Modding my Silverface Champ some more...

[212Mavguy]

Have to say that I need to thank Dan Torres for his book, Inside Tube Amps. I borrowed it from a luthier friend who had it sitting on a shelf unread for quite some time. In the past I had done some preamp mods in my lowly Silverface, increasing the value of the cathode bypass caps for each of the 12ax7 sections and also for the power tube cathode bypass. I had gone too far with increasing the values for the 12ax7 halves, particularly the second half. The reason for the mods was to replace the 3 exploded stock 25/25 caps. The previous owner had a habit of boosting the you know what out of it to push for stage volume. I had gotten the amp secondhand for 75 bucks in poorly operating condition, low volume output and the speaker had a rub.

Although the amp now sounded huge, it had become a bit woofy from my overdoing those caps After taking the second stage 12ax7 cathode bypass cap value down and slightly reducing the value for the power tube bypass it opened up more in the upper middle and top. I presently use it as a head into a 16 ohm 2/12 JBL g125 or 2/15 JBL g135, because after changing out both trannys, especially the huge, oversized new output tranny it became too loud for it's tube health, they rattled around too much from the speaker in the cab, I had tried a 10 inch Peavey Scorpion. The stock POS speaker was toast. The next mod was to increase the value of the resistor going from the bass pot to ground, think that stock was 6.8k, now it is 10k. Have a more likeable mid content now. Will need to go shopping for some more caps for the pre next. Will be taking both sides of the 12ax7 down more, especially the first half. Right now with the 2/12 cab it gets too loud on stage with just a 6v6. It can run 6l6's and el34's because it now has a fixed/cathode bias switch and fixed bias pot. There is just enough coming out of the main tranny to run a 6550, but I have other amps for that kind of wattage. Right now it is a wolf in sheep's clothing. After further voicing in the pre I am confident it will open up even more. Next will be some adaptors to experiment with a diffferent preamp tube (Raytheon 5755) and also try a Bendix 6384. I have an 18 watt boutique that runs Bendix 6094's. That amp sounds spectacular.

Without the book I would not have known what to do. A bit spendy but worth every penny.

 

[Old BF Shred]

IT"S ALIVE!!!!!!![/b] :twisted: :twisted: :twisted:

Nice work Dr. Franken-Mav! :lol: :lol: :lol: :lol: :shock:

 

[212Mavguy]

It's Sundance Film Festival time here, and I got a call from my former roomate, internationally touring performer Chicago Mike Beck, who is running an ongoing presentation of musical artists on stages throughout the festival known as the Radical Rat Review. Said he had an artist coming out from Boston and her guitar player did not have an amp. So I lent her guitarist the Franken-Champ and a 16 ohm 2/12 with JBL g125's for a few gigs, will get some feedback tomorrow night when he returns it, in Boston he has a fully decked out recording studio with several amps. Will be cool to find out how it played out after the latest surgery with a few pedals in front.

This morning I pulled the trigger on some tube adapters, one for a 5755 to 12ax7, and another adapter for the infamous Bendix beast-a-tron 6384 to 6l6. Supposedly the 5755's were quite expensive to produce, and are very well and durably built. The rectifier will be a super beefy Cetron 5r4 tater masher. I thought I'd like to set up the Frank-en-champ tubed a la cold war for giggles.

 

[212Mavguy]

The feedback from Sundance was that it had a bit too much bass still, but still sounded amazing for a Champ. The right amount was with the bass pot turned all the way down. Tried the 5755 adapter, it did not work, the socket rim that stuck up to hold the metal tube shield prevented the pins from seating, the adapter was too large in diameter to work. Found out that the screen voltage was too high to run the Bendix 6384, so I abandoned that too.

So it was time for some surgery to take care of the excess bass. About that time I by chance saw a Blackface garolite turret board on eBay. I did not win that auction, but the seller made up one for me for the same price. YAY! So the surgery went from a couple parts to a full blown yank the old circuit and complete rebuild.

Next I went shopping for resistors and caps, I chose tantalum resistors from a supplier in Hong Kong for the tone curcuit, Kiwame carbon film resistors for the power supply from the same supplier, Jensen copper foil in oil 630 volt caps and the rare and wonderful Rubycon Black Gate caps for the cathode bypasses from Angela Instruments. F&T axials were selected for the filtering, went for 30mfd instead of the 22 as called for, the silver mica 250pf tone cap and filter caps came from Antique Electronic Supply. A week later, all the parts had come in.

