BahHumbug's 5E3 project

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  • springheadspringhead Frets: 435
    edited May 7
    I had the same turret board/jacks proximity issue as you!  I may re-do mine on an eyelet board at some point.

    I'd check all your voltages first against usual known 5E3 values.  Could be one 6V6 not pulling current or any number of things.  I'd then be checking continuity from chassis to all components that should be grounded.  On the leg of the component, not the wire or turret.  Also other usual problem chasing - does it do it with volumes down, what if you remove V1, V2 etc. Plug a guitar lead in, can you measure continuity from the sleeve at the guitar end down to the chassis (i.e. jacks grounded?). 

    Good luck!  Let us know how you get on.

    Edit: Can't see either a heater centre tap or the usual 2 x 100 ohm ground reference in your photo's.  Is your heater supply floating?
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  • BahHumbugBahHumbug Frets: 163
    Thanks for your input @springhead.  Yes my heater supply is floating at the moment, but I get the impression from your question that it shouldn’t be.  
    Its looking very much like Modulus have had a change of power transformer supplier and the layout that I’ve been working from doesn’t match the transformer that came in my kit.  The PT on the drawing has a green/yellow wire, which I assume is a ground reference.  My PT doesn’t have this wire.  I had an updated drawing from them this morning and I’ve just noticed that it indicates 2x100r sitting on one of the power tube sockets.  Those resistors weren’t supplied in the kit, so I’ve just emailed Michael at Modulus to ask if he’ll send me them down.  Hopefully that’ll make a big difference.
    FWIW the hum is there all the time, including volumes at minimum.  Turning the controls changes the tone of the hum, but doesn’t make much difference to the level.  I’ll get a chance to do some diagnosis over the next few days and I’ll do an update when I have some news.

    on the positive side the amplified guitar tone is pretty sweet so I don’t think there is an issue with any of the valves.  Plus I’m continuing to learn a lot :)
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  • springheadspringhead Frets: 435
    Yes the heater fake centre tap with the 100R resistors should help. Sit them on either the left hand 6V6 base or the lamp holder and take the junction down to chassis at your power supply ground point. Mine still has a little bit of hum, not a problem at gig level but I might redo it at point and try and reduce it.  


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  • BahHumbugBahHumbug Frets: 163
    Well, while waiting for some half watt 100R resistors to arrive I read through this article...

    http://www.valvewizard.co.uk/heater.html

    , which points out that it’s possible to do the same virtual heater circuit centre tap with a couple of quarter watt 220Rs.  I had some of those in the parts bin so I soldered them in.  The updated Modulus drawing has them soldered in on one of the 6V6 bases, between the heater pins and the cathode pin.  I tried this and it has worked a treat.  The amp still hums a bit but not enough to be annoying.

    In terms of sounds I’m pleased with what it does.  The cleans are nice.  It has to be taken quite loud to get much overdrive out of it though.  I pretty much have to turn the volumes up to 11 or so to get much drive, which makes it bloody loud.  I’ve tried it through a Vintage 30 and also through the Jensen CH12/70 that’s in my Cornell Romany.  Both are fairly high sensitivity.  I think I’m going to be going old school and getting it a Jensen P12R.  I have no need for it to be particularly loud.  I might consider a resistive power scaling mod too.

    Incidentally, I checked it’s voltages and they are all a bit on the high side.  For instance the volts delivered to the 6V6 anodes is around 406V.  It’s a similar story across the board.  I’ve got a Sovtek 5Y3 installed and looking around the web, the rumour is that that they tend to deliver somewhat higher volts than a typical 5Y3.
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  • springheadspringhead Frets: 435
    edited May 19
    Glad you sorted the hum.  The amount of drive doesn’t sound right. These amps are usually only clean upto about 4. Your volts are a little high. Could be the rectifier or maybe your 6V6’s aren't pulling enough current. What are you measuring on their cathode resistor and what value is it. 


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  • BahHumbugBahHumbug Frets: 163
    The cathode resistor is 250R.  There is 24V on each of the 6V6 cathodes.
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  • springheadspringhead Frets: 435
    Well that's roughly right for cathode voltage, slightly high perhaps.  I get around 21V with 365V on the 6V6 anodes, that's using a NOS Philips 5Y3.  Haven't tried any others.  Not accidentally running it on a 230V winding are you?  You could pull the 5Y3 and measure the PT secondary from centre tap to each leg and check with Modulous if your no load voltage is correct for whatever transformer they supply.
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  • BahHumbugBahHumbug Frets: 163

    I've checked what the transformer is putting out, which is +/- 343V with no load, and +/-330V with everything running normally.  Pretty much in line with the manufacturers specs.  I have a manufacturer drawing for the transformer and its rated for 240V on the primary side, with no options for other voltages.  They state the secondary voltage as +/- 330V at 138mA load.

    So I think everything is ok on the tranny side and I still think its an overperforming rectifier.  Apparently the Sovtek 5Y3 is an 'improved' design, with an indirectly heated cathode (presumably the original 5Y3 had the cathode and filament all-in-one), with the result being that it produces more volts than it should (ie doesn't behave like a 5Y3 should). 

    The higher voltages aren't too much of a worry at the moment.  One thing I've learnt from this is that even with the amp running at nominal levels the quiescent anode power dissipation on the 6V6s would be way above the 12W 'safe' limit that is stated for these valves.  This amp design is well-known for this.  With the voltages on my amp, the power dissipation looks like it is about 20W!  So I'm a bit concerned about red-plating, but haven't see any signs of it yet.  Apparently 6V6s are known for being able to take quite a bit of abuse........

    So next steps:

    1.  I've ordered the bits to do a resistive quarter-power reduction mod as per the Rob Robinette site.

    2.  Try a different 5Y3.

    3.  Get off my backside and start on a cabinet.

    4.  Fork out for a speaker.


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  • springheadspringhead Frets: 435
    I think a more conventional 5Y3 is a good idea. They are usually very lossy so hopefully that'll get your volts down.  If it all measures ok with that in, and you hit around 360-380 on the 6V6 anodes and you're still not getting plenty of break up by 4 or 5 on the volume then you have a signal path rather than DC problem to find.

    I built a rheostat (rather than the usual domestic L-Pad based) attenuator and that works quite well with my 5E3. Rob Robinette's fixed pad will be somewhat similer I guess.   But I also built a variable power circuit into it.  Voltage reduction for the whole amp, same basic VVR circuit that's all over the internet that Lazy J also uses in his amps.  Works really well in a 5E3 to back the volume down whilst preserving the tone and distortion.  I've done one gig with mine and had the power wound down a bit.  Sounded fab!


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