[mlundy57]

I guess third time's the charm. The local tech school has been offering a "Build Your Own Amp" class for over a year and I was finally able to get into one that made. It is a 30 hour class spread over 10 weks that started last night.

I had no idea what amp kit was going to be used so imagine my surprise when I got to class and found out a few things:

1) it's a 6L6GC/EL34 based push/pull tube amp,
2) it's integrated,
2) it's point to point wired, no PC board       All good so far ......
3) it's a guitar amp   

I don't play guitar, and it's mono so I can't use it in my stereo system   

Well, I figure at least I can learn principles, assembly skills and troubleshooting/repair skills.

As the night progresses it turns out the amp is highly modifiable and there are mods I can do to make it work in a HiFi system. However I need to build two kits so I have one for each channel.

The amp is a MOD 101 Guitar Amp Kit   

  https://www.modkitsdiy.com/kit/mod_101_guitar_amp_kit     




This is going to be interesting

Mike

[SteveFord]

That sounds like a fun class!

[poseidonsvoice]

Mike,

Sounds like fun. Take some pics periodically and post here. Would love to see it 

Best,
Anand.

[soundofrockets]

whats the location for the class ? 

[mlundy57]

Mike,

Sounds like fun. Take some pics periodically and post here. Would love to see it 

Best,
Anand.

Will do   

whats the location for the class ? 

Tulsa Tech, Tulsa Oklahoma

Mike

[G Georgopoulos]

The absolute authority in tubes!
Roger A Modjeski
he used to have a circle here on AC
http://berkeleyhifischool.com/roger-modjeski/
 

[mlundy57]

Been moving right along on this














Last night I finished the wiring, hooked one up to a Variac, applied a little current and checked things out. Everything checked out OK so we disconnected it, installed the tubes, connected it to my DAC and fired it back up.

We got sound and no smoke but when I turn the volume up I get cracks and pops.

The other amps in the class preform fine at this point. However, they are being used as the amp was intended, as a guitar amp with 1/4" guitar jacks as both input and output connections, the input device is an electric guitar and the output device is a guitar speaker cab.

I wanted to use the amps (I built 2 kits) for stereo playback so I installed a line level RCA jack for the input and a pair of binding posts for the output.

Initially the gain was tremendous. Both volume controls, the one on the amp and the one on the DAC had to be almost at zero or the cracking and popping occurred. The instructor tried damping the input signal by installing a resistor to ground. This helped, allowing the volume to be turned up more before the cracking and popping occurred but did not fix the problem.

These amps are designed as push/pull, they can also be configured as ultralinear. Power tubes are EL34's.

Does anybody have any ideas or know where I can get a copy of the schematics for a push/pull (or UL) tube amp designed for music playback so the instructor can compare it with the schematics for these amps? These amps are point to point wired though as far as the schematics go I wouldn't think it would matter whether the actual build used PC boards or point to point wiring.

The idea is to see if there is something inherently different between an amp designed for use with a guitar and one for music playback and maybe get some ideas on what to do the stop the popping and cracking.

Thanks,

Mike

[S Clark]

Look up the schematic for the Knight Kit KB-85.  It was an early 60's stereo kit using an EL-37 but would also take a 6L6 GC.  There were also mono versions that should be very similar to what you built. 
http://www.triodeel.com/images/kb85-2.gif
Gary Dodd took one of these and built my amp from it just before his health collapsed.  He thought it was the most musical mid size amp he had built to date. 

[mlundy57]

Look up the schematic for the Knight Kit KB-85.  It was an early 60's stereo kit using an EL-37 but would also take a 6L6 GC.  There were also mono versions that should be very similar to what you built. 
http://www.triodeel.com/images/kb85-2.gif
Gary Dodd took one of these and built my amp from it just before his health collapsed.  He thought it was the most musical mid size amp he had built to date.

Scott,

Thanks

Mike

[Steve]

"2) it's point to point wired, no PC board       All good so far ......"

A lot of diyers and even professionals claim hard wiring is superior. However, I further investigated and found a better solution. From one of my articles:

http://www.sasaudiolabs.com/theory5a.htm

Solution To Hardwiring and PC Board Design Problems:

First, one needs to address this new concept by mating the circuit and physical lay out into one grand design. Both are very important and functionality is a prime concern. Elevating and the proper placement of parts above the chassis solves the capacitance problem to ground and part to part. Of course, computerized layout programs just won't do. Then eliminate unnecessary wires and solder connections by "lead to lead connecting". By this, one obtains several advantages.

    First, this shortens the signal path VS "hardwiring", "point to point" wiring etc.

    Second, this maximizes the high frequency response by minimizing capacitance to ground. This also helps in true inner detail.

    Minimizes the problem of adjacent channel interference, or cross talk.

    All of which improves sonic integrity, and consistency from component to component.

So a pc board can be superior if done properly; by eliminating unnecessary wires, solder connections, cross talk between
parts, and parts to ground, channel to channel etc.

Channel separation, is an interesting subject as it is frequency dependent. Both highs and lows can be prone to problems, with layout and design being critical. A product, or diy spec sheet might claim nice channel separation at 1khz during channel separation, but not mention separation at higher or lower frequencies frequencies, say at 10khz and 50hz for example.

Another tidbit, while the specs may be nice in one channel lab measurements, the other channel may be picking up highs, or lows, and cause the overall sound, in stereo, to be bright and/or too bass heavy. In other wards, the Lab is almost always different than the home environment.

Cheers
Steve

[mlundy57]

Finally got them both working at the end of the last night of class. There were some grounding issues and I had some loose connections. Also, in the experimenting on the first one we managed to mess up one of the tubes.

There is some bleed through going on because I get sound even with the volume control set to zero. One of the other students had the same issue on his amp. Adding in the included mod of splitting the cathodes resolved his problem. There wasn't enough time in class for me to get this mod done so I'll do it on my own.

The amps also have a lot of background noise. The guitar players don't care because when they are playing, the guitar drowns out the hiss. The instructor suggested I try a combination of two things:

1) decreasing the resistance in the feedback circuit from 20k ohm down to 10k, 5k, 1k and 820 Ohms

2) increase the size of the cap between the plate of V1 and the grid of V2 from .001 uF to .022 & .047

The idea is that decreasing the resistance in the feedback circuit will quiet the amp at the expense of bass response while increasing the size of the cap will increase bass response. One of the combinations should result in at least decent bass response and be relatively quiet.

Mike

[THROWBACK]

I'm envious. I'd love to take a class like that. I have built Heathkits, Dynakits, Citation kits and Welborne Lab kits, but I never knew what I was doing except for clipping wires and making good solder joints. I'd love to sit down with Roger Modjeski and start with a clean blackboard.