Champ Reverb Build 6L6/EL34

Discussion in 'Shock Brother's DIY Amps' started by bryankloos, Apr 13, 2021.

  1. Paul-T

    Paul-T Tele-Holic

    Posts:
    629
    Joined:
    Nov 23, 2018
    Location:
    London
    I'm in Europe, bought a Jensen P10R for my Princeton, sounds great.

    If I were in the US I'd buy a Weber signature in a heartbeat. 10A100T would be great too.
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,932
    Joined:
    Dec 30, 2019
    Location:
    california
    With a split ground, usually the PI would ground on the PT side of the amp. In this example the PI grounds with node C. Node C also is associated with the reverb recovery tube, so you will have to choose on which side node C connects to the chassis.

    With a split ground scheme, I imagine two ground bus wires. One terminates near the PT. The other terminates near the input jack. For each node, the negative of the filter cap is the gathering point for the returns of that node. There is a space on the bus between the gathering points (nodes). When a ground bus wire is not employed, the same principle holds true. Each node has a gathering point. Those gathering points will be placed *in order* and terminated on the chassis near the PT side, and near the input jack side.
    I ground my amps this way without noise issues. As mentioned before the grounding of the OT secondary is up for debate particularly when NFB is connected. The Reverb Transformer secondary is isolated from the primary so the ground can be terminated on the reverb jack.

    I find studying ground schemes interesting as there seems to be quite a bit of agreement as to the theory of how to ground an amp but little agreement as to actually applying the theory. Many amps have ground schemes that break the rules yet are quiet. The hard fast rules are bent and broken all the time.
     
  3. 2L man

    2L man Tele-Holic

    Age:
    61
    Posts:
    562
    Joined:
    Nov 23, 2020
    Location:
    Finland
    So you have decided to do Star grounding and you will have four wires going from each amp stage to main electrolyt negative terminal where power transformer negative or center tab also goes. You have done this right when you measure resistance between chassis and ground and it show infinite until...

    ... you connect first pre amp ground to chassis using ground lug next to input jack ot use steel jack without insulation and now ohmmeter show zero.

    Now all operative return current goes direct to power supply and there won't flow any return current to chassis so jack screw is enough to remove potential error between ground and chasis but naturally a star washer should be used. You can mound more input jacks without insulation without building ground loop here when no current flow to chassis.

    Then connect also mains cable earth wire to chassis and there use good quality lug. This makes amp electrically safer when thru this earth wire building fuse burns if amp fuse does not protect. Earth wire is also used to reduce noise what player and guitar absorb from environment. There is practically no current is this noise so it can flow thru guitar cable mesh and thru chassis to mains cable earth and finally to true earth where building is "grounded"
     
  4. Mongo Park

    Mongo Park Tele-Afflicted

    Posts:
    1,155
    Joined:
    Nov 28, 2010
    Location:
    Winnipeg
    Any champ I build foe to a 12” speaker. If you are going with a big bottle power tube I would definitely go to a 12” speaker. 10” will choke out all the glorious tone and punch you are building into this amp.
    Big motor= Big fat tires.
     
    bryankloos and Kevin Wolfe like this.
  5. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    I’m starting to think a 12 would be acceptable in the amp as well.

    What are your picks for a good/great 12” for the cab?
     
  6. Mongo Park

    Mongo Park Tele-Afflicted

    Posts:
    1,155
    Joined:
    Nov 28, 2010
    Location:
    Winnipeg
    I don’t know what can you are using
    I go with the Canibus Rex, but it is a personal thing to suit your tastes. Something I like you might not. a
    Also being in Europe you might find something of better dollar value, maybe Celistion might be easier order.
     
  7. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    Thanks. Ive listened to the Canibus Rex. Sounds nice, very mid forward.
    I'm in the US if that makes any difference.

    I just finished the turret layout and punched the board. I guess its time to start drilling holes, and finish drilling the chassis for transformers, choke, standoff supports, etc. No turning back now!
     
