The "Black bastard" - complete overhaul of the board.

[Toxophilite]

Looks like a cool build.


I'd be very interested to learn some more about your choice of amp name.

[Bash]

Thanks. If all goes well, the final stack should look like shown in this image.
Top to bottom:
-Amp head
-1x12" cabinet
-Rotary cabinet

The name doesn't have much of a meaning. The project started with the idea of combining a Blackface pre-amp with a tweed power-amp (AB763 pre + 5F6 power). A Fender "Bastard", if you like. Since "Tweed Bastard" sounds too much like a spoiled English Lord, I went with "Black Bastard".

But, as it is now the amp will just be a regular AB763, with a few minor tweaks.

[Bash]

Just back from a family trip to Berlin (yes, we played Bowie and U2) and ordered all the parts for the amp.

Finished the intended 1x12 cab, and started on the headshell.
Put a Mojotone Anthem 12" in it. Damn, that speaker sounds great with my Supro DualTone build. Even better than the Celestion A-type that currently is loaded in that amp.

The overly complex backpanel of the cab allows me to add an additional cab in parallel or series. The volume knob is connected to a 100 Watt L-pad (the switch bypasses it). That way I can reduce the volume if I want, because most 12" speakers are more efficient than the Yamaha rotary speakers that I used (on the photograph the bottom cabinet).

[Bash]

At last. Most of the parts arrived, so I can start.

First thing I did: hammering in the turrets on the circuit board.

[Bash]

Finally had time to work on the amp (sometimes life gets in the way).

Most of the soldering has been done:
-The circuit board with all the leads at the bottom
-All the wiring to the front and back panel

Btw when soldering all the resistors and capacitors I made sure that all resistors are oriented the same way (easier for reading the values) and all capacitors have their values pointing up (again, easier for reading in the future).

Still to be done:
-Wire-up the transformers
-Change the circuit board spacers, because the current ones are too high
-Wire-up the circuit board to the rest of the amp
-Finish the headshell (but for that I have to wait for the weather to clear up, because I have to use the router outside).

[Bash]

I updated the layout in the first post, because the wires were connected to the wrong legs of the master volume.

[Bash]

And I updated the layout drawings again, because I made a stupid mistake: the polarity of the two big power capacitors are the wrong way around.

I found out the hard way, when I fired up the amp.
Had nice blue fireworks in the rectifier and a very smelly "pop" from the capacitors. So, now I need to get new ones.

[acheld]

Magic smoke = magic learning . . .

[Bash]

Well, at least it provided me with a learning/Zen moment.
Won't be able to get new capacitors before Wednesday, so I had some time to work on the rest.

[Bash]

Well, I replaced the capacitors, but I got another problem in return.

When I start the amp up, after a few seconds there is a very loud crackling sound (continuously, but without any apparent pattern).
After a approximately 30 seconds I get a single blue flash in the rectifier tube (this used to say "output transformer", which was a mistake) and the mains fuse of the amp is blown.

I've tried to track down the problem, but I cannot seem to find cause of it. All the wiring seems to be OK.

Any guess what the cause could be:
-Faulty rectifier tube?
-Blown capacitor (that knows how to hide it)?
-Shouldn't have connected that Orange cable of the 290CEX OT to ground?

[sluckey]

I get a single blue flash in the output transformer

   Never seen that before. Do you mean like an arc or spark and it's really on the OT? Exactly where do you see this blue flash? Try to describe it better.

[Bash]

Sorry, I meant a single blue flash in the rectifier tube.

The output transformer seems to be OK (really don't know why I type OT instead of rectifier).

[sluckey]

Most likely suspects are the tube and/or filter caps.

[Bash]

Thanks...

I'll have to see when I can replace them.
Best desolder them first and measure them.

OK, so I'll have to replace all the 22uF/50V Elco's in my amp (I replaced the two 70uF/400V today).
BTW
In your schematic you are using 32uF caps. Rob is using 22uF caps. What would be the effect of that value difference?

Don't have a spare GZ34 lying around.
Would it be safe to use a 5Y3, just for testing puposes?

[Bash]

Just desoldered the filter caps; they all have the correct value (so I soldered them back).

Also tried a 5y3 for a few seconds, but had the same noise.

So, I am back to peering at wires and drawings.

Is there a way to measure whether the OT is still Ok?

[pdf64]

Blowing fuses here is not a good way to go about things.
Until everything has proved good, it’s a really good idea to use light bulb limiter to protect the amp from fault current.

[Bash]

Well, at least I found the culprit that caused the blowing fuses: I forgot the ground wire that runs from 47uF, 100V cap in the bias circuit.

No more blowing fuses and nice blue light.

Now I have to solve another problem, because there is a loud hum coming from the amp, which is louder than the guitar.
The loudness of the hum depends on the volume and master knobs.
If I turn the master past halfway I get a whistling sound as well.

So, the quest for finding my f#$@k ups continues.

[Bash]

Found my f^$#^k up.

I didn't ground the input jack properly. In the image on the left you can see what it should be (and what I drew), on the right what I made. Really don't know what I was thinking when I did that 

Tonight I will see whether repairing that will solve all the humming and buzzing (fingers crossed).

[Bash]

Problems solved...
Next to the mistake with the input wiring I also omitted the 47Ohm NFB tail resistor to ground.


Glorious Fendery sounds (still need to bias it properly, though).

[Bash]

I just noticed that there is a weird, buzzy distortion when I turn the preamp volume up. If I turn it up completely it almost becomes a whistle.
As if some part is rattling inside causing it to buzz.

If I bypass the EQ completely it is mostly gone (and you get a huge volume increase) and you get a nice overdrive.
As it is now, the amp is more than loud enough for my purposes, but it shouldn't do that.

Any idea what could cause that?
Not the power section, because I get it also when the master volume is turned down and I turn up the preamp volume.
Its also not any of the tubes, because when I changed these for different ones the buzz persisted.

[Bash]

Finally all problems solved (with regards to this amp)!

By looking a bit more carefully at the schematic (and the wiring inside the amp) I was able to track down what caused the buzzing: a faulty cap.
By forgetting the 47 Ohm resistor to ground, the 100 nF capacitor got a little more than it could handle. It still worked, but could not handle higher output from the preamp anymore.

If you look closely, it developed a faint red line.

I didn't have a 100 nF cap at hand, but three 33 nF ones in parallel do the job just fine.

[Bash]

And the amp is finally done.
Today I had time to tolex the headshell and put everything together.
I'm really happy with the results. It sounds great as well.

Everybody thanks for the help and feedback.

[Bash]

The NFB should be connected directly to a tap on the secondary, not after the selector switch. So, you would still use 47Ω for 8Ω tap or 100Ω for 4Ω tap. This way the NFB will remain constant regardless of which tap the speaker load is connected to.

In that same line of thought you should use 27Ω when connected top the 16Ω tab?

[Bash]

The problems with the build kept bugging me. Furthermore, having a lot of connections to the bottom of the board would make any future maintenance next to impossible.

So I decided to redesign the board and completely replace it (and most of the wiring). No more wires running underneath the board. More logical grouping of the amps functional blocks (Bias board - power section - phase invert - preamp). Layout optimized for having short leads.
And, of course, no more connections at the underside of the board.

The amp is now quieter as well.
Still sounding fantastic.