no sound from an AB763 clone

[chris shaw]

progress on ab763 build big hum

correction re v1 pin 1 plate current-- the voltage drop across the 100k dropper is about 120v so current is 1.2mA as expected
in answer to grounding questions - preamp filter cap is grounded at input ground, as are pots, and preamp cathodes, tried grounding PI at PT, half way along controls and input, big hum remains
DONT KNOW PLATE CURRENT TO POWER TUBES-- DONT KNOW HOW TO MEASURE IT!
most important/ relevant info - when I lift the output from tube 1 at the 250 pf tone stack cap and connected to the inverter input - the hum remained,, -- I then completely disconnected the input at the PI and the hum remained-- so its in the PI or the Power section!!!
its buzzing but if i turn it up to 1 or 2 it turns into a moog growl, which dissappears when I put a signal through

heeeelp please!

[MWJB]

"DONT KNOW PLATE CURRENT TO POWER TUBES-- DONT KNOW HOW TO MEASURE IT!"

Add 1x 1ohm 2W resistor between each power tube pin 8 & ground, make sure resistors are exactly 1 ohm, make sure you have a good ground connection. Read the voltage at pin 8 in mV <200mV range, reading converts directly to mA. Should be less than 30mA per tube (around 20mA per tube if using Russian Reflector/EH/Tung Sol 6V6s).

...or buy some bias probes, they'll save you time & money in the long run.

[MWJB]

Did you use 2x100ohm resistors as a virtual centre tap for the heaters, or did you use a heater winding centre tap on the PT?

[chris shaw]

use of heater winding centre tap

hi MWJB
used red/ yellow wire from hammond transformer as centre tap
Thanks for answer re resistors for current, will look to find bias probes for sale in UK

[bluesfreak]

Chris, you can get a probe kit at Tube-Town - Hot Stuff cool sounds. However, installing a 1 Ohm Resistor between Pin8 and GND on the power tubes is basically the same and it will tell you (based on ohms law) the exact current in mV (e.g. 48mV equals 48mA). There is also a real good bias FAQ in the Info/DIY section of tube-town...

[MWJB]

"used red/ yellow wire from hammond transformer as centre tap" that's the B+ centre tap, for the high voltage DC feeding your OT & tube plates & screens.

I'm talking about the 6.3VAC heater supply, usually a green/yellow wire grounded with the red/yellow B+ CT, or uses 2x100ohm resistors - one grounded from either side of the heater wiring (normally at the pilot lamp).

[chris shaw]

thank you

mwjb, blues freak and pdf64

thanks to you all the amp is beautiful, I have learnt a lot and made some new friends . I am in debted to you all (especially mwjb) and if you were here I would kiss every one of you
the issues have been,
wiring power tubes (?!!)
input resitor but most important
a centre tap to the heater supply - which webers 240 v replacement transformer doesnt have , but I cant see where the need to create one is made clear, and for beginners like me thats a major omission !!!
I put one in , put 1 ohm resitors on output pin 8 and voila - I have a hum free beauty with a bias at about 22m.a = 22mv

(its about 2 mV different between the tubes - have no idea how to correct that )
Im really pleased and proud
thank you all
ps am a jazz player with a love of sweetness -- what to build next - any ideas?

[pontiacpete]

Congratulations, Chris!
way to go....

[chris shaw]

annoying minor niggles

guys
boy this thing is loud
however I have 2 niggles I need help solving
the first is a 'farting' noise under the bottom notes - going from e to about c . when I play a bottom E it runs underneath for a couple of seconds , whereas by the time I get to c it last less than a second. It isnt vibration, I can hear it in the speaker. Ive tried varying bias but no help, happens even at low volume, never on any notes above the low c of an electric guitar-- any ideas?
the second is that the reverb starts to 'screech/ high pitched whistle if I take it above about 25% of full-- almost like high feedback
I have fastened the reverb can onto the bottom of my home mage sapele cabinet -- no isolation sponge or anything -- should i try and isolate it or is it something more 'electrical ?
I know youll solve it!