master volume advice...

[foxfire]

your right, i didn't need an explanation, but it would've been nice to have gotten one...

i do appreciate people urging others to not mod/screw up vintage gear as i often do it myself. i just don't think adding a MV is that big of a deal especially if it make the amp even more useful to the user.   

[montesada]

Wouldn't an attenuator be a better solution?

[foxfire]

Maybe, but I already happen to have the $2.00 worth of parts to do the mod where as I don't have the $200 or whatever it would cost to buy an attenuator. Also it would just be one more then to haul out on the road and have to not forget at the end of the night...

[Soundmasterg]

thanks for the suggestions. i definitely won't be drilling any holes till i am 99% sure that the MV is worth it. just to be clear, your suggesting i try a putting a 100KR between the wiper of the MV and pin 8 of the PI? 
 

Yes, the reason for it is that it isolates the pot from the grid of the phase inverter making sure there is no impedance change. This stops the thin sound at low volumes problem that Marshall JCM 800's were known for. The suggestion comes from Kevin O' Connor's series of books, which are expensive, but offer a lot of great info in them. If there is an input cap to the phase inverter already there, then the 100k resistor would go between the wiper of the pot and the cap. I don't remember offhand without looking at the schematic, and I don't feel like doing that for something so trivial.

If you're looking for more useful tones at lower volumes, then the power scaling that Kevin O' Connor sells is the best approach for that. The master volume won't drive the phase inverter much unless it is up past a certain level, and below that your tone will be clean. The power scaling allows you to get the tone you want, and then turn the volume and the power of the amp down. I'm going to be adding it to several of my amps soon.

Greg

[foxfire]

i'm gonna have a go at the MV tonight and i'll have to start looking in to power scaling...  

the only thing in series between the wiper of the treble pot and pin 8 of the PI is a 100k resistor. the reverb and mid boost switch both get their signal from here after the 100k resistor.   

[Soundmasterg]

I would think that you should located the master volume then right before pin 8 of the phase inverter, with the 100k resistor you would add in between the wiper of the pot and pin 8. The other 100k resistor would be on the preamp side of the MV pot.

Greg

[foxfire]

I just tried it and for like 2 seconds I was getting some good fuzz then it dropped to a completely unusable volume. I double checked all my conections and even pulled the 100k between the pot and PI. I was still getting fuzz but at not much more than whisper. I don't have much in the way of spare parts so I didn't push it... 

[Soundmasterg]

Well it shard to figure out from your descriptions what was happening. It could be that it was oscillating....but again, hard to say.

Greg

[foxfire]

i decided just to get it all wrapped up and back together before i wreck it anymore than i already have. once i have a better handle on what i'm doing in there i'll have a go at it again. i'm at the point where i can't even enjoy it cause i'm just waiting for something to blow or pop...

thanks for your help. you definitely explained it in a way that was easy to understand which was very helpful.

[Soundmasterg]

You're welcome and good luck with it. Keep visiting this forum as well as the others at Ampage, Webervst, Hoffman amps, AX84, etc and you'll keep learning. Lots of good books out there too if you really want to learn.

greg

[Isaac]

A few years ago when I was playing guitar in a band (I'm usually a bass player), I used my Sunn Sentura II amp. I used a distortion pedal, but I also used a Scholz Research Power Soak, and overdrove the input with my wireless. Each had a different sound. The non-pedal sounds were unpleasant unless I kicked in the midboost. With that on, the distortion was smooth and fat. Without it, it was harsh and thin, perfect for playing "Jumpin' Jack Flash"!

[MarkG]

If You want a REAL master volume Sunn....Get a Sunn Coliseum PA :

http://www.youtube.com/watch?v=Vf9WRHMB-DE&feature=PlayList&p=5C86F3CF0BD50C68&playnext_from=PL&playnext=1&index=4

 

[foxfire]

A few years ago when I was playing guitar in a band (I'm usually a bass player), I used my Sunn Sentura II amp. I used a distortion pedal, but I also used a Scholz Research Power Soak, and overdrove the input with my wireless. Each had a different sound. The non-pedal sounds were unpleasant unless I kicked in the midboost. With that on, the distortion was smooth and fat. Without it, it was harsh and thin, perfect for playing "Jumpin' Jack Flash"!

i haven't tried adding a MV since my initial attempt, but now that my amp guy has given it a clean bill of health i guess i should get back in there and mess it up. i find that mid boost removes a lot of low end and creates what i would call a more of a lead tone. distorted, it sounds pretty good. clean, it has a not so great breakup.