Alternative FETs for ROG britannia

feddozz · February 10, 2016, 12:27:26 PM

Hi all,

the comments on a well known blog that supplies effect pedals stripboard layouts say that 2N5458 can be used instead of J201.
Since I can suplly 2N5458 far easier and cheaper than J201, I wanted to know opinions on the interchangeability of these trannies in the britannia.

Thanks in advance.
f

287m · February 10, 2016, 01:27:39 PM

hey, just try and socketed the jfet
dont forget to bias

feddozz · February 11, 2016, 03:48:21 AM

Do you mean that as long as I can bias is fine?
The thing is I would not know what is the end result I am looking for.
I know nothing about JFETs.

I could get MMBFJ201. I know they are different in shape but I presume they are the same transistor.

I haven't seen one before. It may sound wrong but i was wandering whether I could solder some legs on them and fit them in the socket. Anyone been so desperate to try this? I just looked at the dimensions. They are tiny. It's probably impossible. But for the price I could spend 20 min trying this.

Ice-9 · February 11, 2016, 05:39:28 AM

Yes those mmbfj201 are the smd replacement for TO92 j201's you can also buy small adapter boards from many places.

Here is a link to the smd converter board
https://oshpark.com/shared_projects/8bf7XW05

lots of other suppliers for these, do a search on the forum and you should find.

Kipper4 · February 11, 2016, 05:48:55 AM

Smd to through the hole adaptors are available

duck_arse · February 11, 2016, 09:04:55 AM

of the two, the 2N5457 would be a better part than the 5458, which has a higher Vgs(off) spec. even the 5457 is way higher than most J201's.

but there is an adaptor-board for surface mount fets thread somewhere here, I thort.

smallbearelec · February 11, 2016, 02:44:16 PM

Where are you located?

We routinely sort large numbers of JFETs for phasers:

and we get a lot of remainders that are too low in Vp for that application but fine for preamps and cabinet sims. I'm going to post these as a separate line item. For your money, you can have sorted parts that are guaranteed to hit Vp under 1.4 volts. Some go as low as .7 V like the J201.

feddozz · February 12, 2016, 06:27:21 AM

Small Bear, I know you are in the US.
I am in the UK. Could you please give me an idea of the costs involved. Especially postage. Can PM if you prefer.

smallbearelec · February 12, 2016, 10:14:49 AM

Please drop a message to the smallbearelec@ix.netcom.com address. I will look into lowest possible shipping cost and get back to you.

SD

Ice-9 · February 12, 2016, 11:22:40 AM

Quote from: feddozz on February 12, 2016, 06:27:21 AM
Small Bear, I know you are in the US.
I am in the UK. Could you please give me an idea of the costs involved. Especially postage. Can PM if you prefer.

I am in the UK and have a bag full of Fairchild J201's if you need some let me know how many you need for the project and I can see if I can send them. Do you need specific Vp for the Britannia.

feddozz · February 12, 2016, 05:04:29 PM

Ice 9 thanks that would be helpful. The Britannia has 2 J201. As far as I read they do not need a specific Vp, please correct me if I am wrong. They only need biasing 5.5 and 5 V at the drain maybe that's linked to Vp, don't know?

Small bear I'll contact you. It's good to know what's on offer. Either for this or future orders.

anotherjim · February 13, 2016, 07:02:03 AM

I got some smt j201 from Bitsbox in UK and had a rush of blood to the brain and got some little adapter pcbs from them too. Haven't used them yet. But as I like to build on padded perfboard, I probably won't need the adapters as I think I could solder 3 pin smd to the pads just as easily.

Was a thread here a few years ago showing adapters made from a little square of perfboard.

