Kay 752 amp - does this sound normal?

famous beagle

Well-known member
So, this is a continuation of my "Rare Tube" thread. The said amp, a Kay 752, arrived today, and I checked it out. I'm thinking something must be not right with the amp, but I wanted to get some other opinions before I went back to the guy. Here are the issues:

1. The amp is really quiet. Even with it dimed, it's not at all uncomfortable to be sitting right in front of it. The catalog says, "Delivers a big 5 watts of undistorted music power." (See point #2 with regards to the "undistorted" thing.) Now, I know this is a hybrid (solid-state input and power tube/tube rectifier), but my tweed Champ clone is rated at 5 watts as well, and it's not in the same league at all. It's not even in the same sport. The Champ literally seems 5 or 6 times louder than this thing. One other difference is that this amp has a 6-inch speaker, whereas the Champ has an 8-inch, which I'm sure makes a difference too.

2. It has zero headroom. It's literally distorted from the moment you can start to hear signal. To give you an idea, in the "9 o'clock" clip, the amp is barely audible. That's why the hum is so loud in that clip, because I had to crank the mic preamp so high because the signal was so quiet. And, for the most part, it sounds almost just as distorted as all the other clips.

If you listen to the clips in order -- 9:00, 12:00, 2:30, dimed -- it's actually pretty hard to hear a difference in tone at all. It changes a bit in the "dimed" clip because I switched to the bridge pickup; the other three were in the middle position (all on a Hagstrom Swede). And I was probably digging in a bit more, so that probably affected it too. But, with the clips volume matched like this, I wonder if it would have made a difference at all. I mean, I was playing quietly in the 9:00 clip because I was trying to create a clean tone, and it still sounded that dirty.

So, what do y'all think? Does it sound as though this is the way the amp is supposed to sound? I can't imagine the catalog would make the previous claim with this kind of performance. I mean ... I know things were different in 1966, but every other low wattage amp I've ever heard (many of them vintage) has at least some amount of pretty clean space before it breaks up.

Thanks
 

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I'd have thought a 5watt valve power amp should be capable of creating discomfort, even with a 6" speaker.
I have a < 1watt transistor amp here that you could talk loud over, just about. It's 6" too.

I remember the BlackStar HT-1 (1watt valve) being a shitload louder than I expected, too.
I mean, it's no 100w marshall stack, obviously, but I think bedroom-amp is generous too.

That all said, I've no idea what that particular amp should sound like or what a small valve amp was like in 1966,
so feel free to file this under unnecessary shit that Steen posted. ;)
 
I remember having a Gibson GA-5 Skylark that was most likely around 5 watts, from the 60s, and it was a shit-ton louder than this too.

I just had a thought. Could a bad tube (power or rectifier) create this kind of issue? Any tube experts out there?
 
It could be a biasing problem and/or mismatched tubes.
The tone sounds brittle, and it's like I can hear your picking over the amp ...not good!
 
The 50C5 was a common tube used in table radios in the 50's and those weren't real loud. The plate dissipation of a 50C5 is rated about 5 watts. Not sure where Kay got the 'delivers 5 watts of undistorted music power'. My conservative guesstimate would be more like 2 to 2.5 watts with luck for a 50C5. Your Champ uses a 6v6 which can dissipate about 12 watts, thus more power output, about 5 to 6 watts if memory serves me correctly.

Another oddity of that amp is the usage of the first stage transistor. There is no biasing resistor from its base to emitter. It's almost like it could give distortion with any signal put into the amp. It's kind of wired to be a diode Although the resistor that supplies the collector to base appears to be a high value, so maybe base to emitter self leakage is biasing it(?)

I don't know how easy it would be for you to hot wire another speaker in a cabinet to the output to the output transformer leads of the Kay. Maybe might rule out a speaker problem such as a voice coil that's rubbing or something else. Just make sure not to run the output transformer unloaded. Kay catalog I grabbed states 8" speaker in the 752(?)

If you back off on the the pots on the guitar does the amp clean up a bit?

