Vox Kensington Bass V124

[Seawolf]

Hi, I'm really hoping somebody can help me. I got a solid state Vox Kensington bass amp to repair from a friend of mine. I don't know much about these, info is kind of tough to find. I got a schematic for it, but can't seem to locate a service manual. The transformer secondaries were frayed and broken so I fixed that, replace all the electrolytics, and re-wet all the solder joints. There were a number of joints that just had the wire coiled, with no solder so I soldered those. I also installed a grounded 3 prong cord and removed the death cap, took the polarity switch out of the equation. Since I grounded it, it's working better, but it still has this underlying fuzz/distortion when the volume is past 9 with a guitarIand with a bass it's fuzzy through most of the volume knob. Aside from checking the transistors, I'm not sure what else to do unless some of the non electrolytic caps need to be replaced. They all measure fine, but they could still be leaky I suppose. The resistors all seem to be within their respective tolerances as well. I've only been repairing amps for about 2 years, and this is my first solid state amp so I'm certainly no expert. Did this thing just suck from the get go? There's a rather new looking 15" JBL speaker in it so I can't imagine that's causing any issues. Any advice/info is appreciated. Thank you.

[Seawolf]

Since the initial post, I noticed that the JBL he installed is 8 ohm and the schematic says it should be 4 ohm. From what I remember, this isn't a big deal but will reduce the output. Could this be causing my weird fuzz and squeal above 9? Or is it likely unrelated?

[pompeiisneaks]

Not saying someone won't know how to help as there are some SS geniuses still on this forum, but the bulk of the discussion is usually about tube stuff. That being said, have you got an oscilloscope? you could trace the signal and see where it's going amiss. It could be a problem with a transistor that's being poorly biased and amplifying too much causing the distortion. This could be a resistor out of spec or a capacitor that's not blocking DC causing something else like it.

~Phil

[Seawolf]

I do have a scope. Im just not sure it’s working properly. I was trying to use it to calibrate a rack compressor a couple weeks ago and wasn’t able to get a sine wave up on the screen.
It was given to me a couple months ago and of course, like everything else I run into it’s a model that I can’t find any info on. Would I just be looking for clipped waveforms while I’m probing? I plan on buying a better scope soon, but it would be nice to get some use out of this one in the meantime. Thanks for the advice.

[Stevem]

This issue sounds like the output Transistors are not matched close enough,and to make things more difficult most of these amps used Germainum Transistors back then.
It also may be that work was done to the output stage and someone replaced one or both of the outputs with silicon types throwning the bias way the hell off!

How much D.C. Off set do you read on the output with the amp idling?

The values of the bias resistor network on the output Transistors is critical to having clean output!
Check all of these resistors with a good meter and short test leads, and test out the resistors that are under 1 ohm with a cap ESR tester if you have such.

You may also have Driver transistor ( the one before the phase splitter transformer) that's breaking Down just like I just had in a 1965 Kustom amp I just fixed that uses the same output toplogy as that Vox.

One of the best things you can to to these amps in terms of repairs / up grades is to install a modern day output IC pack with there great built in protection circuits and ditch the old output stage!

[Seawolf]

Thanks Steven! That's a boat load of awesome info. As far as whether they are germanium or silicon, I can't say, the schematic I have doesn't give any model numbers for the transistors. The two output transistors are PNP, that's about all I can say at the moment. I would love to find a parts list/service manual, preferably a free one. I will check the offset voltage when I get home tonight. I imagine it's going to be tough to find suitable replacement transistors. Please give me more info on this output IC pack. If that ends up being quicker/easier/cheaper, I'd be happy to just go that !route. Does it just replace the transistors or the entire output stage including the transformer? Thanks again!

[Seawolf]

The offset voltage is reading zero. The amp is pretty noisy and when I had a bass plugged in, with the volume up around 6, the speaker was vibrating at one point. Pretty sure that’s not normal. It was enough that you could see the grill cloth flapping a bit. Haven’t gotten it to reproduce that. When I first turn it one there’s some humming and squealing that slowly gets better after it’s on for a minute or so. I’m going to start testing all the resistors here at some point this evening. I’ll report back after I have some more info.

[Stevem]

Other then the bias resistors on the output Transistors having drifted off value it's all the darn small electrolytic caps that go bad in these amps in the preamp section that make the amp have the issue that you discribe at turn on, that being a ossilation issue.
Also the power supply rail filters need to be changed out ad they also act as decoupling filters between amp stages so they do not interact with each other.

It can be a pain in the Ass changing these all out especially in a amp with reverb and Tremolo just due to how many there are in a guitar amp model, but since yours is a Bass model I would just bite the Bullet and change them all out before you make any other judgement calls on how the amp is performing for you!

Also the difference in the speaker impeadance IS a big deal to a SS amp!
As you go up or down from the impeadance the output stages wants to see you basically cut the output stage's wattage in half, but when you go down from what the amp wants to see you will also have the output device's calling for twice the amount of current and as such they will run twice as hot for any given level of output wattage that it can muster!

A nice up grade to this and any early SS amp like this is to spend 10 bucks and make up a Zobel network to tack on before the speaker jack.

You should go to the geofex.com site as he has a ton of Vox info there and he partisapats on this site also.

[Seawolf]

I replaced those non polar Electrolytics with film caps and she’s working like a champ now. Thanks so much for your input guys. I hope to return the favor some day. Wayyy louder than it was too.

[Stevem]

That's great to hear , your buddy should be very happy that the amp has another lease on life for another 50 years!

[Seawolf]

I'm dropping it off for him today after work. Now I just need to finish his old Lafayette/Univox that I can't find any information on. Hopefully that won't be another post for help.

[Stevem]

This Lafayette / Univox would not happen to be a combo tube model with 2 channels, reverb and 2-12" drivers in a horizontal cabinet would it, or is it the vertical 2-12 model ?
Does it use 6973 output tubes and one preamp tube is a 7247?

Is it SS recto?

If so I one and have been trying to find the time to draw out a schematic for it , but it's output stage is a common Univox type that you can find the schematic for.

In regards to these Univox tube amps and all the .01 uf oil type caps that they use , they all need to get replaced which is sad because when there good these caps sound really great!

[drew]

There were a number of joints that just had the wire coiled, with no solder so I soldered those.

This sounds like you may be describing wire-wrapped connections:
https://en.m.wikipedia.org/wiki/Wire_wrap

[pompeiisneaks]

Yeah I have done wire wrapping, it works great and doesn't need soldering. If you wrap at least 4 times around the physical connection isn't going anywhere. : ) but you need to use a special tool that keeps the wire tight as it goes around.

Solder won't hurt though

~Phil

[Stevem]

RCA use to make all of there tube TVs with the wire wrap to steel pin method , and yes it works great , but was really mostly done to keep good connections happing with all the expansion and contraction these tube electronics went thru, there use on a SS amp is a bit of a waste I woukd say.