Future plan for 300B XLS amp

andrew Ivimey · Post by **andrew Ivimey** » Thu Feb 12, 2009 8:30 pm

Oh dear there's a lot of good stuff here imho and I have no time to digest.. when I get back it'll all be dead busy again. I will read though!

IslandPink · Post by **IslandPink** » Wed Feb 18, 2009 7:57 pm

I can send you the circa.2006 circuit of the C3m-300B if you like, will make it clearer .

MJ

IslandPink · Post by **IslandPink** » Sat Mar 30, 2013 11:42 am

Well here I am again , might as well tack the latest mods onto this thread .

Background :
This amp in one form or another continues to be my mainstay .
In the last 12 months, after hearing The Shadow's amp at Owston , it was converted from C3m/KR300B to C3g/6B4G . It has been very good and with the regulated supply , capable of playing pretty loud even on the Ariel speakers . Bass is fast with good tone . Just occasionally I could do with a little more power .

Ok, so for my horn speaker project I was supposed to be building a multi-channel amp :
http://www.audio-talk.co.uk/phpBB2/viewtopic.php?t=3827
This is going slowly .... so slowly that it's stopping me from progressing with the horns to be honest .
Then I got sidetracked into a possible shunt-regulated power supply :
http://www.audio-talk.co.uk/phpBB2/view ... c&start=45
Which was always intended to be part of the final system in one form or another .
So I decided to do this , and have been buying the parts .
I made sure to spec the transformer with two HT windings, so I could get either ~320V or ~450V out, for 2A3 or 300B circuits.

In the meantime, two other things became relevant :
(i) Owston approaching
(i) Talking to James D about using a simple Series crossover to apply to the Bass/mid horn and the 288/Azura horn, to allow some evaluation & development without the multi-channel amp .

So, anyway, a new plan formed ...

(must take a breather ... )

IslandPink · Post by **IslandPink** » Sat Mar 30, 2013 12:12 pm

So, the plan was formed, and the intention is to create a versatile amp with a bit more ooomph, that can be used to test out the new shunt-reg power supply, and a variant on the driver stage . This will ( all being well ) be coming to Owston in 'stage 3' or 'stage 4' form :

(1) Convert the signal board back to 300B-form again. This involves ripping out the output trannies, filament supplies , C3g-related resistors and suchlike, and putting in the relevant KR300B bits ; C3m drivers , Dave Slagle output trannies & Guido Tent filament supplies .

(2) put another 1uF PIO cap up-front in the series-reg power supply and turn up the control Knob to 450V or so .

(3) Build the Shunt-regulated ( 'T-Rex' ) circuit as discussed on the other thread , using the new Danbury mains transformer ( 390V @ 650mA non-ct ! ) using 6N1P cascode and adjust for 450-ish volts.

(4) After seeing what this is capable of for a while, convert the driver stage from C3m to direct-heated pentode 4P1L . This may require Andrew's new direct-heated filament supplies, or I can improvise.
Operating conditions will be roughly : 32mA , anode load 7.5k , dissipation 6.8W , screen stabilised at 150V with OA2 as per usual .

andrew Ivimey · Post by **andrew Ivimey** » Sat Mar 30, 2013 12:32 pm

So you will let us know tomorrow around breakfast time, I should think.

Onwards!

IslandPink · Post by **IslandPink** » Sat Mar 30, 2013 12:52 pm

He he !
I've got you this time -
After a rather busy day yesterday ( between taking pictures of snow ) I have completed stages 1 and 2 : ( see attached ) .
Amps in-situ in Onken-enhanced Target rack . Built in the traditional 'ventilated/(death-trap)' style .

Reports on the sound will follow .... ( must dash - over to my ma's for Easter now .. )

andrew Ivimey · Post by **andrew Ivimey** » Sat Mar 30, 2013 1:01 pm

well done squire - all those bunnys, all those eggs!

Paul Barker · Post by **Paul Barker** » Sat Mar 30, 2013 1:13 pm

IslandPink wrote:He he !
I've got you this time -
After a rather busy day yesterday ( between taking pictures of snow ) I have completed stages 1 and 2 : ( see attached ) .
Amps in-situ in Onken-enhanced Target rack . Built in the traditional 'ventilated/(death-trap)' style .

