Crate Vintage Club 50 Fixed/Cathode Bias Mod help

[sixstringslinger]

Hi, I recently had a vc5212 given to me by a friend of mine that got fed up with it after it stopped working. One of the tubes went out and blew the 1/4A fuse. I retubed it and changed the fuse and it seems to be working fine. Anyway, after acquiring schematics for the VC30 & VC50 I compared the two to come to find out that the only differences is the VC50 has a higher voltage PT and a couple resistors, capacitors and jumpers for the VC30 (cathode)/VC50 (non-adjustible fixed) bias supplies. All other values and configurations are identical. They're even made from the same board with the empty spaces for the bias supply components labeled. I'm only a novice when it comes to working on tube amps, but I know the basics and can follow a schematic. I was wondering if anyone could help me figure out how to possibly convert my VC50 to a switchable Cathode/Fixed bias as well as to the proper value resistor & pot to covert it to an adjustable fixed bias? Being that the voltages from the PT are different, I don't think I could just rewire the bias supply to the same values as the VC30 to convert it to Cathode Bias. Any help is much appreciated.

[techineer]

The first question that comes to mind is "why would you want to do that?"

However, if you really do want to, here are a few thoughts:

First, a disclaimer: While the information here is believed to be accurate, I accept no responsibility for its accuracy nor any actions you may take or any modifications you perform. Such actions are completely at your own risk, and you should be aware that such modifications could expose you to potentially deadly voltages even after power is disconnected from the unit. Be sure to discharge the filter capacitors after disconnecting power from the unit before working on it.

That said, in order to determine a value for the cathode resistor, you need to know the expected voltage drop across it and the current through it, as well as its power dissipation.

I would expect the voltage across the cathode resistor to be approximately the same as the grid-to-cathode bias that is present for proper operation in fixed bias mode. So measure the grid voltage with respect to ground at idle and use that for a starting value. The schematic shows the voltage at TP17 to be -15.6 volts, so I would expect this to be close to the measured value at the grids.

The cathode current will be approximately equal to the sum of the plate current and the screen currents. The screen current is simple... measure the voltage drop across each screen resistor. (Measure from one end to the other.) They should be about equal. (If not, you probably have a bad tube.) Calculate the current through the resistor by: voltage drop (volts) / Resistance (ohms). The answer is in amperes, and should be a decimal such as 0.05A. Multiply that x4 to get total screen current.

Next to find the plate current: Disconnect power and discharge the filter capacitors. Disconnect the center tap of the output transformer from J16 and connect a milliameter between the center tap lead and the J16 tab it was connected to. (Bridge the break with the milliammeter leads.) Set the meter up to measure milliamps on a high range. Be sure to arrange all leads safely (this is a HIGH VOLTAGE circuit, and your meter probes will have ~400 volts on them) and turn on the power. Adjust the meter range to get the best reading and record it. If the reading is in milliamps, divide by 1000 to convert that to amperes. This is the total plate current for all the tubes.
Shut off the power and discharge the filter caps before removing the meter leads.

To find cathode current, add the total plate current to the total screen current. Leave the result in amperes.

The cathode resistor value in ohms is found by: bias voltage divided by cathode current in amperes. Use this value as a first test. You may wish to experiment with different values around this one for better results.

The cathode resistor power rating must be greater than the idle current power dissipation. Multiply the cathode resistor bias voltage times the cathode current to get the idle power dissipation. Select a resistor with at least twice this power dissipation rating. I would expect it to be 10-15 watts.

Add a bypass capacitor in parallel with it such as C29 in the VC30. At least 220 uF, maybe more.

You will need to add a DPDT (double pole double throw) switch connected so that the fixed bias position makes the same connections as the amplifier currently has, and the other position adds the cathode resistor and switches out the bias supply and grounds the resistor side of jumper J02.

To make the fixed bias adjustable, I would probably change R78 to a series combination of a 10K resistor and a 10K trimpot with center tap connected to one end. Recalculate power dissipations of these resistors as well as R77, as adjusting the pot may cause R77 to overheat. Be sure to use resistors with power ratings at least twice the maximum possible value.

Have fun, but BE CAREFUL!

[sixstringslinger]

Thanks techineer. That's exactly the kind of help I was hoping for. Where's the fun in having someone just give you the answer when it's so much more satisfying figuring it out yourself. To answer your question, I've always liked the idea of have more versatility of having one amp with both Cathode and Fixed Bias options. I play at church as well as gig and home recording. Fixed bias is nice when you need that loud aggressive response, and the Cathode bias is nice when you need it a little quieter, more touch sensitive and are able to push the power amp at lower volume. In regards to wiring a switch to toggle between bias modes, would it be necessary to use a 3PDT to disconnect R77 , the b- connection to J01 as well as connecting the cathode resistor & bypass cap from J03 to ground when in Cathode bias mode? I was wondering if the voltage drop across R78 while J01 is disconnected from the B- supply and connected to ground would cause any problems or affect the sound of the amp while in cathode bias? Does it matter if I leave R77 connected or use the switch to completly disconnect the fixed bias circuit from the rectifier while in cathode bias mode? I don't think it matters, but then again I'm just guessing and would rather find out.

[techineer]

R77 won't be a problem if the switch is properly connected.

Looking at the diagram, the right-end terminal (cathode side) of J03 should be connected to the pole (center terminal) one side of the switch. The bottom end of jumper J01 (TP17) should be connected to the pole of the other side of the switch. With the switch in the Fixed Bias position, the corresponding 'throw' terminals should connect to ground and B-, to replicate the existing circuit.

For the Cathode Bias position, the J01 'throw' terminal should connect to ground, and the J03 'throw' terminal should connect to the resistor/capacitor.

Disconnecting the bias from R77 should in itself have no effect on the sound.... that will be determined by the proper selection of the cathode resistor.

I suggest you install a set of old tubes for a first test, in case of wiring errors.....

[sixstringslinger]

Just to double check, this is how I was going to wire the switch. Forgive me for my 5th Grade MS Paint skills. Would this work as wired?

[techineer]

That was what I was trying to describe. That should work.

However, you should find that the selected terminals on the switch are opposite to the position of the bat handle. Use the ohmmeter to be sure you know which terminals are connected for a given position of the handle.