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Secondhand homebrew amp with regulated B+ - comments invited!

Discussion in 'Shock Brother's DIY Amps' started by buddy_coles, Jan 10, 2021.

  1. buddy_coles

    buddy_coles TDPRI Member

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    Hi all, hope you are keeping safe and well. A few weeks back I bought a secondhand 'home brew' amp from ebay - it looks like the chassis from a Marshall sized head and has clearly not been used for a while! What I thought was a rectifier when I looked at the listing photos turns out to be an 'extra' 6L6GC as part of a HT (B+) voltage regulator circuit using a 0A2 tube, and I wondered what effect a regulated HT supply might have on playing dynamics.
    I've eventually got around to sketching out the schematic (attached), and it looks like a fairly standard 2-stage gain from 12AX7, with a James/Bandaxall tone stack in between, then has a pentode EF86 gain stage, 12AX7 phase inverter, and 2x6L6GCs in push-pull.
    The amp has clearly been built by someone who knew what they were doing (high quality components, neat wiring, massive transformers!), but I am trying to work out if it is worth keeping as-is (and re-housing into a more Fender-esque cabinet) or just using some of the components to build a AB763 circuit in a Deluxe Reverb clone as was my original plan...
    So my question - what might regulated HT do for playing dynamics? Any answers or comments welcomed! My current (highly technical) plan is to replace the electrolytic caps and just see what it sounds like :)
    Thanks, Sid.
     

    Attached Files:

  2. sds1

    sds1 Tele-Afflicted

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    Regulating the power supply will create a more linear amplifier because you eliminating the non-linear response by the power supply to increase in current demand. This would have a profound effect on the amp if the entire supply were regulated.

    In this case though B+ is bypassing the regulator circuit, it's only the screen supply and preamp that will be regulated. The amp will still distort however due to non-linear response to power tube plates demanding current. This is a good thing, for guitar amps we like this.

    I don't think this is much different than the various solid state power scaling solutions out there like:
    https://londonpower.com/power-scaling-faq/

    The primary reason for using such a circuit is not the regulation so much as the power (volume) control.
     
  3. NTC

    NTC Tele-Meister

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    That is a pretty unique beast, not just another copy of a well-known design. Play it as-is for a while and see if you like it or certain aspects of it.
     
  4. buddy_coles

    buddy_coles TDPRI Member

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    Thanks guys,
    @sds1, very good point regarding the B+ not being regulated - my mistake in my description of the circuit!
    There is an engraved (well, scratched...) mark in the top of the chassis stating that grid voltage should be 400V, and bias should be -50V which both sound reasonable. My wonder is whether I would lose some dynamics in the response to pick attack, or whether this would actually be improved as the output would be a bit more linear than without the regulated screen voltage.
    I think I'll plug it in and see...! Caps are on their way so next week I should be able to fire it up. Will post my results and possibly a youtube vid.
     
    sds1 likes this.
  5. buddy_coles

    buddy_coles TDPRI Member

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    Hi again friends - so I have made a little bit of progress with my mystery amp :) All electrolytic caps swapped for new, and using my low-tech current limiting device I tentatively fired the amp up. The mains TX gave what I thought were reasonable readings, though it was a bit confusing measuring the AC voltage from the mains TX as the readings seemed to be double what I expected. I have later realised that what I thought was twice the actual HT voltage was in fact the true HT voltage, which ends up around 710VDC with no load! Wow!

    So, that aside, the voltage regulator circuit seems to work a treat, and the 0A2 tube gives off a very cool neon glow :) I was able to set the screen voltage to exactly 400V and there was no fluctuation at all on my digital multimeter.
    My next problem, then, is the ludicrously high HT voltage! Having had a cursory look on google (what did we ever do in the good old days? I got all the way through school without any internet in existence, and my recollection is that you had to know someone who knew stuff...) it seems that the transformers are probably from a MusicMan amp from the 70s or so, where 700V of HT was provided.

    So I now have two issues:
    1.) The filter /power stiffening capacitor I put in is only rated at 600VDC, so I should probably replace this with a higher voltage component.
    2.) 700V seems like it may properly fry the two 6L6GCs!!! Even with screen voltage at 400V, the amp is grid biased so cathodes are grounded. I have limited knowledge of this kind of thing, but anything over about 500V (cathode to anode) seems out of the allowable range from what I can work out.
    3.) In fairness, I am not 100% certain the output tubes are definitely 6L6GCs, but they look much more like 6L6s than anything else I can find. They got very hot when I fired up the amp for a few minutes, though I did get an acceptable guitar sound out of the speaker so it can't be all bad. I didn't take time to set the bias as I was concerned about the filter cap running above its rated voltage.

    If anyone has experience with 6L6GCs running at crazy high voltage I'd love to hear from you! Otherwise, I wonder if I would do better to get a pair of KT88s or similar. Though I am assuming the tubes in the amp were chosen by the amp builder, so they may yet be ok.

    To be continued...
     
  6. buddy_coles

    buddy_coles TDPRI Member

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    ...and another update! I'm working on this in between trying to do actual work (from home) and look after a family and keep the house standing, so it's slow going :)

    So I have got as far as plugging it in and doing some better checks, and the unregulated B+ is a bit lower than originally measured, but still 640VDC which leaves me concerned for the output tubes! I do have the means of regulating the screen voltage (currently 400VDC) so it may yet be ok once the 6L6s start drawing some current and the anode voltage drops a bit.

    Anyway, with my Tele plugged in, it makes the right kind of noises but sounds like a hi-fi amp, i.e. little expression, very 'flat' sound. The Baxandall tone stack probably doesn't help this as it won't cut the mids like a Fender one would. But one thing that surprised me was that even with the volume control right up, I couldn't get any dirt out of the amp at all. The guitar signal goes as follows:
    12AX7 --> tone stack --> 12AX7 --> EF86 --> PI and output tubes
    I was expecting to be able to drive either the second half of the 12AX7, or the EF86 pentode, but gain is very low it would seem. And after measuring the HT supply to the preamp tubes it becomes clear that I have just 106VDC on the first triode stage, which drops to about 27VDC after the anode resistor!

    So, next steps are to play around with the dropping resistors in the regulated HT supply so that I get more Fender-esque voltages in the preamp. I'll also add cathode bypass caps to the first couple of triode stages to boost the gain a bit. I think with a bit of calculation (followed by the inevitable trial and error) I can get about 300-350VDC at the HT supply which should be ideal. I'm going to put a Fender tone stack (TB, as per AB763) in as well while I have the soldering iron out, and get rid of the 'spare' 12AX7 for the auxiliary inputs. I may end up using this to drive a reverb or trem circuit later on.

    I think as long as I can get the output tubes to play nicely with the high voltage then this should be salvageable - and can then be repackaged into something more compact.
     
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