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Test Fixture for KT-88/6550 Power Tubes

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  • Test Fixture for KT-88/6550 Power Tubes

    I recently pulled a Blackstar S200 stage prop chassis’ out, removed the two xfmrs from each chassis and powered up the Power Xfmr to see if it would be of any use for replacing the bad power xfmr of one of our member's (glebert) Peavey VTB300 amp , which also had four KT88’s.

    Turned out this Blackstar S200 transformer lacks the traditional 6.3VAC high current winding for the heaters in the amp. It’s HT winding is 524VAC C/T, plus it has a 26VAC winding, a 35VAC C/T winding and an 89VAC winding. No idea what the current ratings are on the 26VAC and 89VAC windings.

    I pulled the chassis that had all but the power tube PCB completely apart. Not having any schematics for the S200, and looking at the schematic I had for the HT60, it didn’t offer any significant help, so I didn’t pay a lot of attention to wiring for clues.

    Click image for larger version  Name:	Test Fixture-4.jpg Views:	0 Size:	1.67 MB ID:	960724 Click image for larger version  Name:	Test Fixture-5.jpg Views:	0 Size:	1.53 MB ID:	960726 Click image for larger version  Name:	Test Fixture-6.jpg Views:	0 Size:	1.73 MB ID:	960728

    The S200 PSU PCB assy had the HT supply using stacked 220uF/400V buss caps with 630k ballast resistors across it, then there was an additional 220uF/400V buss cap that had a 100 ohm/10W resistor in series, that had Quick Disconnect terminals that looked like it would connect to the mid-point of the stacked 220uF/400V buss caps to serve for the feed to the Preamp PCB assy’s buss voltages. The S200 has a control that lets you dial the power output from 200W down to 20W. There is a pair of ST W10NK80Z 900V MosFET’s with additional control circuitry that must be that part of the circuit. I assume there’s a tap in there to supply the Screens voltage supply. Not having the documents, and didn’t want to try and back-engineer this PCB to see what it is, I mostly saw this as a source for the basic buss caps for the HT supply, the Bias Supply and it also had bipolar 15VDC supplies along with a 5VDC supply. To be safe, I removed the MosFET's, and a bit of the control circuitry so I just had the basic HT supply terminals to work with. Had to remove more circuits to end up with a -120VDC source voltage for the dual pot bias circuit, off of the 220uF/200V buss cap.

    The power xfmr worked, so I set up the PCB with standoffs to lift it off of the bench enough to have clearance from anything below the board, and saw I could connect the third 220uF/400V buss cap to the mid-point of the Plate supply stack. I was getting 733VDC unloaded at the top of the stack, and 386VDC mid-point, but when I tied the additional 220uF/400V buss cap with it’s series-connected 100 ohm resistor to that point, it dragged the mid-voltage point down to 322VDC. That also placed more than 400V across the upper cap…though this is all unloaded. There were a total of five of these 220uF/400V buss caps, and seeing this mid-point being dragged down with the additional cap added, I removed them all and checked them on my GenRad Digibridge, selecting the two highest values to go in series. That did yield an even balance, where I had more imbalance when I began. Still placing that additional cap across the lower one dragged it down. I was hoping to have around 350VDC or greater for the screens with what I had here. I found date codes on these somewhat physically small 220uF/400V caps as 1430: 2014, 30th week. Size is 22mm dia x 52mm tall, L/S 10mm, so the PCB will take larger dia parts than these if need be.

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    No doubt Blackstar achieved what they needed using their variable HV supply.

    I got the HV by using the BR2512 Bridge Rectifier that was present in the chassis. 25A/1200V Bridge. Negative side of the bridge is grounded. The C/T is presently not used as I have it connected. I have pondered wiring this up using half-wave rectifiers, and connect the 26VAC winding in series with the C/T, grounding the bottom of the 524VAC C/T winding, which would then allow the combined xfmr winding of 288VAC for the lower voltage. Of course, now I'm dealing with 60Hz charging, lower output voltages, and no idea what the current rating of that 26VAC winding is.

    Sure would be handy to have the circuit diagram of this S200 PSU PCB assy. Though there's additional support circuits elsewhere in the S200 system that would no doubt be needed to have all of the clues. Fat chance of getting my hands on those doc's.


    The output xfmrs both measure ok, as far as DCR readings go. I haven’t run signal thru them to verify they work…time enough to do that, and assume they will work.

