Test Fixture for KT-88/6550 Power Tubes

[catalin gramada]

The egg tryin' to tech the chicken, but still: You may try also a flux band over PT. Is very effective and cost almost nothing- just a copper foil over the coil and lamination . The PT is mostly a source of noise in any wrong design. Sometimes phisical decoupling of PT from chassis with a thick isolator layer and isolated screws also help.

[catalin gramada]

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.

Mouser shipped for free in Europe any order over 50 E. FedEx carrier, it takes 2 (two) days from Mansfield TX to my door in Belgium. 100% satisfied.

[nevetslab]

The egg tryin' to tech the chicken, but still: You may try also a flux band over PT. Is very effective and cost almost nothing- just a copper foil over the coil and lamination . The PT is mostly a source of noise in any wrong design. Sometimes phisical decoupling of PT from chassis with a thick isolator layer and isolated screws also help.

I'll tackle that isolation problem if/when it comes up. Not all power xfmr's employ a copper Flux Band. The Power Xfmr in this Blackstar S200 is certainly one of them. It's present on the Fender Hot Rod Deluxe xfmr that I've used, though none on the Hammond 63VA Heater Xfmr. I don't see it used in the Ampeg SVT-VR nor SVT-CL, but IS used in their SVT4-Pro. And so on, and so on.

[nevetslab]

Finally onto the Fabrication of the Preamp/Driver Turret Board wiring. I wanted to do the circuit pattern wiring on the back side of this board, though puzzling me is the swagged-in Turrets, with their 0.074" dia Thru-holes. I couldn't see any practical solution without having fitted solder pegs press-fitted & soldered into the swagged-in Turret holes.



I got my wire size chart out, and saw 13AWG would be the closest size to fit that ID of the Turret. But, fat chance coming up with solid tinned 13AWG Buss Wire. I looked at what 14AWG Tinned Buss wire would cost. About $219/100ft spool. Ain't going that way. I cut off a chunk of the 12AWG THHN insulated copper electrical wire we use in our wiring installations inside the walls, cut the insulation off chopped off a small piece, then with an X-Acto knife blade, chipped away at the end of the test wire, and saw the effective 13AWG wire would be the solution. But, no way I'm going thru THAT!

I drove down to my local DIY Center Hardware Store to purchase about 10 feet of THHN 14AWG Insulated wire, and say hi to the two Store Cats Cinnamon and Daisy, often seen sauntering about the store. I came back with the wire that I already knew would just drop on thru the Turrets, not having a way to stay put. I moved my 8" square 1/2" Steel Plate to the bench so I had a suitable Pounding Surface to work on. With the insulation cut off of the wire, I tried pounding on the end of the wires to flatten it a bit, then found I'd have to rotate it to pound that into a rectangular shape. While that did work, that wasn't any way to get the job done. I then looked thru my assortment of hand tools, trying some large crimp tools and lead wire dimpling tools, but none of those were suitable for heavy buss wire.

I then pulled out my vintage Plierench that I inherited from my great uncle Heini Piffermann back in the early 70's. This could apply plenty of torque in pressing wire like this into an alternate shape. I looked at the various ways of placing this heavy round wire into it's jaws, and tried one location near the top. Pressed the handles closed firmly, feeling some give from the copper reshaping. I removed the wire, and now found a dimpled shape to the round wire. Maybe a solution. I played with this a bit, and found the right pressure and placement of the wire, to create the pattern to enlarge the shape of the wire to now press-fit into the 0.074" ID of the Turret. Soldered the first one in, getting nice flow of solder around the wire. Cut the end off, leaving around a 1/4" of stubble, enough for a wiring turret to work with. Well, I'll be Buggered. Uncle Heini comes thru with a solution here.





So, I counted 37 locations on the back side of this Turret Board that would need these dimpled 14AWG Copper Wire terminals pressed & soldered into place, so I could wrap my hook-up insulated wires around to form the wiring pattern on the bottom side of the circuit board, and have a place to route the connecting wires to/from their destination.....the front panel controls, the input jack, the four vacuum tubes and the power supply wires/filter cap wires and such. So, a working solution has been found for this critical step.

The pattern inside the Plierench:

[mhuss]

The way Hylight did it was to use thinner 22ga wire, stripped to about 3/4" bare wire, which came up from the bottom and folded down over the turret. The components were laid into the turret slots and soldered, not inserted into the turrets. This would be difficult with non-slotted turrets...

[nevetslab]

The way Hylight did it was to use thinner 22ga wire, stripped to about 3/4" bare wire, which came up from the bottom and folded down over the turret. The components were laid into the turret slots and soldered, not inserted into the turrets. This would be difficult with non-slotted turrets...

I've never seen the underside of the Hiwatt Turret Boards to see how they've wired them up. Today I wired up the back side, adding all of the jumpers between posts, then ran the Ground buss,using 20AWG buss wire with Teflon Sleeving. Then added the two HT busses, using 600V 22AWG Cloth Wiring, which was combersome, but got it done. Then populated the top side, using the upper portion of the Turrets, leaving the top hole open for wiring, as well as the bottom of the Turrets that didn't get the solder pegs. I'd rather of had a narrower board and slotted Turrets, but didn't take the time to do further searching for what's available out there. This will work.

I don't yet have a layout for the Power Amp Turret Board. That's next, or maybe after I lay in the Heater Wiring of the two rows of tube sockets. On the Preamp Turret Board, I'll wire up what I can from the bottom side of the board, but will, for the most part, wire the pots from the top side of the board, off of the holes in the Turrets. I'll post photos of this board, both sides sometime tomorrow, as I'll have it for my files.

[nevetslab]

Populated Preamp Turret Board for this Test Fixture now ready for wiring up.



