Epiphone 75th Ann. Electar Century Amp for Jazz

[Thumpalumpacus]

Aren't the internal volume in cubic inches and dimensions of the cab just too small to ever produce big and strong bass tones?

I have a Bluesbaby 22 in a small DIY solid pine cab (Blues Jr sized) with a rather thin plywood baffle and a Fender Blues Deluxe (ply cab with thick particle board baffle and about 1.5 times the dimensions of the Bluesbaby). I have Jensen Neo 12-100 speakers in both, but the Blues Deluxe has so much more 'oomph': bigger bass and a more solid feel to it. I thought it was the amp as they differ a lot (tube vs solid state) but to my surprise I found that playing the Bluesbaby thru the Blues Deluxe's cab mad that sound nearly identical to it!

Yeah, cabinet size is sure important, especially in a closed-back, because where the soundwave is in its oscillation upon reflection matters. Longer soundwaves (i.e. lower frequencies) benefit from larger cabinets. This is why bass cabs are usually deeper than guitar cabs.

[BeBob]

Dennis -- my amp is the 1.6 version which matches the schematic I posted earlier. It has the pull-for-boost function on the volume knob and a front panel jack for a footswitch. The boost function is clearly labelled on my amp, so I doubt that anyone could miss it. I can't speak to whether it was marked conspicuously on earlier versions, but on my 1.6 version it's obvious that the boost is there. Yes, the hum becomes far more obnoxious with the boost on, but I thought it was objectionably loud even with the boost off. IMO a quiet amp is a sign of good build quality and my amp wasn't quiet as it came from the factory.

[BeBob]

Little Jay -- the volume is small, but it's not a sealed cabinet. It's more of a partial open back because there are several vents cut into the back panel to allow the tubes to dissipate heat out of the box.


I've been using mine with the back panel removed, and if I ever put it back on I'll likely enlarge the holes to make it more of an open back configuration.

In an open back cab the baffle size is important -- if the baffle is too small then bass response is adversely effected. This cabinet does have a small baffle, but I don't think that the bass response of the speaker/cabinet is playing a large role in defining the LF response of this amp. My 1.6 version of the amp uses an Electar-branded speaker that looks and sounds like your garden variety inexpensive 12" speaker. It doesn't sound bad. Other amps played through this cabinet have decent bass response, and the Electar amp played through other cabinets still has lousy bass response.

This is because the amp's electronics are designed with several high-pass filters that intentionally throw away most of the LF signal. I think that the ~700Hz HPF was probably designed to emulate the poor bass response of a field coil speaker. But if that's what they were thinking, I don't like the result. Another HPF at ~240 Hz was added in ECN 1.6, presumably as an effort to push the AC line noise down another 12 dB. With all of the HPF the result is that the LF response of the amp is very poor. The low strings sound thin, sharp and tinny, without much fundamental, but with plenty of harmonics. I don't think it's the cabinet's fault.

[Little Jay]

Little Jay -- the volume is small, but it's not a sealed cabinet. It's more of a partial open back because there are several vents cut into the back panel to allow the tubes to dissipate heat out of the box.


I've been using mine with the back panel removed, and if I ever put it back on I'll likely enlarge the holes to make it more of an open back configuration.

In an open back cab the baffle size is important -- if the baffle is too small then bass response is adversely effected. This cabinet does have a small baffle, but I don't think that the bass response of the speaker/cabinet is playing a large role in defining the LF response of this amp. My 1.6 version of the amp uses an Electar-branded speaker that looks and sounds like your garden variety inexpensive 12" speaker. It doesn't sound bad. Other amps played through this cabinet have decent bass response, and the Electar amp played through other cabinets still has lousy bass response.

This is because the amp's electronics are designed with several high-pass filters that intentionally throw away most of the LF signal. I think that the ~700Hz HPF was probably designed to emulate the poor bass response of a field coil speaker. But if that's what they were thinking, I don't like the result. Another HPF at ~240 Hz was added in ECN 1.6, presumably as an effort to push the AC line noise down another 12 dB. With all of the HPF the result is that the LF response of the amp is very poor. The low strings sound thin, sharp and tinny, without much fundamental, but with plenty of harmonics. I don't think it's the cabinet's fault.

It's easy enough to remove thise caps out of the circuit and solder a piece of wire in their place right? I modded the circuit of my Blues Deluxe that way, had some weird caps too. Made the amp a whole lot better.

[BeBob]

Sure, solder away. The key is to know how the individual parts change the features of the circuit so that you'll get the effect you're after. But it's not as simple as just replacing a few caps with jumper wires. The gain structure of the amp is effected by more than just the caps.

If you just remove the filters that I mentioned and replace them with jumpers, I guarantee that you won't like the result. There will be a huge increase in gain, and the noise floor will come up by perhaps 30 dB. That will absolutely ruin the amp for jazz.

[Little Jay]

Oh ok, from your comments I assumed they actually just added those caps to the original circuit, and removing them would change the circuit back to original. I jumped to conclusions too quickly, sorry.

What I did with my Blues Deluxe is turning the circuit 'back' into an AB763 circuit and that is of course a proven design.

I must admit I did not compare the 75th Ann.'s circuit to that of an original Electar Century.... I should have before shouting to take everything out!

[zephyrregent]

This isn't an update but a new video from Epiphone demonstrating their new Masterbilt Century Zenith archtop played through a Century amp. The tone is good for one of these new-fangled piezo/whatever it is pickups and reminds me of vintage archtop recordings, sounding very acoustic.

[Max0817]

Thank you, Gentlemen, for those very informative posts.

I had already been to the amp's page on the Epiphone site during my search for the schematic but I failed to find it, because Epiphone doesn't bother to provide any links to technical data for the amp. The schematic was posted as an attachment to a blog post by a 3rd party user. I never would have found it without knowing to look in the blog posts, because I don't read blog posts.

Thanks for the tip on where to find the schematic. Here it is in case anyone needs it:

Attachment 32161

Anyone have the Epiphone century schematic that Bebop uploaded in good quality that you can share please.

Thanks !!