Stunning little instrument! I always had one in my childhood days. They even sold them at our local general store.

You can play it many ways: either you let the vibrator ring and you change pitch by enlarging or reducing your mouth cavity. Otherwise you can blow through the vibrating harp to amplify the sound.

I would also go with Atilla's advice. 450V should be good.

In my experience, electrolytic filter capacitors are a bit like shock absorbers in a car. If the shocks are old, the bass in your amp is less controlled, and more 50Hz leaks into your audio signal. You do not need to change the values. More capacitance in the power supply gives you a darker amp, not always the outcome you need.

I would also replace all the oil caps, since they are leaky and unreliable. Regular polyester or polypropylene types are good. Orange Drops or yellow Mallory types are the best.

Is there a tube rectifier in the ELK? Maybe a 6CA4?

It sounds lovely indeed. It depends on what styles you play, but the Beatles definitely come to mind. It has that clean fat sound - I guess that is Dick Denny's pre-amp doing it's magic.

The amp plays cd's quite nicely, actually. I burnt the amp in with a Hank Crawford CD, and the sound was delightful. For this the Normal Channel is best. I think that old style analog synth should also sound great through it.

I would play it through a 4X12" loaded with greenbacks, but that's just me liking the sound of the half stack cabinet. Currently I'm using a 1X12"open back cabinet, and that is a very practical option. I would love to use a good VOX fuzz-wah or fuzz box with this amplifier through the 1X12". I think the rounded sound would be good for lead guitar.

Hi All

I've been keeping a low profile, but had to share this with you tube amp lovers. A few years back I acquired an AC50 MkIII head, and I've been waiting to restore it. I got some capacitors from Tube Amp Doctor and some biasing pots from Weber and went for it.

I was lucky to get the exact replacement for the preamp filter caps


I replace the burnt out thermistor with a replacement and also upgraded the bias pots.


The job is done and she sounds wonderful. Hers is some more photos of the finished amplifier:

Not much to report :-\

I have a GEC 2493 with gold pins. That might be worth inspecting. Then some Hewlet Packard 6DJ8's that looks pretty used.

I have a few samples in my stash. Let me look and report back.

Attila Barath wrote: Whats the ratio on open backs one 1/3 or smaller 1/4 open ?

This is a tough question ??? But let's think about the purpose of the back panel.

Think of it like this. The front of the speaker is 180deg out of phase with the rear. So if your ear is equally distant from the front and the back, you'll hear nothing. So the back panel limits the rear reflections and accentuates the front sound.

Another thing. Imagine you have a piece of string. When your speaker is in it's cabinet, the distance from the front of the speaker to the rear of the speaker determines the amount of bass you hear. The longer the string needed, the more bass your speaker has. An infinite long string would be needed to reach around a infinite large baffle, that's why sealed cabinets are called infinite baffle speakers.

The other thing to consider is this. There are many paths from the front of the speaker to the rear. The shortest route determines the the whole bass response of the speaker. So if your speaker is mounted on the edge of you cabinet, the amount of bass would be determined by the path length round the closest side panel. So that is why you see on rectangular cabinets, only back panels at the top and the bottom of the cabinet. It increase the path length from the front to the rear around the narrow section of the cabinet.

But more bass is not necessarily better. It takes more energy to amplify bass, so the more bass you cabinet can deliver, the less loud it sounds relatively. So if you want to cut through the noise, you need to limit your bass response.

Other aspects of the rear baffle size are determined by the limitations of the driver's design. All speakers have a limited distance it can move on the X axis. To stop your driver from jumping out of the magnets groove, you need to dampen the movement when it gets to extremes. This is why closed speakers are great for loud playing.

Then speaker drivers have resonance. It is the frequency it plays the easiest. On the Celestions it is about 70Herz. If you do not want bass boominess and the associated lack of power, you need to bring some out-of-phase sound from the rear to the front to cancel the resonance so you sound is equally loud on all frequencies :bopping:

Look at the VOX rear panel. Think of the speaker as a piston pushing air. The effective area on top of the speaker that pushes the air is a figure to keep in mind. A good starting point is to make the gap in the rear panel as large as the piston area of the speaker. The AC30 has two 12" and the gap is about the the double the size of the Celestion's effective piston area ?

