# Amplifier Conversion Guide - 6L6GC to EL34 Conversion

This guide has been built in place of our K-701 conversion kit which was previously available for purchase. Now, you can simply purchase the required parts below and follow through this guide.

This guide will show you how to convert a standard class A/B dual 6L6GC power amp to a dual EL34 power amp. Changing the screen grid resistors along with altering the bias circuit is required for this modification. Following this guide, you will learn:

• How to convert a power amp using two 6L6GCs into one using two EL34s
• How to convert the Silver Face bias/balance pot into a Black Face style bias pot for increased control of power tube operation
• How to set the bias for 6L6GCs or EL34s

## Required Parts

DescriptionSKUItem IDQtyPriceSubtotal
Resistors - 5 Watt, Wirewound, PowerR-Q1K2$1.14$2.28
Terminal Strip - 2 Lug, 2nd Lug Common, HorizontalP-0201H3$3.45$10.35
Resistors - 0.5 Watt, Carbon FilmR-A10K1$1.32$1.32
Resistors - 0.5 Watt, Carbon FilmR-A1D5K2$1.32$2.64
Vacuum Tube - EL34, JJ ElectronicsT-EL34-JJ-MP1$32.90$32.90

Warning: The adjustments and processes described in this article require interacting with dangerous levels of voltage. Even unplugged, an amplifier can retain dangerous levels of charge. We encourage you to take extreme precautions when performing these adjustments or interacting with your amplifier. When in doubt, seek the help of a trained professional.

## Required Tools

ToolPurposeWe Recommend
Multi-meterA meter for measuring voltage, current, resistance, and sometimes capacitanceS-Z3220
Soldering IronSoldering iron with a 1/8" screwdriver tip and a variable control capable of producing 25-40Watts. This will allow you to do some precise soldering of circuit components and wires (do not use a soldering gun for soldering of electronic circuits).S-TWE1010
Solder60/40 rosin core solder.S-T9000X
Desoldering pump, bold, or braidThis will assist you in removing solder for circuit modifications and correcting connection errors.S-T7874B
Wire CuttersThese are great for cutting leads on resistors, capacitors, etc. before and after soldering.S-T170M
Needlenose Pliers6" long are good for bending component leads and holding components leads while de-soldering.S-TNN7776

## Safety

We will take a moment to again stress the importance of putting safety first when working on this type of equipment. Tube amps operate at high voltages which have the potential to kill. Only work on an amp when you are wide awake and attentive.

• Turn the power switch off
• Turn the standby switch off (down position)
• Unplug the power cord
• Give the power tubes 10 minutes to cool down

When the amp is open:

• Use a multi-meter to measure for high voltage at several test points. (see the "Preliminaries" section for details)
• Use the one-hand rule. (see the "Preliminaries" section for details)

## Soldering Tips

• Work in a ventilated area with a fan to blow the smoke away from your face.
• Allow the soldering iron to heat up to the point where the solder melts quickly when touched to the iron's tip.
• Clean the soldering iron's tip by wiping it across a wet sponge before applying solder to it.
• Be very careful not to unintentionally burn any wires in the vicinity of the soldering iron.
1. Bend the component lead or wire ending and wrap it around the connection point.
• Make sure it is not too close to a neighboring component which could cause an unintended connection.
2. Wrap the component lead so that it can hold itself to the connection point.
3. Touch the soldering iron to both the component lead and the connection point allowing both to warm up just before applying the solder to them.
4. Be sure to adequately cover both component lead and connection point with melted solder.
• Remove the soldering iron from your work and allow the solder joint to cool. (The solder joint should be shiny and smooth after solidifying.)
• Cut off any excess wire or component leads with cutting pliers.
• Clean the soldering iron's tip by wiping it across the wet sponge again after making the solder joint.

## De-Soldering Tips

1. Heat up the old solder joint with the soldering iron
2. Apply fresh solder to mix in with old solder joint
3. Use a de-soldering tool to remove the old solder joint while it is heated.

## Preliminaries

• Turn the power switch off
• Turn the standby switch off (down position)
• Unplug the power cord
• Give the power tubes 10 minutes to cool down
• Make sure the power tubes have cooled down enough to touch.
• Remove the power tubes from the amp and store them someplace safe.
• Remove the amplifier chassis from its cabinet and set it on a safe workplace.

When the amp is open, follow the one hand rule and use a multi-meter to measure for high voltage at several test points. (see test points below)

The one hand rule (pictured above) is a safety precaution for working on an amp that is plugged in or could potentially have high voltages present. Using your multi-meter, clip the ground side to the chassis and use the other side to probe at various test points with one hand. This prevents a fatal shock which can result from current passing through the heart. (Many people even put their other hand in their pocket or behind their back to ensure it is not used).