Filled up the board, and went to install it, the Ceriatone layout called for a star ground. That meant about a dozen chassis ground points needed to be disconnected, and a new ground system needed to be fabricated. Many hours and a few days later I had it up and running, a howing hum ensued upon the rec tube getting up to putting out current. I was truly upset, because I did not know where things had gone wrong, I had taken meticulous notes on the teardown for every wire in the amp. Finally, I looked at the layout drawing from Ceriatone again, and noticed that the ground for the speaker jack was labeled with a wire to go to star ground. So I did that, and things were fine. I had also forgotten to ground the resistor going grom the bass pot, the amp was insanely loud even with a 12at7 in the preap tube slot and the tone controls were not working. I connected the resistor and the amp was of more normal volume and the tone controls worked. However, I noticed that the tone with the tone controls disconnected was more filled with harmonics besides much louder, so after a few minutes' playing I disconnected the tone controls again, and because the mix of parts and values on the board was so good by design, the tone was everything I liked, smooth, warm, lots of mids, and not too fat on bottom.

It is truly a fire breathing baby monster, the loudest 6 watts I have ever heard in my life. Of course, having a 5-7 pound replacement output transformer that is larger than the main tranny, (needing mounting in the bottm of the amp cab) and running into a semi closed back 16 ohm 2/12 with JBL G125's in it had something to do with that as well. EVM 12l eat yer heart out!

The tone is amazingly musical. Breakup is smooth and creamy, not nasty and fizzy. I was really discouraged at first fireup, but now that it is done it is well worth the effort, sounds like a 1500 dollar boutique amp, and why not? The parts in the board are better than most of those amps use.

The point of all this is to encourage those who might be sitting on the fence and wonder what it might be like to build an amp from scratch, whether from a kit, or by sourcing the parts bit by bit. It was well worth the time and money I spent and I dare say that others who have done so feel the same way as as well.

 

[212Mavguy]

Finally have all the stuff done. No more excess bass, now what is there is enough and well handled at any level of gain, tones sound like the instrument plugged into it, very distinct difference for each pickup selection, very lively, and musical. There are more distinct differences in sound due to tube rolling presently than with the original circuit board and parts.

After experimenting with locally available caps for the cathode bypass places in the circuit and getting the best sounding values for tonal objectives/desired voicing, I ordered to Hong Kong again for some more Black Gates. Now with them installed I can say that all de-woofing objectives have been met, the clean tones are to die for no matter what pickup is plugged in, and overall the sound is very lively, agile, detailed, and amazingly sweet at a wide range of gain setiings. Huge, almost metal-ish amounts of gain are there straight into the amp when using 12ax7, so by using an appropriate pedalboard in front the amp's single working volume knob can be used as a master volume with the preamp board in front handling all the eq-ing. It sounds wonderful with at or even au types in the preamp slot, especially with some sort of pedalboard or preamp in front of it to provide dirt.

 

[Old BF Shred]

Hey Mav, sounds like this has been fun for you.
I have considered building an amp, and may very well do that once I know how this economy (and my career future) pans out before I spend any more $.

But, I have recently built a distortion pedal and a compressor. The parts quality is higher than what you'll find in most commercial built pedals and for a lot less $....just cost me time and solder.


Neither pedal took me more than 5 days to complete, including paint and graphics, and they work great. I'd encourage anyone with a basic electronics understanding to give it a try! 8)

Take care

 

[212Mavguy]

Time and solder are a good price to pay! Weber makes some fairly inexpensive kits for a surprising number of amps, headshells, cabs and combo cabs for the diy-er at very decent prices. Other great diy-ers are Ceriatone, Allen amplification, and Torres engineering. Angela Instruments was great to get parts from as well as Antique Electronics Supply and others. I am 100% with you on the premium parts thang. The sound is worth the effort.

Got to use it on stage last night along with my Maverick, they blended together very well and sounded great.

 

[Old BF Shred]

Yes, I have been looking at the Webers mainly...seem to be the best overall deal, though Ceriatone seems reasonable as well.
As I say, I'm on a spending freeze 'til I know how the economy pans out. But one day........ 8)

Later!

 

[212Mavguy]

I'd go Ceriatone. And stick in a Mercury Magnetics OT from the Axiom line... :wink: