AB763 lite w/o trem/rev - european version

[Guitar-Tube-1969]

Do you have any advices for biasing this amp? I',m a little bit confused from the information I read...

[sluckey]

Read this...

     http://www.duncanamps.com/technical/lvbias.html

[Guitar-Tube-1969]

Ok, thanks for the link!

I tried it, the amp was on for about 2-3 hours now. The bias values are:

1st measuring:

V3/3: 411 VDC
V3/8: 25,0mV
=> 411*0,025=10,3W

V4/3: 411 VDC
V4/8: 24,0mV
=> 411*0,024 = 9,9W

2nd measuring in between:
V3/3: 408 VDC
V3/8: 25,0mV
=> 408*0,025=10,2W

V4/3: 408 VDC
V4/8: 23,05mV
=> 408*0,0235 = 9,7W

3rd measuring at the end (same as the first):
V3/3: 411 VDC
V3/8: 25,0mV
=> 411*0,025=10,3W

V4/3: 411 VDC
V4/8: 24,0mV
=> 411*0,024 = 9,9W

Because 6V6 has a max. rating at 12W (hot), it seems ok, isn't it?

-------------------

2nd question: I want to add an extension speaker jack.

The wiring of the main speaker jack at the moment is:

G: wired to the black cable from the OT and to the ground
S: wired to G
T: Green wire from R24 (820 Ohm) and green wire from OT (in my example the three wires for 4, 8, 16 Ohm goes to an impedance switch and one green cable from there to the tip)

Could I use another jack and wire it as follows?

G: wired to G of the main speaker jack
S: not connected
T: wired to the tip of the main speaker jack

Then they shoud be parallel wired and I could:
- Two 8 Ohm speakers, impedance switch at 4 Ohm
- Two 16 Ohm speakers, switch at 8 Ohm

Is that right?

[sluckey]

Because 6V6 has a max. rating at 12W (hot), it seems ok, isn't it?

The info I have says a 6V6 P_AMAX is 14 watts.

Your thoughts about adding a second speaker jack are correct. Just remember, if you only plug into one jack it has to be the main jack.

[Guitar-Tube-1969]

Great, thanks!

Would you recommend to bias them a bit hotter, e.g. 12 Watt?

[sluckey]

You are biased at 70% now. That's what I have my fixed biased amps set for. But you can bias hotter if you like the sound. If so, you may have to change tubes more often.

[Guitar-Tube-1969]

Hey all,

I've got a problem with my power tubes. Both tubes show  a silver film at the inner glass (see attachement). The tubes were new and only have about 20h playing time. What's that?

In addition the internal speaker lets bell my rectifier tube on a certain frequency. It disappears when using an external speaker, so it seems to be a physical problem, any ideas?

Thanks - Axel

[PRR]

(Nearly) ALL tubes have that silver splash. Yours did from new, right?

It is "getter". A monoatomic film of thirsty metals to absorb the last traces of gas inside the vacuum.

Related question:

In time (decades) the getter can turn brown. As long as there is ANY silver left, it is still doing its job. Throwing-out a tube just because its getter is 10% brown around the edges is like throwing out a soap-bar when 10% is gone (and 90% of soap is left).

[Guitar-Tube-1969]

Great, thanks!

Do you have any idea, to prevent the tubes (either rectifier or power) to resonate at a certain frequency?

[Guitar-Tube-1969]

Here is a final foto with faceplate and cab - Thanks to all of you!!

[sluckey]

It's possible that the particular tube just resonates (rings) at a certain frequency. Since it only does it with the internal speaker I would try another tube first. This is called sympathetic vibration. I've seen silicone rubber rings for small tubes that help. Maybe the same rings are available for larger tubes. Can you stop the ringing by holding the tube with an oven mitt or pot holder?

[sluckey]

That amp turned out very nice! 

[Guitar-Tube-1969]

Hi Sluckey,

sorry for my delayed answer but we play a short gig and I used the amp. Sounds good!

To the resonance problem: The ringing stops when I'm holding the tube. It stops too when using an external cab only. So it seems to be a physical problem, isn't it?

Axel

[Guitar-Tube-1969]

and: thanks for your compliment!!

[Guitar-Tube-1969]

Hey all!

I've uploaded a video on youtube with a clean sound example (first solo, at 1:40 with a backing track). The video contains some photos from the final assembly.

Have fun - Axel

https://www.youtube.com/watch?v=j9rUS5LV6r4

[sluckey]

Nice playing! Did those rings on the octal tubes solve the 'ringing' problem?

[Guitar-Tube-1969]

Thanks.

To be honest, I'm not sure. I've changed the rectifier and power tubes to JJs and did not check it without the rings. The is no ringing at all the moment and I'm happy.