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  • #61
    hi guys,
    it's going to get me a couple of days to get a hold of a few 5W resistors to play with (220 to 270 Ohm) [always takes time to try to consolidate my MOUSER shopping cart]
    in the meantime I think I am going to try to test the amp providing the -50 VDC with an external power source and see how it sounds now that I should be providing something in the vicinity of 400 V (provided I have not blown the 7189A's when when I plugged the amp to measure voltage after installing the bridge rectifier)
    I will come back to you with the results
    thx,

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    • #62
      get me a couple of days to get a hold of a few 5W resistors to play with (220 to 270 Ohm)
      From post #56:

      As your plate voltage is higher than 375V you will need to reduce cathode current by using a higher value cathode resistor than 220R to stay below 100% dissipation.
      - Own Opinions Only -

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      • #63
        Originally posted by Helmholtz View Post
        From post #56:
        yes, thank you for the reminder. you mentioned 220-270R in post #50 that's why I had in mind to buy a bunch of resistors covering that range. Understanding that 220R will not work because the PLATE VOLTAGE is higher (400V instead of 375V) I should get a bunch of resistors that are higher than 220R.

        I did a search in MOUSER for 5W, 1% tolerance resistors and got the following values (all wire wound): 225, 240, 250, 270, 280, 300 Ohm (about $15 in total)

        I was trying to find something like a multi turn cermet trimmer but found nothing rated higher than 1W as using a trimmer would be much more practical to determine the right value.

        Any further advice or agreement with the list of resistors to buy?

        thx!

        Comment


        • #64
          470 is always a good value to have on hand, used in many applications. 330 also a standard value.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


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          • #65
            Originally posted by g1 View Post
            470 is always a good value to have on hand, used in many applications. 330 also a standard value.
            thank you, sir
            you mean... to add to the list or shift the resistor selection more towards those values? say: 270, 280, 300, 330,...,470
            maybe we already know the R values below 250 will surely not work
            thx!

            Comment


            • #66
              I say it a lot and to everyone... It is just a guitar amp, not some precision lab equipment. Step back a bit from ten-turn bias pots and 1% resistors. Your mains voltage may be nominally 120v but it generally varies several volts either way and when it does, your B+ varies 3 to 4 times the voltage. MAins goes up 5v and your B+ can soar 15-20v. And your tube circuits won't dutifully scale themselves to every change. These days, 1% resistors are not so expensive as they once were. But I myself wouldn't spend extra for them. I mean if you are a 70% guy, would you hear a difference between 70% and either 69% or 71%?
              Education is what you're left with after you have forgotten what you have learned.

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              • #67
                Originally posted by Enzo View Post
                I say it a lot and to everyone... It is just a guitar amp, not some precision lab equipment
                I get your point Enzo, what I was thinking is that if I’m trying 270, 280, etc resistors there’s less than a 5% difference in between them. Thus the 1% would have ensured there was actually a difference between 2 different value resistors.
                If you guys say: ”just buy 250,270,300,330 5% and see what gets you closer to the power dissipation target” I’ll follow.
                that’s what I’m shooting for
                thx!!
                Last edited by TelRay; 02-10-2020, 01:23 PM.

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                • #68
                  I generally stick with standard values for stocking, in this case that would be 270,330,390,470.
                  I would try a couple of those, and if I felt I needed something in the middle, I would then order it (or use combination).
                  But resistors are generally cheap, and you only want to pay shipping once, so I understand wanting to cover all the bases. If you are just getting cheap generic sandbox style resistors, you can get one of each of many values. If they are more expensive type, I would try to calculate what you want first. You have a 220 in there now? Do you have something similar you can put in series with it? Or a couple 1K's to try in parallel? (you will need to do some math to make sure the wattage is adequate, but it's good exercise )

                  But like Enzo implies 'close enough for rock 'n roll'. (Or maybe no where near for jazzers )
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #69
                    You have a 220 in there now?
                    The original amp works with hybrid bias (schematic see post#1): 100R cathode resistor(s) and additional -17V grid bias. Problem is that the replacement PT doesn't provide a suitable separate bias winding. So I suggested to go full cathode bias. Though the resulting B+ with the new PT isn't quite clear yet, I expect that something like 250R will give suitable biasing below 100% dissipation.
                    - Own Opinions Only -

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                    • #70
                      Thus the 1% would have ensured there was actually a difference between 2 different value resistors.
                      I get that. But the other side of that coin is your 1% resistor to fit between the 5% values is only a 1% difference. WHen guys want to experiment with things like cathode bypass caps, my rule of thumb is to use at least a factor of 2 making a difference. In other words, we don't compare 20uf against 21uf. We compare 20uf with 10uf. Imagine turning one of your volume or tone controls 1%.