There are some videos on Youtube of a Kay 703 which uses a 50L6 output tube. The 50L6 is rated at 10 watt dissipation and the amp is realistically rated by Kay at 4 watts. Also is all tube, rather than a hybrid.

Kay 703 Vintage Amp Test/Demo 1963 Small Amp - YouTube
 
Yes I've seen a lot of those videos on the 703. I was hoping the 752 would be somewhat similar with regards to a bit of clean headroom, as can be heard in those videos.

I do have an external cabinet (2 x 10) that I can hook up to the 752. How do I know the output impedance of the 752?

Thanks
 
It does not clean up a bit when I back off the guitar volume. No matter what I do, I can not get a clean signal out of the amp. If there's audible signal coming from the speaker, it's distorted like that.

The 1966 Kay catalog does say an 8" speaker, but this amp has a 6" speaker. The baffle board and grill cloth look original too, though it's possible they're not. The whole amp is in pretty good physical shape, so it's hard to say.
 
Another oddity of that amp is the usage of the first stage transistor. There is no biasing resistor from its base to emitter. It's almost like it could give distortion with any signal put into the amp. It's kind of wired to be a diode Although the resistor that supplies the collector to base appears to be a high value, so maybe base to emitter self leakage is biasing it(?)
It is self biased, just not as common as to what I've usually seen with a voltage divider biasing.
 
I'm thinking if you have one that's 8 ohms that should be OK.
If it's your amp that has this schematic I couldn't tell if the speaker impedance was printed next to the speaker symbol. Maybe you can see it better?

Is there any indication the amp has been recapped (electrolytics) or just has original parts?
 

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That is the right schematic, and I can see it better on my amp, but I don't see any impedance on it. I looked everywhere.

I'm pretty sure I do have an 8 ohm speaker, so I'll give it a shot.

I haven't opened up the chassis yet to look at the components, but the tubes are definitely not new. They could be original.

If the power tube (or rectifier) was really bad, could it possibly account for this type of behavior?
 
If the speaker is correct, you can take a reading across the terminals

I was thinking about that. But I'm not exactly sure how to do it, because the speaker has three terminals, which I've not seen before.

It's kind of hard to see in this picture, but there's one green wire and two black wires, with the middle one being a little thinner than the outer one.

From looking at the schematic, I'm guessing the middle terminal is ground, the green wire is positive, and the other black wire is negative. Would that be right?

Then if I were to hook these wires up to another speaker cabinet that only had two terminals, would I just need to attach the ground wire to a ground point in the 752 amp?
 

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That third wire is marked on the schematic chassis ground.
L+R in your picture should be output to voice coil, and the centre that chassis tie.

That's easy to confirm with a meter or simply by looking at the voicecoil wires down the back of the cone, but it seems clear cut.

If you're popping in a test speaker it probably wont have a chassis tab so i'd just crimp a ring terminal and fit it with one of the speaker mounting bolts.
Hit the mating surfaces with sandpaper first and confirm you've made a good contact afterwards.

For measuring the existing speaker impedance, disconnect the wires first. Apologies if that's obvious.
 
I can't tell anything from the schematic . I haven't done any line transformer speakers since the early '80s, and I don't think I got that back then : ) Got me !
 
Make note of which wire comes off which terminal. A single speaker doesn't care about phasing, but not a bad idea to put it back the way it was.
 
I usually loop the hot/positive/special

EDIT: there may be ribbing, also

Sorry ... I don't know what you mean by "loop the hot/positive/special." "Loop" it around what? And what is "special?"

Thanks.

And I guess I also don't know what you mean by "ribbing" either. :)
 
That third wire is marked on the schematic chassis ground.
L+R in your picture should be output to voice coil, and the centre that chassis tie.

That's easy to confirm with a meter or simply by looking at the voicecoil wires down the back of the cone, but it seems clear cut.

If you're popping in a test speaker it probably wont have a chassis tab so i'd just crimp a ring terminal and fit it with one of the speaker mounting bolts.
Hit the mating surfaces with sandpaper first and confirm you've made a good contact afterwards.

For measuring the existing speaker impedance, disconnect the wires first. Apologies if that's obvious.

Nothing's obvious for me! :) Thanks for the tips!
 
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