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SOMA 84
#1
Why is the input jack not insulated from the chassis? 

What is the 270R from the OPT 16 Ohm tap to ground on the schematic, is it on the wiring layout?  Gathering parts for this.
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#2
I now see the 470R on many of the schematics.
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#3
Hi Sinusoids

What makes you think the input jack is not insulated from the chassis? Every TUT3 and TUT5 project has ONLY insulated jacks.

The back view of the jack looks as if it is not, and these are Switchcraft long bushing types where you add the shoulder bushing on the inside face of the chassis and insulating plastic washer under the outside metal washer and nut.

The R across the output in every TUT3 and TUT5 project limits how open the load can be.

have fun
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#4
You have "non insulated jack" written on the wiring layout. Under the input jack. I was wondering. Good to know about the 470R. I know you recommend 3/8" internal lock washer under the shoulder as well so I have those. I'll insulate the jacks as usual.

Thanks!
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#5
hey

Soma-84 is the exception that makes the rule Smile

I designed this for a friend who then built it using transformers I supplied. He chose to ground the circuit at the input jack and it was quiet so he left it that way. It is not how I would do it as witnessed by all the other projects.

You can get away with certain things like this in low-gain amps, but were you to modify the amp later it would be wise to insulate the input jack and take the ground tie from the middle of the circuit.

All those old projects use Switchcraft jacks which are good and bad. The large contacts make the jacks robust but the weak part is the crimp holding the whole thing together. If you over-tighten the jack, the crimp loosens and the wafers and layers of the jack rotate out of alignment and do not stay tight any more. It is a good reason to switch to plastic jacks which are already self-insulating. With plastic jacks, Switchcraft becomes a poor choice as theirs always seem to have smaller contacts than the competition.
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#6
Any recommendations on plastic jacks? They seem to break if a straight plug bets bumped.
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#7
Hi

Neutrik is pretty good, or Cliff - depends if you need PCB mount or solder lugs.

If bumping into straight quarter-inch plug ends that are inserted into jacks is a habit you can't break - or maybe it's a hobby? - get some 90-degree cable ends. "Expensive yes, but I like it too"

Have fun
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#8
I have the SOMA 84 built, circuit card should have been bigger.  With 70mV 1kHz in I measure 7.25V out, rms.  Sounds great, however, I evidently made a mistake somewhere.  Tubes are known good.         .
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#9
This seems like low power for the tubeset/transformers, just checking in to see if that is case.  Also, I realize that I have the IEC connection on the input/quiet side of the amp.  This is a reused chassis, there was an existing hole the IEC filled.
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#10
Hi Sinusoids

I assume you are having a hum, or that the amp is?

The leads from the IEC should be tucked back in the chassis held by friction or with a cable clamp. The photo shows it flying over all the tubes to get to the PT. Redressing this might reduce the hum.

It might have been wise to reverse the front panel layout and the card if you did not want to knock out a new IEC hole. However, most compromises of layout can be fixed in some way, or compensated for in the circuit.

You can test where the hum is coming from by turning the Volume and MV controls down and see which has an effect. The main filter could stand to be a higher value; even though there are three RC sections it is hard to beat what 100uF will do in the first position. Active hum filtering is the next big improvement to be made in the plate supply, which has to be clean with the UL connection - the screens are sensitive to supply ripple.

There are a few ways to add balancing for the output tubes, as well, and this would accommodate tube aging and different tube and OT samples. Note that the hum balance point for an OT is NOT the DC balance point for the tubes - these points are always off by a few milliamps.
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#11
Kevin,
It seems like I have low power. With single coil pickups there is no real output until the Vol and MV knobs are more than 1/2. I measure the max output at less than 8W rms into a dummy load.

No hum, I have not gotten around to securing the wires yet. Just wanted to see if I could minimize the empty holes in the chassis.
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#12
Hi Sinusoids

The designed output power is only 10W for Soma-84. Gain is a bit on the low side as it was designed for a specific player.

You can get a bit more gain by rewiring the MV using a dual pot to replace the 909k grid-leaks and remove the 100k series Rs to the output grids. To retain some of the tone you might want to increase the 9k09s to 15k or 22k.

The parallel input stage can also be rewired as two cascaded stages without cathode bypasses and with attenuation between them - this can be tweaked for whatever extra drive you need.
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#13
Thanks for the info. It sounds great as is, quite the range of tones, just wanted to be sure that power level is correct. My wife's electric violin sounds nice through this amp as well.
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