AA764 Help!

[charisjapan]

Hey all!

It's been some time, but I have been here, just not posting much. In fact, I've been here at Shock Brothers a LOT, trying to find info about getting a Triode AA764 Vibro Champ working... to no avail :(

I had a Fender Japan SS amp with twin 8" speaker that sounded meh, so I am using as my donor (recipient?).

After putting it all together, I have sound. YAY! But not good, in fact pretty unusable. Basically, treble and bass pots seem to be doing their thing, and speed does it's thing when the vibrato plug is shorted, which is how it's supposed to work. So I have volume and intensity pots that seem to be messing with one another, even when the vibrato is off. When vibrato is on (shorted), the intensity seems to work a few places in the sweep, then cause volume variations, then not affect intensity at all. the volume sweep is also spotty, and at the end of the sweep cuts out vibrato altogether.

This is my first build, and I'm happy to get any sound at all, but...

I have checked everything that I can with a cheapo multimeter, and all the resistors check out to spec. Everything is grounded properly (I think). All the solder points seem to be solid. What am I missing?!?!

Here's my Flickr set that shows all the steps I've taken, including tearing out the board and re-connecting everything, and changing pot locations to be in sinc with the Triode layout and the original Fender schematic. My Triode tech seems flummoxed, which is not a good sign. Please help!

http://http://www.flickr.com/photos/...57629267256750



RCS_1699 by charisjapan, on Flickr


RCS_1686 by charisjapan, on Flickr


RCS_1669 by charisjapan, on Flickr

[Wally]

Chris, ambitious. Odd layout to look at, though. I am going to think that there might be some problems with induced noise due to the lead dress. I don't know if that lead dress arrangement could have anything to do with how your trem function is working, but from my point of view things should be laid out differently. I would have put the pots and input jacks on the front panel. Things would be a lot simpler to 'see' and work on if it were laid out more like a FEnder VibrChamp...even if the lead dress as it is does not have deleterious effects on the fucntioins and the sonics.

Anyway....what are your voltages? Be aware that that bias vary trem circuit functions in connection with the cathode bypass circuit on that PI...and that bias point can have an affect on how the trem circuit functions. Lots of things to suspect....but voltages tell one a lot. Both 12AX7's are suspect...and this is an easy thing to change out to see if things change, right? But...check voltages first. NOte: bad tubes can cause voltages to be improper. All caps and resitors in the circuit are suspect. All connections are suspect. The pots are suspect. Step by step,......eliminate possible problems.
Do not accept that your work is correct. IT is also suspect. Ignoring this is a that is a trap that can prevent one from finding an error.
IN fact, I would suggest rebuilding the amp with the controls and input jacks on the front panel and leave the tube socket where they are...at the back.
This would at the least simplifiy any work...now and in the future. At best, this would eliminate some induced problems connected to the sloppy lead dress..

I say this with best of intentions. Good luck with it.

[icd]

Have you tried just disconnecting the trem/vibro circuit to check that the volume issue goes away - at least then you might isolate the problems to the specific circuit involved in the vibro/trem. The use of the cascaded caps. is a bit tricky at times and does not always perform as speced.

[charisjapan]

Originally Posted by Wally

Chris, ambitious. Odd layout to look at, though. I am going to think that there might be some problems with induced noise due to the lead dress. I don't know if that lead dress arrangement could have anything to do with how your trem function is working, but from my point of view things should be laid out differently. I would have put the pots and input jacks on the front panel. Things would be a lot simpler to 'see' and work on if it were laid out more like a FEnder VibrChamp...even if the lead dress as it is does not have deleterious effects on the fucntioins and the sonics.

Anyway....what are your voltages? Be aware that that bias vary trem circuit functions in connection with the cathode bypass circuit on that PI...and that bias point can have an affect on how the trem circuit functions. Lots of things to suspect....but voltages tell one a lot. Both 12AX7's are suspect...and this is an easy thing to change out to see if things change, right? But...check voltages first. NOte: bad tubes can cause voltages to be improper. All caps and resitors in the circuit are suspect. All connections are suspect. The pots are suspect. Step by step,......eliminate possible problems.
Do not accept that your work is correct. IT is also suspect. Ignoring this is a that is a trap that can prevent one from finding an error.
IN fact, I would suggest rebuilding the amp with the controls and input jacks on the front panel and leave the tube socket where they are...at the back.
This would at the least simplifiy any work...now and in the future. At best, this would eliminate some induced problems connected to the sloppy lead dress..

