NRBAD - New Re-Built Amp Day

A couple of years ago I built Rob Robinette's 5E3 Micro circuit into the chassis of a Harley Benton GA5 combo, basically an even cheaper Epi Valve Junior. It sounded pretty good except for a squeal when the gain and/or volume were maxed. The layout was a compromise, partly because of the positions of the holes I had already drilled for extra pots, and the PT from the GA5 didn't really get close to the optimum plate voltage.

After finishing my TW Rocket build in the Spring, I decide to go back to the 5E3 Micro and do it right. I bought a Hammond chassis that (after a bit of woodwork) would fit in the cab, but I had to cut and rivet extra aluminium plates to accommodate the cage nuts. I bought a used PT on eBay that had come from an old valve radio. I had originally hoped to site the OT next to it, but I found the OT would pick up hum from the PT (regardless of orientation) so they ended up on opposite sides of the chassis. I re-jigged the turret board layout a bit, crucially moving the first grid stopper from the board to the valve socket.

Everything worked on first test, except for some sputtering, which I thought probably came from the output valve. The 12AU7 has a maximum plate dissipation of 5.5W across both triodes, but I found it was doing a rather toasty 6.4W. I changed out the 820R cathode resistor for a 1k2 (which gave 5.8W PD), and settled on 1k5 (just under 4.8W PD).

No squealing, and the sputtering has gone too. It works with either a  12AX7 or a 12AY7 in V1, depending on how much gain you want. You can dial in pre-amp drive, power amp drive, or both. Clean output power is estimated to be about 0.6W, which is great for low volume practice, but cranked you can wake up the neighbours.

All that remains of the donor amp is the cabinet and IEC mains inlet. Here's the spec:

PT - unknown, 290-0-290V HT, 6.3v filament
OT - Hammond 125 DSE
Speaker - Jensen C8R (8 ohm)
V1 - Tung Sol 12AX7
V2 - Brimar 12AU7

Obviously pics or it didn't happen:




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Comments

  • PeteCPeteC Frets: 409
    Nice one ! And great work on the cabinet too.   I never really got on with my 5E3 as it was simply too flabby    The chassis ended up being used for the Trainwreck express build.    
     but i am tempted to try this Mini version of the old 5E3.   

    Clips ?? 
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  • Nice! Now you just need a decent 12" speaker
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  • KeefyKeefy Frets: 2272
    PeteC said:
    Nice one ! And great work on the cabinet too.   I never really got on with my 5E3 as it was simply too flabby    The chassis ended up being used for the Trainwreck express build.    
     but i am tempted to try this Mini version of the old 5E3.   

    Clips ?? 
    I can't claim any kudos for the cab. It's all that is left from the Harley Benton GA5, I just sawed/chiselled away some of the battens to get the new chassis to fit. I am pleased with the faceplate though - the name is nearly straight!

    There is plenty of bass response but no flab as such. It sounds pretty grown-up - I will be using it mic'd up on my next home recording but don't hold your breath.

    In other news I am now fed up of turret board, my next build will be on phenolic eyelet board.
    Nice! Now you just need a decent 12" speaker
    Ha! I looked at putting a 10in speaker but there isn't room in the cab.
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  • PeteCPeteC Frets: 409
    Good stuff.  
    Yep - phenolic board and eyelets is much easier and more reliable imho.  
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  • KeefyKeefy Frets: 2272
    Thought it was time to do an update on this.

    For the OT I used a Hammond 125DSE which at 10W is hugely over-specced for this amp. I wanted it for a future build so I removed it and fitted a smaller (3W) OT.

    When I fired it up again I noticed it was prone to a shrieky oscillation. I realised this had started (in the form of bursts of noise on loud notes) before the OT swap. Chop-sticking led me to believe it was a microphonic 12AU7 but I subsequently eliminated both valves as being the problem. Eventually I traced the noise to the 22uF first stage cathode cap. I replaced this with a 25uF of a different make and that fixed the problem.

    The noisy cap tested fine for capacitance and ESR so I presume the internal parts were not wrapped tight.
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