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Yesterday I ran into an unusual sequence of Mains Fuse blowing when switching out of Standby on a Hiwatt Custom 100W Signature Series amp. I had just replaced the original EL34 tubes installed in this amp with the J/J Platinum Matched set Rob (from Hiwatt/Toronto) brought with him.
I rebiased this set at 35mA plate current per tube. After removing the bias probes and putting the freshly-sleeved hold-down clamps into place, when I went to switch out of Standby, the HT fuse blew. I found a 2A Fast Blow fuse in the holder, calling out 3A, so replaced it with a 3.16A Fast, tried again and it blew again. Checking the four freshly installed/biased power tubes, I found one was now pulling fault current, so now, I was down one tube from the new set. sigh...... I drove over to our new building to see if there was a matched quad set of J/J EL34 power tubes, and lucked out....brought that new set over and set up again to bias those up.
Prior to dealing with the newest set of tubes, I had to restore ballast resistors on the two sets of filter caps wired in series, as each set was missing a ballast resistor. I removed the one 220k ballast resistor from the input series pair of 220uF/400V caps and installed 100k/2W ballast resistors (don't yet have the 220k CF resistors), and moved that 220k resistor to the HT3/HT4 series cap array, so there would be a ballast resistor from the output side of the second 50uF cap to the ground of that dual cap, and already had the 220k resistor on the lower cap.
Side note on the absence of the ballast resistors.....the input caps, without power tubes installed, 486VDC on upper cap, 458V on the lower cap (exceeding the voltage rating of the lower cap, due to no equal ballast resistors). With the 100k ballast resistors and power tubes installed, I now have 460V on top, 231V on the lower (series connection).
Then, set up to bias the new power tubes, again setting these for 35mA plate current. Powered down, restoring the hold-down clamps, then set the chassis up on end to allow getting voltage measurements on the circuit to add to my 100W amp schematic.
Powered back up in S/B, then after the tubes had warmed up sufficiently, I switched out of S/B, and the AC Mains fuse blew! Shut down, replaced that fuse, then powered back up in Standby, and this time, after letting the power tubes warm up, I quickly turned the variac to 0VAC, switched out of S/B and began turning up the variac, and saw high mains current flowing immediately as I shut back to 0VAC.
I removed the hold-down clamps, then removed the outside pair of tubes, leaving the middle tubes in place. Repeated that modified power-up sequence, and found the middle pair of tubes are pulling high current. I removed those, and put the outer pair of tubes back into their sockets, and repeated the power-up sequence. Here, I was able to turn the variac all the way up to 115VAC. Powered down, then moved that outside pair to the middle tube sockets, and repeated the modified power-up sequence. It powered up fine in those sockets.
Then, to tempt fate, I left those installed in the middle sockets, and plugged the two tubes that I had removed from those sockets into the outside pair of sockets, and repeated the modified power-up sequence. I was able to power up to full line voltage this time! NO high current draw as I first saw.
THIS IS WIERD! I did have the Output xfmr set for 4 ohms, with 4 ohm dummy load attached. I changed that to 8 ohm, with 8 ohm attached. Repeated the modified power-up sequence with the variac, and still got to full line voltage with all the tubes installed. Went back to Standby, and let it idle at that stage for 10 minutes. Tempting fate, this time I just switched out of S/B as normal, and no problems....no Mains fuse blowing. I cycled that step repeatedly, both in 8 ohm load and 4 ohm load, so I'm still at a loss to account for why the Mains Fuse was blowing.
This morning, I plugged in my Tektronix Current Probe amp AM503A with the A6302 20A Current Probe attached, and set up the scope to measure the inrush current in Storage mode. As one would expect, there IS a short-term high inrush current switching out of Standby, which lasts around 250mS. In short-term spans, I've only seen 10A peak current flow, which rapidly drops to 3A peak current in Operate. In Standby, there's no peak current being drawn...the mains current looks sinusoidal, as you'll see in the scope photo. I also moved the current probe to measure the HT current Inrush, sensing that from the bottom side of the discrete diode bridge, current passing to ground thru the HT Fuse holder.
This Mains current inrush issue, blowing the Mains Fuse I've not seen on any amp before. I can't imagine it's a result of correcting the absence of ballast resistors on the two sets of series-connected filter caps. Though that's on only major change made on the chassis. I didn't see anything like that occurring on the Custom 50W. IT does have 220k ballast resistors on the first filter stage, but the second filter stage HT3/HT4 cap array, it's missing the second 220k resistor. I didn't have any 220k resistors to add...they're on order, but coming from Farnell/UK, and won't see them until the end of next week at the earliest.
I've also changed the 1N5408 3A Bridge Rectifier circuit diodes to UF5408's, as well as changing the 1N4007 bias rectifier to a UF4007, just to be consistent. Hiwatt had been building with the slower 1N series rectifiers, but have changed to the UF series rectifiers, though I don't know when.
