From billbolto–(at)–naustralia.com.au Fri Apr 25 23:40:43 CDT 1997
Article: 47089 of alt.guitar.amps
From: billbolto–(at)–naustralia.com.au (Bill Bolton)
Newsgroups: alt.guitar.amps
Subject: Re: 6L6GTA Bias in SF VR??
Date: Fri, 25 Apr 1997 23:55:36 GMT
Reply-To: billbolto–(at)–naustralia.com.au
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nielsen–(at)–cmail.orst.edu (Simply Steve) wrote:
> And I am still looking for a schematic for it. The one Lee was kind
> enough to fax me (for free!) is, alas, unreadable.
Specific Vibrolux Reverb schematic differences I have found by
observation between between the SF AA270 schematic and a ’81 Blackface
II are:
1. Voltage on centre tap of OT = 451V
2. 2K2 decoupling resistor in B+ after choke instead of 4K7
3. 330K resistors on phase splitter grids instead of 1M
4. 680R resistor of phase splitter cathodes instead of 470R
5. 2000pf cap across the 220K grid resistor on the reverb return
6. The volume boost stuff, adds a 10K resistor between the ground
end of the 220K on the grid to the driver tube. A 1K resistor
connects to this junction of the 10K and 220K and goes to the pull
boost switch. The other side of the pull boost switch goes to the
secondary of the reverb OT. A footswitch, which shorts to ground,
also connects to the junction of the 10K and 220K resistors.
7. 100K1W pre-amp late resistors in place of 1/2W
8. Miscellaneous measurements at idle after cap replacement:
After 2K2 decoupler = 422V
After 10K decoupler = 356V
Junction of diode and 50uF in bias supply = 56V
Centre tap of bias balace pot = 45V
Grids of 6L6 = 41V
Bias current = 31ma
Cheers,
Bill
Bill Boltonbillbolto–(at)–naustralia.com.au
Sydney, Australia