I say this with best of intentions. Good luck with it.

Wally,

Thanks a lot for the reply. Hehe! I assume good intentions here, unless I have a LOT of evidence to the contrary.

Yeah, I know my lead dress leaves a lot to be desired, and that the unconventional layout is certainly a potential problem. My explanation (excuse) is that none of the VC controls would match the labels on the front panel. (dumb reason) I thought that I could first do everything on the back just to get started, then mayeb get a new panel made... Until I found out how much a custom-size, custom labeled panel would cost! Anyway, to fix that, I plan to order a chassis with plate from Ceriatone. They're close to Japan (Malaysia) and the white is pretty! It won't fit in this combo, but I'll just make a head, and plug it in.

I will get those voltages and post. I did it twice before, but was very ignorant as to what I was doing, and they were weird, user error. I've since studied a bit here and other places, and think I can get reasonably accurate numbers now.

I'll try out different tubes (they just arrived) and measure again. I came to the conclusion that everything is suspect... especially ME!

I checked resistors (easy) and continuity as best I could understand. I have no clue how to check pots or caps, and my little multimeter is probably not much good for anything but VDC VAC and Ohms. Any suggestions?

Anyway, I really do appreciate your input, and want to get this thing singing!

[charisjapan]

Originally Posted by icd

Have you tried just disconnecting the trem/vibro circuit to check that the volume issue goes away - at least then you might isolate the problems to the specific circuit involved in the vibro/trem. The use of the cascaded caps. is a bit tricky at times and does not always perform as speced.

Hey, icd, thanks for your reply.

I responded to Wally's post, and will measure and post voltages and switch tubes as per his advice.

Please bear with this Noob! I'm a carpenter at best, a reasonably good mechanic, a 15-years-ago PC-tech, and play very bad guitar. A few months ago, I wouldn't have a clue what 'cascaded caps' meant, and even 'circuit' would be a bit vague. For your suggestion, I need some clarification, but I understand the concept (I think). A buddy told me that an AA764 was basically 'paint-by-numbers.' So (with apologies to Triode) I put numbers on their board layout. Where would I disconnect to isolate the trem? (my guess would be turrets 16, 18, 21, 23, and possibly the wire between pin 6 of the second AX7 and pin 4 of the 6V6?) Forgive a stupid question, but could you just pull the second AX7 tube?

OK, enough guessing on my part!



Triode AA764 Vibro Champ 40-18027 120V Numbered by charisjapan, on Flickr

[icd]

Pulling the 12AX7 would be the easiest 1st step to try. Second step would also to be to remove the yellow wire from the "intensity" pot - tape the end and keep it out of contact with anything else. Looking at the schematic, that should isolate the trem/verb part of the circuit. I am no expert on this type of trem/verb circuit, but I think that there is a little art as well as engineering involved as it relies upon controlled instability to give the effect. Thus it can be sensitive to voltages, lead dress and tolerance of components. However the issues you report seem more like engineering problems. Let us know how you get on. WATCH OUT FOR SMOOTHING CAP VOLTAGE RETENTION - DRAIN THOSE CAPS !

I guess you have done it more then once, but check each and every component value against the schematic and check each and every wiring connection just to be sure the circuit is as designed. No need to disconnect anything but just eyeball and checkoff against the circuit each element. Good luck !

[charisjapan]

Wally - voltages with volume,treble,bass at "5" (12 o'clock)

5Y3GT
2 - 348V
4 - 281AC
6 - 281AC
8 - 346V

6V6GT
3 - 346V
4 - 329V
5 - -3.0 at "0" +105mV, at "10" -7V
8 - 17.57V at "0" 20V, at "10" 16V

V2 AX7 (Vibro)
1 - 162.4V
2 - 0.4mV
3 - 1.539V
6 - 328V
7 - 162.8V
8 - 165.8V

V1 AX7 (Preamp)
1 - 205V
2 - 0.0
3 - 1.495V
6 - 204V
7 - 0.0
8 - 1.462V

And yeah, a 'proper' VC chassis would be best for lead dress. I have to admit that I'm not too thrilled about taking it all apart (again) and drilling out the front panel for these inch pots and working in those tight spaces. (sigh)

icd - I pulled the V2 tube. While the weird tremolo stuf is gone, the sound is still pretty bad, with strange oscillations through the volume sweep. It's a bit louder now, but not pleasantly so. Even at 12 o'clock, any kind of pick attack makes harsh sounds with pops and rude noise. At 3 o'clock it's just loud noise. I pulled the yellow intensity, no change. I switched the AX7s around, no change. Next I can try all new tubes, but that doesn't appear to be the problem. To test the speakers, I hooked up my Champ 600, that sounds great! Switched out the 6V6, and there was no change.

