The HEATHKIT MA-12 a MULLARD 5-10 In Disguise.

[vinylkid58]

Originally Posted by The Space Egg Corp

Firefox told me it was 'embarressed'

That used to happen all the time XP.

Originally Posted by The Space Egg Corp

Hope your OK Jeff and nothings happened like a catastrophic stay snaping crash or anything, am just asking as I burnt my baked potato last night while trying to get back on the 666 (sorry, typo) WWW last night.

Haven't had a serious crash in a couple of years. They usually involve interactions with cars or pick-up trucks.

Originally Posted by The Space Egg Corp

Funny you should mention the GEC 912-plus, I downloaded the GEC book of tube amps from Pete Millett's exellent site yesterday (haven't had a chance to look at it yet but I guess the 912 might be in there).
Got any pics of it ?

No, I was hoping you had a pic. I have a copy of that book from Audio Amateur Press. The amp is in chapter 3.

Originally Posted by The Space Egg Corp

The bass & treb use 2 Meg!!! pots, and the vol 1 Meg!!!, so would probably have to be stepped, as me thinks decent pots may be hard to get (I think they're doing this so the EQ caps are very small).

Personally, I would skip the EQ stage.

jeff

[The Space Egg Corp]

Originally Posted by Mr. Zenith

According to most data sheets I've seen the EL84/6BQ5 is only good for 300 V on the plates and screens. The schematic for the AA-151 here shows 355 V on the plates, and 360 V on the screens - Yikes! All I can figure is either the plate current is low, or those poor tubes are being beat to death!

Am I missing something here?

Hi Mr. Zenith

I seem to remember a comment by someone I used to work with years ago suggesting that the EL84 could be pushed way beyond it's usual operating parameters.

Have enclosed the 1961 & 1964 Mullard datasheets here.
They present operating graphs for both 250v & 300v.
So 355-360v is a bit above their expected application levels.
They also state the limiting value for the EL84 as 550v.

So maybe my old work mate was right. This is a very robust tube for it's size.
Looks like there's a fair amount of headroom here.
Maybe this has something to do with how good it sounds at an average level.
The EL84's in the amps I have here do get fairly hot at the 315v of Mullard's own design.

I was once told the VOX amps loved by The Beatles ran a lot of EL84's.
I haven't checked this in any VOX schematics, but in view of the limiting value of 550v stated in Mullard's 1960's datasheets, they may have run them even higher than Heathkit in the AA-151.

Thanks for pointing this out, as the user of the re-boxed AA-151 (photos earlier in this thread) seems delighted with the sound of it and seems to love the EL84 in general.
I assume their amp is running 360v from the original Heathkit transformer.

Might be worth checking out what VOX did and cranking it up a bit on my refurb.
I will certainly be fitting 450v components just in case.

Cheers...Simon...

The EL84 glows BLUE when operating.
Those clever boffins at Mullard predated the trendy blue LED by 50 years !

[The Space Egg Corp]

Originally Posted by Mr. Zenith

Yikes! All I can figure is either the plate current is low, or those poor tubes are being beat to death!

Thanks Mr. Zenith

Your comments on the EL84's driven to the max, reminded me of the VOX EL84 amps.

Did a bit of research today on 'The Jennings Organ Co' VOX AC-15.

2x EL84's with EF86 input tube, and an ECC83 thrown in as well, Great !

VOX seem to be stateing their output at 15w, 5w higher than the Mullard 5-10, and 3w higher than the Heathkit MA-12.

No transformer feedback on the VOX AC-15 either.

Do you have any favorite EL84 amps Mr. Zenith ?

Cheers...Simon...

[The Space Egg Corp]

Originally Posted by vinylkid58

I was hoping you had a pic. jeff

The Leak 'TL/20 Plus', another nice little EL84 amplifier made in West London in the swinging sixties.

[The Space Egg Corp]

Heathkits first major DIY success story was their late 1940's oscilloscope kit based on WWII surplus CRT tubes.

The keen diyer could build their own high quality oscilloscope at a fraction of the cost of a ready made one.

