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Topics - steve_m

#1
Amplifier Discussion / Help on Lead 12 combo repair
October 14, 2006, 05:11:01 PM
Since I'm more of a tube guy, I don't even begin to know where to start fixing this amp.

I have a Marshall Lead 12 combo. I bought it used a few years ago to gut and build into something else, but it sounds pretty good as is and makes a nice light practice amp. The stock speaker really sucks and makes the amp sound crappy. When the crappy stock speaker is replaced with a good Celestion or similar, it really opens up the amp's tone and makes it sound much better. The mini stack head was used by a lot of the LA guys in the mid to late 80s in rack setups as the tone generating machine, IIRC.

It has never really worked right. When I got it the low input channel was intermittent, it would cut in and out while playing the amp. The high input is fine. I tried reflowing the solder on the low input jack, but this made the problem worse. Now it squeals when the gain is dimed and the volume is turned up.

Any suggestions where to begin? There aren't that many parts connected to the low gain input: a jack, a resistor and an opamp.

Any ideas where to start to fix this thing?

Schematic is here:

http://www.drtube.com/schematics/marshall/3005.gif

The combo is model# 5005, the mini stack is model# 3005.

TIA,

steve
#2
Amplifier Discussion / PT for LM3886 project?
July 13, 2006, 10:49:12 AM
I found this transformer the other day online:

http://www.allelectronics.com/cgi-bin/item/TX-245/790500/24_V.C.T._"#64;_5_AMP_POWER_TRANSFORMER_.html

Would a pair of these work for an LM3886 project? I know they aren't toroid, but the price is certainly attractive. With two, there could be a separate power supply for each amplifier.

steve
#3
Preamps and Effects / Driving an SS Power Amp?
July 05, 2006, 04:56:26 PM
I had this idea to build a low watt tube head and use it as a tube distortion pedal. I just finished a circuit that has an adjustable line out. With a 150mV sine wave put into the input of the amp, I get about 160mV out of the line out. The line out is post OT, i.e., connected to the secondary of the output transformer. The amp itself is a copy of the Valve Junior circuit; single ended, 5w with an EL84 tube. The preamp tube is a 12AX7.

My question is: how many mV does it take to drive one of those LM3886 amps to full output? I'm trying to tweak the line out on my tube head.

TIA,

steve
#4
Amplifier Discussion / SS Amplifier kits
June 20, 2006, 12:13:28 PM
Found this site while looking over my bookmarks:

http://www.ampslab.com/products.htm

No affiliation with them, don't know anything about them, but the do offer SS kits that might be adaptable to SS guitar amps.

steve