• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Need 3-5W tube amp

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Here is the semi-final schematic. I just need to clarify a couple ratings. Please let me know if anything else should be changed. Should there be resistors in the heaters line? Thank you!

m-2.png
 
I would make C1 a 400V part. Also, more like 1uF unless you want to limit LF. (-3db at 19Hz instead of 1.59Hz). It will protect the preceding stage in case of an arc to grid. C2 can be as low as the speaker output voltage (25V is probably enough).
Why is R6 there? R7 looks high. Usually around 100k.
I doubt this will have enough power to drive the screen grid of the 6P15P.
 
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Here is the semi-final schematic.

Repeat, the value of C2 is currently unknown. A signal generator and an o'scope are needed to determine the correct value. The R4/C3 Zobel network may not be needed and, if needed, the parts' values will change.

The suppressor grid (g3) "sits" between the screen grid (g2) and the plate. In a 6Π15Π, g3 comes out to the base at pin 1 and pin 6. In a 6BQ5/EL84, g3 is internally connected to the cathode. Connecting either pin 1 or pin 6 of the socket to pin 3 allows for proper operation of both O/P tube types.

What about volume control?

The circuitry driving the amp has to comply with IHF "standards" and be capable of driving a 10 Kohm load. Add a 10 Kohm audio (log.) taper pot. across the terminals of the I/P RCA female. The wiper of the pot. feeds the 0.82 μF. high pass capacitor. Please notice that the "textbook" 1:10 driving/driven impedance ratio rule is obeyed.

Also, more like 1uF unless you want to limit LF.

We very much want to block infrasonic noise. Keep that part 0.82 μF. A global NFB loop encompasses the O/P trafo. If the amp tries to follow infrasonic noise, the error correction signal will cause the "iron" to saturate. :mad: :down:
 
I'll make C1 400V, C2 25V and add volume pot. I'm not sure if I need to change tube drawing to reposition g3.
kodabmx, the presented schematic was based on the design by Mr. Gillespie (see the post #38)
R6 and value for R7 came from that design.
I'll update the schematic later today.


Alas I have only multimeter in my disposal :(
 
FUGLY describes the power pentode symbols. :bawling: :down: :mad: Go back to showing only the control and screen grids. :yes: During construction, don't forget the pin 1 to pin 3 jumper that allows both O/P tube types to function correctly.

Alas I have only multimeter in my disposal

Then we use brute force methodology. ;)

The open loop gain of the voltage amplifier is made to increase with increasing frequency by "peaking". That means part of the anode load is inductive, along with being resistive. The schematic shows a 470 Kohm resistance. Make that 470 Kohms up by assembling a series wired composite of a 100 Kohm wirewound part, a 150 Kohm carbon film part, and a 220 Kohm carbon film part. While the inductance of wirewound resistors is frequently a liability, it's an asset, this time around.

The selected Edcor O/P trafo spec's state up to 18 KHz. Above that freq., its response rolls off. "Peaking" the voltage amplifier increases gain, with increasing frequency. The O/P trafo's response decreases, with increasing frequency. We are adding with 1 hand and subtracting with the other. We rely on the NFB error correction signal to maintain linearity. However, we will "run out of steam", eventually. That is dealt with by eliminating NFB beyond 80 KHz. Don't try to lift excessive weight. Remove C2, C3, and R4 from the schematic. Add a 180 pF. C0G/NP0 ceramic cap. across the O/P trafo secondary.
 
Yes, but there's a pipe organ that makes 8Hz (Can't remember what it's called, but I have some music from that organ on vinyl) and a lot of modern music has lower bass. Even the low note on a 5 string bass is 30.5Hz. Just my 0,02$ My "standard" -3db frequency is 1.59Hz and I don't have saturation issues with my OPTs.
EDIT: Maybe that's why my tube amp has better LF bass than my 300W Yorkville...

400V is a safety thing that I do but strictly speaking on a grounded cathode gain stage you can usually omit the input cap altogether. We are talking about a 20 cent part though. X2 MKP safety capacitors make excellent audio coupling caps.
 
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OK, if I'll feel (or should I say 'hear') that there is not enough bass I'll know where to look :)
Here is the schematic which I hope incorporates all comments. Please let me know if anything else should be changed before I start buying process (hopefully coming weekend).

m-3.png
 
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C2 must be rated 400V because at turn on, with cold tubes, it will see about 300V. I use 630V caps in my builds, just in case. C1 has a odd value. If you have trouble finding it, 0.1 uF will be just as good. You must have a ground reference on the heaters. I like to have a DC elevation circuit (see my schematic), but you may also use two 100 ohm resistors. Pick the first output tube socket, add the first resistor between pin 4 and ground, the second one on the other tube socket between pin 5 and ground.
 
Thank you pcan, I've changed C2 to 400V. In this case C1 and C2 can be the same type capacitor. There are two inline 100 Ohm resistors in the heaters' line (see PS schematic). Do you mean to add two more connected to the ground?


Does the russian tube 6P15P (metric system) fit well into the socket for 6BQ5?
 
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Do you means the schematic on post #106? It does not contains the resistors I am referring on my previous post. To avoid a humongous hum, the heater filament of the tubes must have a reference to the ground. A simple way to do so is to connect one tap of the filament transformer to ground. This is the way most tube radios were built, but a better way is to have both ends of the filament at the same potential relative to ground. To reach this goal, you connect two resistors in series between the filament transformer secundary winding, and then connect the point between the two resistors to ground. The usual value for 6.3 volt windings is 100 ohm. You have selected a 12.6 volt filament transformer, a 150 ohm resistor may be a better choice.
 
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