#1
The Dad used to play guitar way back when, but he only played about a year. I did a little bit of cleaning-up and work on his guitar but it is still pretty bad. It is my back-up, back-up, back-up guitar. But his amp has been sitting in our junk room for a while now, it has not been played in probably 50+ years. I was really on the fence about working with it because there were peanuts in the bottom cavity of it, mouse-matter everywhere, it could potentially be an electricity hazard, etc. So I made the deciding factor today: If it had tubes innit I would put some effort into it. I asked him, it did. So here we go!














Schematic Found Online

After pulling the tubes to examine them, I sprayed some contact cleaner on the pins and re-seated them. I plugged the amp in, turned it on (by clicking the tone knob clockwise of all things) and heard this discouraging low buzz/hum. I would guess this is likely because of a bad output transformer (if you can see in the pictures, it looks tiny) as well as there is not a ground on the input power, it is just a 2-prong plug. There may be other issues as well. From what I understand, the mains power feeds directly into the amp. And looking at the circuit, it feeds right into the tone control, which probably explains why I can change the tonality of the hum.

The tubes started lighting up, though. The middle tube is actually the power tube, and it took the longest for me to see the most fire. The tube closest to the left is a preamp tube, and the right-most tube drives the tremolo circuit. The buzz and hum at the beginning of start up is probably not good for the tubes, since they are being hit with signal before they even get warm. Adjusting the tone knob from minimal position (turning the amp off is turning it all the way counter-clockwise) then going to maximum reduces some of the hum but not all. In fact it just seems to take some of the bass out of the hum. The hum is independent of the volume control.

I gave it a few moments to warm up and decided to try to plug in a guitar. And would you know, it passes signal. And it sounds pretty okay, too! (aside from the hum). It cannot get loud at all, and turning the volume up just kind of increases gain. It sounds like what you would expect a vintage amp to sound like. The cleans actually sound pretty good and the amp is responsive to dynamics.

And this is all with a speaker cone that is 40% broken apart! You can see in the pictures that it has probably broken apart from age and mice (mostly the fault of the latter I would think). I wanted to try putting in a spare speaker I had lying around but I do not have any of the same size. I looks like a 7"? I have two 6.5'' and a 12''. I might could try fitting the two 6.5'' speakers in. But I am not really sure about matching the power and ohms. I doubt the amp puts out more than what the speakers together can handle. But I am curious about the ohmage- the schematic looks like it says 3.2 ohms. I measured the + and - pieces and it read something around 1.5 ohms, which I know the reading is usually a bit off. Could I just put in two of the speakers without further harming the amp? I know tube amps do not like mismatches but can handle speaker ohm ratings higher than the output rating.

Also, what info can I get on the tubes? They are all 7-pin tubes, Japanese-made it looks like. The brand looks like HE but I am not sure. I cannot find that brand online. NOS 12A(X)6's are not that expensive so I do not think they are worth much.

So if anyone could help me on the tubes, speaker swaps, or the hum/buzz, I would be very appreciative!

Here is a sound clip of the amp: https://soundcloud.com/celestialcannons/kent-trem-tone-amp
Last edited by Will Lane at Feb 4, 2016,
#2
You could use a couple of 8 ohm speakers in parallel to get them to 4 ohms. The old Fender Champs had 3.2ohm speakers, and it's pretty common practice to replace them with a single 4ohm speaker. Ted Weber has several small 4 ohm speakers that would work. Personally, I'd be tempted to rout it out to put at least an 8" or 10" speaker in there.

Those are some funky tubes, though. That's good and bad, because the demand is much lower for them but they could be hard to find. If you can find them, they should be cheap. (old stock is going to be your only option with these)

The hum could be one of the tubes, but I'm thinking it probably just needs a recap. Sitting that long without being powered up has probably dried up any of the electrolytic caps. That's a pretty cheap job and the parts are standard. If you are confident with a soldering iron you could DIY. A good amp tech should be able to recap it for you otherwise.
#3
I actually really like the tremolo sound on that thing.


Have you tried it with drive pedals yet? I bet you could get some cool ratty tones out of it.
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#4
it looks like someone drilled holes in the speaker cone?

but damn, it sounded pretty good to me.

congrats!
#5
That hum sounds like dead/dying power supply filter caps.

Otherwise it sounds pretty damn sweet;

HNAD
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#6
Quote by H4T3BR33D3R
I actually really like the tremolo sound on that thing.

