AB763 build finished

[Planobilly]

Hi Guys,

The AB763 was built off the Hoffman schematic without issue. I have made several changes to the basic schematic. Master volume, connected the reverb and tremolo to channel one, increased the gain on channel one, changed a few other things here and there.

I am pretty happy with the results. The Hoffman schematic was a little easier to read than the original Fender schematic. I ordered some of the parts from Hoffman and they came to me very fast. Good service. For my use, I would have preferred the turrets to be slightly larger in diameter but not a big deal.

I put a mid control on channel two and there is not enough mid control  to suit me. Not sure just yet how to re-configure the tone stack to get what I am looking for. The amp is very quiet at idle with only a very faint bit of motorboating/crackling at max gain...not sure what is up with that. There is no audible sound at anything less than 95% gain. I have in mind to modify the NFB loop with a three position switch so we will see what happens at less NFB.

I built this amp just to be easy to experiment with, so some things are not made to look good. Here is a photo of the stock amp before the mods.



Cheers,

Billy

[pompeiisneaks]

If you're getting a bit of motorboating at max volume, you may have the bias a bit hot.  Have you checked both tubes?  As for mid range, that's odd, I was under the impression that guitar amps always do mids well and you're fighting either for more treble or bass.  Maybe you wired the mid backwards?  Try putting it to '0' and see if its better? 

That's a very clean looking layout.  Well done!

~Phil

[eleventeen]

I like (in my nerdish way) how you fab'ed the chassis out of sheet metal and angle bracket, no bends. I have considered that approach at times. It looks like you used pretty thick sheet metal. How do the corners look from the outside?


Fine layout and execution. No criticism from me.

[Planobilly]

Hi Eleventeen,

The chassis is not easy to build in this way or at least a lot of work. The outside corners look ok. I machined the holes for the tubes and transformer with my small CNC milling machine. Writing the G code to do this is a bit of a pain.

The idea was to be able to easily change any panel on the amp. I built this amp just to experiment with.

I still have not sorted out the motor boating issue. This is generally caused by some sort of positive feedback. Could be lead dressing or improper grounding or one of several possible issues. The cause is normally not easy to find. Oh well...I will find the cause sooner or later.

Actually, now the motor boating is intermittent and does not seem to be gain related. Or course it gets louder with increased gain but it  is occurring at any gain position. I found one coupling cap that was microphonic, but that had nothing to do with the issue.

Cheers,

Billy

[SILVERGUN]

Hey, that looks nice...
 
But the three things I don't love are...
1) The placement of the filament wires in proximity to the preamp tube wiring
and
2) More importantly, the OT wires appear to be laying on top of the PI wiring - could be source of motorboating (you might be better served to run them on the outside of the chassis and drill a hole over near the speaker outs where they can enter the chassis)...I usually use some hi temp, hi quality wire loom whenever I do that and the wires will be out near the tubes themselves.
and
3)It is unclear to me  how the ground is attached to the chassis for the preamp wiring ground buss.....I see the black wire on the far left side of the pic but cant tell where it goes....ideally it would have a direct connection back to the filter caps (-) for those stages.

[Planobilly]

Hi Silvergun,

Thanks for the heads up. I need to re-read what you said and take a look at things. I am pretty sure the motorboating is due to lead dress somewhere. I will try some of your suggestions.
1. I guess I could fly the black heater wires over the top of tubes sockets.
2. I ASSUME you are talking about the secondary OT wiring..??
3. The grounding for all the pre amp goes to the buss along the pots.  There is a buss in the back for the jacks, the buss in the front (along the pots) for everything else except the filter cap ground and ground connection for the heater voltage which has no center tap on the transformer, all of which go to the star.

Cheers,

Billy

EDIT: I put a in new set of output tubes and everything went back to normal no noise and no motor boating. Learn something new every day.

[HotBluePlates]

... EDIT: I put a in new set of output tubes and everything went back to normal no noise and no motor boating. Learn something new every day.

Oddly, I had a similar issue with noise in a bum output tube recently.  That was a new one for me.

If the noise returns, you may try unsoldering the feedback wire from the speaker jack to the board.  This is to verify the "negative" feedback isn't really positive feedback.  I just wonder if the motorboating drops at reduced output power because of a high series feedback resistor value; that could also mean weak output tubes (not putting out full power) might not motorboat (or only at high output levels).  But I'm really not sure.

... I put a mid control on channel two and there is not enough mid control  to suit me. Not sure just yet how to re-configure the tone stack to get what I am looking for. ...

This one is easy: make the Mid control a much higher value.

The stock AB763 tone stack has a large mid scoop of ~24dB at ~500Hz.  Bass & Treble are only cut ~10-14dB when the controls are half-up.  This is for a 6.8kΩ resistor to ground at the tone stack (stock AB763 with no Mid control), or for a stock AB763 with Mid control set at ~7 (these use a 10kΩ linear pot instead of the 6.8kΩ to ground).

If you use a 25kΩ linear pot (like Marshall & the 5F6-A Bassman) or even higher, you will be able to dial in more midrange.  If you had a switch for disconnecting the Mid pot from ground, it will greatly boost the mids and overall gain by disabling the tone stack.

You could choose to use a large-valued resistor (100-270kΩ) between the Mid pot's ground and actual ground, with a switch to short the resistor, to implement this tone stack lift.  You might experiment with a 250kΩ pot wired as a rheostat to determine the best value to your ears for this Mid Boost and/or alter the value of the Mid pot to give the range of adjustment desired in each setting.

