AB763 build, weird behavior at times

[taiwanluthiers]

Hi

I've been building a AB763, using the Fender Twin reverb design.

https://robrobinette.com/AB763_Model_Differences.htm#Twin_Reverb

I've followed the layout as much as I can except I had rows of power tube behind the controls, and preamp tube behind the board. Not ideal but I wanted the tubes to show...

The problem is that after all the tweaks I've done to make it behave correctly, I still have problems (I shielded/twisted cables together because before this they would have weak signal or strong tremolo ticking).

One problem I have is that the amp would make this pulsed sound (like machine guns) at certain volume positions, then go away once I turn it up.

I also tried the type 3 PPIMV mod but all it did was introduce all kinds of weird positive feedback squeal when connected, so I had to take that off.

[tubeswell]

Yikes. Fender, Marshall and Vox laid out their amps the way they did for a reason.


Putting high-current signal components (like output tube sockets) in close proximity to passive low-current components (like volume and tone pots) is a recipe for cross-talk and unwanted feedback from electromagnetic induction.


Only way forward I can see it to can that amp and start again. Others may have more interim suggestions...

[AlNewman]

Hate to say it, but back to the drawing board.  All those long runs, with the output tubes right next to the signal chain is bad news.  Heater wires, high voltage, etc. next to the sensitive signal chain is a recipe for disaster.  The machine gun noise is oscillation.  Even just the input jack placed right above the output tube would be enough to make it squirrely. 

Maybe you could install some shielding between the signal side and the output tubes, but it would require a complete rebuild, and even then there'd be no guarantee it would work properly.

[pdf64]

I've followed the layout as much as I can except I had rows of power tube behind the controls, and preamp tube behind the board. Not ideal but I wanted the tubes to show...

It's not just 'not ideal', it's pretty much the worst conceivable layout and lead dress.
Sorry but I can't see any benefit in trying to sugar coat it.

Maybe just leave the heaters wired up for display purposes, and hide a working amp underneath?

[taiwanluthiers]

Ok, I get it, it sucks.

I shouldn't have picked this layout but I don't have another chassis either.

However I eliminated the motorboating by shielding the input jack and routing it away from the tubes.

I still have problem with tremolo ticking however. I need to get a bunch of coax cables to reroute it. I chopped up some shielded RCA cables for the input jacks.

https://photos.app.goo.gl/jJ4FPT5ucUgfWkMZ7

But also I see lots of audiophile tube amps where it's nothing like guitar amps when it comes to layout... like this one: https://www.tubedepot.com/products/elekit-tu-8200-stereo-tube-amplifier-kit

Granted it's all PCB and all that, but I see all kinds of layouts with various tube amps...

[Willabe]

But also I see lots of audiophile tube amps where it's nothing like guitar amps when it comes to layout... like this one: https://www.tubedepot.com/products/elekit-tu-8200-stereo-tube-amplifier-kit

Granted it's all PCB and all that, but I see all kinds of layouts with various tube amps...

Look at the size of the stereo amps chassis you posted and the size of your chassis. And that causes all those long wire runs in your amp.

And your amp has a lot of control pots with lots of long wire runs, that stereo amp doesn't have anywhere near the amount of wire and doesn't have all those long wire runs going to all those pots that are right next to the power tubes. 

Huge difference between the 2.

[AlNewman]

The new picture you posted is already way ahead of the first picture, it looks like you've done a lot of work.

If your only issue is a tick at the tremolo, you're winning.  I think that's a pretty common problem, a cap or something.  Google is your friend.

[taiwanluthiers]

Yea, I shielded a lot of stuff but before the problem and feedback was way worse. I'm just thinking about welding up a metal box to cover the power tube leads that can be bolted to the thing.

I'm also thinking to just change everything signal related to coax cables, including the tremolo controls.

Maybe a better design is just having the preamp and power amp in different boxes?

[mresistor]

also with a pcb all the runs are on the same plane..


taiwan   just look at well established 50-60 year old Fender Marshall and other companies layouts for their products      use them as a guide.


Also take a look at Hoffmans layouts on this website

[HotBluePlates]

Ok, I get it, it sucks.

I shouldn't have picked this layout but I don't have another chassis either.

The chassis is equivalent to having a hose that brings drinking water into your house right-at where raw sewage is dumped.  No matter how much you try to clean it, you're gonna get some poo in your water & be sick.