    So, what to use for the circuit to run this as a test fixture for checking/screening KT88’s and 6550 power tubes? I thought I’d wire it up as a Hiwatt DR201, as there’s enough space in the chassis to do so. I ordered a couple 300mm x 60mm 60-terminal turret boards for the hand-wiring of the circuits, along with ceramic power and preamp tube sockets, the pots for the preamp section, and additional parts that I’d need to complete a working amp, along with adding chassis-mounted test sockets for checking cathode currents, plate voltage, pair of chassis-mounted bias pots, and fused cathode circuits with LED’s as Orange uses on their AD200 amp. I also had to buy a 63VA 6.3VAC C/T Hammond PT166S6 for the heater supply. Thankfully Antique Electronic Supply offers free shipping on orders greater than $100. Wish other suppliers did that.

    I do have the wiring diagram for their 50W/100W Preamp Turret board, and enough detail photos of the DR201 power tube Turret Board & chassis wiring to figure out the rest. I'll just have to see what limitations this Blackstar power xfmr presents. This goal is, of course, just putting a power tube test fixture together out of spare parts, while using something that sounds good. Onward.

    I do see J/J power tubes are beginning to be available. Not much out there yet in standard preamp tubes thus far.

    Attached Files
    Last edited by nevetslab; 05-19-2022, 10:23 PM.
    Logic is an organized way of going wrong with confidence

  • #2
    Originally posted by nevetslab View Post
    Not much out there yet in standard preamp tubes thus far.
    CE sent out an email "flyer" last week, claimed they were now selling PSVANE tubes, all the usual suspects, prices looked a bit high but not totally out of sight when I checked their website. I put a few in my shopping basket and the "sorry out of stock" message did NOT pop up as it has on every inquiry for the last couple months. It's 'any port in a storm' vibe scoring tubes now, kind of like the toilet paper crisis 2 years ago. PSVANE quality, I can't say anything about because I've never encountered them before though CE did carry a few in past years. I know they're mostly aimed at hi fi market.

    This isn't the future I signed up for.

    Comment


    • #3
      Originally posted by Leo_Gnardo View Post

      CE sent out an email "flyer" last week, claimed they were now selling PSVANE tubes, all the usual suspects, prices looked a bit high but not totally out of sight when I checked their website. I put a few in my shopping basket and the "sorry out of stock" message did NOT pop up as it has on every inquiry for the last couple months. It's 'any port in a storm' vibe scoring tubes now, kind of like the toilet paper crisis 2 years ago. PSVANE quality, I can't say anything about because I've never encountered them before though CE did carry a few in past years. I know they're mostly aimed at hi fi market.
      YUP....I did see their email flyer as well. I probably should have put a couple in this purchase I just made yesterday as I had free shipping at that point.
      Logic is an organized way of going wrong with confidence

      Comment


      • #4
        Originally posted by nevetslab View Post

        YUP....I did see their email flyer as well. I probably should have put a couple in this purchase I just made yesterday as I had free shipping at that point.
        CE never gave me free shipping they must like you!
        This isn't the future I signed up for.

        Comment


        • #5
          Originally posted by Leo_Gnardo View Post

          CE never gave me free shipping they must like you!
          When your order is over $100, then it's free shipping.
          Logic is an organized way of going wrong with confidence

          Comment


          • #6
            Originally posted by nevetslab View Post

            When your order is over $100, then it's free shipping.
            Is that a new policy? They always hit me hard, like $35-45 typically. Orders usually 400, 500, sometimes near a grand.

            I am aware those ordering from the same outfit, under Antique Elec. are offered the 100+ free shipping deal. Wonder what their third entity Amplified Parts has for a policy? So much faffing around with a company that's mostly competent but plays these pretend games with names & shipping rate policies, it is annoying.

            Am I going to have to open another account & set up shopping carts in both then compare merch + ship totals? Looks like that will be the choice.

            One time a salesman told me I could set up my own account with Oops or FDX & maybe chip a little off the charge thataway.

            A while back they also proudly announced "we'll ship priority mail" but when I look at the rates for that, definitely not the flat rate per size box like I get at the Post Office.

            Meh, I'm gonna go get lost in a Twin Reverb chassis & get my mind off of this quandary.
            This isn't the future I signed up for.

            Comment


            • #7
              Originally posted by nevetslab View Post

              When your order is over $100, then it's free shipping.
              All my orders have been over $100 - never gotten free shipping. maybe $100 in tubes only?


              cheers,
              Jack Briggs

              sigpic
              www.briggsguitars.com

              forum.briggsguitars.com

              Comment


              • #8
                Originally posted by jack briggs View Post

                All my orders have been over $100 - never gotten free shipping. maybe $100 in tubes only?


                cheers,
                I went to Antique Electronics Supply's website, pulled up their Shipping and Delivery page: Shipping and Delivery | Antique Electronic Supply (tubesandmore.com)

                I was surprised to find it now states domestic US orders over $89 can receive USPS 1st Class shpg for free. I've never noticed it being exclusive as to what was ordered.
                Logic is an organized way of going wrong with confidence

