Dansette Major De Luxe volume problem - 2

[GeoffK]

My mod method would have saved track cutting, just the addition of a variable resistor or even a 'select on test' resistor to get the correct gain and left VR9 in place to adjust the output level to fine tune it, but pleased that your mod worked.

[awc]

Hi Martin

My technical knowledge doesn't extend much beyond ohms law, I'm afraid, so I can only go for the most part on results achieved for work suggested by those more competent than me.

My object here was not only to solve the problem of the increasing difficulty of getting hold of a high output cartridge for this player, but coming up, with the not inconsiderable help of worthies on this forum, with a cost effective solution I could use for future players that pass my way and to assist others with similar needs.

In this respect running the preamp off of batteries is not a long term solution, due to the practicalities of disconnecting the same after using the player and reconnecting when using it again. Making use of the 6.3v tap on the mains transformer to give about 9v dc through a simple rectifier circuit which of course goes on and off with the mains switch, would seem to solve the problem.

For AC/DC players, the easiest solution would probably be to connect up a separate small mains transformer giving say, 12v which when rectified would give about 17vdc and connecting up the preamp though isolating caps.

Please let me know if you can see not only any flaws in this approach but indeed any better solutions.

Thanks again.

Alex

[Michael Maurice]

Originally Posted by Hartley118

Now an EL84 in tip-top condition needs about 12 volts peak-to-peak input signal on its grid to give its maximum output,

Martin

But the most you'll get out of a crystal or ceramic cartridge is about 1 volt peak to peak, so to that end it is somewhat academic.

[awc]

Hi all

I have now made up a second Maplin kit as modified by Herald1360 and incorporating the further mod suggested by Hetrodyne instead of the mod recommended by Harley118.

The end result would seem to be similarly acceptable and with the variable resistor substituted for R2 (10k) set at a level which allows for maximum volume at acceptable distortion, it would seem that R2 should be reduced from 10k to 1k. Either solution would therefore seem to be ok.

As LM358's seem to be available very cheaply and no doubt a Veroboard could be adapted to suit the circuit quite easily to make construction costs even cheaper than the Maplin kit, that this is what I shall be doing in the future when refurbishing these one valve record players and using a meduim output cartridge.

Thanks again for all the invaluable help.

Alex

[awc]

Just in case anyone is rushing off to build one of these preamps, I have to report that having substituted the temporary 10k pot R2 for a fixed resistor of 1k (the reading on the pot when I removed it) that the volume isn't loud enough. I am at a loss to understand why this should be, but it is the case nevertheless. Further experimentation would appear to be required, but in the meantime I can confirm that the Herald1360/Hartley118 combination definitely works.

Alex

[GeoffK]

A resistor R2 equal to R1 would give a AC voltage gain (due to C3) of 2. Did you measure the variable resistor right?

[Hartley118]

Hi again Alex,

I was a bit surprised that you ended up with a gain of just x2, which is what a 1k feedback resistor would give you. I'd expect you to need something more like x5 to x10 gain because my limited experience of those generic ceramic cartridges suggests they only give a few hundred millivolts output. The original TC8H or similar crystal cartridge would have given several volts output from a typical vinyl 45 in order to get a cheerful noise out of the EL84 .

As ever, when you get a surprising result, try it again!

Martin

[Chris55000]

Hi!

Just thought of a little power-supply tip!

If you split the bias-resistor for the EL84 into two, using a 1 x 220R 3W from pin 3, plus 1 x 270R 3W from the bottom of the 220R to chassis, making 490R 6W total, you can use the 18-22V at pin 3 to power the LM358 preamp i.c., ***providing*** you decouple pin 3 to chassis with a cap of about 220uF 50V.

The - G.B. for the EL84's grid (pin 2) is obtained at the junction of the two resistors, with a 100uF 35V capacitor decoupling this point to chassis.

If the Volume Control IS the EL84's Grid Leak, disconnect it's bottom end from the chassis, replacing the direct connection with a 270R 3W and 100uF 35V wired in parallel. The 220R 3W then connects from Pin 3 of the Valve to the bottom tag of the Volume control, with a 220uF 50V from Pin 3 to chassis-earth. You can then lead the Positive Supply for your Preamp PCB direct from Pin 3 of the valveholder.

This will reduce the Anode-Cathode and G2-Cathode voltages of the EL84 slightly, but as the EL84 is such a high-slope valve (11.3 mA/V) the drop in sensitivity will be hardly noticeable, and you'll save having to remember the pre-amp battery!

Supplying the preamp this way ensures that it only gives it's full output voltage once the EL84 valve has fully warmed up, and will help to reduce any distortion that might otherwise be noticeable during the warm-up period of the output valve!

