Cathode Current Sense Resistor Calibration.
If you have installed the tubes, remove them.Even though the diagram shows the leads from the banana jacks going to the terminal strip lugs, they should not be installed yet. Connect them as described below.
Cut two 1 inch lengths of the heavy copper wire you used for the ground buss. Remove the insulation and solder each one into the eyelets, lower hole, in the lugs of the terminal strip where the 1 ohm sense resistors are shown. Refer to the photo below.
If not already done, connect 4 lengths of violet wire from the EL34 sockets, pins 1 and 8, to the terminal strip on that side where the short heavy piece of copper wire is soldered into the eyelet. DO NOT SOLDER THESE WIRES TO THE COPPER WIRE, SOLDER THEM TO THE TERMINAL IN THE USUAL WAY. On the other side of the chassis solder 4 more lengths of violet wire from the other EL34 sockets, pins 1 and 8, to the terminal strip on that side where the short heavy piece of copper wire is soldered into the eyelet. Solder them to the terminal not the copper wire.
Connect a length of black wire from the front end of the ground wire to one of the front most banana jacks. Connect another length of black wire from the banana jack where the black wire is connected to the other front banana jack. Solder all three connections.
Connect a length of violet wire from one of the remaining banana jacks to the short length of copper wire soldered into the eyelet of the terminal strip on that side. Hook the end of the wire around the wire near the lug and solder it. Be sure to leave enough room for 3 resistors to be soldered to the copper wire. In a similar manner connect another length of violet wire from the remaining banana jack to the short copper wire on the other end of the terminal strip. Solder all connections.
If not already done, and it shouldn't be, connect two 1.1 ohm resistors (shown in purple) between the ground wire and the short copper wires soldered into the eyelets of the terminal strip. Bend a hook in the resistor lead, hook it over the copper wire, and squeeze the hook shut. Solder the resistor leads to the copper wires. Leave enough space to solder two more resistors to each short copper wire.
At present the only source of 1.1 ohm resistors I can find is DigiKey. Type resistor 1.1 ohm into their search box.
Now, dust off your transistor power supply. You need one that will deliver 200 mA, or more, and is capable of being adjusted from 5, or less, to 20 volts, or more. You need a 100 ohm 5 watt resistor. If you have to make it up by paralleling or seriesing two or more resistors, do it. The value is not critical to this calibration. Just get it close.
Connect the negative of the power supply to the chassis or the ground wire.
Connect the positive of the power supply to one end of the 100 ohm resistor.
Connect the other end of the 100 ohm resistor to the positive terminal of a meter that will measure 100 to 200 mA accurately.
Connect the negative of the meter to one of the octal tube sockets, pin 8.
Set the meter you are going to use to measure the cathode current to its 200 millivolt range.
Connect this meter to the two banana jacks on the side to which you have connected the transistor power supply and the rest.
Turn on the power supply and bring up the current as read on the current meter connected in series with the 100 ohm resistor, to 150 mA. This is to make sure the 200 mV meter doesn't go out of range.
Read the meter that is set to measure 200 millivolts. If it happens to read 150 you don't need to do anymore. That is highly unlikely so continue.
Now there are two ways to go about this. One is to calculate the value of the next resistor needed. The other is by trial and error and may involve a lot of soldering and unsoldering of resistors. Both of these methods assume you have a stock of 5% ¼ watt resistors.
The Calculation Method.
At this point you pays your money and you takes your choice. We still have all the meters connected up and the current meter is indicating something close to 150 mA, right?Increase the current until the voltmeter is reading somewhere in the 190s of millivolts. Divide the voltmeter reading by the milliammeter reading.
R = V / I
For example. V = 196.1 mV and I = 173.5 mA
R1 = 196.1 / 173.5 = 1.1303
R1 is the 1.1 ohm resistor and we will add another resistor in parallel which we will call R2. Your result will be different.
R2 = 1 / (1 - 1 / R1)
R2 = 1 / ( 1 - 1 / 1.1303) = 8.765
The correct value to use is the next higher 5% value. In this example it is 9.1 ohms.
Permanently solder the resistor value you determined in the above steps to the copper wires.