So... do you think it sounds reasonable to say that it's not the tremolo circuit causing the problem, but it DOES make whatever it is worse? Which brings me back to bad chassis layout/lead dress. Or do those voltages tell another tale? I've looked over the board and all the connections pretty carefully, and I'm pretty sure that the ground is correct.

Finally, I hear ya about scary caps!! As a carpenter/electrician, I've been knocked on my can a few times, and don't need THAT kind of 'inspiration.' ;)

Thanks so much for your help. charisjapan

[icd]

Unless someone else corrects me, I think those voltages look close enough to the specs to be fine, I can't see an immediate problem there..

You could try disconnecting the negative feedback link. That's the wire that runs from the speaker connector to 2.7K resistor on the turret board. If the sound improves, then it is likely that the OP transformer is wired backwards. It won't hurt to run without the -ve feedback, but of course tape the loose wire to keep it safe.

[charisjapan]

Originally Posted by Wally

IN fact, I would suggest rebuilding the amp with the controls and input jacks on the front panel and leave the tube socket where they are...at the back.
This would at the least simplifiy any work...now and in the future. At best, this would eliminate some induced problems connected to the sloppy lead dress..

I say this with best of intentions. Good luck with it.

Wally,

The way of a fool is right in his own eyes,
But a wise man is he who listens to counsel.

Thank you for good advice. I listened, I learned, I now hear sweet sounds!

And icd, thanks for the encouragement.

For the life of me, I can't find any difference in the connections, but now I have a wonderfully tubey amp! Switched things around closer to the original Fender layout, snipped about 3 feet of extranaeous wire, and added the power cable socket. Dimed guitar + dimed amp = a LOUD 5W! And man, the "vibro" (tremolo) is excellent.

One little thing... When Vibro is not engaged, the Intensity pot turns down the volume at the end of the sweep. Not completely, but definitely affects the volume of the amp. Anyone?

Btw, a few pics:


RCS_1841 by charisjapan, on Flickr


RCS_1845 by charisjapan, on Flickr


RCS_1852 by charisjapan, on Flickr

[charisjapan]

RCS_1862 by charisjapan, on Flickr

[icd]

Great, I'm glad you solved it. I'm afraid that I can shed no light on the volume issue.

[charisjapan]

Originally Posted by icd

Great, I'm glad you solved it. I'm afraid that I can shed no light on the volume issue.

I wish I could credit myself with solving it. But it sure looks better than the 'spaghetti mess' before!

As to the volume/intensity connection, I can't see anything in the layout that would make them interact! However, it only happens when the Vibro is disengaged, and I'm finding that I like it on all the time. With intensity at '0' it's completely transparent, and doesn't make any real tremolo until 11 0'clock or so. From there it's got a nice taper all the way to a very strong effect.

Best part is, I had just about written this project off, and needed a small amp for a Gospel gig this Sunday. Tried it out at our venue, and it fills the room nicely at 3 o'clock volume. It also reacts well to my Tele partscaster. Cool.


RCS_1868 by charisjapan, on Flickr

[Wally]

I'm glad that the problems disappeared with a more correct layout. Thsiis why layouts are soemtimes protected by copyright law. A fellow member here builds a great littel boost pedal. The plans for this effect...and almost all other pedals that are being built today...have been common knowledge for decades. What is important is not particularly the schematic but the lalyout...and he has had to protect his copyrighted PCB from folks who have 'reverse-engineered' the board.
When you changed the layout, you eliminated some...who knows how many....points where unwanted 'signals' were being induced by improper relationship of the wiring. Wtih a more proper layout, these problems disappeared. Wires are now not in close proxiimity in a certain arrangement that would induce unwanted currents. Kudos for going to the trouble of reworking things.

Vibrato/volume....most of us turn the vibrato off if we are not going to use it. I have never incurred the situation that you describe. IT may or may not be 'normal'....I jsut never have 'been there, done that'. Have you worked up a switch to turn it Off? If so, what happens when you do that? That Vib circuit is connected to the speaker out lead through the cathode bypass on pin 8, V1....and there is a relationship there that can only be avoided by turning the circuit 'off' with a switch.

Voltages....the on ly thing that I see that is 'odd' would be the screen grid in the 6V6....it should be closer to the plate votlage. Everything else looks spot on, doesnt' it?

