Wednesday, July 29, 2015

The coolest lil' guitar amp ever made. WHO MADE THIS????

This is easily the most adorable amp that's come across my bench in my entire career. I don't know who it was built by, I just know it was made during the 1940's. It's a 3 watt amp using a single 6V6 driven by a 6SF5 triode with a 4 pin 80 rectifier tube. This drives a 6" field coil speaker. It was brought to me in good working order but the client wanted more breakup. It had none at all. Just a nice mellow voice. He also wanted a switch pot (it had no on/off switch originally, nor did it have a fuse holder!) and a line out to drive a bigger amp.

So basically I re-did this amp more like an old tweed Champ. I replaced the 6SF5 with a 6SL7/5691 dual triode.
Here is the original ultra simple preamp circuit. No cathode resistor, single MASSIVE 300k load resistor and an old wax .01 cap.

First step is disconnect everything except the filament wires which are the same for the 6SF5 as they are a 6SL7.

I also removed the 500K pot since I'll be replacing that with a 1 meg switch pot.

Next add in whatever will be closest to the chassis, in this case the 220k load resistor on the 6V6 and the new .022 cap feeding it:

Now add the pot. Notice it is turned a bit sideways. If I had it straight the lugs for the 117VAC would be dangerously close to some of the pins on the 6V6 so, ya do what ya gotta do!

Once the pot is installed you can build the circuit around it quite easily...

I like to use this fiberglass tubing called 'spaghetti' that you can buy from Antique Electronic Supply. It works like a charm and actually looks rather correct in a vintage circuit. They used to sell it in black but I don't know that you can get it any longer.

When doing a mod like this you never know how it will turn out until you play it. You may need to do some tweaks. The original circuit was ultra quiet. When I fired this one up it was quiet as well but when you turned the volume up it started to do this motor boat oscillation thing. BUH BUH BUH BUH in rapid succession. Easy fix. I merely needed to add another filtering stage. This amp originally had 2 filter caps and the plate load resistor for the preamp tube came off the screen grid of the 6V6. I added a 56K resistor there into a 10UF 500V cap. Oscillation gone.

Motor boating, as that oscillation is called, usually shows up with a faulty filter cap. I did once have a brown Princeton amp that had this issue and the only thing that fixed it was adding another stage. That customer had 4 of those, all sounded great, but the one had that issue. I figured it out by looking at later Princeton amps. They have four filter stages, one isn't actually being used. The brown amps have 3 just like the tweed ones do. I figured the fine folks at Fender must have started adding this stage cause it was a problem for some amps though most were just fine. So if you have a brown Princeton and it has that issue, and you've re-capped it already, you're not crazy. They were evolving at that point into what they became and they hadn't worked all the bugs out.

So how does this sound now? Tom Waits comes to mind. Like a fat radio. It's only a 6" speaker, and it's a field coil speaker at that, so it has a nice warmth to it. It now breaks up nicely and if you want it clean, merely roll back the volume on your guitar. It still retains it's sweet original voice this way, just now you have somewhere to go. If I had to venture a guess I would think this amp was built by Magna /Esty in California. It has that nice mother of toilet seat covering, pearl colored too. But really I do not know who made this amp. If you do, tell me!

Happy amping! JB

Monday, July 6, 2015

Wicked cool Deluxe Reverb on Ebay. BUY IT!!!!!!

This was one of mine. I don't remember it but I'm sure it sings. It has the nice Mogami hookup wire I used to get. Whoever buys this amp will be one happy camper!

Thursday, July 2, 2015

Carr Rambler, weak tremolo!

Here is one of my favorite modern day amplifiers. A Carr Rambler.

They are inspired by the well sought after Fender Princeton Reverb but this company has some pretty different things going on. I admit I've never been a huge fan of the black and silver panel Fender amps, too icy for my taste. I prefer tweeds and old Marshall amps. But I always dig the sound of these amps, largely because they are cathode bias rather than fixed. What this means as an owner of one of these is you don't need to bias the amp, feel free to drop in whatever 6L6 tubes you want. No trip to the shop necessary. What it also means is the tone is ultimately much warmer. These have that lovely magic midrange I need from an amp.

They also use Solen polypropylene capacitors for the power supply:

 They are huge and you will never need to replace them. They have the added advantage of giving the amp a smoother tone overall. Electrolytic caps are the 2nd from the worst, tantalums are at the bottom. The fact that Carr went this far to make a boutique amp truly boutique is impressive. I myself like a mixture of high quality components and trashy components. These don't use anything special in the signal path, just good old Mallory 150 series caps. I like those myself.

Here is a shot of the chassis:

Truly hand wired. A lot of love goes into making these amps. They are shockingly quiet amps. I could barely tell it was on until I played some music through it.

This came in cause my customer thought the tremolo a bit weak. Yeah, I agree. When I get a Princeton with a weak tremolo I usually check the resistors and caps. If they are all good then I replace the 1 meg resistor feeding the intensity knob with a 470k or lower. Tricky part is to get it strong without too much "wump wump woof woof" sound coming through. This amp has a very similar circuit except that it has no resistor. It is indeed a bit weak. So I replaced the 100k load resistor with a 220k, that helped. Next I put a .22 cap in parallel with the .1 cap that is feeding the intensity knob. To tame the  "woof woof" stuff I put in a .022 cap from pin 6 (connected to 220k load resistor and feed cap) to ground.

Also replaced one crackly 12AX7 and replaced a weak reverb driver tube with a nice NOS 12AT7. I never use new 12AT7 tubes as a reverb driver. They can die fast and there are plenty of very high quality NOS ones around.

Thanks for reading today! Feel free to ask for more specifics if you have the same issues.