Hi I'm somewhat new to tube amps but have admired them for years. Nostalgic I guess. I ordered a PCB from china single ended KT88 HiFi. I was hoping someone might have some information on this schematic.
I have looked at many schematics but have not come up with anything as of yet. Hoping for explanation of circuit, but have figured out lots so far.
I'm not sure about the filament jumpers to the KT88 tube. Maybe if you have a different supply voltage. I have figured out why the 6SL7 filament voltage is floating at 80v.
I still need to do some research on the OTP connection and 3.5k specification. But if you have info you want to share it would be appreciated.
I have looked at many schematics but have not come up with anything as of yet. Hoping for explanation of circuit, but have figured out lots so far.
I'm not sure about the filament jumpers to the KT88 tube. Maybe if you have a different supply voltage. I have figured out why the 6SL7 filament voltage is floating at 80v.
I still need to do some research on the OTP connection and 3.5k specification. But if you have info you want to share it would be appreciated.
R31 and R32 create an artificial center tap for the heater winding. They are not needed if the winding has real center tap as shown in the schematics.
OPT switches (SP1 and SP4) allow to change the operation mode of the output tubes - triode/ultralinear/pentode.
Heater switches (SP2 and SP5) are most likely there to accomodate 12.6V heater tubes (1626 or whatever) as an option insead of KT88.
What's exactly unclear about the "3.5k specification"?
OPT switches (SP1 and SP4) allow to change the operation mode of the output tubes - triode/ultralinear/pentode.
Heater switches (SP2 and SP5) are most likely there to accomodate 12.6V heater tubes (1626 or whatever) as an option insead of KT88.
What's exactly unclear about the "3.5k specification"?
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I’ve read from other posts about pcb’s not matching with schematics - perhaps one of them being a more recent version. I’ve no direct experience myself, but if you have time on your hands you could check before you build.
Looks like a nice amp design, from my limited experience.
Looks like a nice amp design, from my limited experience.
Thanks for all the info, I picked up the board off fleabay.
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1pc Single-ended tube amplifier bare diy PCB for EL34 KT88 6L6GC FU7 FU50 6550
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1pc Single-ended tube amplifier bare diy PCB for EL34 KT88 6L6GC FU7 FU50 6550
Is there anyone who can explain to me the meaning of the scheme applying 80v at the central point of the 6.3v for the filaments. Thank you for your help
The V1a and V3a (upper 6Sx7 -half- tubes) cathode is at higher potential (about 160-180V), than the tube heater-cathode maximum value, so must to use elevate the heater -in this case with 80V-.
Usually V1b, V3b is one tube (normal heater potential) and V1a and V3a is another tube with elevated heater potential.
Usually V1b, V3b is one tube (normal heater potential) and V1a and V3a is another tube with elevated heater potential.
jdarg,
240k Rg is too large for a KT88 that has Fixed or Fixed Adjustable grid bias.
The cathode self bias switch setting shows 360V @ 100mA = 36 Watts
When using that circuit in fixed or fixed adjustable bias, if the current is also 100mA, Rg should be 100k or Less.
Note:
Using a 6550, the maximum grid resistor, Rg, is even lower resistance.
"Run Tubes Hot, with high resistance Rg, Equals "Long Life NOT".
240k Rg is too large for a KT88 that has Fixed or Fixed Adjustable grid bias.
The cathode self bias switch setting shows 360V @ 100mA = 36 Watts
When using that circuit in fixed or fixed adjustable bias, if the current is also 100mA, Rg should be 100k or Less.
Note:
Using a 6550, the maximum grid resistor, Rg, is even lower resistance.
"Run Tubes Hot, with high resistance Rg, Equals "Long Life NOT".