Hell-marriage amp (5842-> IT -> 2A3)

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Dave the bass
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#1 Hell-marriage amp (5842-> IT -> 2A3)

Post by Dave the bass »

I know this is gonna come as a bit of a shock to the worldwide Audio Kingdom....but.....for one time only...... I appear to have built 'Not the best amplifier in the World'... I know! I know!... I'll get the smelling salts :shock: :-)

Amp cct.
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PSU cct.
Image


I knew this amp had 'history' and was well fore-warned, but I wanted to see what it was all about. Our-Will described it variously as sounding "Electronic" and "a marriage made in hell of 2 valves" or summat IIRC.

So, being a sucker I thought I'd see what all the fuss is about...Been super busy this weekend and built it...
Image

Image

Fixed Bias for 2A3 and IT betwixt 5842 and 2A3, simples innit. Yup, not a lot to lead us off the path to Aural Nirvana and righteousnessnessnessness.

Wrong!

Built as-is there's next to no HF. Bummer! Its dull, lifeless and has gargantuan bass (probably exacerbated by the lack of anything else in the audio spectrum).

I've built it exactly as per the cct using a 302AX our-Mr Counter bought a while back to for this actual amp as he was going to build it a few years back too, I bought the TX off him when it was advertised here. Also got 2 x Hammond 126B's (1:1) and Silver AE o/p's and lovely Full Music meshies + a few 5842, all known good. All voltages are as expected although i did have to sub in a 8K2 instead of the spec'd 47K/33K in the fixed bias network to get the required -45 volt on the bias pot range. Built with a 47K the bias on the grid of the 2A3 is about -68, odd. 8K2 gives swing from about -30V to -52V-ish.

So whats causing the distinct lack of HF? An idea's anyone? Only thing I can think of is the 22uF sitting on the wiper of the bias adj pot is acting as a low pass filter maybe? Or summat is pulling down the HF response here maybe? I've had to pack it all away for now as JTS (who described in 2 words consisting of 'crap' and 'Its'!) finally wanted the dinner table back so we could all have dinner, pfffttt.... girls :lol:

I built it more as a spare/fettling amp to give me summat to 'scope around on over christmas and play a bit with IT's and different front-end valves, just a project-amp really.

Anyway, looks like I've got some HF to find over christmas! That'll keep me busy.

Dunno if you'll see this thread our-Will but do you remember if your version lacked HF too? I know you weren't a fan of it.

Anyone else build this cct per-chance?

I'm sure it's not a 'build issue', I've triple checked everything.

DTB
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pre65
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#2

Post by pre65 »

Does the 2A3 cathode have to have a ground connection ? (other than through the heater circuit).

And does having the 2A3 heater transformer centre taps connected together help ?

And do you not need a grid leak resistor with an interstage ?
Last edited by pre65 on Sun Dec 12, 2010 8:53 pm, edited 1 time in total.
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thomas
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#3

Post by thomas »

Well done DTB!!
Just sought refuge here from the family viewing (the fiX- Factor.....!!)
Cct looks ok to me (as if I'd know..!) well at least 2A3 looks well fed and watered?...I'm not familiar with the 5842 is there any chance of subbing another driver valve in there? Whenever I build something new I always make some silly mistake or other I finally work out a day or two later ( I accidentally shorted the grid leak R's on my WE91 when I built it- took me 3 days to trace the fault! oops!)
Cheers
T
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#4

Post by david C »

well done Dave, thank god I never built it, I would have been lost :shock:
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#5

Post by Cressy Snr »

Good work Mr Bass

There has to be some big capacitance somewhere where it shouldn't be if
the HF is being pulled down that much.

Have you tried reversing the secondaries of the interstage transformers?


Steve
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#6

Post by Dave the bass »

pre65 wrote:Does the 2A3 cathode have to have a ground connection ? (other than through the heater circuit).
Aye, i know what you mean, it does through the CT of the heater TX dunnit. Its fixed bias so I know it looks 'odd' not having a cathode resistor or nowt there but you're introducing grid bias via a separate -45V supply via the secondary of the IT. the valve see's 250V across it and its grid at -45, its happy. Its amplifying Ok natch, but there's a distinct lack of zingle and spish.
pre65 wrote:And does having the 2A3 heater transformer centre taps connected together help ?
All CT's are taken the same star earth point.
pre65 wrote:And do you not need a grid leak resistor with an interstage ?
It has! :-) There's a 100K grid leak and a 100R grid stopper too. Are ye thinking of Grid chokes per chance? In place of a grid leak?

I'll get there, it can't be the design? Can it?

DTB
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andrew Ivimey
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#7

Post by andrew Ivimey »

or indeed the primaries!

I built something similar a couple of years ago ... involved 2A3 5482 and AE interstage. It too sounded 'less than teh best amplifier in the world' - all bassy and crappy - never did persevere... but if you do I'll have my answer ;-)

onwards!

and it appears that there is only a 1ohm resistor between cathode and ground - that's only there so you can measure the current, I think.
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#8

Post by pre65 »

Dave the bass wrote:
pre65 wrote:And do you not need a grid leak resistor with an interstage ?
It has! :-) There's a 100K grid leak and a 100R grid stopper too. Are ye thinking of Grid chokes per chance? In place of a grid leak?

DTB
I meant on the 2A3 ! I see some IT designs use the grid leak across the IT.

I think Mike does on his Aurora version 926,142,831 (f) revised. :wink:
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#9

Post by Dave the bass »

david C wrote:well done Dave, thank god I never built it, I would have been lost :shock:
:lol:

Cheers MrC, having the right mains TX certainly helped make the build easier.

DTB
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#10

Post by david C »

SteveTheShadow wrote:
Have you tried reversing the secondaries of the interstage transformers?

somewhere deep in the inner reaches of my brain I remember
Philip Ramsay saying
that you have to get the interstage secondaries the right way round

HTH
David

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#11

Post by Dave the bass »

Winding swappages? Nope, never thought of that 'un I must confess... Doh! if it turns out it is summat daft like that... I'll errrrr...... umm..... pinch Phil and make him cry, ha!

Ta for the ideas Messer's Creswell and Ivimey, we want onwards, lots and lots of onwards.

@ our-Phil. Re-Grid leak R's across the IT, Oooer, no I haven't notice that before TBH. I suppose if the resistance of the winding was higher than the spec'd max grid leak resistance i could see that being a reason but these are low, DCR is 175R.

I've got a pair of our-Mikes Danbury's to play with too at some time. The Hammonds are just straight forward 1:1. Thought I'd make things easy for meself to start with.

DTB
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#12

Post by pre65 »

How have you wired the interstages ? (what colour goes where)
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#13

Post by thomas »

I didn't fit gridleaks to the secondaries of my breadboard IT 2A3 and 300b jobbies.....don't think sound has suffered as you have described.... :roll: :?:
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#14

Post by Dave the bass »

pre65 wrote:How have you wired the interstages ? (what colour goes where)
Here ya go Phil, like this.

Image

and...

Image

DTB
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#15

Post by Dave the bass »

thomas wrote:I didn't fit gridleaks to the secondaries of my breadboard IT 2A3 and 300b jobbies.....don't think sound has suffered as you have described.... :roll: :?:
You pleased with your breadboard Thomas? First time you played with IT's too I'm guessing?What IT you playing with our-Thomas? Danburys?

I'm sure the Hammonds are Ok, summat else is playing up, I wonder if that 8K2 resistor is affecting things impedance-wise and casuing the HF to drop off.

DTB
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