KT88 heaters

[Giaime]

Yes, I see, you need it big. But why so high B+ voltage? With 400V you could have done just well...

 

[croccodillo]

Somebody told me that KT88 works better with high voltages...
And using 550V I can have an output voltage swing of almost 400V (from 150V to 550V)@150mA.
Moreover, the tubes, when in idle, sit in the middle (I do not know how to explain better the concept), at 350V, and can go down to 150V (-200V from idle) and up to 550V (+200 from idle), reproducing a (ideally) perfect signal at output.
If I'm right this should lower the distorsion and increase the output signal correctness (sorry for my bad english explanation...).
Again, this should also increase the damping and improve the sound.

Ciao,
Giovanni

 

[Giaime]

Oh yes I see. But the rise in plate voltage in a tube amp is derived from the inductive loading of the output transformer, not necessary from the power supply. I mean, with a 400V supply, plate voltage can go up to 600/700V because of the inductive loading, that's not actually necessary to use 700V supply. Even with parafeed topology you're using.

Obviously it won't hurt to waste a few volts

Correct me if I'm wrong...

 

[croccodillo]

I'm a newbie with tubes, and I'm still learning how to use them.
I was born as a "sand" guy, and I'm used to think in that way: that means that my solutions derive from sand topologies, and my mental simulation of circuits has a sand base...
Thus, your observation make me think, since up to nw I never considered OPT inductive load voltages: thanks a lot!!!

 

[Giaime]

I'm a newbie too Giovanni, but I made a little simulation on TubeCAD (very very good program, go purchase it immediately): a PSE with KT88 (not parafeed): the results with a 350V power supply are that plate voltage can go anywhere from 1 to 678V, with an average of 265V. So you have to think with tubes in mind

I'm not a sand-state expert, but I think that a SS output stage can put on the load a maximum voltage equal to the power supply voltage minus a few volts, with tubes that's very different.

I think you have to choose a proper operating point for your KT88, then choose a B+ voltage of some volts higher the quiescent plate voltage (IMHO 400-450V not more) and let the OT do the rest

 

[dhaen]

I think you have to choose a proper operating point for your KT88, then choose a B+ voltage of some volts higher the quiescent plate voltage (IMHO 400-450V not more) and let the OT do the res

Sounds like a good starting point to me

 

[SY]

Ah! deserter...... he's using solid state in the driver stage...I've always suspected this stage in any amp determines alot of the quality aspect more than the output tubes. I wouldn't be suprised after auditioning he does an about turn away from mosfets. Anyone else using such SState driver applications in thier designs ?


omo richj

I do. MOSFET source followers using IRF820 to drive the 6LF6 outputs. And a proto using a DN2540 cascode source follower. Works a charm.

 

[Giaime]

I do. MOSFET source followers using IRF820 to drive the 6LF6 outputs. And a proto using a DN2540 cascode source follower. Works a charm.

Me too Stuart, and I plan on a PowerDriven EL84 PP when I'll finish the preamp

Let's avoid some of the rigid "tubes or nothing" rules... IMHO sand state stuff, slaved to the Glory of Vacuum Tubes, made to Serve them, is a very good idea. It keeps the intellectual superiority of vacuum tubes with many of the advantages of sand

 

[Tubelab_com]

First of all if you use a CCS to supply the voltage to the plate of the output tube, the OPT is not in the DC path. The inductive kick of the transformer can not supply the positive swing above the supply voltage. This is known as "chokeless parafeed". Since there is no choke or other inductance in the plate DC path, the head room must come from the power supply, and yes you need a lot of voltage. I had a similar design running from a 400 volt supply and I was only getting 3 watts. This was discussed heavilly in the following thread by most of the same contributors. There is a picture of my test circuit there.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=67437

Ah! deserter...... he's using solid state in the driver stage

I wouldn't be suprised after auditioning he does an about turn away from mosfets. Anyone else using such SState driver applications in thier designs ?

This was overwhelmingly the initial response that I got when I put the Powerdrive concept on my web site. I even got hate mail saying that I should change my name to Transistorlab! Now 2 years later, after people actually tried it, that is all gone. The people who have actually listened to PowerDrive like it a lot. It has become the bigest source of questions in my email. Fortunately I am in the process of expanding the how to material on the PowerDrive page to make it easier to understand, and adapt to your own design.

http://www.tubelab.com/powerdrive.htm

A major update will be posted in a few weeks.

