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Old 14th May 2019, 9:14 pm   #1
aferguson
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Default Dulci DPA-15 amplifier info

Hi

I have just acquired 2 Dulci DPA-15 monoblocs which will be getting some gentle restoration. I can find very, very little about them online so far (a bit more about the DPA-10 but even then not much). The circuit has similarities with the Mullard 5-10 but also some differences. I've drawn a schematic from the two models that I have of the DPA-15 and was wondering if anyone has additional information on the DPA-15 and if there are any hints or ideas of issues with the schematic.

In case attachments don't work, links for images and schematic:
Schematic: ****************/gvgF2Q0
Underside 1: ****************/vYtGxfq
Underside 2: ****************/qprczs9
Topside: ****************/rpByWPY

Any hints or advice gratefully received.

best wishes

Andrew
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Old 14th May 2019, 10:43 pm   #2
GrimJosef
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Default Re: Dulci DPA-15 amplifier info

I've had very little experience with Dulci amps, but on the basis of the experience I have had you might want to satisfy yourself that you're happy with the safety earthing of the chassis.

As far as the schematic goes, the only thing I've spotted which is there in reality but not on the schematic is the mains inlet fuse.

Cheers,

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Old 15th May 2019, 12:52 am   #3
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Default Re: Dulci DPA-15 amplifier info

The transformers are very correctly the right size!!!

They should sound superb.

Joe
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Old 15th May 2019, 7:09 am   #4
Trevor
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Default Re: Dulci DPA-15 amplifier info

C2/ C5 / C8 / C9 should be as a matter of course be changed ( leaky ) all electrotlytics in the power supply checked changed
Amp is pretty much a standard Mullard 5 10 circuit build quality is better than an RSC/Linear a poor man's Leak. However the impedance selector and output transformer connection cannot be correct as it would be essential to use taps on the output winding that are not shown.

Having said that I would be delighted to own a pair, makes up a lovely retro system.

Trev
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Old 15th May 2019, 7:24 am   #5
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Default Re: Dulci DPA-15 amplifier info

Dulci and Elpico were both made by ''Lee Products ltd''.
The DPA15 was manufactured between 1961 and 64. And 15 Guineas when new.

Looks like a straight copy of the 5-10 circuit as you have drawn it. Just your C2 ground position is different. (I think Mullard changed it from the ground bus to the junction of R2/C3 early on.)

You may have missed the number of taps on the output transformer too that correspond to the 'impedance' switch positions?

Alan
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Old 15th May 2019, 8:09 am   #6
aferguson
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Default Re: Dulci DPA-15 amplifier info

Thanks everyone for your comments and insights. There are several resistors out of value as well, not surprisingly. As for the impedance selector, my apologies as I haven't fully traced that yet. I'll get that completed ASAP.

Bit of cleaning up then on to parts replacement, optimise the power supply ground, check the valves and then on to the variac.

Thanks again

Andrew
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Old 15th May 2019, 9:52 am   #7
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Default Re: Dulci DPA-15 amplifier info

The circuit does not seem any different to the Dulci DPA10, apart from the input attenuator network and a suspicious 100pF Miller capacitor between anode and grid of the ECC83 left hand side (noise reduction??). Have they cranked up the HT to gain more power or is the DPA15 just a badge engineered DPA10?

Ron
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Old 15th May 2019, 12:59 pm   #8
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Default Re: Dulci DPA-15 amplifier info

A pair of EL84s with 300v HT is good for about 17W, maybe a bit less in UL mode, so the circuit could be near identical to a 10W version.
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Old 15th May 2019, 1:16 pm   #9
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Default Re: Dulci DPA-15 amplifier info

It's worth mentioning, that although earthing the chassis via the mains lead is normal, when the amps are used alone, normally they would be earthed via the signal cable(s) connecting then to the pre amplifier, which would be earthed.

Having multipleearths could result in hum, so only earth one unit if this is found to be the case.
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Old 15th May 2019, 1:29 pm   #10
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Default Re: Dulci DPA-15 amplifier info

Problem with that is the conflict between safety earthing requirements and signal grounding ones. Especially if power- and pre- amps have their own separate power supplies. A safety earth should never rely on a signal connection for its integrity. Even apart from any standards issues it has to be capable of carrying enough current for long enough to trip whatever protective device(s) there may be in the ac supply to the equipment.

In an ideal world, the two grounds would be functionally separate except for a single point of contact somewhere.
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Old 15th May 2019, 3:24 pm   #11
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Default Re: Dulci DPA-15 amplifier info

I'm very much with Chris on this. Back in the day standards were different. But I would never remove a safety earth that was in place on any piece of kit, vintage or modern.

Relying on an audio interconnect lead is particularly risky, not least because in the event of a relatively low-current leak (say a few hundred mA) which was being kept under control by this earth, anyone putting one hand on the chassis and using the other to pull out the plug will, if the plug has a metal body, find the few hundred milliamps suddenly diverted through them - more exactly up one of their arms, across their chest and down the other arm. Nearly 40 years ago I saw a colleague of mine have exactly that experience (with lab equipment rather than audio) and to date, after a lifetime of working with electronics, it remains the worst electric shock I have ever seen.

