Mini Jubilee 2525c

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JD
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Mini Jubilee 2525c

Post by JD » Wed Nov 28, 2018 11:05 pm

Anyone try this amp? I've been looking for a low power "at home" combo and picked up one. Really impressed with the sound for the price and size.

Curious how they get 15w out of a pair of EL34s though.

More curious what other power tubes I could try. Not because it doesn't sound good; just for fun. Power tubes appear to be plug and play with no bias adjustment.
Last edited by JD on Sat Feb 09, 2019 10:41 pm, edited 1 time in total.

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Re: Mini Jubilee 2525c

Post by JD » Sat Feb 09, 2019 5:22 pm

Wondering if some of you amp gurus can offer some opinions.

Bought this little amp without tubes (owner kept his vintage glass) and I installed a set of used but good Groove Tubes 12AX7-C (Chinese) preamp tubes and EL34-M power tubes. I had these in another amp that I sold and know them to be good tubes. Amp sounds incredibly good for the size and cost.

I like to play the amp "dimed" just using the guitar's volume and tone controls to control volume, gain, and tone. Channel 1 (clean channel) sounds glorious this way. Issue I'm having is when I switch to channel 2 (lead channel). The amp feeds back pretty severely unless most all of the amp's knobs are all dialed back about 1/4 of the way (i.e. 3/4).

I have eliminated the cable and guitar/pickup as the source, as it is identical with other (quality) cables and guitars. I thought maybe it was the Chinese Groove Tubes so I ordered and installed a set of Mullard reissue CV4004/12AX7 and EL34 tubes. The Mullard preamp tubes (with either the Groove Tubes or Mullard EL34s) were even worse, now feeding back on both channels.

Switched back to the Groove Tubes and works as before - channel 1 ok dimed but channel 2 feeding back when channel switched.

Wondering if this is just inherent to the 1x12 combo design, or if I need to continue to look for a better preamp tube. Maybe another brand, or perhaps vintage tubes? Maybe part of the amp's design using PCBs? Something else?

Of course someone is going to say just use a pedal or don't play it wide open, but honestly channel 1 sounds soooo good this way this is how I'm going to play it. I'd like to be able to use channel 2 for some heavy distortion (almost like a pedal) when needed without it freaking out.

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Re: Mini Jubilee 2525c

Post by JD » Sat Feb 16, 2019 9:59 am

Well if anyone is curious I've done some tube swapping and it seems this gain channel feedback isn't associated with any tube brand and is inherent to the amp's design, or maybe just the one I have.

What is interesting is that, to my ear, no combo of tubes sounded better in this amp, both tone and noise floor, than the Groove Tubes (rebranded) 12AX7-C and EL34-M combo (all Chinese mfg). Maybe because Marshall used these preamp tubes stock for a number of years that sound has sort of become familiar to me. Although an older set of big bottle 6CA7 Westinghouse branded tubes sounded pretty good as well.

Maybe since this amp doesn't really fully push the output tubes it is difficult to really hear differences.

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Re: Mini Jubilee 2525c

Post by danman » Sat Feb 16, 2019 8:00 pm

As with the new DSL20, Marshall is using lower plate voltages along with cathode biasing in these newer amps to achieve the 20 watt rating. I have heard many good reviews about the great tone of the Mini Jubilee but I have never heard any complaints of the type off feedback you are experiencing. Maybe try a 5751 in the v1 spot and see if that helps.

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Re: Mini Jubilee 2525c

Post by JD » Sun Feb 17, 2019 5:19 pm

Thanks, Danman.

I can turn the amp on and dime all the knobs on either channel and it's quiet as a church mouse without a cable plugged in. Only when a cable is plugged in (with or without a guitar connected) does the amp squeal, and that is only on channel 2 (high gain channel) with all knobs dimed. If I dial back just the tone stack about 1/4 way (treble, middle, bass, and presence) it is fine. Severity varies with preamp tubes, but It is independent of power tubes. I've tried a number of modern production preamp tubes and they all do it to various degrees, but some are definitely worse than others. As noted, the cheap Chinese Groove Tubes seem to sound the best from a tone perspective but are about in the middle for this squealing issue.

I've just ordered and installed a set of NOS GE USA short plate tubes and that seems to be the best solution. Since I do adjust the tone knobs back from 10 to get the best sound, I can now switch between channels without feedback or volume loss. Unfortunately the tubes cost about 10% of the cost of the amp itself.

The other thing I noted is the amp ships with only a shield over V1. I found the exact shield online and ordered two more for V2 and V3 as the chassis will mount these with no modification. I hope that is "good enough".

Otherwise I'm quite happy with the sound of this little amp.

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Re: Mini Jubilee 2525c

Post by JD » Tue Feb 19, 2019 7:52 pm

If anyone is interested or has a similar issue, I got in two preamp tube shields. They are available online and made by Belton. The chassis mounts these with no modification. Hard to understand why Marshall didn't factory install these. These shields made a remarkable audible improvement for both the noise floor and this odd feedback issue. So other than playing around with a couple more tube combos for curiosity, it's probably as good as it's going to be without removing the chassis.

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