Have you tried it with drive pedals yet? I bet you could get some cool ratty tones out of it.
No I haven't! I would be afraid it would be too much for it! And yes the tremolo is fairly pleasing, at least at moderate settings to me. At higher rate and intensity settings, it just kind of putt-putts.
Quote by gregs1020
it looks like someone drilled holes in the speaker cone?

but damn, it sounded pretty good to me.

congrats!
What you are actually seeing is the other side of the cabinet/grillcloth. The cone is the darker parts around the holes. There are just chunks of cone completely gone!
Quote by Stickymongoose
You could use a couple of 8 ohm speakers in parallel to get them to 4 ohms. The old Fender Champs had 3.2ohm speakers, and it's pretty common practice to replace them with a single 4ohm speaker. Ted Weber has several small 4 ohm speakers that would work. Personally, I'd be tempted to rout it out to put at least an 8" or 10" speaker in there.

Those are some funky tubes, though. That's good and bad, because the demand is much lower for them but they could be hard to find. If you can find them, they should be cheap. (old stock is going to be your only option with these)

The hum could be one of the tubes, but I'm thinking it probably just needs a recap. Sitting that long without being powered up has probably dried up any of the electrolytic caps. That's a pretty cheap job and the parts are standard. If you are confident with a soldering iron you could DIY. A good amp tech should be able to recap it for you otherwise.
Quote by Cathbard
That hum sounds like dead/dying power supply filter caps.

Otherwise it sounds pretty damn sweet;

HNAD
It very well could be. The ValveKing I have has a similar hum and I deduced it was the filter caps, but it is much more subtle.

A chap on TGP said this:

This amp has what is called a line-derived power supply. That means that the AC from the wall is rectified directly, without the safety that the galvanic isolation of a power transformer would provide (since this amp does not have a PT). It will not be really safe to use without the rather expensive modification of adding an isolation transformer. Looking at the pictures, my recommendation would be to put it out for the trash guys to tote off, since you would never recoup the money it would cost to make it safe.

The hum comes from the fact that the amp is single-ended (no cancellation of hum due to a push-pull output stage) plus the fact that the power supply is half-wave rectified, which results in the ripple hum being 60 Hz rather than 120 Hz (and consequently harder to filter out).
I could replace the filter caps, but it looks as if I would really need a proper transformer too, in order to make it safe and properly eliminate noise.
Last edited by Will Lane at Feb 4, 2016,
#7
It's a half wave rectified direct mains? WTF?
Try upping the reservoir cap to near the maximum allowed by the rectifier. The datasheet for the rectifier will tell you what that is.
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#8
Quote by Cathbard
It's a half wave rectified direct mains? WTF?
Try upping the reservoir cap to near the maximum allowed by the rectifier. The datasheet for the rectifier will tell you what that is.
You've lost me. I think I understand the basics of those units but I couldn't easily find them. I'd either have to have someone help me or do it for me, and that may be more trouble that it is worth.

I should also mention that silver can to the right is what the schematic calls a filter. I thought it was a tube at first. There are a lot of wires going to it.
#9
The reservoir cap is the one immediately following the rectifier tube. Is the filter a multiple tapped capacitor maybe? Having up to three capacitors in one package is fairly common.
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#10
Quote by Cathbard
The reservoir cap is the one immediately following the rectifier tube. Is the filter a multiple tapped capacitor maybe? Having up to three capacitors in one package is fairly common.
Ah, that is where the issue is. There is no tube rectifier. In fact I do not see much in the schematic that looks like a rectifier. Not sure about the cap.
#11
Oh yeah, it uses a diode so you can up the reservoir cap to buggery. Replace the 80uF (or does that say 60?) with something much larger, say 220 or 470 uF.
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#13
It will filter out the hum much better. The only reason they would have used such a small value would be to save some money. Increase the 40uF one by the same amount too.
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#15
Neat little amp! HNAD!
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#16
This little thing sounds sick. A little work and I bet that'll be an awesome amp to have around.
I don't give a shit if you listen to me or not
#17
Without a power transformer, I'd be scared to use it TBH.
What you need is a centre tapped 220V transformer (so 110+110). Add an extra diode for full wave rectification, beef up the filter caps like I explained and you'd be all set.
It must have been a cheap amp in its day. I mean really how expensive is a PT for a 5 to 10W amp? It would have to be so cheap that that cost mattered. Nobody would dream of doing it that way these days. It wouldn't be legal in most countries today.
Is it a Sears amp?
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#18
nice score

sucks it's dangerous, though because it sounded really good in the clips. nice playing too.
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#19
Quote by Cathbard
Without a power transformer, I'd be scared to use it TBH.
What you need is a centre tapped 220V transformer (so 110+110). Add an extra diode for full wave rectification, beef up the filter caps like I explained and you'd be all set.
It must have been a cheap amp in its day. I mean really how expensive is a PT for a 5 to 10W amp? It would have to be so cheap that that cost mattered. Nobody would dream of doing it that way these days. It wouldn't be legal in most countries today.
Is it a Sears amp?

My tube radio doesn't have a power transformer... I also don't plug a guitar into it.

I like to listen to Coast-To-Coast in style.
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#20
Well, you aren't physically connected to a radio, you are to a guitar amp.
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