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Scratch Build, -from the beginning...

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Sep 24, 2020.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hammond 290ax 100mA dual secondaries 550ct and 650ct.
    Classictone 40-18016 120mA 660ct.
    I am confused as to whether 6L6 or 6V6 are in the plan. I thought we are now looking at 6V6 as that is what came with the amp???
    You can run the OT with two parallel tubes to use the 3k-ish primary. Maybe the original amp ran all three 6V6 paralleled with that OT. That would still be in the ballpark for the OT.
     
  2. Jewellworks

    Jewellworks Tele-Meister

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    the backstory:
    im WANTING to use leftover pieces parts from 2 different amps. the original plan was to use the PT from the Ampro that had the 3, 6V6GTs, as well as the 6SJ7, the 6J5, and the 5Z4 (that i also want to use in this Vintage style build).
    the OT came from a small 10w PA amp that ran a single 12ax7 and a 6L6GC. it was original to that amp, and labeled as an 8 ohm output tap (along with the 70v and 25v taps previously mentioned in this thread, but i wont be using).
    the 6L6G Power Tube came from another PA amp, and there were 2 of them in PP.
    what im REALLY WANTING to do is use the 5Z4 rectifier, 6SJ7 and 6J5 as preamps and 6L6G in this vintage style amp. the cabinet is small and can only handle a 6" speaker. 6" at 8ohms was just delivered today. if i can get away with the PT from the Ampro (and it looks like i cant) and the OT from that other small PA amp, then great! if i have to buy a PT and an OT to make all this work together, then thats what ill do.
    -although, unless its says 8 ohms on the OT, i dont know what else to look for in the specs, so it will match well with my setup. mine already says 8 ohms but apparently its not. even when i crunch the numbers using a 6L6GC that came with that amp, it comes out too high. (10.3) -its confusing
    im wanting to use these tubes and older transformers because im looking for a Vintage LOOK as well as Vintage Sound. (plus i already have them and would like to use them. less to buy) i dont want this to sound like a modern amp. thats why i have the 6SJ5 as a pentode in V1.
    and i want to try the Bone Ray tone control on this. so its 6SJ5, tone stack, volume, 6J5, 6L6G, 8 ohm speaker
    ive got a 5Y3 as plan B for the rectifier
     
    Last edited: Sep 27, 2020
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    But as I pointed out two of the 6L6gc in parallel will put you in the right place for the OT. 6k6 primary at 8ohm secondary. At most it would be 3dB more output than a single 6L6 so why not. Checks off all the boxes *if* you have enough room for one more octal socket.
     
  4. Jewellworks

    Jewellworks Tele-Meister

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    ive got a 5Y3 as a Plan B for the rectifier
     
  5. Nickfl

    Nickfl Friend of Leo's

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    You seem kind of stuck on the idea that 5K is the correct load for a 6l6. That is just a number pulled from the datasheet that works in one situation. Different voltages and different bias conditions would use a different load and there is a bit of leeway anyway, tubes are fairly forgiving.

    The back of a napkin calculation for determining a center biased class A load is the square of the plate voltage divided by the maximum plate dissipation of the tube. Datasheets often go with a slightly different load to maximize power or minimize distortion. Keep in mind that datasheets were not made for guitar amp building, so their numbers may or may not be what you want to use.

    So yes, 5K would be theoretically ideal for a 6l6g at 310 or so volts. But what good is that when you have a estimated 470v power transformer and an output transformer that is going to either give you 3.3 k with an 8 ohm speaker or 6.6 k with a 16 ohm?

    Lets layout a couple of practical possibilities for the parts you have rather than keep hammering at the theory behind it.

    If you want to use the speaker and OT you have, you are stuck with a 3.3K load. So, to match that load you could use a much lower voltage; 250v would be ideal so maybe 265v B+. That wouldn't really work with the PT you have because you would have to drop about 200v off of it, which is just not reasonable.

    Alternatively, you could add another 6l6G in parallel, which would half the required load resistance and work well with your 3.3K load at about 370V B+ (350v plate). You could get your PT voltage down there with a big zener diode or a 20w resistor, or just get a more appropriate PT.

    You could keep the PT without having to drop the voltage if you switch to a 6l6GC. You would still have to either get a different speaker or use two power tubes in parallel though because a 6l6GC wants to see 6.6K at 450V.

    If I was building an amp with the topology you are envisioning, and my starting point for the power amp design was a 6l6G and I was specifying other parts from there, I would go with 370v B+ and a 6.6K or so load.

