Test Fixture for KT-88/6550 Power Tubes

[g1]

Bob P's last name was not Pease. Two different people. Bob Pease passed in 2011, Bob P was still posting here after that.

[Enzo]

Well Pease was still my hero, whether posting here or not.

[trobbins]

I can't see a schematic, but there was a comment about a fuse in the cathode leg. If the fuse blows then that may risk the Vhk voltage limit depending on the fault current path and the reason for the fault. It may be a cautious move to add a parallel resistor or high voltage zener across the fuse to restrict Vhk rise to something allowable.

[nevetslab]

I wish it were true but I believe that Bob P, listed as located in Chicago, is a different person than Bob Pease of National Semiconductor who passed away in 2011. Bob P's last MEF post was on 03-18-2019.

OK, my mistake. I wasn't aware of Bob Pease' passing. Then, there were some comments in his threads that also made me doubt that it was Bob Pease, so thanks for clearing that up for me. Sorry for the mistaken identity.

[nevetslab]

I can't see a schematic, but there was a comment about a fuse in the cathode leg. If the fuse blows then that may risk the Vhk voltage limit depending on the fault current path and the reason for the fault. It may be a cautious move to add a parallel resistor or high voltage zener across the fuse to restrict Vhk rise to something allowable.

The fuse in the cathode circuit came from Orange as a protection and failure indicator. They use a 22k resistor from the cathode thru an LED to ground, which if the fuse opens (between the 1 ohm cathode resistor and ground), it will light up. Orange has that on their AD200 as well as their Rockerverb....I think it was. The Rockerverb can switch between EL34's and 6L6GC, while the AD200 uses KT88's. I hadn't considered that fault/problem. Thanks for bringing that up!

[nevetslab]

I can't see a schematic, but there was a comment about a fuse in the cathode leg. If the fuse blows then that may risk the Vhk voltage limit depending on the fault current path and the reason for the fault. It may be a cautious move to add a parallel resistor or high voltage zener across the fuse to restrict Vhk rise to something allowable.

I've attached the circuit diagram for the Orange AD200 output stage, which shows the LED Fault Indicators if either cathode fuses open. I have a 22k 1W resistor from the cathode to the LED in the circuit I have on the rear panel. Genelax listed the Vhk max rating for the KT88 to be 150V. Does this circuit seem to keep all ok if the cathode fuse opens?

AD200 Main Board Schematic sheet 1.pdf

[g1]

Marshall also used the fuse with indicator LED for power tube cathodes in 900 series. Theirs had 100K in series with LED.

[nevetslab]

Moving forward, having received great assistance from you all in solving the HT Tally LIghts, and cobbled the LV LED Drive circuits in the discarded circuits of the Blackstar PSU PCB assy, I've now isolated the two Power Supply Buss Grounds from System ground, so I can place the HT Fuse Holder between them. Verified all the other grounds were restored by adding a jumper, then connected the two HT Buss Cap's ground and the System Ground to newly installed Solder Turrets.



I also checked the Output Xfmr polarity, so now have the wire colors identified for wiring up the Primary to the Power Tubes. Closing in on being able to wire up the Power Tube Turret Board to the tube sockets, and then tackle the listening to the residual hum thru the Output Xfmr with the three power transformers energized/isolated. I had previously placed mounting holes for the added Heater Xfmr and Screen/Preamp Xfmr (Fender HRD P/T), but don't know if there's field issues with their placement. I don't have a lot of latitude in moving those additional xfmrs. Blackstar pressed in Chassis Fasteners for their Power and Output xfmrs, so I'm pretty well locked in there. I always thought their choice of placing all of the weight at the one end of this long chassis was really a dumb idea. Particularly with providing a centered simple cabinet lift handle. I'll add proper lifting handles when I get to the packaging.

I also have this seventy-degree C Heat Sink radiating constant heat, and the only cooling provisions are vents punched into both ends of the chassis. But, the chassis width and the inside width of the cabinet are so close together that it offers NO ventilation relief. So, more thermal engineering required there as well.

It was a good friend of mines' father who told his sons one day......"if you can't cobble, you ain't worth diddly squat"

[nevetslab]

This morning, having revised the plan for the rear panel HT Plate and Screen Test Points to be scaled down by 1000:1 dividers, I realized I had forgotten to select the high value resistance dropping resistors of the HT Pilot Lamps for correct voltage. I had used 300V rated 4.7M and 1M resistors to feed the MPS A-06 LED driver. So, pulled that apart, and had to use a pair of 1.8M 1W in series in place of the 4.7M, and a pair of 1M 1W in series in place of the 1M resistor, as these 1W CF resistors are rated 500V each. So, put those in series inside shrink tubing, joined with the shortened Cloth-covered source wires and moved on to tackle the 1000:1 Voltage Dividers.



