Brighten up a Univox U45b.

sys

Member
Messages
9
Hi all,

I posted this question a few weeks back but I think I worded it a bit oddly, so would like to try again/

TL;DR - looking to add high end response to this amp - any suggestions welcome.

I have a 60's Univox U45B which was recently serviced, recapped. converted to 3 prong, and converted to 220v by a local tech. Otherwise original, including the original Jensen C12S (a quick google search reveals this to be a rather inefficent 12 watt speaker).

Amp is killer, and does everything you'd want a 10 watt 1x12 EXCEPT that it's too dark most of the time. I run the tone full up all the time, and always wish I could get a little bit more air and presence.

So I'm looking to mod it. Here are the three options I have been able to come up with, but I am wildly inexperienced with amp modding (read - never so much as changed a speaker), and would appreciate feedback from you more experienced folks. Worth noting that I do have someone who could do the work for me if it ends up involving any serious work with high voltages.

1) Change the speaker. Seems obvious... kinda. As someone noted last time I asked about this amp, Jensen's usually don't lack presence, and I am a little unclear whether that fact this one is inefficient/low rated would affect the high end. So would just changing to another speaker (like a Jensen C12N or equivalent american voiced 50w ceramic) bring the high end back?

2) I found this video of The Guitologist changing C1 from 0.01 to 0.0047, to add a little treble to the amp. I am a little hesitant since he mentions this just "shifted the bass cut-off slightly" - can any one tell me if this will add some high end, or just cut bass?

3) I usually plug into the ACC (accordion) input since its very slightly brighter than the guitar input (though still far from enough). Looking at the schematic, looks like all three inputs are going through the same value resistor, but I assume it means that one of the resistors has probably drifted over the years. Which leads me to believe that changing out the resistor on one of the inputs can affect the high end. Would this be a good way to do it? And if yes, what value should I shoot for?

Thanks in advance and apologies if I sound a little confused and uniformed - it's because I am ;)
 

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pdf64

Member
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8,801
Assuming it’s all working ok, to brighten up a dull amp, fit a bright cap, ie a lowish value cap between input / hot and output / wiper terminals of the vol pot.
I suggest to try 220pF in the first instance; if too bright, change to a lower value, if not bright enough, change to a higher value.
Voltage rating preferably 250V or above, though 100V might be ok.
Note that the effect of a bright cap will reduce as the vol control setting is advanced. A benefit of that though is that stability won’t be compromised.
Regarding the inputs, as a test, I suggest to try disconnecting the shorting link to the switching terminal of 2 of them. ie the wire link between the jack’s sleeve and switching terminals.
If I’m interpreting the schematic correctly, that should greatly increase the input impedance of the remaining, unmodded input jack, thereby allowing your guitar’s pick up to achieve its intended treble resonance when that input is used.
 
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Muzzy

Silver Supporting Member
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3,311
Often old speakers lose their detail from age and just being tired. Those are nice sounding amps when they're running right.
 
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305
if too bright, change to a lower value, if not bright enough, change to a higher value.
Isn't this the opposite of what you should do? If it is too bright wouldn't you increase the value from 220pf to say 300pf and if it is not bright enough lower the value from 220pf to 100pf?
 

pdf64

Member
Messages
8,801
if it is not bright enough lower the value from 220pf to 100pf?
If that was the case, having no bright cap fitted would be extremely bright :)
Whereas we know from many BF Fender models that cutting the bright cap / switching it off (ie breaking the circuit between one of the cap leads and the vol control terminal) makes the amp less bright.
 

71strat

Member
Messages
10,129
Rawson Sparfield Starfinder 12in speaker. Will wake up any dull amplifier. If it will fit. Heavier than a JBL E120, or EV 12L. 100w/100db. 2in voice coil

All but an exact repro of the Fane Cresendo A from the late 60s, early 70s. 20,000 Gauss. 30hz-16,000hz frequency response. 70 Resonant Frequency/fs.

Has an incredibly strong Ceramic magnet. JBL E120 is 13,500 Gauss. Starfinder has a magnet almost 30% more powerful, and a huge frequency response. But, is very expensive.

Remember those WEM cabinets Dave Gilmour uses?? Loaded with Cresendo A???? Theyll do everything from Clean, to Mean, and are very efficient. Forget theyre 100w.
 
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sys

Member
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9
Rawson Sparfield Starfinder 12in speaker. Will wake up any dull amplifier. If it will fit. Heavier than a JBL E120, or EV 12L. 100w/100db. 2in voice coil

All but an exact repro of the Fane Cresendo A from the late 60s, early 70s. 20,000 Gauss. 30hz-16,000hz frequency response. 70 Resonant Frequency/fs.

Has an incredibly strong Ceramic magnet. JBL E120 is 13,500 Gauss. Starfinder has a magnet almost 30% more powerful, and a huge frequency response. But, is very expensive.

Remember those WEM cabinets Dave Gilmour uses?? Loaded with Cresendo A???? Theyll do everything from Clean, to Mean, and are very efficient. Forget theyre 100w.

Thanks for the suggestion - I'll keep it mind but might be a bit extreme for the time being.

Now just debating between doing the bright cap mod and a new speaker - but if I go down the new speaker path, I think I'll start with just getting something more in the Jensen lineage, and see if the issue is just the original speaker having aged out.
 

71strat

Member
Messages
10,129
Speed Shop Tweed Deluxe amps has a great Jensen repro thats Alnico. 20w


A12Q Tweed Deluxe guitar amplifier speaker.

Designed after the original P12Q speakers from 1958 to 1960, the RecPro Audio Tweed Deluxe Speed Shop A12Q captures the true vintage Jensen sound and performance from the era, but without all the hassles and risks associated with vintage speakers.
 
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If that was the case, having no bright cap fitted would be extremely bright :)
Whereas we know from many BF Fender models that cutting the bright cap / switching it off (ie breaking the circuit between one of the cap leads and the vol control terminal) makes the amp less bright.
Thanks, I knew there was more to it than met the eye.
 

Devin

guitar enjoyer
Silver Supporting Member
Messages
4,372
I think I have the same amp. I also use it with the tone all the way up. I would consider trying a treble booster or Klone before modding the amp. But it sounds like you want to mod it. So I would start with a speaker for sure. Maybe a bright alnico. Mine was chopped when I got it so I converted it to a head and it sounds great through a cele blue.
 
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sys

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I think I have the same amp. I also use it with the tone all the way up. I would consider trying a treble booster or Klone before modding the amp. But it sounds like you want to mod it. So I would start with a speaker for sure. Maybe a bright alnico. Mine was chopped when I got it so I converted it to a head and it sounds great through a cele blue.

I usually run a Wampler Black 65 in front of it, which, yes, adds a lot of presence, but I still feel like the amp is not at it's full potential. I also run a treble booster into it on occasion, and its a fantastic sound, but also not exactly what I am looking for.

Thanks for the info regarding the celestion blue - I am leaning more and more towards a speaker change.
 




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