Soldano SLO-50 rebuild and mods.

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Hey, long time no post!
I recently rebuilt a 50Watt turret board version of the SLO amp that I had taking dust on the shelf.
This was one of the first guitar amps I built back several years ago and now called for a facelift. Continue reading for some more pictures of the amp, and what was done to turn it into an awesome piece of gear!

I had to deal with 2 issues. Firstly, the amp was quite noisy, mainly due to a wrong grounding scheme and lead dress.
Secondly, the Clean channel sounded too sterile and the Overdrive channel had so much gain and harsh sound it was almost useless no matter which settings were adjusted.

To address the first problem I replaced all open jacks to Amphenol isolated and rewired the whole thing. I noticed this amp has been built a lot by diyers around the world, but couldn’t find too much right information about a proper grounding scheme, which I think is mandatory in this kind of high gain designs. Even the vendor itself seems to get away with something much less than optimal.

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The result here is a little messy, but quite efficient. There’s no audible hum in neither of the two channels even with preamp and master volumes turned all the way up. However, there’s some audible hiss in the Overdrive channel with controls dimmed, something to expect given the tons of gain of the channel.

In regards to the power tubes, I kept the Svetlana Winged-C 6l6GC and adjusted the bias accordingly.
To improve the sound of the Normal Channel, I chose a Blackburn 60’s Mullard Ecc83 in the v1 position and kept v3 -v5 (Russian TAD 12ax7). I changed the first few stages plate resistors in both channels to Allen bradley and Philips carbon comp types, and the coupling caps from Mallory 150’s to Plessey 470A. Also got rid of Silver Micas in favour of Polystyrene .

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As per the Overdrive channel,  I chose a JAN GE 5751 tube in the v2 position and that made a huge change. Still too much gain though, and the preamp and master controls were still kinda useless. Then reduced the 1M resistor of the voltage divider before v2’s second half grid to 330k, soldering a couple of 1M resistors parallel to the original 1M. That made the volume controls quite useful but it was quite harsh still. For that reason I reduced the v2 cathode resistor from 39k to 10k which gave the Overdrive channel a less aggressive feel and  increased the power rail resistor value that sets up the v2 voltage to 40k, getting a B3 of about 270v.

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I’m really happy with the result!!!  The amp sounds great, still lots of gain in the OD Channel if needed but without any harshness and the controls are quite more usable. What an improvement !!!! The normal channel is great too. For lack of a speaker cabinet, I’m connecting the head to my 5e8a combo cabinet loaded with a pair of alnicos (Weber 12A150 and 12A150B), and they’re quite a good match to both channels.

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~ by fperezroig (22nanofarads.com) on December 29, 2013.

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