801A VT62 Power and Distortion

[IslandPink]

For those who can get past the doorman at TriodeMafia ( a few of us ) , Steve's ( as was ) practical results are in the 'Projects' section , thread title "801A / A2" .

Picture below shows distortion products from the amplifier complete , he was not entirely happy with the results . This amp used choke-loaded D3a in triode mode as a gain stage, then a crafty arrangement of a 5687 cathode-follower active-loaded underneath by a 6AK6 , but there are some 423A VR tubes in there to allow direct-coupling, somehow . Would need to look at the circuit more to understand it .

He also mentions that the 801A quiescent point of 480V at 50mA is 24 watts dissipation, a little excessive !

Perhaps the ACF-2 driver as tried by Nick might drive this cleaner ?

[Cressy Snr]

Yeah what the hell is going on with that place?
I'm buggered if I can see any forums even though I have registered.
Waste of bloody time!

Do you have to be blessed by his Fordship or something in order to access
the inner sanctum?

[Paul Barker]

The problem is Steve the forum is dead as a door nail.

One post every 10 years.

I do have access but you have not missed anything in the last 5 years.

[Mike H]

Fanstastic Tool. Here is your sweet spot!

That looks about how mine is set up IIRC, apart from the "500V" bit.

Pa is 15W I think. (As a consequence.)

[Paul Barker]

That must be good sounding.

[Dave the bass]

That must be good sounding.

It is.

I've heard it a few times, it really is pretty awethumne. 1st time was round Grandad Phil's, MrI and me looked at one another with this look on our faces....


DTB

[ed]

wtf happened to my reefer....I could swear it was there a moment ago

reminded me of a cheech and chong moment:

knock knock knock
'who is it'?
'it's dave man, let me in'
'ah sorry man, dave's not in at the moment'
'but I am dave, let me in'
'nah, sorry man, dave's not in at the moment, come back later'
'but this is dave man, let me in'
'nah sorry etc etc

[Mike H]

That must be good sounding.

It is.

I've heard it a few times, it really is pretty awethumne. 1st time was round Grandad Phil's, MrI and me looked at one another with this look on our faces....

Sounded excellent on Thomas's Lowthers as well .... multiple thuds of chins hitting floor ....

[Paul Barker]

For those who can get past the doorman at TriodeMafia ( a few of us ) , Steve's ( as was ) practical results are in the 'Projects' section , thread title "801A / A2" .

Picture below shows distortion products from the amplifier complete , he was not entirely happy with the results . This amp used choke-loaded D3a in triode mode as a gain stage, then a crafty arrangement of a 5687 cathode-follower active-loaded underneath by a 6AK6 , but there are some 423A VR tubes in there to allow direct-coupling, somehow . Would need to look at the circuit more to understand it .

He also mentions that the 801A quiescent point of 480V at 50mA is 24 watts dissipation, a little excessive !

Perhaps the ACF-2 driver as tried by Nick might drive this cleaner ?

Id screen shot the circuit, it is exactly as you described above Mark. I couldnt remember where Id got the screen shot. At least Ive got that, I have no screenshot of the discussion and Was googling to see if I could find the source, but to know avail. A couple of things bothered me, the bias voltage is printed as -32 on the schematic, that was a misprint, it should have said -52.. I couldnt make sense of why run the 5687 CF at just 4 mA. But Nick has kept nudging me a couple of times that the A2 grid current doesnt require the driver to specifically provide it. Im a passanger in where the grid current is provided from. But what the heck with the triode el84 CF there is no shortage of drive compared to a 5687 at 4mA.

I fully understand the direct coupling level shifter and the concept of biasing the output from the voltage amp of the input, same basic idea of the ACFII. Like the ACFII the level shifter could come from silicon diodes.

But Stephie was at the time on a complexed capacitorless trip which I bought into and build stuff without capacitors from the px25 amp at the first Eggborough, and that last beautiful amp I had running here at home only heard by one other, it was best sound Ive had. But especially the capacitorless power supply is massive and as its shunt regulated, very heavy and over sized.

At my age and frailty now and slowness, and dropping things all the time, which is also affecting my work life. So I’m building hybrid of the capacitorless hybrid structure Stephie showed us all that were following Stephie, I certainly was.

This amp Im working on, is much simpler to get right, no worries waiting for vrtubes to strike, sonsequence if they dont cascade of all valves in series, dropping like flies. Poof! like a mosfet! My precious 112’s 30’s etc are grossly deflated from that amps first turn on. I won it in the end. But nowadays Im not that bothered to go to that extreme.


Ill have LCL couple volt amp with MH41 mu of 80, grid chokes mean small coupling cap, and no high dc resistance, and anyway LCL coupling sounds good too. The operating point of 801a in Stephies design is 10k 500v, 40 mA -52v 20 watt dissipation for 9.5 watts power, and actually the distortion area is only in the last 3 watts, Ive decided the operating point is a good start.

[Nick]

Im a passanger in where the grid current is provided from.

The cathode where else? the clue is "g1 starts looking like an anode". And remember the current is made up of electrons flowing OUT of g2.

[andrew Ivimey]

Is Dave there? priceless !!

I can confirm that chins thumped the floor in unison.

[Nick]

Perhaps the ACF-2 driver as tried by Nick might drive this cleaner

Maybe, but that is just assuming that "if it was perfectly driven it would be lower distortion", I don't know of any reason to believe that (it may be true, but there is no evidence I can point to).

My solution in the 211 case was to make sure there is enough 2nd in the driver to cover up the higher harmonics.

The best solution I can think of for a low impedance driver stage would be a mosfet source follower driven by a floating op amp with the feedback from the mosfet source back to the op amp, but it would be fragile.

One thing that just occurred to me is you could do something like I do in the ref phono.

d3a as pentode driver, with current source in the anode with a parallel resistor to give a high load resistance without needing high B+, so get loads of gain. drive a cathode follower (5687) from that via a cap and voltage setting to the valve grid. Current source under the cathode follower. Drive g2 on the d3a with a mosfet source follower with its voltage set by a potential divider but AC couple some of the output of the 5687 cathode (output valve grid) back int the g2 of the d3a to provide feedback and reduce the gain to the value you want and also add error correction on the A2 drive.

[Paul Barker]

problem is I sold my D3a,s. to Andrew. but I like the local feedback opportunity it facilitates.

thanks for the explanation of the grid current’s journey.

im not using 5687 any more, el84. my 5687 just arrived. Thats how it works, you order a part for a project, when it comes its redundant. But no harm done brand new GE 5687 in stock, no shame.

anyone here with ability to check match got two surplus d3a to sell?

[Paul Barker]

As usual, big thanks Nick.

[Nick]

anyone here with ability to check match got two surplus d3a to sell?

I can probably find a couple for you.