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

What people are working on at the moment
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#181

Post by pre65 »

simon wrote: I have some 6EM7s, principally for a Barker-Kaufman one day, but I really should try it as VA and CF sometime, perhaps as a replacement for the 12GN7 driving the GM70s.
CCS loaded 12GN7 (as a triode) works well Simon. :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Dave the bass
Amstrad Tower of Power
Posts: 12273
Joined: Tue May 22, 2007 4:36 pm
Location: NW Kent, Darn Sarf innit.

#182

Post by Dave the bass »

I liked that Anode choke load version you did Popz, that was a cracker (IMO).
"The fat bourgeois and his doppelganger"
simon
Thermionic Monk Status
Posts: 5600
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

#183

Post by simon »

Yeah, choke loaded trioded 12GN7 works for me :-)
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8867
Joined: Mon May 21, 2007 9:42 pm

#184

Post by Paul Barker »

simon wrote:Interesting stuff. I would never have thought that an unbypassed cathode resistor would have such a large reduction in gain. I might have guessed at 30% but not 80%! This gives me some interesting things to consider. How do you work out the gain for an unbypassed cathode resistor?
In a nutshell it depends on the size of the cathode resistor as to the extent of the problem. So each time you half the resistor you near enough double the gain. I always like the sound of shared cathode resistors in any case.

The gain formula is complexed but the cathode resistance has a very influential part to play. Once bypassed it's effect is zero.

Using compound curves and your known quiescent operating voltage and current you identify the actual transconductance and anode resistance. Insert them into the following formula which incorporates all other resistances involved. The usual way s is portrayed in data sheets, such as 1 or 6 that sort of figure has to be devided by 1,000 for the formula.

Image

For direct coupled I use a grid resistance of 1 meg, infinity doesn't affect the formula significantly more than 1 meg. Grid resistance is the least influence as long as it is at least 200k which it normally is.

Just to give you an idea of the influence. Say the parallel resistances are all 5,000 ohm the s is 6 with bypassed cathode gain is 5000/166 = 30
Add a 1,000 cathode resistor 5000/1166 = 4.2
share the cathode resistor between channels 5000/666 = 7.5
make two stages and share a single resistor across all four cathodes (therefore no direct coupling possible) 5000/4166 = 12

By which time it is almost fixed bias with some odd cross influences. But sounds OK ish. Wasn't the best amp I built.

I have never found a problem sharinf cathode resistor across channels in a single stage and in some ways it is a cheap trick that influences stereo separation for the better, not everybody likes that fact but I don't get hung up on what happens just enjoy the sound. What the heck it's done all over the studio recording with microphone trickery.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
simon
Thermionic Monk Status
Posts: 5600
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

#185

Post by simon »

Thanks Paul, yes that's quite clear the cathode resistor makes a huge difference in the denominator.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8867
Joined: Mon May 21, 2007 9:42 pm

#186

Post by Paul Barker »

Paul Barker wrote:[make two stages and share a single resistor across all four cathodes (therefore no direct coupling possible) 5000/4166 = 12
Edit:

5000/416 = 12

Just in case anyone is now confused but was following up to that point.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8867
Joined: Mon May 21, 2007 9:42 pm

#187

Post by Paul Barker »

OF course the tricks to reduce resistor size whether filament bias or shunting a ccs through the cathode resistor from a known stable source or a tightly regulated B+ could be tried.

the shunting of a ccs through it was decried quite volubly in the past with the claim "you would hear the ccs". Might be ways to mitigate the "influence of the ccs" with an equivalent but inverse "influence" on the ccs to the resistor on the next stage cathode in a cascade.

the meddler could go wild with all the options.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
User avatar
Dave the bass
Amstrad Tower of Power
Posts: 12273
Joined: Tue May 22, 2007 4:36 pm
Location: NW Kent, Darn Sarf innit.

#188

Post by Dave the bass »

Surgery completed, the Marriage-amp survives!

Welcome to your new home Mr 5687...
Image

The amp...
Image

The PSU(s)...
Image

Sound? Smoother and more detailed than a 6C45 being thrashed. Really noticable on a track where there's masses going on, its easy to listen 'inside' the envelope. Very very HiFi in comparison to the previous driver valve. Very interesting. I'm transferring it downstairs to the proper listening room for some serious furrowed brow listening sessions.

Whahey!

DTB
"The fat bourgeois and his doppelganger"
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#189 Re: Hell-marriage amp (5842-> IT -> 2A3)

Post by RhythMick »

I know this is an old thread but I just wanted to say thanks to all for superbly informative and educational posts.

I've built my own amps now (I'm a newbie of 2 years) which take a balanced input, convert to SE through input transformers, and then are 5842 -> IT -> 45 -> OPT -> Lowther DX4 speakers (in parallel with active subs).

