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This past week I finally got to see one of the Hiwatt DR201 200W Bass Amps. Came over from Lon Cohen's facility nearby, with No Output. Tubes? was the suggested hint, along with a pair of J/J KT88 used boxes, one marked 35mA, other as 36mA. I got the iphone camera out to begin by photographing the unit for my database. Same proportions as the Custom 100W and 50W heads, but larger and much heavier.
When I tipped it up to remove the rear panel and then the chassis mtg screws, I found one of the chassis screws missing, with the other one of the front screws ready to fall out. No Lock Washers.
I removed the chassis carefully, and moved it over to my test bench, then spotted the shredded adhesive-backed aluminum ground foil on the floor of the cabinet. Looking at the chassis on my bench, I saw the probable cause of the HT Fuse having blown. Nearest O/T side KT88 was frosty white.
Inside views of the chassis. I noticed this one didn't have the 1 ohm Cathode Resistors, which Hiwatt is finally beginning to install in their amps. Also noticed no ballast or bleeder resistors across the parallel'd 100uF/500V PS Buss Caps. I stopped to see what I could download from the internet on the schematics for this amp. Various renditions out there from over the years of production.
I made a list of the items I had found, sent that off to Hiwatt for getting their approval to proceed ahead, as I had just received an email from them asking if this had the 1 ohm cathode resistors installed. I had received a string of their 1 ohm 1W 1% Carbon Film resistors for adding those to their amps. And had their 220k 1W CF resistors for installing on the buss caps to make this safe to work on, as without them, it can take many hours to drain off. Got the approval to proceed ahead. One feature I did like in their wiring was the pair of bias adj pots for each half of the output stage.
Before I made that move, after having looked at the wiring, not yet realizing the upper Buss cap (220uF/400V was on the opposite side of the chassis from the three dual-100uF/500V buss Caps, all having a common ground buss on the neg terminal. They couldn't be running the power tubes off of less than 500V, surely? Nope. That single 400V 220uF buss cap was the top of the stack, and found from it's positive terminal, the BRN and YEL wire soldered to the middle pair of power tube sockets Pin 3 were 9.86 ohms and 7.96 ohms difference from that cap terminal. I checked the where that RED wire went, seeing a RED wire at the S/B switch. That DCR difference from the buss cap to there was insignificant to the plate taps from the C/T.
I removed that bad KT88, along with the other three tubes, one of which no longer had it's alignment peg (broken off/gone). My bias probes/meter aren't rated for over 600V, so I began by using the measured DCV reading between the 220uF/400V buss cap to the two plate terminals where the O/T primary leads attached, then divided that value with the DCR value to get the plate current and wrote that down. Did that for the five power tubes, finding the three remaining tubes from the chassis did work, and had assumed the two from the client also worked. I ended up with what I could make up for two pairs. Difference in their discrete plate current was 17% on one side, 7% on the other. Plugged them in and adjusted the bias for 50mA total on each side, targeting for 25mA/tube, though didn't know just how well that really was. I didn't check the screen currents in the process.
After revising the cathode wiring from being grounded to chassis at each tube socket, adding the 1 ohm resistors standing up Japanese style, and adding the 220k bleeder resistors across the buss caps, I was finally able to check the cathode current thru each tube. One side was way off, running nearly 50mA per tube while the other was not quite 25mA per tube. After landing them at 24mA & 26mA on each pair, I was quite pleased, as well as somewhat befuddled with the method of using the primary plate wires' DCR and voltage drop from C/T to calculate/set the bias. Far easier with having those cathode resistors to measure across. Or, at lower voltages, using bias probes that have the cathode sense resistor inside the probe body.
When I tipped it up to remove the rear panel and then the chassis mtg screws, I found one of the chassis screws missing, with the other one of the front screws ready to fall out. No Lock Washers.
I removed the chassis carefully, and moved it over to my test bench, then spotted the shredded adhesive-backed aluminum ground foil on the floor of the cabinet. Looking at the chassis on my bench, I saw the probable cause of the HT Fuse having blown. Nearest O/T side KT88 was frosty white.
Inside views of the chassis. I noticed this one didn't have the 1 ohm Cathode Resistors, which Hiwatt is finally beginning to install in their amps. Also noticed no ballast or bleeder resistors across the parallel'd 100uF/500V PS Buss Caps. I stopped to see what I could download from the internet on the schematics for this amp. Various renditions out there from over the years of production.
I made a list of the items I had found, sent that off to Hiwatt for getting their approval to proceed ahead, as I had just received an email from them asking if this had the 1 ohm cathode resistors installed. I had received a string of their 1 ohm 1W 1% Carbon Film resistors for adding those to their amps. And had their 220k 1W CF resistors for installing on the buss caps to make this safe to work on, as without them, it can take many hours to drain off. Got the approval to proceed ahead. One feature I did like in their wiring was the pair of bias adj pots for each half of the output stage.
Before I made that move, after having looked at the wiring, not yet realizing the upper Buss cap (220uF/400V was on the opposite side of the chassis from the three dual-100uF/500V buss Caps, all having a common ground buss on the neg terminal. They couldn't be running the power tubes off of less than 500V, surely? Nope. That single 400V 220uF buss cap was the top of the stack, and found from it's positive terminal, the BRN and YEL wire soldered to the middle pair of power tube sockets Pin 3 were 9.86 ohms and 7.96 ohms difference from that cap terminal. I checked the where that RED wire went, seeing a RED wire at the S/B switch. That DCR difference from the buss cap to there was insignificant to the plate taps from the C/T.
I removed that bad KT88, along with the other three tubes, one of which no longer had it's alignment peg (broken off/gone). My bias probes/meter aren't rated for over 600V, so I began by using the measured DCV reading between the 220uF/400V buss cap to the two plate terminals where the O/T primary leads attached, then divided that value with the DCR value to get the plate current and wrote that down. Did that for the five power tubes, finding the three remaining tubes from the chassis did work, and had assumed the two from the client also worked. I ended up with what I could make up for two pairs. Difference in their discrete plate current was 17% on one side, 7% on the other. Plugged them in and adjusted the bias for 50mA total on each side, targeting for 25mA/tube, though didn't know just how well that really was. I didn't check the screen currents in the process.
After revising the cathode wiring from being grounded to chassis at each tube socket, adding the 1 ohm resistors standing up Japanese style, and adding the 220k bleeder resistors across the buss caps, I was finally able to check the cathode current thru each tube. One side was way off, running nearly 50mA per tube while the other was not quite 25mA per tube. After landing them at 24mA & 26mA on each pair, I was quite pleased, as well as somewhat befuddled with the method of using the primary plate wires' DCR and voltage drop from C/T to calculate/set the bias. Far easier with having those cathode resistors to measure across. Or, at lower voltages, using bias probes that have the cathode sense resistor inside the probe body.
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