Save your money, buy another chassis that allows you to put the major pieces where they go.  This amp will be a Bucket of Fail until that happens.

Maybe a better design is just having the preamp and power amp in different boxes?

You can do that (maybe), or you could go the intermediate step of getting the proper chassis.  It "keeps Output away from Input."

I've learned the hard way that when I take a short cut, I encounter problems.
Then I try awkward solutions to address the problems, but I run into other problems.
After I put bandages on all the problems, I spent more money & time than if I did not take the original short cut.

A Foolish Man does not learn from his mistakes.
A Smart Man learns from his mistakes.
A Wise Man learns from the mistakes of others.
   I'm hopeful you'll be wise enough to learn from my mistakes, and avoid short cuts.

[Lectroid]

A Foolish Man does not learn from his mistakes.
A Smart Man learns from his mistakes.
A Wise Man learns from the mistakes of others.
   I'm hopeful you'll be wise enough to learn from my mistakes, and avoid short cuts.

@HBP,
Is this a quote?  I've long told my kids, then my grandkids after them: "Anyone can learn from their own mistakes.  Smart people learn from the mistakes of others."  It was just lan observation I'd made in my own life.

Here you've said very much the same idea in different words.  Is it a quote from someone else, or homegrown?

[taiwanluthiers]

I understand.

I made my own chassis, I couldn't find any dimension or other layout information online however, just pictures but no dimensions, so I've had to work off of guesswork, and I wanted the tube showing.

If I really have to, I can move the tubes to where the output transformer is right now, move the OT where reverb transformer is (the OT is where the big blank space where I've routed the input wires with coax cable is). Then I can just weld up the holes or something... I rather not have to but I will if this is what it takes to get a functional amp. I'm not sure where the OT is originally be. I didn't know it mattered.

I won't buy it from Mojotone because the cost plus shipping for the thing to Taiwan would have been enough to make this entire amp, twice!

Right now the only problem I got is the tremolo not working right, ticking and buzzing like crazy. I got some RG176 coax cables that will hopefully fix this.

They built computers out of tubes back in the day, the amount of interference must be INSANE. There has got to be no such thing as a "good" layout when you got like 20,000 tubes.

[Willabe]

I made my own chassis, I couldn't find any dimension or other layout information online however, just pictures but no dimensions, so I've had to work off of guesswork, and I wanted the tube showing.

Search on line, or find someone who has a Fender Twin Reverb amp who will measure it up for you. 

If I really have to, I can move the tubes to where the output transformer is right now, move the OT where reverb transformer is (the OT is where the big blank space where I've routed the input wires with coax cable is). Then I can just weld up the holes or something... I rather not have to but I will if this is what it takes to get a functional amp. I'm not sure where the OT is originally be. I didn't know it mattered.

Is this your 1st amp build?

If so, you picked a very big amp for a 1st build, plus you did your own layout, that's a lot to try and figure out for a 1st build.

It would do you a lot of good to read up on tube amp layouts before you go forward. You might have, maybe, gotten away with it on a much smaller amp. 

[taiwanluthiers]

I do not know anyone with a Fender Twin Reverb in Taiwan, and I doubt music stores will let me do those measurements.

I've done far more difficult things, more dangerous things than build tube amps. It isn't rocket science, and it's certainly simple compared to what TSMC does.

If the idea is to move the tubes away from control/inputs, I can do that. I want to put plexiglass in front of the cabinet and I can feel a considerable amount of heat coming off the tubes, and this may be a fire hazard. I think the OT seems to have minimal influence on the signal at all, and so it's probably not a problem to move the OT where the power tubes currently is. The layout certainly suggests that the OT is probably above where the turret board is (and I've had to make my own too, I think next time I will just buy fiberglass boards and drill holes manually, the stock turret board has the wrong hole placements causing me to have crappy component layout)

I haven't built other amps because the cost of the component is just too high. Even component cost for a 5E3 is still a significant portion of a Twin Reverb, and so I deemed it a poor value. It's ONLY good for one sound and has little headroom (I need headroom because this amp will be used mostly for playing jazz).

One question, how do I add an effect send/return to a tube amp? Do I just feed out a line from the phase inverter to a jack for the send, and the return goes to the power tube's input? Or is it more complicated than this?