IT and OPT are Tribute, the 5842 is self-biased but the 45 is fixed (trimmable) via the IT secondary. The mains TX is a custom build with separate windings for the 45, 5842 and Bias - each of which is 6X5GT rectified then LCLC smoothed.

The last improvement (maximised the inductance in the LCLC smoothing) made a huge difference and now the amps sound superb to me, also my brother who has a pair. There's a little hum - the heaters are AC and currently fed directly from windings on the same mains TX, this thread has prompted me to try separating those off - thanks.

I'm in the UK and would love to know of any hifi meets - I'd like to listen to other builds and get some constructive feedback on mine.
User avatar
andrew Ivimey
Social Sevices have been notified
Posts: 8307
Joined: Mon Jun 11, 2007 8:33 am
Location: Bedford

#190 Re: Hell-marriage amp (5842-> IT -> 2A3)

Post by andrew Ivimey »

Sounds good! Do the output valves have centre tapped secondaries with the centre tap taking the cathode resister to earth?

I'm surprised you have hum, you see. I have 45pp monoblocks where the custom wound trafos have quite long leads to the output valves but no hum. With my 2a3 amps I've always used Hammond heater transformers mounting them as close to the valves as possible with centre tap to earth via the cathode resistor with no hum. Using 300bs (5 volts @ 1amp so pretty much the same wattage as 2a3s) I cancel hum with a variable resister across the heater and the cathidecresistor to the centre tap - It works fine... even with the sensitivity of the dread Lowther.

When I got up to the dht valves that use 7+ volts at increasing rampage I started to use d.c. heating up to 60watts of GM70 and beyond, the gorgeous ridiculousness of Gu48s.

But with 45 / 2a3 world I love the simplicity of simplicity.

Interstate trafos are always to be preferred!!!!!

There is an Owston meet in May, I believe.
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#191 Re: Hell-marriage amp (5842-> IT -> 2A3)

Post by RhythMick »

At present I'm sticking to single ended - however I have PLANS .... mwah mwah mwah ...

Seriously I figure I'd like to get this amp right before launching into the world of PP, though I do plan next to build an all-DHT PP ... 26->IT->71A->IT->45.

Back to the current amp. The 45 has a humbucker arrangement :22R on each filament pin to either end of a 10R linear wiper - I found that idea on the web somewhere and it focuses the wiper on the centre of the range and gives a better chance of finding the sweet spot. Output pin down to ground via 10R sense resistor (it's fixed bias).

As I said I'm still a newbie builder and it probably comes from poor layout, which I can work on. I'll also offload the heater windings onto separate transformers, I have them all ready to go just need to get my derriere into 1st gear...

Blimey Owston is just darn t'rowd. How do I get involved ?
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#192 Re: Hell-marriage amp (5842-> IT -> 2A3)

Post by RhythMick »

Andrew I just re-read your post and something you said makes sense (kinda) but surprises me. Let me explain.

I'm referring to your comment about mounting the trafos for the DHT filament as close as possible to the valve base. I get that this brings hum reduction from the secondary windings by keeping them as short as possible - but it brings mains AC into the signal section ? In my naive way I've been religously keeping mains way out the back and signal across the front.

SO, thinking it through. Is it because with the heater trafos the secondary carries all the current ? And it's current loops that generate hum ?

The 45 is 2.5V @ 1.5A - so the primary (240V) will draw 15.6mA. So in this case better to have the lower current primary going to the front and keep the higher current secondary short ? Is that right ?

Blimey if that's true then THANK YOU. Bit of an eye-opener to me that one.
simon
Thermionic Monk Status
Posts: 5600
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

#193 Re: Hell-marriage amp (5842-> IT -> 2A3)

Post by simon »

RhythMick wrote: Mon Feb 19, 2018 10:17 pm Blimey Owston is just darn t'rowd. How do I get involved ?
A little nearer the time Steve will ask for numbers so he can arrange the buffet, just put your hand up. It's usually about a tenner for the Saturday. Then you just turn up, listen to music on people's kit, drink beer and talk shite.

(The last two aren't mandatory but are encouraged :-))
User avatar
andrew Ivimey
Social Sevices have been notified
Posts: 8307
Joined: Mon Jun 11, 2007 8:33 am
Location: Bedford

#194 Re: Hell-marriage amp (5842-> IT -> 2A3)

Post by andrew Ivimey »

In a word, Yes :-----)
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
RhythMick
Old Hand
Posts: 747
Joined: Fri Jun 23, 2017 12:24 pm

#195 Re: Hell-marriage amp (5842-> IT -> 2A3)

Post by RhythMick »

Thanks Simon. Can I bring kit?

I guess people might be nervous about connecting other people's DIY amps to their speakers so I could bring lowthers, amps and my autoformer passive pre. Need a source with balanced outputs but I can bring one.
Post Reply