On the 24/08/06 I received an e-mail from a pleasant young man in France who was requesting some possible help with two Kelly 50 watt amplifier heads. One was an original, standard, familiar Kelly 50 whilst the other, on an aluminium chassis, was possibly an early one or even a prototype. Philippe had originally bought the latter without the case and subsequently made one himself.

I donít know if this is a "first", but I sorted both of them out for him OVER THE INTERNET using e-mails. This is how it all transpired:-



I'm Phil from the south of France, I'm trying to repair an old Kelly guitar amplifier (50 watts with 2 x EL34 and 3x ECC83). It works but, the EL34's are burning and the glass all red! I have measured the bias and I find 80 and 90 m/v, is it normal? I don't know how to checked bias current, there is no trimmer like a Marshall! Please, can you help me? Sorry for my bad English. I joint 2 photos of the amplifier.

Amitiers de France.






Hello Phil, it seems to me that you maybe have a bias problem? I don't think you have very much bias at all if you are seeing 80 - 90m/a!! (I think you mean m/a, not m/v?) You should have around 35 - 45 volts negative on pin 5 of the two EL34's! Take the EL34's out and do your test like this, you will do no harm like this and especially to the EL34's (if they are not damaged already?).  If you get back to me with some good photos of the underneath, I can tell you how to sort this. Cheers, John.


First, thank you for an answer; it is very kind of you. I have enclosed an underneath photo as requested.



So I have measured tensions at pin 5 without tubes and I found: tube 1 = 37.5 volts and tube 2 = 37.8 volts with pin 5 negative pole and chassis positive pole. After I took my dual bias tester with tubes on , standby on, speaker on, volumes off and I found: 95.5 millivolts one and 73.3 millivolts other than first tube glass broken! Is this correct? 3 photos with zip extension attached.

Best regards. PHIL.


OK Phil, I do not quite understand what you are saying in the last part of this e-mail where you talk about the milli-volts?The first part of your mail I do understand and this is correct at about -37 volts on both tubes.Can we please forget about the duel bias tester, there is nothing you can do with this anyway, just follow what I say.

Put the EL34's back in properly (without the bias tester) and power the amp up with the standby switch off, then check the voltages again at pin 5 on both tubes?Now switch the standby on (all controls off and a speaker connected) and check the same voltages again?

Let me know both of these results and we can take it from there?


Cheers, John.



First measure with tubes and standby off = 65 volts one 66 volts other tube. Second measure with tubes, standby on, volume and tone on = 36 volts one 36.5 volts other tube. Apology it's not milli-volts but milli-amperes with bias tester and my controller. PHIL.


OK Phil, a voltage of 36 - 37 volts negative is fine and correct. You say that one of the EL34's is running too hot and glowing red? I think maybe you need to try some more EL34's?

Also now your milli-amps makes more sense, as the Kelly does not have a facility for the bias adjustment, the only thing you can do with your bias tester is to try some different EL34's until you find a pair with approximately the same currant reading when you switch form 1 - 2 on your bias tester.

I hope this has been of help?

You know where I am, cheers, John.


Thank you very much John for your help it's great!! Philippe.


Hi Phil, no problem with the help.

Cheers, John.



I have put two olds Mullard EL34ís play guitar since a long time and it's seem good; nothing burned in just a noise from the speaker (in French, bruit de fond).

I have another Kelly 50 watts guitar amp with aluminium chassis maybe more older but it don't work, a big, big noise come from the speaker when standby is on and no amplified.

Best regards, PHIL.






Hello again Phil, Ok I am glad the amplifier seems to be all right now. Thank you for all the photos. As regards your other Kelly, if you want to send me some good quality photos of the underside, showing all the components, I might be able to fix it for you, via e-mails?

Cheers, John.


OK, thank you for that John, I have attached some underneath photos for you. When I buy it (in Leeds UK, the transformer smoked a lot and one rectifier diode was dead, I had to change the transformer and two diodes by two BY127 and I don't now if it's the good choice? The transformer to! It's a 350-0-350 volts 125 VA with 6.3 volts 5 amperes. I had to change several components with exactly same values but nothing had change! The amp don't work when standby is on, immediately one enormous big noise come from the speaker but nothing burn. With your method I found 19.5 volts on pin 5 first tube and 24.5 volts negative on the second tube. Maybe power caps are dead?? There are 32 + 32Ķ farads and 16 + 16Ķ farads.

A bientot. PHIL





Hello Phil, I have now extracted all your photos to a new folder so I can check them properly. The BY127 diodes are fine, so is the specification on the power transformer. I have read your e-mails and I think I can sort your problem quite quickly but first I need you to please do the following:-

Remove all the tubes and then tell me the voltages you have at the three points, circled in red on the attached photo?



Let me know the answer to this as soon as possible and then I will respond with the next part.


Cheers, John.

Hello John, I found:-

Pin 5 tube 1 = 27 volts

Pin 5 tube 2 = 2.1 volts?

Red wire = 27.7 volts.



