Lunchbox ECL86

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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

Vvr problem!! just done a test of the voltage sweep on the vvr unit and this is what I got,minimum 31V,one third turn 1200V!!,two thirds turn 600V,maximum 312V. luckily all the tests i’ve done so far we’re on full 312V.
C5F5D870-E6CF-47E9-B0EC-078F89A7B102.jpeg
this is the vvr,the 100K goes to the pot wiper and the diode to the input lug with a 100K to ground from lug 3.
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TriodeLuvr
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Re: Lunchbox ECL86

Post by TriodeLuvr »

Well, it can't really be +1200V, can it? May be it's oscillating and fooling your meter.

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Re: Lunchbox ECL86

Post by Daviedawg »

Was it under load or isolated from the circuit when you tested?
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

it was in circuit under load Dd,going to remove the valves and retest on both diode & ez81 rectifier and use both of my multimeters to see if it's an oscillation like Jack mentioned.very quickly as i don't want to damage anything i.e. OT or filter caps etc.
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Re: Lunchbox ECL86

Post by TriodeLuvr »

I think you need a scope for this. Or try it with your meter on an upper range of AC volts.

Jack
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

I swapped the 100K to ground from the pot to 220K to increase the minimum voltage,removed the NOS ez81 and tested again with diode rectifier.first pic is minimum voltage,second pic about a third switched off straight away! the same with both my multimeters
DC68F187-8E8D-45CC-A4ED-79A73DA84F33.jpeg
61ACEA79-FAC5-4607-AB2A-A98715330678.jpeg
one area of uncertainty I can think of is the 12V zener was at a stretch across the turrets,I might remove it and extend the leads to get a good mechanical connection but don’t know if that is the cause or another “duff” mosfet?
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Re: Lunchbox ECL86

Post by TriodeLuvr »

Do you have a schematic for the VVR that also shows how it's installed?
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

files/18_watt_liteiib_vvr_pa_pi_141.gif
here's the schematic Jack,i did have a layout but struggling to find it.
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Re: Lunchbox ECL86

Post by TriodeLuvr »

OK, I ran a simulation of the circuit using a STMicroelectronics STW11NM80 MOSFET (almost identical to the Toshiba 2SK2698). The circuit seems to work well. Do you have a scope? It would be interesting to know if there's a waveform of some type at the "bottom" of the pot when it's turned up.

Jack
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

i have the exact vvr in my 6V6 amp Jack and it works perfectly, i'm gong to reset the 12V zener and if it's the same that mosfet is coming out! i do have an old analogue scope but have'nt even started learning to use it yet,besides i'm not risking over 1000V on that sweep into capacitors rated at 500V!
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Re: Lunchbox ECL86

Post by TriodeLuvr »

Well, it does sound as though one or more components have failed. However, I don't believe the meters are accurately registering the voltage. I also don't believe it's possible for this circuit to output a greater voltage than is applied to the input. In any event, it might be easier and faster just to replace the MOSFET and Zener and try again.

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Re: Lunchbox ECL86

Post by Bieworm »

Geoff.. I noticed you often put the pots above the tube sockets. Is there a particular reason you do that? Ever had communicating wires in that configuration?
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

purely for space bieworm,you can see how much room is in there and my first build had that layout because i copied the valvepower/old linear cage type amps.no problems with hum,oscillations etc but you have to be careful with lead dress,i lay the bias/cathode wires first and lay a lollypop stick across them for a height gap then the plate wires and signal wires last so if you do have to move them around they are on top and easy to get at.
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

Back on track! fitted new mosfet & zener and getting a nice voltage sweep now.going to rig the speaker up tomorrow and see what it sounds like,tweaking and attenuating time!
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Re: Lunchbox ECL86

Post by Bieworm »

Looking forward to the results Geoff!!!
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Re: Lunchbox ECL86

Post by wmwelch55 »

In my experience, the MOSFET needs a heatsink. Mine gets pretty hot.
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

Yeah it has a 2mm silicone heat transfer pad underneath the mosfet and the fibre board is 14mm thick.i will monitor it and see if there is a problem.taking some time out to learn the basics of using my oscilloscope now.
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Re: Lunchbox ECL86

Post by zaphod_phil »

geoff 1965 wrote:
Wed 06/16/21 12:05 pm
Yeah it has a 2mm silicone heat transfer pad underneath the mosfet and the fibre board is 14mm thick.i will monitor it and see if there is a problem.taking some time out to learn the basics of using my oscilloscope now.
The MOSFET will need to be mounted on a metal surface, with the silicone pad, to transfer its heat away, not the circuit board.
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geoff 1965
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Re: Lunchbox ECL86

Post by geoff 1965 »

Okay will do,might need to alter the board to suit.i’ve also sourced some FQA9N90 mosfets which are a lot closer in spec to the NTE2377 on the schematic.
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Re: Lunchbox ECL86

Post by musicheals »

Why not place a smaller heat sink between the mosfet and the fibre board. For better heat transfer you could mount it without an isolation pad, if the heat sink is not connected with the chassis....

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