6L6 mod gone wrong - help!

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Big amp is in storage so I took my Ampmaker SE5A to 1st rehearsal of new band on Sunday.

I was pleasantly surprised that it MORE than kept up with acoustic, piano, bass & drums.

So I got to thinking that I might even be able to gig small venues with it. 

I decided to mod it to take a 6L6 instead of an EL84 in the hope of achieving a bit more headroom, so I ordered an octal socket and:

-       hooked it up to green heater supply,

-       shared the signal stuff and B+ with the existing nonal socket

-       connected 1 & 8 to ground via the new looking 47uf cap and the 470r resistor (yellow lead that goes under the turret board). 

It works with the 6L6 tube in (and the EL84 removed) but at a VASTLY reduced volume.

Can anyone spot what I might have done wrong?


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  • ICBMICBM Frets: 40586
    You’ve used a 4.7uF cap instead of 47uF. I can’t see the resistor value...

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • DannyPDannyP Frets: 932
    Drat. Thanks for the spot!

    Does that account for the low volume?


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  • springheadspringhead Frets: 440

    Your 47uF cathode cap.  Looks more like 4.7u and non polarised.  Also quite old.  Try just running with the resistor only.  And in that state measure all the voltages on the 6L6.

    EL84 is a very sensitive valve, needs less drive than a 6L6, that may be a factor, plus load impedance. 

    Drop that cap and measure voltages first though to see if your 6L6 is running with sensible dc conditions.

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  • DannyPDannyP Frets: 932

    Your 47uF cathode cap.  Looks more like 4.7u and non polarised.  Also quite old.  Try just running with the resistor only.  And in that state measure all the voltages on the 6L6.

    EL84 is a very sensitive valve, needs less drive than a 6L6, that may be a factor, plus load impedance. 

    Drop that cap and measure voltages first though to see if your 6L6 is running with sensible dc conditions.

    Yes that cap is defo wrong (although it was brand new from Cricklewood Electronics)

    I'll give that plan a try tonight!
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  • DannyPDannyP Frets: 932
    Had a play last night. Removing the new cap did increase volume but not by much.

    I was a bit tired yesterday and didn't want to electrocute myself. I'll get the multimeter out and stick one hand in my pocket over the weekend...
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  • springheadspringhead Frets: 440
    You might find all’s well and you won’t get much more volume as the 6l6 might be constrained by the circuit. I.e. 470R may be biasing it too cold for maximum power but perhaps the OT can’t handle higher anode current. Also the preamp may be at its limit for max signal and turning up the volume some more might just be adding distortion rather than increasing the drive to the 6L6 - as it’ll need more than the EL84.  6L6 at full clean power vs EL84 is probably not that much louder.  I think the power handling is roughly double but that’s only a 3dB increase.  Might sound different in a pleasant way though!

    Not trying to put a downer on your plan just saying that be wary of chasing a fault that doesn’t exist.  

    There may be an ampmaker forum where someone has done similar and safely lowered the cathode resistor value or modded the preamp for more clean drive etc.  


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  • DannyPDannyP Frets: 932

    Not trying to put a downer on your plan just saying that be wary of chasing a fault that doesn’t exist.  



    No, thanks for your help – don't worry about putting a downer on things :)

    I don't have rose-tinted specs regarding a 6L6 magically transforming a 5watt amp into a gigging amp!

    For about a fiver's worth of parts, I thought it was worth having a fumble about! (I already have 6L6s knocking about for my Blues Deville)

    I'll take some voltages over the weekend and see where I get. I've built an 18-watter for my uncle since making this SE5A - just really enjoying learning about what goes on in valve amps!
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  • ICBMICBM Frets: 40586
    It's also worth mentioning that aside from the lower 'gain' of the 6L6 - meaning you need more input signal to achieve the same output - it has a drastically lower impedance, roughly half that of the EL84, so if you don't have a selector on the amp that can be set to twice the speaker impedance, or a speaker of half the impedance, then you're running it mismatched and will actually *reduce* the power, even if you can drive the output stage fully.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • springheadspringhead Frets: 440
    Without doing the maths I suspect that’s why this design is running the 6L6 in this condition, well below it’s max rated power. It might just be that it’s an operating point that’s happily near enough the same load that the El84 is running at. 
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  • ICBMICBM Frets: 40586
    edited May 24
    Without doing the maths I suspect that’s why this design is running the 6L6 in this condition, well below it’s max rated power. It might just be that it’s an operating point that’s happily near enough the same load that the El84 is running at. 
    The maximum power is almost always set by the circuit - in particular the HT voltage and current capability and the output transformer ratio - and not the power valve. Even putting a KT88 in an EL84 circuit won't get you any more power unless the EL84 was actually restricting it in the first place.

    So by using a "higher power" valve which is also a different impedance and not compensating for that, you will get less power output not more.

    Think of it as being like putting a lorry gearbox in a car. Even though the box is *capable* of transferring much more power when used in a lorry, would you get more useful power to the wheels in the car?

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • springheadspringhead Frets: 440
    Yes I know.  That's what I was saying.  The 6L6 is being constrained by the reflected load, the relatively high cathode resistor and probably low anode volts.  But it might well be a condition at which the 6L6 is relatively "happy" but not able to make maximum power/minimum distortion.  It may be possible to squeeze a bit more power out of it in this circuit, depending on the OT load and maximum current capability.  However it's unlikley to sound very much louder but could perhaps contribute an alternative distortion character to the EL84, if sufficient drive is available.  
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