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Peavey Encore 65 Misc Parts Question

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  • #31
    And a shout out to Enzo who did mention "DC Resistance of the winding" in #19. I didn't connect the dots on that. So thank you.

    Last topic on this amp, I wanted to post some notes on the Bias Control for this amp. I guess we refer to this as the non-adjustable fixed bias circuit? Note: I am using JJ 6L6GC 30 Watt tubes.

    I made some notes in the picture below. As you can see, the 6L6 tubes are running "cool." I don't see that altering R67 or R58 buy me much. The problem must be that the transformer is loaded such that we can't get the desired -57 vdc at the junction of CR12 and R67. So as we lose voltage across the divider and 220K Control Grid resistor(s), I can only get -52.2v at the Grids. I don't know if the difference of 3 volts would make a substantial difference, in raising the plate current, dissipation, or overall sound of the amp.

    I have seen other threads where it was suggested a redesign of this circuitry (some type of full wave negative rectifier) could get you there. After reading your comments on my other thread (Pre Planning For Modifying A Fender Super Reverb Amp), it is clear that "current" in the secondary winding must also be considered. So I don't think this is an option.

    Unless someone has a rebuttle, I think this is as good as it gets.

    Reference - Rob Robinett Tube Bias Calculator
    Attached Files

    Comment


    • #32
      As you can see, the 6L6 tubes are running "cool." I don't see that altering R67 or R58 buy me much. The problem must be that the transformer is loaded such that we can't get the desired -57 vdc at the junction of CR12 and R67. So as we lose voltage across the divider and 220K Control Grid resistor(s), I can only get -52.2v at the Grids. I don't know if the difference of 3 volts would make a substantial difference, in raising the plate current, dissipation, or overall sound of the amp.
      If you want hotter bias (higher plate current) you need less (negative) bias volts. That's easy to achieve by adjusting resistors. Just increase R67 value.

      BTW, the voltage drop across R56/R57 is most probably caused by your meter (indicating 10M meter resistance) and not real as grids don't draw current.
      Last edited by Helmholtz; 03-29-2020, 09:54 PM.
      - Own Opinions Only -

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      • #33
        Originally posted by Helmholtz View Post
        There seems to be a misconception. A DCR difference at the plate of a few k won't change tube current. As the internal plate resistance of the tube is much higher than the plate load DCR, it's the tube that dictates the current (acting as a constant current source).
        I was thinking more about the possibility of a partial short in the transformer, or a leaky parallel cap.
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

        Comment


        • #34
          I was thinking more about the possibility of a partial short in the transformer, or a leaky parallel cap.
          Both effects would reduce or even kill reverb drive but neither of them could noticeably change tube current as the tube acts as a constant current source with an equivalent internal plate source DCR above 40k.
          Last edited by Helmholtz; 03-29-2020, 10:22 PM.
          - Own Opinions Only -

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          • #35
            As was said, -57 on the grids will be even cooler. And so what is wrong with being a bit cool anyway?
            Education is what you're left with after you have forgotten what you have learned.

            Comment


            • #36
              I apparently wrote my notes backwards! So it should go like this...

              The more negative the Control Grid voltage, the lower the plate current.
              If you make the CG voltage less negative, that will increase the plate current.

              So there are a couple ways to do this. As Helmholtz suggested, I could increase R67. The other option is to lower R58. Perhaps if time and energy allow, this would be a good candidate to add a variable bias pot into the circuit. I have seen examples of how to do that. R67 stays the same. R58 is replaced with a fixed resistor and pot. But for now, a good experiment would be to tack a resistor in parallel with R58 to see if I hear a difference.

              Enzo.... What is wrong with running cool? I guess nothing. That is why I asked the question wondering if the change in current (operating point) would have change overall sound of the amp. So this would be an experiment so to speak. And since I do not rely on an amplifier operating like a Shredding Machine, running cool is probably OK !!

              Comment


              • #37
                So there are a couple ways to do this. As Helmholtz suggested, I could increase R67. The other option is to lower R58. Perhaps if time and energy allow, this would be a good candidate to add a variable bias pot into the circuit. I have seen examples of how to do that. R67 stays the same. R58 is replaced with a fixed resistor and pot. But for now, a good experiment would be to tack a resistor in parallel with R58 to see if I hear a difference.
                I recommend to increase R67 because this actually improves bias supply filtering while lowering R58 has the opposite effect.
                - Own Opinions Only -

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                • #38
                  You increase R67 because it works. Decreasing R58 won't. R67 is only 470 ohms. The bias voltage is the result of the voltage divider the two resistors form. R58 is 47k. If I knock R58 all the way down to 4.7k - 1/10 its present value - it only drops the bias 5 volts. In other words with 470 ohms you have no leverage for the divider. Plus the divider draws more current from the supply. I can raise R67 by a factor of 10 for the same 5 volts, but it is easier on the system and if you make that resistor variable you get a better range control.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment


                  • #39
                    Thanks Enzo...

                    Earlier, I found an old thread from 2010 titled "Not Enough Bias Voltage?" In that thread, you offered advice on the Bias circuit for the Peavey Butcher amp. The bias circuit in that amp is identical to that of the Encore 65; even though the schematic looks backwards!! So I will try out your suggestion.

                    Comment


                    • #40
                      MEF Members (and future visitors):

                      I want to close the loop on a couple items discussed in this thread.

                      First, the bias of the 6L6s. I rigged up a temp circuit as suggested by Enzo. It worked, yeah! In the end, at the moderate volume level that I play at, I really couldn't hear any difference between a 50% or 70% bias point. But there certainly was a difference when I yanked the Scorpion in favor of a Jensen C12K 100W (4 ohm) speaker. So for now, the original bias circuit will remain, the Jensen too!

                      Next, the Opto-Isolators (LDRs). Now keep in mind, the ones in the amp were working. And I guess it is rare for these things to blow. But my mission was to determine if a replacement is available should Peavey discontinue carrying the parts.

                      I ordered one of each from Peavey: the 40101 (21L265) and the 40102 (21L628). Trying to find datasheets for these parts was impossible. But I had a hunch that I knew what the replacement parts are. So I rigged up a circuit to test the theory.

                      While waiting for Peavey parts, I ordered a VTL53C/2 from eBay to see if it came close to the 40101. I dropped that into my test circuit. I changed the voltages and R1 of the test circuit to force 40ma through the LED (as per the specs- see photo). I determined the resistance across the outer legs of the LDR to be ~ 1500 ohms. Is that close enough to the 2000 ohms as shown in the datasheet? I found my first substitute. A few days later, Peavey sent me the replacement part for the 30240101. Lo and behold, it was one in the same, a VTL53C/2. So my theory and testing proved it. Mystery part #1 solved!

                      As for the 40102 (the Red Dot), I found specs for the VTL5C2/2. That is what I thought the part could be. The Peavey part arrived and it had the original 21L628 number on it. So I changed up my test circuit (as shown) to push 40ma through the LED (as per the spec for the VTL5C2/2. Per the voltage measurements, I calculated the resistance across the outer legs of the LDR to be ~ 500 ohm. Is that close enough to the VTL5C2/2 spec of 700 ohms? Unfortunately, this part is getting harder to find.

                      Unless anyone can weigh in to the contrary, I think the LDR mystery might be solved.

                      Look for these parts just in case Peavey ever runs out.

                      70240102, 21L628 -> VTL5C2/2 (Edited)
                      30240101, 21L265 -> VTL5C3/2
                      Attached Files
                      Last edited by TomCarlos; 04-05-2020, 02:35 AM.

                      Comment


                      • #41
                        ...
                        Last edited by g1; 04-05-2020, 02:42 AM.
                        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                        Comment


                        • #42
                          Thank you G1 - I went back into that post and fixed my errors.

                          Comment


                          • #43
                            Originally posted by TomCarlos View Post
                            Next, the Opto-Isolators (LDRs). Now keep in mind, the ones in the amp were working. And I guess it is rare for these things to blow. But my mission was to determine if a replacement is available should Peavey discontinue carrying the parts.

                            I ordered one of each from Peavey: the 40101 (21L265) and the 40102 (21L628). Trying to find datasheets for these parts was impossible. But I had a hunch that I knew what the replacement parts are. So I rigged up a circuit to test the theory.

                            While waiting for Peavey parts, I ordered a VTL53C/2 from eBay to see if it came close to the 40101. I dropped that into my test circuit. I changed the voltages and R1 of the test circuit to force 40ma through the LED (as per the specs- see photo). I determined the resistance across the outer legs of the LDR to be ~ 1500 ohms. Is that close enough to the 2000 ohms as shown in the datasheet? I found my first substitute. A few days later, Peavey sent me the replacement part for the 30240101. Lo and behold, it was one in the same, a VTL53C/2. So my theory and testing proved it. Mystery part #1 solved!

                            As for the 40102 (the Red Dot), I found specs for the VTL5C2/2. That is what I thought the part could be. The Peavey part arrived and it had the original 21L628 number on it. So I changed up my test circuit (as shown) to push 40ma through the LED (as per the spec for the VTL5C2/2. Per the voltage measurements, I calculated the resistance across the outer legs of the LDR to be ~ 500 ohm. Is that close enough to the VTL5C2/2 spec of 700 ohms? Unfortunately, this part is getting harder to find.

                            Unless anyone can weigh in to the contrary, I think the LDR mystery might be solved.

                            Look for these parts just in case Peavey ever runs out.

