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Simplified 5E8a Layout

Discussion in 'Shock Brother's DIY Amps' started by nathanh, Dec 27, 2018.

  1. Nickfl

    Nickfl Friend of Leo's

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    You need to have a connection from the bridge rectifier to the negative side of the first filter cap and reference it to the chassis ground too for your split ground scheme to work. You also need the power tube cathodes to be grounded.
     
  2. nathanh

    nathanh Tele-Meister

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    Left them off to keep it neat. Revised.

    Note: not everyone is a fan of the back of the pot ground bar. I don't know if thats how I'm going to do it either but don't sweat it for now.

    This is a bit of a long board however. If the circuit is fine then I guess the next step is to make sure everything is to scale. This part will definitely be my weakness.

    Edit: Actually I need to get the B+ correct first.


    5E8a V7.png
     
    Last edited: Dec 31, 2018
  3. Nickfl

    Nickfl Friend of Leo's

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    Re: getting your B+ right, did you say what tubes you plan to use? @Snfoilhat made a good point earlier in the thread that the 6l6g tubes in the original 5e8a are very different than the 6l6GC tubes in the HRD and that will likely effect your voltages quite a bit. If you want to use 6l6g tubes there are ways to bring the voltages down, but you will need to do some measurements of the transformer to get an idea of where you will be starting from. Alternatively you could use 6l6GC tubes even though they are different from the 5e8a schematic, which will probably keep your voltages very similar to the HRD and from there your sag resistor alone will get you in the ballpark of 5e8a B+.
     
  4. nathanh

    nathanh Tele-Meister

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    Ya know I'm curious to how similar the 5881's were to the 6L6G's from then. The JJ 5881s can take the voltage if I get it close to 415v if Im reading correctly and 6L6G's seem pricey.

    What do ya think?
     
  5. nathanh

    nathanh Tele-Meister

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  6. Nickfl

    Nickfl Friend of Leo's

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    That spec sheet shows 440v at 65.7ma, thats probably pretty close to what you would be running it at with 6l6g, maybe closer to 75ma, but thats negligible in terms of transformer load. If you need to drop 25v at 75ma, that is going to take a 330R resistor which will be dissipating close to 2 watts at idle, but could be dissipating more like 12 watts under full load... so you'd probably want at least a 15W, or better yet 25W resistor in that position.

    I think a resistor that large is going to cause more power supply sag than the tube rectifier in the 5e8a, so you may want to use a smaller sag resistor and drop the rest of the B+ voltage with a zener diode chain. This will also be easier to fit in the layout than a giant 25 watt resistor than may even need to be heat sinked and bolted to the chassis.

    If you use a 150R 10W resistor to drop about 11v that should sag pretty similarly to a gz34 rectifier and then 2-3 10v, 5w zener diodes will be able to drop the rest of the voltage you need to shed to get you down to 5e8a levels.
     
    Last edited: Jan 1, 2019
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  7. Nickfl

    Nickfl Friend of Leo's

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    5881 are close to the same as 6l6G/GB, 23watts for 5881 vs 19 for 6l6gb. So I'd consider them a perfectly acceptable substitute if your using new production tubes. That being said, take a look at vintage 6l6g/gb/gt tubes on ebay. NOS/ANOS are way cheaper than vintage 5881 or 6l6GC, which are $$$. I built a Supro Thunderbolt clone a couple of years ago and tubed it with a pair of sylvania 6l6GB tubes for about $25 off of ebay. Thats the route I'd go for this build if I were you.

    The other question that you have to ask at this point, is what is the primary impedance of your output transformer and what does the 5e8a call for? It looks like the HRD uses a 4.2K primary (assuming you are using the same speaker load as the stock HRD). Determining the correct primary impedance for a 5e8a seems to be harder, I'm not getting an easy answer with a cursory google search.

    I see some OTs being listed as suitable for the 5e8 with 4.2K primaries, but that doesn't mean much as transformer manufacturers tend to be a little fast and loose with those recommendations. Fender, like most manufacturers, leaves this crucial bit of information off of the original schematic and layout, so you are going to have to do some digging to make sure that HRD OT is right for this build.
     
  8. nathanh

    nathanh Tele-Meister

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    Copy that. I'll do 3 zener's and that should put me close enough to not care too much. Is the order here: rec, diodes, sag resistor?

    Since I tour around quite a bit I might just go with the JJ 5881's. Though I think I have some old GE's lying around that might be G's. I'll have to peek.