                Comment


                • #9
                  Originally posted by nevetslab View Post

                  I went to Antique Electronics Supply's website, pulled up their Shipping and Delivery page: Shipping and Delivery | Antique Electronic Supply (tubesandmore.com)

                  I was surprised to find it now states domestic US orders over $89 can receive USPS 1st Class shpg for free. I've never noticed it being exclusive as to what was ordered.
                  I did not know that, but I buy from CEDs site. Despite Antique Electronics Supply being a mirror site - their pricing policies are different fro CEDs.

                  cheers,
                  Jack Briggs

                  sigpic
                  www.briggsguitars.com

                  forum.briggsguitars.com

                  Comment


                  • #10
                    Yes, AES is the retail outlet, and CEDist is the wholesale outlet, of the same company.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #11
                      I did a tube hifi repair about 5 years ago and the customer bought along a pair of Psvane 12AX7s that the thought would be helpful and ordered them before asking (the fault was not 12AX7 related). They were in fitted wooden box and at the time I did look up the ridiculous price - I forget what it was, but astronomical. He got them from an "I saw you coming" hifi site. I compared them with some TAD Shuguang tubes I had to hand and they looked identical. I did try them, but to my ears no improvement whatsoever and a waste of money compared to buying regular tube.

                      Comment


                      • #12
                        Having some empty shop time for a short stretch of time, I got busy modifying the Blackstar S200 Chassis to retrofit the Hiwatt DR201 Power Amp circuits into it, with added provisions for externally measuring the power tube cathode currents (via discrete 1 ohm/3W cathode resistors & pin jacks), along with HT Protection Fuses & Failure LED indicators, added recessed rear panel bias pots (with discrete bias voltage test jacks), and added rear panel Indicator lamps to show presence of Power Tube Plate HT, Power Tube Screen/preamp HT, Bias Voltage. Essentially a KT88 Power Tube Test Fixture for matching/testing power tubes, as well as having a ballsy amp, if it works out as I hope it will.

                        I peeled off all of the front panel Escutcheon that was punched for the large cluster of controls, and selected the lower set of holes for DR201 front panel controls (Normal & Brilliant Vol Pots, Bass, Treble, Middle, Presence & Master Volume)....reamed those out for the 3/8" pot bushings, then cut a new T-6061 1/16" Aluminum Panel, clamped it into place and using the freshly opened pot holes as a guide, punched the mating holes thru the Aluminum panel. Then added a set of #4-40 mounting screws to hold the new panel into place. After deburring, c'sking the mtg holes, I painted the new face plate Gloss Black to match the rest of the chassis and mounted that.

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                        The insides of the Blackstar Chassis was loaded with pressed-in standoffs and snap-in standoffs for their front and rear PCB assemblies. I only made use of their rear panel Output Impedance Select Switch and dual Phone Jacks, and sliced that portion of the PCB from the rest of that long board. Most of those tall snap-in standoffs had to be knocked loose with a hammer, then finally driven out of the front and rear panels. The cluster of standoffs on the underside of the chassis top were easier to remove...gripped them with my vintage 8" Gas Pliers and snapped them out after a little side-to-side loosening. Had some minor metal flattening work to do to restore the flatness of the panels.

                        Then, having ordered ceramic octal sockets, I lucked out having their mtg centers match that of the chassis pwr tube sockets. The Preamp Tube Socket openings required mtg holes. The sockets I ordered had provisions for the tube shield, but the mechanics of the frame that cradled the ceramic 9-pin socket was poorly formed. After grinding the mounting hole slots into the chassis holes and mounting the first pair of tube sockets, the third one had the ceramic socket slip out of the mounting cradle, so I had to pull them off, size up the fault and corrected that, now properly swagged into place and re-mounted them.

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                        I now have the wiring pattern for the Preamp Turret Board, so I'm about to start the inter-connect wiring on that. Still have the Power Tube Turret Board to work out. I haven't yet mounted the Heater Wiring board that will be in that gap between the PSU PCB assy and the Power tube Turret Board (between the tube sockets). That will be the first chassis wiring that gets laid in.

                        Not having the schematics/service documents of the Blackstar Power Tube Adjustable Power System that's on that PSU board, I'll have to cobble the Screen Supply/Preamp/Driver HT circuits using a Fender Hot Rod Deluxe P/T that I may have to load down for 50mA to get the HT voltage right. Sure would be easier using that Blackstar circuit. I'm using a conventional 6.3VAC Heater Xfmr, as Blackstar uses DC supplies and series-connected tubes to run their heaters.

                        So, this is the work-in-progress as it presently stands.