The scope kit was hugely popular, and the profits fueled the companys expansion to a larger factory.

Business boomed for Heathkit throughout the 50's 60's & 70's, promoting further expansion of their new factory.

Heathkits problems came when taking on the DIY computer market, here's one of the early analogue computer kits.

[The Space Egg Corp]

A.R.Bailey of Radford Electronics Ltd detailed 'improvements' to Mullard's designs in the September 1962 edition of 'Wireless World' magazine.

Radford Electronics Ltd produced exellent output transformers, and a kit version of their MA-15 amp was available as # CMA-15.

[kenpeter]

RE: #62
312 plate minus 12 cathode is 300V. Where is the overspec?

Excatly how (Cathode or GND) and which amp did you measure 360V?

[The Space Egg Corp]

Originally Posted by kenpeter

RE: #62
312 plate minus 12 cathode is 300V. Where is the overspec?

Excatly how (Cathode or GND) and which amp did you measure 360V?

Hi kenpeter

Not wishing to be pedantic concerning the 'real world' use of the EL84 and it's industrial cousins such as the 7189A seems worthwhile.

Whilst Mullard present examples of several EL84 operating points in their 1960's datasheets, manufacturers 'real world' use of the tube seems to vary widely according to how they see fit.

I am sure diligent manufacturers seek to optimise the EL84's sonic qualities in their products in numerous ways, as would the diligent DIYer to achieve their desired result.

It's curiosity over this that prompted a quick look at EL84 guitar amps such as the VOX AC15.
Having regretably never heard a VOX AC-15, I can't comment on it's sonic qualities. However, having extensivly experienced the sound of it's big brother the VOX AC-30 (4x EL84) the tubes are clearly exploited in a very different way to that of 'HI-Fi' amplifiers of a similar ilk.

I'm led to believe Carvin guitar amps run 400V+ in their EL84 amps, if this is the case there is no 'over spec' in their view.

Perhaps tube life is not optimised outside of average operating conditions, but perhaps the tube that burns the brightest sounds the bestest ?

Cheers...Simon...

Joe Strummer - High Operating Points ?

[kenpeter]

My Fisher X-100-3 w sockets labeled "7189" seems happy
enough with new JJ-EL84. Old 7189s found with amp were
unevenly lit, mixed bag that could not even claim to warm
at the same rate. Only one was an original Fisher (Mullard?).

I recall measuring 390 at the plate and 40 at the cathode...
Unfortunately, I don't recall measuring the screen.

Circuit is not a 5-10 clone, not that matters for purpose of
discussing EL84 operating voltage.

No screen glow, and the plates burn a dull brown/orange.
I've seen none of the blue that others report.

No, wait: I have seen blue w. Ruskie 6P14P-EB. But they
they didn't sound quite right at the original 7189's bias.
Nothing exploded due to voltages, they just didn't cross
smooth... Since the JJs worked with no hassle, I never
bothered dial in for any other EL84 types . Lazy I guess....

[The Space Egg Corp]

Originally Posted by kenpeter

My Fisher X-100-3 w sockets labeled "7189" seems happy
enough with new JJ-EL84. Old 7189s found with amp were
unevenly lit, mixed bag that could not even claim to warm
at the same rate. Only one was an original Fisher (Mullard?).

I recall measuring 390 at the plate and 40 at the cathode...
Unfortunately, I don't recall measuring the screen.

Circuit is not a 5-10 clone, not that matters for purpose of
discussing EL84 operating voltage.

No screen glow, and the plates burn a dull brown/orange.
I've seen none of the blue that others report.

No, wait: I have seen blue w. Ruskie 6P14P-EB. But they
they didn't sound quite right at the original 7189's bias.
Nothing exploded due to voltages, they just didn't cross
smooth... Since the JJs worked with no hassle, I never
bothered dial in for any other EL84 types . Lazy I guess....

Hi again kenpeter

Are you an EL84 'fan' then ?

If you were about to be shipwrecked on a desert island (with AC supply, ha ha), would you save your EL84 amp from the sinking ship ? or perhaps something else ?

Cheers... Simon...