1000:1 SE OT under 5W?

Discussion in 'Shock Brother's DIY Amps' started by drneilmb, Dec 10, 2019.

  1. drneilmb

    drneilmb TDPRI Member

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    I'm getting interested in @tubelectron 's Radio-Tone as a really cute one-tube micro amp https://www.tdpri.com/threads/amps-from-scratch.956173/#post-9148201. It uses a single ECL86 as the output stage and the datasheet suggests output impedances between 7 and 10kohms. I'd want to use an 8 ohm speaker cab with this, and I've been looking for a reasonably priced output transformer. I'm not finding much in that impedance ratio and power output.

    I see the Hammond universals 125ASE or 125BSE, but they're real transformers that cost real money. I see this 8W 5000:8 cheapie https://www.tubesandmore.com/products/transformer-output-8-w-single-ended but I'm worried about too much current at 5k ohms (maybe no problem since the ECL86 is over-rated for the 2W this amp asks of it?). Is there a Hammond 1750 series replacement transformer that does what I would need? Maybe https://www.tubesandmore.com/products/transformer-hammond-output-replacement-fender?

    Any suggestions? I'm new to DIY amps and I haven't got a pile of transformers sitting around, so I have to buy them off the rack and I'm not very familiar with the options that are out there and what they should cost (and what the trade-offs are for lower cost). Thanks for any help you can offer.

    -Neil N0FN
     
  2. dan40

    dan40 Tele-Afflicted

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  3. Dacious

    Dacious Poster Extraordinaire

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    You can do what I did - a line level output transformer has multiple taps, and an 8ohm output option. No, they're not intended for single ended tube amps. But they work a treat, my last one cost $10.

    https://www.ozvalveamps.org/optrans.htm
     
  4. drneilmb

    drneilmb TDPRI Member

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    I like that idea very much, but I don't see anything there about single-ended transformers that are air-gapped to be able to carry DC. As that page says:

    "You must keep everything balanced. You can't, for example, use one transformer in each anode circuit because both will saturate on unbalanced DC."

    Is there a way to work around that with those line transformers?
     
  5. drneilmb

    drneilmb TDPRI Member

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    It looks like there is a 3.5W single ended 7.5k:8 fender replacement from Hammond called the 1750AX: https://www.hammfg.com/electronics/transformers/classic/1750. I don't see them for sale from AES or Hawk Electronics, but $19 from Mouser.

    That looks reasonable, but what do I know? Should I be worried about the power rating? Mouser has it listed as a "1964 BLACKFACE 6G15 REVERB 3.5W Output".
     
  6. tubelectron

    tubelectron Tele-Holic

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    @drneilmb - maybe I can help ! ;)

    First, here the schem of my Radio-Tone (I do not remember if I posted it, so...) :

    [​IMG]

    It is derived from the Magnatone model 411 :

    [​IMG]

    As you can see, the transformer that I use is a 5K/5R Audax (cheap French-made from the 60s that I had in stock). The speaker that I use is a 8R, so it provides a 8K load.

    One that would be suitable is the Hammond 1750AX : 7.5K/8R

    https://www.tube-town.net/ttstore/hammond-1750ax-for-fender-blackface-reverb-125a12a-47605.html

    https://www.hammfg.com/files/parts/pdf/1750AX.pdf

    There is also the Hammond 1760C Champ output transformer with 3.2/8/16R secondary :

    https://www.tube-town.net/ttstore/hammond-1760c-for-fender-ot-125a35a-022905-upgrade-champ.html

    https://www.hammfg.com/files/parts/pdf/1760C.pdf

    All can be found in USA from tube amp part supplier and have similar compact size to my Audax, but yes : as is the 125ASE, they are not so cheap - like most transformers for tube amps.

    That said, if compactness is not your premium condition, any old radio output transformer designed for EL84/6BQ5, EL90/6AQ5, 6F6, 6V6, 6K6 would be convenient also !

    -tbln
     
    Last edited: Dec 11, 2019
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  7. drneilmb

    drneilmb TDPRI Member

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    Yes, I have been studying it, thank you! I think that I will use the Hammond 1750AX. I don't have any old radio transformers, but I would have been happy to re-use one if I did.

    While you are here, I'm wondering about the power supply filtering. The magnatone take the B+ straight off the transformer and uses 20/10/10uF for the three reservoir caps. You have two nodes of RC filtering before the B+ and use 47/22/22uF for the reservoir caps. I have read that older amps tended to use smaller caps, but is there a reason you had to add the two additional filter nodes? Is it just your preference to have that nice clean supply?

    I ask because I'm going to use a switching DC-DC converter for the HV supply and I'm thinking of adjusting it to the B+ voltage and skipping the 220R/47uF filter nodes.

    Thanks again for this elegant little circuit.

    -Neil N0FN
     
  8. tubelectron

    tubelectron Tele-Holic

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    Your 're right : I really can't stand supply hum... the 3x47µF + 220R offers the expected hum silence from the amp, that the rudimentary filtering of the Magnatone 411 is unable to provide.

    Well, I never used DC-DC switching converter... :oops: So I have no experience about this !

    Which one would you use and how would you proceed, by the way ? I'm interested... o_O

    if it was me, I would provide in the amp some room for a possible additional RC filter, just in case that the DC-DC converter would not be so silent (hum or RF disturbancy). ;)

    A+!
     
  9. drneilmb

    drneilmb TDPRI Member

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    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=322374615632 is similar to the unit I've got. For my Deluxe Micro, I've been very happy with the ripple characteristics of that unit. The switching frequency is very high (72 kHz?) and the ripple on the 390V output is only 300mV at that frequency.

    I'm quite surprised how well these work for Micro amps. The Radio-Tone will be my second attempt with it and if it works well, then I'll use another one in a JCM800 Micro.
     
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  10. drneilmb

    drneilmb TDPRI Member

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  11. tubelectron

    tubelectron Tele-Holic

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    Thanks for the ref. of the DC to DC booster : I'll buy one just to see how it works...

    And good luck for your "Radio-Tone-Clone" ;)

    I'm indeed interested to see your built evolving and the result... :cool:

    This is a simple circuit, but if you're in trouble and I can help, you can contact me, of course.:)

    -tbln
     
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