WEM “Control” ER 15

WEM ER 15 faceplate

Here is a WEM ER 15 …

15 Watts, 2 x 12AX7, 2 x EL84 and one EZ81 as double diode.

This is a mid-60′s valve amp from Watkins / WEM.

WEM ER 15 plateHere is the schematic (as found on the net ! ) :
WEM er 15 schematic

WEM er 15 above viewWEM er 15 bottom viewPTP (bottom view) …

Mullard telefunken valvo wem er 15 tubesNice tubes. Still alive ?

You could also see this site … vintage hofner !

OK, one fuse dead and the EZ81 died with a very cool blue flashlight after a few minutes (unfortunately for a mojo Valvo). New fuses, new JJ EZ81.

Voltage check without tubes, then with tubes. Everything seems to be OK. EL84 bias current at 46mA (cathode resistor is 130Ω on schematic, my amp has a 100Ω … quite hot).

Then, I plugged a cab instead of 16Ω resistor used for testing stuff.

Wow, this thing sounds !!! With a strat, very nice clean tone, and crunch at maximum volume. Pots are a little scratchy … to be fixed.

Some say that the Marshall 18W is based on this amp (in fact, I’m wrong here : Marshall 18W is based on Watkins Dominator). The WEM has two channels, why not mod one to exact spec of Marshall 18W ? ;-)

WEM er 15 biasFocus on 2nd stage (PI) bias … (voltage with tubes)

PI voltages compared WEM ER 15 vs Marshall 18WPI compared (WEM ER 15 Vs Marshall 18W)

Power filter capacitors replacement :

WEM ER 15 power filter capacitor replacement 1Back to spec : 15uF

WEM ER 15 power filter capacitor replacement 50uF50uF + 50uF (old/new)

WEM ER 15 power filter capacitor replacement doneDone !

Modification of one channel to Marshall 18W :

WEM er 15 Marshall18W jpg

WEM ER 15 marshall 18W mod

WEM ER 15 marshall 18W mod 2(The tiny 1M resistor is the only I had in my toolbox-pedal stuff. I replaced it … a few days later)

It works well, more crunchy at max gain, but the 2nd genuine Wem channel stays still so lovely. Gonna try to mix both with a special Y cable plugged into both input jacks …

Bias adjustment :

Before filter capacitors change, EL84 cathode resistor was 100Ω, and Ik was 46mA. After filter capacitor change, all voltages has increased so I changed Rk to 120Ω, for a new Ik = 44mA (Va=320V)… I could try 150Ω to preserve EL84 life … one day.

Wem er 15 EL84 Rk

Coupling capacitors replacement :

On channel WEM, quite every capacitors leak DC current (up to 12V DC measured at Bass pot …) so I replace these capacitor with MKP :

WEM ER 15 old capacitor DV current leakageBefore …

WEM ER 15 new capacitorsAfter …

Also replaced Ra resistors and all Ck (noise hunting) :

WEM ER 15 restore done !

Two improvements this morning for the Marshall 18W channel : shielded input lines + a grid stopper (68k) on the tube socket (to prevent radio and mobile phone frequencies I noticed from time to time) :

WEM control ER15 Marshall 18W shielded inputWEM control ER15 Marshall 18W shielded input and grid stopperPerfectly quiet now !

Many thanks to forum ProjetG5 members

WEM er 15 PRS Orange Honk machinePRS, my Honk Machine, WEM ER 15, Orange : great combination !

Ready to rock !

Wem control er15 ready to rock

Some more pix and measurements for Jonathan. Hope this will help !

WEM ER15 BackWEM ER15 under the hoodWEM ER15 under the hood 2WEM ER15 cabWEM ER15 cab 2WEM Front schematic

WEM Side

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23 Responses to WEM “Control” ER 15

  1. jonathan says:

    Thanks for your post.
    I bought a Control ER 15 about 13 years ago and it’s still working ok. However, mine was turned into a combo amp with one speaker (12″, I think) at some point. Your project encourages me to refurbish the amp a bit and I also consider turning it into an amp head again. I’d be interested in the measurements of the original amp (also thickness of the material) and a photo of the back side. Are your knobs and the lamp original?


  2. t-geek says:

    Hi Jonathan !
    Nice project you have there ! I’m on vacation right now. I will measure and post the dimensions in late August (and photo). I replaced the light bulb (easy to find). Knobs on the “Wem” channel are original, knobs on the “marshall 18W” channel are new (Marshall spare parts). The Marshall 18W channel is a quite easy mod, and it sounds amazing.

  3. jonathan says:

    Thanks for your quick answer.
    After some research yesterday and finding some more pictures (http://www.mercatinomusicale.com/mm/a_wem-amplificatore-a-valvole-vintage-1960-control-er-15-power-bass_id2459640.html and http://www.vintagehofner.co.uk/britamps/watkins/pickaback/pab7.html) I got the impression that either
    a) there was a version of the Control 15 ER that was built into the Pick-A-Bass amp
    b) somebody (the owner before me?) did this mod (and also replaced the rectifier tube by diode.
    I think b is more plausible, as there seems to be no evidence that a version like a ever existed. But I will ask in a forum (and post pictures…).

