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Thread: Univox U-102 Tube Amp

  1. #1
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    Univox U-102 Tube Amp

    I picked up another Univox guitar amp. Its an early 60's Univox U-102. There's very little information on this amp. I was lucky to find a schematic. It has a 12" Jensen speaker that dates to 1965. Runs on a 6AU6, 2 - 12AX7s, a 6CA4 and 2 - 6AQ5s. Its all original with two prong power cord. The Suzuki caps and the remains of a Sansui label on the top of the cabinet give it away as made in Japan.

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    Stray Cap DrGonz78's Avatar
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    That is a beautiful amp! Looks to be very well made and it's in great condition. Thanks for sharing some pics. Would love to a picture of the speaker when you get to it. Score!!

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    Quote Originally Posted by DrGonz78 View Post
    Would love to a picture of the speaker when you get to it.
    Thanks! With little to go on I figured I'd take a chance. I plan on replacing the electrolytics and oil filled caps. Here are pics of the speaker. Someone wrote 16ohms on the basket, but it measures 5.7.
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    Don't forget the joker g1's Avatar
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    Maybe at some point someone reconed it with an 8 ohm kit?

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

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    Quote Originally Posted by g1 View Post
    Maybe at some point someone reconed it with an 8 ohm kit?
    I just wrote it off as someone mistakenly labeling as 16ohms. But now you have me thinking. So I pulled the speaker. Here are pics of the cone.

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    Supporting Member TomCarlos's Avatar
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    Just curious... back in the day when this amp was new, was Jensen selling the Gold Label speaker? (It looks like a Gold Label in the pics above). Take a look at this ad on Reverb.com. The speaker does say Special Design but it looks different - like the P12R square alnicos. https://reverb.com/item/1502030-vint...-great-tremolo

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    Quote Originally Posted by TomCarlos View Post
    Just curious... back in the day when this amp was new, was Jensen selling the Gold Label speaker? (It looks like a Gold Label in the pics above). Take a look at this ad on Reverb.com. The speaker does say Special Design but it looks different - like the P12R square alnicos. https://reverb.com/item/1502030-vint...-great-tremolo
    I love the mysteries that come with these old amps. The markings on the speaker indicate 1965 production date. Unless its 1975. Here's a link to another U-102 with a blue label Jensen. This is the listing where I scored the schematic. Otherwise the schematic was nowhere to be found.
    https://reverb.com/item/3379137-univ...inal-reduction

    This is another U-102 listing with what appears to be a gold label Jensen.
    https://www.worthpoint.com/worthoped...ombo-140690892

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    Supporting Member TomCarlos's Avatar
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    Did you have a third link in your post? I do not see a link to the schematic. Can you please post that for us. Yes, this is a hard one to find!!

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    Quote Originally Posted by TomCarlos View Post
    Did you have a third link in your post? I do not see a link to the schematic. Can you please post that for us. Yes, this is a hard one to find!!
    It was in the first link. The amp posting has pictures of the amp and one of pic is of the original schematic.

    I downloaded the image. Here it is.

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    Supporting Member Jazz P Bass's Avatar
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    Here it is in pdf format: UnivoxU102.pdf

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    I finished recapping the U-102. I also replaced the rectifier tube socket and put on a grounded power cord. The volume is good. It actually sounds pretty good but something is a little off. I don't get a full spectrum of tone adjustment when I turn the tone knob. A small turn of the knob from all bass goes to all treble. Then 3/4 of the rest of the turning is all treble. Also getting breakup on the low notes all the time. I was short a cap when I replaced the 6AQ5 cathode bypass cap. The schematic calls for a 20/25. It had a 25/25. All I had on hand was a 47/50. Is that where my problem lies?Click image for larger version. 

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  12. #12
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    These are the voltages I measured:
    First filter cap - 331.7V
    Second filter cap - 294.8
    Third filter cap - 247.5
    6AQ5 plate - 325.6
    6AU6 plate - 154.2
    1st 12AX7 plate- 206.6
    2nd 12AX7 plate - 286.9

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  13. #13
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    I take back everything I said. The amp sounds incredible!

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    That 12AX7 Gain Stage/Phase Splitter is interesting.
    The first 12AX7 triode is a gain stage with a bootstrapped anode load resistor.
    The second 12AX7 triode is a Paraphase (Seesaw) Phase splitter.
    Cathode signal is fedback (as positive feedback) to input stage to bootstrap the anode load.

    Note the 50K anode load of Paraphase Splitter anode and the 100K cathode load resistor which is effectively in parallel with the 100K bootstrap resistor (the one from the top of the anode load to B+ node), so 50K load at cathode too.

    I have not seen this before.

    Cheers,
    Ian

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    Senior Member trobbins's Avatar
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    That it exactly.
    CC Gain stage anode load and bootstrap resistor both 100K in lieu of 250K and 40K in the Wireless World Article.

    In the last SS Amp I did with bootstrapped VAS load I found the ratio of anode (Collector) load to bootstrap resistor was important for harmonic mix. In that case 33K VA load and 22K bootstrap resistor.

    Wonder if same is true for this Jeffrey circuit?

    Cheers
    Ian

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  17. #17
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    The second 12AX7 triode is a Paraphase (Seesaw) Phase splitter.
    I think that (apart from bootstrapping part of the first triode's plate load) the PI essentially works as a Concertina/split-load type (the Jeffery article calles it "cathode follower PI"), and this:

    ... so 50K load at cathode too.
    ..is necessary for equal signal amplitudes at cathode and plate of the split-load PI (second triode of 12AX7).

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    Last edited by Helmholtz; 11-05-2019 at 10:19 PM.
    - Own Opinions Only -

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    My Bad - I meant Cathodyne but brain short circuited and typed Paraphase.
    Cheers,
    Ian

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    I really like the sound of the Univox U102. I had a guitar play over yesterday and we made some recordings of all my amps. He wants this one. I told him not for sale. So now we have to look for another one.

    Unfortunately some pops got into the recordings but you still get the amp.

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