Help with Mullard 5 20 project

[5F2A]

Hi,

My names Andy, I'm a long time stalker but first time user of the forum I'd love to get some of the more experienced members opinions on a few things, I'm relatively new to DIY hi fi but I've got a fair bit of experience with guitar and instrument amps.

I'm currently in the research phase of a Mullard 5 20 project. I'd like to make a stereo amp and possibly integrate a preamp. I want to be able to play back vinyl and cd through the amp so I'm going to need a phono stage in there as well.

The issue I'm having is what preamp to attempt to integrate. The Mullard preamps seem a little dated and the general consensus on the net is that they aren't worth the time as they were voiced for equipment from that era???

So my question is, where is a good place to start with regards to integrating a pre? Are there any that anyone recommends or would I be better off with an entirely separate pre?

I'm sure there will be other questions to follow, but I don't want to bombard anyone too much just yet

Much appreciated,
Andy

 

[vinylkid58]

I want to be able to play back vinyl and cd through the amp so I'm going to need a phono stage in there as well.

To avoid a whole host of problems, keep the phono stage separate.

jeff

 

[JonSnell Electronic]

The pre amps back in the 50's were fine for 78RPM records and a crystal cartridge! We didn't have RIAA equalization then, or so my Dad tells me.
I must agree, keep a separate pre amp and use two EF86 valves if you can, adjusting the feedback accordingly over the whole stages.

 

[zigzagflux]

Agreed, keep the phone stage separate, and depending on your long term goals, have the preamp in the same chassis as the phono stage. Then implement a volume control with the preamp.
Keep the 5-20 as straight gain, no volume control. A great design, really, if you implement a few modern tweaks, such as CCS and possibly LED bias.

 

[Junk Audio]

Agreed, Keep the preamp separate. But The Mullard 5 tube 20 watt amp has quite a bit of gain, So make sure you put a volume control in. I have a pair that I built and I have to use the volume control or I couldn't turn my preamp up past 9 o'clock. If you can find a pair of Acrosound TO-300 Outputs, I highly recommend them for the 520 circuit. Also take a look at the Avantic Beam Echo DL7-35 chassis. If you can have it made, It works well for hiding transformers and has a great look. Here's a couple of links

Avantic DL7-35 Amplifier Beam Echo Ltd.; Witham, Essex, buil

Twenty-Watt Amplifier

Here's mine

An externally hosted image should be here but it was not working when we last tested it.

 

[crispycircuit]

If you do go with the 5-20 you could use a passive preamp or buffer preamp and build a seperate phono preamp. The Mullard circuit is high gain and doesn't work well with preamps. Input volume control is a must. The circuit does sound great and isn't fussy about layout....

 

[famousmockingbird]

I am currently building the same amps and had some oscillation issues with the output stage. I am going to apply some snubber networks from plate to screen tap on the output transformer. I will let you know how it turns out. I was told from some of the more experienced builders on here that they implement them on all high gm (like the EL34) output tubes running in ultra linear.

 

[Junk Audio]

I built these amps with Acrosound TO-300's and they're quiet as a church mouse.

 

[Johan Potgieter]

I am going to apply some snubber networks from plate to screen tap on the output transformer.

I generally agree with the above posts. Not to prematurely talk doom, but you will also find a host of so-called 'improvements' (often called tweaks), all predicted to give you audio nirvana. I would initially keep it as is. (There is never a dearth of tweakers ready to change anything with various degrees of success.)

My remark regaring the quote is that any R.C networks applied to the output transformer are highly dependant on the characteristics of the individual transformer, not simply a 'one-for-all' kind of thing. That is also generally true for the phasing capacitors used in connection with the NFB resistor. Not to make matters complicated, but hopefully you have a friend with an oscilloscope and signal generator to assist with this (or you have your own!).

Good luck!

 

[5F2A]

Thanks for the comments guys, really helpful

Would a passive pre do the job if connected up to a line level source (DAC, CD player)? I don't have a huge amount of experience with passives.

From what some of you are saying the circuit has enough gain to not need a preamp as such and just needs some kind of attenuator on the input?

Please correct me if I'm wrong!

 

[Junk Audio]

if it's going to be monoblocks put a volume control on the back of each one. and also have a passive pre which can be as simple as a job box potentiometer set up. and yes you can run a cd player with a passive volume control. But If you can't figure out how to make a volume control, This project may be a bit much for you to start with.

The reason I say put a volume control on each amp is that at some point in time you may use an active preamp and you will be glad you have volume controls.

 

[famousmockingbird]

I believe the input sensitivity is somewhere around 220mV for the rated 20 watt output power on the Mullard 5-20. With most modern sources you are looking at 2Vrms for output so an active preamp with gain is a waste, a passive preamp is just what the doctor ordered for this type of amp. Also because of such high gain in this amp you will want a really quiet source.

 

[5F2A]

All good with the passive pre, was just double checking as I haven't used one before

With regards to individual volume controls, am I right in thinking omit R1 and replace R2 with a 1 meg pot as a voltage divider? This is the way it is done in the 5-10 from what I can see.

Here's a link to the schematic for reference:
http://usr.audioasylum.com/images/0/2260/Mullard_5-20_Schematic1.jpg

If anyone has a better way feel free to set me straight

 

[Junk Audio]

Leave R2 and make R1 a pot. anything 100k or above will work fine.

 

[Junk Audio]

R2 will be center tap of R1.

An externally hosted image should be here but it was not working when we last tested it.

 

[crispycircuit]

Log or Audio tapper.

 

[TheGimp]

Audio taper.

R2 serves as the resistor in conjunction with the tube input capacitance to form a low pass filter.

It helps suppress am radio demodulation and amplification.

 

[DF96]

Better still, leave the circuit as it is and add a pot to the input. Include a coupling cap from the pot wiper to the grid. This keeps grid current away from the pot and stops it going noisy after a few years.

 

[5F2A]

Thanks for all the pointers guys, really helpful!

I'm now debating whether to build monoblocks or a stereo amp, swaying towards the stereo option...

But it does bring up a few issues:

- I would need to add another GZ34 or a SS recto to cope with the extra current demand, I have a soft spot for valve recto's so that's the way I'd want to go Ideally I'd run the heaters in series so that if one valve went the other wouldn't try to carry on. But this would mean a 10v winding which is probably going to mean pricey custom PT. Would running them be a poor choice?

- What would be the best way to implement a balance control? I understand the basics and have looked through various designs but don't really have any experience with stereo amps.

Sorry to be a pain chaps, I really appreciate your help!

 

[Junk Audio]

Lots of people talk about building, Get something built, Then ask questions.