EL84/6SN7 tube amp

[riktw]

Hello

i got some tubes laying around and i want to make an amplifier with them
with some help from the internet, especially this sites i tried to design one.

How to design valve guitar amplifiers
How to Design an SE Power Amplifier

i want to make an EL84 amplifier, EL84 as triode and an 6SN7 as input stage.
i already got the transformers (got them on a dutch ebay site, 30 euro's for 2 OPT and a power supply transformer) and i got the tubes.
The OPT's have a 1:30 winding ratio, i got 4 ohm speakers so that means 30*30*4 = 3600ohm

for the questions:
the EL84 stage requires a 20V swing to get max power, the 6SN7 stage amplifies 15 times.
so i would need 1.33V input fox max output.
as far as i know a CD player delivers roughly 2Vpp but do i need 1.33Vpp fox max output of 1.33Vrms?
i expect at least 50 different mistakes but did i make some and so yes what and where?

here is my design with the formula's i used in them:

Thanks, rik

 

[artosalo]

At first, you seem to get some 2 watts out from the amplifier with no so small distortion. Is this satisfying you ?

You need 1,33 Vpp to input of 6SN7. This is 0,47 Vrms.

Your design procedure seems fully right.

 

[aardvarkash10]

have a read of this for a good easy explanation of the concepts.

Your output transformers are not ideal for 4 ohm speakers using EL84 tubes - the EL84 prefers closer to 8k impedance. They will be fine if you use 8 ohm speakers.

 

[artosalo]

the EL84 prefers closer to 8k impedance.

Philips recommends 3,5 kohms load impedance for triode connected EL84 ( with 250 V anode voltage and 36 mA anode current).

 

[nigelwright7557]

You need an input capacitor.

I would also recommend an RC between EL84 supply and the 6SN7.
This will reduce hum and feedback.

 

[riktw]

i will put an RC network in the power supply, 100uF 47ohm 100uF should work good enough
input capacitor is a good point indeed, shall add it.

and yes, 2 watts is enough, if this works properly i can start thinking about making a bigger tube amp with good new transformers.
the GM70 tube is one i really like lookwise, who knows if this works out perfectly i can build a nice GM70 amp
this is pure practice

 

[aardvarkash10]

Philips recommends 3,5 kohms load impedance for triode connected EL84 ( with 250 V anode voltage and 36 mA anode current).

Mea culpa - I was thinking pp.

 

[Miniwatt]

You need an input capacitor.

And why is that?

 

[nigelwright7557]

And why is that?

Because you dont know what he is going to connect it to.
It might have DC on it and upset the valves working conditions.

 

[kevinkr]

I don't use input capacitors, as they say the best cap is no cap, however and this is a big however; I always make sure that my sources (a line stage in most cases) have no more than a couple of mV of DC on their outputs.

All of my digital sources are balanced and transformer coupled so no DC there. The line stage is also transformer coupled so again no DC there.

 

[riktw]

thanks you all, time to get parts and get building

 

[nigelwright7557]

I don't use input capacitors, as they say the best cap is no cap, however and this is a big however; I always make sure that my sources (a line stage in most cases) have no more than a couple of mV of DC on their outputs.

All of my digital sources are balanced and transformer coupled so no DC there. The line stage is also transformer coupled so again no DC there.

I have a guitar effects pedal that puts out 4V5 DC so I would need a capacitor on the input. 4v5 on the grid would change the voltage on the cathode too and screw things up.

 

[kevinkr]

I have a guitar effects pedal that puts out 4V5 DC so I would need a capacitor on the input. 4v5 on the grid would change the voltage on the cathode too and screw things up.

I would say so.. Guess they forgot a coupling capacitor..

 

[Troncones]

If you use a pot to attenuate the input should you us a cap to make sure there is a grid resistor?

If the pot was set at full volume it's R would be 0 and the 1M would be of no value as a grid resistor.

(0 X 1M)/(0 + 1M) =0