GU81 Triode Connected Push Pull

[Paul Barker]

I found a pair of these last time I was at the lockup.

I’ve found a thread on diyaudio about it.

I calculated the ri using the Rosenblit formula. Change in volt(50) /change in current (.1) 500 ohm along the same bias line. I calc’d it on the -80v load line.

Someone came up with a spice model which I dont know how to use but they stated gain was 3.5 so transconductance by calculation becomes 7 mA/V

Well I dont have any SE transformers 1k5. And SE transformers couldnt cope with the dc current. So I looked at the Sowter 8k peak to peak connected to 4 ohm speakers triode connected. The distortion was better than ul and Im not short of power so chose class A1 triode connected. Im not hungry for power.

I have respected the g2 max of 600v. Also because I can generate sufficient bias voltage with various power transformers I own to use auto bias.

There is a justification by the way to consider the set point of +600v g2 in the sense if you fix bias -300v the g2 will see 900v it will be safe. As Im using auto bias I could use 900 anode volts with g2. I shant because I don’t need to.

I would say here gu81 doesnt appear particulary distortion free there are better valves that are triodes already, such as 212/833/211.

But these valves are cheap and cheerful and I have a pair.

So I shall use that pair to make a 30 watt mono amp, if only for fun.


I found bases for sale £59 each which is affordable for a mono amp. The valves are cheap.

If I love it I have enough parts to make a second channel.

I shall enjoy the process.

Lets have some fun.

[Paul Barker]

3 pictures but only two valves. Probably bought them 25 years ago for £30 each. At last enough internet savvy for people to try them. So I can join in.





A little bigger than 833b

[Paul Barker]

Very affordable today. I consider the base a bargain too considering it accepts some heavy voltage and current.

[Paul Barker]

When Ive picked them up from the lockup I can show you them relative to the 833b.

I’ve ordered the two bases and 4 top caps for the monoblock.

If it’s a success I’ll build a second buying the valves bases and top caps that requires.

Not too expensive to try it out.

[Paul Barker]

If I were to go se when I go stereo it would go parafeed, possibly big inductor and the sowter pp connected peak to peak, again into 4 ohm. That way I would have no need for any more valves bases top caps uncle Tom Cobbly and all.

Or if I love the pp. version: buy the rest.

I probably have not identical but two 30 henry inductors 250mA dc capable. Which I’d have to deprive the power supples of.

As these as triode are only 500 ohm. That should do. But I could half resistor load with the 30 h choke load as Im not short of high voltage transformers. A hybrid load.

[Paul Barker]

As expected distortion looks better employing the 250mA capability of the chokes, and I’ll have enough power supply headroom for half resistor load. At 250mA dc the distortion is better.

Ive added more anode voltage employing the concept if fixed bias potential difference of screen tolerates 900v. Makes sort of sense but I’ve only employed 750v of it.



Though Ultralinear doesnt half look good. This isnt consuming the extent of classA1. Ive only shown equivalent power comparrison.

[andrew Ivimey]

I read somewhere where they used g3 as the anode - all the looks but without mega power SPl wise.

[Paul Barker]

Yes. But also connect g3 and g2 to anode but g3 burns up so connect it with a 10k power resistor.

Then there is ground g3 and anode use screen as the anode. All talked about.

Same Ideas as were ruminated with 813. I didnt hear any benefits compared to standard expected method with 813. Boys and their toys get carried away.

Nobody I can find has drawn grid lines for any of those ideas but essential to know the 3 characteristics which change every time.

So I’ll not waste time on the unknown characteristics. As I said 813 better as an 813. Likewise here I would summise.

Not to mention 813 lacks the sweet pure sound of the 211. I described it then as a tractor. Quite rough. 211 more like a Daimler double six. I’ve driven both.

Of course this thing could be a tractor compared to an 833b, we dont yet know. So far in my experience to beat the copper gm70 the 212 should be operated in much higher voltages the gm70 could only dream of. But if you are holding down the gm70 to its dissipation and max voltage and you have the capability to operate the 212 full wack, the 212 is superior.

833 sounds better than both until you give the 212 its high voltages and its not clear. Operated where it ought to be the 212 is youre baby you dont need youre 833. But if you cant come up with the voltages or prefer not to die with one small slip. The 833b at low power is amaising.

This valve allows way more current and power than all those. A bit like Josh at vsac drove the gm100 hard by going parafeed. Likewise here! Josh at vsac sounded good, and very similar to my 212 in those days. But the best sound at vsac was in Bruce Edgars room. I asked about the amp and I think he said it was el34 parallel push pull cathode follower output.

The best sound was clearly Buces speakers. Put Josh’s amp in Bruces room . Joshes speakers he made himself out of steel. His trade was working professionally with steel structures.

Its not all about the amp.

If I could have had Bruces speakers it would have been the best upgrade barr none.

Clive considered buying them.

No illusions this is just a bit of fun.

My 4ohm celestian speakers are mud compared to Bruces speakers, so lets not call this about best sound. Its just another way to make mud more noisy.

[Paul Barker]

I read somewhere where they used g3 as the anode - all the looks but without mega power SPl wise.

actually just read g2 max is 120 watts and 200mA, so yes ground anode and g3 may be sufficient. But grid 3 burns up if combine with anode as an anode helper, unless connected through 10k power resistor to limit current. Anyway g3 is usually meant for cathode potential or below, in pentode mode.

I may try g2 as anode though but shooting in the dark without curves. Might get 30 watts though and thats all I’ll probably get from the anode, due to not able to harness the current available due to the transformers I have. Maybe g2 amp worth considering.

By the way I think I’ll just build the pp version anyway. I think the sowter pp opts should be ok with 30 watts.

Then if I like it I’ll have to buy two more valves and two more bases and 4 more top caps.

I think I’ll start the 211 se amp first.

[andrew Ivimey]

Vt4c gets my vote.

[Paul Barker]

Yes.

Well the bases and top caps are in the mail for mono.

So I could aim towards 30w pp, which I could ultralinear and pop in schade feedback. Worthy of a trial. Will need to get 2 pcs 130 watts 12v torroids and put them on same phase of each valve for simplicity reasons.

Its not an amp to leave on 24/7 unlike my el38 amp which is always on.

[Paul Barker]

Anyway : what I may try, whichever decade I get round to this, is :-

Without any information of mu s or ri, Super Triode g1//g2 combined plus make g3 “anode helper” through a 10k cathode resistor. Mainly because in standard triode connected I would have to build much more gain into my CF direct coupled driver amplifier, because 2.5 to 4 mu is total pants.

Wish me luck in 10:or 20 years when anything with the r81 ever happens. At least I have two valves and bases and four top cap connectors. Its a start. Also I probs would parafeed for stereo using the 8k pp Sowters on 4 ohm speakers for 4k with some series connected old microwave transformers that I have wound by then as chokes. Probaby get 20 to 30 distorted watts. But it would be warm upstairs in the winter. I know I’m mad.

The worst of it is completely shooting in the dark not a clue what the new characteristics may be, except clearly more mu more ri and nobody knows how much s. That may remain what it is, or not. Until somebody charts loadlines for such connection.

I dont see the point using g2 as the plate, when I have about 20 211’s at home, who’s characteristic curves are available….

[Paul Barker]

There is almost enough evidence to admit me to Rampton.

[Paul Barker]

Well bases arrived in good time. I’ll leave then in the protective wrap until use.

[Paul Barker]

Well bases arrived in good time. I’ll leave then in the protective wrap until use.