I like the Mullard description for the EF86 as being in 'starvation' mode in this circuit. It is fascinating and there are many reports of it sounding excellent. The EL84 is a great valve in my opinion and packs a lot into that little envelope. The only issue I had with this circuit is the need for such large amounts of feedback, which is not so popular these days. I have had remarkable results from a triode connected single 84, and while many say its no good as a triode - well I didn't just take their word for it and tried it. Glad I did too. Also glad I found this channel too. Subscribed.
Hi, Thanks for answering back. I built it many years ago. The grid of output tube has high + voltage from plate of previous tube and with 560 ohm of cathode of its output tube is to have overcurrent. So its 560 ohm was burned by heat of overcurrent at that time.If I remember still, I replaced it with 6.8 k ohm 5w. Then it was ok, I think.
With the amp you don't need the capacitor and resistor if you are using an external mixer circuit. The resistor is only varied to get the correct HT for the preamp. Thanks, Clive Kavan
Why do amplifiers like this not have a coupling capacitor between the preamp and output valves? I don’t understand how the DC on the grid of the EL84 is dealt with.
Hi, we checked the value which is correct, and according to the Mullard Circuits For Audio Ampliers book (available from our ebay shop www.ebay.co.uk/itm/163158603335) ''The use of direct coupling between the stages necessitates a higher cathode voltage in the output stage than is required with RC coupling. The value of R13 is thus greater than is usual for the cathode resistance''. Hopefully this clears things up, but we will double check with Phil the technician when we next see him, Vintage Audio Workshop
I've got the book, im about to have a go at this project, thanks for your video
Just getting back into messing with valves and will be building a mono-block 3.3 to start with, great to see a detailed video on this little amp ;)
I built 2 of these amps in 1969 when I was a radio tech at Bletchley Park. The circuit looked a lot more daunting and complicated back then :)
Did you wind your own transformers? Paul
I like the Mullard description for the EF86 as being in 'starvation' mode in this circuit. It is fascinating and there are many reports of it sounding excellent. The EL84 is a great valve in my opinion and packs a lot into that little envelope. The only issue I had with this circuit is the need for such large amounts of feedback, which is not so popular these days. I have had remarkable results from a triode connected single 84, and while many say its no good as a triode - well I didn't just take their word for it and tried it. Glad I did too.
Also glad I found this channel too. Subscribed.
glad it helps out, we have many other videos concerning the EL84 valve for the near future, one of the amp circuits has zero nfb... thanks
The 1k resistor across the secondary of the output transformer is to prevent parasitic oscillation, according to the accompanying literature.
Hi, Thanks for answering back. I built it many years ago. The grid of output tube has high + voltage from plate of previous tube and with 560 ohm of cathode of its output tube is to have overcurrent. So its 560 ohm was burned by heat of overcurrent at that time.If I remember still, I replaced it with 6.8 k ohm 5w. Then it was ok, I think.
Hello, if I won't be using an external preamp, can I dispense with R16 and C10? Thanks.
With the amp you don't need the capacitor and resistor if you are using an external mixer circuit. The resistor is only varied to get the correct HT for the preamp.
Thanks,
Clive Kavan
@@vintageaudioworkshop Ok, thanks for your answer, much appreciated.
@@KK-no7be very welcome
Why do amplifiers like this not have a coupling capacitor between the preamp and output valves? I don’t understand how the DC on the grid of the EL84 is dealt with.
We will do a video on this subject in the very near future and include the EL84 in depth, Thanks!
R13 560 ohm does not seem to be proper value.
Hi, we checked the value which is correct, and according to the Mullard Circuits For Audio Ampliers book (available from our ebay shop www.ebay.co.uk/itm/163158603335) ''The use of direct coupling between the stages necessitates a higher cathode voltage in the output stage than is required with RC coupling. The value of R13 is thus greater than is usual for the cathode resistance''. Hopefully this clears things up, but we will double check with Phil the technician when we next see him,
Vintage Audio Workshop