Deluxe Reverb AB763 Tremolo Issues

[plaidzebra]

Hello everyone, I hope you are having very happy holidays.

I'm having trouble with the tremolo for a Deluxe Reverb I'm building for my uncle. Whenever I activate it with a footswitch that I made, it produces an oscillation in the speaker but doesn't affect the signal from the guitar. This happens when the speed knob is at 1-4. Whenever the oscillation occurs, the intensity knob will slow the oscillation down if it's turned between 9 and 10. If the speed knob is above about 5, the oscillation cuts out completely.

I have recorded the pin voltages for the tremolo 12AX7, shown below: (I recorded the pin voltages while the tremolo was on, but not while it was oscillating)

1 - 335 V DC

2 - 258 V DC

3 - 295 V DC

4 - 3.1 V AC

5 - 3.1 V AC

6 - 448 V DC

7 - 335 V DC

8 - 348 V DC

9 - 3.1 V AC

Because this is bias vary tremolo, I also recorded the pin voltages for the power tubes.

V8:
1 - .03V DC

2 - 3.2 V AC

3 - 446 V DC

4 - 447 V DC

5 - -39 V DC

6 - -39 V DC

7 - 3.2 V AC

8 - .03 V DC


V7:

1 - .03 V DC

2 - 3.2 V AC

3 - 447 V DC

4 - 448 V DC

5 - -39 V DC

6 - -39 V DC

7 - 3.1 V AC

8 - .03 V DC


If it matters at all, the amp is also hotter than the one that I built for myself over the summer. I think it's because I used better tubes, and I haven't fully investigated it yet. Could this point to an issue with the bias section of the amp?

Thanks in advance for any help!

[sluckey]

Since this is not a stock AB763 trem circuit, it would be helpful if you also post a schematic. Hi-rez pics are helpful too.

Pin 2 and 3 voltages for the trem tube are waaay wrong, depending on your schematic.

[plaidzebra]

The schematic is the one Doug supplies on his website.

http://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf

[sluckey]

OK. Recheck and post the voltages on all pins of V5. Do so with the trem switched off (ie, footswitch jack grounded).

I think you are missing a ground connection for the trem circuit. Make sure the turrets with the green circle are all tied together and have a good connection to chassis ground. (See attached pic) Doug's layout shows those turrets connected to the pot ground buss and that's fine.

[plaidzebra]

D'oh! I forgot that dang ground wire. And I was so sure that I had checked the schematic so well. I even used a highlighter... 


Thanks Sluckey, I really appreciate the help. On an unrelated note, got any recommendations for a Champ project? That's probably going to be my next project, way fewer places to mess up.

[plaidzebra]

Just to be clear, that ground wire solved all the issues I was having. Tremolo sounds just as good as I've heard it.

[sluckey]

On an unrelated note, got any recommendations for a Champ project? That's probably going to be my next project, way fewer places to mess up.

I've never been caught up in the "Champ" thang. I think it's a good learning project for someone without an electronics background that is building their first amp. You know, learn some neat electronics stuff, develop some basic building skills, little or no frustration, all for a low price. But you have successfully built two AB763s! That's quite an accomplishment. Are you sure you want to step back to a lil ole Champ?   

[Willabe]

I thought the same thing but, the Champ (5F1 with a single tone control/5E3) I have set up on the bread board sounds really good and at a low volume. 

Might not be much of a challenge for someone who's built a couple AB763's but it's nice to have around the house/apartment/studio.   

[shooter]

a Champ project

fwiw, I've taken a few "champish" front-ends and melded them with *big bottles*,or PSE, keeps the *tone* from the champ pre and gets you closer to a gigging volume, especially if you have a 2 or 4 by cab

[plaidzebra]

I mentioned it in a previous thread, but I'd really like to delve into why circuits work and sound the way that they do. I'm going to take a few electrical engineering classes once I get back up to school in a week, and hopefully that'll help out with some of the basics.

The idea I was throwing around in my head was to build a Champ using a solderless breadboard so that I could change out preamp circuits very easily. Lots of people on this thread have so much experience with how different circuits sound, and I'd like to be even half as familiar with tone circuits as them. Budget permitting, I'd also like to swap out power sections, power tubes, preamp tubes, transformers, etc. etc. I don't know why, but a KT88-powered amp with just a volume control and a 15-inch speaker sounds like a LOT of fun.

Someone pointed me to a champ-like amp with a 6550 for a power tube and I have that saved in my bookmarks. The only thing preventing my experimenting with this is money (okay, time isn't easy to come by either when I have classes  ). I have a job lined up with the machine shop at school, maybe that'll give me a little extra spending money for these projects.