Allow the resistors to cool thoroughly before taking the readings. Now read the meters again. Example, V = 190.3 mV and I = 188.6 mA
R12 = 190.4 / 188.6 = 1.0095
Where R12 is the parallel combination of R1 and R2.
R3 = 1 / (1 - 1 / R12)
R3 = 1 / (1 - 1 / 1.0095 = 106.26
Use the 5% value that is closest to this value whether it is up or down.
Permanently solder the resistor you determined in the above steps to the copper wires in the same way as the 1.1 ohm resistor was soldered. The two meters will now likely agree within 5 counts in the last digit.
Trial and Error.
We still have all the meters connected up and the current meter is indicating something close to 150 mA, right?The voltmeter will likely read more than 10% higher than 150. Tack solder the leads of a 10 ohm resistor to the copper wires so it is in parallel with the 1.1 ohm resistor.
Make sure the reading on the current meter has not changed. It did when I did this. Check the voltmeter reading again. If it reads low remove the 10 ohm resistor and replace it with an 11 ohm. If it reads high replace the 10 ohm with a 9.1 ohm. Get the reading as close as you can to 150 but high, not low.
Unsolder the resistor you tack soldered and permanently solder it to the copper wires in the same way as the 1.1 ohm resistor was soldered above.
Tack solder a 100 ohm resistor to the copper wires so it is in parallel with the other two resistors. Make sure the reading on the current meter has not changed. if the reading on the voltmeter is high replace the 100 ohm resistor with a 91 ohm. If the reading is low replace it with a 110 ohm resistor. Once you have found the correct resistor permanently solder it in place as above.
There shouldn't be any need to go to a 1000 ohm resistor in parallel. Most people take the readings of a DMM as accurate down to that last digit. If you carefully read the specifications for a DMM you will find that it is only 0.5% on the current ranges. That's plus or minus 1 mA on the 200 mA range. So even though it is telling you 151.3 mA it's only accurate to 1 mA.
Repeat the steps above for the 1.1 ohm resistor on the other side of the amplifier chassis.
Initial Tests.
Let's assume you have gotten through the construction phase without mishap. Before giving it the smoke test it is wise to do a little preliminary resistance testing.Power Supply Resistance Tests.
The best instrument to make these tests would be an analog VOM such as the Simpson 260. Failing that some sort of analog VTVM. Failing that I guess it will have to be a DMM.WARNING!!! The transformers and filter choke used in this project have a large amount of inductance. When the current through an inductor is changed quickly, such as by disconnecting a test lead or changing resistance ranges, a voltage spike of several hundred volts may be generated. These spikes will not bother a VOM or a VTVM but they can do serious damage to a DMM. If a DMM is what you are using, be very careful. I will give cautionary notes as I go along.Primary Testing.
In the paragraphs below a result preceded by a # sign will be from a VOM or VTVM and one preceded by a $ sign will be from a DMM.With the fuse removed and the switch turned off measure the resistance from one side of the primary circuit to chassis. The resistance should be infinity.
Install the 4 amp Slow Blow fuse and turn on the power switch. Do not plug the line cord in. Instead measure the resistance from one prong of the line cord to chassis. It should be infinite.
If you are using a DMM make an arrangement of clip leads so you can short out the meter, connect the meter for testing, unshort the meter for the test, and then short it again.If using a DMM, take the precautions above, connect the meter to the two prongs of the line plug, set the meter to its lowest resistance range and unshort it. The resistance should be # 1.1 ohms, $ 1.5 ohms. Remember to reshort the DMM before disconnecting the clips from the prongs of the plug.B+ Test.
Connect the meter negative to B minus and the positive to B+1. You will see a low resistance which will rise as the capacitors charge. The reading on a DMM set to the 200 k ohm range will move up slowly. Changing to a lower range will speed things up some. The resistance reading should go as high as 150 k ohms. Notice that in the first stage of the filter there are two 100 microfarad capacitors in series. There are two sets of seriesed capacitors. With your ohmmeter negative still connected to B minus connect the positive lead to the positive terminal of each capacitor in turn. The reading should settle at about 115 k ohms on each capacitor. Note: Because of normal capacitor leakage the reading may not go this high. If the reading is a short or is less than 10 k ohms you must correct it before turning on the power.Similarly connect the positive of the meter to B+2. The meter should settle at around 150 k ohms.