    As it stands it looks like you either need to replace the PT, replace the speaker, or use two 6l6G in parallel. Getting a different PT is probably the simplest option. I know you want to use what you've got, but when you are harvesting parts to reuse for amp builds you rarely end up using all the parts from something on the same build, especially as you start pulling parts from things that are further removed from guitar amps like a projector or a PA. Sometimes you just have to accept that some stuff is going to have to go in the parts bin for next time.
     
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  6. Jewellworks

    Jewellworks Tele-Meister

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    how do i calculate how much my B+ will be after rectification? so i can compare rectifier tubes?

    how does this tie in with the OT, and a 16ohm speaker? what math is this? what (if anything) do adjust in order get it to work with an 8 ohm speaker? and WHY is 6.6k "ideal?"

    im having a very very difficult time seeing how all this relates to one another.

    i would like to keep the tube set that i selected, and the 6", 8ohm speaker. im ready willing and able to buy a different, more appropriate Power Transformer, as well as a more appropriate Output transformer to use with this tube set and speaker. but i dont understand the math and dont understand the specs

    sorry @Nickfl i posted this before i saw you had posted something. then yours popped up...
     
  7. Jewellworks

    Jewellworks Tele-Meister

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    weve already determined the voltage is too high with the PT i was hoping to use, and im shopping for a different one. that part is settled. ive been given a few choices by other contributors, including yourself.

    where does 3.3k come from with an 8ohm load? and 6.6k with a 16 ohm load? where do these numbers come from?

    thats the plan.

    why 6.6k load? because you have a 16ohm speaker you want to use? i have an 8 and cant i buy a different OT to work with that? as well as a more reasonable PT?
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    In an earlier post you measured the OT. You came up with an impedance ratio of 414.52 to 1.
    When you adjust the ratio to fit an 8ohm secondary you multiply each side of the ratio by 8.
    414.52 x 8 = 3316 so adjusting the ratio would end up as 3316 to 8 ~3.3k to 8
    Likewise 414.52 x 16 = 6632 so adjusting the ratio would end up as 6632 to 16 ~6.6k to 16.

    We know 6.6k primary is safe for 6l6 tubes because other amps have used them such as the fender bandmaster with 410 B+.
     
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  9. Nickfl

    Nickfl Friend of Leo's

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    To get a ballpark B+, look at the rectifier datasheet. There is a graph where you can plot the AC voltage (from your PT) and the DC current draw (you mentioned 70ma for your design earlier in this thread) and you can get an idea of what voltage that rectifier will give you under load.

    Next determine your plate voltage by subtracting your cathode voltage from you estimated B+. I assume you are going with cathode bias, so for a single 6l6g it should be 15-20v.

    You can then get a ballpark plate load by using this formula:

    plate load = (plate voltage x plate voltage)/ max plate dissipation

    So, for 350v and a 19 watt 6l6g:

    (350x350)/19 = 6447.37

    So basically 6.6K

    You can work this backwards too.

    For instance with your 3.3K reflected load with your existing speaker and OT:

    3300 * 19 = 62700
    √62700 = 250.4v
    250v + 15v cathode voltage = 265v B+

    With these tools you should be able to estimate your power section.
     
  10. Jewellworks

    Jewellworks Tele-Meister

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    Ok good. Now I know. Just hope I remember... ;)

    So what about getting a new OT? Like maybe this one?
    https://www.tubesandmore.com/products/transformer-output-8-w-single-ended

    5k impedance on the primary. Which is what the 6L6G data sheet says. Or is it more complicated than that? If I end up with 352 plate volts, and my impedance is 6521 (as noted in an earlier post) ... The transformer wants to see 5k... ? Is this "close enough"?? I'm not understanding the relationship.

    How else do I shop around for an OT? They never give the winding ratio, or ohm readings on the various leads. I don't even know what I'm looking for:confused:

    I've read and re-read this post all day long, and I'm still not seeing how to calculate the estimated voltage after rectification. The power transformer data sheets (that Hammond and Classic tone) show their estimations with 3 different rectifier tubes, but how is it done? I'd like to know what it is with a 5Z4, with any given PT
     
  11. Jewellworks

    Jewellworks Tele-Meister

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  12. Nickfl

    Nickfl Friend of Leo's

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    The transformer doesn't want to see anything. The transformer has no impedance and no requirements of it's own, it just reflects what it sees from the speaker. The square of the winding ratio is the impedance ratio, the speaker impedance multiplied by the ratio is the primary impedance.

    The relationship is between plate voltage, the tubes max plate dissipation rating, and the plate load, as described by the equation in my last post.

    If you use a transformer that is going to give you 5k from your 8ohm speaker, you'd ideally want a B+ closer to 325v.

