• 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.

Schematic for a small SE EL84 amp

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Way back when Mullard produced a very good book on audio with circuits. If I remember correctly there was a 5-10 amp and a 10-10 amp. These used EL84's and EF86's. From What i remember they were very popular and gave a good sound.
Does anyone have any info on these amps or if the book is still available.
Many thanks
Phil.
 
Yep here it is... the famous mullard EL84 SE
tomcat:rolleyes:
(hope it works; the attachement that is!)
 

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Hi Mike,

It's hard to say whether your transformers are suitable until you know the turns ratio. Its very easy to measure. I've never used 6BM8s.

First, turn off the amp and make sure it is discharged. Desolder the B+ and the leads to the 6BM8 plates.

Next get a low voltage AC transformer, say 3V and place it across the 8R taps. Measure the ACV with a voltmeter across the 8R taps. Then measure the ACV across the primary, plate-to-plate. Divide the primary ACV by the secondary ACV. This gives you your turns ratio. Square this number and then multiply it by 8 and you have the primary impedance plate-to-plate.

The spec in the article calls for 8kR plate-to-plate. My trans is 7kR and even though my horns should be above 8R, I think 7kR should be OK. The Dynaco ST35 was 7.1kR.

HTH
Brett
 
Mind sharing your circuit?

Brett,
Would you be so kind to share your
output stage circuit that is similar to the
6W amp?
I wonder why that differential pair
output state topology
is not so common - perhaps it is the need to
get equal currents thru both tubes?

rgds
Yv
 
RMS: I'm not sure about the pot in the NFB loop only to theorise that a) to be able to vary the amount of NFB to taste (it does make a diff to the sound), or b) to adjust to make up for the parametric spread in components and prevent oscilation, as whoever at the time and just as now, would not be able to access the exact same components, so this feature allows for that methinketh humbly....
tomcat:rolleyes:
 
RMS,
I have had in my house on lone a pair of the Mullard 3 monoblocks and got wonderfull sound from them. There was no pot in the feedback circuit and a volume pot had been installed between the input jack and the 6267/EF86. I don't know how good your opts are but if you have the bandwidth this is a nice amp. I was driving 92db efficient speakers and got lots of volume before they started to clip the amps.
 
planet10; my amp is finished!

Hey Dave, thanks to your help my amp was a success!

I thought everyone should know that I built that first schematic at the begining of this thread and its working fine. For the most part all the values for the ressistors and caps that I used are fairly close if not the same to what is in the map. Though I did use an extra cap in series in the first filterstage to take the surge of the solid state rectifier diodes. At first testing no sound at all, then I cliped the negative feed back lead that goes to ground and it worked and I cliped the other feed back lead and there was no difference in sound so that is all gone now. Its also plenty loud enough and drives a 12" speaker very clearly.

Here are the test results:

B+1=331VDC, B+2=262VDC, B+3=235VDC

EL84= pin9=261V, pin7=311VDC bias=47mA, pin3=6VDC
 
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