  8. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    Slow and steady progress. I've drilled the chassis front/back and tube sockets. I need to cut out for the PR, and determine OT, RT and choke locations. After that I'll start drilling the G10 for turrets. Tomorrow will be rainy so I'm hoping to get a little further along.

    IMG_6656.jpg
     
    2L man likes this.
  9. Fixcinater

    Fixcinater TDPRI Member

    Posts:
    54
    Joined:
    Mar 29, 2021
    Location:
    San Diego, USA
    I have a modded (mostly for more gain) 5F1 circuit in a 1x12 cab, it’s glorious compared to my old SF Vibrochamp. Sounds like a real amp in person and yet isn’t super loud.

    I’d go 12” if possible.
     
    bryankloos likes this.
  10. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    Yes, I've decided to go with a 12". Not sure yet what speaker I will use, but given the size of the chassis at 19" to accommodate the verb and TMBR, I'll have plenty of room to fit in the 12.
     
  11. Fixcinater

    Fixcinater TDPRI Member

    Posts:
    54
    Joined:
    Mar 29, 2021
    Location:
    San Diego, USA
    I think you'll love it, with the direction of the other mods you've mentioned, the 12" should be just another step in that direction. And if not, you can always slap an adapter ring in there and go to a 10".

    As for the circuit, almost all of my mods that I've kept are going in the other direction, so I'll just say if you aren't digging the tone, or feel like you need more mids/aggression, try it with a lifted tone stack and/or lifted NFB. I generally like the easier transition to OD that those give me and those are easy to audition.
     
    bryankloos likes this.
  12. Mongo Park

    Mongo Park Tele-Afflicted

    Posts:
    1,155
    Joined:
    Nov 28, 2010
    Location:
    Winnipeg
    Looking excellent, sorry I confused the posts I thought you were a little further east.
     
  13. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    First time making a turret board. It went well.
    Assuming my evening doesn't get away from me, I'll start mounting hardware this evening.
     

    Attached Files:

    radiocaster and 2L man like this.
  14. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    37
    Posts:
    3,678
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    Just saw this thread for the first time. In regards to the rectifier choice - I'm not going to argue the technical aspects of "sag" & "Class A", BUT in real world use, the headroom, tone, feel, and even dB output difference in rectifier choice in single ended amps cannot be argued. I just finished up a 5881 5f2a-styled amp and have built several 6v6 Tweed Princetons with light mods. There is absolutely a difference in the end result depending on rectifier choice. Blame it voltage difference or whatever you want, the difference in a 5y3 and gz34 in there is like 2 completely different amps.

    I also hooked an ipod into this last amp and stood there and measured the B+ while I turned up the volume on the amp. It absolutely lowered the voltage as the volume increased and when heavy bass notes played. I wish I'd written it down, but I only have so many hands. Increased volume & heavy bass brought down the B+ voltage by several volts sometimes.

    To quote Aiken: A true class A amplifier has no sag because the current draw at full power is the same as the current draw at idle. However, most class A amplifiers aren't biased exactly at the midpoint of the range, and will tend to clip asymmetrically, especially when going into grid clamp on the output tubes, so there will be an offset current component, but it will be much smaller than in a class AB output stage. https://www.aikenamps.com/index.php/what-is-sag

    Also: The current in a true class A amplifier is constant, so it doesn't exhibit this bias shift, unless driven to clipping, where all bets are off. https://www.aikenamps.com/index.php/the-last-word-on-class-a


    To sum up why I went all-in on this is because I see "rectifier choice doesn't matter in a Champ" in nearly every post about Champ & single ended amplifiers. Whatever you want to blame it on, the difference from 5y3 to gz34 is pretty stark. I'd hate to think that anyone building an amp like this got finished with it and was unhappy with the amount of headroom or overdrive from it, but didn't run through different rectifier options because they thought it wouldn't make a difference. Bias it right and you can get 3 different amps out of a simple rectifier swap.
     
    Kevin Wolfe likes this.
  15. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,429
    Joined:
    May 24, 2010
    Location:
    Canada

    When current passes through the resistance there is a voltage drop. Different tube rectifiers have a different internal resistance, the 5Y3 greater than a GZ34. The greater the current capacity of the rectifier the less the voltage drop in the identical circuit. So right off the bat the 5Y3 will drop more voltage than a GZ34. This can change the sound of the amp on its own with the power tube having a different supply voltage.