The build looks good :bopping:

Nice one EZ :yup:

It would be interesting to hear what you think of the difference between the 18 watter and the 5E3. Success with the build.

Nice find. Some people wait a lifetime for such a find.

Some tips: Take care not to power it up in the state it is in now. It is quite easy to check the transformers and speaker without powering the unit up. Those capacitors looks like they've crossed over a while ago ???

Yeah, off to the tech is the only way to protect your investment. If you want to learn, ask the tech if you can watch him/her work. Most tech's hate it when someone looks over their shoulder when they work, but it is a sure way to learn the basics. Then you can ask questions on the forums, and get some answers.

May she play again. :yup:

Nice project :yup:

That "Live from Mars" double cd really does it for me ? Nice album title too :bopping:

Thanks people... It is great to get all the wishes, but it is not my birthday... yet! :'(

I'll have to check my profile setup. Sorry.

Hey Vic

I like the diamond cloth wall hanging behind the amp ?

Here's the link to 7189 at tube depot

http://www.tubedepot.com/so-el84m.html

Mr Valve should also have some in stock.

Since you go where angels fear to tread, let me chip in too. This biasing thing is quite difficult to grasp, but really simple in principle.

Note about high voltages:
* Check that your life insurance policy is up to date and the funeral plan sorted.
* Try to measure with one hand only; the black probe should be clipped with a crocodile clip onto a convenient earth.
* Stand on a rubber mat or wear rubber shoes.
* I have noted than when I get shocked, I tend to smack myself in the face or on the chest ? So your muscles contract involuntarily when electricity flows through your arm. So always keep your arm free from wires and proximity to sharp metal edges when the voltages are above 80VDC. I was working on an amp one day, while being barefoot. I had the guitar in my hands while probing around the amp; you know... tweaking the values for best tone. I do not know what I did, but when I came by, the guitar already completed it's upward journey, and was coming down, and I could actually catch it.
* After you have been shocked, you should leave the work bench and take some time to recover. Call it a day. Apart from the body effect of a shock, your brain also stops functioning properly (you are in shock). If you were stupid before being shocked, you are more so after being shocked, and might do the same silly thing again. Yeah, been there. Shocked twice in a row :-[

Here's the link to the 70% thing http://www.aikenamps.com/Why70percent.html

Most amps are actually biased quite a bit colder than 70%. 50% is closer to factory preset. But 70% has a better clean tone typically.

I have just worked on a Checkmate 25 and can post more details:

This amp does not use EL84's! It needs 7189's which are similar to EL84 but can take significantly higher voltages without self destructing.

7189 Ratings
Vh Ih VaMax Vg2 Max Pa Max Pg2 Max
6.3 0.76 400 300 12 2.0


7189 Application Data
Class Va Vg2 Vg1 Ia Ig2 Zout Pout
AB1 400 300 -15.0 15.0 1.6 8,000 24.0

You can get 24 watts out of 7189's - thus "Checkmate 25" This is only possible with fixed bias; which the Checkmate 25 has.

Compare that to EL84

EL84 Ratings
Vh Ih VaMax Vg2Max VhkMax PaMax Pg2Max IkMax
6.3 0.76 300 300 100 12 2 65


EL84 Application Data
Class Va Vg2 Ia Ig2 Rk Zout Pout THD
AB1 P/P 250 250 62-75 7-15 130 shared 8,000 11 8
AB1 P/P 300 300 72-92 8-22 130 shared 8,000 17 10

You can get 17 watts out of two EL84's in push-pull with MAX 300V between the anode and the cathode. Thus the "18 watter"

Nice one Mark :goodtimes:

It is great to follow your thinking. You seem to be very comfortable with tube technology. It seems the Hammond 125ESE does the job hey?

A question: did you bypass the cathode resistor on the first triode? What value volume pot did you use? Also: what size/type coupling caps did you use? Size of output tube grid leak resistor?

You do not mention a choke? What size/rating resistor did you use to smooth the screen grid supply? Any hum?

A photo of the power supply etc. would be lekka :rolleyes: