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Old 20th May 2016, 11:47 am   #161
vampyretim
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Default Re: Solid state preamp for EL84

Right, so I have done as Lawrence suggested.
I disconnected the grid and put a 1 meg resistor across it.
I connected the scope to the output of the preamp and I got a clean 2.1V that wasn't affected by the pot at all.
So that means that the grid is drawing current then?
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Old 20th May 2016, 12:16 pm   #162
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Default Re: Solid state preamp for EL84

At the moment that looks a possibility, connect the volume control to the grid again etc so it's as it was before with your preamp connected.

Measure the grid voltage and the cathode voltage both with respect to chassis.

If the valve is drawing grid current it could be a defective valve or something external.

If the grid draws grid current the grids input impedance plummets, that low impedance in conjunction with the relatively high source impedance will cause the positive peaks of the input waveform to clip.

Lawrence
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Old 20th May 2016, 1:15 pm   #163
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Default Re: Solid state preamp for EL84

Hi Lawrence,

the signal at the output of the preamp is fine until the valve warms up at that point the positive peaks start to clip.

I then measured the grid at -5V and a mere .4 of a volt across the cathode.

During the time this thing's on it sounds very unstable and I'm half expecting something to blow.

How's best to proceed?

Many thanks.
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Old 20th May 2016, 1:40 pm   #164
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Default Re: Solid state preamp for EL84

Turn the volume to minimum and measure the grid and cathode voltages again.

Lawrence.
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Old 20th May 2016, 1:48 pm   #165
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Default Re: Solid state preamp for EL84

Grid is now 0 V.
K is .75 of a volt

Peaks are no longer clipping but I'm getting a horrible hissing sound which seems to respond to me taking the Cathode voltage.
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Old 20th May 2016, 2:09 pm   #166
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Default Re: Solid state preamp for EL84

Ok, the zero volts at the grid is good, 0.75 volts on the cathode is not so good, there are no voltages shown on the Dansette Bermuda schematic I'm looking at but the cathode voltage should certainly be much higher than 0.75 volts.

You need to concentrate as to why that voltage is low.....Possibilities:

1) Low anode voltage.

2) The resistance between the cathode and chassis is abnormally low, that could be the cathode bias resistor gone low value value or the cathode bypass capacitor being electrically leaky, or a combination of both, disconnecting that capacitor and measuring the cathode voltage would prove the capacitor as yes or no for leaking. According to the schematic I'm looking at the cathode bias resistor should measure 120 ohms or there about.

3) Low emission valve.

If the anode voltage is reasonable (180 volts or higher) then I would concentrate on the cathode components.

Lawrence.
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Old 20th May 2016, 2:45 pm   #167
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Default Re: Solid state preamp for EL84

Thanks Lawrence,

I know how to fault find valve amps I was just unsure as to whether the preamp was affecting the valve.

The cathode resistor is fine. I will check elsewhere.

Many thanks.
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Old 20th May 2016, 2:51 pm   #168
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Default Re: Solid state preamp for EL84

No problem, keep at it but be aware that the clipping could possibly be due to too much signal appearing at the grid in conjunction with incorrect grid bias for the amplitude of the signal being applied.

Lawrence.
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Old 20th May 2016, 3:46 pm   #169
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Default Re: Solid state preamp for EL84

I substituted the valve for another and the cathode voltage shot up to 6.5 volts I had a ton more gain but I still have positive clipping.

I'm going on holiday for a week but am keen to get cracking once I'm back.

Till then thanks to all of you for all your help, I'm very grateful for it.
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Old 20th May 2016, 3:53 pm   #170
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Default Re: Solid state preamp for EL84

As you now have a lot more gain it might be worth reducing the amplitude of the signal feed to the grid, either by reducing the gain of the preamp or by some means of fixed attenuation, anyways, think about that when you get back, have a nice break.

Lawrence.
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Old 23rd May 2016, 9:03 pm   #171
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Default Re: Solid state preamp for EL84

Can't remember if anyone's covered this idea or not.

Is there or could one make a step up transformer with a suitable step up ratio?
I appreciate there could be impedance matching issues but has anyone tried it?