Ground loop issues can quite easily arise with two monoblock amps used in a stereo system. Assuming they're identical then each one might contain one of Chris's 'single points of contact' which, obviously, would then no longer be 'single'. The resulting ground loop can be eliminated in this case either by breaking the ground in the signal line with a suitable quality audio isolation transformer or by inserting a 'lifting' element (the most effective is a pair of high-current silicon diodes wired in anti-parallel) between the safety earth to the amp chassis and the signal ground. The chassis can then remain securely grounded and in the event of a fault trying to raise signal ground to an unsafe level the diodes will conduct and limit the voltage excursion to (typically) less than 1V.

Cheers,

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Old 15th May 2019, 5:01 pm   #12
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Default Re: Dulci DPA-15 amplifier info

Lots to think about there for sure. Thanks for the detailed advice. What I would like to do before altering the ground is have the amps back up and running and checked over as things stand, then see about grounding alterations needed to both maintain safety and minimise negative effects on sound quality.

Sadly I've limited time to allocate to this so it will be a slow but hopefully steady process. Not entirely clear what makes this a 15W rather than a 10 but will be interesting to see all the voltages checked with tube load in place.

I have to say they are lovely little amps. Unfortunately the nice arch shaped cages have a reasonable amount of corrosion on them and I'm torn between limiting clean up to rust management and going further with a paint job. I might be able to get close to the grey hammered finish with some trial and error. It may be sacrilegious but I'm not one for the full "warts and all" approach to restoration. I think it should be possible to perform cosmetic enhancement to get closer to the condition of an "excellent used" amp of this age rather than leaving it "as is".

Quite a few resistors turning out to be fairly far off spec so I'd say many of the passives will be replaced. What are your opinions on resistor replacements given the horribly spartan look of modern metal films? I've used PRP resistors a fair bit and quite like the look, and there are some quite nice brown Vishays.

I'll try to get some additional pictures up as things go.

Thanks again for the input.

Best wishes

Andrew
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Old 16th May 2019, 9:58 am   #13
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Default Re: Dulci DPA-15 amplifier info

Certainly a common earth bond for amps and pre amp is ideal, in which case the
signal earth may not be needed both ends. However as this is a Class I device, if you
have any doubts about its safety it should be subject to a portable appliance test.

Finally to say that raw mains appears on the octal socket pins 3/4 and the 2 pin socket,
and some method may be devised to prevent accidental contact.
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Old 21st May 2019, 8:02 pm   #14
aferguson
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Default Re: Dulci DPA-15 amplifier info

Thanks for all the helpful info. I've attached an updated schematic and a picture of the work so far on amp 1. Things are moving slowly as I haven't had a lot of time on my hands, but progress is being made.

Best wishes

Andrew
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Old 21st May 2019, 9:19 pm   #15
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Default Re: Dulci DPA-15 amplifier info

First class piece of work both in the chassis tagstrip work and in the schematic
Damm near professional
Trev
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Old 21st May 2019, 9:53 pm   #16
aferguson
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Default Re: Dulci DPA-15 amplifier info

You're too kind Trevor. My cheapo Chinese soldering iron was trying to fall apart in my hand and I only just got finished before it did! I know, that's what happens when you don't buy a decent iron up front. Limited budgets are a right pain!
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Old 22nd May 2019, 7:23 am   #17
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Default Re: Dulci DPA-15 amplifier info

Very neat Andrew.
What make of resistors are you using?
Only change I would make is to move the 270 ohm away from the 47uF cathode bypass, so both have free space around them. Mounting the resistor as you have, directly heats the capacitor when the amp is in its working position.
Alan
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Old 22nd May 2019, 8:24 am   #18
aferguson
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Default Re: Dulci DPA-15 amplifier info

Good morning Alan

Thank you for your comment. The resistors are all 1W Vishay. In the original amps there was literally no gap between the resistors and capacitors so I deliberately separated them (see attached photo) which may not have come across in the first image. That said, I could perhaps move the resistors off to one side more so that their heat does not rise onto the capacitors directly.

Thanks

Andrew
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Old 22nd May 2019, 10:31 am   #19
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Default Re: Dulci DPA-15 amplifier info

Hi Andrew,
No it was not clear from the first shot, sorry.

But now I see a couple more potential problems.
The 270 ohm resistors were originally much larger wattage, 3 watts?
1 watt here is only just enough, far better to over specify and use 2 or 3 watt types for this location.

Second (and it may be a camera angle thing too) but the 0.1uF decoupling cap is not soldered to the buss?

Alan
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Old 22nd May 2019, 10:44 am   #20
aferguson
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Default Re: Dulci DPA-15 amplifier info

Hi

Sorry, I meant that the standard resistors (usually 1/2 watt) are all 1W. All resistors specified > 1W are 3 W types, including the ones you mention. The decoupling cap is indeed soldered...sadly I wrapped the lead several times before soldering so you are seeing the final emergence from that. I will have to be neater next time!

Thanks

Andrew
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