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                        The knurled push-on control knobs have a plastic insert set for 6mm D-shaft. I loaded a similar-sized blunt-end cutting burr into my Foredom #28 handpiece to see if I could grind that D-shaft detail out. That worked. Then, using a 0.250" dia reamer with a slight beveled end in the cutting tool, I loaded it into one of my Tap Wrenches and manually spun it in to see if could open it up for 1/4" shaft. The thought was to then drill & tap #4-40 for a set screw. As it turned out, the fit is just right to push the knob directly onto the 1/4" shaft and it's friction-fit. I had to do this process on a number of knobs, as some didn't fit right or had excessive non-concentric 'hop' as you rotated it. Just a lucky break, not yet knowing what I was going to use for control knobs!

                        Onward.........
                        Attached Files
                        Last edited by nevetslab; 06-16-2022, 11:07 PM.
                        Logic is an organized way of going wrong with confidence

                        Comment


                        • #13
                          The six-position Barrier Strip arrived this morning, along with the Keystone dual vertical 1/4" male Quick-Disconnect terminals and single vertical version of that. As luck would have it, you can't use them out of the bag and install them onto the barrier strip without first flattening the mtg surface, which protruded beyond flat, causing the terminals to pivot. And the mtg hole of it is too tight for #6, so I had to first flatten the mtg surface with a large pin punch and steel plate, then grip the part with my vintage Gas Pliers while reaming out the hole a touch, then debur the hole so I now had flat proper sizing. THEN, installing the barrier strip screw thru the dual and single vertical Quick-disconnect terminals, I got the strip set up for 'plug-and-play' wiring on the inside AC Mains area.

                          Click image for larger version  Name:	Test Fixture Chassis-32.jpg Views:	0 Size:	2.06 MB ID:	962471

                          I was fighting for space, and to get that barrier strip to fit between the end fuse holder and my recessed bias pot mounting plate, I had to file a relief into the aluminum plate to get the barrier into place. I was also able to share the mounting hole of the lower recessed plate with that of the barrier strip. I didn't have any pem inserts which would have made life simple with that barrier strip. I did find I had a very tiny hex washer nut...#6-32, but, it was a locking nut...NOT a fiber insert, but something else...tiny dimples pressed in at two locations. The overall size was such that if I could re-thread the part, and enlarge the mounting hole in the Bakelite body of the barrier strip, I could press the part in without cracking the Bakelite. Managed to do that. What a PITA, though.

                          I haven't yet drawn up the AC Mains Wiring Diagram to feed the three xfmr primaries, properly fused and with inrush Thermistors in place for the two HT xfmrs. The Main HT xfmr already has that part mounted on the PSU PCB, while I have the 10 ohm/5A Thermistor inbound for the Fender P/T.

                          I swapped locations of the Heater and Fender P/T's.

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                          While I haven't yet phased the Output Xfmr Primaries to the Secondaries, now having three Power Xfmrs on this chassis makes me suspect I'll have to have them phased as well. Never have had this sort of clap-trap wired up before, but would suspect the random parallel-wiring of three 120V primaries would probably have a proper phasing to yield minimum stray hum fields.

                          Any thoughts on that subject?
                          Attached Files
                          Last edited by nevetslab; 06-18-2022, 12:07 AM.
                          Logic is an organized way of going wrong with confidence

                          Comment


                          • #14
                            You probably know this, but for everyone's benefit: When arranging the transformers, do you know about the "headphone trick" ? Just means, tape off all PT secondaries, apply mains AC to PT primaries, hook OT secondaries to a set of headphones (or a scope), and adjust the relative positions of the transformers while monitoring the AC (hum) at the OT output. Any hum heard at this stage will also be heard through the speaker(s) as soon as you apply power to your amp, so get it as quiet as possible.
                            --
                            I build and repair guitar amps
                            http://amps.monkeymatic.com

                            Comment


                            • #15
                              The original two xfmrs have already been set by Blackstar, they having Pem Nuts to pick up the mtg screws. I've known about the relative orientation of the cores between Power and Output needing to be 90 deg, but I've never gone thru this procedure. I should have done so before drilling the mtg holes for the Fender P/T and the Hammond Heater Xfmr, though I have their cores in that same orientation relative to the O/T. I'll have a listen to this to see what I can do with these added xfmrs to the mix. Many thanks for the tip! I can also see if there's any change in primary phase of the additional two power xfmrs.

                              I assume in this Headphone Trick test, the O/T is being used to sense excess hum field from power xfmrs conducting eddy currents into the steel chassis. Presently, the O/T primaries are terminated in insulated 1/4" quick disconnect. Coil them up? Leave them laying about? I haven't yet tried this, as after I took the last photos, I removed the transformers so I could resume the chassis wiring prep work.
                              Last edited by nevetslab; 06-19-2022, 01:13 AM.
                              Logic is an organized way of going wrong with confidence

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