Hello again Phil. Firstly we are talking all negative voltages here, yes?

 If so, then the low voltage on one of the pin 5's is very wrong, far too low! Here is what you need to do next:-

 Lift one leg on both the big brown capacitors, circled in the attached photo, do the same test again and tell me the voltages this time? Cheers, John.




With the two caps disconnected I find:-

30 volts (pin 5, tube 1)

30 volts (pin 5, tube 2)

31.5 volts on the red wire.



Hello my new friend, this is good.

Firstly we need to get rid of these two capacitors, they are duff, you can put in here a .047uf or a .1uf @ 400 volts or more (the 1000 volts is not necessary). Please do this first and then do the tests again, telling me the voltages before we carry on?

Bye for now, I will answer with the next step tomorrow after you have replaced these two capacitors and given me the three voltages again.

Cheers, John.


Good morning John, I hope you had good dreams last night? I have changed all the caps to mustards, 4700 pf and I find:-

First tube: 30.7 volts negative.

Second tube: 30.7 volts negative.

Red wire: 31.7 volts negative.

Can you say to me what is this black rectifier at the side of the other BY127ís; can I put another BY127 in here? PHIL.


OK, back to your Kelly number 2, the bias voltages are much better now but, all a little low. This is because of the power transformer change but is easy to put right.Don't worry about the black rectifier, this is working fine, there is no point in changing it.

Now look at the two resistors circled in the attached photo, these are 68k each and in parallel this gives us 34k.


There are two things you can do here, firstly remove the old resistors and put a single 39k in. Check the negative voltages again; we need about 37 volts on all the negatives. If the voltage is not enough try a 43k resistor. The other alternative is you can do a mod here (you could also do this on the other Kelly should you wish too?).

Using a 100k pre-set potentiometer, solder the middle leg to one of the side legs. Now solder this into the amplifier, in place of the original two resistors. Just solder the two outer legs across where the other resistors were. Still with no tubes in and with this pre-set potentiometer set to the middle position power up the amplifier and then set the control to give you about negative 37 volts on the two pin 5's.

Once you have done either of these options, e-mail me back and then we can finish the amp with the last part.

Cheers again, John.


Hi john with a new resistance in place:-


Tube one pin 5: 36.7 volts

Tube two pin 5: 36.7 volts

Red wire: 37.6 volts

Too much?


Hi Phil, no this is fine, this will be ok, let us move on now.

 Next put the two EL34's back in place, no other tubes, with a speaker connected, power the amplifier up with the standby switch off. Check the bias voltages again and if they are about the same then switch on the standby, but be ready to turn it off quickly if the loud noise is still there? Let me know and we can then move on.



Hi John, with two EL34 and no ECC83's, standby on, no more noise!

36.2 volts

36.2 volts

37.2 volts

Very good yes!? Phil.


Yes Phil, this is good! You do not need to check the bias voltages anymore now.

Next, put the phase splitter ECC83 in place, this is the one located right next to the EL34's. Leave the other tubes out. Now I have a strong feeling that when you power up this time the noise may be back? If it is, I think I know what the problem here might be.

This time, power up with the standby switch on, keep your finger on the power switch, ready to turn it off if you hear the noise start to come on? Let me know the results? I am with the computer waiting for your answer.

Bye for now, John.


John, just with the one ECC83 big noise come back! Phil.


OK, I thought it would. First remove the ties (circled in black on the attached photo) and move the resistor (circled in yellow) directly onto pin 5 of that output tube. Use a new piece of wire to link the resistor back to the tag board, but don't lay this new wire close to the one it is tied to in the photo, leave it up in the air. This is not the cause of the noise but is not a good idea and needs to be changed. The resistor should be located as close to the pin 5 as possible.

Next, the possible cause of your noise. Look at the two red circles in the photo. These are the blue and purple leads from the output transformer to the pin 3's on the output tubesí plates.



Simply, swap them over and your noise should be gone!? If I am right, you can put the rest of the tubes back in, turn all controls down to zero, and providing the rest of the amp has been built correctly?...........away you go!!

All the best, John.


Hi John , I had resolved the 3.3k ohms directly onto pin 5, put the blue wire to the violet wire and the violet to the blue, put all tubes in, power on but noise is biggest that before the amp is screaming!!! Phil.


OK Phil, sorry about this, I thought the noise you described was a kind of screaming, this is why I told you to change those two wires around thinking the feedback was the wrong way around? You will need to put them back where they were now. This is obviously not the noise problem. Can you please describe the noise, is it a loud hum?

Once you have put the wires back where they were, it might be worth you just trying the amplifier again now you have moved that resistor; this was not in a good place where it had been put. Use only the one ECC83 again; leave the other two out as before.

Let me know ASAP, I will study the photos again now to see if I can spot the problem.

I await your reply, cheers, John.


OK John, I had put all wires like before, check 3.3k place and it's all good as before. With ECC83 splitter noise is still here, it's like a big sound back (enorme bruit de fond in French) it's a big ronflement. PHIL.


