Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SILVERGUN on May 14, 2020, 08:50:48 am

Title: Fender reverb volume boost
Post by: SILVERGUN on May 14, 2020, 08:50:48 am

2deaf posted this (https://el34world.com/Forum/index.php?PHPSESSID=a69quo58b3enkb4ikc7a8a7u53&topic=25955.msg282224#new) schematic and I perused it to find the actual "volume boost" function and just figured I would post this as a mod for the typical Fender reverb circuit.
I figured if it was new to me that there are possibly others who might find it interesting.
Thanks for posting 2deaf.
Title: Re: Fender reverb volume boost
Post by: 2deaf on May 14, 2020, 01:32:09 pm
The boost is the same as the DELUXE REVERB-AMP drawing number 014641. 
Title: Re: Fender reverb volume boost
Post by: mresistor on May 14, 2020, 02:30:21 pm
Well    I just plugged into my 81 BF VR and I'll be darned..    that boost sounds pretty good..   it does what it says it does and boosts the volume up considerably.    It even has a 1/4" boost pedal jack on the back of the chassis.    Could be very useful in the right setting ..


If anyone has a schematic of the 1980-82 Vibrolux Reverb sharing it would be appreciated..  it's not in the schematics here..     I suppose by the way it sounds that the circuit is just like the one 2deaf has posted up ..
Title: Re: Fender reverb volume boost
Post by: Dave on May 21, 2020, 03:53:05 pm

That's very interesting. I wish I could visualize why it works.


Dave
Title: Re: Fender reverb volume boost
Post by: SILVERGUN on May 22, 2020, 09:48:12 am
Quote from: Dave on May 21, 2020, 03:53:05 pm

That's very interesting. I wish I could visualize why it works.


Dave

I'll throw a dart at it.... It appears to be more of a switchable signal dumper than an actual boost circuit.


It looks to me like the switch is normally closed and some of the signal in the reverb path goes around the xfmr, pan and recovery stage and only sees 1K to ground, so this is the "non-boosted" mode.
When you open that switch the path is more like the normal reverb circuit and you get a volume "boost"
Title: Re: Fender reverb volume boost
Post by: Dave on May 22, 2020, 10:13:31 am
Oh, I see it now. I just had to look at it after a night of sleep. Normally, the entire signal from the reverb transformer is piped through the reverb tank and then out to the recovery stage. The switch is in parallel to the tank connection and takes part of that signal and adds it to the recovery stage output for a boost.
I bet you could play with that 1k resistor and get a big old boost out of it. Might sound horrible, but I bet you could.
Dave
Title: Re: Fender reverb volume boost
Post by: SILVERGUN on May 22, 2020, 10:55:08 am
Well, now we've got 2 Dave's with 2 different explanations so I have to hope one of us gets corrected.
Title: Re: Fender reverb volume boost
Post by: Dave on May 22, 2020, 11:28:07 am

Sorry brother, I wasn't trying to get into a competition. I just read your post and went back and looked at it with fresh eyes and saw something different.


Dave
Title: Re: Fender reverb volume boost
Post by: SILVERGUN on May 22, 2020, 11:51:04 am
Quote from: Dave on May 22, 2020, 11:28:07 am

Sorry brother, I wasn't trying to get into a competition. I just read your post and went back and looked at it with fresh eyes and saw something different.


Dave

You're ok with me Dave...I forgot to put the smiley face on the end of my last post.
I don't mind being wrong and I feel lucky to be in a place where I can get corrected when needed. At least I'm not being wrong in the back of an ambulance.
No competition here.  :thumbsup:
Title: Re: Fender reverb volume boost
Post by: shooter on May 22, 2020, 12:20:29 pm
Quote
me Dave

 :laugh:

Doug needs a Dave filter  :icon_biggrin:
Title: Re: Fender reverb volume boost
Post by: mresistor on May 22, 2020, 12:28:22 pm
No offense  but I'm going to go with Daves answer..  I think it diverts the reverb drive signal and mixes it back into the dry as increased gain.

Title: Re: Fender reverb volume boost
Post by: SILVERGUN on May 22, 2020, 01:00:35 pm
Quote from: mresistor on May 22, 2020, 12:28:22 pm
No offense  but I'm going to go with Daves answer..  I think it diverts the reverb drive signal and mixes it back into the dry as increased gain.

No offense taken...I'm just stuck on the idea that 1K is the path of least resistance
Title: Re: Fender reverb volume boost
Post by: Dave on May 22, 2020, 01:06:44 pm

That 1k... the way I see it is just dumping most of the "boost" signal to ground because without it, the boost would be gargantuan.


Dave
Title: Re: Fender reverb volume boost
Post by: mresistor on May 22, 2020, 02:02:53 pm
Notice the elimintation of the bypass cap and the lower value cathode resistor on the reverb driver. Some people have said that the boosted signal can damage the reverb transformer over time..   humm.. 
Title: Re: Fender reverb volume boost
Post by: PRR on May 22, 2020, 02:48:50 pm
> That 1k... the way I see it is just dumping most of the "boost" signal to ground because without it, the boost would be gargantuan.

The 1k is across the ~~5r tank driver winding. It's got little function.

The "gargantuan" boost is moderated by the ~~55:1 ratio of the reverb transformer.

Full understanding involves knowing why the fancy-channel on these amps has an added gain stage and that odd 3.3MEG resistor; not everybody wants to think this hard. (Too hot here.)

The reverb driver plate is the *third* stage of gain from the guitar. With high-gain tubes, the 3rd stage is pretty sure to peak in overload. Probably >200V peaks! 4V peaks at the RT secondary. Maybe less in bass/mids due to the 500pFd and tank loading. The 220k into 470k||3.3Meg cuts this down slightly, slams V4A grid.