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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.
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.
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.
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.
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.
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.
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