Similarly test B+3. Same result.
Bias Supply.
Connect either lead of the meter to chassis and the other one to C+. The reading should be 100 k ohms.Connect the positive side of the meter to C+ and the negative side to C minus. The reading should be in excess of 100 k ohms or even as much as ten times higher.
Heater Winding.
Connect either lead of the meter to either side of the heater and the other to chassis. The reading should be 10 meg ohms.Disconnect the meter lead from the chassis and connect it to the other side of the heater winding. The reading should be zero.
Power Supply Voltage Tests.
Set your meter to measure 1000 volts DC. Connect the negative side of the meter to B minus and the positive side to B+1. Turn the power switch off, plug in the power cord and turn the switch on. The meter should read approximately 600 volts. Turn the power switch off. Note that the voltage decays slowly. Do not touch the innards of the supply until the voltage has fallen below 50 volts.Move the positive lead to B+2. Turn the switch back on. The meter should read approximately 400 volts. It may go a little higher but as long as it doesn't go past 450 volts you are OK. The zener diodes should feel warm or maybe even hot. Turn the switch off and follow the above precaution.
Move the positive lead of the meter to B+3. Turn the switch on. The voltage should be about 400 volts but it must rise to this value slowly. Turn the power switch off.
Connect the positive lead of the meter to C+ and the negative lead to C minus. Turn the power back on and change the meter range switch. The voltage should read about 100 or a little higher volts. Turn off the power switch.
Amplifier Tests.
If you haven't done so make the cable with a male octal plug on one end and the female octal socket on the other. You need 8 conductor cable and two of them should be number 14. If you can get 10 conductor number 16 you can parallel two sets for the heater. Two number 16s in parallel make a number 14. If you can get 14 conductor number 18 you can parallel 4 for each side of the heater circuit. Four number 18s in parallel make a number 14. I have some Ham-M rotor cable I'm going to use. But it didn't work.The limitation is not just the wire but the pins on the octal plugs and the contacts in the octal sockets. They are just not rated for 15 amps. What I finally ended up doing was to mount a two terminal barrier strip on the power supply and amplifier chassis. On the power supply chassis I cut the heavy green wires from the filament transformer off at the octal socket end and routed them through small holes drilled next to the barrier strip. I stripped and tinned the ends and wrapped them around the screws. You can tell I placed the barrier strip between the octal socket and the filament transformer.
At the amplifier end I placed the barrier strip next to the power plug and unsoldered the filament wires from the octal plug. I ran short pieces of solid number 14 wire from the screws on the barrier strip through small holes in the chassis and soldered the filament wires to the number 14 wires. Then I ran a length of two conductor number 14 wire from the barrier strip on the power supply to the one on the amplifier chassis.
I tried to get two conductor number 14 stranded at Lowe's but the guy cut me three conductor number 14 solid. Oh well, I used it anyway. Someday I will find some 2 conductor number 14 stranded and replace it. Right now it's working fine. In fact it warms up faster than before.
If you use stranded wire you must tin the ends, that is melt solder on the wires to hold the strands together where the wires go under the screws on the barrier strip. Form the wires into a loop to go around the screw before applying the solder.
Amplifier Resistance Tests.
Do not put any tubes in the amplifier chassis. Do not connect the amplifier to the power supply. Do not pass "Go" do not collect 200 dollars.Set the meter to measure resistance. Connect the negative lead of the meter to the chassis. Connect the positive lead to B+1. The meter should indicate infinity. It should also measure infinity on B+3. On B+2 it should indicate approximately 380 k ohms.
Connect the positive lead of the meter to each side of the heater circuit in turn. Each one should indicate about 4.7 k ohms. Measure between the two heater connections. The reading should be approximately 300 ohms.
Move the meter's positive lead to the chassis and the negative lead to C-. The meter should indicate about 6.1 k ohms.
Amplifier Hot Tests.
