1971 Fender amp - help?

JVanDe7 · Post by **JVanDe7** » 10 May 2012, 15:07

I have a Fender Musicmaster bass amp that has "OCT '71" printed in the chassis. It uses 6AQ5A power tubes and a 12AX7 preamp tube. I replaced the stock speaker with a Ted Weber Sig Series Alnico 12S. It's a fantastic amp but I have some questions about the circuit (which is small/simple)...

1. What is the purpose of C1 (0.0047uF)? Some people like to remove that cap and put a jumper wire there instead. I don't understand what the desired effect would be?
2. C3 is a .01uF ceramic disc coupling cap? I'm told that increasing this to a 0.022uF cap will increase bass response. Is that true? Is this modification advisable? Any negative consequences?
3. C2 is a cathode bypass cap? I'm told that people like to increas this 2uF cap to 10uF or even 25uF. What would that do to the sound of the amp?

Please and thank you.

devastator · Post by **devastator** » 10 May 2012, 16:06

C1 is a cap which forms a hi pass filter (reduce bass) with the 470K resitor (needed for the proper biasing of the tube ) .

C2 is a cap to increase the gain of the stage . (playing with that values can bring different frequencies boost the more large is the value the more full is the frequency)

C3 is a cap to reduce basses after the 1st gain stage (it a litte bit different here beaucase the frequency cut is dependent of the tone pot )

if you want more bass you can play with caps but be aware that more basses can bring unexpected things. Especially if you like crancking the amp to get overdrive . You have to find the best compromise for your ears.

JVanDe7 · Post by **JVanDe7** » 10 May 2012, 16:18

Thanks for the prompt response!

I don't really want to modify the amp. In fact, when I bought it, the previous owner had removed C1 and put a jumper wire there instead. I recently removed that wire and put C1 (Mallory 150 series 0.0047uF) back in. It sounds more tight and punchy now. I like it that way but I was curious what the purpose was for C1. Thanks for clearing that up.

I'm comfortable modifying the amp. I just don't want to mess with it because it sounds good already. But the curiousity in me was starting to get the best of me and I needed to hear more about what those mods would do.

I don't really want to increase the bass response of the amp. It has enough already, especially with the new speaker instead of the tired old stock speaker.

The only mod it sounds like I would be interested in is C2. If that increase the gain... that might be nice to squeeze a little bit more gain from the amp but I kind of like having the circuit stock. People mod the shit out of everything nowadays just for the sake of modding. It's getting to the point where a guy can't buy a piece of used gear without finding out that some novice decided to try some mods on it and clipped some capacitors out or rewired the tone stack or what have you.

Sorry for the rant. Thanks for the help!

devastator · Post by **devastator** » 10 May 2012, 16:47

actually the tube can work with only a cathode resistor but the gain won't be "max". Putting a cap in // with that resistor brings more gain . that thing is you can choose the frequencies you want to boost. Take a look at the famous "plexi marshall" , you can see there's a 0.68uf (680n) instead of the usual 22uf/45uf (JTM45 , bassman ect..) That value is chosen to reduce the bass boost and create a bigger mids/highs boost (with a 2n2 output cap , or with the 470K/470pf filter in the signal path) .

take a look at the valvewizard "gainstage" PDF , there's a part about that with formulas and graphs :

http://www.freewebs.com/valvewizard1/gainstage.html

deltafred · Post by **deltafred** » 11 May 2012, 06:57

devastator wrote:
take a look at the valvewizard "gainstage" PDF , there's a part about that with formulas and graphs :

http://www.freewebs.com/valvewizard1/gainstage.html

That is a great article.

devastator · Post by **devastator** » 12 May 2012, 18:22

yes great article , really understandable .

Books are really good too according to some people .

JVanDe7 · Post by **JVanDe7** » 12 May 2012, 22:24

Another question-

If I wanted to order up some carbon comp resistors to replace the originals that may have drifted out of spec... what rating do they need to be? Is 1/2 watt alright? Most of the resistors ar small but there's a 220R that is HUGE for some reason.

I was just working on the amp and I replaced the 100K in the tone circuit with a 100K carbon comp 1/2 watt I had. The original was like 93K and my replacement was 101K. It improved the sweep of the tone pot noticeably.

