Schematic for the Hiwatt DR201/KT88 Test Fixture done

[nevetslab]

After updating the minor corrections to the Hiwatt DR201 Bass Amp schematic (which I've attached below), I've revised that drawing to reflect all of the Test Fixture & Power Supply retrofits added to fully document that project. I haven't yet gone back in to get updated circuit voltages thruout the Hiwatt DR201 amp. I did record those prior to completing the project for temp reference.



Hiwatt_DR201_Clone-KT88_Test_Fixture-1.pdf

Hiwatt DR201 Bass Amp Schematic-3A.pdf

[mhuss]

Looks good.

[nevetslab]

In reviewing my notes on the Heater Xfmr, with the 7.2A load presented by the four KT88 power tubes and four preamp tubes, my voltage on the secondary is still too high. 7.2VAC with the xfmr drawing 0.58A. I had dropped the variac to 106VAC to get 6.3VAC. Using a 20 ohm 50W resistor in series with the primary of the Heater Xfmr, that would drop 11.6VAC, which would be just about right. As I've been considering removing the 20 ohms from the input resistor string on the Fender Screen Supply P/T to increase the loaded DC voltage back up to around 400V from the 375V level it's been running at, that would be just about right to get the Heater secondary voltage where I wanted it. That's about 7% high. Not sure what the long-term effects of having over-voltage on the heaters, though would guess it would reduce the life of the tubes.

[nevetslab]

Yesterday I pulled the amp chassis back out of the cabinet and moved it over to the bench, to disconnect the 20 ohm/50W Dale Power Resistor from the Primary side of the Fender HRD Power Xfmr being used for the HT Screen Supply for the KT88's and the Preamp HT circuits. Next, I removed the jumper wire that connected the 40 ohm/50W resistor with the 20 ohm/50W resistor, and then removed the other end of that 20 ohm resistor to the end of the 40 ohm resistor, it still being connected to the AC Terminal Block's Neutral of the two power xfmrs. I then unplugged the Neutral side of the Heater Power Xfmr from the Barrier Strip (where it shares terminals with the Fan), and prepared a new Mains Neutral wire to connect that wire to the input of the 20 ohm/50W resistor, and the output got a new Neutral wire that plugged it back into the Barrier Strip Neutral terminal, so the Heater Xfmr now has the 20 ohm resistor in series. That was just enough to drop the Heater voltage at 120VAC Mains from 7.2VAC down to 6.5VAC.

And, the HT Screen Voltage increased from 375VDC to 393VDC at Idle. I had to re-bias the power tubes, as the plate current came up in that move. The HT Plate Voltage is now lower...anywhere from 704VDC to 700VDC. I swapped two of the power tubes, now having one that was reading higher matched with the other that was higher, so re-biased for 24mA per pair nominal.

While I had the chassis open, I added a Ground Wire to attach to the frame Ground screw of the NMB Fan to the Amp Chassis, snaking that in with the Fan Connector cable. Probably not necessary, but, seeing as there was a grounding provision on the metal frame, why not.



I then finally sat down and recorded all of the DC Voltages thruout the Hiwatt DR201 Amp circuit, along with the bias circuit, supply utility circuits of this test fixture and then opened up the Multisim schematic and updated that with the details. Produced an updated pdf of that which is attached below.

Then, I moved the chassis back into the cabinet and opened up the fan exhaust vent to allow measuring the heat sink temperature, having altered the head load with these last changes. After it had been running for an hour, it had stabilized at 61 deg C, up from 57 deg C in the previous wiring. More than adequate, and puts the heater voltage where it should be, HT Screens at a better place, along with a bit more headroom on the Preamp circuits. This project is now complete.

Hiwatt_DR201_Clone-KT88_Test_Fixture-2.pdf