• 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.

Combine EF80 with EL84

newbadboy

Member
2008-03-07 2:27 pm
Combine EF80 with El84

This is the first time i'll build a tube amplifier.

I have two EF80 tubes and will get two EL84 tubes from a friend. I have also ordered to output transformes. I need help with combining these tubes and the transformers to an amplifier.

thank you
 
Hi and welcome to the DIY valve world!

I assume you will build a stereo SE amplifier? Have you seen thiswebsite ? It is one of the websites that offer a walkthrough for the design of a SE valve amplifier. You can follow the same steps with the data from the EL84 and the EF80 (I would probably triode connect the EF80, curves in triode mode can be found here

The above does help, but do not expect to have it ready on the back of a napkin. Better to sit with lots of paper and a pen, a calculator comes in handy as well!

Good luck, Erik
 

newbadboy

Member
2008-03-07 2:27 pm
I've checked out the boozhound link. The power stage is pretty easy to understand i think. But when they calculate the pre-stage they use som alreday finished example from the datasheet for the tube. The EF80 tubes i have does'nt have this ready example with suggested component values ....!?
 
newbadboy said:
I've checked out the boozhound link. The power stage is pretty easy to understand i think. But when they calculate the pre-stage they use som alreday finished example from the datasheet for the tube. The EF80 tubes i have does'nt have this ready example with suggested component values ....!?




Download and read this . You could then use the triode curves that Eric linked to to determine your operating points.

Best of luck.
 

newbadboy

Member
2008-03-07 2:27 pm
pedroskova said:





Download and read this . You could then use the triode curves that Eric linked to to determine your operating points.

Best of luck.


Ive read thru the manual. It only describes as i understood the preamp stage (in my case ef80). I will also use the el84 to the power stage and this is not described?!

I would also like to power the whole amplifier with 155VDC only. What do i loose/gain by having this low supply voltage??
 

Shoog

Member
2002-08-15 10:16 pm
Eire
I would also like to power the whole amplifier with 155VDC only. What do i loose/gain by having this low supply voltage??

It probably wont sound as good as it could. Every valve has its sweet spots and all of the designs out there have chosen their operating points to achieve those sweet spots. You may be heading for a fall trying to squeeze the EL84 into an inappropriate role.

I would suggest that you might want to consider using a EL86 which is very close to the EL84 but likes lower voltage and higher current. The PL84 is a 15V heater version of the EL86 and can be had very cheaply in large quantities. Heater requirements will be easily met with surplus trannys.

Consider a version of the Baby Huey as a pentode makes a perfect driver for this design.

Shoog
 
newbadboy said:



Ive read thru the manual. It only describes as i understood the preamp stage (in my case ef80). I will also use the el84 to the power stage and this is not described?!

I would also like to power the whole amplifier with 155VDC only. What do i loose/gain by having this low supply voltage??

There are plenty of SE EL84 schematics floating around the web...and this forum...that should give you a good idea about what is required in building this kind of amp.

With respect to your psu, using a simple 1:1 isolation transformer will bring you close to the conditions that would suit the EL84. Don't limit yourself just because you happen to have another transformer on hand. In the long run, saving a little money now probably isn't worth it.

Also, I'd recommend perusing the learning resources thread permanently located at the top of this forum. There are a lot of links to good sources for learning about tubes.

...ditto the safety thread.
 
Originally #17 posted by newbadboy
Is this schematic possible???

http://www.ladda-upp.com/bilder/3908/ritning-steg

Hello newbadboy,
looks good. Place a 1K grid stopper at the g1 of the EL84.
I used 2K2 at the kathode and 150K at the anode of the EF80 (Triode).
270 Ohms at the kathode of the EL84 (Triode) as recommended
in the data books. I used a PL84 output transformer.
Don't forget the cathode decoupling for the EL84.
I took 100µF and 22nF 560K at the grid of the EL84.

Kind regards,
Darius
 
#19

Hello newbadboy,

C1=47µF
C2=22nF
R2=2K2
R3=560KΩ
R4=270Ω

You can take higher values for the caps but this can make
the operating point creeping.
Negative feedback eliminates the influence of the components.
A 7400 (TTL NAND) works linear with negative feedback
for example.
Thus it makes no sense to me to build up a tube amp
with negative feedback.
Give the triodes a chance to linearise themselve. ;)

Kind regards,
Darius