Provided you use adequate value decoupling electrolytics as I suggest you shouldn't run into any unforeseen feed-back woes!

Chris Williams

[Michael Maurice]

I'm restoring a radiogram and need to build such a preamplifier for an Ipod connection, I decided to measure the EL84 cathode voltage is approx 7 volts, so I'm going to use the heater supply through a 1N4007 or equivalent decoupled with a couple of thousand uF that should give me just under 9V.

One thing and it could happen, if the EL84 develops an internal short, it could blow that op amp to pieces.

[Herald1360]

Yes, but compared with the cost of an EL84, a cremated LM358 is probably not too much of a worry. If it is, just fit a suitable watty zener clamp across the preamp rails. That'll probably cost more too.......

[awc]

I have just found a little rectifier on board a small piece of paxolin consisting of four diodes in a bridge with two 1000uf caps separated by a 150 w/w resistor. Connected across the heater tap on the mains transformer, this gives about 8.0v smoothed DC. The little opamp seems happy with this and the whole thing works with very little hum, something which usually seems to plague me.

Options with this player were very limited as it uses a printed circuitboard so circuit changes would be difficult/messy. If the amp had been chassis mounted, I may have tried a valve substitution using a ECL82 or 86.

I am now going to repackage every thing back in its box as of course final sound quality is often affected by the acoustics of putting a speaker in its cabinet.

Will report final findings in due course.

[awc]

Well, I'm afraid the mains transformer wasn't up to the job of powering the heater of the EL84 and the rectifier of the opamp power supply. After playing just one LP I heard a sizzling noise and this was the mains tranny overheating and oozing wax out the bottom. I'll have to find a another small transformer and power it up separately, I guess. Oh why didn't I take up stamp collecting instead......

[Edward Huggins]

When adapting an EL84 amp, use the ECL86 as the anode loading is similar on the primary of the OP tranny. Not so with an ECL82. Edward

[Hartley118]

Originally Posted by awc

Well, I'm afraid the mains transformer wasn't up to the job of powering the heater of the EL84 and the rectifier of the opamp power supply. After playing just one LP I heard a sizzling noise and this was the mains tranny overheating and oozing wax out the bottom. I'll have to find a another small transformer and power it up separately, I guess.

Well that's a surprise. Your rectifier/smoothing capacitor board seems like a good solution, but it sounds like it's got a fault, such as a short-circuit capacitor. No way would the tiny current drawn by the op amp overload the transformer. When you disconnect the rectifier board, does the overheating stop? If so, check the current that it's drawing.

Martin

[awc]

Just measured it and the supply to the opamp seems to draw about 315ma on load.

[awc]

Just measured the current on the PP3 battery and it's about .43ma. Just a bit of difference!

[Edward Huggins]

Having suggested the option of an ECL86 in Post #33, on second thought I'm not quite sure if the Midget OPT in the Dansette "Major" would justify it! You seem to be really struggling with this and have already spent time and money on it. Why don't you just source a correct cartridge (import) and try to keep the player original as possible. At the risk of being boring, I did sugest this earlier. Edward

[awc]

Hi Edward

Thanks for your comments and whilst I could take the import route you suggest, I am really looking at solving the upcoming long term issue of an extreme scarcity of high output cartridges at a cost effective price and also sustainability of continued use of the players in the longer term.

I felt that if the players could be made to use the "red" generic cartridges now, if and when a further cartridge replacement was necessary in the future, then the players would already be kitted out to enable another medium output cartridge to be fitted.

I also was looking for something which restorers of my limited technical ability would be able to do by either using a ready made kit or wiring something up following a simple circuit on a bit of circuit board.

The opamp here, utilising the mods suggested by Herald1360 and Hartley118 in my opinion fits the bill both in terms of using cheap, easily obtainable components and needing relatively modest ability to construct and wire up.

Once I have overcome the last remaining issue of knocking up a suitable power supply for the opamp which also fulfils the above criteria, I think I will have achieved my goal.

Alex

[KeithsTV]

I think that the problem with your power supply derived from the 6.3V heater supply is that one side of the heater winding is connected to the chassis. If you're using a bridge rectifier with the negative output connected to the chassis this will cause a short circuit on the heater winding.

At the current you say the op amp circuit draws you can use a simple half wave rectifier and a single capacitor. If there are not enough volts to the op amp you can use a voltage doubler to increase the op amp supply to around 12V.

Keith

[ms660]

Is there a fit all kit for this reoccurring problem? If not I'm surprised, job for a young entrepreneur with a bit o'nouse maybe.

Lawrence.