I guess it would be too costly in any case.

A.
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Old 24th May 2016, 11:01 am   #172
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Default Re: Solid state preamp for EL84

Are you now getting a decent volume out of the player ?

If so, as Lawrence pointed out, you may need to make some final adjustment to the pre-amp gain to suit the now (hopefully!) properly working EL84. Ideally, find a record with lowish modulation (as quiet as you can find, usually something with a very long playing time) and then set the gain so that the player just starts to distort (by ear, out of the speaker) at the maximum volume setting.

Ignore the clipping at the pre-amp output, it is always going to clip asymmetrically with a single ended amp

dc
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Old 24th May 2016, 6:07 pm   #173
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Default Re: Solid state preamp for EL84

Tims on holiday until next week I think.
We will have to wait with bated breath.

A.
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Old 28th May 2016, 12:51 pm   #174
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Default Re: Solid state preamp for EL84

Hi guys,

Am going to get back at it on Wednesday.

Will keep you all informed.

Many thanks,
Tim.
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Old 22nd Jun 2017, 10:12 pm   #175
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Default Op Amp preamp for a single stage valve amp.

Hi all,

I requested help with building a little amplifier to amplify the signal coming off medium to low output ceramic cartridges when being used in single staged valve amps.

After attempting a single transistor design that someone had kindly suggested on the thread I have now attempted another design (recommended on the same thread) using an Op Amp kit that was given to me by a forum member.

I don't know if anyone else has now had luck using an Op Amp as a preamp but I can say it's hands down the winner and in my opinion the best way of doing this sort of thing.

I used a velleman Op Amp kit and a small voltage regulated power supply using its own miniature mains transformer. It works much better than I was expecting and was much less hassle than a single transistor design.

I just want to thank everyone that contributed their ideas and steered me in the right direction.

I have bought another velleman kit and intend to keep using this design but am scratching my head a bit with what to use as power supplies the Op
Amps need a regulated supply of 10 to 30 V. Can anyone recommend anything suitable?

Thanks again,
Tim.

Last edited by Station X; 27th Apr 2018 at 1:50 pm. Reason: Threads merged.
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Old 22nd Jun 2017, 10:53 pm   #176
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Default Re: Op Amp preamp for a single stage valve amp.

Assuming you are using a separate transformer, then I'd suggest a silicon bridge and something along the lines of a 317 or other adjustable regulator. Or use something like a 7812 fixed voltage job.

Andy.
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Old 22nd Jun 2017, 10:57 pm   #177
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Default Re: Op Amp preamp for a single stage valve amp.

A couple of PP3 batteries works well too and as the power consumption is very low you'll get months of use, probably better for stuff only used occasionally.

Andy.
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Old 22nd Jun 2017, 11:34 pm   #178
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Default Re: Op Amp preamp for a single stage valve amp.

You don't actually need a regulated power supply. Audiophile phono designs in particular tend to have ridiculously OTT PSUs, but the current drawn by a single op-amp preamp is pretty constant (and low) so you just need to ensure that the on and off load voltages stay within the op-amp ratings.

If I were building one of these I'd use a transformer scavenged from something like a radio or ghettoblaster. These are typically centre tapped and can easily provide +-9V relative to the centre tap, which should be perfectly adequate whether you need a single or dual rail supply.

(Actually I'd probably steal the power from the existing amp somehow, but you seem to have decided against that).

On the subject of your old thread, the mods will normally reopen a closed thread on request.
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Old 23rd Jun 2017, 10:31 am   #179
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Default Re: Op Amp preamp for a single stage valve amp.

The opamp current is almost certainly low enough for you to steal it from the HT supply.

A conservatively rated WW resistor, 12 or 15 volt zener, a 470uF 25V electrolytic and a bit of stand off tagstrip will do nicely.

If the opamp has a dual rail supply, it gets a bit more involved but still not very.

Which Velleman kit are you using?
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Old 23rd Jun 2017, 10:49 pm   #180
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Default Re: Op Amp preamp for a single stage valve amp.

If the op amp would work at 8V (single supply) then you could get the supply off the output valves' cathode.
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