Steve's DIY Guitar Amp Pages

New! Valve Junior Modifications eBook

$19.95 .PDF file, 11 megs	A complete guide to the basic mods for a Valve Junior head. This guide covers most of the most basic mods to get your Valve Junior sounding its best. Covered are: Preamp Mods Power Amp Mods Power Supply Mods Biasing Replacing the output transformer Replacing the input and output jacks Tools And More! Many full-color photos illustrate the mods. This book is especially for beginners.
To order click on the button below: You will be taken to PayPal to complete the transaction. PayPal will send me an email and I will email you the URL with customized login and password to download the file.

Valve Junior Tone Control

Here is a simple tone control based for the Valve Junior on the Tweed Princeton's tone control:

Note that the stock volume control has been replaced with a full-size pot with solder lugs. The connector that was plugged into the stock volume control's PCB has been cut off. The wires are soldered to the to the pot's solder lugs.

Valve Junior Rebuild: The Voodoo Baby

I took on this project in response to a post on The Gear Page. One of the members wanted help in rebuilding his Valve Junior amp. He had tried several mods, and had drilled several holes in the face plate. I responded that I would help him rebuild it. We agreed that I would:

• Rebuild the circuit using high quality components
• Add a standby switch
• Add a gain boost switch
• Add a tone control

Once I started ordering parts, I talked him into letting me design a face plate as there were several unnattrative holes drilled in the front. He supplied the artwork and the concept of the "Voodoo Baby." I ordered the face plate from BNP Lasers, and couldn't be happier with their work. I highly recommend them. The face plate came out great, and was inexpensive.

Note: using acetone to clean the chassis will remove the writing. Luckily, I only erased the writing on the front of the chassis which was eventually covered by the new face plate.

Parts used:

• A mixture of carbon comp and metal oxide resistors
• Orange drop coupling capacitors
• High Grade F&T power supply capacitors
• A new eyelet board to house the circuit
• Heyboer output transformer

For the circuit, I used the "Gold Mods" for the Valve Junior as listed on the 18watt.com forum.

The amp came out great. I'm still testing/tweaking the circuit. Initially, I left the Heyboer's leads their original length when I soldered it into the circuit. The amp oscillated until I cut the leads to fit.

	This photo shows the completed amp from the front. I designed the face plate with CorelDraw 9 using supplied artwork. I sent the CorelDraw file to BNP Lasers to engrave the face plate in black plastic, etched to white. I like the contrast between the black face plate and the white knobs.
	This is the amp from the back. It looks almost stock, with the exception of the new Switchcraft jacks. They are much sturdier than the knock-off Cliff style, PCB-mounted jacks that come with the stock amp.
	I got the board from Ken at Turretboards.com. The board is well made, and and is .125" thick. I normally use .063" boards for my scratch builds. The tube sockets are Beltron. The board is a little cramped, but I got all the parts to fit.
	This photo shows the stock power transformer to the left and the Heyboer output transformer to the right. The Heyboer is huge, and fairly close to the tubes.
	This photo shows the bottom of the chassis from the back.
	Notice how close the tubes are to the Heyboer output transformer. I purchased the transformer from Dobermanamps.com. It is a particularly beefy unit. According to Doberman, it is capable of 160mA DC through the core.
	This photo shows the underside of the chassis. The eyelet board has been wired into the amplifier.
	This shows the wiring looking at the front of the amp.
	Photo of final wiring. Notice the DC filament board in the middle towards the back. The board consists of a a bridge rectifier and a 10,000µF capacitor. I don't normally use DC filaments, but in this case it was necessary to kill the minor hum.
	Close up of the DC filament board.

One final note: the artwork and faceplate are my intellectual property. Please do not use them without my express permission.

Biasing an Epiphone® Valve Junior

It is a good idea to check the bias of your Valve Junior to make sure it is set right.

The EL84/6BQ5 is rated at 12 watts maximum anode dissipation. Since the Valve Junior is a single ended Class A amp, you will want to set the bias as close to this number as possible.

To check the bias you will need three numbers:

1. the value of the cathode bias resistor, which is R14
2. the cathode voltage as measured from the cathode to ground
3. the anode to cathode voltage

Start by writing down the value of R14, the stock value is 220 ohm.

Caution: the next step requires that the amp is turned on and warmed up. Turn the volume knob all the way down and don't plug anything into the input. You will want to plug in a dummy load or a speaker cabinet to keep the output transformer loaded.

Measure the voltage from the cathode to ground. The cathode is pin three on the EL84's tube socket.

Next, measure the plate to cathode voltage. The easiest way is to put the red lead of your DMM on pin 7 of the EL84's tube socket and put the black lead on pin 3.

Another way is to measure the anode voltage to ground and subtract the cathode voltage. To do this clip the black lead to the grounding bolt where the green wire with yellow strip is bolted down. Then touch the the red lead's tip to pin 7 on the EL84's tube socket. This is also much safer than the first method. With the first method you must put both hands in the live chassis which us much more dangerous. With the second method you can clip your negative lead to the ground and use only one hand to measure the voltages.