    That being said, the ideal is just a guideline. 5k would probably be fine at 350v but you'd need to draw a load line to see what the differences would be. Itd expect 5k to likely work and sound similar to 6.6k.

    Look up the datasheet for the 5z4 tube. There is a graph on the datasheet that you can use to estimate voltage from a given power supply.

    FWIW, I've found the classic tone datasheet estimates to be unreliable.
     
  13. Nickfl

    Nickfl Friend of Leo's

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    That hammond is push pull, you're looking for a single ended transformer, right?
     
  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I think you should have a look at
    5881 in a 5f2a
    Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Jul 11, 2020.

    The 5881 a replacement for the lower power 6l6. I know you are contemplating a different preamp so only part of the thread will be similar to your build.
    It can be calculated but there are many variables.
    I just look for figures that are somewhat normal such as the rectifier chart rob robinette lists (credit given to 300guitars.com). A 5y3 drops about 60v and a 5Z4 drops about 20v.

    I know in a 5E3 circuit a PT with 660VCT and a 5y3 is fairly consistent with a B+ of ~370v. I would expect a 5Z4 would deliver a B+ of ~410v.
    The OT datasheets usually do give a ratio such as 5k:3.2ohm You have to do the math for other interesting ratios. I recently bought a 5k:3.2ohm OT for a SE 6V6 build. I intend to use a 8ohm speaker. Doing the math 5k/3.2 = a ratio of 1562.5:1 so 1562.5 x 8 = 12.5k. I am interested in the high primary because I will be using a PT with only a 40mA HT rating. The 12.5k primary will allow the 6V6 to run a cooler bias so the circuit will not over tax the PT. I am not interested in *Center bias* for my build because the PT cannot handle the current draw the circuit would demand.

    You should make sure the power rating is similar to the intended use so a 5W OT would be pretty wimpy for your project.

    One of the reasons to have a look at the 5881 in a 5F2A thread or another recent thread
    Champ OT: 5k or 8k primary? Has it ever been answered?
    Discussion in 'Amp Tech Center' started by gabasa, Sep 15, 2020.
    sometimes an amp gains character using a little higher OT primary resistance.

    Crunching the numbers for a 6v6 the ideal OT primary would end up around 5k. A general consensus for the 5F1/5F2A: an OT with 8K primary sounds better. Everyone has different tastes so the 5K may suite others.

    So I suggest you have a listen to the sound clips of the "5881 in a 5F2A thread". You may decide that amp does or does not have a sound you like. You will have to make the judgment call as to what the preamp is adding to the sound of that amp vs. the preamp you intend to use.

    Now you can crunch numbers and come up with the ideal amp on paper but the ideal *guitar amp* is not going to be that amp. You must consider all of the vintage tube circuits were designed for clean operation, like Hi-Fi clean. Electric guitars don't sound good with Hi-Fi amps. You will have to un-learn what we learned in kindergarten and draw outside the lines. There is a lot of fudge factor and 20% variance for wiggle room.
     
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  15. Jewellworks

    Jewellworks Tele-Meister

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    im going to step away from this for a little bit. i need to think about my options. and clearly, ive got to do some reading to get a better understanding how all the pieces of this puzzle fit together. i cant say enough how much i appreciate @Nickfl, @Lowerleftcoast @andrewRneumann @printer2 , and anyone else i may be forgetting to mention. i apologize if ive been a source of aggravation due to my ignorance.

    im considering changing to a PP design, using 2, 6L6G's (which i have), keeping my OT (right?), adding another tube (6N7) as a cathodyne phase inverter, (which will add a gain stage), OR using an interstitial transformer (which i have) as a PI. (
    ll still need a new PT, correct? the one i have has too much voltage that wont be sucked down with the current draw? -then again, im adding possibly 2 tubes, so... ?
    that would give me a much more "interesting" amp, and certainly more useful than a dedicated 6" in an old radio cabinet. its also appealing because i get to use more of my leftover pieces parts...
    what about the wattage? that OT came from a SE at 10watts... arnt we looking at about 20 watts with the PP?
     
  16. Nickfl

    Nickfl Friend of Leo's

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    Single ended and Push Pull transformers aren't interchangeable, so you would have to get a different output transformer if you want to switch to a PP design. The main difference is that a push pull transformer has to have a center tapped primary, though there are some internal construction differences as well.
     
