Dared Dv-200 parallel PP KT88 help.

[tukham rob]

My first post here so point me elsewhere if I’m wrong.
Brother in law brought me the above monoblock in bad shape with no schematic. Could not find schematic so went ahead with repairs using the “good” monoblock as reference. Determined a coupling cap had shorted internally and taken out a number of cathode fuses as well as grid resistors. I cleaned up the damaged board and replaced the components that the magic smoke had leaked out of and a few others for good measure.
When I started to bring it up on the Variac the amp had high current and by trial and error I discovered a bad KT88. Interesting because I expected to find a bad OP tube so tested all on my Stark 9-66 and they all had tested good. The bad one still does test good but that is another problem to solve.
Put in a good KT88 and brought the amp up with speaker load but no input. Amplifier draws about 1.2A. Bring up the other monoblock and it draws @2.4A. Swap tube sets and higher current stays with 2nd amplifier. Plate current reflects that with higher plate current on amp#2.
Left on vacation and finally got a schematic. There are no biasing pots but there are trim pots off of the two driver tubes. I am wondering how to set those but doubt that will be the cause of the uneven current between the two MBs.
I will try and upload the schematic from my smartphone but will not be surprised if that has to wait until I’m home next week.
BTW the schematic is as I received it from Dared. The mark ups are theirs not mine.
Any thoughts and ideas are welcome.
Thanks
Rob

 

[gadget73]

looks like R1 and R2 are your bias adjust pots. The output tubes are direct coupled to whatever is upstream of it. 6P1P I think? the schematic scales down a fair bit and by the time I blow it up things get fuzzy. The pots adjust how much current flows through that tube, which will change how negative the cathode is which changes where the KT88 grids sit. The cathode on the small tubes will be tied to the negative supply, so current flow through the tube pulls it up.

anyway if you know the plate voltage you can make some guesses about what the current ought to be. Have to measure across the 10 ohm resistors. No individual bias for this either, its one pot per side of the output transformer so the tubes should be matched on each side, and ideally it would be an entire matched set.

anyway, worth checking voltage on each cathode resistor to confirm all the tubes are conducting to some degree. Anything not doing work will be obvious, either no voltage or way high because of an open resistor.

 

[tukham rob]

looks like R1 and R2 are your bias adjust pots. The output tubes are direct coupled to whatever is upstream of it. 6P1P I think? the schematic scales down a fair bit and by the time I blow it up things get fuzzy. The pots adjust how much current flows through that tube, which will change how negative the cathode is which changes where the KT88 grids sit. The cathode on the small tubes will be tied to the negative supply, so current flow through the tube pulls it up.

anyway if you know the plate voltage you can make some guesses about what the current ought to be. Have to measure across the 10 ohm resistors. No individual bias for this either, its one pot per side of the output transformer so the tubes should be matched on each side, and ideally it would be an entire matched set.

anyway, worth checking voltage on each cathode resistor to confirm all the tubes are conducting to some degree. Anything not doing work will be obvious, either no voltage or way high because of an open resistor.

Thanks for the reply. Your answer confirms what a suspected that the pots bias the 3 Kt88’s on each side of the PP and there is no bias for the individual tubes on each side of the PP.
considering you have to take the amp apart to get at the trim pots it’s not a great design for the end user. I suppose just try best matching tubes for each side and then adjust the pots for total cathode current on each side.
Still not sure why the one monoblock is drawing twice the current of the other. Will be home on the weekend and will get back to it next week.

 

[capbeefheart]

Convert everything to watts and compare primary to secondary. Or look on the back of the amp, it may tell you the power consumption of the device and then use that to calculate out the current it should be running at.

It's late and quickly looking at schematic the secondaries you got about 70 watts in heaters, then about another 140-150 watts for everything else. So 220 watts total.

Convert that to the 120v primary voltage and you get 1.8 amps of current for 220 watts.

One amplifier could be biased way cold, or a tube or two could be dead and not conducting current. The other amp could be biased very hot and all tubes work and pulling extra current. The margin of error high and low around 1.8 amps could give you both currents you are seeing. Once you get into each amplifier and get them functioning correctly both amps should fall inline with each other as far as energy consumption.

 

[tukham rob]

My first thought was a bad tube tubes drawing less current. However when I switched the full tube set the current readings stayed the same. I believe the repaired unit is low current and unrepaired is correct. Perhaps I missed a bad component during repairs. More checks when I return home.
Thank you.

 

[Robert Wilso]

My first post here so point me elsewhere if I’m wrong.
Brother in law brought me the above monoblock in bad shape with no schematic. Could not find schematic so went ahead with repairs using the “good” monoblock as reference. Determined a coupling cap had shorted internally and taken out a number of cathode fuses as well as grid resistors. I cleaned up the damaged board and replaced the components that the magic smoke had leaked out of and a few others for good measure.
When I started to bring it up on the Variac the amp had high current and by trial and error I discovered a bad KT88. Interesting because I expected to find a bad OP tube so tested all on my Stark 9-66 and they all had tested good. The bad one still does test good but that is another problem to solve.
Put in a good KT88 and brought the amp up with speaker load but no input. Amplifier draws about 1.2A. Bring up the other monoblock and it draws @2.4A. Swap tube sets and higher current stays with 2nd amplifier. Plate current reflects that with higher plate current on amp#2.
Left on vacation and finally got a schematic. There are no biasing pots but there are trim pots off of the two driver tubes. I am wondering how to set those but doubt that will be the cause of the uneven current between the two MBs.
I will try and upload the schematic from my smartphone but will not be surprised if that has to wait until I’m home next week.
BTW the schematic is as I received it from Dared. The mark ups are theirs not mine.
Any thoughts and ideas are welcome.
Thanks
Rob

My first post here so point me elsewhere if I’m wrong.
Brother in law brought me the above monoblock in bad shape with no schematic. Could not find schematic so went ahead with repairs using the “good” monoblock as reference. Determined a coupling cap had shorted internally and taken out a number of cathode fuses as well as grid resistors. I cleaned up the damaged board and replaced the components that the magic smoke had leaked out of and a few others for good measure.
When I started to bring it up on the Variac the amp had high current and by trial and error I discovered a bad KT88. Interesting because I expected to find a bad OP tube so tested all on my Stark 9-66 and they all had tested good. The bad one still does test good but that is another problem to solve.
Put in a good KT88 and brought the amp up with speaker load but no input. Amplifier draws about 1.2A. Bring up the other monoblock and it draws @2.4A. Swap tube sets and higher current stays with 2nd amplifier. Plate current reflects that with higher plate current on amp#2.
Left on vacation and finally got a schematic. There are no biasing pots but there are trim pots off of the two driver tubes. I am wondering how to set those but doubt that will be the cause of the uneven current between the two MBs.
I will try and upload the schematic from my smartphone but will not be surprised if that has to wait until I’m home next week.
BTW the schematic is as I received it from Dared. The mark ups are theirs not mine.
Any thoughts and ideas are welcome.
Thanks
Rob

Rob, do you have a better quality picture of the schmetic? We cannot read the fine print from that picture. If not can you tell me where you got it?
Bob Wilson

 

[tukham rob]

Rob, do you have a better quality picture of the schmetic? We cannot read the fine print from that picture. If not can you tell me where you got it?
Bob Wilson

I was sent a photo of a hard print schematic by email. It looks clear enough when I look at it and I tried a screenshot to my photos as well. Either way when I look at them the resolution is OK but when I upload to here the resolution seems to drop and they become difficult to read. If you can give me your email somehow I can try that way.

 

[Robert Wilso]

My email address is rwilson5717@gmail.com Than you for reaching out.
Bob Wilson