                      Put differently, if we are chasing down tube current. I think we want to move in 5ma chucks rather than 0.05ma chunks.
                      Education is what you're left with after you have forgotten what you have learned.

                      Comment


                      • #71
                        Originally posted by Helmholtz View Post
                        The original amp works with hybrid bias (schematic see post#1): 100R cathode resistor(s)
                        correct! and now I am going to change to full cathode bias and try the following R values: 250, 270, 330, 390, 470

                        Originally posted by Helmholtz View Post
                        Though the resulting B+ with the new PT isn't quite clear yet, I expect that something like 250R will give suitable biasing below 100% dissipation.
                        with the power supply as is (with the new bridge rectifier) and isolated from the rest of the amp (I cannot have it connected as there will be a lot of current going through the power tubes without the negative bias voltage) I am getting about 338 VDC



                        what do you guys think, is there still hope to use this power supply?

                        Comment


                        • #72
                          I am getting about 338 VDC
                          Now this seems very low. Is this with a reservoir ecap connected??

                          I don't think it is possible to produce a negative voltage from a HT tap when using a bridge rectifier, so your proposal won't work.

                          I just saw that there is a BJT stage that requires a -32V supply. So while full cathode bias would provide a solution for the power stage, I think you need a different PT that allows for the -50V supply.
                          - Own Opinions Only -

                          Comment


                          • #73
                            Originally posted by Helmholtz View Post
                            Now this seems very low. Is this with a reservoir ecap connected??
                            no, the ecap is not connected yet as if I connect the 12-pin connector i would be engaging the power tubes as well. so this is purely what's coming out of the diodes without being smoothed by the ecap. this will add a bit of voltage, right?

                            Originally posted by Helmholtz View Post
                            I don't think it is possible to produce a negative voltage from a HT tap when using a bridge rectifier, so your proposal won't work.
                            no, nooo... I wasn't suggested I could get negative voltage there, -50 V it's just the nominal according to the schematic. I am actually getting 0 V

                            Originally posted by Helmholtz View Post
                            I just saw that there is a BJT stage that requires a -32V supply. So while full cathode bias would provide a solution for the power stage, I think you need a different PT that allows for the -50V supply.
                            then I am back to looking for a new Power Transformer?

                            if that is the case, the HAMMOND 370BX seems to be the closest (601.4 VAC) being able to provide the 400 VDC with the original FULL WAVE RECTIFIER

                            https://www.hammfg.com/files/parts/pdf/370BX.pdf

                            however and for the BIAS VOLTAGE: all the Hammond transformers I've seen, they all deliver 50 VAC and I am failing to see how 50 VAC are going to become -50 VDC

                            https://www.hammfg.com/electronics/t...rs/classic/300

                            Comment


                            • #74
                              this will add a bit of voltage, right?
                              Yes, considerably. My x1.4 formula only applies with smoothing cap and without load. Without smoothing cap many meters won't give a meaningful DC reading as the voltage is varying heavily.

                              then I am back to looking for a new Power Transformer?
                              I am afraid, yes.

                              however and for the BIAS VOLTAGE: all the Hammond transformers I've seen, they all deliver 50 VAC and I am failing to see how 50 VAC are going to become -50 VDC
                              Too high AC is no problem as DC level can be easily reduced by a voltage divider or (preferably) a 50V zener + series resistor.
                              Last edited by Helmholtz; 02-11-2020, 10:07 PM.
                              - Own Opinions Only -

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                              • #75
                                thank you, sir

                                if you guys can take a look at the Hammond 370BX and agree with buying that one I will order it (because they range from 75 mA to 575 mA for HV and from 2.5A to 9A with the 6.3VAC)
                                Of course I am open to other transformer suggestions.

                                thx!

                                PS: it doesn't matter I am full circle back to getting a new PT. I've learned a few things I did not know about the rectification process I am thankful for
                                Last edited by TelRay; 02-11-2020, 10:22 PM.

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