I'm now digging thru surplus Data Cables I have on hand to see what is inside the cable jackets of those. One I opened up might work for the Heater Wire....GRN/YEL, and three BLK jacketed 18aWG copper multistrand...looks like 19 strand....not 7 strand, thankfully. I could use the GRN/YEL with the GRN 18AWG 19-strand PVC cable for twisting up the Heater wires, at least for the Power Tube sockets. I haven't yet seen how these twist up and hold their form.

There is a Pacific Radio store two short blocks away that does inventory a lot of different types of wire and cable. I haven't stopped by to see what they have, and what their prices are. I was hoping to be able to make use of what I have on hand, or what I might be able to scrounge, as I'm exploring at present.

[nevetslab]

I've got the Preamp Turret Board wired up to the pots and preamp/driver tube sockets now. I thought I had all of the Tube socket wires connected, cut and prep'd. I began by wiring up the Panel Pots which as I figured, was going to be tedious, not having the space that Hiwatt has in their chassis. The Turret board was really too wide/ 60mm. 50mm or 45mm would have been fine, but this is what I bought. I got all those pot wires cut and stripped, then once I had all of the tube socket wires installed, cut and prep'd, I began wiring the pots up. Made liberal use of the DMM with the beeper activated to keep on track, along with my labeled drawings.



As I finally got onto the Tube Socket wiring, having first added an isolated Turret between the input tube socket and the chassis wall, used to terminate the 1M Grid Bias Resistors,which were routed to the Ground post at the end of the board, and tied over to Chassis ground close by on the end wall, I continued using the DMM to identify each of the wires per their socket destination, and soldered them into place. As I got the Driver tube socket wired up, I realized I was missing the last three wires for V4 tube socket. I was afraid that would happen.....loosing track and didn't have a check list to keep me honest. So, those last three wires came from the top side of the board, instead of underneath, as all the others were. No doubt won't be the first mistake.

One point of concern I have is the where I have tied the 220uF/400V Buss Cap Filter below the first Volume pot....that connection and terminal is right below the wiper of that 500k pot. In the Hiwatt chassis, that Turret board and the pots have a good inch of space between them. Might have to fold and insulate a brass shield to slip there. I won't know any of this for awhile. I'm about to route the heater wiring off of that 6-terminal board positioned between the two rows of tube sockets.

So, slowly making progress. I ended up using 22AWG Teflon 19-strand wire, and using the cloth-covered 20AWG wire to distribute power. I added a phone jack to interrupt the feed from the Preamp circuit to the input of the power amp. I added a 1.8M resistor on the Tip Normal, so when plugging into that, I still have a DC discharge path from the Tone Circuit caps that are sitting at plate supply potential at the source end. That might mess with the pot taper some.

Onward......

[xtian]

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.

You just need to make sure the output transformer is not energized when you do the headphone trick. You will be listening for 60 cycle hum radiated by the power transformers, coupled into the output transformer. You will observe the level of hum rise and fall depending on the relative positions.

[Helmholtz]

You just need to make sure the output transformer is not energized when you do the headphone trick. You will be listening for 60 cycle hum radiated by the power transformers, coupled into the output transformer. You will observe the level of hum rise and fall depending on the relative positions.

I came up with this method some time in the 70s. Used a cheap piezo earbud having high impedance and sensitivity.
Optimum relative orientation is always with the coil formers (not cores) at right angles.
Also used the method to study the effect of different magnetic shielding materials/methods

[nevetslab]

I came up with this method some time in the 70s. Used a cheap piezo earbud having high impedance and sensitivity.
Optimum relative orientation is always with the coil formers (not cores) at right angles.
Also used the method to study the effect of different magnetic shielding materials/methods

Is this method you're referring to still using the Output Xfmr as the field pickup as xtian has stated? Connecting the HiZ Piezo earbud as a listening device? Of is this piezo earbud being used as a sensor....and where?

This coming week, depending on how far I get with the layout & building of the Power Tube Turret Board, I'll put the power xfmrs into the chassis, first as I need to work out the wiring pattern.

[nevetslab]

I just got the Heater Wiring installed on the Preamp/Driver side circuit, and wired up the 6-position Turret Board for feeding the two Heater Harnesses to the tube sockets. Feed from the Heater Xfmr comes in to the middle two terminals of that board.



Last week I worked out the Bias Voltage range for the two-pot circuit, being sourced from -120VDC, and trimmed the circuit for an adjustment range of -70VDC to -46VDC




Also wired up the Output Stage Cathode Resistors, mounted on Tip Jacks on the Rear Panel, along with a pair of HT Fuse holders and LED's to indicate if there's a failure on any of the four power tubes.

[Helmholtz]

Is this method you're referring to still using the Output Xfmr as the field pickup as xtian has stated? Connecting the HiZ Piezo earbud as a listening device? Of is this piezo earbud being used as a sensor....and where?

It's exactly the same method as described by xtian.
The OT is used as a sensor/pickup for the radiated leakage hum field of the PT.
The leakage field induces a small hum voltage in the OT.
A high impedance earphone would be connected to the OT primary, while a low impedance earphone might produce more signal/hum when connected to the secondary.

[xtian]

I also observed that a choke can bend the magnetic field. I set up a PT and OT several inches apart and listened until I had minimized the hum. Then I placed the choke between the PT and OT, and the hum got louder again. I had to find a way to position all three hunks of iron to solve the equation.

[xtian]

Don't forget that this isn't just an academic exercise. Instead of using headphones or a scope to listen to the hum coming from the OT, you can use an actual guitar speaker. In a quiet room, you can put your ear close to the speaker and actually hear the hum, and this is exactly the hum you'll get with your finished build, whenever your PT is energized.