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Old 6th May 2003, 04:49 PM   #11
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Guys,

Quote:
Originally posted by James D.
THe GM70 looks terrific.
It does indeed.
There's a Yahoo group for it with a lot of great members. The group started out dedicated to the GM70 but has evolved into the Boatanchor Amp Club open to all with an interest in big heavy amps. There are some building 212 and GM100 monsters.
Here's a GM70 datasheet
I'm still hunting for some copper anode versions.

Quote:
Originally posted by dhaen
I am using AC heating. For SE, 20 volt AC heaters would be a bugger.
How much hum do you have at the output? Even in PP I couldn't make mine quiet enough, but if you're doing OK in AC, maybe I'll try again.

Quote:
Originally posted by dhaen
I forgot to add that another thing against the GM-70 is the socket.
Most people seem to convert (drill out) septar sockets, which seems a little unreliable to me. Only later, did I find a US company that can supply the real thing.
Would you mind sharing who can do the sockets?
I'm using a big metal cap clamp to hold mine in place, and some brass crimp-on connectors pushed on to the pins. I figured it's worth the hassle because these aren't tubes you can roll.
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Old 6th May 2003, 08:08 PM   #12
dhaen is offline dhaen  Europe
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Brett,


Hum:

Last time I checked it was the magic 5mV. My speakers are quite inefficient, though I can't put a figure to them.

GM-70 socket:

This is the post I remember from AA:

http://db.audioasylum.com/cgi/m.pl?f...et&r=&session=
It actually doesn't look promising because of the minimum order.
There has been a lot of sicussion on AA & DIY which is searchable.

Cheers,
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Old 7th May 2003, 01:10 AM   #13
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Hi John,

5mV is a fraction loud on very high eff speakers, but I still might revisit AC, and see if I can better the 2mV I had.

I've spoken with Doug and other Boatanchor members and the Brenner's socket is apparently a modified septal, and not all that great. Then there's the $US100 min order. My solution might be the best, and certainly the cheapest.

Toodlepip
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Old 7th May 2003, 05:16 AM   #14
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Default neutralising caps

James/Gurus,
Would you pls enlighten me on how these
caps work and what they neutralise?
What are the component values in your pp amp?

Thanks in advance
Yv
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Old 7th May 2003, 07:42 AM   #15
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Default Sorry all guru's are busy at present, but will I do?

Yves,

Neutraliising caps are fitted to couteract or cancel the Miller capacitance of triodes.
I believe the term comes from RF electronics, where the technique is widely used.
The idea is to feed an "equal and opposite" signal to the grid, thereby "neutalising" the efect of the Miller capacitance.

Miller capacitance is due to the proximity of the grid to the anode (plate). It is worse in high mu valves.

Cheers,
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Old 7th May 2003, 07:46 AM   #16
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Default ...it's a positive improvement...

Hi Yves,

As they are connected anode to grid between a push-pull pair they are providing positive feedback and frequency dependent feedback at that to 'help' drive the input capacitance of the triode. This would be a modern description.

Traditionally it is designed to increase the high frequency current drive into the triodes input capacitance providing extra current drive at higher frequencies when the driving stage may be starting to run out of steam. Hence the intention is to 'neutralise' the high input capacitance of the output triode.

It is an old technique (1920s first use) more commonly found in RF circuits before pentodes took over. However Western Electric, amoungst others, used it in PP audio circuits.

Lynn Olsens mentions it in his Rosetta Stone article. A Rosetta Stone for Audio . He found that it caused too much overshoot on squarewaves but I have found it helps in my circuit. It is very component variable and I am using quite different components from Lynn. As always try it and see YMWV... (Your Milage Will Vary).

ciao

James
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Old 7th May 2003, 08:14 AM   #17
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Default ...practice makes...

Hi John,

The 22uF capacitor connected to the negative grid supply was arrived at experimentally... Origionally I had tghe 2uF and 22uF the other way around but tried swopping them and prefered the sound that way around. It doesn't make sense to me and my lab. is in pieces so I can't measure what is really going on...

Any suggestions welcome - 'cos I'm confused!

ciao

James
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Old 8th May 2003, 11:16 AM   #18
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Default very clever...

...the neutralising cap technique, and those who
chipped in too Sure, you qualify, John

Regarding the 2uF and 22uF, does it have anything to
do with the resonance of the capacitor with the inductances
of the transformers? If there is a particular frequency band
that sounds particularly good with the swap, maybe it
is an indication of this effect.


Yv
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Old 9th May 2003, 12:12 PM   #19
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Hi Yves,

I drew out the signal input circuit loop to look for resonances, see below. Is this what you had in mind?

I can't immediatly see where the 22uF Cbypass would help reduce a resonance effect. Have you any idea what is going on here? Any suggestions would be appreciated.

ciao

James
Attached Images
File Type: gif input signal loop.gif (3.6 KB, 380 views)
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Old 9th May 2003, 12:50 PM   #20
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Default An observation...

Hi,

The Q of the "resonant circuit" will depend on the source impedance of the bias power supply, and the position of the volume control.

James, can you estimate the inductance?

Cheers,
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