Ralph Power amplifier

[Carl Ekblom]

Sorry, it should be PEG 124!

[stigla]

Carl;

Interesting about the Rifa caps... Must investigate that.....


Well, guess there's nothing more to be said here.... Thanks to everyone! Espacially Mohan, Yves and Andrewbee. This is what this forum is all about!

I've updated the schematic on page 1 to reflect changes i've done / expereinced. (hit the 'refresh' button)



Stig Larsen of Norway!

[LuisMCP]

Originally posted by stigla
Hello

I am wondering if anyone have buildt this amp:

http://www.meta-gizmo.com/Tri/interstage.html


I wonder if the rated 275V at the plates are correct. The amp states to give ~8W, and I think maybe the platevoltage should be 375V.
But then the 200uF PSU cap has a wrong Rating too..
The schematics give no currents, but I think it runs at 60mA per tube, thus giving a 60V bias through the 500R cathode resistor.
Anyone knows?

I really want to build this thingy, so I would appriciate some help Maybe it gives 8W in class AB?

-Stig

Hello, Stigla:

Lundahl not recommend the LL1635 for SE to PP interestage xmer, instead use the LL1660 xmer.

Regards.

[Brett]

<b>Lundahl not recommend the LL1635 for SE to PP interestage xmer, instead use the LL1660 xmer.</b>

Yes, that's true. The LL1635 is for SE->SE or PP->PP only because it's winding pattern is not optimised for the SE->PP application.

There was a great section on www.diyparadiso.com about selection of the appropriate OP and IT transformaer from the Lundahl range. Their site is being reorganised so it was not available when I last checked tonight. Try again soon. Datasheets are available at www.lundahl.se

[stigla]

Hello!

Waking up an old tread here

Amps are running very good. I have dared to elevate the bias current of each tube to 60mA and around -47V gridbias. (two 750R's in parallell for the cathode resistor) and I really find this operating point more pleasing to my ears than the original.
I abandoned the Ultrapath at the input. Either the value I used was terrible wrong (22uF) or the particular ECC99's of mine was alergic to Ultrapath. Kind of removed details and softened the highs... so I sort of lost the interst in that and I did'nt really have any other values around to try out... Maybe I'll dig into it some other day.

Haven't got to try other caps for the PSU than the RIFA's either but I cant really recognise the things Carl Ekblom was talking about. But it is an interesting point and I will definitivley go into it some day. Maybe my ears are used to them 'couse I've got'em in my SS amp too!!

Had some problems with Arc-overs inside my 6D22S's on one channel. When I would fire up the amp, the Power tranny would "hum" for a sec or two, then a small "zzzzittt!" would appear inside one or both diodes, and then everything would work normal from there. (warmup time etc...) After some days this stopped happening, and I havent seen it since...

WEEEEEEELLLL anyway, enough "blah-blah". I have some really questions to ask you guys;

So. Take my schematic there, at the bottom of page 1. Would it be possible to throw in a couple of KT88, without any modifications besides a new Cathode resistor, heater arrangements and maybe some resistors from the plates to the screens?

What about running them in triode mode?

I'm just curious, if it's not TOO much work, I think it would have been fun to try out. Hopefully some more Output power, but thats not the main goal.

I've seen KT88 work with 5K loads in other amplifiers, and my OP-trannys can handle a good 50W, so the main limitation as I see it, would be the PSU. Too low voltage perhaps. Maybe I can gain som voltage by applying fixed bias through the IT sec. and connecting the cathodes to GND?

Of course, I want class A operation.


Stig

[Brett]

Hi Stig,

Glad to hear you're enjoying the amps, and thanks for letting us know.

<b>So. Take my schematic there, at the bottom of page 1. Would it be possible to throw in a couple of KT88, without any modifications besides a new Cathode resistor, heater arrangements and maybe some resistors from the plates to the screens?</b>

Not really if you want significantly more power. For this tube I think you'd be needing 450Vp-k at 80-90 mA for class A and then you'd get about 16-20W output, about 5dBW more than you have now. The bias would be roughly the same, ~-45V, but the Rk's would need to be adjusted for the new Ik. Because a trioded KT88 isn't as linear as a 6B4G, you will also get 5-10x the distortion.

Running with the same B+ as you have now, I reckon maybe 10-12W, but I haven't run the curves, just glanced at them and guessed.

If your 6B4Gs are Sovteks, they have a high plate dissipation of 30W, not 15, and can take 450V on the plates. I'd change the PSU and run them a bit hotter if you really need the power, for say 10W. The difference betwen 6W and 10W is only 2.2dBW, hardly worth the effort.

Below are a set of KT88 triode curves for you to look at and try some loadlines on. An a full datasheet can be found at http://frank.nostalgiaair.org/sheets/084/k/KT88.pdf

[fdegrove]

Brett,

And have you taken a look at the curves of a pseudo trioded KT66
and its brethren the 807?
If you need some more power than what a DHT Se amp has to offer I would most certainly consider using them.

Cheers,

[Brett]

Originally posted by fdegrove
Brett,

And have you taken a look at the curves of a pseudo trioded KT66
and its brethren the 807?

Yes indeed I have. The STC datasheet from www.retrovox.com.au is excellent and has a full set of triode curves plus graphs showing the H2 and H3 as a function of input voltage fpr some op points. The curves look excellent, and I love the ST bottle and topcap, but they seem to be a particularly unloved tube. The DHTs, and the popular tets and pents are all you hear about.

I might just build some now as the small EL84PP amp I have will take them happily for about 3 X the power out. I have a whole lot of NOS VT100 JAN tubes from WW2, and a number of matched sets of later ones, maybe 50 in all. But I have no sockets or topcaps.

If you need some more power than what a DHT Se amp has to offer I would most certainly consider using them.

I run horns (104+ dB/W), so power isn't an issue.
By the end of the week, my Allen Wright amps will be running, giving 12W. The transformers cleared Customs today. As part of this package are some custom power trans for a set of 813PP, trioded of course, which should give me 40W+ of thoriated tungsten DHT glory. I can't justify the power, but what the hell. The Fertin thread has me interested in trying some baffles, but even they only need a couple of watts.

Beautiful aren't they? Make 845's look like toys. More linear too.
(pic from Bas Hornemans site)

Cheers

[fdegrove]

People,

My guess is that as long as the real McCoy was around people did not want to complicate their life too much.
Keep on posting stuff like this though and you'll see what happens to the stock price.
Add to that the top cap and you start to see why some people are uncomfortable with this tube.

Rgds,

[planet10]

I always figured the 807 didn't get much respect because of the top cap. Ceramic sockets are available at ApexJr. I have a couple dozen of them & their relatives.

dave