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Simplified 5E8a Layout

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

  1. Wally

    Wally Telefied Ad Free Member

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    Agreed...and the grid is a basic variable, eh? My wall voltage can vary from 123VAC in the mornings to as low as 111VAC during the afternoons. That is problematic for my purposes.
     
  2. nathanh

    nathanh Tele-Meister

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    Do you guys think my logic about V1 is correct?

    To reiterate, in the original the Normal high and lo corresponds to each side of the tube: V1a = lo, V1b = hi. Bright v2a = lo, V2b - hi.

    Since there are two channels and 4 inputs you can only jumper collectively to the sum of 1 whole tube. So 1 input, jumpered on the pins using one one entire tube. I was able to ditch the 100k mixing resistors as well. I don't imagine losing the mixing resistors has an effect now.

    So if the voltages stay the same, theoretically the preamp section is the same as if I jumpered the channels on the original?
     
  3. Wally

    Wally Telefied Ad Free Member

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    IMU...in my understanding...there is no 'sum' of the tubes. When jumpering, one is running the two channels in parallel. IN any case.....
    IF you use on 12A_7 for the input stages of the two channels, you will have to use an ABY switch, a Y cable to parallel the two channels, or do an internal switch to accomplish that.
     
  4. nathanh

    nathanh Tele-Meister

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    yeah thats kind of what Im saying.

    You cant use all of V1 originally without a Y cable. You can jumper however to V2 with a patch cable and use half of V2 and V1 at the same time.

    So my solution is to just use one tube with the channels jumpered at the tube pins. Just wondering if this is actually any different sonically than if I had left it normal and used a patch cable.

    Screen Shot 2018-12-29 at 12.47.05 PM.png
     
  5. peteb

    peteb Friend of Leo's

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    Sorry, I’m probably not adding anything....


    The two channel amp was originally designed to accept four inputs, instruments or mics. There are four inputs and four triodes in the first stage. Later designs simplified this and four inputs were used with two triodes in the first stage. Maybe this means the intended use was now only two instruments at once, one per each channel.


    I assume that you are interested in using only one instrument.




    This means that you could have 1, 2, 3, or 4 inputs and you could also use any number of triodes when all you really need is one.



    Not trying to be captain obvious,




    But what are your goals?




    Btw, the circuit has a lot of potential and I like the idea of having dual rectifier tubes with the ability to use different rectifiers including using one or two at once.
     
  6. nathanh

    nathanh Tele-Meister

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    Thats the spirit!

    A single input version that uses one rec instead of two but retains as much of the original sound as possible. I have a dead HRD I was considering building this into and I figured the good people here at TDPRI could help. I want to simplify without changing the sound. Im thinking the single rec however is a no go.
     
  7. Wally

    Wally Telefied Ad Free Member

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    Nathan, I was not teching back when I owned my 5E8A almost 25 years ago. I didn’t even know enough to dig into my tech’s understanding and do the experiments with him. It would be interesting to make observations on the differences between 2 x 5U4, a single 5U4 and perhaps a single 5AR4/GZ34. Whoknows, one might set up for two rectifiers with a switch to drop one out. One could switch between one or two 5U4’s or between a single 5U4 and a single 5AR4/GZ34???.watch out...I’ll drive off the cliff...........
     
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  8. peteb

    peteb Friend of Leo's

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    Despite providing endless combinations, the two rectifier sockets are just cool.



    With one rectifier socket powered for a 5U4, you still have quite a few possibilities. 5Y3, 5U4, GZ34, and SS.

    That’s pretty much the full range you will get with two sockets.


    You need at least 3 triodes leaving two for the first stage. It seems like it would be a waste to not either have two inputs into two triodes or have the one input switchable or something so that it either goes thru one triode or two in parallel? Not sure what the affect would be here. I do know the sound of combining or bridging channels, it is fatter or fuller sounding.
     
  9. nathanh

    nathanh Tele-Meister

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    The way I see it is to always have them in parallel. My bassman feels anemic after I turn down the second channel. It’s not actually thin, but relative to having both up it’s less than. It would be a shame not to use both sides of V1!

    Man I didn’t think about mismatching recs. I bet a 5U4 type and a SS would be very cool.


    Sent from my iPhone using TDPRI forum mobile app
     
  10. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Check out the inputs as drawn. Imagine inserting a plug into input 1. It will lift the switch to ground, which is good. Follow the leads. You can see that the signal now has a clear path to the control grid of V1a. But notice that it also has a path through the closed switch to jack 2, and from there to the control grid of V1b. This is a different Hi-Lo setup than in the typical Fender amp of this time and later. Robrob has a cool write-up of the Fender typical setup for either 2 or 4 inputs. It uses voltage dividers to provide Hi and Lo. This amp parallels V1a and V1b with input 1, and uses V1b alone for input 2.

    Another note -- you may miss the absence of grid-stopper resistors (the 68ks). Some home builders doing repros of old Fenders (like the weird 3 input amps) build them pretty faithfully but use a more modern input scheme. By modern I mean like 1957.

    twin_5e8a_schem.png
     
    Last edited: Dec 29, 2018
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  11. Snfoilhat

    Snfoilhat Tele-Afflicted

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  12. nathanh

    nathanh Tele-Meister

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    Great info and I love that site. This section deserves a re-read for me tonight.