                            70240102, 21L628 -> VTL5C2/2 (Edited)
                            30240101, 21L265 -> VTL5C3/2
                            Holy coincidence Batman!!!!

                            A couple years ago I became the lucky inheritor of a much used, abused, rode hard & put away wet, Rockmaster amp. Which is basically the 120w head version of the Encore (& w/o reverb). I managed to get it up & running again. Other than the case being about completely shot & butt-ugly, it's only real issues were a blown fuse, a couple bad solder points, & in need of fresh power supply caps. & since I'm one of those "path of least resistance" types, I just fitted the chassis with fabbed-up rack mount ears.

                            There was one item that I didn't fix. The pump circuit switching. & like you I set about researching the LDRs to source 'em, & came up with about the same exact info you did. Or should I say lack thereof.

                            I had run across a few old posts (in fact, by Enzo I believe if memory serves) about calling Peavey. At the time even those posts were over half a decade old, & since we're talking about an amp with a born-on date in the early 80s, I just figured no way they're still available. So I never bothered.

                            So, as I was sitting having my morning coffee, for whatever reason the mood struck to see if I could find out anything on replacements for 'em that was new or that I missed the first half dozen times around. Imagine my surprise when Google sent me here.

                            Guess I actually DO need to call Peavey. I can't believe they still have those things available.... in 2020.

                            As a way of saying "thanks for the info" & maybe help you out a bit if possible.... here's what I know about 'em. (some of which may or may not be accurate... I'm by no means an LDR expert)

                            From what I've been able to find, I'd say you're spot-on with your assessment. It seems in the world of LDRs "in the ballpark" would be considered a "tight tolerance". "In the same zip code" is considered close enough. So yea, 1.5k would be close enough to 2k. Ditto 700 & 500. Especially since, as far as Vactrol is concerned, there's only one other option, the 5C4/2, & that one's min on resistance is only 150 ohms. So, I think you've called it right.

                            What I've heard, the problem of getting 2 single LDRs to "match up" with anything that vaguely resembled precision was the reason behind the creation of the duals. Peavey using them in this circuit may have been more of a convenience thing than a necessity. My own idea, push come to shove & if i could figure out/guess the value of the dead one, was simply to use 2 singles in place of the dual. It's not like this is a stereo compressor where each channel absolutely has to exactly mirror the action of the other to the microsecond.

                            As for measuring the "dark" resistance. If you refer to the footnotes of the description sheet as referenced back from the data sheet, you'll find that dark resistance listed is as measured 10 seconds after turn off. Which is why you can't simply just measure that cold. Apparently it keeps climbing past that after it's off.

                            (footnote 3 on the last page of description sheet) "... ultimate resistance is many times greater than the value at 10 seconds."

                            One last tip I've got... If you do ever have to end up replacing one, make sure you heatsink 'em as you're soldering 'em in. Apparently, these things really don't take to being heated up much very well.

                            Anyway, thanks again for the posts on it. I'll have to give Peavey a call this week.
                            DISCLAIMER: The above "Redneck Engineering" views as expressed may differ from those of someone with actual engineering/electronics experience/education.

                            Comment


                            • #44
                              Just a couple more things I forgot to mention above....

                              In the reverb circuit of the Encore a single should definitely work as a straight-up substitute there. The only reason for the use of a dual used that way is likely because it was a part already in inventory, and it would work. So, no need to source out another unique part.

                              Also, if you do any searching for info on other LDRs (like I was doing when thinking of using 2 singles as replacements), it can get somewhat confusing. Largely in the case of the 5C4, 5C4/2, & 5C10. Depending on where you look (including Vactrol's own datasheets) you can find those listed as having a dark resistance of 400Meg or 400 ohms. Both of which, as it turns out, are wrong.

                              The actual dark resistance of the above is 400K.

                              Also, while most have a forward voltage drop of 2v, for some it's 2.8v.

                              I've attached the "most accurate" spec sheet I was able to find. It's from an old PerkinElmer catalog. They at least managed to get most of the specs listed correctly.

                              I say "most" because even they have one error. The last column of the specs chart where it reads "Turn-Off (Decay) to 100 ohms". Should read "to 100K ohms".

                              But, at least everything else is correct.
                              Attached Files
                              DISCLAIMER: The above "Redneck Engineering" views as expressed may differ from those of someone with actual engineering/electronics experience/education.

                              Comment


                              • #45
                                In the reverb circuit of the Encore a single should definitely work as a straight-up substitute there. The only reason for the use of a dual used that way is likely because it was a part already in inventory, and it would work.
                                I don't follow. I see a single in the reverb return circuit. Turns the signal off and on into the summation stage V3b.

                                The doubles both seem to be in the PUMP circuit.
                                Education is what you're left with after you have forgotten what you have learned.

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