[charisjapan]

Wally,

Thanks for the reply and kudos! I gotta admit, it feels good to get my first amp build running.

I did decide to put a switch in where the Vibro RCA connector is. Actually, the Vibro (tremolo) is engaged by default, and shorting to ground the wire from turret 20 on the board. Maybe later I will try to 'interrupt' pin 8 on V1 with a switch, but I will be using the amp for a gig on Sunday, and didn't want to mess anything up, now that I got it going!

Thanks again!


Triode AA764 Vibro Champ 40-18027 120V Numbered by charisjapan, on Flickr

[celeste]

YaY, I was so out of ideas on the other board and felt really bad you did not have a working amp yet. I would really like to know what the issue was, to put with my collection of odd problems, but having a working amp and not knowing is still way better then a non working amp and buckets of information. Have a great gig tomorrow.

[charisjapan]

Originally Posted by celeste

YaY, I was so out of ideas on the other board and felt really bad you did not have a working amp yet. I would really like to know what the issue was, to put with my collection of odd problems, but having a working amp and not knowing is still way better then a non working amp and buckets of information. Have a great gig tomorrow.

Thanks for the YaY, celeste. The gig was fun, though I was pretty nervous. I didn't freeze up, so it was great!

Indeed, working amp is a LOT better than a miserable failure of an amp build!

And it is working fine. I am really loving the tremolo, so warm and real. I put a switch in the place of the footswitch jack.

As to the volume/intensity thing, it's my first 'odd problem' and challenging me. The only thing I can see in the layout is that both pots are conneceted to V1 AX7 at pins 7 and 8, but have no clue what that means. (but i'm not going to lose any sleep over it, and it's late in Japan, so...

Be well!

[charisjapan]

I can't believe it's only been a little over 2 months since I "finished" my first build! And now I'm closing in on the completion of my second, a 5E3 with a 'real' chassis and cab.

I was already salivating after another build when I looked at my VC yesterday. It's kind of on 'permanent loan' to my church band, and everyone that uses it loves the tube tone and just-right volume sweep for our venue. But it looks bad compared to the 5E3, and I came to the conclusion that I would feel a lot better about really finishing the AA764 than starting another project. So....

I haven't really found a nice chassis for a VC, but Memphis Amps, who made my tweed cab, make custom chassis. I'm asking for a quote, as well as a slightly-downsized 5E3 cab for a 10" speaker. Both transformers are already upgraded, 5-15W for the PT (I'm seeing 342V, a nice number) and a 15W OT. I will probably upgrade the resistors and caps, any suggestions? (nothing too expensive, please!) Also, what do you think of keeping the 40-20-20-20 525V Twist-lock, or going to 3 separate filter caps? What values? I have a perf-board and plenty of turrets, so I'm willing to get a little 'out-of-the-box' for layout, but I want to keep the basic SE circuit. Just food for thought, I was told it would be pretty easy to use a 6L6 power tube...

Please, help me 'honorably' finish this amp!

[telex76]

I got my VC clone chassis from here.http://store.marshamps.com/product_i...roducts_id=278. I guess you can get it from where you are located.

I used an empty 25 Watt Frontman cab, it fit perfectly. Just had to drill new holes for the chassis straps. I went with a ten inch speaker since the cab's baffle was already for a 10.. I also used the triode kit.

[Keyser Soze]

Just a thought on the intensity pot issue.

That pot is in parallel with the bias resistor (1.5k) of the 2nd triode of the pre-amp. If the pot were defective it could be shorting the cathode to ground.

Try reading the voltage from pin 8 to ground with the pot turned up all the way, and then down all the way. If you notice any significant difference between the two readings then this is the source of your problem.

[charisjapan]

Originally Posted by telex76

I got my VC clone chassis from here.http://store.marshamps.com/product_i...roducts_id=278. I guess you can get it from where you are located.

I used an empty 25 Watt Frontman cab, it fit perfectly. Just had to drill new holes for the chassis straps. I went with a ten inch speaker since the cab's baffle was already for a 10.. I also used the triode kit.

Thanks, Telex!

I do remember looking into Marsh Amps, but $85 plus about $50~60 shipping was pretty steep. The Frontman 25 is hard to find used here, so that is also kind of difficult. I'll just have to see what Memphis Amps quotes for the chassis + cab.

I liked the Triode kit, but was wondering if there was an alternative to the cap can, like Ceriatone? Did you just stick to the Triode layout? Try any mods?

charisjapan