    Have a nice vacation

  4. t-geek says:

    Stay tuned ;-)

  5. Shali says:


    I have one of these and am looking to sell.

    Could you tell me how much they sell for in working order. The case is in very good condition with no major damage just a couple of minor bumps and slight rusting on metal brackets.
    Any help you can give would be greatly appreciated.ml


  6. vinic says:


    Thank you for your article. great job!

    I now plan to refresh mine but may I ask you those two question :
    - what power dissipation rating would you use for resistors? I have some that obviously melted a little :)
    - how do you wire the “grid stopper” resistor? I don’t see precisely how you did that…

    Many thanks in advance

    • t-geek says:

      Hi Vinic !

      For grid stoppers, as for anode and cathode resistors, 1W are OK. For power supply : 2W resistors, for R15 and R16 : 5W (or more).

      Grid stopper : I can’t say it any better than Aiken ;-) “In order to take advantage of the parasitic suppression benefits of these grid resistors, they must be placed as close as possible to the socket pin of the tube, preferably soldered directly to the pin with a very short lead.”
      (see http://www.aikenamps.com/InputRes.htm)

      Cool for refreshment project, let us know !

      • vinic says:

        Thank you for that fast answer!

        That white wire was confusing, but I just gather my neurons and understood it was due to Marshall mod… :P

        Uploaded some pics of the opened head here : http://vinic.free.fr/wem_ER15/
        As you can see, R24 and C21 have been melting, and C5 is a good witness saying “time to change your capacitors!”

        Oh, and another question : is the center tap of the output transformer wired? Mine was (recently rewired on the pics) but it goes nowhere on the 3 way plug to the Power Bass cabinet…

        And for anybody who reads this, if you have a spare knob (genuine, just like t-geek ones, silver), please make an offer! :D

  7. t-geek says:

    Yes, the center tap of the OT (AT THE PRIMARY) is to be connected to the High voltage (push pull principle). Plug to the loudspeaker cab is at the secondary of the OT.

    Nice pictures ! On IMG_1096, nice burnt resistor (EL84 grid stopper ?) … look also at the capacitor at the right of the picture, maybe to be changed (C22 ?)

  8. vinic says:

    I was mentioning secondary of the OT, see the green/yellow wire on IMG_1086. I can’t see if your is wired to LS cable shield or something…

    And yes, the bunt resistor is the EL84 grid stopper (R24 – 10k)
    And you’re right, it is C22 that has exploded, not C5 as I mentioned above!

  9. vinic says:

    Job done!
    Pics here : http://vinic.free.fr/wem_ER15/
    Thanks again for all!

    PS : next step : Blues Jr mods after seeing your link to Billm… few kits ordered!!!

    • t-geek says:

      Great job !
      Nice cab and nice telecaster ! How does it sound ?

      Cool for your Blues Junior !

      • vinic says:

        It just sounds great!!!
        A lot of harmonics are back now and as I play it stereo’ed with my Blues Jr., it’s like I had plugged a third amp!
        I’ll post some samples ASAP ;)

  10. vinic says:

    Sorry to bother you once more.

    I try to know if this amp is class A or AB to get the correct bias value (range) and check/correct it on my amp. If you have the answer, could you explain please?

    By the way, your 44mA bias current is VERY hot!!! as it is supposed to be around 34mA (A) or 26mA (AB)…


    • t-geek says:

      Yes you are right, 44mA is too much. On the schematic, it seems that EL84 pin3 voltage “must be” 10.5V accross Rk = 130Ω so 81mA for 2 tubes and then 40mA per tube. Let’s say 2mA for screen grid current => 38mA for Ia … still quite hot. Maybe this thing is a full Class A amplifier ? Should measure the impedance ratio of the output transformer an draw the load line to be sure …

      And you don’t bother me ;-)

      • vinic says:

        I’ve just had a short time to make some measures. 10,5V is what I have, so, as you say, it “must be” OK.

        But… I have a plate voltage of 375V (instead of 320V nominal), getting down to 302V on C5 (instead of 250V nominal) (NB : the PT delivers 307Vac, sounds OK there…). So I decided to change the primary option from 220V to 240V, which gets the voltages down to 345V and 280V respectively. Still a bit hot, especially considering that EL84 has a Va max at 300V…

        I see that your PT optional jumper is set on 220V… Is your Va=320V measured or taken from schematic?

        …there’s something I don’t understand with that difference between the schematic and measures… but well, it works so why scratching head? :D

  11. Fernando Garcia says:

    Hi and nice page with lots of information!!, now where could I get hold of a new audio transformer, the one in my amp seems to have melted.
    Thanks for any information you may be able to provide me and as I am an electronic engineer I will be able to do the change myself. thanks again Fernando.

    • t-geek says:

      Hi Fernando !
      I don’t know the spec for the OT, but I think a 8k – 15W would fit, such as a Hammond 125E or Hammond 1650E …
      Or you could try Hammond 1750Y VOX Outputtransformer for AC15 (6.3k instead of 8k, not sure you have to care about the difference of impedance …)
      Or even an OT for Marshall 18W ;-)
      Let me know and be careful with the high voltages, check the PSU caps !

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