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  17. printer2

    printer2 Poster Extraordinaire

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    What the heck, may as well give the rest of the goods. The SE OT has a constant magnetizing current at idle, half way between no current (no magnetic lines of flux due to no current) to saturation (maximum current). Because the dc in the windings magnetizing the core the core can get saturated quite easily. To combat this a gap is put in the core. This air gap acts as a sort of magnetic 'resistor' with the iron as a resistor but with a much lower resistance. With a P-P OT the two primary windings have the same current (at idle) going through them but the magnetic flux from each winding opposes each other and they cancel out. Because of this no air gap is needed and the core can be smaller. One other effect of the air gap in the SE core is to reduce the primary inductance, necessitating the greater amount of iron. A simplistic description, OT design can fill a book.

    And Jewel, yeah new people's questions can be aggravating but one day you will be the one answering the questions and we will get to put our feet up, so there is a payback. On changing designs, I am on roughly the 20th iteration for the chassis at my feet, I think I generally have it nailed down now.
     
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  18. Jewellworks

    Jewellworks Tele-Meister

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    Image2.jpg
    am i charting this correct?
    im looking at a 320-0-320 PT . the curved lines represent the AC, correct? weve estimated having a 70ma load. so that means ill have about 380ish volts DC on the plate of the 6L6? is this before or after the filter cap? (im assuming after because its "under load") or is this saying ill have 380 when using a 4mfd (4uf?) filter cap. i was gonna use a 22uf. apart from the filter cap being "too large" (-say, 50uf) does it make a design difference? i want to make sure im looking at this chart correctly.

    so in the case of this 320-0-320 :
    320x320/19=5389 plate load
    this is in ohms, correct?
    -using the @andrewRneumann / merlin approach, "we need to ballpark the voltage that will drive this current through the load resistance, and Merlin suggests just use the peak voltage. The keyword there is “peak”. That is 1.414 times the RMS voltage."
    320x1.414=452
    "you should be able to use Ohm’s Law again to calculate the effective load resistance “Rl”.
    using an online Ohms Law Calculator, i put in 452V and 70ma and got 6457 ohms. almost 1k difference.

    and again, i apologize for my ignorance, but... with these plate load numbers, i want to find an SE OT (at 8ohms to match my speaker) that closely matches? 5k, or 6k?

    ive been reading, and calculating, but i need to get over this hump and buy something. im sure the answer is right in front of me. this is as close to seeing the forest through the trees as ive gotten in 2 weeks.
     
  19. Jewellworks

    Jewellworks Tele-Meister

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    i re-read this, and theres 2 things:
    1) are we talking about using 2 parallel 6V6GTs? or 2 parallel 6L6GCs? in either case, thats not what im shooting for. what i have, and CAN do, is run 2 parallel 6L6G's (one is actually a 6L6GA, but after hunting down a data sheet, theyre identical, except the GA has a smaller bottle. specs are identical)
    2) and i dont have to buy a different OT? i can use the one weve been talking about? with the 6.6k/16ohm, 3.3k/8ohm? remember, ive got a 6", 8ohm speaker. so it would be 6.6k/8ohm in the end.

    if thats the case, i can do that. :D

    BUT, after adding in the 2nd 6L6GA, my total load resistance goes up by another 56.5, for a total of 125.8 ma. -and that is literally off the chart for a 5Z4 (in purple). -am i looking at this right?
    727369-166f15e669bebe8e955b9383bc723ee9.jpg
    the rectified voltage doesnt change, but once it starts drawing more current, the voltage will drop. the Hammond 290AX is 100ma, but is this even safe for the 5Z4? seems not...
    then i looked at a few more rectifier tubes i have on hand, and i came to the 5AU4A -theres not data sheet that i can find for the "A", but theres a "B", and it looks perfect.
    Image4.jpg
    about 360V with current draw for days.
    unless im missing something else? like i need to use a 40uf filter cap like it says?

    im open to buying a different rectifier tube as well. this is just what i have in my junk drawer. i looked at the G734. itll work too, but is capable of much higher voltages and seems like a waste for my lil amp... i dunno... any suggestions?

    im actually looking at a 320-0-320 PT that came from a Hammond AO-20 M3 organ that was running like, 10 tubes. if the 920AX @ 100 wont handle the extra load, im pretty sure this one on ebay will...

    am i close?
     
  20. Jewellworks

    Jewellworks Tele-Meister

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    so just for fun, i decided to look up what the 5Z4 was doing with the original AMPRO circuit it came from. running 2, 6V6GT (PP) and 1 6V6GT (SE?) as well as the 6SJ7, 6J5, (and there was another tube i cant remember) and on the low side of things (not including the tube i cant remember) it had a load of 122.8ma! -of course, the power supply was putting out a much larger voltage (430-0-430) but still.. how did this 5Z4 rectifier not melt? its drawing way more amperage than its rated for, and even the RMS is off the charts.
     
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