j-FET / Triode Phono front-end

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IDM
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#256 Re: j-FET / Triode Phono front-end

Post by IDM »

Hi,

I finally got round to building the jFET phono as shown in Mark's schematic and am having a few issues I hope someone might be able to help.

Initially I built exactly as the schematic showedand the sound was really detailed but thin, like it was missing an octave or more of bass. Also the gain was too low for my kontrapunkt B cartridge. On measuring I found that each of the FET's was only drawing about 4mA. I decreased the source resistors to 4.7R, this increased the current to nearly 7mA each so 14mA on the stage. Massive improvement, it now sounds usable.

The first stage valves have G1 biased to about 14V, I haven't tried changing them yet.

I also only measure about 50-60V on the anode of the output valves (12k anode resistor), so the output stage seems to be drawing nearly 20mA. Does that sound right? Should I decrease the anode resistor to get the voltage up? I get similar results with both batter bias and when I swapped in a cathode bias resistor (70R)

However, the gain is still low (using D3A in the first valve position and Russian 6J9p in the second). Also the bass is still light (much better than when I first tried but not yet matching my old phono (Andrews C3G pentode + ECC88) with an SUT.

I would be grateful for any thoughts.
Cheers
Ian
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IslandPink
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#257 Re: j-FET / Triode Phono front-end

Post by IslandPink »

So as you clarified it's 6J9p ( not 6N9p) - but as this is an E180F equivalent , is it correct that this is in the 2nd stage position or is it in fact the first stage, above the jFets ?
Then, can we just re-cap what the anode voltages are on the 1st and 2nd stages ?

Edit : OK page 4 has the circuit as far as I can tell. Where are you deviating more than eg. 5 or 10v from these numbers ?
I also notice there's a decoupling cap to ground ( 220uF ) to stabilise the G1 voltage divider - that could affect bass significantly if not there - have you got that ?
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IDM
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#258 Re: j-FET / Triode Phono front-end

Post by IDM »

Hi,
The valves above the jFETs is a D3A with 103V on the anode and drawing 14.8mA (the jFET source resistor dropped to 4.7R to achieve the current draw).

The second stage valves are 6J9P with 62V on the anode with a 12K anode resistor.

I was wondering about putting D3A in the output position as well as it has a higher triode mode mu and I therefore assume more gain, is that right?

Cheers
Ian
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IslandPink
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#259 Re: j-FET / Triode Phono front-end

Post by IslandPink »

Yes - that was my choice and I was able to get the gain more or less the same as 1:10 SUT , D3a, D3a with that combination. You will get several more dB this way which hopefully will be enough . Also if you used the resistor divider and battery as per my circuit for the D3a then you should get back to the right current and anode voltage for st.2 if you put the D3a there. With a bit of luck these two things might get you the bass performance as well - but see how it goes. Bass should definitely be better than with the step-up, not dramatically so for the Kontapunkt, but at least as good. Check for that cap on the voltage divider on St.1 anyway.
Is the power supply limited at all because if so, the 20ma in each 6J9p could have pulled the voltage down a bit and been leaving it unregulated ( if you have it regulated ? ) .
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IDM
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#260 Re: j-FET / Triode Phono front-end

Post by IDM »

Hi Mark
I will try the D3A in the second position and report back.

The PSU is a CCS with two voltage stabiliser tubes (I have a 56nf cap across the stabiliser tubes to keep the noise down). You were right about the current limiting and voltage drop as the phono stage drew more current. It did drop out of regulation and I had to tweak the CCS to give more current to keep it in order.

I am not sure quite what you mean by "Check for that cap on the voltage divider on St.1 anyway" what should I check?

Thanks,
Ian
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#261 Re: j-FET / Triode Phono front-end

Post by IslandPink »

Oh I just meant, check that you have it there, that's all. Just thinking about things that might spoil the bass response.
Good info on the PS with VR tubes. I used a cap & resistor across the pair, this came from Gary Pimm who experimented with ways of nulling the inductance of the VR tubes. Think it's about 0.1uF and 390R .

Watch out for pinout changes, I can't remember off-hand if the E180F and D3a are the same.
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IDM
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#262 Re: j-FET / Triode Phono front-end

Post by IDM »

Hi,

Latest update. I am now running D3A in every valve position and this brings the gain up to being pretty much fine (certainly much better then with the E180F/6J9P). Bass also seems improved.

I am running the two valves on the output with battery bias as per Mark's schematic. The main issue now is that both channels are still drawing 20mA which means that the voltage on the anode is still low at 55-60V. I have looked at loads of schematics and all indicate that the current should be less. I am a little bit floored by this. One diagram showed the D3A with IR LED bias and a CCS on the anode. The designed claimed this gave higher gain. I would be grateful for any thoughts on how to get this stage right.

I should say that the phono is now starting to sound pretty good, I just want to nail down all these issues before critical listening.
Cheers
Ian

Cheers
Ian
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Nick
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#263 Re: j-FET / Triode Phono front-end

Post by Nick »

Its all down to the Ids of the jfets. If you are using two in each cathode, simple suggestion would be to just use the one.
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#264 Re: j-FET / Triode Phono front-end

Post by IslandPink »

On stage 2, I'm a bit concerned it isn't right, and thinking. Can you double-check the battery is definitely giving 3V ? If so, the resistor divider should give about -1.3V or -1.4 bias when connected as drawn. Let me double-check the 2015 phono drawing which is what I use. OK so 750/(750+1000) = 0.43 . Then -3 volts x 0.43 = -1.3V approx. Hmmm...
Double-check the D3a ? - G2 connected to anode, G3 to the cathode ?
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#265 Re: j-FET / Triode Phono front-end

Post by IDM »

Hi Mark,
On stage 2 I have both G2 and G3 connected to the anode via 100R grid stoppers. Is that the problem should I move G3 to the cathode? I must admit I did that part on auto-pilot!

Hi Nick, the problem is in stage 2 not stage . I agree that the IDSS guides the current of stage 1

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Ian
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#266 Re: j-FET / Triode Phono front-end

Post by IslandPink »

Not sure how much difference the G3 options will make. I think I've connected them to cathode as per normal pentode operation.
There's got to be something wrong if you have a 300V supply , 12k anode load, a bias of -1.3V and a D3a in the socket. They are close-tolerance valves, so you'd not expect the current to be up by 70% from datasheet. It did occur to me the battery could be the wrong way round ( ie. neg terminal to ground ) but then I'd expect the plates to be glowing !
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IDM
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#267 Re: j-FET / Triode Phono front-end

Post by IDM »

Hi
I checked the battery bias and the grid definitely measures about -1.3V.

I faffed about and moved G3 to the cathode and still get 60V on the anode and a current of 19mA. I think I will stop tonight and try more tomorrow if I have time.

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Ian
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Nick
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#268 Re: j-FET / Triode Phono front-end

Post by Nick »

20ma @ 60v looks very close to the 0v line looking at the d3a triode curves. Not sure if that says anything or not. Are you looking at the stage with a scope, one possibility is oscillation. I would be tempted to use some stoppers on all the grids.
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#269 Re: j-FET / Triode Phono front-end

Post by IslandPink »

...and what's the voltage on the cathode ? ...
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IDM
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#270 Re: j-FET / Triode Phono front-end

Post by IDM »

I had an early morning teleconference to Australia so been up since god knows when, so had a look at the datasheet I have from Philips for the D3A, which includes triode curves. It indicates that -1.3V is about 19-20mA of current and that the bias for 12mA is -1.6V or 15mA -1.5V. I am off to work in a minute but will play with the potential divider tonight to see if I can get the anode voltage right.
Cheers
Ian
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