[Willabe]

I do not know anyone with a Fender Twin Reverb in Taiwan, and I doubt music stores will let me do those measurements.

No, ask somebody on line.

[mresistor]

here is an auction     https://www.ebay.com/itm/324770244093?itmmeta=01JP8QAP9TRPJVKT8QXE9V4JEV&hash=item4b9dd0a5fd:g:7hAAAOSwiR9hK70V  it has some dims


Mojo has specs/dimensions listed https://www.mojotone.com/Blackface-Twin-Reverb-Style-Chassis


There is a guy in Thailand that sells chassis http://www.solandfa.com/products-Chassis.asp

[Willabe]

Mojo has specs/dimensions listed https://www.mojotone.com/Blackface-Twin-Reverb-Style-Chassis

Click on the specs button to the right of details button.

[taiwanluthiers]

As if almost by magic... I replaced all the tremolo control wire with coax cable, routed it away from the power tubes, and the ticking is gone.

https://photos.app.goo.gl/69uMCFZ2UxtcoPhT6

The only issue is now the type 2 trainwreck ppimv doesn't seem to do anything. The amp works normally as if the mod hasn't been applied, but turning the dual gang pot does nothing. I don't have any 1.8M resistors on hand, need to head over to radio shack for some, so I used 2.2M resistors for the safety resistor...

I might also replace the vibrato channel controls with the RG176 coax as well, and route it away from the power tubes. It would look neater too.

[tubenit]

If I were in your shoes and could not afford or have access to another chassis, I'd reuse the one you have. 

I'd eliminate normal channel getting rid of one preamp tube and make it a AB763 with only two power tubes which
I would mount on that back row near the power transformer.

So the back row would then have 5 12A_7 tubes and 2  6L6 tubes on that back row.   I'd cap the octal holes between the board and the potentiometers with a piece of aluminum sheet metal.

In other words, I'd gut most of the amp including removing the board. You can create a faceplate for the front of the chassis hiding open holes that you had for potentiometers.


As everyone has said, I also believe it will be a mistake to continue using the current layout as it is.   

With respect, Tubenit

[tubenit]

By the way,  I lived in Tienmou just outside Taipei, Taiwan for 2 yrs in 1961 and 1962. It was a short hike to the top of Doghead mountain from my home.   It was a great experience!  Tubenit

[taiwanluthiers]

The amp is functional now, no oscillations or noise, just that the Lar-Mar master volume mod I tried to do doesn't seem to do anything (as in it acts as though it's not even there no matter how I turn the pot).

For now I just soldered extra wires onto that mod, stuck the pot somewhere inside the chassis, and tacked a type 3 master volume onto it (those work well enough). I am unsure what I've gone wrong because I've checked the connections, used the right 250k log dual gang pot, but the amp acts like it's not even there.

If I can figure out how to make it work I can just snip the type 3 master volume off.

I feel like I would probably have to sacrifice tremolo if I wanted to add an effect loop to it, it looks like it requires another tube.

[stratomaster]

You wired something incorrectly if your pot isn't doing anything.  Remove the power tubes and measure the negative voltage at the grid. Messing around in this area of the circuit has the potential to nuke your tubes if not done correctly. So don't reinstall them until you've sorted this out.

There are also two 220k mix resistors that you might have accidentally replaced instead of the grid leak resistors you meant to replace with the pot.  That would exhibit behavior closer to what you're describing.

[tubeswell]

Hi there, which thread would you prefer responses in?


https://el34world.com/Forum/index.php?topic=32569.0

[taiwanluthiers]

Speaking of reusing the chassis, I can simply move the OT, reverb tranny, and the choke to where the power tube is now, no need to remove tubes. I can find space where the OT used to be, and the OT will cover the holes from the power tubes.

I've also learned coax cable shields better than twisted cables, no idea why. With twisted cable I can still get oscillations at high volume, but with coax it goes away completely.

[Esquirefreak]

I think Tubenits avice is very good. Take everything out and pretty much mirror everything when reassembling. It would look kinda like a Hiwatt lauout. I think you would have to re-design the board, though - to keep the inputs away from the main cap board. Although not ideal, I think the chassis is reusable. But it's probably a waste of time to fix the wiring, rather than ripping it out and start over.

/Max