    Care to school me in OT's in this case? I assumed (I know I shouldn't in general) that since the HRD was pushing two 6L6GC's at 40 watts then all should be well enough plus the 4ohm tap will keep the NFB in line.
     
  9. Wally

    Wally Telefied Ad Free Member

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    You probably don’t want to go to the expense, but you can buy an OT from Mercury Magnetics that is built from the blueprints of an original OT from a 5E8A.
     
  10. nathanh

    nathanh Tele-Meister

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    Man...
     
  11. nathanh

    nathanh Tele-Meister

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    @robrob are your layouts to scale?
     
  12. nathanh

    nathanh Tele-Meister

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    @Nickfl Check out this setup. Seems to save a little space and I've got tons of room where a tube rec would be.

    Edit: I need to flip D4

    5E8 v9a.jpg Screen Shot 2019-01-01 at 5.48.50 PM.png
     
    Last edited: Jan 1, 2019
  13. Nickfl

    Nickfl Friend of Leo's

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    I like the rectifier diode circuit board layout, however those zeners need to face the other way, they drop their rated voltage when reverse biased.

    I also noticed that you have two 16uf filter caps in parallel at the node after the choke. Thats the way they did it in the original, but probably just as a cost saving measure as high capacitance value electrolytics were expensive back then. Just using a 32uf will save you space and simplify your layout.
     
  14. Nickfl

    Nickfl Friend of Leo's

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    I am sure it will work, I'm just trying to figure out if it will be exactly what you want for the amp you are building. In general, tubes are pretty forgiving about the conditions they will function under, so there is a relatively wide range of workable possibilities that may sound fairly different from one another. Fender seems to have most often gone with a 4K primary for 2x6l6, but the Super got 6K and it seems the bandmaster may have been at 2k (!) using the same OT with an unusual speaker load. All of these work, but they change the way the tubes operate and therefore how they sound.

    As mentioned above, the 6l6gc is a 30W tube while the 6l6GB is a 19 so that is a difference, plus B+ voltage is a factor in deciding what primary impedance you need. So those differences both mean that the HRD transformer will work a bit differently with the 6l6gb in your build as the somewhat lower voltages. This may very well be exactly what you want, actually I suspect it will be a good fit since the 5f6a definitely used a 4K primary and at least one supplier (mojotone) has a 5e8a specific transformer with that same rating.
     
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  15. Nickfl

    Nickfl Friend of Leo's

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    I am sure it will work, I'm just trying to figure out if it will be exactly what you want for the amp you are building. In general, tubes are pretty forgiving about the conditions they will function under, so there is a relatively wide range of workable possibilities that may sound fairly different from one another. Fender seems to have most often gone with a 4K primary for 2x6l6, but the Super got 6K. Both work, but they change the way the tubes operate and therefore how they sound.

    As mentioned above, the 6l6gc is a 30W tube while the 6l6GB is a 19 so that is a difference, plus B+ voltage is a factor in deciding what primary impedance you need. So those differences both mean that the HRD transformer will work a bit differently with the 6l6gb in your build as the somewhat lower voltages. This may very well be exactly what you want, actually I suspect it will be a good fit since the 5f6a definitely used a 4K primary and at least one supplier (mojotone) has a 5e8a specific transformer with that same rating.
     
  16. nathanh

    nathanh Tele-Meister

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    @Nickfl glad you dig. I considered mounting the the bias pots to the chassis as well but it could be counterintuitive unless I ditch the balance circuit. I'll think on it I guess.

    Re 32uf. Great idea. So where does the choke come in play now?

    As for the OT I think I'm not as familiar with its values as I thought. I'll look for some literature today!
     
  17. nathanh

    nathanh Tele-Meister

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    Like so?
     

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  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    They aren't exact (except the new AB763 layout) but they're close.
     
  19. Nickfl

    Nickfl Friend of Leo's

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    The choke stays put as does the first 16uf filter cap, it is the two after the choke that will become one 32uf.
     
  20. nathanh

    nathanh Tele-Meister

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    Cool, I need to get it in the ball park so I don't design a board thats too small or excessively too large.

    Copy. I'll adjust the layout then.
     
    Last edited: Jan 2, 2019
  21. nathanh

    nathanh Tele-Meister

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    I'm using an 8 ohm speaker as the main speaker and the 4ohm + Ext for my extra cab with an 8ohm speaker as well.

    Im not sure about the negative feedback resistor for the 8 ohm slot. 5E8a V13.jpg
     
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