According to the datasheets, they both have identical maximum ratings with the exception of the EL having a 3 watt lower dissipation figure. Try again. Bob Pletka of Eurotubes said: I have said many times in print that I personally am NOT an electronic design engineer or tech by trade, Click to expand Yet you continue to try and sound like a tech in order to increase sales. Bob Pletka of Eurotubes said: however having a decent working knowledge of how and why tube amps work is not rocket science.
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Use shielded wiring from the input jacks to the volume control and if more than an inch or so from the volume control to the tubes. Ground only one end of the shielded wiring. If signal and power wires must cross, separate them as much as possible and do so at right angles.
Tightly twist AC carrying wires together such as to tube filaments. Use terminal strips, lug strips or other methods of fastening components in place so they will not shift over time and possibly short out. Be sure to observe the color codes on the transformers.
If you get the output transformer wiring wrong blue vs brown the amplifier will have positive feedback instead of negative feedback and will not perform properly. Use quality components. Cheap parts, especially the coupling capacitors will seriously degrade the performance of the amplifier. The Russian K40Y9 paper-in-oil capacitors work well as the coupling capacitors in this amplifier.
I used audio grade capacitors for bypassing the filter capacitors. You do not need to use the brands specified on the schematic, I left them there to show what I had used.
Do not use carbon composition resistors. Use either metal or carbon film ones. One half watt size is sufficient except for the ohm one in the power supply.
It should be a 2 watt or larger and can be a wire wound type. Filter capacitors may be larger, but avoid using smaller ones. I recommend volt rating on the capacitors as the power supply can deliver slightly over volts with no load at VAC input.
If your AC mains are above volts the value will be correspondingly larger. Be careful using the LR8 IC regulators. They will fail if you wire them wrong, short them out or abruptly apply a different load. If they fail, they usually will act just like a low value resistor and pass all the voltage to the load. They also have internal sensors for temperature and excessive dissipation. If they sense either condition they will shut down and not restart until you remove the power and correct the condition.
The first time this happened to me, I thought the regulator had failed. The "K" series LR8 regulators are the most robust, but the "N" series that look like TO transistors may be easier to use.
Be careful using the LM voltage regulator IC. They are robust, but can be cooked. If you wire them wrong they will not act as constant current sources CCS and the amplifier will not work.
In amplifiers like this one, be sure that no part of the heater circuit is connected directly to the ground. It will defeat the reference voltage about 70 - 80 VDC that is applied to the heater circuit. The reference voltage protects the driver tube from heater to cathode failure and helps reduce the overall noise level in the amplifier. Place the driver tube away from the power transformer.
Space the tubes at least one diameter of the tube apart for good heat dissipation. Check your wiring before you first apply power to the amplifier. Figure 1: Mini Block Push-Pull EL84 6BQ5 Valve Amp Schematics Warning: This project uses potentially lethal voltages and should not be undertaken by anyone who is not familiar with working with such voltages or may not be comfortable with projects that entail such voltages.
The inside layout is not critical. I used a perfboard for the power supply circuit and point-to-point wiring for the remainder of the circuit. I mounted the LM voltage regulator IC directly to the chassis with an insulator. I used a three pin ribbon connector to connect the LM to the circuitry. The grounding system I used was a main signal ground buss running down the length of the chassis. The actual buss was made from a strip of self adhesive copper tape that was used in window breakage alarm systems.
The power supply board had buss wires down each edge that tied together in the center of the board. From there I ran a single wire to the junction of the signal buss and the input and output terminals.
I have found that this type of arrangement works well to prevent ground loops. The chassis is connected to the same ground location by a 0. For more detailed information about grounding, see my suggestions about grounding and shielding for your DIY audio projects. The level control at the amplifier input can be deleted and replaced with a k resistor to the ground if you are going to use them as mono blocks. With the level control in place the amplifiers can be used as stand alone monaural amplifiers.
The cases are made out of bamboo kitchen bins with one end cut off. The bottom of the bins slide out and make it easy to access the interior of the chassis. For additional valve amplifier design and construction tips, see my blog entry about design and construction tips and suggestions for vacuum tube amplifiers.
Also, I have also posted some suggestions for a tube amplifier wiring color code. It uses a constant current source to establish tube operation instead of a more conventional cathode bias resistor or fixed bias arrangement.
The use of the CCS forces the amplifier to always stay in class-A mode. To ensure that both output tubes share equally in the power output a balancing control is placed in the cathode circuit. It is important to proper operation of the amplifier that the tubes be balanced at idle. This should be done on initial power up and checked after a few minutes to allow for initial tube variances. After that it should be checked occasionally and reset as needed. The reading ought to be about 41 millivolts mV on each set of test points 82 mV total of both sets.
If you can not obtain satisfactory balance, check for wiring errors. If it follows the tube, then one or the other is out of tolerance and needs to be replaced.
I find that with well matched tubes the deviation is quite small even after many months of use. Above that level it is possible to have the tube passing the greater current to approach maximum thermal dissipation and in extreme cases fail. When you do the balance measurements you are actually measuring millivolts across the one ohm resistors. This translates directly into the number of milliamperes that particular tube is passing. I highly recommend matched pairs of output tubes from quality manufacturers.
Others may work, but performance can not be assumed to be good. If the tubes balance and you get no sound then you need to go through the wiring to find the problem. In the event the amplifier seems to have no bass or sounds excessively detailed you probably have the output transformer wired incorrectly. Temporarily disconnect the negative feedback wire and see what happens. If the sound level goes up then it was correct, if it goes down then it is backwards.
You will then need to swap the anode and screen leads from the transformer from one tube to the other. If there is excess hum you will first need to determine the type. Is it at the mains frequency or a multiple of it? If it is at the mains frequency it is either due to lack of shielding on the input signal wiring, or a ground loop. If it is at a multiple of the mains frequency then it is power supply related and most often indicates a problem with a filter capacitor.
Power Tube Differences: EL84, EL34, & 6L6
Only the machines are the same, and in the Tesla days they were new, and now they are old. The used materials definitely are not the same. Also wires, and nickel technology came from Tesla plants that are all closed now. They are regarded something different, which is correct. There was enough NOS around. Many people thought this situation would always stay that way, and the TESLA management thought so too.
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Use shielded wiring from the input jacks to the volume control and if more than an inch or so from the volume control to the tubes. Ground only one end of the shielded wiring. If signal and power wires must cross, separate them as much as possible and do so at right angles. Tightly twist AC carrying wires together such as to tube filaments. Use terminal strips, lug strips or other methods of fastening components in place so they will not shift over time and possibly short out. Be sure to observe the color codes on the transformers.
Vacuum Tube - EL84, JJ Electronics
New Matched Quad (4) JJ EL84 / 6BQ5 Vacuum Tubes