Dynaco ST70 Distortion.
- Thread starter rudes
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How did it sound before changing board power supply cap and bias circuit ?
can you post some clear and sharp photo's of the inside of the ST-70. In what condition are the tube pins in the tube sockets, are they clean and tight?
What model/type of preamp are you using.
What model/type of preamp are you using.
excellent point. The 7199 and 6GH8/6U8 boards have different pinouts and will require the correct tube to be used.
Thanks for the replies guys!
As stated, I am using a new ORIGINAL circuit board, like the Dynakit. Yes the one using 7199. I even tried different 7199s that I have. I tried a different 5AR4. It is weird that I can bias the amp with no problem and all the high voltages are where they are suppose to be on all the tube pins. Is it possible that the output transformers are at fault? They both pass the "ohm test". Both channels are very badly distorted.
As stated, I am using a new ORIGINAL circuit board, like the Dynakit. Yes the one using 7199. I even tried different 7199s that I have. I tried a different 5AR4. It is weird that I can bias the amp with no problem and all the high voltages are where they are suppose to be on all the tube pins. Is it possible that the output transformers are at fault? They both pass the "ohm test". Both channels are very badly distorted.
Are you 100% shure that the new board is for 7199 ? Check documentation.
Other then that, inject sinus , follow signal with scope. Double-check voltages around 7199 for reasonable value.
Is there a 270k resistor on connected to pin 2 or pin 6
If it's pin 2 then it's for a 7199, if it's pin 6 then it's wired for a 6U8
Also on pin 6 if it's for a 7199 there should be a 620 ohm resistor and a 0.05uf cap. If it's on pin 7 then again, it's a 6u8
Nothing to add that hasn’t been mentioned. For all the doubters
I built/installed the same board a few months ago with no problems.Unfortunately I do not own a scope. I have new gold input jacks, and yes I do have a good 10 ohm ground reading. I have O ohms from common speaker output to chassis. 270K connected to pin 2 of 7199. So far everything suggested, I have already done. Any other suggestions appreciated!
Are the speaker terminals banana jacks or screw terminals. Make sure you have the wires connected correctly AND that there is no contact between the ground wire and + going to the speaker. Not my first thought since both channels are involved.
I'm feeling that the problem is in either your PS or driver board.The 0.1uF voltage on the driver side of the coupling capacitor should be a few hundred positive volts. The voltage at the output tube side of the coupling capacitor, to pin 5 should be negative voltage.
Do you have access to a tube tester???
I'm feeling that the problem is in either your PS or driver board.The 0.1uF voltage on the driver side of the coupling capacitor should be a few hundred positive volts. The voltage at the output tube side of the coupling capacitor, to pin 5 should be negative voltage.
Do you have access to a tube tester???
The value doesn't really matter all that much. 15 ohm resistor needs 1.5 volts of bias. 15.6 ohm needs 1.56 volts. Assuming 2 tubes per resistor. One tube per resistor is half that, 0.75v and 0.78v respectively. Its 50ma per tube however you slice it. Me personally I'd probably use 10 ohm resistors because the math is easier.
if the output stage biases OK, voltages to that stage are likely good and the issue is somewhere in the driver stage. Both channels makes me inclined to think its a common issue. I'd be looking at voltage feed into the driver board, and voltages at each pin on the 7199 sockets vs the manual. Also worth confirming the connections between the driver board and the output tubes are correct. If those are reversed it will do really stupid things. Usually it will scream and howl but if its switched left to right somehow it might do other non-sensible things. A good clear pic of the underside would be a big help here.
if the output stage biases OK, voltages to that stage are likely good and the issue is somewhere in the driver stage. Both channels makes me inclined to think its a common issue. I'd be looking at voltage feed into the driver board, and voltages at each pin on the 7199 sockets vs the manual. Also worth confirming the connections between the driver board and the output tubes are correct. If those are reversed it will do really stupid things. Usually it will scream and howl but if its switched left to right somehow it might do other non-sensible things. A good clear pic of the underside would be a big help here.
Mayby a cheap used tek scope is what to get now. It's exactly whats needed when this
kind of problems occur.
