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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 27th Oct 2018, 10:51 pm   #1
yestertech
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Default DULCI DPA 10 Amplifier

I'm a sucker for a neat looking valve amp especially one that's crying out for attention. I picked up this DULCI at a recent Audiojumble. Its a pretty exact copy of the Mullard 10 watt amplifier circuit. It looked rather forlorn and was minus all the valves. I was in 2 minds ,in case either of the transformers was faulty, but the seller generously allowed me to remove the baseplate and test these with a conveniently placed multimeter. All was well so I took the plunge. After a quick clean, and some basic cold tests, the only problem areas to be sorted before power up were

1. open circuit main electrolytic ( both halves )
2. Leaky coupling caps to the EL84's ( already replaced at some point but 'past it ' )
3. Remaining HUNTS caps ( 2) to be replaced for good measure.
4. Cathode bypass caps missing on the EL84s ( 270 ohm cathode resistors had been replaced before, so perhaps 'forgotten' )
5. New mains lead ( an 8 inch lead is little inconvenient ! )

The electrolytic was a dual 50+50, lug mounted into a paxolin plate, riveted to the chassis. After careful extraction and a rummage in the parts bin, a 'Radiospares' 50+50+50 was found and tested fine on the ESR meter. Taller than the original, but reasonably 'period'
The new Caps were some 100nF /800 V orange drop which I found in a long forgotten drawer. The others being normal VISHAY type yellow jobs.

The 100K anode loads on the phase splitter were out of spec. so these were also changed. Mullard state that the mismatch caused by not reducing the value of one of these ( as per the Leak TL12 + ) was insignificant - more on this later.
A set of valves was fitted and power applied slowly. Up came a nice reassuring hiss ( over sensitive input, as designed, alas ) and the generator, power meter and scope attached to see what was what.

All seemed to be well and it looked clean on the scope at the rated 10 watts. The frequency response checked at 1W was virtually flat down to 10Hz and extends upwards to 30Khz so not bad at all A slight 'droop' is noticeable after 10Khz but this seems common with a lot of these designs. THD at 1Khz came in at 0.4 % ( spec is 0.3) at 10 watts. There was a problem with the ECC83 phase splitter, which would crackle as soon as the valve was touched. Having checked the base and all the soldered connections and applied some DE-OXIT, this turned out unusually to be the valve itself ( ostensibly a NOS Mullard ) - that's a new one on me. At first, I could not quite believe it, but fitting another ECC83 effected a cure - Weird !

As small diversion, I decided to check the balance of the drives to the EL84s and found they were not quite exact. To see if this really mattered, I attached a 1Meg pot across one of the 100K Anode loads and adjusted this until the drives were equal. This was then removed, measured and padding resistors of the same value ( 630K ) were wired across the anode load. Re-testing showed this had made an improvement and the THD was now down to 0.25% at 10 W.

As my bench isolated AC supply is slightly low at around 225 volts, I then repeated the test with 'full mains' This produced marginally more output approaching 14 W and the THD was still less than 1%. For a 'budget' amp. this is really rather excellent.
Just to test for true hum and noise the input was grounded and I could then only hear a very faint 100Hz hum with my ear right up to the bench test speaker. Providing the source impedance can be lowered ( with a suitable resistive pad ) then first stage noise won't be a problem.

All I need to do now is find another one !!

A.
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Last edited by yestertech; 27th Oct 2018 at 11:14 pm.
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Old 27th Oct 2018, 11:45 pm   #2
ronbryan
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Default Re: DULCI DPA 10 Amplifier

Maybe you also need one of these! The grey hammer finish matches your DPA10 amp. Only mono though.

I used this with my home constructed Mullard 5-10 in the 1960s.

Ron
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Old 28th Oct 2018, 2:18 am   #3
Radio Wrangler
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Default Re: DULCI DPA 10 Amplifier

No mention of Dulci amps for a long time, then two threads come along at once!

David
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Old 28th Oct 2018, 11:26 am   #4
Restoration73
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Default Re: DULCI DPA 10 Amplifier

Can't believe it's ten years since I did two amps and the basic (non EF86) preamp

https://www.bvws.org.uk/publications...olume33number4

I'm currently restoring a Quad II
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Old 28th Oct 2018, 9:55 pm   #5
Valvepower
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Default Re: DULCI DPA 10 Amplifier

Hello,

Nice job.

Ive a pair of those in the workshop awaiting restoration just need a round tuit

Regards
Terry
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Old 29th Oct 2018, 10:31 pm   #6
Phil G4SPZ
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Default Re: DULCI DPA 10 Amplifier

That looks really neat above and below chassis. Who needs stereo? A good use for a single valve amp is as a powered sub-woofer, preceded by a simple low-pass passive filter and followed by a nice big speaker. I use my single Quad II in this mode and it certainly enhances listening.

Well done!
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