    You can look at the power supply as a constant voltage source with a resistor in series. This is called it's Thevenin equivalent.

    [​IMG]

    Now let us look at the 5881 datasheet. At 350V zero signal plate current is 53 mA and screen current is 2.5mA. At maximum signal current the plate is 65ma and the screen is 8.5 mA. So 55.5mA compared to 73.5mA, a difference of 18.5ma. So an increase of 33%. Lets look at the 5Y3 plate curves.

    [​IMG]

    Each division is 2V along the bottom. So going to the 52mA line and going up to the 350V line along the left side of the graph we have our idle point. Pretty darn close to the 300V AC curve, good we will use that to see what the voltage will be at maximum signal. Now we move to the right to the 73mA line. So it looks like we dropped down a division and a half. Each division is 10V. So from zero signal to maximum signal the voltage dropped 15V, from 350V to 335V.

    So this is where I am going to get lazy, actually I have other things to do.

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/

    Select 5881 from the drop down list. It will bring up the 5881 curves and load line for (it is so convenient) 350V in Class A. It gives a power out (at max G1 : 12.25 (W))

    This is the lazy part, At the V+ (V), it is set to 350 (V). Change this to 335V.

    Then look at the power (at max G1 : 11.19 (W))



    So does the voltage drop in Class A from idle to full power? Yes, with a 5Y3, 350V to 335V. This is 95.7% of the idle voltage, a drop of 4%. So you you are right. A Class A SE amp does have a voltage sag. Is the 4% significant? Probably not.


    The difference between a 5Y3 supping the current and an 5AR4 will probably be more than the 15V sag in Class A at full output. Changing tubes can change the feel of an amp. But this will be because of the different operating points due to the supply voltage rather than the sag. I think.
     
    Lynxtrap likes this.
  16. Mongo Park

    Mongo Park Tele-Afflicted

    Posts:
    1,155
    Joined:
    Nov 28, 2010
    Location:
    Winnipeg
    Excellent explanations, well worth incorporating into your build. To this end you should have a transformer that has 3 amps on the 5 volt winding so you are not limited by your rectifying choice. A lot of transformers are built for the likes of a 5Y3 as it is a common tube to use in a single end amp which uses 2 amps.
     
    jsnwhite619 likes this.
  17. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    A little more progress... I was short a 250K pot and a few 1/2" lock washers. Oh well, I needed to order a few more bits.
     

    Attached Files:

  18. Kevin Wolfe

    Kevin Wolfe Tele-Holic

    Age:
    63
    Posts:
    570
    Joined:
    Jun 13, 2017
    Location:
    Lakeland FL
    I gotta agree with this ^^^. The voltage difference alone makes for two entirely different amps.
     
    jsnwhite619 likes this.
  19. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    Busy with work and my son's cancer treatment, but did manage to get a little work done on the board.
    All the tube sockets are mounted as well as the iron. Its taking shape.
    Next will be wiring the pots.


    IMG_6730 .jpg
     
    Gleesonjf, rdjones and Fixcinater like this.
  20. bryankloos

    bryankloos Tele-Meister

    Posts:
    100
    Joined:
    Oct 28, 2015
    Location:
    Weston, CT
    Hello All,

    Probably premature, but I have a question on my build. I'm finished with the board and have most of the power circuit wired up. I've tested the output of the tube rectifier at 347Vdc. Strangely, the SS rectifier is giving me an output of 287Vdc. These voltages are taken at the base of the rectifier tube, and the common/last diodes on the SS rectifiers at the TBD resistor (currently a 150/2W). There are no downstream connections to C1 or the choke yet...

    Does this seem off? Am I using the incorrect SS rectifier configuration? Its built with 1N4007 in I believe a Full Wave CT configuration...

    Thoughts?

    Thanks.
     

    Attached Files:

    Last edited: Jun 4, 2021
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.