Mullard 5-10 circuit replace EF86 with ECC83

[Gillian]

Hello,I would like to build the Mullard 5-10 in stereo form but would like to replace the EF86 with an ECC83 or 82.
First of apologies for my lack of knowledge those glowy glass things,but do I need to make serious circuit changes
The Gain of the Mullard cuircuits was very high as well ,so is generaly not required so much these days.
I am quite clued up on semi design but do not know that much about valves.
Any help would be appreciated or any curcuits that use this arrangement,even a web link.
Thank you
Gill

[Sideband]

Maybe I'm missing a point here but why?? If you build the 5-10 with controls, it will take the output of a CD player or Ipod without changing anything......I've done it!

Rich

[Richard]

Hi

The other option is to run the EF86 as a triode, lower gain and lower noise.
There is another thread here about it, I will see if I can find it

Richard

Here it is, some info and links here
https://www.vintage-radio.net/forum/...ad.php?t=99399

[G8HQP Dave]

The gain of the 5-10 is set by negative feedback. Changing this to reduce gain may then require less open loop gain in order to maintain stability. I used half an ECC81 instead of the EF86. An ECC83 will give a little more open loop gain; and ECC82 much less.

See http://lundahl.se/wp-content/uploads...fier_30wpp.pdf for an example of a similar amplifier using a triode-connected EF86.

[Ed_Dinning]

Hi Gillian, a Google should allow you to download a copy of the Mullard book "Circuits for Audio Amplifiers" that contains the 5-10 and other famous amps and pre amps.
There is also a wealth of info on the web on upgrades that can be applied to this circuit, including DC biasing that allows removal of the cathode resistors from the output valves.
If you have no success PM me with your e-mail and I'll send you some scans; there will be a bit of a delay as I will be going away later this week.

Ed

[Leon Crampin]

The only possible reason I can think of to dispense with the EF86 is its price - as others have pointed out it's in a feedback loop and hence the stage gain is useful.

If you object (as I would) to paying for a "golden ears" grade EF86, rewire the base and use a 6BR7. No other changes are needed as the valves have otherwise similar characteristics. I have found the 6BR7 to be less noisy than the EF86.

In a screened box, the 6BS7 is even better...

Leon.

[Radio Wrangler]

The Mullard 5-10 has a rather sensitive input, so it has high closed-loop gain. It also has, for a valve amplifier of its period, quite a large amount of feedback. 30dB gets quoted.

Consequently the open-loop gain is very high, and this fits with having a thermionic 'long-tailed pair' as the phase splitter. This circuit has gain, unlike the split-load phase splitter.

Making the finished amplifier less sensitive at the input means reducing the closed-loop gain. This means that the attenuation in the feedback path is less, and so if the open-loop gain stays the same, the gain around the loop is greater. As Dave said, this will reduce the stability margin and the amplifier becomes more prone to oscillate.

The original transformers would have been carefully designed to minimise phase lag at high frequencies in order to maintain stability margin. If less carefully designed magnetics are used, then the stability is further reduced.

Switching to a triode input stage would tend to reduce the open loop gain (which is good if you're making the thing less sensitive) but the triode will have greated effective Miller capacitance, and unless it's in a lower impedance input circuit, the pole which it contributes to the open loop response will be reduced in frequency.

To do these changes and to keep the characteristics of the 5-10 would be a significant design task, but that could be part of the fun of the thing.

The 5-10's I've come across weren't bad for noise, so I agree with Leon about sticking with the EF86 unless price is the limiting factor, but then the audiophiles are pumping up the prices of ECC81/82/83 as well. There are other audio pentodes which they haven't latched onto (yet) I seem to come across loads of 6SJ7s at radio rallies when I'm hunting 6SG7s for the AR88 RF/IF stages.

There are design decisions involved in what you've said that can only be navigated by knowing why you want to do this.

In this sort of amplifier, the feedback acts to minimise 'valve sound' up to the point where clipping begins.

David

[Refugee]

I would look into reducing the closed loop gain.
In the cathode circuit of the EF86 there is a potential divider with the decoupling capacitor across the top end.
I would fit a temporary pot in place of the top resistor with the negative of the decoupling capacitor on the wiper and adjust it for the required gain and if it works well measure the pot and fit fixed resistors not forgetting to add the value of the lower one to the final value of the new lower one.

[PJL]

I must admit that I can hear absolutely no noise from any of the Mullard 5-20 designed power amps I have but this maybe because I can't hear anything over 10KHz. The EF86 in high gain preamps is another matter.

[bikerhifinut]

Just a mad thought.
Why not check out the Leak TL12+ circuit and compare it to the Stereo20.

The TL12+ uses EF86 with a LTP of ECC81, while the Stereo20 uses ECC83 with an ECC83 LTP in a more or less identical circuit topology based heavily on the 5-10.

Might give you a few pointers? Plus there's a bit in the Morgan Jones book on Valve amplifiers that tells how to alter the negative feedback network on amps with EF86 when altered to triode mode.

Regards

Andy.

[G8HQP Dave]

Originally Posted by Refugee

I would look into reducing the closed loop gain.

As several of us have said, you can't reduce the closed loop gain of the 5-10 by much without hitting stability problems (at both HF and LF ends). There is already a small subsonic peak in the unmodified circuit - a classic sign of marginal LF stability.

Better to decide what closed loop gain you need, then reduce the open loop gain by the same ratio as the closed loop gain is reduced. This will maintain stability.

[Refugee]

I have a couple of pairs of very similar amplifiers that have a very similar preamp with an EF37A and the feedback loop also looks the same. They do get a little unstable at LF with the wick up a bit and something with a lot of bass in it playing.
I put it down to the 60 year old capacitors. It is now on the to do list to drill them out and fit the 10mm diameter modern caps on the old cans. There is a thread about them in the component section. I have got the parts so it will be done soon.
I tried using an old DVD player for CDs and found that I had to fit a couple of pots in the scart plug shell to get round the problem and that worked fine.

[Gillian]

Thank you everybody for all the links and help .
I have also been lookonig at the Dynaco ST35 circuit which has a double tried in input/phase splitter arangement.
I do not require any of the tone control network of the 5-10 circuit as this will be dealt with by a seperate srpp triode line level preamp and phono stage.
The dynaco circuit arrangement may suit me better.
Or indeed a conglomeration of both design topologies.
Gill

[bikerhifinut]

found this website, full of out of copyright downloadable text books.

http://www.tubebooks.org/technical_books_online.htm

http://www.tubebooks.org/Books/York_Amplifiers.pdf

second link is to a textbook which has a 10watt EL84 amp using an EF86 wired as triode.

[Ed_Dinning]

Hi Folks, as well as the Mullard Circuits for Audio Amps, they, and Phillips, published a large number of app notes (anyone got a full listing?) on both valve an transistor circuits. Additionally there were full page ads in various professional magazines on valve circuit apps.
The other manufacturers did likewise and there is also information from GEC and RCA.
Plenty to choose from but quite a bit of similarity.

Ed

[Radio Wrangler]

The RCA 'Radiotron Manual' is worth reading and is kicking about on the web. The GEC info is also out there and worth reading

Terman's books 'Radio Engineering' etc are a real treat and are worth finding and spending real folding money on. Ironic really, from the man who kicked off what became Silicon Valley.

After designing the Dynaco amplifiers, David Hafler did an excellent semiconductor one in the late 70's, the DH200. Well worth looking at if you've not come across it. He was a competent engineer and could design good amplifiers in both technologies.

David

[Gillian]

Thank you,bikerhifinut.I have discovered an absolute smorgasbord of books and info on your link.Wohoo!