I've attached the one schematic I have, and layout diagram. I haven't loaded the circuit into my schematic capture program so I can add voltages and other details, but now have the voltages marked up on my copy of this schematic.
I rebiased this set at 35mA plate current per tube. After removing the bias probes and putting the freshly-sleeved hold-down clamps into place, when I went to switch out of Standby, the HT fuse blew. I found a 2A Fast Blow fuse in the holder, calling out 3A, so replaced it with a 3.16A Fast, tried again and it blew again. Checking the four freshly installed/biased power tubes, I found one was now pulling fault current, so now, I was down one tube from the new set. sigh...... I drove over to our new building to see if there was a matched quad set of J/J EL34 power tubes, and lucked out....brought that new set over and set up again to bias those up.
Prior to dealing with the newest set of tubes, I had to restore ballast resistors on the two sets of filter caps wired in series, as each set was missing a ballast resistor. I removed the one 220k ballast resistor from the input series pair of 220uF/400V caps and installed 100k/2W ballast resistors (don't yet have the 220k CF resistors), and moved that 220k resistor to the HT3/HT4 series cap array, so there would be a ballast resistor from the output side of the second 50uF cap to the ground of that dual cap, and already had the 220k resistor on the lower cap.
Side note on the absence of the ballast resistors.....the input caps, without power tubes installed, 486VDC on upper cap, 458V on the lower cap (exceeding the voltage rating of the lower cap, due to no equal ballast resistors). With the 100k ballast resistors and power tubes installed, I now have 460V on top, 231V on the lower (series connection).
Then, set up to bias the new power tubes, again setting these for 35mA plate current. Powered down, restoring the hold-down clamps, then set the chassis up on end to allow getting voltage measurements on the circuit to add to my 100W amp schematic.
Powered back up in S/B, then after the tubes had warmed up sufficiently, I switched out of S/B, and the AC Mains fuse blew! Shut down, replaced that fuse, then powered back up in Standby, and this time, after letting the power tubes warm up, I quickly turned the variac to 0VAC, switched out of S/B and began turning up the variac, and saw high mains current flowing immediately as I shut back to 0VAC.
I removed the hold-down clamps, then removed the outside pair of tubes, leaving the middle tubes in place. Repeated that modified power-up sequence, and found the middle pair of tubes are pulling high current. I removed those, and put the outer pair of tubes back into their sockets, and repeated the power-up sequence. Here, I was able to turn the variac all the way up to 115VAC. Powered down, then moved that outside pair to the middle tube sockets, and repeated the modified power-up sequence. It powered up fine in those sockets.
Then, to tempt fate, I left those installed in the middle sockets, and plugged the two tubes that I had removed from those sockets into the outside pair of sockets, and repeated the modified power-up sequence. I was able to power up to full line voltage this time! NO high current draw as I first saw.
THIS IS WIERD! I did have the Output xfmr set for 4 ohms, with 4 ohm dummy load attached. I changed that to 8 ohm, with 8 ohm attached. Repeated the modified power-up sequence with the variac, and still got to full line voltage with all the tubes installed. Went back to Standby, and let it idle at that stage for 10 minutes. Tempting fate, this time I just switched out of S/B as normal, and no problems....no Mains fuse blowing. I cycled that step repeatedly, both in 8 ohm load and 4 ohm load, so I'm still at a loss to account for why the Mains Fuse was blowing.
This morning, I plugged in my Tektronix Current Probe amp AM503A with the A6302 20A Current Probe attached, and set up the scope to measure the inrush current in Storage mode. As one would expect, there IS a short-term high inrush current switching out of Standby, which lasts around 250mS. In short-term spans, I've only seen 10A peak current flow, which rapidly drops to 3A peak current in Operate. In Standby, there's no peak current being drawn...the mains current looks sinusoidal, as you'll see in the scope photo. I also moved the current probe to measure the HT current Inrush, sensing that from the bottom side of the discrete diode bridge, current passing to ground thru the HT Fuse holder.
This Mains current inrush issue, blowing the Mains Fuse I've not seen on any amp before. I can't imagine it's a result of correcting the absence of ballast resistors on the two sets of series-connected filter caps. Though that's on only major change made on the chassis. I didn't see anything like that occurring on the Custom 50W. IT does have 220k ballast resistors on the first filter stage, but the second filter stage HT3/HT4 cap array, it's missing the second 220k resistor. I didn't have any 220k resistors to add...they're on order, but coming from Farnell/UK, and won't see them until the end of next week at the earliest.
I've also changed the 1N5408 3A Bridge Rectifier circuit diodes to UF5408's, as well as changing the 1N4007 bias rectifier to a UF4007, just to be consistent. Hiwatt had been building with the slower 1N series rectifiers, but have changed to the UF series rectifiers, though I don't know when.
I've attached the one schematic I have, and layout diagram. I haven't loaded the circuit into my schematic capture program so I can add voltages and other details, but now have the voltages marked up on my copy of this schematic.
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