I had previously cut that long 30-turret 10mm wide strip into a pair of 12-turret strips with #6-32 stud-standoff mounts. Just what I needed for stringing a pair of 1M 1W resistors in series plus the 2k shunt resistors. The two series-connected 1M 1W resistors gives me the voltage rating of 1000V together. Measuring the 1M resistors I had, all were only coming up to 965 to 985k range. So, I selected some so when placed in series, I could add 39k 1/2W 1% in series with them to get to 2.0M. My 2k 1% 1/2W resistors were measuring a touch low as well, so when I got thru, I had what let me trim that 39k series resistor with a much higher value, and trimmed the resistor string to 1000:1 to within the LSD numbers reading six digits on my system DMM. Overkill, but fun to do having the instrument that would do it. Had to watch while the trim resistor that I soldered in on both strings cool down to room temp, with the reading slowly getting to that last digit range. I set them up using 100.000VDC source (Kikisui's voltage pot is a 10-turn pot), so you could get very accurate, even though it just has simple analog Voltage and Current meters on the front panel.



So, this chassis seems to be filling up. Getting close to the final wiring, once I remove that PSU PCB assy and mount the two fixed xfmrs, and add the other two power xfmrs, energize the three of them safely and have a listen to what the Output xfmr picks up in way of conducting 60Hz field emissions of the three power xfmrs. Then see what latitude I have in physical placement for minimizing the fields. Uncharted territory for me.

[nevetslab]

Now on the home stretch. I've had the power supply PCB assy lifted out of the chassis, while now in the process of wiring up the AC Mains and Standby switches. I came up with a Molex Series 1625 12-position male/female in-line connector that allowed me to terminate the Rear Panel LED's along with the HT Test Points to one side, then wire up the PCB side for sourcing the LED's, and added another 5-position in-line connector to wire up to the 1000:1 Divider circuit that scales the 700V and 400V HT buss voltages down to 700mV and 400mV ,so I can, for the time being, remove and disconnect the PSU PCB. At some point, that will become difficult.

I next wired up the Load end of the Power Switch to the first two positions of the 6-position Barrier Strip, each position having three 1/4" male Fast-on terminals. I've wired up the three adjacent Fuse Holders to have two of them on the Primaries of the Power xfmrs: One for the Heater & Fan circuit and one that's common to the two HT Power Xfmrs, and the third for the HT Fuse in-line with the bottom of the power supply Grounds, with the other side of that fuse holder attached to Chassis Ground.



Now starting to wire up the Power Tube Turret Board. I'll also have to lift that PSU PCB out of the chassis so I can mount the four transformers to the chassis, and then do the AC Mains Lead dress, along with the lead dress of the Secondary wiring, and get all that tied down. Before I get it all nailed down, I can at least get the Primaries powered up and go thru the physical moves on orientation of the original Blackstar Power And Output xfmr orientation with regards to field pickup, using the un-energized Output Xfmr to pick up the field and listen/watch it thru the secondary on headphones and scope. Then do the same with the Heater Xfmr and the Fender HRD Screen/Preamp power xfmr. I hope at least Blackstar got it right or reasonable. I will be able to swap phase on the primary at the barrier strip. Not sure if phasing of the primaries/secondaries are part of that exercise.



I haven't yet soldered the end of this shielded cable from the Output Impedance Select PCB assy that feeds the 8.2k/100pF feedback network. I have more wiring to do, and need to lift that PCB assy up until I'm thru making connections on the bottom side of that turret board, then I can wire up the back end to the power tube sockets.

onward..........

[nevetslab]

I now have the Power Tube Turret Board wired up to their tube sockets, and drive signal lines soldered in to the Preamp Board, along with Feedback tied in. Tied in the Screen Supply voltage and Ground (above the HT fuse). Now ready to install the HT Plate/Utility Power Xfmr and Output Xfmrs. I'll have to lift the PSU PCB out of the chassis to do that, as next step is routing the Primary and secondary wiring of those two xfmrs, as well as that of the Heater and Screen/Preamp xfmr (Fender HRD xfmr).



I'll tackle that task in the morning. I haven't yet revised the Fuse values on the stock labels, nor revised what the Fuse Functions are. I still have a wiring schematic to do. Have it on different sheets, and still working off of the Hiwatt DR201 schematic, as that's what this is patterned from, with all the test-related refinements. Onward........

[nevetslab]

I've removed the PSU PCB assembly, and had to cut the lead from the HT PS GND Turret Terminal that was soldered to the HT Fuse post. That will now be a male and female Fast-on Terminal so that board can be removed. With the PCB now out of the way, I installed the four transformers, feeding the wiring in thru their respective grommets for the xfmr's wiring. Then, started with the Heater Wiring, and using Tie Wrap Hold-down Block, I started in the middle of the chassis underneath the PSU PCB area, and worked outward to get cables routed. I set up my adjustable 4" Machinist Square to the height of the chassis Standoff's that support the PSU PCB, so I could be sure the wire bundles underneath aren't violating the height limitations.







Heading home to catch the last Dodger game before the All Star Game break next week. Meanwhile, I'll resume this transformer installation wiring tomorrow, and perhaps even get it powered up to see what I have. I still have to see what the residual fields are with three power transformers installed. Just how much latitude I have to move anything.....not much. Onward.

[nevetslab]

Yesterday found me having overslept, and I decided to just stay in bed, catching up with my cat naps thruout the day. That also put off my weekend grocery shopping to this morning, so I got a bit of a late start today.