The stock schematic shows the anode voltage as 310v and the cathode voltage as 9v.

Using Ohm's Law calculate the dissipation:

First find the cathode current. In Ohm's law, current equals voltage divided by resistance or I=V/R. In the case of the stock schematic divide 9v by 220ohms and you get approximately .041A or 41mA.

To find the dissipation multiply 301v times .041A which equals 12.3 watts. (P=VI)

Checking the bias with this method doesn't take into account screen dissipation. I usually ignore this so if the final dissipation is slightly over 12W it is close enough.

However, if you like to get a more specific number for anode dissipation, you can estimate screen dissipation as 10% of the cathode current. In the case of the stock schematic, the estimate would be .004A or 4mA. Multiplied by 301v you get 1.2W. Subtracted from 12.3W, the total is roughly 11.1W anode dissipation.

Finally, if you want exact numbers you will need to install a screen resistor from the screen supply in the power supply to pin of the EL84's tube socket.

If the dissipation is much more than 12W it is a good idea to increase the value of R14 until the dissipation is closer to 12W. If the dissipation is under 12W, decrease the value of R14.

Unfortunately, in North America the power transformer in the Valve Junior is rated for 115 volts coming out of the wall socket. The voltage coming out of the wall in my home is about 124v. This increases the voltages in the Valve Junior. The result is that the EL84 tube will be over the maximum dissipation of 12W. One fix is to increase the value of R10 from 220 ohms to 1K ohms. Some of the newer Valve Juniors already have R10 as 1K ohms so it is a good idea to check.

Especially for beginners:

The safest way to probe is to only use one hand inside the chassis.

When using your DMM you will need to acquire a retractable hook that fits over the test leads. They come in red and black. Use the black lead and hook it to the bolt where the green wire with the yellow strip is fastened down to the chassis.

Use the red probe to measure the various voltages. Place your other hand in your pocket or behind your back to resist the urge to put it in the chassis.

Measure the cathode voltage on pin 3 on the EL84s tube socket. Measure the B+ voltage on pin 7 of the EL84's tube socket. Subtract the cathode voltage from the B+ voltage to give you the anode to cathode voltage.

Remember to keep your hand off of metal part of the probe. Also, it may go without saying, but don't perform these measurements in bare feet, or on bare concrete, etc.

Thanks to Zaphod_Phil from the 18watt.com forum for his corrections.

EL84 Pinout

Pin 1: internal connection
Pin 2: control grid
Pin 3: cathode
Pin 4: heater/filament
Pin 5: heater/filament
Pin 6: internal connection
Pin 7: anode/plate
Pin 8: internal connection
Pin 9: screen grid

Epiphone® Valve Junior Scratch Build

1. it was noisy due to heater hum
2. the preamp circuit had odd values
3. the anode voltage was way too high
4. the output transformer was mismatched to the EL84 tube

On the positive side, the amp had a sturdy chassis and a well built cabinet. The PCB (printed circuit board) was fairly easy to remove and modify.

Epiphone addressed these problems and released an upgraded combo version of the amp as well as a head version. They used a different PCB which addressed the hum issue. The added a bridge rectifier and large-value capacitor to effectively convert the AC preamp circuit to DC. The head had 4, 8, and 16 ohm output jacks for matching to a variety of speaker cabinets. The combo amp's OT had one 4 ohm tap to match to the 4 ohm speaker.

After reading about mods for the Valve Junior for a few weeks, I decided to purchase one to mod. While waiting on my Valve Junior head to mod, I decided to build one from scratch. It is an easy project and took me about two or three days. I also added a few extras to the circuit.

Below are pictures of my Valve Junior build.

	The circuit fit well into a Bud AC-407 chassis (11" x 7" x 2"). I used the Triad R6-A PT and a Hammond 125ESE OT. The face plate was made of a .125" thick piece of polystyrene plastic which is too thick; a .062" thick faceplate would have worked better. From left to right, the controls are: on/off, standby, pilot light, volume, input.
	The back panel has an adjustable post OT line out, a switchable conjunctive filter, a fuse and a power cord. There is a chassis-mounted snap-in capacitor which is a 150uF, 450v unit. One experiment I plan on trying in the future is hooking up the line out to an FX processor and a high-powered stereo solid state power amp.
	This is an underside shot from the front. The circuit was modified from the stock schematic. I rebiased the power amp, changed some of the values in the preamp, and considerably beefed up the power supply.
	This is an underside shot from the back. While the wiring is not as neat as I would like, it is dead quiet. There is only a minor amount of thermal hiss when the amp is cranked all the way up.
	This shot shows a close-up of the guts. The board is made of Grade XX Garolite from McMaster. The 47uF/450v capacitor on left is in parallel with the 150uF chassis-mounted cap. Notice the black/green twisted wires going to the grids of the tubes: I used 20 gauge, 300v solid wire in black and green twisted together to form a pseudo-shielded lead.

Stock Valve Junior Schematic (redrawn)

Valve Junior Gold Silver Mods by Layboomo

Compare it to the Gibson® Les Paul Jr. schematic:

Stock Gibson® GA-5 Les Paul Jr. Schematic

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