Don't put the tubes in yet. Make sure the power switch is turned off and connect the octal cable between the power supply and the amplifier chassis. Turn on the power and measure the voltage between the chassis and B+1, B+2, B+3, and C-. The voltages should be 600, 400, 400, and -50 respectively.Set your meter to negative voltage or connect the positive lead to the chassis. Connect the other lead to the center terminal of one of the bias pots. Rotate the pot from one end to the other. The voltage should vary between -30 and -50 volts. Fully counter clockwise should give -50 volts. Test the other bias pot in the same way. Leave both pots set fully counter clockwise, -50 volts.
Final Hot Tests.
If you have a scope, plug in only the first 12AX7 in each channel. Use a Y wire to connect your oscillator or function generator to both inputs. Set both volume controls fully clockwise and the sleep switch to normal if you installed it. Turn on the power. Test for signal at the plate and cathode of the phase splitter in each channel. The voltage should be about 20 times the input voltage.Turn the power off and plug in the second 12AX7 in each channel. Turn the power on. Test for output on each plate of each second 12AX7. It should be about 200 times larger than the input signal and be slightly effected by the balance control.
Turn off the power and plug in the 6CG7. Turn it back on and test for signal at each cathode of each channel of the 6CG7.
Turn off the power and plug in all the EL34s. Connect two 8 ohm 50 watt resistors to the speaker terminals or if you don't have such resistors connect two 8 ohm speakers. Connect your DMM to the two banana jacks on one side and set it to measure 2 volts. If you have two DMMs connect the other one to the other two jacks. Also set it to the 2 volt range.
Set both AC Balance controls to the center of their range.
Turn on the power and watch the tubes light up. Both meters should read much less than 0.2 volts. adjust the bias pots to bring the meter readings up to 0.2 volts. There will be some interaction caused by loading of the B+ voltage.
Turn off the power and connect the inputs to your preamp. Turn the level controls fully counter clockwise, if you installed it turn the sleep switch to normal. Turn the power back on, and set your preamp to a normal level. Slowly bring up the input level controls until you hear sound. If it works, congratulations.
AC Balance Adjustment.
If you have distortion measuring equipment connect it up to the amplifier along with 8 ohm 50 watt resistors. Apply a 1000 cycle sine wave and adjust the power for approximately 1 watt, about 2.8 volts across the 8 ohm load. Measure the distortion and adjust the AC Balance pot for minimum distortion. Repeat for the other channel.If you don't have distortion measuring equipment set your DMM for AC volts and your audio generator to 1000 cycles. Connect the DMM between chassis and grid of one of the EL34s. Move it to one that is on the other side of the push-pull pair. The two in front are on one side and the two in back are on the other side. Do not use two DMMs unless you can verify that when they are both connected to the same voltage source they give the same reading. Adjust the AC balance control for equal voltages on both sides. Repeat for the other channel.
The End.
I'm getting sleepy.
Testing the Amplifier 7 years later. May, 2013.
I brought the amplifier to my workbench and didn't touch any of the adjustments. I'm going to run it through a full testing procedure to see how it compares with the original spects.The idling current is down to 135 mA on the right side and 170 mA on the left.
The value of B+1 is 531 volts.
This is to be expected as the ESR (effective series resistance) of filter capacitors increases with age. The original output for a 300 mA load was 551 volts. Power output just shy of clipping is 66 watts on the left and 60 watts on the right, one channel driven.Distortion at various powers and 1000 cycles is listed in the table below.
Power (w) Distortion (%)
LeftDistortion (%)
Right50 0.320 0.80 25 0.300 0.60 12 0.286 0.46 6 0.280 0.362 3 0.280 0.31 1 0.280 0.28 OK. Let's give it a tune-up and see what changes.
I would say just right off that an amplifier of this kind needs to be tuned up more often than every 7 years. Maybe yearly?
Power (w) Distortion (%)
LeftDistortion (%)
Right50 0.295 0.420 25 0.220 0.225 12 0.140 0.115 6 0.090 0.060 3 0.054 0.037 1 0.032 0.027 The amplifier I have temporarily replaced it with has a THD of 0.04% and 0.05% at 1 watt. My immediate impression was that the replacement was better. I guess I can detect the difference between 0.28% and 0.05%.
Introduction. Power Supply Schematic and Circuit Description. Amplifier Schematic and Circuit Description. Power supply chassis layout. Amplifier chassis layout. Initial tests and adjustments. (You are here.)
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This page last updated April 3, 2006.