Output transformers for 205D SET amp

Design and use of the various types.
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bigfunhorns
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Joined: Thu Aug 23, 2012 3:33 am
Location: New Zealand

Output transformers for 205D SET amp

Post by bigfunhorns »

Hi Dave,
Your permalloy AVCs volume controls continue to do sterling work
in my set up.
My DIY set amp is designed to use a D3A with CCS set to 10mA,
cap-coupled to a fixed bias 300B and grid chokes on the 300Bs.
The fixed bias is set to 60mA and the 5K primary amorphous-Nickel
output transformers are gapped for 60mA.
I recently changed the 300Bs for 205D DHTs, fixed bias set to 22mA,
B+ 370Vdc (filaments 4.5V dc). I'm really enjoying 1200mW
of triode 'muscle amp'! magic with my Big Fun Horns/Lowther PM6As.
Anyway, my question Is: I assume the 60mA gapped TXs are a mismatch with the 22mA 205Ds. What would you recommend as a
suitable 'no holds barred' output transformerand what would they
run to $ wise? Thanks in advance Kevin.
dave slagle
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Location: NYC
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Post by dave slagle »

Hey Kevin,

The 60ma transformers aren't that much of a liability but the 5K load is since the Rp of the 205 where you are running it is around 4300 ohms. If your 6A's are 16 ohms you can run them on the 8 ohm taps and that should help a fair bit.

Beyond that the first thing I would do would be to ditch the cap coupling for a 1:1 nickel IT. If that puts you in the right direction, then I would look to a 49% nickel 12-15K output transformer.

dave
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bigfunhorns
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Joined: Thu Aug 23, 2012 3:33 am
Location: New Zealand

205D set amp

Post by bigfunhorns »

Hi Dave,
You read my mind concerning the ITs! but I thought i
should ask about the output TXs first.
I'd be very interested in trying a pair of ITs and
I'll contact you about costs and delivery etc.
Kevin.
bigfunhorns
Posts: 10
Joined: Thu Aug 23, 2012 3:33 am
Location: New Zealand

ITs for 205D set amp

Post by bigfunhorns »

Hello Dave,
Trying to decide on the best way to use the ITs I ordered.
I've previously only built cap-coupled set amps.
My amp uses fixed bias on the output tubes but i could
re build it for cathode bias.
Do you have any thoughts or 'bias' either way?
Are the ITs bifilar wound as i understand the primary and secondary windings have to be in phase.
Do the ITs have the filament-standing current-cancellation winding
and if so is the polarity of the dc filament current important?
I have a box of Siemens 'post' D3As which i have used, triode connected as driver tubes. From your experiance with your ITs are there
any tubes which work better? I have a box of NOS E180F (7221) and
Raytheon 5842 triodes.
I realise that i will have to experiment to get the sound i like but any
advice is welcome.
Any idea when they'll be on the way to me, Kevin.
dave slagle
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Post by dave slagle »

Hey,

wire it up for the D3A / 7721 they have very different flavors from each other. you can then also use the 7788 which gives you a third flavor.

I'll send you some "magic" diodes for under the driver cathode then just set your B+ to give 10ma or so.

you can keep fixed bias and apply it through the secondary of the IT and no offset winding is used.

dave
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bigfunhorns
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Location: New Zealand

IT Driver Tube bias

Post by bigfunhorns »

Hi Dave,
Thanks for the tubes advice. I'll be sure to try and get some
7788 (E280F) ? tubes.
The designer of my DIY set amp suggested changing the D3A driver tubes from cathode bias to battery grid bias.
I think the 'Troels Gravensen' grid bias is superior to 'conventional' capacitor/resistor cathode bias.
I look forward to receiving the diodes and will be very interested to
try cathode diode bias which is reputed to be very good.
Kevin.
bigfunhorns
Posts: 10
Joined: Thu Aug 23, 2012 3:33 am
Location: New Zealand

ITs arrived

Post by bigfunhorns »

Hi Dave,
Removed the couplings caps and grid chokes and Installed
the 49% nickel ITs.
So each channel consists of battery grid bias (2.4V) on the triode wired D3A with cathode directly connected to ground without resistor or bypass cap. B+ is about 360Vdc and i have retained the CCS set to 11mA (which drops around 200V) and is connected to the IT primary and gives about 180V dc on the D3A anode. Bias supply connected to secondary of IT set to give 60mA bias on 300B triodes with B+ 0f 360Vdc.
Output Trans' are 5K primary amorphous nickel gapped for 60mA.
I understand that using a CCS with an IT is not 'normal practise and
that using resistors to get the correct anode voltage is accepted practise.
First Imprssions compared with 80%nickel grid chokes and teflon coupling caps.
ITs are Less overtly detailed and 'lite up but more listenable and
'musical' with improved timbre and tonal colour.
Very in-the-room feeling and non tiring.
One problem. I seem to have a rising bass response which is giving
a very punchy, solid bass from my Lowther Fidelios!!!! (no really)
but low bass (which isn't usually noticable) is now a bit excessive.
I've pulled the Fidelios away from the room corners but still abit too much low bass. Am i making some obvious mistake? is the CCS to blame?
any ideas Kevin.
bigfunhorns
Posts: 10
Joined: Thu Aug 23, 2012 3:33 am
Location: New Zealand

Post by bigfunhorns »

CCS gone and excess bass gone too.
Pure laziness not wanting to have to experiment with the right
resistor values to give the correct anode voltage and current.
Anyway 13.8K ohms gives me 10.5 mA and 185V dc on my D3As.
Amp still warming up but already liking what I'm hearing, Kevin.
suqun1
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Post by suqun1 »

If that puts you in the right direction, then I would look to a 49% nickel 12-15K output transformer.
pilav
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Joined: Mon Jun 16, 2014 4:21 pm

Post by pilav »

dave, this is an interesting thread and since I’m planning to use these output transformers in my circuit and getting the right amount of current is the key here. You have mentioned that D3A, 7721 and 7788 have three very distinct flavors. The term “flavor� is a little conducing for me here. Can you please loosely describe what sort of flavors can these be and what difference can I expect in my audio output?
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