Baby Huey enters into puberty: 6550 KT88 possibly GU50?

[zintolo]

Compared with e.g. the EL 34, it needs one and a half times to twice the drive voltage plus grid current (i.e. a low impedance driver) for almost the same output power.

Thanks Kay, indeed GU50s could become useful in BH design if parelleled.
As for the low impedance driver, it has foreseen since the beginning indeed.

 

[zintolo]

B-H and GU-50 is not a good mix. Nor is UL with low Vg2 and unchanged tap%.
If you need more power with the B-H use EL34 or EL156 and Vb=500V.
Mona

Thanks Mona,
what do you think about the second option:
Ub=800V, Ug2=600V,Raa 7K8,UL=40%

 

[zintolo]

By applying a positive feedback of 6% on g2 , it is possible to get 75w 2.5% Dtot. If 2 pairs of these low cost tubes (3 euro) is used, 100w 1% is at reach.

Thanks kokoriantz,
so you suggest positive feedback on screens, and negative on the PI.
The quartet will also help raising gm of the tubes.

 

[zintolo]

How long do you think the screens will survive that 😱
Mona

Can I ask you why you think they will not survive?
Thanks

 

[zintolo]

On simulator I get average screen current of 11.9 ma, for 75w output. This makes 11.9ma x250v =3W. With g3 14v , the screen current is lower.

Perfect, you answered to my previous question.
Have you tried with 570 V B+, 60 mA and 8 kOhm Raa?
I was playing with this: Loadline calculator for power stages with reactive load - Vacuum Tube Amplifiers - DIY

 

[zintolo]

Perhaps at 75W it's still acceptable.But when the anode goes realy low the screen current goes up dramatically especialy with the higher voltage caused by the pos fb.
Mona

So it would need a bigger resistor in series?

 

[zintolo]

Depends on how we define low distortion

I remember gingertube saying that this amp is not targeting zero dot thousand zeros one distortion, but a small amount of pleasant distortion.
Not easy to accomplish with a not well known tube, compared to the EL84.

 

[zintolo]

By using a more precise model from Nakabashi Tube Models for TINA , I get 70W with 0.8% distortion

Thanks again. 70 W with such a low distortion and no nfb seems a good result.

 

[U130421]

What are the power /Dtot of BH EL84 &EL34?

 

[Ketje]

Thanks Mona,
what do you think about the second option:
Ub=800V, Ug2=600V,Raa 7K8,UL=40%

Well, I think you can get away with that for some time.Thanks to the rather low Raa the anode doesn't go lower then 150V (without grid current !) keeping the screen dissipation within reason
But I don't think the 100W out is reached.
Mona

 

[zintolo]

What are the power /Dtot of BH EL84 &EL34?

IIRC we are around 35 Wrms compared to around 15 Wrms of the EL84 one, with a THD around... 0,1%?

 

[zintolo]

Well, I think you can get away with that for some time.Thanks to the rather low Raa the anode doesn't go lower then 150V (without grid current !) keeping the screen dissipation within reason
But I don't think the 100W out is reached.
Mona

From the signals shown at that link ( https://www.diyaudio.com/forums/tub...l-separate-g2-winding-supply.html#post3568913 ) seems it can give something like 125 Wrms in AB1.

 

[U130421]

I think It is better to start 2x70w with single pair , Than , pass to monblock 100-120W with 2pair of GU50.

 

[Francois G]

Thanks Mona,
what do you think about the second option:
Ub=800V, Ug2=600V,Raa 7K8,UL=40%

Zintolo,
Is this even an option? I believe specs are Ug2 = 250V, although I have also seen max of 300V.

 

[Ketje]

From the signals shown at that link ( https://www.diyaudio.com/forums/tub...l-separate-g2-winding-supply.html#post3568913 ) seems it can give something like 125 Wrms in AB1.

Yes, classAB2 that is with grid current, demands a driver stage that can deliver current. Not an B-H and not very healty for the screens.
Mona

 

[zintolo]

Zintolo,
Is this even an option? I believe specs are Ug2 = 250V, although I have also seen max of 300V.

From what the author of the thread said, it was reliable:
https://www.diyaudio.com/forums/tub...l-separate-g2-winding-supply.html#post3568913

If you dig the forum, he has built a bass amp with a sextet of GU50.

 

[Francois G]

Yes, classAB2 that is with grid current, demands a driver stage that can deliver current. Not an B-H and not very healty for the screens.
Mona

Mona,

It is class AB1, not AB2, according the link Roberto provided, for the OP’s “aggressive” case. I just read it and it’s interesting with possibilities, if true. Please read it and tell us what you make of it.

 

[Kay Pirinha]

From what the author of the thread said, it was reliable:
https://www.diyaudio.com/forums/tub...l-separate-g2-winding-supply.html#post3568913

If you dig the forum, he has built a bass amp with a sextet of GU50.

Yes, but this amp isn't UL.
Best regards!

 

[Ketje]

Mona,

It is class AB1, not AB2, according the link Roberto provided, for the OP’s “aggressive” case. I just read it and it’s interesting with possibilities, if true. Please read it and tell us what you make of it.

In this treat it is AB2 with 400V on the screen and yes AB1 for 600V,
https://www.diyaudio.com/forums/tub...l-separate-g2-winding-supply.html#post3568913
I don't now if he mend AB1 is without grid current.If so I don't see him get more then 90W sine.
Mona

 

[U130421]

zintolo, You are designing a high power BH , it is only two stages. With 600v screen you will need at least three.