if you need to fix the Fenders, I am very familiar with them. Ha ha, that's an under statement, I got into electronics because of the Twin Reverb. I know the classic Fenders really in side out.

 

[jebbo]

Alan0354, here a pic of the little beast so far.

 

[Alan0354]

From the socket, looks like it's 2 EL84 output tubes. Any other power tube has bigger socket.

 

[jebbo]

It's actually an ecl-86 a triode with a pentode output in one bottle. The pentode section has an amplification factor of 21 and plate dissipation of 9 watts.

 

[jebbo]

Hey Alan0354, according to the schematic output plates will have 296 volts going to them. What do think the output watts will be in push pull? If you want to see the schematic, google Peewee Hammond (AO-44 conversion)

 

[Alan0354]

I have no idea, I am not very good with tube calculation. You can put a master volume like this if you want:



This will give you a master volume to get show dirt.

 

[jebbo]

My son is not a fan of master volume controls. He believes a true overdriven sound should come from an overdriven output tube. He has his likes and dislikes and he's kind of stubborn.

 

[Alan0354]

It's a simple mod. As is now, your gain might be a little too high because you literally add the whole Fender front end driving the original amp. The volume control might be very sensitive. I look at the master volume is too optimize the drive of the front end. You put the master at half, you crank up the front volume, you get a little more drive sound ( not distortion), it sounds better than if you don't have a master and turn the front volume very low.

The two amps I designed have power scaling, that is I adjust the power of the output tubes so I can overdrive the power tubes at any volume down to like 3W.

 

[jebbo]

Well well tell me more about this power scaling you speak of !

 

[Alan0354]

This is my design. London Power sell the kit similar to this you can put into the amp. Believe it or not, I came up with this idea in 1978 and put it into my Fender Twin for gigging. I did not know electronics at the time, I just use a variac to adjust the B+ of the power tube and the grid and leave the rest at normal voltage. It worked. I was too young to think about patenting the idea. London Power did not come out with the idea until much later. But what can you do?!!!






This is built into my amp and it works. But it's not easy for DIY unless you are experienced. So look up London Power.