ab763 clone's volume is cutting out at high volume

[Dandrix]

hello:

I was biasing my new ab763 clone project and i discoverd at nine or ten on the volume that the amp would cut out and make a scratchy like distorted sound. The biasing current increased to -56 ma as well. The biasing current i was setting it too was 32ma, i'm using Tung-sol 6l6gc-str. Would there be a standard fix for this situation.


Maybe overdrive is causing the output tubes to go into grid blocking after being over driven, the signal causing a temporary bias shift?
could that be the problem? what would have to be done fix that?

I found this link but not sure if this is my root problem?
Blocking Distortion
thanks for the help !

[MWJB]

I suspect it is they way you have grounded or laid out some wires. Does it happen on both channels?

Can you post some pics of the circuitry?

[Dandrix]

hello MWJB

i have it on right now, i don't have a tube in v1 but, i will put one in and find out.
i was fooling around with the vol/treble knobs and noticed that, with treble at zero and the volume at ten. if i move the treble up to six it cuts out again. i'll have to recheck the wiring again.

[Dandrix]

MWJB,

normal channel works fine, put all knobs on 10. did not cut out.

vibrato channel volume on ten/ treble on zero / bass on zero

if you turn the treble to six it will cut out.
turn the bass up to ten, and turn treble to six it will make a farting sound.

[Dandrix]

[MWJB]

MWJB,

normal channel works fine, put all knobs on 10. did not cut out.

vibrato channel volume on ten/ treble on zero / bass on zero

if you turn the treble to six it will cut out.
turn the bass up to ten, and turn treble to six it will make a farting sound.

The Normal channel may not be as seriously affected due to having lower gain.

You have parasitic oscillation, as you dial in the highs the amp is feeding back at a frequency too high for you to hear...it isn't shutting down, it's working hard, you just can't hear it.

I can't see any shielded wiring from your input jacks to the preamp tubes, nor from volume pots to preamp tubes. Shorted &/or shield all grid wires running to 12AXT7 pins 2 & 7. Shorten the wires running to power tube pins 1, do not twist these wires together, you can shield these too if you like. Ground one end of the shielding only.

All connections may well be correct & match your layout, it's the physical run & length of wires that is the issue.

Describe your grounding scheme in detail.

The pics are really too small to be useful, better ones please.

[Dandrix]

[ATTACH=CONFIG]21687[/ATTACH][ATTACH=CONFIG]21688[/ATTACH][ATTACH=CONFIG]21689[/ATTACH][ATTACH=CONFIG]21690[/ATTACH]

[Dandrix]

MWJB sorry about the picuters size.

it came out to be that the physical run length for the wires going to the brite switch that is located in the rear of the chassis. i unsolder those connections and that solved the problem.
would like to use that switch down the road (or to complete my build). Am i out of luck? i did use inter connected wire, what else is a guy to do?

[Raybob]

... Am i out of luck? i did use inter connected wire, what else is a guy to do?

Shielded wire, grounded at one end only.

[MWJB]

"it came out to be that the physical run length for the wires going to the brite switch that is located in the rear of the chassis. i unsolder those connections and that solved the problem." Just checked in...yes, that was the first thing I was going to raise, once the signal has passed through the tone caps, any wire to from pots & bright switches is now grid wire for the following stage, too much length here is bad news.

[pdf64]

Consider a volume control that incorporates a pull/push switch eg Watford Valves :: Product - CTS 24mm 1 Meg Log Push Pull Pot for the bright cap.
Pete

[Dandrix]

"it came out to be that the physical run length for the wires going to the brite switch that is located in the rear of the chassis. i unsolder those connections and that solved the problem." Just checked in...yes, that was the first thing I was going to raise, once the signal has passed through the tone caps, any wire to from pots & bright switches is now grid wire for the following stage, too much length here is bad news.

update: i moved the bright cap to the volume pot. then, ran the the shielded wire back to the bright switch. i noticed the no difference in treble when switch up/down. does distance (to much length) have anything to do with this? any suggestions?

[überfuzz]

update: i moved the bright cap to the volume pot. then, ran the the shielded wire back to the bright switch. i noticed the no difference in treble when switch up/down. does distance (to much length) have anything to do with this? any suggestions?

You lost me now. Didn't you state that you found that the treb switch was funky due to wire length..? Hart to grasp that considering that you can't have more than, say, 25 cm of excessive wire, tops.

Ah well, some suggestions maybe: I never even touched my bright witches (always in bright mode).

[Raybob]

update: i moved the bright cap to the volume pot. then, ran the the shielded wire back to the bright switch. i noticed the no difference in treble when switch up/down. does distance (to much length) have anything to do with this? any suggestions?

Is that twin lead shielded cable, or two runs of single lead?

[Dandrix]

At first i only used push cloth wire to run my signal wires. The problem i was having in my first thread was due to that, ( length or NOT sheilded) had something to do with that i'm sure. alittle of both? Now i'm using Mogami W2330 Interconnect Wire to fix my original problem. and moved the 120pf bright cap to the volume pot. but, i noticed the no difference in treble when switch up/down. does distance (to much length, WIRE) have anything to do with this? any suggestions?