I'm using a blue ESR meter for caps 1uf and above and I love this meter! But what about those little tiny caps? What are techs using to measure these little guys? Can anyone recommend a cap meter? I'm currently using a cheap Mastercraft Meter but I question the cap readings. I also know these little caps need to come out of the PCB in order to test properly. So, I'm putting it out there, what do people recommend? What works for you?

Thanks.

Thanks guys, definitely some suggestions I hadn't considered. 

I've been putting off buying an amp cradle due to cost. Everyone using one? Or have you found a cheaper alternative to working on a tube amp chassis....thanks!

Thanks g1. Amp works great! :dbtu:

Enzo, definitely handy. The new universal transformer works but I'm concerned about the higher voltages. At junction of CR11 and CR13 I'm getting -45.6VDC.

Also the plate, screen, B++, and B+ voltages are higher. At CR8/9 junction I measure 357V. At junction R58/59 I measure 351V,and at R59/60 I measure 352 V. At  R60 on the B+ side I measure 349 volts.

I haven't put the tubes in yet as I wanted to make sure the voltages are within range.

The amp uses russian Tungsol El84 tubes and according to the data sheet, if I'm reading it correctly, these voltages are too high....

Correction: brown/black together....white/blue together.  Thanks Enzo!

Hello Folks!

I'm trying to replace a Power Transformer for a Peavey Classic 30, 120V operation , with a Peavey Universal Transformer. The original transf. had two yellow secondary wires that attached to a connector which slid over a set of two pins on the board. There were also two red secondary wires which went to another connector and that connector slid over the other two pins on the board. Easy stuff.

But the universal transformer in the attached pic has two yellow wires with separate connectors and two reds with separate connectors. I'm guessing I need to remove the connectors and solder each of the four wires to the pins. Makes sense?

Also, the tech notes that came with Peavey transformer tell me to connect both the blue and brown to the hot side of the supply and the blue  and white to the neutral side which, again, appears easy enough.

I've never used a universal transformer replacement so I'm a little confused on this one.

Thanks!

QuoteMy remarks above relate only to the almost universal use of 6.3 or 12.6VAC heater circuits in valve guitar amps

Roly I've always had it in my head that tube heaters were only capable of handling a small voltage but not a large one--I checked a 12AX7 data sheet and it showed max rating Vh-k 180volts.

Sorry  I didn't post the schematic immediately and that created some confusion.However, out of that came same great info and learning. Thanks Enzo, Matec and Roly (nice long detailed answer :tu:)  :tu:

JM...wired up fan like you suggested. Voltage across fan--4.11V, voltage across 180 ohm res. 13.86, so 77ma through resistor, Total Voltage approx.18volts.  I tried measuring the fan resistance but I'm getting a really large number--177k--so I must be doing something wrong.

Quoteastonishing discovery that "90% of the cooling effect comes in the first 10% of the rev range

". Cool fact Roly, thanks, I wouldn't have know that.

Turns out the manufacturer has decided that the fan is not needed and the heatsink will be enough....guess that stuff happens all the time...

Thanks Matec.So when an EL84 fails is it possible that it's filament will still be intact and feed voltage to the other series connected output tubes? 

From what I understand the EL84 output tube filaments in a classic 30 are wired in series so if one tube malfunctions then that stops the voltage going to the other tube filaments so the other tubes will also stop working. If that is true is there a way to wire the filaments in parallel? Or is that something we wouldn't want? Was the output section designed that way for a good reason?  Thanks.

Roly, thanks for your response. I would have gotten back to you sooner but your post pointed me in many directions so I've been reading all about Bang-bang controller, hysteresis, control theory, Schmitt Trigger, and Driver LM3914. Great info.

QuoteAround 12V we can consider the fan to be a resistor of R = E/I ohms, 12/0.15 = 80 ohms.
{Long rambling aside:
but we need to remember that if it's one of the almost universal "brushless" type then it's actually a tiny switch-mode controller in the hub that tries to run the fan at a fixed speed over a wide range of voltages, and may not behave like a resistor at all by the time you get to say 6 or 18 volts.

At first glance that made sense 18-6=12V but then I tried ohm's law to calculate the voltage drop across R108 and did not find 6 volts theoretically but instead found .15AX150 Ohms=22v. Isn't R108 connected in series with the fan, therefore .15A?  Is .15A a max. rating, or will the fan work at a much lower current/voltage. Or, as Enzo pointed out, "a 9V battery would probably spin a good fan." So I applied 9V and the fan spun, so it does work at a lower voltage/current. What am I missing in terms of ohm's law? Please see attachment. Thanks.

QuoteAre there part numbers silk screened on the board?  Like R102, C216, etc?  Look at anything on the heat sink, do you see RT101?  The sensor needs to be on the heat sink somewhere.  In my head I see one of those little blobs with a mounting tab, a screw hole through the tab.  Look for a small thing with two thin wire leads screwed to the heat sink.  Other ones may look like a diode and could be in a hole in the heatsink or maybe just leaning on it in a blob of white goo.  Yet others look like a small transistor with two legs.

Enzo, I've attached an image. Between the two output transistors on right is the sensor. I removed the back panel. If you look at it from the circuit board end you see two little wires going to the heat  sink. The sensor is black (you can't see it from the image and I put this back together so I can't re-do the pic).

QuoteStop thinking of the transistors as putting a voltage on the other side of the motor.  Think of them as a switch that completes the circuit through the motor

.
Thank-you, I will.

QuoteIf the transistors are off, and thus the motor as well, then the transistors would not conduct.  Zero current.  Use Ohm's Law to calculate the voltage drop across the resistor at zero current.  Zero volts.  So when the motor is off, then the full 18v should appear at those collectors

. My bad, I didn't have the fan plugged in when measuring. So the full voltage is at the collectors. Thanks Enzo for taking the time to delve into this. As always, I've learned something new!

Thanks Roly I see what you mean by a proportional controller. Your description makes sense. But what I'm trying to wrap my head around now is this: the fan is rated at 12V/ .15 A. The -ve is 18. Do we use ohm's law to calculate  the voltage drop across  R108 to see that we will get -12 ve to the other side of the resistor? And how does this tie into the fan working once D102 is forward biased and the transistors turn on and provide a voltage to the other side of the fan (should that be 12+ve?).

From what I get from your response is that, at resting, with no overheating issues, we will find zero voltage on Q101 and Q102 collectors and bases and zero voltage on the cathode of D102 (measured found no voltages so hopefully that is correct).... Thanks. (By the way this unit is operational)

Hope this is okay to post as this is an audio amplifier rather than a guitar amplifier, but same concepts. I've attached a schematic (see bottom of schematic)

Trying to understand how the thermostat works. It is supposed to turn on the fan when the fan gets too hot. The fan is rated at 12V but the voltage on one side of the fan at R108 is 18v (won't that fry the 12v fan?).
What does this thermostat look like? Is it a thermistor?  I find no voltages on Q101,102. Is it safe to assume that there is no voltage as the thermistor is an open circuit until  the temp gets too high and therefore no voltage on transistors? Hmmmm...
Thanks!