As I entered the building, it now being full heat upon the valley, I was ever thankful for the fell of air conditioning as I came in. But, assulted with day-old stench of what was no doubt good pot smoked in one of the studios yesterday, but DAMN. Stinks the facility up to high heaven. CenterStaging had invested in air scrubber units....larger than R2D2 in Star Wars, that does eventually get the air restored, but it takes a couple days of it running. So far, that hasn't been set up. Sigh....

I moved the chassis back onto the test bench, as the next step was attempting to mount the PSU PCB assembly back into the chassis with all the transformer wiring in place.

In a word....IT DIDN''T FIT. NO HOW, NO WAY. In spite of having used my machinist square set for PCB bottom height, and all of the harness bundles did comply, it didn't take into account the wiring on the back side of the PCB assy that feeds the HT Transistor Level Sense to drive the HT LED indicators. So, in several stages, I began removing the cable ties from the blocks, eventually pried most of the hold-down blocks off the chassis, and ran without hold-down blocks, to see if I could get the board to fit.

I did finally get it to fit. I was having serious doubts prior to that. The obvious solution would be to increase the standoff height, but.....the power and standby switches were NOT about to allow that to happen. it's now all wired up, apart from the HT Screen supply wires being attached to the discrete diode bridge I installed on Turrets. I'll use male-female 1/4" quick-disconnects for that on wires, so that too can come apart.

Back at it.....more to come.

[nevetslab]

I've been powering up the system. I began with the Heater Xfmr, leaving the HT Fuse out so only the Heater Xfmr would be up. Initially, I found with no load, it was pulling 0.74A/68W @ 120VAC, with 7.66VAC No load. I intstalled four new KT88 power tubes in it, for a 6A load. I expected the current to increase. Instead I now see 0.54A @ 54.4W @ 120VAC, with 7.2VAC output. I added the four preamp tubes (three ECC83's, one ECC81), for a 7.2A load. That changed the to 0.58A/65.2W @ 120VAC, and 7.15VAC on the heater buss. To get this xfmr with the full tube load down to 6.3VAC, I had to drop the AC Mains to 106VAC.

When I first powered up the Heater Xfmr, I think the Hum Balance pot wasn't centered. After centering it, with no load, I only had 0.22A/4.8W @ 120VAC, so it must have been from the Hum Balance not being centered. I had the HT Fuse removed so only the Heater Xfmr is powered.

I shut it down, cooled the tubes off and removed them for the time being.

I then looked to see what the Bias supply looked like. My adjustment range for the two bias controls is -69,8VDC to -46.8VDC. BIAS Status Light is lit, as is the HT Plate Status Light. I began with the HT Screen's HT secondary disconnected, to just look at the HT Plate. With No Load, I have 736VDC on the caps, as well as at the plates of the tube sockets. Thru my Divider, I"m seeing 740mV, so a little off, but close enough. I did set the divider up using 100VDC, where now I have over 700VDC across the resistor string, so there's more heat. Close enough.

The Screen Supply is reading 391.4VDC, and 392.5mV on the 1000:1 divider test point, so close enough for that. When I switch back to S/B, with no load, it takes time for the supply caps to drain off. There are two primary Supply caps for the Screen, then following the 1k/7W decoupling resistor to feed the Preamp Plate Supply, there's another 220uF, a 22k decopling resistor and a 47uF cap, and a 10k decoupling resistor to the input stage plates, where there's a final 220uF cap.

I haven't seen the supply voltages with tubes installed, to see what the potentials are under load, and then when switching from Operate back to Standby.

I did find my LED Circuit for the HT Screen Supply has a problem. Anode is connected to +15V, but for some reason, I'm seeing 23VDC on the top of the 1.5k collector resistor (off of an MPS A06), so gotta pull the PSU PCB to solve that.

So far, no major surprises.

[nevetslab]

While watching the supply voltages come up and drop at vastly different rates switching back to Standby from Operate (only one set of filter caps on the HT Plate Circuit, while several filters on the Screen/Preamp Supply circuits), I realized my first wiring of the second stage HT Screen Filter was placed AFTER the Standby Switch on the Power Tube Turret Board. This is not the case in how the Hiwatt DR201 Bass Amp (from which this amp is patterned from) is wired.

Hiwatt DR201 Bass Amp Schematic-2B.pdf

Having watched and timed the discharge rate with existing wiring, the discharge time to get from 390VDC Screen to near zero volts takes a good three minutes. So, I revised the wiring, moving the Screen buss on the bottom side of the Power Tube Turret Board from that 220uF/450V Filter to a separate Turret adjacent to the feed Turret I was using to connect to the Preamp Turret Board's 1k decoupling cap and it's subsequent filter/decoupling resistor stages, and connected the HT Screen supply buss there off of the Standby Switch. That DOES replicate how the DR201 is wired.

After getting that wiring change made, I watched the discharge timing. It was virtually identical to what I was seeing before. So, for well over a couple minutes, there's falling DC potential on the Screens of the KT88 power tubes while there's NO Plate Voltage present. I seem to recall reading someplace that this can damage the power tubes. I haven't found where I read that. I just posted a separate thread regarding this question.