All right Phil, we need to approach this noise problem from a different angle now. On the attached photo, circled in red, can you remove the red wire that is on pin 2 of the splitter ECC83. Then with another piece of wire, ground this pin 2 to earth, on the buss where I have circled also in the photo, not directly to the chassis.



Try this and let me know if there is still the noise?


Cheers, John.


Hi John, now with pin 2 on earth rail and ECC83 on, noise is -80%!!



Hi Phil, OK. Can you be a bit more specific, you say -80% yes this good but, the other 20% that is left, can you tell me if this is still the same noise, but much more quite?

I await your reply?



OK John, the noise is the same but with less intensity, enough to play at home but not possible for the stage for example. It is a ronflement in the speaker.



All right on this then Phil, thank you. I was hoping this is what you would say. I think I might have now found the problem?

Have a look at the 2 attached photos. The one of the power transformer with the red circles, on the 4 screws that hold the transformer together and the 2 that hold it to the chassis, it is clear that there are no wires coming out of the transformer, being grounded on any of these fixings.

However, I can not see the other side of the transformer; are there any wires coming out of the bobbin and grounded to any of these screws on the rear side?



Next on the other photo where I have circled the 2 black 6.3 volt heater winding wires going to the first EL34, where is the CT (center tap) ground wire?



I await your reply again ASAP?



Hello John, I'm not yet in bed, so, there is not wire on the other transformer side to the ground and no more wire center tap 6.3 volts there is:-

0 volt / 6.3 volts just two black wires direct to the tubes.

Is this trannie bad?



No, the transformer is fine, however I know now the final problem.


With the amplifier on and still the pin 2 of the ECC83 to ground, just the one ECC83 in place, with a screwdriver just short either pin 2 or pin 7 (only one of them) to the ground rail (see attached photo, yellow line) and then tell me if the noise stops?



Get back to me ASAP with the answer?

Cheers, John.


Hey John! Noise stop immediately with the screw-driver!! Cool!


Wicked and cool Phil, I thought so...............great.

OK, what you need to do now is solder a link here for now, you can put the red wire back on the pin 2 of the ECC83 and as I said before, if the amp has been built correct (providing there are no other component problems too?). You should be able to put the other ECC83's back in and........away you go this time!!

What you will need to do when you can is to use two, either carbon 2 watt or higher, or, wire-wound of 2 watt or higher. Both of a value of somewhere between 33ohms to 50 ohms (they must both be the same!) and solder them as in the attached photo (drawn in yellow).



I told you I could sort it out with e-mails!!!


Cheers my friend, John.



You are the best in the world! This amp work now like his first day, it's a miracle! Sound better that my other Kelly, best attack in little volume and now less back noise. I haven't the good resistors at home so I made a bridge between pin and ground for now.

You are a great professional and I took very much of your precious time with head and computer so I would pay something for this great job, how can I proceed, maybe paypal system I have an account.




Dear, Philippe, I have been more than glad to help you and it has been a challenge and fun too. If you wanted to send me something for a drink of whisky then the paypal address is, but I will leave this to your good self?

After a few whiskies now I am off to bed, I will be back to you tomorrow to see if all is OK?

Bye for now, John.


The next day:-

Good day Philippe, I have just got up about 1.30pm!! I am glad the amp is all good for you now. I have saved all our communication mails in a separate folder and wanted to ask you if I could put the whole thing, along with the photos on my web-site, would you mind?

Anyway, have fun with the amp.

My best regards, John.


Hello John.

I am glad me too, Kelly have a good health now! Very good sound, better than the other original Kelly!! I send to you a little thing via paypal, it's not a big thing.

Hey! No problem for the pictures, you want to put the whole thing in your web-site, whaow! I am very proud of this. I saw your 1kw amplifier on your site it's enormous, fantastic great realization (in French, je suis sur le cul!) now for the technical side of tubes amps, if I have well understood, first operation: Check negative tension of power tubes, from 35 volts to 45 volts approx (really for all EL34's tubes amps with 2 or 4 tubes?) after checking all ground solves. Can I in the future ask to you some technical questions? (Sorry for my poor English school). Thank you for everything.

Grande amitier de France.



Philippe very kindly sent £75 to my paypal account for all the help I had given him over a four day period. He is a very nice young man and this whole thing had been fun throughout. Below is the final e-mail I sent to him on the 28/08/06. I hope you all found this interesting!

Cheers, John.


Hello again Phil, firstly let me thank you for the paypal contribution, I can buy 5 bottles of whisky with this! Cheers.

Yes, on any amplifier in fixed bias you should always make sure the bias voltage is there first, on all the output tubes (no matter how many) before putting the standby switch on. And also yes, most amps run on a negative bias of about 35 - 42 volts (EL34's generally hover around -37).

I will now piece together an article on your two Kelly amps and display them on the site, in time. I will let you know when it is done?

No problem with any further help, you know where I am.

Once again kind regards, it has been fun. Cheers, John.