Need reverb circuit troubleshooting help AB763

[Planobilly]

Hi Guys,

AB763 2 Channel Bright switch and Mid Pot layout with no mods except added master volume and 25uf 50V cathode caps.

The issue...The hum increases from normal (very little) to a good bit as the reverb control is increased.

The reverb tank is disconnected.

V3 A and B plate voltage=425VDC
V3 A and B cathode voltage=7.7VDC
With no signal there is 5.5 mv AC at the grids
B voltage=427VDC with 116 mvAC ripple

V4 A  Plate=180VDC
V4 A  Cathode=1.3VDC

The reverb transformer is grounded to a ground along the back of the chassis and grounds the reverb jack, tremolo jack and output transformer.

I have checked and doubled checked all the wiring, changed tubes, checked all component values, resoldered everything without results.

Any ideas on what to do next?????...lol


Cheers,

Billy

Current photos

[PRR]

> The hum increases ...as the reverb control is increased. The reverb tank is disconnected.

Isn't that normal? You are turning-up the gain on an open (nothing connected) input.

[Planobilly]

Hi PRR,

The hum increases as the reverb control is turned up with or without the tank being connected.

I am going to change the heater wires next and place them in a vertical position to reduce the over all length. The distance from the tubes to the back of the chassis is larger than normal and any benefit from placing the wire along the chassis is offset by the longer length of the wire.

 I am beginning to think that this hum may be directly related to the way I have the heater wires twisted and placed. Also the ground scheme I used is not like the original Fender design which may also contribute to the issue. 

[eleventeen]

Before you take that all apart, it's my belief that heater wire dress is remarkably NON contributive to hum. Now...it is possible that SOME OTHER [signal] wire is running close to your heater string (which looks fine to me, to the extent I can see it) in a way which is inducing hum, but we have seen probably every variation possible of heater wire here and my impression is that if it is done soundly (no bad solder joints, no excess 6" loops running around all over the place) it doesn't make much if any difference in a guitar amp. It *may* in a gourmet high-fi amp.


With no heater center-tap, real or dual-100 ohm, virtually ALL tube amps will hum grossly, totally unacceptable, not subtle at all. We know this. It's the way it is.


I have been fairly impressed with the efforts of folks who have by one means or another (usually meaning, make another bias supply except ~~20-40 volts positive going instead of negative going) to raise the heaters. Such a feed would go to the midpoint of your 2 x 100 ohm resistors.



I am just making this comment to possibly save you the trouble of taking all that apart. It doesn't look bad to me at all. The "ground scheme" as you mentioned is 100x as likely to cause hum relative to heater lead dress. IMHO.

[sluckey]

The reverb recovery circuit is the most sensitive circuit in that amp. It needs a good ground. You have your reverb jacks isolated from the chassis then the jack grounds are connected to a ground buss that also shares the speaker ground and then heads to the power end of the chassis. That's bad!

Separate the reverb jacks from that "power" ground bus. Just cut the buss at the "X". Then remove those insulating bushings from the RCA jacks so the jacks can provide the chassis ground connection for the two RCAs and the Reverb footswitch jack.

The Tremolo footswitch jack should be OK connected to that "power" buss.

Hopefully this will be all you need to do to minimize the hum. Fender does call for a 7025 (low noise 12AX7) for the recovery tube. I had to roll several 12AX7s through that socket to get the quietest one. I finally gave up and just used a 5751.

I'm surprised to see two cliff jacks used for the footswitch connections rather than Fender's typical RCA jacks or even a two circuit (stereo) phone jack. What are your thoughts about that?

[Planobilly]

Hi Steve,

Thanks for the advice. I will make the changes you recommend.

What I was experimenting with so far as the Cliff jack and grounding scheme in general, was to isolate everything from the chassis and only have two connection points. One would be the AC safety ground and one would be one central ground point.

I then had in mind to place as many separate buss bars as would be needed to prevent ground loops. I obviously do not know what I am doing because it is not working.

The reason for all this is to learn how to build very effective ground systems.

I have built a good many small low wattage Fender style amps ( Bassman, Deluxe, Vibrolux and 18 watt Marshall) without any issues.

My attempts to build more complex amps up to 100 watts have been failures due to issues of hum and hiss. The amps sounded good and did what I wanted if I could just get the issues of hum and hiss under control.

Perhaps the approach that I have taken is simply not workable compounded by the fact I only started doing any sort of electronics about a year ago.

I have a good amount of stock of new JJ tubes. I totally get your comment about about the 5751. It would be really wonderful if we could buy new 7025 tubes made to the quality of the best years of the RCA production. Well...it is what so we all have to deal with what is available.

Cheers,

Billy


Note: Eleventeen was correct about the heater wiring. I re-did the heater wiring which only reduced the overall hum by a small amount. Here is a photo of the new heater wiring.

[Planobilly]

Update

I made the changes that Steve recommended. No change. I also ran a ground wire from the reverb jacks directly to ground at the star with the RCA jacks still isolated from the chassis with no change.

I changed V4 from a 12AX7 to a 5751 with no change.

I remover the HT fuse and connected the center tap directly to the filter caps. That reduced the hum about 5%.

I replaced the 16uf B voltage filter cap with a 33uf which reduced the hum by another 5%. The ripple went from 116 mv to 65mv.

I don't know what uf value would be needed to reduce the ripple to say less than 5mv or what other negative effect that could produce.

Connecting the isolated buss bars to the chassis in various placed seems to have no effect.

There are two carbon comp resistors on the board because I ran out of metal film of that value, one is the 3.3M mixing resistor between V2 and V4.

My idea of using isolated ground buss bars seems to not make any difference one way or another.

The overall noise in the basic circuit is acceptable, and the reverb noise is now down to a level that is more or less usable. Perhaps there is some other condition I have missed. Perhaps the way I twisted the heater wire is less than the best......

On a brighter note Fedex just delivered the new speakers I ordered from Weber. Be interesting to see how they sound!

Cheers,

Billy

[Williamblake]

Maybe i got this wrong but when you turn the reverb recobvery pot up and your hum increases, do you have the grid of the reverb recovery connected to anything?

I am ressurrecting this thread because of reverb noise, the less you have the better your reverb is. What i found out so far is that the placement of the tank is key and maybe add shielding if your available place is that small. But there is only so much you can do and for the initial question  this doesnt even enter into it. Next best thing i found out is adjusting the reverb recovery cathode bypass and coupling capacitor to cut off low frequency is very pleasant too. Then i use two tanks to get some humbucking effect. And i too think Heater elevation is a must on any amp. If you also tinker how much base your springs can take you will probably have good reverb.  I have a base cotrol at the dwell and that is really nice. Never hesitate to chopstick it all if you have bad hum of course. And connect the reverb chassiss to your chassis an the reverb output to circuit 0V (ground).
That all being said i wonder why the reverb transformer output is traditionally left floating and not referenced to ground. I have already read on this nice forum you should but it doesnt make a big difference.

[sluckey]

That all being said i wonder why the reverb transformer output is traditionally left floating and not referenced to ground.

It's not floating on the old Fender amps we love to clone. That RCA jack is solidly mounted directly to the chassis and provides a very good ground.

[silverfox]

Rectifier tube? I didn't see any diodes and it looks like tube rectifier at the secondary. If it were SS Rectifiers I always add, can't remember how many pf, disc capacitors across the diodes to filter spikes, but I don't think that applies to a tube rectifier.


Your wiring looks good compared to many circuits that are very quiet yet have absolutely abysmal wiring dress. Some of the old Fenders are really atrocious but satisfactorily quiet.





silverfox.