What are some mods to get a little more gain from a Fender?

[cgoodson]

I built a 60's style Fender Vibrolux Reverb and would like the vibe channel to have just a bit more gain; it seems a little wimpy to me. What are some good ways to go about this?

BTW: Hello everybody, this is my first post on here.

[Smitty]

By more gain do you mean more voltage amplification (traditional interpretation) or more overdrive (popular misinterpretation)? Louder or raunchier? If you want more breakup at lower volume then you may want to try a 12ax7 in the phase inverter. If you like that sound and want to keep it, then try replacing the cathode resistor with an 820R or 1K.

What is your negative feedback setup. 820R into 100R? You may want to try a soul control. Replace the extension speaker jack with a 10K pot to control the amount of NFB.

Removing V1 will give you a little more gain in V2.

What are your plate and cathode voltages and resistors for V1, 2, 3 and 6?

Smitty

[Don Symes]

The quick/simple way to add a little gain is to pick a triode - the tone stack recovery stage, for instance - and raise the value of that plate resistor by a couple of standard values (100k, raise to 120k or 150k), and maybe raise that stage's cathode cap by a bit (this may add too much bass/flubbiness).

Actually, that's kind of a sensitive stage, so maybe the FIRST triode instead.

Before that, though, make sure the vibe's light source (neon bulb, LED) is really going all the way dark and the LDR element is going to its full 'dark' resistance. You may need to fiddle with the 100k in series and the 10M and 0.022uF components in parallel to get there.

Confused, but I hope this helps!

(and welcome!)

[cgoodson]

By more gain do you mean more voltage amplification (traditional interpretation) or more overdrive (popular misinterpretation)? Louder or raunchier? If you want more breakup at lower volume then you may want to try a 12ax7 in the phase inverter. If you like that sound and want to keep it, then try replacing the cathode resistor with an 820R or 1K.

What is your negative feedback setup. 820R into 100R? You may want to try a soul control. Replace the extension speaker jack with a 10K pot to control the amount of NFB.

Removing V1 will give you a little more gain in V2.

What are your plate and cathode voltages and resistors for V1, 2, 3 and 6?

Smitty

I'm thinking in the more traditional way, I'm not really looking to make a distortion box out of it. Just a little more punch.

I've been planning to add a "soul control", I've played around with negative feedback before and I like some of the tones. It's been a few months since I finished this amp but I think I went with 820 Ohms, I'm sure it was whatever the schematic showed and I think thats the usual for the 60's Fenders. If I put the pot in series with the 820 when I turn it all the way down that will give me the "stock" sound....won't it? Is 10k the prefered value for that?

Another thought- I've never tried more negative feedback maybe I should lower the 820 when I add the soul control. Would it be a bad idea to mount this in place of the ground switch? It's not like I have it hooked up or anything, but would this add noise considering it would be closer to higher voltages?

As far as my voltages go, I plan to crack it open tomorrow(I need to use it today) and I'll check them out and post.

[cgoodson]

The quick/simple way to add a little gain is to pick a triode - the tone stack recovery stage, for instance - and raise the value of that plate resistor by a couple of standard values (100k, raise to 120k or 150k), and maybe raise that stage's cathode cap by a bit (this may add too much bass/flubbiness).

Actually, that's kind of a sensitive stage, so maybe the FIRST triode instead.

Before that, though, make sure the vibe's light source (neon bulb, LED) is really going all the way dark and the LDR element is going to its full 'dark' resistance. You may need to fiddle with the 100k in series and the 10M and 0.022uF components in parallel to get there.

Confused, but I hope this helps!

(and welcome!)

"Before that, though, make sure the vibe's light source (neon bulb, LED) is really going all the way dark and the LDR element is going to its full 'dark' resistance." - Oh crap, thats makes a difference, I definately need to check it then. What's the symptoms if it isn't?

As far as raising the value of the plate resistor in a stage; is there there a "best" stage to do this, or would it be better to raise them a little in all of them for that particular channel? I think I'm going to do this one for sure, I just want to know I'm going about it the best way.

[cgoodson]

One other place I remember reading about(Gerald Weber's book I think) that was a possible "more volume" tweak is the resistor that controls how much signal goes to the reverb circuit. I rarely use my reverb knob past 3, it's usually on 2. Any thoughts on this? Will it effect the quality of the reverbs tone if I mess with this? What would be a good value to replace this resistor with? What's the answer to the ultimate question of life, the universe and everything? :eek: Sorry, I got carried away...

[Smitty]

You could put the soul control in the ground switch location. I've done that before. Just be careful with the lead dress.

You can lower the value of the 820 ohm resistor with the Soul Control. You just won't be at a stock setting with the knob turned all the way down.

If that circuit is working properly and you want a little more volume, you may want to swap in a Bassman output transformer.

What speakers/impedance are you using? Efficient speakers are like free watts.

[misterdontmove]

This mod works well. It's subtle but allows the amp to transition into overdrive very smoothly. I have it mounted in the ext. speaker jack of my Super Reverb (i never use that jack anyway). You could also disable the trem if you dont use it ! The mods listed are great because they each add a little bit of grind (season to taste so to speak). Just do one at a time so you can work back a little bit if you need to .

misterdontmove

[Ray Ivers]

Welcome!

To add to the other suggestions... you could get one of the 50K reverse-audio pots w/switch that Ted Weber sells at webervst.com, and use it to replace your 'Intensity' pot, then move one pot wire to the switch and run a short jumper from the switch to the pot. This mod - along with removing the negative feedback - are the only mods I've done to my '66 VR, and together they made for a huge sonic improvement IMO.

