Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: princeton65 on May 17, 2011, 09:27:42 pm

Title: Silverface to Blackface Reverb Circuit
Post by: princeton65 on May 17, 2011, 09:27:42 pm
What are the differences between the reverb circuits in silverface and blackface amps?  I have a blackfaced 79 vibrolux reverb and I just cant seem to get the reverb to be "reverb-y" enough.  Is it not a matter of changing a few resistors and caps to go between the two styles?  Any help would be greatly appreciated.

Matt
Title: Re: Silverface to Blackface Reverb Circuit
Post by: sluckey on May 18, 2011, 05:21:46 am
Reverb circuits are identical. Maybe your tank isn't all it could be? See the 470K resistor connected to the wiper of the REVERB pot? Change it to a 220K or 1ooK for more reverb.
Title: Re: Silverface to Blackface Reverb Circuit
Post by: Fresh_Start on May 18, 2011, 09:17:23 am
Contact cleaner on both ends of the reverb cables.  Try different tubes for the reverb driver (V3 - 12AT7) and reverb recovery (V4 - 12AX7).  As sluckey said, some reverb tanks just have more 'verb than others.

HTH

Chip
Title: Re: Silverface to Blackface Reverb Circuit
Post by: princeton65 on May 18, 2011, 01:46:46 pm
I will try some different tubes, I hit the RCA jacks and plugs with deoxit/scrubbed, and clipped off the .002 capacitor to ground on the reverb output jack.  Any knowledge on what the 25uf electrolytic from pin 3 of v5 to ground and the 220k resistor from the reverb output jack to the reverb pedal jack are for?  Do these affect the reverb tone?  I wouldn't say that the range of the reverb control is not sufficient, just that the character of the reverb is weak, and gets lost easily.  I need to get a fuse to fire it up and check it out after clipping that cap to ground.

Title: Re: Silverface to Blackface Reverb Circuit
Post by: Fresh_Start on May 18, 2011, 02:36:27 pm
It sounds like you have something similar to this 1977 Deluxe Reverb:
http://www.thevintagesound.com/ffg/schem/deluxe_reverb_10-6-77_schem.gif

Regardless of which Silverface circuit you're working with, I suggest that you find the right one for your amp and compare it to a Blackface AB763.  The schematic collection here is great (see top of page), but the Fender Field Guide might be helpful too:
http://www.thevintagesound.com/ffg/

Look at the cathode resistor on the reverb driver triodes.  Is it 2.2K with a bypass cap or 820 ohms without a bypass cap like that D.R. linked to above?  That could make a big difference in the amplitude and frequency spectrum of the signal going to the reverb tank.

It might be worthwhile to check voltages throughout the reverb send & return circuit.

HTH

Chip
Title: Re: Silverface to Blackface Reverb Circuit
Post by: princeton65 on May 18, 2011, 03:54:24 pm
2.2k with bypass cap, so I guess that is blackface spec. 
Title: Re: Silverface to Blackface Reverb Circuit
Post by: sluckey on May 18, 2011, 04:36:11 pm
Quote from: princeton65 on May 18, 2011, 03:54:24 pm
2.2k with bypass cap, so I guess that is blackface spec.  
All vibrolux are the same.
Title: Re: Silverface to Blackface Reverb Circuit
Post by: Fresh_Start on May 18, 2011, 07:04:10 pm
Quote from: sluckey on May 18, 2011, 04:36:11 pm
Quote from: princeton65 on May 18, 2011, 03:54:24 pm
2.2k with bypass cap, so I guess that is blackface spec. 
All vibrolux are the same.

My bad!  I saw the Blackface Reverb in the title and missed VIBROLUX!!!

Sorry,

Chip