Jensen C8R ?

[madison]

Anybody used one of these in a Champ?
Give me some input.
I am testing one out new Champ build.
I don't want to pass judgment too fast on it but I am thinking they aren't so great?

[jjasilli]

http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Jensen/C8R/10/1

[madison]

Thanks for the link.
I did see that before.
I've decided I don't like these speakers.
Maybe because the 25 watt rating.
I think if 25 watts were pushed into them they might sound good but at 5-6 watts on a Champ they have a strange sort of speaker break up riding on top of the the normal tube distortion.
Just not my thing I guess.

[jjasilli]

I know what you mean about that unpleasant speaker breakup - and went through a number of speakers for different amps searching for the right match-up.  Recent experience with my latest VibroChamp re-build (posted) now has me wondering if the problem is not necessarily the speaker per se, but lack of correct negative feedback.  That is, experimenting with the NFB loop can help make an amp and speaker make nice together.

[madison]

You nailed it.
I did in fact mess with the NFB loop.
I was hoping it would "fix" it.
It didn't help. :(
I also played with the cathode resistor/bypass cap and even a tone control.

The break up on this speaker was a tad "too much" or too loose? IMHO.
It's too bad, the cleans were very good.

EDIT:
I just played with it again, especially the NFB.
Disconnecting the NFB completely made it worse and increasing resistance really didn't help much either.
I would say harsh sound on the Jensen is just slightly more pronounced than the other Weber 8" I was testing it against.
Then again, run the amp though 12" sounds fantastic.
Must be the character of 8" speakers?

[jjasilli]

Sorry, I should have been more clear.  What seems to be working for me is increasing NFB strenght.  This could be done by:  decreasing the value of the NFB series resistor; and/or running the NFB off a higher ohm OT secondary tap (if any).  Note that because of the somewhat complex power equation a 16 ohm tap is about 2X that of the of the 4 ohm tap.

What I did to experiment is to use a 50K pot, wired as a variable resistor, as the series resistor.  Maybe put the pot in series with a small fixed resistor like 470R or 820R .  The pot is big enough to kill all NFB signal, or to let a heavy dose of NFB pass through the small fixed resistor.  See if enough NFB will kill the nasty speaker distortion.  Then replace the test circuit with a fixed resistor.

Also a suppression cap on the power tube was of further help in smoothing out the overdrive tone.  Checkout this thread:  http://www.el34world.com/Forum/index.php?topic=6088.0

[madison]

Yes, I understood what you were driving at.
I tried exactly like you said with the pot on the NFB.
Didn't really clean it up.

What you described with the stadic distortion in your thread is what I think I am experiencing too.

Here is another idea.
I only have a single 8 ohm secondary.
But I have 3K/5K/7K primaries.
I am using the 7K right now.
For kicks I am going to try one of the other primary taps?
I kind of wish I could go up to 8-9K on the primaries to try that too.

I am also thinking about that suppression 330 pF cap you have on your Champ.

Any other suggestions?
Thanks!

Oh, I bet your Vibro Champ rocks.
Really like it.

Unfortunately, these Champ cabs I made is just a tad too small to put in a new alnico 10" I have sitting in a box.

[jjasilli]

Unfortunately, these Champ cabs I made is just a tad too small to put in a new alnico 10" I have sitting in a box.

Maybe depends on the magnet.  My 10" alnico Emi is fine with the speaker rotated the "right" way; and that's with a big bottle 6L6 hanging there.  Before that was a 10" Weber ceramic which only had about 1/8" clearance with the big power tube.

In the VibroChamp combo, I cut the 10" hole with a jigsaw into the existing baffle board.  (Usually I use a router on a hole cutting jig.)  The speaker hole is way off-center with the speaker frame nearly up against the right side-wall of the cab, looking from the rear.  I traced the new hole so that the left edge of the original 8" hole is still the left edge of the new 10" hole.  I broke 2 jigsaw blades on those darn staples that fender used to mount the baffle board to teh combo walls.  Also inadvetently tore up the original tube chart with the jigsaw blade.

The amp does sound great though.