Reports on the sound will follow .... ( must dash - over to my ma's for Easter now .. )

Very nice. I am reminded that when you built your first amp you took a lot of care over the woodwork, and made suitably shaped pieces to fit over any live contacts. you should go back to that idea.

IslandPink · Post by **IslandPink** » Sun Mar 31, 2013 11:41 am

Yes Paul, I'll do that - I'll have the two big 10H Sowter chokes to mount a partial cover from .

As far as the sound goes, it's interesting to go back from 6B4G ( Sylvania or Ken-Rad ) to the KR300B ( XLS ) . I couldn't exactly put my finger on the differences when I did the mods to do 6B4G .
Going back, the circuit with the KR300B's has more detail and slightly more treble tone/resolution, but it's a bit leaner in the bass, not so flattering . It's quite a different presentation. The bass timing and dynamics are still there though .
Apart from the differences in the valves, there are a number of potential reasons in the circuit though - will be interesting to do the various mods and see what happens . I can see several important factors :
1) Amorphous core trannies instead of M3 ( Purvine ) before.
2) C3m instead of C3g ( valve itself)
3) Guido Tent supplies instead of home-brewed Thorsten style ones
4) C3m driving big 300B at 18mA and 11.8k anode load may be struggling more than C3g at 17mA and 7.5k anode load driving 6B4G .

I suspect 1) and 4) are significant factors myself ... we shall see .

IslandPink · Post by **IslandPink** » Sat Apr 13, 2013 3:12 pm

So , now to get some info on which changes did what . Discussions with Nick and Bud Purvine suggested that a lot of the change could be due to the amorphous cores .
I got the go-ahead from Bud to put his M3-core ( 'level 2.5' ) transformers ( for 2A3/6B4G ) back on the amp ; they are somewhat overbuilt for the 2A3 and will handle 75-80mA OK with good inductance.

So, this sounds better - a bit more tone in the mids , and generally a more fluid sound, at the expense of a little edge sharpness .

OK, I've decided to forge ahead with the power supply change, and then see later if a change of driver might fill out the bass and lower mids on the KR300B's .

IslandPink · Post by **IslandPink** » Sat Apr 13, 2013 3:15 pm

As if by magic, I'm back to report on the next stage .
Attached , picture of status this morning vis the power supply ( on kitchen table ) .
Goodbye old friend !

(This is the series-regulated PS just before dismantling )

IslandPink · Post by **IslandPink** » Sat Apr 13, 2013 6:06 pm

Steady progress, see pic below.
Danbury mains ( 390V @ 650mA ! ) in situ .
No problems so far .... except this is going to be one heavy beast to move around ....

IslandPink · Post by **IslandPink** » Sun Apr 14, 2013 7:22 pm

Ok, it's finished !
( I think ... )
But dead as a Dodo

Nothing coming through to the secondaries on the mains transformer .

I wonder if Mike or anyone else who has a Danbury mains transformer can help ?

I have a IEC socket & switch and it feeds the primary side of the mains TX . The switch neon is lit. The two output connections of the switch are showing 247V AC mains relative to ground .
Mike/Dave's info on the primary TX side reads :

Blue Brown Black Red
0V 238 248 258

So, I have Blue and Black connected to the switch , for my mains .
Nothing doing on the secondary side - no HT , no filament AC , so nothing's lit .

Very frustrating ....

Dave the bass · Post by **Dave the bass** » Sun Apr 14, 2013 7:40 pm

If it was me... I'd do a continuity test on primary and secondary windings. If they're both OK apply 240V AC tothe primary and leave the secondaries un-connected to the PSU cct. to test if anything is pulling the TX sec/secs down firstly.

DTB

Post by **Nick** » Sun Apr 14, 2013 7:57 pm

Dave the bass wrote:If it was me... I'd do a continuity test on primary and secondary windings. If they're both OK apply 240V AC tothe primary and leave the secondaries un-connected to the PSU cct. to test if anything is pulling the TX sec/secs down firstly.

DTB

What he said.

audio-talk

Future plan for 300B XLS amp

#61

#62

#63 New Plan

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#70 Stages 1 , 2 , 3

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#72 Progress

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