If either/both sound like something you'd be interested in trying (they're both very simple) post back and we'll take it from there.

Ray

[Don Symes]

"Before that, though, make sure the vibe's light source (neon bulb, LED) is really going all the way dark and the LDR element is going to its full 'dark' resistance." - Oh crap, thats makes a difference, I definately need to check it then. What's the symptoms if it isn't?


The main symptom would be that the vibe channel isn't as loud as the normal channel. The test is to simply clip the wire that runs from the 'clockwise' terminal of the intensity control to the 220k/0.1uF node near the triode just to the left of the PI - I'm using the AA270 Vibrolux Reverb schematic from the
Fender Field Guide (http://www.ampwares.com/ffg).

As for which triode, I'd pick either the first of second rather than messing with more than one.

HTH!

[cgoodson]

"The main symptom would be that the vibe channel isn't as loud as the normal channel."

That's exactly how my amp is. I assumed that this was a normal thing because of the reverb and tremelo circuits getting some of the signal. I guess I need to work this out before I go modding eh?

[cgoodson]

What is your negative feedback setup. 820R into 100R? You may want to try a soul control. Replace the extension speaker jack with a 10K pot to control the amount of NFB.


What are your plate and cathode voltages and resistors for V1, 2, 3 and 6?

Smitty

I happened to have a 10k pot w/switch built in, so I added a soul control and then wired the switch to disconnect the feedback alltogether. I'm really having a hard time hearing much of a difference at any setting, except with no feedback.

BTW, I built this following the AA964 schematic in an early 70's Fender chassis that was so hacked that I couldn't figure out what it was. I'm using all the proper tubes according to the layout. All plate resistors are 100k 1% metal, except for V6's PT2 resistor; which is 82K.

Here's my voltages-
V1
PT2=191V
KT2=1.29V
PT1=194V
KT1=1.35V

V2
PT2=188V
KT2=1.33V
PT1=196V
KT1=1.35V

V3
PT1&2=411V
KT1&2=8V

V6
PT2=173V
KT2=78.3V
PT1=173V
KT1=78.3V

[cgoodson]

"The main symptom would be that the vibe channel isn't as loud as the normal channel."

That's exactly how my amp is. I assumed that this was a normal thing because of the reverb and tremelo circuits getting some of the signal. I guess I need to work this out before I go modding eh?

Quoting myself:p

I think its actually okay here, I looked at the LDR in the dark and didn't see any signs of it lighting up; it's very obvious when it does. Also, after comparing the channels, I don't think the volume difference is that drastic. I did however try disconnecting the tremelo, and that does yield a nice boost, the problem is that I'm one of those wierdos who likes tremelo(especially with some sort of modulation pedal running with it). I might end up getting one of the pots Ray mentioned.

[cgoodson]

Welcome!

To add to the other suggestions... you could get one of the 50K reverse-audio pots w/switch that Ted Weber sells at webervst.com, and use it to replace your 'Intensity' pot, then move one pot wire to the switch and run a short jumper from the switch to the pot. This mod - along with removing the negative feedback - are the only mods I've done to my '66 VR, and together they made for a huge sonic improvement IMO.

If either/both sound like something you'd be interested in trying (they're both very simple) post back and we'll take it from there.

Ray

Do you run yours without any negative feedback at all?

[Ray Ivers]

Do you run yours without any negative feedback at all?

Absolutely - I do this with pretty much all my tube amps nowadays. After years of trying out many different levels of NFB, I always found myself liking zero NFB most of all (YMMV).

Ray

[Smitty]

OK. Yeah. You're using the power supply dropping resistors from the Vibrolux AA964. 10K to B+3 and 10K to B+4. You could use the values from the AB568 Vibrolux: 4K7 and 10K, but you may prefer the values from the AB763 (Twin Reverb and Super Reverb) 1K dropping from Screen to PI and 4K7 dropping from PI to gain stages. That'll wake things up a bit. You should notice a big jump in plate voltage and current. Which will give you the gain you're looking for.

Swap those two resistors and report back.

[cgoodson]

OK. Yeah. You're using the power supply dropping resistors from the Vibrolux AA964. 10K to B+3 and 10K to B+4. You could use the values from the AB568 Vibrolux: 4K7 and 10K, but you may prefer the values from the AB763 (Twin Reverb and Super Reverb) 1K dropping from Screen to PI and 4K7 dropping from PI to gain stages. That'll wake things up a bit. You should notice a big jump in plate voltage and current. Which will give you the gain you're looking for.

Swap those two resistors and report back.



Thanks Smitty, that did the trick!!! This was what it was needing the whole time; the preamp was running too cold. Now I'm getting plate voltages around the 240-250 range. The amp has more volume, warmth and presence; basically it sounds more like a Fender should. Also, it breaks up nicely when cranked, before it stayed clean even with everything on ten.

I guess my mistake was assuming that if I followed the schematic exactly, I would automatically have a great sounding amp. Not that it was too bad; it just lacked the balls it now has. I should have done a little more research in this area, which is what I plan to do.

BTW: I should mention I went with 1k/4k7. I think this is what the amp originally had in it...

[Satamax]

Nother good trick i've seen on fenders is using the bright switch to bypass the tonestack. Instead of going brighter. Can't remember, who exactly but i think it was Enzo, who was saying making the two chanels able to use the reverb is nice too, as well it puts the two inputs in phase, so you can jumper the inputs. Or ABY them.