Kelly amp 50W head

I got this nice Kelly amp. 50W, 3x12AX7, 2xEL34. To be checked and somewhat restored.

John Kelly was a tech working for Selmer (end of ’60s, beginning of ’70s).

John Kelly valve amp 1

selmer treble n bass 50Kelly, Selmer Treble ‘n’ Bass : twins ? Not the same guts, anyway … (Selmer pix found on the web)

Great links found on the web :

I tried to draw a schematic from the guts of the amp as I saw them.
(Schematic upgraded October 2011)

Kelly amp 50W Head

Kelly amp 50W Head pdf

Voltage measurement, with tubes :

  • A0 = A1 =  400V
  • A2 : 394V
  • A3 : 330V
  • A4 : 270V
  • EL34 plate : 400V
  • EL34 grid2 : 395V
  • Bias current : 40mA

Kelly 50 Head amp PTPKelly amp 50 head enclosureVoltage Selector Kelly amp 50 HeadPrimary voltage selector …

Kelly 50 Head tube amp back

12AX7 Mullard Kelly Amp Head Tube

A pretty ECC83 Mullard …

Power filter capacitor C22 was obviously out of order … so I replaced all filter capacitors

Kelly amp filter capacitors C18 to C21Old C18 to C21 capacitors

Kelly amp filter capacitors C18 to C21 off

Kelly amp  capacitorsOld ones, new ones …

Kelly amp  new capacitorsNew capacitors in place

Kelly amp  C22

Kelly amp C22 replacedC22 replaced

Kelly Cathode capacitors replacedCathode capacitors replaced

Kelly amp C bias oldKelly amp C bias replacedBias voltage capacitors (50uF/63V) replaced

The problem is : very low power output … (less than 1W). So I inject a 1000Hz 70mV rms input signal and follow up and measure at each stage the signal / gain. Before distorsion, I get 20V rms on the grid (Vgrid1 / cathode) of each EL34 (that seems ok) but only 14V rms plate to plate EL34, and 0.9 V on loudspeaker side (0.05 W under 16 ohms !)

Kelly amp V3A grid without tube

Kelly amp V3A grid with tube

EL34 Grid : 60VccEL 34 grid : 60V

EL34 1 and 2 gridsEL34#1 and #2 grid signal

Output transformer seems to be dead, that’s why there is this very low power output. To be fixed or replaced.

And then, I finally replaced the OT with a IGPW (ex-Shinrock) transformer for JMP50. It works (and sounds !) very well !

Kelly amp shinrock IGPW transformer OT

IGPW Shinrock JMP50 output transformer

Many thanks to forum ProjetG5 members for the help !

kelly amp ac30British oldies amps !

2 pictures of a Kelly 50W with a choke in the power supply, thanks to Greg Cunningham :

Kelly 50W with choke 1Kelly 50W with choke 2

Kelly Amp 50W PSU with choke

Power supply with choke schematic drawn by Greg Cunningham, thanks Greg !

Found on eBay, pictures of a Kelly slave amp !

Kelly-Slave

Kelly-Slave-inside

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28 Responses to Kelly amp 50W head

  1. Billy Cunningham says:

    Lovely article and photos. I have an old Kelly 50w with twin 12″ speakers for restoration. Thanks for the guidance and the circuit diagram. Keep us posted on your progress.

  2. t-geek says:

    Thanks Billy.
    Currently waiting for an answer from a french OT supplier (Magnetic / ESO) to see if it is possible to fix the OT or if I have to change for a new one … stay tune ;-)

    Have you seen the small article about the Wem ER 15 ? Pretty amazing sounding amp, and very cool mod of one channel to mimic the Marshall 18W …
    http://www.tonegeek.com/musicgear/amps/wem-control-er-15.php

  3. Paul B says:

    Stumbled across your site whilst searching for info on Kelly amps. Here’s a link to a Kelly DP100 I’ve just renovated (the schematic isn’t uploaded yet though).

    http://valvesonixamplifiers.moonfruit.com/#/kelly-dp100/4560974097

    Best regards,

    Paul

    • t-geek says:

      Hi Paul,

      Nice amp you have there ! How does it sound ? Is there any chance Kelly have used Partridge OT like on the Selmer amps ?

    • Eduardo Enes says:

      Hi Paul,

      Just receive an DP100 Kelly amplifier and would like to start refurbishing it.

      Saw your link but did not work.

      Can You help me to obtain more information related with Kelly DP100 head?

      Thank You

  4. Greg Cunningham says:

    Hi T.G.,
    I repaired one of those last month, but it’s power supply circuitry was slightly different, with a choke included. I’ll send you the PSU diagram when I get it drawn up with Multisim. The rest of the amp is much the same as yours according to the diagram which was very helpful indeed, thank you.
    BTW; I just noticed my brother Billy was here some time ago at the top of the reply’s. small world. lol.

    See pictures above, at the end of the post

    Cheers, Greg

  5. t-geek says:

    Hi Greg,

    Thanks for your pictures w/ a choke ! I updated my post. Yes, it would be definitely great if you send me the schematic of the power supply with choke.

