Carr Mercury

[skyboltone]

Ok, so we talk about mostly Dumbles and trainwrecks.

But, while looking at a different board, folks are talking about the Carr Mercury. It's got everything I need. Has anybody stripped one of these guys down to his underpants to see what gives? It's got a single ended EL-34 and sounds really good.

http://www.carramps.com/carrframeset.html

That's the direction I need to go. Listen to the clips.

Any help?

Dan

[LeftyStrat]

Take a look at a couple of Fender champ schematics, specifically AA765 (Blackface two tone control champ), and the 5F1 (No tone controls).

The boost switch in position two just bypasses the tone stack. I imagine position three probably adds a bypass cap to the second stage (not present on the 5f1), probably something relatively small, say 1uf, i.e. "low end focus is refined" (from the manual). More fun would would be something like the "Texas Tea" control on the Kendrick Climax that gradually bypasses the tone stack, making it continuously variable from blackface to tweed.

The champ runs a 6v6, but sounds great with an EL34, though you may want to fiddle with the cathode resistor. Use silicon instead of the rectifier tube, add a reverb circuit from a blackface princeton; parallel 12at7 driving the tank, 1/2 12ax7 as recovery. Other 1/2 is mixer stage (third stage in the princeton schematic). The cut is probably a conjuctive filter on the output.

The low power switch probably an lpad of sorts. Personally I would maybe do something like powerscaling and have continously variable output power.

But start with the 5f1 schematic and you're most of the way there. You may even find that it is as far as you need to go. Oh, but use higher value caps in the power supply, maybe 40/40/20. For iron, maybe a Hammond 125ESE and 270DX.

Man, I'm still on my first cup of coffee and I've already dissected an amp design. "Hi, my name is LeftyStrat, and I am an ampoholic."

That does sound like a great little combo. And from the sounds is definately a champ derivative, regardless of the marketing hype.

[LeftyStrat]

Oops, left one thing out. Drop the negative feedback from the 5F1 schematic. You'll like it.

[skyboltone]

Oops, left one thing out. Drop the negative feedback from the 5F1 schematic. You'll like it.

Thanks Lefty: Like I always say, If one cup is good two's gotta be better.

[sliberty]

I have to admit tha tthe cleaner sounds on the Mercury are just beautiful! The dirty compressed tones defintely remind me of a Champ though, as was stated.

[keithrick]

I played a Carr Rambler in GC last night and yeah the clean was incredible!!!! The reverb sounded so good I am going to have to go back to the drawing board on my ODS reverb circuit.

BTW-I was thinking Fender while playing didn't get a chance to crank it though.

[David Root]

Check out the schematic on this one, at schematicheaven.com; it's a tweed Champ circuit with no feedback & tons of filter capacity. I'm thinking of doing something similar with a single KT66/EL34 for about 10W, but maybe using choke feed on the plates and screens, which will probably sound better than resistors and need less filtering.

One question about this schematic. What is H.P., the middle output jack?

[xtian]

Client brought in his Carr Mercury, and I'm impressed. Great build quality. Super quiet at idle, lots of great tones. The power scaling is interesting--there is an inductor (looks like a voice coil) inside an additional "doghouse", that also contains the bias adjustment pot.

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[FUCHSAUDIO]

I'm glad you are "impressed". I honestly think Carr amps sound pretty good. They feel and sound like a really good old BF Fender (likely because the circuits are basically similar).
I do have a real problem with any amp covered in useless silicone snot.

[xtian]

I do have a real problem with any amp covered in useless silicone snot.

Well, not entirely useless. This is a combo amp, and all the chassis-adjacent wire runs are tacked down to avoid rattles. But, yeah, there's a lot of unnecessary goo, too.

[cdemike]

This will have next to zero impact in a class A amp like this, but I do really like how Carr uses film caps for B+ filtering -- zero maintenance other than tube changes!

I'd probably go with variable NFB rather than omitting it completely, though that would require some thought into how that might interact with a switchable cathode bypass cap at the second stage. Something like a 250KL pot would basically eliminate NFB at its upper range if you inserted the NFB signal directly at the cathode like on the 5F1, but adding a partial bypass to that stage would function kind of like a 5F6-A/Marshall-style presence control by partly defeating NFB in addition to bypassing the cathode. If you decided to go with the AA764 version where NFB is inserted at the junction of the cathode resistor and a series 47R resistor, the NFB signal might be a little weak with no bypass cap. That would also function kind of opposite to a presence control as well, since the bypass cap forms a high pass network rather than a low pass network like in a presence control. That could actually be pretty cool since the the corner frequency for the highs boosted by the bypass cap and corner frequency for the high pass filter shunting NFB treble to ground will most likely be different, so you could get some voicing options not otherwise available to you. Fully bypassing that stage would provide as close to neutral a response possible by boosting all frequencies via the cathode bypass and passing nearly all relevant frequencies from the NFB loop. I'd probably go with a 5KL if you decided to the AA764 variant to have more useable range from the NFB control. 30% taper 1MA and 25KA pots would also work if you really wanted to be sure you're defeating NFB.