Add me to the list

steve

Thanks for the reply.

I'm pretty sure I got everything wired correctly. The jacks & resistors are PCB mount. I probably should pull the PCB again and check my solder connections with a magnifying glass.

Oddly enough, the low gain input sounds much better than I remember. Usually it is kind of thin and anemic sounding; now it is instant Angus Young

The High gain input is saturated and compressed sounding compared to the low gain input, but not as much volume.

Regards,

steve

Quote from: teemuk on November 17, 2006, 06:07:35 AM
Something to do with switching arrangement: No plug in high input jack grounds the signal of low input through both R1 and R2 (parallel connection so resistance is about 24k). I can't read value of R3 (68k?) but this one forms a voltage divider with R1 and R2.

When the plug is inserted to high input the jacks's internal switch mechanism (note the arrow) is broken and the voltage divider is now R2 and R3 (only small attenuation). Are you sure you wired the jack's switching arrangement properly?

Update:

I got three Cliff jacks from AES in the mail today on the advice from a tech who posts on the Ampage.

I replaced all three jacks with new ones and replaced the 33k, 68k, and 330k resistors connected to the input jacks.

The amp now works and the squeal is gone. Oddly enough the low gain input is louder than the high gain input with the same volume and gain settings.

Thanks to all who replied.

steve

Thanks for the reply, I'll try that.

steve

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

Quote from: joecool85 on August 05, 2006, 10:02:25 PM
Yeah, you'll want to drain all the power caps.  Even if the SS section uses a different power section, its still safer to drain all power caps in there.

On a side note, SS preamp and tube power section??  I'd only heard of the other way around.

I had a Marshall 3203 "Artist" head that had an SS preamp and a tube power amp. Two EL34s and a 12AX7 phase inverter. Sounded pretty good at low volumes.  The 4203 was the combo version. These were made in the mid to late 80s. Cool amps.

steve

Joe,

I believe a transformer can only have two wires on the secondary, i.e. no center tap. You have to use a full wave diode bridge instead of the instead of the more common full wave diode rectifier setup.

Hammond has a good .pdf file you can download which shows the various setups:

http://www.hammondmfg.com/pdf/5c007.pdf

steve

Quote from: joecool85 on July 22, 2006, 07:08:28 AM
I don't think you can use that...on that same note, I don't think I've ever seen a transformer with only two secondary wires.

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

Joe,

Sorry, forgot my manners. I got my kit last week, thanks for getting it out to me so fast. I appreciate it.

regards,

steve

Joe,

Thanks for the kind words. That amp was completely scratch built from parts I had laying around. I'm developing the amp for an e-book I'm writing. The face plate is a piece of textured poly styrene. I "built" the amp in CorelDraw first before I drilled any holes. I made a 1:1 drawing with actual size components first so that I didn't mess up the chassis. I haven't ruined a chasiss since I began using CorelDraw

Now I need to get the chipamp kits built so I can see if my line out idea works

steve

Quote from: joecool85 on July 11, 2006, 07:44:41 AM
WOW! Looks great! So did you build that all yourself? Is it a kit?

Matthias,

Thanks for the kind words. Actually, that amp was thrown together in a hurry. It isn't my best work. I usually don't cut the transformer leads and this one was extra sloppy

This one looks much better:

http://www.diycustomamps.com/images/6v6_amp/

That one is a custom build for a friend of mine.

steve

Quote from: matthias on July 11, 2006, 01:29:42 AM
hello steve,
Man that looks good!

Here are some pix of my VJ scratch build:

http://www.diycustomamps.com/images/vj/

I reconfigured the post OT line out: I have a 47k 2w resistor on the 8 ohm tap of the 125ESE power transformer. This is connected to a 5kA pot and an output jack. With the new circuit I'm getting about .854V out of the jack with the volume control and the 5k pot dimed.

I'm hoping that this will sound cool driving an SS power amp. The amp sounds pretty good driving a 12" speaker cab.

steve

Thanks for the help guys. I've been building tube circuits for a while, but all of this SS stuff is new to me.

Regards,

steve

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

Thanks, Joe.

steve