deluxe AB763 problem

[shagual]

Hi,

I just finished a deluxe reverb AB763, did check up all the voltages and I'm everywhere between 5% of the original, reverb works, vibrato works etc.

I've been measuring it's power with a 1KHZ 100mvac signal and it pulls out approx 32 watts on both channels. (bass & treble controls around 6)

B+ voltage is 420 and tubes are biased around 22mA.

One thing I noticed is the serious drop of output on the vibrato channel when I turn up the treble around 8 and higher with the volume on 10.

Anyone an idea?
Maybe the brightcap?

Thanks!

[Amp Kat]

That's called sag and it happens to all amplifiers almost out there or at least all Fenders. Some say and use it as compression or whatever you want to call it but it can desirable to some at that point.

[shagual]

I'm having serious drop of output, like going from full power to zero when just turning the treble a bit up.

[MWJB]

You said that you have 32W "on both channels? Are you saying that your 32W measured reduces as you turn the treble up, or that it is just an audible effect? With the volume at full clockwise rotation the bright cap is shunted by the pot, so has no effect.

Pics of the build please.

[shagual]

indeed, I measure approx 32 watts on both channels.
Turning up the treble cuts off all signal.

I'll check the wiring of the tonestack this evening.

[pdf64]

You may be getting ultra-sonic oscillation - have you got an oscilloscope?

[MWJB]

If you don't have a scope, you might still be able to detect oscillation by a hike in plate current (despite a lack of sound out of the speaker) & a drop in plate voltage.

[shagual]

just did some tests.

I was inserting a 1KHZ signal to measure the amps power.

when pushing the treble from 8 to 10 (volume was on 10) , I measured my ouput frequencys were shifting from 1KHZ to 5KHZ with a loss of wattage on the dummy load.
Did some adjustments on the wiring of V2s grids and the problem disappeared.

I'll be installing some shielded wiring tomorrow.
Any further advice?

Thanks

[1ampman]

Do you have any pics of your layout???

[shagual]

http://users.telenet.be/pierssens/ab763/DSC00055.JPG