    Nice amp !

  6. Paul B says:

    Hi Eduardo,
    Here is the schematic from the DP100 that I fixed. I think it is correct. The one I fixed had leaky caps, a burnt out bias circuit, burnt / desoldered / cracked 1K 5w resistors on the el34s and arcing between the HT wiring and the valve bases! Nice!

    http://homepage.ntlworld.com/pburford/dp100.JPG

    I added a PPIMV in place of the standby light. Sounded quite crunchy when it was turned right up.

    • t-geek says:

      Thanks, Paul !

    • Eduardo Enes says:

      Thanks Paul,

      Now I can start to check if there is any changes on mine. I would like to keep it as original as possible.

      At first glance it seams all components are original, so maybe I need only to do some recap. let´s see.

      Tubes are ok! They all tested 90% on my tube tester (Original EL34 Mullard) and between 85/95 % each triode in each of the four 12ax7 each triode (original Mullard also). I guess I was lucky on that.

      Cheers, Eduardo

  7. Michael Crabtree says:

    Hey thanks for the very precise detail and pictures of your project.
    I too own a Kelly 50w head which I run through an old Fender 2×12 loaded with
    Celestion creamback g12m 65w. Sounds great. The amp kicks out a bit of hiss on both channels but it really sounds nice when you crank it up. A very unsung make of amps in my opinion. I guess that keeps them affordable!….. For now at least… ; )

    I have also had the privilege of playing through both mark 1&2 models of the Selmer Treble N Bass amp heads. While I think the croc skin is king, I prefer the overall tone of the Kelly compared to the mark 2 Selmers.

    Thanks again.

    Mike

    • t-geek says:

      Hi Michael,

      Thanks for your comment. Yes Kelly looks like Selmer Treble n Bass but different schematic hence different sound.
      I play mine with a jack between the 2 channels and it sounds great !

  8. Michael Crabtree says:

    Cool! I’m gonna try that out. ; )

    • t-geek says:

      It’s a well known trick (did you say Hendrix ? ;-) ) that works on many amps, of course. I use it with my 1987 and 1917 Marshall, but also with my Kelly, Vox, Bandmaster and even my Bassman when playing bass guitar …

  9. Michael Crabtree says:

    Thanks, sounds like you have a nice amp collection.
    Yes, I had heard of that trick with channel linking.
    Can’t wait to get into the studio and have a blast next week.

    I play the bass as well. Looking for a new amp head to go with a Hiwatt 1×15 cab.
    Not sure what the best option might be. I’m on a restrictive budget which doesn’t help matters. ; )

  10. Michael Crabtree says:

    Both very tasty, yes.
    Beyond my means. At least right now anyway!

    Thanks for the suggestions all the same…

    Kind regards,

    Mike

  11. I just aqquired a John Kelly 50 watt 1×12 combo inLondon.I’m heading over next week,and have arranged with John to bring it in to him for a service.
    Can’t wait…
    He’s an Amp God in the UK ….
    AND he comes from Ballyfermot Dublin Ireland like myself.
    Doesn’t get any better.Watch this space for Pics of me and John in his workshop with the amp.
    :]

  12. Great page to run across!
    I have two Kelly 50s, and a couple of 2×12 columns (w/15w Fanes) I can only presume were meant to be for PA. In fact, the guy I bought them and the first head from advertised it as a PA, and threw in a microphone.
    I was, first, very happy to see the power transformer was multi-tap, then rather disappointed to find that what I thought read 110 actually said 210 (I’m in the states). I guess I’ll have to continue using step-up transformers on them.
    The tech I work with reckons they look like Hiwatts underneath, and act like plexis on the scope. Not a bad thing. :-)
    They have been in storage for a long time, as work keeps me traveling. I have some time at home now, so bringing things back to life. I’m looking forward to seeing what these sound like.

  13. Martin says:

    Kelly 50W head.

    I absolutely love my Kelly 50W head. Had it for about 35 years? Pretty reliable except for a couple of rectifier diodes going short. (one in HT and one in bias) and a few caps and one resistor end cap coming off.

    I also drew out a circuit but although very similar, it has quite a few differences to yours, notably the 100uF cathode caps are 25uF on mine. And I suspect you have a typo for the bias resistor from the HT being only 100R?? mine is 180kOhms.

    I never use the Normal channel as if it very average compared to the Normal. Anyone modded the amp to get extra gain or preferably a tremolo?

    Let me know if you want photos or the circuit diagram?

    • t-geek says:

      Hi Martin, thanks for your post.
      Ever tried to use Normal and Bass channel bridged ? Sounds good to my ears !
      Yes of course, any picture / schematic welcome.

      On my amp, bias resistor seems to be 100R (I don’t remember if I measured it) . Maybe replaced by a previous owner ?

  14. Ed Gain says:

    i got the kelly amp with Nolan plate & written Multi-Purpose 50, wondered what year that plexi has been made exactly?

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