Fender Concert Rivera Mod

[dscottguitars]

Hello,

I thought I would share this modification I made to my amp. It is a 1983 with a 1x12 Celestion Vintage 30, using Russian 6L6's. I cleaned up the first channel by increasing the plate voltage on V1-taken from point D vs E on the stock schematic. I also increased the cathode resistor to 1500 vs 680 and the bypass cap from 25uF/25v to 68uf/160v.

On channel two I moved the tone stack after the first preamp tube V2A, dropped plated voltage on that tube to dial in distortion-little to fuzz-then shape it through the tone stack out a pot to a full wired (parallel) triode (I liked the 12AU7). It is the other half of the second preamp stage V4B, of the same tube as V4A, and was not used-not wired. I took advantage of that and copied the circuit for the input preamp tube V1. Same configuration. This allows one to amplify the chosen shaped distortion cleanly until really overdriven (if desired). You can get almost as clean as channel one and anywhere in between to Ozzy fuzz without it sounding muddy.

I also added a fixed bias adjustment pot to dial in the correct bias voltage in addition to the stock bias balance pot. (the one I used in 100K with a 4.7K wired parallel, that's all I had of that type of pot) I then added a switch to change from the fixed bias to cathode bias. Cathode bias has more sustain, breakup, compression, sag and less gain. It is used in most lower power amps up to 18-20 watts. Good for blues, classic rock and situations where you don't want so much bite with every hard stroke. Fixed bias is in most higher powered amps and is better for up front picking, heavy metal, jazz, country, more gain and pick response.

I really like the sounds available now, very versatile. And the distortion channel, IMHO sounds much better than Rivera's.

Here's the changes:




Daniel Scott

[Chuck H]

Whoa!?

[jrfrond]

As long as it works and sounds good, that's all that counts.

The only thing I dispute here is tripling the value of the front end bypass cap. The stock value of 22uF yields a -3dB point damn close to the bottom of the audio spectrum. Even Fender bass amps used this value. Marshall used 320uF in their tube bass amps, which was way overkill. With guitar, you won't start to notice any rolloff until you get down to less than 4.7uF or so.

For better distortion characteristics, it's actually best to REDUCE this cap to 1uF or below to tighten up the low end a bit for nice in-your-face distortion without "rolling".

[pdf64]

Schematic
http://www.webphix.com/schematic%20h...cert_ii_83.pdf
Cathode bias with such a high B+ (475V) can be a problem; 'no-signal' plate dissipation can be rather high (have you checked this) and high signal levels result in 'squish' - the cathode voltage rising and causing crossover distortion, which in this case tends to take big bites out of the waveform, resulting in a thin overdriven tone. Have you scoped the output signal?
Not meaning to shoot you down! Thanks for sharing your work. Like John says, 'As long as it works and sounds good...'
Just bringing potential issues to your attention - Pete.

[Chuck H]

Anyone else notice the tone stack cap values and the gain switch being directly off the V1 plate, pre coupling cap, as in 157V on the switch and probably a very loud POP when used.

[dscottguitars]

Hi, thanks for your input and interest.

Jfrond: I used that value based on what I read about the values used. Because the tube is wired in parallel the normal value would be 22uf@25v, but wired full it needs to be doubled for the parallel circuit to 47uf@50v. I chose the 68uf@160v because a higher value is supposed to provide a cleaner response and is used on bass amps and acoustic amps. I did not want distortion at this point, only to amplify the incoming signal. The stock set up was not that clean.

Pdf64: I have no equipment to do any of that. I got the idea and value from Gerald Weber's book "Tube Guitar Amplifier Essentials". It seems to work but I'm not a 'guitarist' either. I like the way it sounds, a little more distorted, much wavier-a slow wah wah, sag? I also don't know anything about plate dissipation and the other stuff you mention. I really a novice. I used values suggested in the book and from other schematics hoping to get what I wanted by trial and error. I would like to know more about all that, in time I guess.

Chuck: The tone stack values I used to get more bass. The changes in the plate voltages I made is why I changed the resistors in the stack. I had an extremely difficult time finding a combo that worked and gave a response when turning the pot. It is still not that great, seems to have an effect for the first half but then no change from there. And the switch is what I put in the drawing to represent the change there, but it is still stock. The actual circuit here just splits and the switching network is complicated with optocouplers. In the amp it is a union at the point where I have the switch in the drawing.

One thing about how it works and sounds: it's good to me, but I hope to have a 'real' guitar player check it out also. Playing in my room and in a band are two different things. I am a rhythm player and usually play acoustic guitar. I'm a wannabe electric lead player.

Thanks again all...

Daniel

[dscottguitars]

Corrected schematic...