DIY BF Deluxe Reverb troubleshooting

[Meddon]

Hi all,

I'm not an experienced amp builder, I've only built a few simple amps before, like a 5F1, a 5E3 and a single ended Supro clone. Now i'm afraid I'm in over my head.
I've built a AB763 clone from the Weber 6A20 kit, but there's no sound from the speaker.
I've checked the layout and wiring, and I can't find anything wrong. I've also read the voltages on the preamp, driver and power tubes, but I don't see anything wrong there either:
V1 (12ax7): pin 1 +159 v, pin 3 +1.3 v, pin 6 +162 v, pin 8 1.48 v
V2 (12ax7): pin 1 +157 v, pin 3 +1.3 v,pin 6 +163 v, pin 8 1.48 v
V6 (12at7): pin 1 +168 v, pin 3 +70 v, pin 6 +160 v
V7 (6v6): pin 1&5 -36 v, pin 3 +400 v, pin 4&6 +398 v, pin 8 20.3 v
V8 (6v6): pin 1&5 -36 v, pin 3 +400 v, pin 4&6 +398 v, pin 8 20.3 v

The only time I hear a sound from the speaker is when I measured the voltages on pin 3 on the output tubes, then there was a faint crakle to be heard when the probes on the DVM touches the pin.. The rest of the time it's comletely silent, there's not even any hum.
The Normal channel preamp section is working, I checked that by running a cable from the the coupling cap into another amp.

I would very much appreciate feedback on how to proceed with troubleshooting this amp!

Thanks!

[pontiacpete]

shouldn't pin 8 on v7 & 8 be grounded?

[Meddon]

Thank you, Pontiacpete!

Fixed the grounding, now there is sound in both channels.

Now I gotta figure out why the tremolo drops the volume of the amp when engaged, and why there's no reverb at all...

[überfuzz]

Does is oscillate or just lower the volume?

[Meddon]

Well, it does both. When the Intensity is at 12 on the dial the trem throbs good but also ticks like a snare drum (regardsless of whether the guitar makes a sound or not) but the volume is way down compared to when the Intensity is for instance at 5 or 6. Then the volume is at more or less normal level, but the volume fades as the Intensity is increased.
I've been looking at the amp and the layout drawing for so long that my eyes are sore, but I can't find the error. Also, the reverb doesn't work; when I knock on the Reverb tank it makes the crashing sound, but it does not add reverb to the guitar. I guess there must be something wrong somewhere around the Reverb send-circuit and tube, but I'm at a loss to find that fault too.

[überfuzz]

I've seen this tapping behavior before in AB763 circuits... I've only seen it in a couple of clones, but Fender has a fix for it. There are threads in this forum describing it.

You're sure the volume dies in a way it shouldn't when the tremolo is on? The point of the tremolo is to lower the volume/amplitude in an oscillating manner.

I guess you have voltages suitable for a AB763 scheme. Try to lift the resistors in the tremolo circuit and check them, while you're at it check or replace the tree caps in the oscillator. If all seems dandy, do the Fender fix.

Fender fix of tapping tremolo (Check this up, I think it's like this)
A 0.01 cap is soldered from ground to the 10M Ohm - light-bulb eyelet connection.

[txstrat]

There are not noo many things which can be wrong in the reverb send circuit. Both channels are working so I assume the signal goes to the reverb circuit. The signal has to pass the 500pF cap - check the cap by clipping another small cap parallel to it.
Also check the 1M grid leak resistor, the cathode resistor (that was the culprit in my second build) and the cathode bypass cap.

[Meddon]

Replaced the cap and the reverb worked. If it was a poor solder joint or bad cap I'm not sure, but it surely working now! Thanks for the assistance!

[Meddon]

I checked the resistors and replaced the three caps, then I did the Fender fix, but no luck, the tremolo stays the same.
The voltages seems fine except for pin 8 at V5. On the schematic it should be +5 v, but in my amp it reads +18-19 v. Could that be the cause of the tremolo trouble?

What happens is that there is a pop when I rotate the Intensity dial from 1 to 2, that's the switch behind the pot, I guess. When I rotate the pot further the volume gradually decreases while the tapping sound becomes more noticable. It's like the sound bleeds away as i rotate the pot. There really isn't any noticable tremolo effect to be heard unless the Intensity is at 12.

[Albert Kreuzer]

The only things I can think of without having a look at your amp are a bad optocoupler ("roach") or possibly a miswired intensitiy pot.

[überfuzz]

Oh, I guess I weren't clear on one thing. Don't do the fender fix if you have other problems in the tremolo circuit. Only if you get it working properly but with a tapping.

Have you double and triple checked the wiring of the tremolo circuit? What scheme or layout did you use? The way you describe the problem as of now I'd bet on the wiring... Keep in mind, I'm not an expert, just a dude trying to help.

[Meddon]

I have triple checked the wiring but see no errors. But that doesn't mean there aren't any, I have looked at the circuit for so long my eyes are sore! I'm gonna let the amp just sit on the bench for a few days before I take a last look at it before I order a new optocoupler or tear the circuit apart and rebuild it.

Your input is very much appreciated, it's very helpful to have someone much more experienced to discuss matters with!

Cheers,

[überfuzz]

What scheme or layout did you use?

[Meddon]

I used the one at Weber website: https://taweber.powweb.com/store/6a20_layout.jpg

[loudthud]

The jump in volume when you turn up the intensity control is normal for this circuit. The switch would be a mod to a stock DR to increase the gain on the vibrato channel when the tremolo is not used.

[überfuzz]

Yeah, I guess I'll throw in my towel. I've never seen Webers DR. However, if it 's new there shouldn't any maul functioning stuff there... So either Loadthud is right on the money or... eumm ehh well someone did something wrong. ;-)

[überfuzz]

Oh, by the way. If you're looking for the original DR behaviour you can take a look at the AB763 circuit. It looks as if the pot is wired differently...

[Meddon]

I looked at Fenders layout and schematic for the DR, but the only difference I can see is that Webers kit has a switch on the back of the Intensity pot. Are there other differences?
I think I'll let the amp sit for a week while I build the cab, then hopefully a fresher set of eyes will spot the error I must have made somewhere..

[loudthud]

The switch is a mod to the stock Fender circuit to completely remove the tremolo from the signal path and thus increase the gain. Weber has included this as a standard feature.

[überfuzz]

When you get back in there why don't you give us an update.

1. Current status of the amp - I've lost track
2. What you've repaired/replaced.

[Meddon]

I gave up on the troubleshooting and rebuilt everything on the circuit board except the normal channel preamp and the driver stage. I reused the caps and resistors, but used a different wire. I found that the wire that came with the Weber kit was easily broken, it's not very fleksible. If that was the culprit of my troubles with this bulid I can't really say. But after the rebuild the amp, reverb and tremolo and everything is working like it should.