## Test Points

#### Inside view of amp chassis - not all components are shown

Warning: If there is a high voltage present at any of these test points, there may be something wrong with your amp. Please have it checked by a qualified repair shop.

• TP(1), TP(2): These test points are both on the standby switch. When the amp is in operation, these two points have a very high voltage present. By 10 minutes after turning off the standby and power switches, all the voltage should have been drained.
• TP(3): This is the anode side of a solid state diode.
• TP(4), TP(5): These test points are the cathode pins of the preamp tubes. It is not a bad idea to check these two pins on each one of the preamp tubes.
• (Tube Rectifier TP: If your amp has a GZ34/5AR4 tube and no standby switch, then check the GZ34 pins 2 and 8 for voltage.)

## Bias Potentiometer Conversion

If you have a silver face amp, you might have to convert the bias/balance pot into a black face style bias potentiometer for increased control of power tube operation.

Exhibit A: If the bias pot in your amp looks like this, then follow the steps below to convert it.

• Step 1: De-solder the end of the resister mounted to the wiper (middle) lug of the bias pot and leave it hanging temporarily.
• Step 2: De-solder both wires going to the outside lugs of the bias pot. Solder both wires to the wiper lug of the bias pot.
• Step 3: Solder the disconnected end of the resistor to an outside lug of the bias pot.
• Step 4: De-solder the wire connected to the center-tap lug of the bias pot. Solder that wire to the other outside lug of the bias pot.

Determine your hot and cold bias directions:

• Set your multi-meter to measure resistance
• Clip one lead of the meter to the wiper lug of the bias pot and the other lead to the outer lug with the wire connected.
• Turn the bias pot control all the way in the direction of 0 Ω and leave it there.

#### Connection points for determining hot and cold bias

Cold = the direction of 0 Ω between these two points

Hot = the direction of 10 kΩ between these two points

## Power Tube Conversion

Here is a schematic representation of this mod. It is not necessary to be able to read the schematic in order complete the mod.

#### Before Modification

Because the EL34 draws more filament current than the 6L6GC, it is not recommended to do this mod on amps with more than two power tubes.

#### After Modification

Depending on your amp model, you may not need to make all of these modifications and some of the components may be located in different places. Pay attention mainly to the connections.

• Step 1: De-solder and remove the 470Ω resistor.
• Step 2: De-solder and remove the wire and resistor connections at pin 1.
• Step 3: Solder a connection between pins 1 and 8 with a small piece of wire.
• Step 4: Fasten a terminal strip to the mounting screw of the tube socket. (Alternate mounting method for screw also shown)
• Step 5: Solder and connect the wire and resistor to the ungrounded lug of the terminal strip.
• Step 6: Solder and connect the 1kΩ (5W) resistor between pins 4 and 6.
• Step 7: Fasten a terminal strip to a grounded screw in the circuit board.
• Step 8: Solder and connect the 10kΩ resistor to the terminal strip.
• Step 9: Solder and connect a wire from the bias pot (outer lug with the resistor connected) to the ungrounded lug of the terminal strip.
• Step 10: Set the bias all the way to cold. (See instructions above to determine your hot and cold bias)

## Setting the Bias

You should re-bias your power tubes after changing them. It is a good idea to begin with the bias pot set all the way to cold (i.e. with maximum negative voltage applied to the power tube control grids) to avoid damaging the tube from too hot of a bias setting.

Make sure to get matched power tubes. This way the current measured from both power tubes should be close to the same value. We recommend Apex Matched Tubes. See also our article on biasing a tube amplifier.

• Step 1: De-solder the ground wire connection from pin 8.
• Step 2: Set your multi-meter to measure DC current. Clip one lead of the meter to pin 8 and the other lead to the ground wire.
• Step 3: Insert the EL34s and keep the one hand rule in mind. (With clips in use, you should be able to just use one hand for turning the bias control.)
• Step 4: Plug the amp in and turn the power switch on. Let the tubes heat up for about 2 minutes before turning the standby switch on.
• Step 5: Turn the standby switch on (upward position).
• Step 6: Slowly turn the bias pot towards hot until you get a reading of about 35mA (ideal bias point for 6L6GC).
• Step 7: Use your ears to determine the best sounding bias point for the EL34s (e.g. 42mA), but do not exceed 70mA. If you see the outer tube plates begin to glow, it probably means you have the tube biased too hot.
• Step 8: Unplug the amp and turn off both power and standby switches. Allow the tubes to cool.
• Step 9: Re-solder and connect the ground wire to pin 8. Close the amp back up and you are done.