1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Stereo Tube Amp Question

Discussion in 'Shock Brother's DIY Amps' started by robrob, Jan 2, 2021.

  1. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    Some tube datasheets have charts to calculate THD (even and odd) using voltage and plate load.
     
    awasson likes this.
  2. NTC

    NTC Tele-Holic

    Posts:
    595
    Joined:
    Mar 17, 2003
    I thought so at first, but the "screen tap" is on the other side of the transformer, resulting in negative feedback through the screen grid. I wonder about the math on that. This certainly is not normal practice. I would like to see the rest of that schematic. Not much signal should pass that 50 uF cap, though....

    On second thought, that is just a filtered supply with the OT acting as a choke - the 1500 ohms and 50uF cutoff frequency is .47 HZ - no signal getting past that.
     
    robrob, Wally and Mexitele Blues like this.
  3. Mexitele Blues

    Mexitele Blues Tele-Afflicted

    Age:
    43
    Posts:
    1,620
    Joined:
    Mar 7, 2018
    Location:
    Westminster, CO
    You're absolutely right, I was looking at one thing and imagining another. Still very interesting, I have not seen that arrangement before.
     
    Wally likes this.
  4. timfred

    timfred Tele-Meister

    Posts:
    148
    Joined:
    Mar 14, 2016
    Location:
    Northern CA
    This was my first thought, and now I’m convinced. It’s a cost savings way to get a free choke out of an OT. I converted 2 of these to guitar amps. The first one I left mostly alone except for adding a preamp and it sounded awesome. The second I could not get to sound decent until I ditched the OT in favor of a Champ style one.
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    745
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
  6. tubelectron

    tubelectron Tele-Afflicted

    Posts:
    1,742
    Joined:
    Nov 25, 2010
    Location:
    France
    Well @robrob , do not expect too much Hi-fi performance with the Hammond "Guitar Series" transformers, but OK : you may not need to go further, and nonetheless this will not prevent your amp from having a satisfactory tone, right ? :)

    Personally, in the Hammond range, I would consider the 1628SEA (SE 5K 40%ULtap 30W) or eventually the 1629SEA (SE 6K5 40%ULtap 30W) which will be more able to respect bass extension due to their higher primary inductance. But OK again : it's definetly not the cost of a 125CSE ! :oops:

    https://www.hammfg.com/files/parts/pdf/1628SEA.pdf

    Below is another experiment I made with a 6550 (KT120 projected) mostly with re-use parts :

    [​IMG]

    As you can see, the output transformer (at right) is by far the bigger (3.5K 1K2ULtap 50W) :

    [​IMG]

    Here are the square waves obtained with this circuit and that big OPT at 4.5W True RMS output : well, it's not bad for a SE amp, but not so amazing... o_O

    [​IMG]

    Here in comparison are the waveforms from my OTL (Output Transformer Less) 6080 amp project at 6.25W True RMS output : much better obviously, thanks to the absence of any OPT in the amp...

    [​IMG]

    The big black casings on the pics below are the housing of the HV filtering of the OTL power stage - not an OPT ! ;)

    [​IMG]

    All this just to show you the importance of the output transformer particularly in an Audio / Hi-Fi SE tube amp, even more than in a PP... :confused:

    But again : it depends of course on how far you plan to go with your little 6V6 SE project ! ;)

    -tbln.
     
    Last edited: Jan 5, 2021
    awasson, robrob and Paul-T like this.
  7. tubelectron

    tubelectron Tele-Afflicted

    Posts:
    1,742
    Joined:
    Nov 25, 2010
    Location:
    France
    :):):) Thanks @Tom Kamphuys ! I wonder how you could retrieve my quick post... :eek:;)

    I do not post too much on www.diyaudio.com because I'm fed up to be criticized by some ugly and dogmatic so-called "experts" who are simply jealous of my builts, because my rule is : "no pics ? just blah-blah ? = no proofs, it never existed."

    But it's me, OK ? :D

    -tbln.
     
    Paul-T and Mexitele Blues like this.
  8. dankilling

    dankilling Tele-Afflicted Silver Supporter

    Posts:
    1,566
    Joined:
    Oct 28, 2014
    Location:
    Lehigh Valley, PA
    Nickfl and NTC like this.
  9. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    For this little amp I'm not looking for antiseptic tone so a true audio OT isn't needed to meet my design goals. I did look at the Hammond 30 watt audio output transformers but they are too large and expensive.

    Dude, I was all set with my output transformers and you post that link. I think I'm going to have to use them. They look great, cost less than the Hammond and have ultralinear taps that I can put on a switch (or just ignore :D )
     
    awasson and dankilling like this.
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    Working on the layout:
    [​IMG]
     
  11. tubelectron

    tubelectron Tele-Afflicted

    Posts:
    1,742
    Joined:
    Nov 25, 2010
    Location:
    France
    Thanks for the link @dankilling ! :)

    Yes, this is a cheaper intermediate alternative... :cool:

    AFAIK I don't think that this brand is distributed here in Europe, conversely to Hammond - but I should re-check. o_O If not, the shipping cost to France with customs will be prohibitive :eek:, compared to Hi-Fi Hammond models that I can buy from Germany !

    -tbln.
     
    dankilling likes this.
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    Updated Layout:
    [​IMG]
    Decided to go with a small headphone jack for input and move the volume control to the right side since most of us are right handed. I moved the power switch to the rear of the unit for improved 60Hz hum isolation.
     
    Last edited: Jan 6, 2021
  13. pmacaula

    pmacaula Tele-Meister

    Posts:
    199
    Joined:
    Jun 2, 2019
    Location:
    Toronto
    Rob - am curious about the external humdinger/hum balance pot. Isn't this typically a set once and leave thing ?
    Is it because this is an end user amp (or at least more than most DIY amps would be) ?
     
    robrob likes this.
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    That's a really good point pmacaula. I'll change it to an internal or screwdriver type pot. This is why I post my layouts publicly during the design phase :D
     
    pmacaula likes this.
  15. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    745
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    @tubelectron Don Audio (Europe) has Edcor transformers.
     
    robrob and Paul-T like this.
  16. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    745
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    I think I miss grid stoppers. Is that intentional?
     
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    The power tube does have a grid stopper. I left off the V1 grid stoppers because volume pot resistance acts as a grid stopper unless the volume is maxed.
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    Progress:
    [​IMG]
     
    awasson and Nickfl like this.
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,346
    Joined:
    Dec 29, 2012
    Location:
    United States
    Near final design:
    [​IMG]
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.