 

[Giaime]

First of all if you use a CCS to supply the voltage to the plate of the output tube, the OPT is not in the DC path. The inductive kick of the transformer can not supply the positive swing above the supply voltage. This is known as "chokeless parafeed". Since there is no choke or other inductance in the plate DC path, the head room must come from the power supply, and yes you need a lot of voltage.

Oh, excuse me George, I was wrong. I didn't think that the OT couldn't be on the DC path, because of the coupling cap. So if you want 600V max at the plate, you need a 700 or more volts supply if you use a CCS, right? That's quite exhagerate for me, much better a CCS at the cathodes instead, and conventional output tranny. You can even save a C in the signal path (and that's very good)...

I guess that if Giovanni has chosen this, there are good reasons. I didn't follow the original thread, so I don't know.

 

[Shoog]

I'am still a little concerned about those CCS's sitting on a large heatsink which will be exposed to the outside world and riding at an unhealthy 500V. If ever anything goes wrong (ie a heatsink to tab short) it could be lethal. I suppose if the heatsink is earthed and the case is safety earthed to a RCCB things should be OK as the amp will trip out the mains prity quick.Alternatively could you get your Mosfets in isolated packages.
Even though you save yourself a few volts of overhead - I still prefer a meaty tube CCS. Tubes are built to shed heat in a way that transistors are not.

Shoog

 

[Giaime]

What you suggest, Shoog, remembers me a CCS KT88 + 807 you've made some time ago... is still working? How does it sounds? Maybe if you post the design here you'll convince Giovanni of that idea...

 

[Shoog]

I have been working on tweeking it and its sound better than ever. I upgraded the output transformers to Power toroidals (made a huge difference), I also increased the amount of plate to plate feedback (160K down from 470K), added an extra gain stage to make up for the unity gain of my preamp. I tried the plate to plate at 100K but it introduced resonances into the output stage.
Using the same 807's after about 8months of 16 hour days, and the bias point seems rock solid. KT88'a aren't even sweating.
A real stunner - I cannot image me building anything that will sound better so I am tentatively going to say my quest is over.

Shoog

 

[SY]

I'am still a little concerned about those CCS's sitting on a large heatsink which will be exposed to the outside world and riding at an unhealthy 500V.

Shoog, not to mention the stray capacitance inherent to grounded heatsinks...

 

[Giaime]

What's that, SY? Could you explain?

Are you saying that the mosfet + heatsink act like a capacitor and suck out the sound from the plate of the tube?

 

[Tubelab_com]

Oh, excuse me George, I was wrong.

Not needed. I also ran simulations that showed that the OPT could supply the needed headroom even though it was coupled through a capacitor. I had to actually build the amp to find out that this ins't so. Some voltage is there but there is extreme distortion.

The point of this forum is for us all to learn from each others experiences, so we all don't have to repeat the ones that don't work.

I upgraded the output transformers to Power toroidals (made a huge difference),

Are we talking about mains transformers here? If so how many VA are the transformers that you are using, and how much power is your amp making. 240 volt primary?

 

[Shoog]

240V primary with 6V secondary. I would estimate that the VA rating is in the region of 120VA. An 807 SE, so we're probably talking 4-5watts output - more than loud enough with my speakers.
I used some 12V transformers and unwound half of the secondaries. I then taped the whole assembly up with insulation tape. Very clean detailed bass and, very smooth sound.
I just wasn't prepared to pay the outragous prices been asked for parafeed output transformers. I don't know if real parafeed transformers sound significantly better, but I haven't got the money to find out. I am more than satisfied with the sounds of my cheap mains transformers. I even went and built another parafeed headphone amp with toroidals as outputs - sounds great !

Shoog

 

[Tubelab_com]

I don't like the prices for many transformers either. It seems that most vendors raise the prices for any transformer they make that has the word "tube" in the description.

I have used some Talema toriodal mains transformers with good success, but I was using a 56VA transformer on a 2 watt amp. It did sound good though. I will try some larger mains toroids when I build a proper power supply for the KT-88 - 6LW6 amp.

 

[Shoog]

Sourcing mains transformers in the correct ratios at high VA's means custom wound - which brings cost up again. Thats why I ended up using some that I had and unwinding to spec.

I think transformer manufactures are laughing at what the Parafeed crowd will put up with (anyone for cobolt cores).

Shoog

 

[Tubelab_com]

As long as the audiophile crowd keeps buying.............