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Old 22nd May 2019, 11:00 am   #21
snowman_al
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Default Re: Dulci DPA-15 amplifier info

Glad to hear that. All good then, you have it covered.
I'll go back under my stone for a while! ;-)
Alan
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Old 22nd May 2019, 11:50 am   #22
aferguson
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Default Re: Dulci DPA-15 amplifier info

Not at all Alan. Multiple eyes on things prevent simple mistakes that are all too easily made. If I'd been clearer with my explanations and pictures it would have made things easier. Feel free to offer advice and comment anytime.

Best wishes

Andrew
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Old 22nd May 2019, 12:31 pm   #23
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Default Re: Dulci DPA-15 amplifier info

Hi Andrew,

Excellent! Your work looks of a very good standard, these amps should sound really good.

I don't know an awful amount about Dulci but you show C15 in the schematic running from pin 6 of the EF86 (anode) to pin 6 of the ECC83 (also anode)

Is this correct? I ask because it seems odd and drawn like this pin 7 (grid) of the ECC83 is also connected directly to the anode of the EF86 which according to the schematic is at a potential of 88v! I have redrawn that part of the schematic and attached it to show what I mean.

Cheers
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Old 22nd May 2019, 1:41 pm   #24
aferguson
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Default Re: Dulci DPA-15 amplifier info

Hi

Thank you for the query. With reference to C15, this is an odd wee cap placed between 6 and 7 of the ECC83 in both of the amps that I have. The DPA-10, a close relative of these, is considered very similar to the Mullard 5-10, and does not have this capacitor. I don't know if it was a later addition to improve some aspect of performance. If this capacitor is removed, we are left with a circuit very similar to Mullard 5-10 and indeed 5-20 (see attached).

I'd be very grateful for comments form everyone on whether the circuits in my amps, reflected in the schematic (C15 removed in the attached), have been doctored along the way with resultant errors. They look to me pretty much like the Mullards other than C15.

Thanks

Andrew
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Old 22nd May 2019, 2:42 pm   #25
Hunts smoothing bomb
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Default Re: Dulci DPA-15 amplifier info

Hi Andrew,

I didn't read the schematic thoroughly, if I did I would see that the cathodes of the ECC83 are sat at 90v so with the grid being at 88v then it's actually -2v with respect to cathode so no problem!

Wish I learnt to read!!

Cheers
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Old 22nd May 2019, 4:44 pm   #26
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Default Re: Dulci DPA-15 amplifier info

Hi,

I figure the capacitor between 6 and 7 of the ECC83 (anode and grid) was added for stability reasons, it may well have been added by test when the circuit exhibited HF instability – but I suspect it may well have been permanently added after the original ETR schematic (on Pauls disc) was produced so the schematic didn’t show the addition/revision. .......Ah the echoes of the recent Leak TL50 thread

I’ve attached a picture the two DP15’s I have in the workshop awaiting restoration showing the additional capacitors – one of the few projects I do have to hand!

As regards differences I do remember there was a slight difference in the HT supply with one less RC stage in the Dulci.

Regards
Terry
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Old 22nd May 2019, 8:13 pm   #27
Edward Huggins
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Default Re: Dulci DPA-15 amplifier info

How nice to see this. We were Agents for DULCI and I still have the circuit diagrams for the DPA10 and it's 2 valve (EF86s) pre-amp, based on Mullard's publuc domain circuits.
It is elegantly printed on a large, glossy, white fold out. These, and the contemporaneous Tripletone amplifers, might not have hit the spot with Quad and Leak afficionados, but they were beautifully hand wired, had nice hammer finished chassis and used good quality components.
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Old 29th May 2019, 9:18 pm   #28
aferguson
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Default Re: Dulci DPA-15 amplifier info

Thanks Edward and Terry for the valuable info. I love an underdog and I'd love to see these up and running again!

As I work on these I have decided to do some temporary modifications to evaluate performance. I'm afraid I'm not wedded to the idea that all amps should stay in their absolutely original state (warts and all). I've been corresponding with Claus Byrith about the amp, following on from my interest in his mods to the Mullard 5-20. I was keen to try running the EF86 in triode mode, and Claus has kindly looked over my plans and made some helpful suggestions. I plan to implement these and see what the results are like, given that I have another original still sitting untouched, and can always go back to "plan A" if things don't work out.

The rough schematic for the modded version is attached. You will see a temporary cathode resistor and pot on the EF86 to establish the value required to reach Claus's recommended anode voltage of around 70-75V.

You will also notice some component removals, and several value changes.

I will need to do the work and then put this on the scope to see how the feedback resistor/capacitor values are behaving with the output transformer. I haven't gone beyond square wave examination before so if I end up in Bode plot land it could be Christmas before I struggle out!!!!!

Lots of tweaking required but I'll let you know how things go when I can.

Best wishes

Andrew
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Old 30th May 2019, 10:43 am   #29
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Default Re: Dulci DPA-15 amplifier info

The Bode plot is the easy bit..... cooking up a suitable feedback network to deal with what it tells you, achieve a useful degree of negative feedback and keep it all nice and stable is when the fun starts.

Square waves can tell you quite a lot- you don't want noticeable overshoot or ringing, nor a really slow rise and beware of a rising edge that starts off fast, possibly with a kink or ring part way up, then breaks over into something slower and more controlled- it's likely conditionally stable and may take off big time if ever driven into clipping!

Useful toys like circuit simulators can make cut and try methods for tailoring the feedback network response less fraught- you only get virtual magic smoke!
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Old 30th May 2019, 7:33 pm   #30
Valvepower
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Default Re: Dulci DPA-15 amplifier info

Hello,

Whist looking through some stuff to find a circuit for the PYE Super Black Box I came upon the original circuit for the Dulci DPA10 preamplifier and power amplifier and remembered this thread, and I felt for completeness I should post it on the forum.

I did a Triode EF86 mod on a Sterns 10Watt ECL86 amplifier see this thread:
https://www.vintage-radio.net/forum/...ad.php?t=99399

Regards
Terry
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File Type: pdf Dulci_Power_Red.pdf (212.3 KB, 16 views)
File Type: pdf Dulci_Pre_Red.pdf (234.2 KB, 11 views)

Last edited by Valvepower; 30th May 2019 at 7:42 pm. Reason: Add Stern link
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