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[SripBoard Design] Starving Student Millett Hybrid Vacum Tube Amp - Page 13

post #181 of 269

Your power supply is 48v?

post #182 of 269
Thread Starter 

Yes thats what is written on it however, when testing with voltmeter, I read 68Volts. Strange.

post #183 of 269

That's ok it will drop under load

post #184 of 269
Thread Starter 

Thats what i thought but then remembered Volt meters have theoretical infinite resistance.

 

EDIT: Same resistance between input and R14/R15


Edited by TestSubject - 12/27/11 at 5:03am
post #185 of 269

Measure between the grid and cathode for voltage.

 

Measure voltage between the gate and source on the mosfets.

post #186 of 269

Allow the tube heaters time to warm before taking measurements.

 

 

post #187 of 269
Thread Starter 

0.42V between grid and cathode on the non working channel 1.7V on the working channel

 

5.12Volts on both mosfets

 

EDIT: It seems like one channel doesnt work at all, even after unplugging the non working channel , I hear no difference.

 

REDIT: I am not the only one with a similar issue apparently. LINK Checked resistance between cathode and ground, Same on both tubes

 

Quote:
Left channel works gerat, right channel seems to only get some kind of cross talk - signal comes through but you have to crank it way up - the left channel is ear bleeding just to get audible sound out of right channel.

 


Edited by TestSubject - 12/27/11 at 7:47am
post #188 of 269

Check the resistance between the input RCA jacks and ground.

 

68v is a bit high for your power supply. Is it 48v dc or ac?

post #189 of 269
Thread Starter 

Do you mean the resistance from the Ground of the RCA and circuit ground? If so, they are both equal.

 

The Cisco PSU says 48Volts DC.

post #190 of 269

Measure resistance between RCA signal input jack(the center pin) and circuit ground.

post #191 of 269

Draw 2 copies of the schematic by hand. Mark down what every resistance should be in your favorite color pen. Be sure to use ohm symbols. 

 

If you can draw it from memory it is OK to use a computer printed copy, but I find drawing it by hand a few times helps me really "get into the circuit" so I'l pass that along. 

 

With the amp off go through the amplifier and measure EVERY resistance you can. Measure from the furthest ends possible - IE if the resistor is on the board and connected to the tube with a long bit of wire measure from the ends of the wire. It doesn't matter that you used a 1Kohm resistor where a 1Kohm resistor belongs if you didn't solder one of the 3 joints its hooked up to.... Measuring one way only verifies resistor value, the other gives a pretty good assurance that you soldered it correctly too. Mark this down in PENCIL, making sure to use the ohm symbol. 

 

It is OK for certain things to be slightly off from the values on the schematic, as long as both channels measure similarly. Depending how you measure, you may find stuff in the voltage divider to be off by a lot, but I doubt that's where your problems are. 

 

After a bit of poking around with the resistance-meter you should have found something that makes absolutely no sense by now. Figure out why and fix it. Does the amp work? 

 

If measuring resistances doesn't find anything (unlikely) turn the amp on. Measure voltages from EVERYWHERE to ground. Plate, cathode, gate, drain, source, etc. Mark this down on the schematics in pencil, making sure to note volts. Measuring voltages is often faster than measuring resistances, BUT it requires poking about in a live amp so Id save it for a last ditch effort. 

post #192 of 269
Thread Starter 

OK, Sorry about the misunderstanding. Both at 102K Ohms

post #193 of 269

It would be easier if you did what Nikongod asks.

 

Then post your findings.


Edited by digger945 - 12/27/11 at 8:41am
post #194 of 269
Thread Starter 

No worries, already on it

post #195 of 269
Quote:
Originally Posted by digger945 View Post

It would be easier if you did what Nikongod asks.

 


When I am the ruler of the world you can be my head negotiator. 

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