First Amp Build....!

[benella77]

I am looking to build my first valve amp and have a keen intarest for the Mullard designs, primeraly because I have a 1962 record player that uses what I believe to be a 2w Mullard design with ECL86 and EZ80 tubes and it sounds great.

I would be looking for something around 20-30w max output. I have had some people suggesting the Mullard 5-20 amp but with modern alterations......

A full parts list and build list with schematics would be a great help..

Any suggestions please...?

 

[baldrick]

Mullard designed a classic 20W circuit using a PP ultralinear EL34 with an ECC83 (12AX7A) phase splitter and an EF86 input tube. Rectification is with a GZ34 and the power supply is choke regulated.

This design is easily adaptable to modern transformers and you could use modern inexpensive offerings from Hammond or DynaClone to end up with a nice sounding design.

I built one of these years ago and remember it sounding very nice.
Here's the schematic for it from Mullard:
Mullard EL34 pp amp

 

[Eli Duttman]

The 5-20 topology is "classic". However the small signal tubes shown yield WAY too much gain for use with a CDP. A 12BH7 or 6FQ7 section as the voltage amplifier and a 12AT7 as the LTP phase splitter provide: a sensible gain structure, a superb harmonic distortion spectrum, and higher gm to reduce exposure to slew limiting.

BTW, the PSU filter is CLC, not choke I/P, and exhibits the mediocre regulation associated with all modest value cap. I/P filters.

 

[baldrick]

Perhaps there is a lot of gain in that circuit.

You're right about using a 12AT7 for a phase splitter. I've also been very pleased with a 12AU7 in place of the 12AX7.

There is lots of room for improvement in that circuit. I'd add fixed regulation for instance as a first "improvement". You could also increase the power supply capacitance from 2 x 8 uF to 2 x 47 uF for better regulation.

However it's a good starting point and a simple (and inexpensive) enough build for a first project.

I would leave the EF86 as the input stage. It's a very good valve....it doesn't get the respect it deserves.


Full Mullard 5-20 link

 

[Eli Duttman]

I would leave the EF86 as the input stage. It's a very good valve....it doesn't get the respect it deserves.

A good case can be made for the EF86 being the very best small signal audio pentode of all time. It's better than many a triode is. Unfortunately, stage gain is simply too large, when a CDP provides the signal source. However, the EF86 makes a VERY nice triode and (in that mode) could be satisfactory.

 

[bigwill]

However, the EF86 makes a VERY nice triode and (in that mode) could be satisfactory.

Seconding this!

 

[pointy]

i think for an ef86 to work better in triod mode you run them very close to 76% of max anode working volts............i may be wrong

 

[gingertube]

The "modern" Mullard 5-20
http://www.lundahl.se/pdfs/claus_byrith/amplifier_30wpp.pdf

A lot of people have built this as their 1st tube amplifier project.
It has good construction notes an descriptions of the circuit etc.. You will note that the EF86 is run in triode mode. Oh and just because its on the Lundahl site does'nt mean you have to use Lundahl transformers (thou' they do have a good reputation).

Cheers,
Ian

 

[baldrick]

The Lundahl version is a nice upgrade on the 5-20.
It's a shame that it is solid state rectified.
The explanatory article is excellent reading for a beginner, though.

A pair of GZ34 tubes for rectification would be a better choice in my opinion.