    In the meantime, is my single input being connected to both sides of V1 (v2 has been eliminated in this layout) the same as having two sets of inputs (V1a and V1b) and jumpering channels?
     
  13. Snfoilhat

    Snfoilhat Tele-Afflicted

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    If you want to reproduce the Input #1 of the 5E8A, then you should have a 500k grid leak (there are two 1M resistors to ground in parallel) and no grid stopper.

    If you want to reproduce the Input #1 of a more modern amp (5F8 and later), then your layout is correct with the exception that your grid stopper should be 34k not 68k, because on input 1 there are two 68k resistors in parallel.

    That's my understanding of it, FWIW
     
  14. Nickfl

    Nickfl Friend of Leo's

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    I think that would be no different than just using a SS rectifier by itself. If you run diodes in parallel with the tube rectifier, you are in effect bypassing the tube with a solid state rectifier and you will just get the higher voltage and lack of sag that is characteristic of the SS rec, the tube will have no effect.

    In my opinion you are spending too much time worrying about the choice of rectifier. Rectifier sag makes a difference in the overall sound of the amp, but it is a subtle one. Voltage makes more of a difference and rectifier choice does impact that, if you are already stuck with a specific power transformer. Since you are making a new design you are not limited by your part choice and you really need to choose a tube operating point as a starting point and make your component choice from there. Agonizing over rectifier tube choice as if that is a defining characteristic of the amp is the wrong approach in this case as it only really changes the characteristics of an amp when you switch rectifier types in an existing circuit. The rectifier tube contributes very little to the overall tone of the amp in and of itself, it is only when changing from one type to another without changing anything else in an amp that it can have a big impact, because it will alter the voltages and therefore bias points of tubes etc...

    You said you are building this in a dead Hot Rod Deluxe, what is dead and what are you using from that old amp? Are you using the transformers? If so that would be the starting point for designing your power supply.

    Personally I would not bother with the dual rectifiers, extra expense for no real reason. I'd either go with a gz34 or SS, possibly with a sag resistor. The weber copper caps are fine, but they are just diodes and a sag resistor in a can, if you are going with that why not just do that inside the amp and save the expense of the socket and copper cap?
     
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  15. nathanh

    nathanh Tele-Meister

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    Great points. This is truly a thought experiment to me for now as it helps me learn but it is evolving into something I might be able to build, money pending. As for the HRD all of the trannies work and you are absolutely correct about starting there. I'll be going back through and figuring out how to compensate.

    It's good to hear your opinion on the recs. I was a little hung up on simplifying but not changing the tone for sure. I guess the rec isn't as responsible in this case as I thought. I think I'll go with having a socket and cap in case I want to try a tube later.
     
    Last edited: Dec 29, 2018
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  16. peteb

    peteb Friend of Leo's

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    From an option and tone standpoint, on the rectifier




    The best option is to have two rectifiers and a switch. wire them so that you can have two (or three) different options available on the fly. Popular choices would be a GZ34 and a 5U4 or a 5Y3.



    Newer amps have this option but it ties back to the twin.
     
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  17. nathanh

    nathanh Tele-Meister

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    Copy that! Not a bad idea at all.

    Does anyone care to double check my preamp section to make sure I didn't get anything wrong when trying to compile them? Im not so sure on the aforementioned 1m grid leak (500m according to @Snfoilhat), the cathode cap/res, and the preamp not having the extra 100k mixing resistor pairs.

    Files attached.
     

    Attached Files:

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  18. Nickfl

    Nickfl Friend of Leo's

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    If you are going to use the HRD transformers (I would) you aren't going to be able to use a tube rectifier. The HRD is SS rectified and the power transformer doesn't look like it has a 5v winding for a tube rectifier. You could add a dedicated filament transformer just for a tube rectifier, but I wouldn't bother. Just go with a diode rectifier and use a sag resistor to drop the voltage down a bit and simulate the rectifier you wanted to use. You could still use a copper cap, but thats $20 for the weber part and a couple bucks for a socket vs $0.25 for diodes and a resistor to do the same job...

    With the HRD power transformer your B+ may be a touch high, 430 on the HRD vs 405 for the 5e8a, but its close enough that you could knock it down with the sag resistor and/or some zener diodes. The biggest problem I see with the HRD transformer set is that the HRD choke is just filtering the screens, while the 5e8a choke is filtering the entire B+ supply. You are going to either need to alter the 5e8a design or get a choke rated for more current. That HRD choke is probably only rated for about 50ma, but for the 5e8a you would need a choke rated to around 200ma.
     
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  19. nathanh

    nathanh Tele-Meister

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    Just now pulling up the HRD schematics. I don't think I've ever cracked one open before this. With adjustments I think the power section is totally doable. Thanks for the direction!

    Roger Roger on the diodes and sag resistor.

    I'll start working on a design then based on the stock PT. I could definitely stand to upgrade the OT however. Challenge accepted.
     
  20. symbiotic

    symbiotic TDPRI Member

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    Do you know you need an upgraded OT? Doesn't the HRD already pump out plenty of clean volume from its two 6L6's?
     
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