And to repeat, if you go crazy with the amount of Mid Boost mentioned above, the Treble and Bass controls will have no effect.  The amount of usable range for those controls shrinks as the amount of Mids is increased.  This is because of how the tone stack overall cuts signal (and mids especially), then allows some amount of less-cut to give the impression of boost.

If you change the value of the 100kΩ resistor in the tone stack, you can alter the amount of mid scoop and its center frequency.

The Duncan Tone Stack Calculator (free online download) will allow you to see the effect of tinkering with circuit values, either in advance or to visually grasp the changes you hear.  That tone stack doesn't always sound the way you'd expect because of how interactive the controls are (i.e., I've found with some guitars I like a 25kΩ Mid pot at max, Bass off and Treble adjusted to taste as a final step).

[Planobilly]

Hi guys,

 Everything works well and the amp sounds good to my taste. I backed out all the mods I did so the amp is back to standard.

The issue is that I have a low frequency oscillation which can be heard at idle. I think this sound would be considered "motorboating" It appears with all gain controls set at zero. I seems to be associated with channel one. When the channel one gain is increased to about 60% it goes away and when the gain is increased to about 65% it comes back.

THe amp is built very close to this schematic. http://el34world.com/Hoffman/images/Hoffman_AB763.pdf

All the voltages are normal and I have changed all the tubes. I have moved all the wires around to no effect. I tried installing a low wattage tube (5751) in V1 to no effect. I have re-flowed many solder connections. I have changed 95% of the capacitors in the amp to no effect on the oscillation. The cap changes did make the amp sound better....used orange drop caps.

Everything is built per the schematic except the cathode caps are 25uf and not 22uf. 25uf is what Fender put in their amps. I made a change to the schematic with the value of the resistors in the power supply which I changed to 10K to provide correct plate voltages.

The oscillation is not very loud and I thought it was caused by a tube. I guess I had the channel one pot set just where there was no oscillation.

I have pretty much done everything I know how to do to try to resolve the issue. At this point I really need some guidance on how to proceed.

Thanks,

Billy

BTW...When I get the motorboating issue solved I re-install the mods and play with the tone stack.

Also, here is the latest photo...

[shooter]

Have you tried putting a large bypass cap on V1's cathode?  try something from 100 - 250uF.  tone WILL change, but your hunting gremlins now, not gigging.

[Willabe]

I have pretty much done everything I know how to do to try to resolve the issue. At this point I really need some guidance on how to proceed.

You might have missed this from HBP?

If the noise returns, you may try unsoldering the feedback wire from the speaker jack to the board.  This is to verify the "negative" feedback isn't really positive feedback.

If the motor boating goes away with the NFB wire disconnected then flip/flop the 2 OT power tube primary wires and reconnect the NFB and see if the motor boating stays gone. Very common problem on new builds.

[Planobilly]

Update

I think I have the issue localized to V3. When I remove the V3 tube the issue goes away. Removing any other tube has no effect.

Also the plate voltage on V3 is 435 V. That is what is indicated on the original Fender schematic but it seems high, perhaps not.

I had removed the NFB but no change.

What should I expect with the higher value cap on V1??

Billy

[Planobilly]

Tried the 100uf cap on V1 idea and no change.

Perhaps I should move this issue to it's own thread...

Update, I placed a .1uf ceramic cap in parallel on the V3 cathode which reduced the issue a bit. 

[shooter]

What should I expect with the higher value cap on V1??

I suggested it based on this comment;

tried installing a low wattage tube (5751) in V1 to no effect

I had MBing on 2 amps, I could see it on the scope inside the noise on ground.  using a large cap *masked* the problem just fine.  I no longer hear it, so.....!!

The cap is more a *troubleshooting* tool to isolate where the problem is showing up.  In my case I *killed* it with about 30-40uF and just re-tweaked tone, all is good.

When I remove the V3 tube the issue goes away

That's your verb driver, and from what I understand it needs to run hot, if you leave 3 in and disconnect the input signal from the *left side* of the 3.3M is the problem there?

[HotBluePlates]

I think I have the issue localized to V3. When I remove the V3 tube the issue goes away. Removing any other tube has no effect. ...

If you replace V3 and unplug the reverb tank, the problem likewise goes away?

[Planobilly]

Sorry guys...it is the removal of the V4 tube that removes the issue.

With V4 installed disconnecting pin 2 of V4 has no effect. Disconnecting the grids on V4 also has no effect.

I just replaced the 500pf in series with the 3.3 resistor. That reduced the issue down to a level where it is harder to hear and increasing the gain does not make it much louder now.

The oscillations are still there but are less audible. 

I have some new output tubes coming today or tomorrow. I will see if that has any effect.

There may be more going on here than just the oscillations. Intermittently I also hear a sound, sort of like when you touch a probe to something. I guess this could be all related to the basic issue, just not sure.

Now that I found one of the silver mica cap bad I think I will check all of them.

Thanks,

Billy

[Planobilly]

Update
After turning the amp off and back on the issue came back. Now pulling v3 does not stop the issue or pulling V4. Pulling both V3 and V4, I can not hear the issue. I can not seem to get the amp to do the same thing every time.

[Planobilly]

I found the bloody issue!!

There was a new AT&T router installed in my shop last week and that was the cause of the problem.

What a pain in the butt hunt for the cause this has been.

Cheers,


Billy