Inexpensive balanced push-pull amp with DHT driver.

[Cressy Snr]

Not exactly miniature battery DHT drive, well not yet anyway.

I have transplanted the 6005 tubes into the balanced pp EL84 chassis, to check them out soundwise before committing to building something with them.
I don't think I am going to bother with the parallel single ended KT120 amplifier project.
The existing KT120 SE is just so damn fantastic sounding that I think chasing more power might just spoil everything.

The balanced PP concept however, offers such promise and potential rich pickings I would be a complete fool not to pursue it further.

The good news is that the 6AQ5/6005 sounds very very good as a 2 valve per channel balanced PP.
Power is down on the EL84 as these are only operating in triode mode (6WPC max probably)
but they have that EL84 liquidity, with a bit better detail and tone than the new production EL84.

We know that the 6AQ5/6005 is roughly a 6V6 in a miniature bottle and it sounds like a 6V6, ie bloomin' lovely.

Here's a little pic of the ugly little sod.



The 25SN7s (13D1) literally tower over the output valve lineup.
The missus wants the KT120 back (can't say I blame her given the ugly hacked about, fire engine red monstrosity that this amp is.
I've promised her I'll put back the KT120s tomorrow after I have evaluated the PP amp a little further.

[Cressy Snr]

Just been checking out prices for NOS 6V6

Makes the 6AQ5 7pin miniature equivalent a no-brainer purchase.

At 250V a-k and -15V bias at 30mA, they are running class AB, dissipating 7.5W at idle

The recommended class A1 triode configuration is 250Va-k and -12.5V at 49.5 mA, dissipating 12.375W; flat out at the maximum anode dissipation.

As JamesD has already mentioned, these tiny powerhouses are screaming out to be driven direct-coupled in AB2.

[Cressy Snr]

I haven't done a grainy glow shot for a while:


So here's one of a row of 6005s behind a pair of 13D1s

Oh... the little tubes have warmed up nicely and are sounding wonderful.
Treble harmonics on brushed ride cymbals and hammond organ swells are achingly beautiful. The tone is delicious.

Larrrrrvely

[Paul Barker]

I haven't done a grainy glow shot for a while:delicious.

Larrrrrvely

Too much information

[Cressy Snr]

I haven't done a grainy glow shot for a while:delicious.

Larrrrrvely

Too much information

Now now

[JamesD]

Steve,

Thank you for the 6005 info. I have been wondering what to do with these for a while as they are so cheap and there quality is excellent!

They just beg to be driven A2 as well. Attached are some load lines I tried against the triode curves..

[Cressy Snr]

By 'eck those loadlines sure do show the potential of this little beam pentode
Agreed on the cheapness of these things. Fantastic price/ performance ratio.

They are so inexpensive in fact, that I am about to start on a parallel push-pull, triode, class AB design with a six-pack of them per channel (three per phase) balanced all the way through of course. Maybe using a pair of 6AU6 per channel (to keep the seven pin miniature format) configured as a pentode differential driver stage, with the ubiquitous gas tube regulated screens; I haven't quite made my mind up about drivers yet. 6AU6 will idle at 10.6mA, with 250V a-K and 150V screens.
Power supplies will consist of a solid state rectified, choke input, 2350uF storage behemoth for the 12 output tubes, consisting of 5 x 470uF 385V 26000 hr BHC caps, backed up by a tube rectified CRCRC supply for the driver stages.

The Hammond 1650s will operate 8 ohm speakers whilst configured for 16 ohm operation; perfect for a 6AQ5 triple per side.

[Cressy Snr]

Just been discussing the merits of this latest 6AQ5 PP with my daughter Amy.
This one apparently does not sound like metal, in fact it sounds like music.
I sorta see what she's on about.

Now I have two amps of completely differing topology that manage to sound like music.
I know now, on whom to test out sound quality and it ain't me
A result methinks.

[Paul Barker]

It certainly is good when the family comment.

[Cressy Snr]

School's out

So tomorrow I will be doing the final mods to this amp experiment.
This will involve making up a couple of adapter plates to fit the octal
holes and allow a B7G base to be fitted in each driver stage position.
this will, in turn, allow the fitment of a pair of directly heated Mullard DCC90
double triodes configured as differential drivers.

When I used these last, it was in a directly heated, battery powered, triode phono stage.
When, a few years ago now, Simon and I auditioned this phono stage at his old place,
It proved to be a stunning piece of kit.
The DCC 90s were highly microphonic in the context of a phono stage but sounded
fantastic. Hopefully in a different context, as differential drivers, we will get the
same fab sound carried through the 6005 power valves.

If it works well, the whole amp wil be rebuilt over the summer months, (holiday, notwithstanding)
on a brand new chassis, scaled back to a quartet of 6005 per channel, giving about 15W output.

Watch this space.

[Paul Barker]

I was just looking at your chart above, and my wife came in. "What's that you're looking at!"

"Well that's his one, and now get that piece of paper over there and that's my one. Good isn't it?"

"You're a bit obsessive with it now aren't you!"



I need to get out more.

[Cressy Snr]

Indeed
Anyway..
Adapter plate made, sockets mounted, twin 2.5V heater transformers mounted and twisted filament pair run to each DCC90 socket.

Just have to do the twiddly bits now like..

Oh dear...oh my...oh dear Lippy.... I just know it's gonna hum like buggery

[Cressy Snr]

I just know it's gonna hum like buggery

Yep ...what a bleedin' racket. The 50Hz hum is deafening.
Absolutely no fscking use at all.

Silly thing to do using a 2.5V series connection of the filaments,
no worky like that! Needs a parallel configuration with the filament centre grounded, you daft bastard or you get, guess what...hummmmm
because there's no self-cancellation

Ah well I'll change the filament transformer config...tomorrow and try again.
Bloody DHTs!....Jeez!

[Cressy Snr]

Nah!
No point whatsoever in trying to use AC on these battery filament DHTs.
All the literature on the 3A5, I have found says that DC is essential on this valve.
Like the 26, these valves simply hum like buggery unless DC fed.
In the back of my mind, I knew this already, so I'm not sure why I pressed on.

I have a DIYHS DC filament supply somewhere. I'll have a go with that this morning. Ideally I'd have one per channel, but as our Ant has my other one in his nuvistor phono stage, this will have to do. At least it will tell me whether it is worth continuing down the miniature DHT driver route.

I'm not holding my breath.

The 25 volt Brimar 13D1 British versions of the American *SN7 family, driving push pull 6005s have to be heard to be believed, just stunningly good. The DCC90 is going to have to be something real special to beat the 13D1, and I doubt if the expense of a dedicated DC filament supply per valve is going to be justifiable; not when in this application, you can get just as good a sound from carefully auditioned and chosen indirect types.

[Paul Barker]

Are you using one half per channel Steve? In the drawing of the valve half the filament serves a side each so you will have problems yes. It would have to be used parallel in one channel or a fancy arrangement.

Fancy arrangement 2.8vct secondary hum pot between ct and each pole. cathode potential for each half taken individually from individual hum pots.

Though it would probably hum at that because of no cancellation, and the low level signal at this stage. You have got on great with idht's up to now elsewhere.