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Mullard 20 Watt Amp

Thank you for the quick responses, have heard the term before of Mullard 5-20 (but was unsure of the use not being a valve person). Will have a look through the suggested threads.

Wanting to build one, with a integrated pre-amp. Or would it be more advisable to have a separate pre amp?
 
The answer is in this paragraph from the original article:

The sensitivity of the amplifier measured at 1k Hz is 6.5 mV for an output of 2O W when no feedback is applied, and approximately 22O mV with feedback, the loop gain being 3O dB. The loop-gain characteristic of the complete amplifier for the full frequency range is shown above. The sensitivity, with feedback, at the overload point (27 W) is approximately 26O mV.

So if your source such as a CD player has an output voltage greater that 260mV then you can replace the R1 with a 100k pot. The standard output voltage of a CD player is 2V so adding a preamp will simply add noise and distortion to the signal chain.
 
Haven't built a valve amp from scratch before, although I am now.

Seems like the 5-20 is a good amplifier just don't know if I can or should fit it all in to one chassis?

Working on this now, not my design and this is my first build but I am working alongside a master of valves.
 

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The answer is in this paragraph from the original article:

The sensitivity of the amplifier measured at 1k Hz is 6.5 mV for an output of 2O W when no feedback is applied, and approximately 22O mV with feedback, the loop gain being 3O dB. The loop-gain characteristic of the complete amplifier for the full frequency range is shown above. The sensitivity, with feedback, at the overload point (27 W) is approximately 26O mV.

So if your source such as a CD player has an output voltage greater that 260mV then you can replace the R1 with a 100k pot. The standard output voltage of a CD player is 2V so adding a preamp will simply add noise and distortion to the signal chain.

Thankyou, source is mainly going to come from a PC with a DAC inline. All of my music is streamed now for convenience
 

teemumm

Member
2014-04-22 3:18 pm
I have build one stereo amplifier which was based on 5-20. I however did two modifications to it. The EF86 was triode-connected and power supply was done with bridge rectifier instead of tube rectifier. After the rectifier I had CLC-filter (100uF/1.5H/100uF). C-filter would be sufficient, but I noticed some occasional motorboating with it and adding the 1.5H choke to the power supply solved that problem.
 
Ok, I have made my mind up and I want to build the 5-20.

Any pros and cons of doing a stereo version over two monoblocks?

Would be interesting to see if I can have a two monos and the pre amp all in one chassis. Or would interference just mess with everything?

Also, point to point wiring, tag board, or design some PCBs?

Just how good does the original design sound, want to try keep it as close to original as possible, will change some parts for modern designs like a bridge rectifier
 
In replying I have the usual difficulty - one can suggest a number of small changes, each with its "claim-to-fame', until the OP is thoroughly befuddled.

Mine would be to rather replace the overrated ECC83 phase inverter with a lower-µ tube like ECC82 or rather better: 12BH7 (plus some resistor changes). Not to bore with a technical analysis, but there are distinct advantages in keeping a pentode as input tube and the ECC83 is a poor power tube driver. (I thus respecfully differ from the well-known Bayrith-adaptation.) NFB and gain can be re-computed appropriately. Any l.f. instability as Teemumm experienced temporarily, can be controlled far easier with e.g. the pentode screen bypass capacitor etc.

But as said: Choices/alternatives! The above my preference.
 
Further:

One can certainly build two monoblocks on one chassis. One can use a larger single power transformer to save cost. I prefer point-to-point wiring for tube amplifiers; more freedom for routing wires. It is neat to use component boards though.

Combining this with a pre-amp may require some experience to avoid interference. With a separate pre-amp it might also be more convenient to be able to place the (heavy) power amplifier 'out of the way' so to speak.
 
Thank you for your responses.

Going to have a word with my boss tomorrow and see what he says.

I like things to be neat so will look at PCB designs.

I am now working for quite a famous tube amp manufacturer, but they don't do HiFi amps (but did some rare 5-20s).

Think it would be neat to have it all in one chassis, but I can see the benefits of having it all separate.

Would still like to hear how people think they sound.
 

DAK808

Member
2012-05-27 7:34 pm
I just refurbished a pair of Knight Basic 25 watt amplifier which is the classic ef86 input + 12ax7 splitter/driver powering push pull el37 tubes (6l6gc), with all Mullard tubes. This is a monoblock amp and the pair I redid required some new chokes and minor components to be fully operational. Anyway, they sound very good and can play loud. After listening to it for about a week I replaced it with my RH6L6 single ended amp with the EL37 power tubes. As soon as the music started I noticed a smoothness and naturalness to the single ended sound vs the Knight that to me is preferable. The difference is not huge but getting an amp to convey the subtle nuances or the emotional quality of the music was better portrayed by my single ended amp compared to the Knight/ Mullard circuit. But for some speakers or music that would require over 20 watts of power I guess tube push pull is better than a sand amp. cheers, 808
 
If you go to a lower mu valve for the LTP then you will need a CCS tail to ensure balance. ECC83 has just enough gain to work without a CCS in the tail.

It is unlikely that you will need a preamp with the 5-20. Building a stereo pair of 5-20s on one chassis is fine. I used one PSU to feed them both.
 
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