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Tragic1281734187

SoCal 50 Overdrive channel

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I got one of these a couple of weeks ago and instantly thought it needed a little more gain. The o/d channel sounds very good for AC/DC or Airbourne style rhythms but doesn't work for leads.

 

I have scoured the web for mods but there isn't much out there.

 

At the moment the amp is on the bench and I am experimenting based on what little I know about valve amps.

 

The most important thing which can never be overstated is that the high voltages in these amps can kill you. Do not attempt anything unless you know what you are doing. I touched the HT line on a valve head once.....450v up your arm hurts a lot.

 

I have tried two things so far. R15 is the cathode resistor on the 3rd gain stage and it has no bypass capacitor which affects the gain. Fitting one, I used a 680nf 400v, increased the gain a lot, too much and too fizzy for me though.

 

Now the schematic (http://www.armorik.net/~porci/EPIPHONE_SO_CAL50H_SCHEMATIC_VER1-DPPT.pdf ) shows R15 as 2.2k but mine is 10k. This resistor affects the gain also. I strapped another 10k 1W resistor across it to reduce it to 5k and this sounds a lot better to me.

 

There are a few more anomalies between the schematic and the pcb so I can only assume there are a few different versions of this amp out there.

 

I am still searching for mods if anyone has any.

 

Col

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I am still searching for mods if anyone has any.

 

Col

 

Col... sounds like you could build a 'Robo-Angus' if you set your heart on it... nice work... & welcome to the wonderful world of the Epi-heads[cool] !

 

eggmuffins

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To give it more crunch and roar, I put a shunt over the 10k R13 to drop its value to maybe 3k2 using the a 4k7 on 'other side' of my bright switch.

 

This was on my BC30, same basic circuit. See the BC30 mods thread.

 

This is not a drastic change but it does add another flavour to the amp. Up for bright on C4 (connected, C6 is permanently lifted) and fat crunch.

 

This R13 is the cathode resistor to V2A but note the main drive stage is done by V1A/V1B i.e. use a low-noise high-gain bottle here (E83CC), the emphasis on low-noise. The idea is to drive a normal-gain V2 (ECC83) into overdrive. The PI driver might like a little more current like an ECC803S but not an ECC81 or ECC82.

 

Changing the cathode caps on V1 or V2 can also alter the performance, in fact adding one to V2A has more effect than changing the others but I did not like it. Also removing the snubbing caps at C7/C11 (I put them back in). Note that on the WEM Dominator/18W Marshall, one channel has snubbers, the other does not. Snubbers are often put in on a prototype to fix layout problems and so find their way into production. This amp has similarities to the Fender Bassman, WEM Dominator and 20W Marshall, always borrow from the best.

 

Recommend changing the 1Meg R5 and R6 to 470k (by shunting with 330k), it opens up the bottom end, for me a real improvement - cf 20W Marshall 2022 circuit.

 

Do check the board values of components as they are often different from the published schematic, some 'mods' have already been done: the SoCal50 schematic shows R6 to be 470k already.

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Oops!

 

I've advocated modifying the cathode resistor on V2A above but the switch may actually be on V1B. I have modified both at some time and I'm afraid that my diagram has so much scribble on it now that it is hard to tell. When I look under the hood next I'll sort that out and re-post.

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This R13 is the cathode resistor to V2A

 

 

This is R15 on both my schematics. R13 is one of the channel LED current limiters. Is there more than 1 schematic out there I wonder.

 

Col

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My eyesight is poor, it could easily be R15. There are several schematics, I'm working from the BC30 diagram I got from Epi Europe, it is incorrect as to many component values. I have a SoCal50 diagram, they look very similar, but it is low-res. E&OE as they say ;-)

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