Have any sellers of PCB's for projects with JFETs added pads for smt (SOT23?) versions in addition to TO92?

duck_arse · February 13, 2016, 08:37:56 AM

Quote from: anotherjim on February 13, 2016, 07:02:03 AM
I got some .... padded perfboard, ....

can't hurt yourself with that.

anotherjim · February 13, 2016, 10:34:47 AM

feddozz · February 20, 2016, 03:08:59 PM

I woild like to celebrate a little gesture of kindness. Me and ice-9 discovered we are practically neighbours.
Yesterday Mick pushed some J201 through my letter box. More than I needed for the Britannia!
Thank you Mick!

On a different note, has anybody tried this layout?http://effectslayouts.blogspot.co.uk/2015/10/run-off-groove-britannia.html?m=1
It looks quite big, will it fit in a 1590bb?

feddozz · March 07, 2016, 03:39:24 AM

Hi all,

I completed the circuit and I have a few questions:

1. It seems that it is the case with other ROG circuits but at zero gain I get no sound. Does it apply to the Britannia too?

2. I read that in this situation, a low value resistor connecting lugs 1 and 3 on the gain pot should solve this, allow me to roll the gain pot off ang get as low gain as possible without losing sound.

3. Does this circuit cleans up at next-to-zero gain?

4. Q1 which is a 2N5457 is supposed to be biased to 6.0 V at the drain. the best could get is 6.7 and then I run out of trim range. Is it close enough? Should I try a different 2N5457? The other FETs are biased as per schematic.

P.S. all FETs are as per schematic, no alternative were used.

Thanks
f

B Tremblay · March 07, 2016, 07:02:34 AM

When the Gain is at minimum, there will be no sound. If this is not to your liking, then a small resistor (10k or so for a starting point) placed between lug 1 and ground will set an audible minimum.

Yes, the circuit is clean at lower Gain settings. If memory serves, settings less than around 10:00 were without overdrive in my builds.

Regarding the bias of Q1, it is probably fine to be a little high. That particular FET may be responsible, so trying a different one may yield better results.

feddozz · March 07, 2016, 07:16:48 AM

Mr Tremblay,

I am quite fascinated by a few of the circuits that ROG has been sharing with the community, so thanks for this. Also, I am enjoying a Ruby for my practice at home.

Quote from: B Tremblay on March 07, 2016, 07:02:34 AM
When the Gain is at minimum, there will be no sound. If this is not to your liking...

After a bit of thinking, I must admit this is quite confusing when not expected. The first time I hooked it up I was not quite sure which was the volume and which the gain because they both cut the sound when set a 7 (24hr notation 7 to 17). Initially, the fact that I was not expecting this behaviour threw some doubts in the correctness of my build. The fact that it is not boxed yet and I have no labels against the pots did not help either.

No criticism but I'd be actually curious to know the reason for this choice.

GibsonGM · March 07, 2016, 08:45:05 AM

This is how early (and some modern *copies*) amps were set up, Feddoz! Best I can make of it is that you are setting the GAIN with, of course, the gain pot....ROG is simply being as close to the original setups as you can be.

One would set the gain, then you can dial it in with the volume. There is an interplay of gain (DRIVE) and output level (speaker/power amp distortion...).

As you go forward and begin to see how tube stages are made, it might make a little more sense - there is a balance between preamp and power amp distortion that is subjective...but I agree, a small resistor along with the pot to prevent total silence is often a good thing!

PRR · March 07, 2016, 08:02:23 PM

> at zero gain I get no sound

Despite the labels, both are "volume controls".

Just like the knob on a hi-fi or PA, or most guitars and guitar amps, you can turn them down to ZERO output.

If you don't like no sound, don't do that.

The idea is you can turn-up the first ("Gain") to get heavy distortion, then turn-down the second ("Volume") to get a sane/clean feed to your amplifier. Or turn-down to get the Britania ugly-stage "clean", and turn-up after to get a happy level to your amp.

I agree that in *this* case, there is no reasonable situation where you you would need to turn-down to zero. As B Tremblay says, add a bottom-end resistor. However maybe the build is complex enough already?

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Alternative FETs for ROG britannia