Problems with a high gain AB763

[deuce42]

Could I ask some advice with problems with my higher gain AB763? I've read as much as I can but cannot seem to find the problem. I also note that I've converted an original silverface Super Reverb to this circuit and had wonderful results.

In short I bought a used build of a deluxe reverb clone and wanted to change it to a high gain circuit that I really like with the following changes:

1) Single channel and no trem, therefore I'm able to make it really crunch and overdrive at high volume.
2) 45w output xformer like a Super Reverb or Bassman
3) Diode rectifier.
4) Changed V1 cathode bypass to a Marshall like 2.7k/0.68uf value.

The noise on this build is unbearable whilst the mod to these specs I did on my my original super reverb mod is great.

In terms of my grounding I note the following:

1) Input socket touches case to ground it.
2) All preamp grounds connect to bus wire going to input socket. This includes the cathode of the 20uf filter cap powering the preamp.
3) Cathode of another 20uf filter cap powering the PI goes to the same wire as the preamp ground.
4) All power amp grounds go to the same lug as the mains earth ground on the other side of the chassis.
5) I've experimented isolating the output sockets from the case and also where they touch the case. Neither makes a difference.
6) Power transformer has a centre tap wire which is connected to the poweramp and mains earth ground.
7) Secondary output xformer wires are being kept away from tubes.
Filter caps are all brand new and I've tried different tubes and the output xformer and power xformer are on separate sides of the chassis with the power xformer being on the other side to the amp's input.

I seem to have two horrible noises. One is the low end buzz and the other higher pitched one that increases when I increase the volume on the amp or guitars volume. I'm guessing I therefore have 60hz and 120hz hum going on.

For the life of me I can't work out what is going wrong. The only other thing I can note is the build does not have twisted heater wire, simply two flat buss wires (with rubber around them) running along the sides of all tubes. I am guessing since the original amp was a 2 channel blues deluxe it ran very clean and maybe noise was not an issue so not twisted heater wire was necessary.

Could you guys offer any of your wisdom with where the noises could be coming from?

Thanks for reading:)[/quote]

[deuce42]

sorry double post!

[Deric]

Does the filament winding on the PT have a CT?

[deuce42]

Yes the power xformer heater wires also have a centre tap wire and this is connected to the lug with the power amp and mains earth ground.

[Deric]

Was it quiet before you modded it?

Schematic and goods pics would be a big help.

[M Fowler]

I seem to have two horrible noises. One is the low end buzz and the other higher pitched one that increases when I increase the volume on the amp or guitars volume. I'm guessing I therefore have 60hz and 120hz hum going on.

O suspect filament imbalance: twisted heater wires is needed and use a balance pot 100R to 500R.

The higher pitched noise: try switching the OT primary wires to the power tube sockets. Out of phase issue.

Mark

[JazzGuitarGimp]

Deuce,

From what I've read, it seems the general consensus with regards to grounding the PI in a two-point ground system is to connect it to the power amp ground.

Also, with the speaker jacks grounded to chassis, it is of the utmost importance to connect the OT secondary's ground wire to the output jack, and not to the power ground point. The desired result is to not have any high-current paths traveling through the chassis, and the speaker ground is one if the highest-current paths in the amp.

Cheers,
Lou

[deuce42]

Thanks guys, I'll try to take some pictures and post them.

The schematic is just a normal AB763 but no second channel and diode rectifier instead of tube. Oh an the first cathode bypass has different values. The rest is the same AB763 schem.

I've tried to tie the PI filter cap negatative to the preamp ground with the same noise. Then tried to tie it to the power amp ground but no difference there.


I've reversed the order of the output xformer primaries at pin 3 of the power tubes but then a squeal happens so I suspect the original order was correct.

Have now tried twisted pairs of heater wire and the low end hum comes down a bit but still continues. The higher hum still continues as I turn up the volume. If I move to the side of the amp the noise comes down a fair bit but standing parallel it hisses and hums.

I've also connected the output xformer ground only to the speaker jack.

I'm just stumped for an answer to the problems

[Stevem]

First off all the wires that carry AC voltage within the amp need to be twisted as tight as you can get them, and the B+ wires with there high voltage need to be made as short as possible.
Getting the lowest levels of 60hz made noise is impossible without twisted wires and or real sheilding on them!
Second make yourself a test plug by taking a 1/4" Jack and shorting it out, plug this in and crank the amp, this is then the noise level of the amp itself!

Next get a peice of cardboard or wood that is big enough to cover the top of the amp and then tape aluminum foil on it to cover one side.
Place this covered side on the top of the amp , press down on it to make ground contact and report back on any improvements on the noise level .

Also note that it would seem that you are having a oscillation issue and this could be due to layout and or gain.

In high gain amps you need more stages of filtering and having more than two preamp gain stages on one filter node is asking for it!

In the stock fender they had the minimum amount of preamp filter node's in fact to me the amp was never made so that both channels could be used at the same time, even putting the phase difference issue aside in the two channel amps with reverb!

feeding your first gain stage thru another filter node of a small 2K resistor and a 20 uf cap may be the oscillation fix, but anyway it's something that should be done just because!

Also any two grid wires that are within 2 to 3 inches of each other and span 3 gain stages ( making them in phase with each other ) are likely candidates for the source of osscilation issues.
If two grid wires like this that are unsheilded run parallel with each other it can be issue time!
If two such wires need to get passed each other they need to do so at as close to 90 degree angle as you can work out.

[Davidg]

I second all the above suggestions and would add that u need to make sure u have your preamp nodes grounded correctly. Rule of thumb is u want anything going to ground after signal comes out of the plate goes to ground with cathode of the following stage.(grid leak or voltage dividers,etc...). Also if u are using pots as a ground buss u may need to isolate them from chassis??. (PICS help alot!).
Also may have some luck grounding PI to speaker jacks as well. How high gain are we talking here?? The more gain u get out of it the more precise ypur grounding scheme will have to be!! (Again, pics and schem help us to help u!)

[Structo]

Make sure you don't have the heater center tap and a artificial ground with two 100R resistors.
Use one or the other but not both.

Check to see if your heater wires are phased properly, that is, the same wire going to the same pin on each tube.

[ranjam]

For the high pitched 'noise', disconnect the negative feedback wire. If the noise disappears, reverse the plate leads of the output transformer.
For the low end buzz, and the squeal if it isn't solved already, make a small notch in a wooden dowel about the size of a pencil. Move some wiring around while the noise is obvious. If moving any wiring around make the noise louder or changed the pitch, you have a lead layout issue.

[lumox0013]

if your speaker jack is not grounded to the chassis that can be a problem. I used a cliff jack for the speaker once and had major issues till I ran a ground wire then all the sudden the thing was peachy. cheers