Re-valving my amps, advice needed.

Lawrence · Post by **Lawrence** » Sat Dec 11, 2004 10:05 am

Hi there guys,

I'm think that the tubes in my amps need changing (a '76 Superbass and JCM 900) and I just had a few questions:

Can you just drop in replacement preamp valves of the same type without any problems, also what brand of valve do you guys find best?

What could I do to reduce the gain in the preamp stage of the JCM 900 as I find it can get a bit fizzy?

Is there anything I can do to up the gain in the power amp stage of the JCM 900? Would new/different brand tubes be the first thing to try?

And finally, how difficult is it to bias tubes in the power section as I've not really had any experience of working on tube amps? Should I discharge the filter caps every time I work inside the amp and if so, how do I go about doing this safely?

I know it's a lot of questions but I'm sure you guys can help.

Thanks a lot,

Lawrence

Flames1950 · Post by **Flames1950** » Sat Dec 11, 2004 10:34 am

You can drop new 12AX7's into the preamp without any adjustment, they are self-biasing in the circuit. In fact, you can interchange 12AX7's, 12AY7's, 12AT7's, 12AU7's in the same circuit in most cases, with the only difference being the gain of the tube.
Which may help you with question 2, but a lot of JCM900's use diode clipping as well as tube distortion so it may not truly help to the extent you'd hope for. The later SL-X 900's are the only one's that don't rely on some diode clipping IIRC, the Dual Reverbs are diode clippers and so are the earlier High Gain MV 900's.
What tubes are in the 900 now, 5881's? Some guys like the EL34's better in there (which may require modding the bias circuit to get biased correctly) and others seem to like a real 6L6 instead of the 5881's.
Typically a lot of us might recommend feedback circuit changes to bump power amp gain/distortion, but with the amount of distortion already available in a 900 I'm a little leery of doing so. The low end can get mushy as you turn the power amp loose with feedback changes, and I don't think extra mush is desired for a 900.

Guest · Post by **Guest** » Sat Dec 11, 2004 3:13 pm

Thanks for the reply flames.

Yeah the 900 probably doesn't need much more distortion in there.

I had a look in the back of the amp and there are EL-34's. They're some no-name brand so I'll just switch em out for some JJ's, I think that might improve the sound by quite a bit.

How hard is biasing the amp considering I've not really worked on amps before?And also how do I safely discharge my filter caps before working on the amp?

Lawrence

Jim SS · Post by **Jim SS** » Sat Dec 11, 2004 8:08 pm

I have a JCM 900 dual reverb. I find with the MV at low volumes it is very fizzy, however, with the power brake on and the MV at about 6-7 I get a very nice distortion. These amps can sound very good if tweeked just right. Not quit as good as my plexi, but nice none the less. Actually the best sounding amp I have is the new Marshall 20 watt hand wired amp.

Jim

myker · Post by **myker** » Sat Dec 11, 2004 9:07 pm

I have also owned one of those dual-reverb 900 series amplifiers. the pre-amp is full of those diode distortion bullshitters! shite! i would also try the power brake and turn the pre-amp down as well! less diodes = more natural. the difference between white lion and led zeppelin!
or sell it and buy an 18 or 20 watter, or build your own, and run it full steam!!
mike

Lawrence · Post by **Lawrence** » Sat Dec 11, 2004 9:09 pm

Yeah unfortunately, due to the lack of an attenuator, its very rare that I get to run this thing at a decent volume (the neighbors dont share my enthusiasm for rock n roll

). I think it'll be easier to get a nice tone out of the amp once I get some new tubes in there.

I am thinking of getting a weber mass attenuator, anyone ever tried them?

Lawrence

flemingmras · Post by **flemingmras** » Sat Dec 11, 2004 9:34 pm

I'd get George's PPIMV(post phase inverter master volume)kit. It's a dual pot master that goes AFTER the phase inverter instead of BEFORE it. Instead of sounding buzzy like a prephase master does, this sounds VERY VERY DAMN CLOSE to what the amp would sound like on an attenuator.

Get the 500K version, drill a hole in the chassis of your amp, and if you follow the directions it won't take you more than about 5 mintues to install it.

Hope this helps.

Jon

Lawrence · Post by **Lawrence** » Sat Dec 11, 2004 9:52 pm

Yeah I like the sound of that, is there any way i could just replace the MV's for both channels with the PPIMV 'cause if I can avoid drilling any extra holes that would be nice.

Oh yeah and thanks for the online electronics course Jon, looking forward to the next lesson.

Lawrence

Flames1950 · Post by **Flames1950** » Sat Dec 11, 2004 10:56 pm

I don't think you'll be able to convert the two existing MV's into PPIMV's, especially since as near as I can tell they're probably board mounted pots that you'd have to contend with working a way around. You could add a hole for a single PPIMV, leaving your existing two alone.
But for the $200 that MASS will cost you, you'll find it money well spent. I've got two MASS'es and a Power Brake, I find the MASS'es to be WAY more transparent, plus they have a DI out with EQ/volume controls, you can add options like a MASS bypass, DI bypass, multiple load settings, etc. And if you turn the MASS speaker volume to zero it acts as a load box. Way cool.

Lawrence · Post by **Lawrence** » Sat Dec 11, 2004 11:08 pm

I was just thinking that maybe i could have the volumes set independently for the different channels but its no problem. Hang on, didn't Jon post something not long ago about having two or three different footswitchable settings for a PPIMV?

And when I get the cash I am definitely getting the weber mass 'cause its got all those features for a great price. I just wondered if anyone else had used them and what they were like.

Lawrence

Flames1950 · Post by **Flames1950** » Sat Dec 11, 2004 11:31 pm

The MASS ain't gonna win a beauty contest, it's just a black and gray box sitting on your amp; but since it uses actual speaker motors for its loads I just find it to have a way more usable attenuation range than the Brake.
I also find that since it can act as a load box you have some versatility in setting it up. You can run it between the amp and the cab and adjust the speaker volume to taste (just having it in line with the speaker volume on 10 is still a difference in volume from no MASS at all) OR you can run it off the other speaker jack in parallel to the cab (don't forget, 16 ohm MASS + 16 ohm cab = EIGHT OHMS impedance setting on the amp!!) Set the speaker volume to zero and just let it waste half the amp's power, while the amp reacts normally with the cab.

thunder970 · Post by **thunder970** » Sun Dec 12, 2004 9:08 pm

For the 900 buzzy sound, I put my preamp gain at about 12(little over halfway) and crank the master. The buzzy starts to go away at 4, at 6 it sounds great and 8-10 it don't sound like the same amp but you have to speak VERY LOUD to me for about half an hour afterwards. but I like movin air