I also replaced the 470K resistor (grid resistor?). I had a 1/2 watt carbon comp 470K that measured 469K and the original 470K in the amp was nearly 500K!! I replaced it. The amp seems better for it but it could be in my mind. It seems less noisy and more focused or something. Does that 470K affect the bias?

I would like to replace all the original resistors, or at least measure them and see if they are close or not. But can someone tell me what watt rating they need to be? thanks!

PokeyPete · Post by **PokeyPete** » 13 May 2012, 02:50

Since you have a working amp......if you're very careful....

measure the voltage drop across a resistor, square it.....then divide by the resistor value for the wattage needed.

devastator · Post by **devastator** » 13 May 2012, 12:00

The original was like 93K and my replacement was 101K. It improved the sweep of the tone pot noticeably.

93K seems pretty close to 100K I'm not sure that it will make a hearable difference. (pots are in general +/- 20% tolerant) but who knows.

I would like to replace all the original resistors, or at least measure them and see if they are close or not. But can someone tell me what watt rating they need to be? thanks!

the Resistors have to be calculated for the power they will handle . Look the different pages on the valvewizard to find the formulas (that's pretty simple) . Starting with 2W resistors for general purpose is a good thing (it's simpler when you order parts) You have to take bigger for the power section (look at the 220R cathode resitor ) .

Bear in mind that working on amps can be dangerous if you haven't experiences with them. There're High voltages inside (more than 200VDC) . I recomend you to read informations about electric security and working on amp before you start , for you own safety (and to avoid your fingers to smell bacon

) . It's not rocket science but being prepared and have all risks known is better to work

JVanDe7 · Post by **JVanDe7** » 13 May 2012, 13:14

devastator wrote: 93K seems pretty close to 100K I'm not sure that it will make a hearable difference. (pots are in general +/- 20% tolerant) but who knows.

Maybe the fact that I replaced the .01uF ceramic disc coupling cap with a .027uF VitQ paper in oil cap had more to do with the noticeably better tone pot sweep? That coupling cap supposedly increases bass when its value is increased.

Or maybe the 470K resistor I swapped in there (grid resistor?) for the way out of spec original? (it was at 498K even though it was marked/banded as a 470K).

Or maybe it's all in my head haha.

phatt · Post by **phatt** » 13 May 2012, 14:21

JVanDe7 said;
"The amp seems better for it but it could be in my mind."
Why yes, it probably was.

Re exact values;
No 500k or 469,9999999999999999k is neither here nor there but the fact you went to the trouble of changing the resistor is the main reason why your brain tells you it's better. The plate cap was changed to almost 3 times it's size and that is what made the difference.

Depends a lot on where in the circuit you make changes. So at one point you could double the value of a resistor and hardly tell the difference,, Yet at another more critical point a 20% difference could blow the Amp.

Re resistors;
*It's a Valve circuit so high voltage is the issue not wattage.*
Go check the *voltage breakdown* on 1/8 watt all the way up to 2 watt resistors for clues.

Yes the cathode R is very big,( Not because of high voltage) but because it does pass big current.

Meanwhile The 100k plate resistor needs to be at least 1/2 watt because there is a substantial voltage across the plate load resistor and although a 1/4 watt will work their, there is a limit to the voltage flash-over. So play it safe and use 1/2 watt or larger.
I use 1 Watt simply because the pigtail leads are easier to work with.

Oh yeah just in case you are heading in this direction,,,and before you waste money regards to all this cork sniffing mojo you know, orange coloured parts, oxygen free wire, carbon comp verses the universe golden ear testers. lol.
Go here; http://www.guitargear.net.au/discussion ... ic=32906.0

Read post by *Roly* 9 down. Very knowledgeable chap and funny also.
Have fun, Phil.

JVanDe7 · Post by **JVanDe7** » 13 May 2012, 16:24

Not heading in a cork sniffing direction. I suspect you offered that advice because of my "paper in oil" cap choice. Actually that cap came from a Les Paul I recently purchased. The previous owner had put two Sprague Vitamin Q 0.027uF caps in there. I used 0.015uF film caps instead and had those VitQ caps laying around unused. Since they cost $20+ each I wanted to try to use them for something, and the amp seemed like a good fit since they are rated for 400VDC.