VT4C Single ended audio amplifier
Paul van der Meij Siddeburen - Netherlands
| schema amplifier |
| schema power supply |
| components |
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Stereo VT4C amplifier with power supply |
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Some engineers are fascinated of tube amplifiers. They invest a lot of time in their own design. I am also one of these “audio freaks”. Why buying an expensive commercial amplifier when you can build high end for a little thousand euros???
Note: The power supply in these amplifier produces 1,1 KV There is no such thing as a slight shock of a 1,1 kV power supply IT IS LETHAL!!! I do not accept any responsibility for any damage, being the result of the described circuit diagrams, building instructions, or any other information presented on these web page.
The story; for a few years ago I have built a tube amplifier with 4x KT88- E188CC balance design. In this project I used tutorial UGT’s. The sound was impressive but I did not experience the sound where I am searching for. When I finished the KT88 push pull I started to read about single ended amplifiers specially with VT4C tubes!!!
The VT4C tube was a very old friend from my radio time.
At the internet I found a lot of data about pentode- Mu stage driver designs.
In these I understood the advantages but not the necessity for the use of pentode use, certainly not in a triode-end amplifier!!!
So I used the good old 6sl7 in combination with the 6sn7 tube. As a experiment I have build a test design on a wooden board. After a lot of testing and calculating the amplifier schematic was born .
The performances of this configuration was very sufficient for driving the miller capacity in the VT4C. I tested the driver with a 10K ohm load resistor and 1 uF output capacitor. The driver was able to make over 100V rms output with very low distortion!!!
The relationship between frequency and output voltage:
column 1 = 1V drive column 2 = 2V drive column 3 = 3V drive
Let's take a look at the amplifier schema ………. oopsss a choke in the 6sl7 anode???
Yes! the choke in the driver is the most important choice in this project. Let’s calculate for the chosen inductance of 1200H.
We take the formula; XL = 2∏.f.L, in our desired audio spectrum the 6sl7 sees the next choke impedances (~XL): 5Hz = 47K ohm, 100Hz = 942K ohm, 1kHz = 9,4M ohm, 10kHz = 94,2M ohm, 50 kHz = 471M ohm. With the formula; R dynamic = XL/1-Av(~0.9) we calculate the anode impedance reaching from 470K ohm (5Hz)to 4710M ohm (50Khz)// R10 (R grid from V2). The 6sl7gt triode has the next specifications;
Class A1
Plate voltage; 250V,
dc grid; -2V,
dc plate current; 2,3ma
Load R; 44K ohm
Mu; 70
Conclusion: in Mu stage we can expect full linearity from this tube.
The V2 grid is coupled with a 0,1uF capacitor to the 6sl7 anode and receive full signal over the choke. So there isn’t necessity for high amplification in the follower V2.
Some important rules for Mu stage design;
¨ The voltage over R9 is ~ 1/2 V+
¨ The higher the anode impedance of V1 the lower the total Z output.
¨ The higher the current in V2 the lower the Z output.
¨ The anode dissipation of V2 may not exceed 30%.
Because the miller capacitance in the VT4C tubes we need a low as possible drive impedance. So I took 2x 1/2 6sn7 triode in parallel connection to double the current and lowering the Z output of the driver.
In the USA the VT4C is still available as new old stock, an alternative can be our Chinese manufacturers.
This triode has the next specifications;
Class A1
Plate voltage; 1250V,
dc grid; -80V,
Peak AF grid; 75V,
dc plate current; 60ma
Load R; 9200 ohm
Power; 20W
The amplifier has fixed bias. The stabilized filament supply is optional, you can also use 10V AC, in this a good mass balance on the filament is necessary!
Power supply;
The supply is compensated for the left and right channel, the output is subdivided with 2 chokes and capacitor each. This obtains sufficient channel separation and acceptable dimensions in the living room.
The PY500a and PY81 tube-bridge’s offers universal transformer choice without the need for middle connections. I used tutorial transformers for high voltage and filament supply. The power supply is external connected to a big separation transformer for a clean 50Hz.
KT88 amplifier
Test driver
Some building pictures