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Old 2nd July 2009, 09:03 PM   #11
kegger is offline kegger  United States
diyAudio Member
Join Date: May 2004
Location: michigan
Thanks everyone..

I don't have an easy way in this amp with it being a PCB to do much with the
screen voltage, and G3 is tied in the PCB, also it's a customers/friends amp so
using a scope to bias it won't really work here either.


So yah the cathode bias and KT88 is the order of the day here.

Called VTL and as most would expect no support to address any issues nor give
any info to the heater amperage or high voltage amperage ratings on the tranny.
It can allways be upgraded!!! YES!
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Old 3rd July 2009, 11:11 AM   #12
kegger is offline kegger  United States
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Join Date: May 2004
Location: michigan
Default It's workin..

I'll post all the changes once I'm finalized in case anyone may find the info useful.

But running KT88's and 6550's I have roughly 60v on the cathodes for 46ma per tube,
I "massaged" the bias circuit to get about -8.5v with a range of about 2v, you can see
the mixed bag of outputs and I can balance each side within it's pairing dead nuts on.

I've got to up the cathode bypass caps and baring any disasters I think I'm done here.
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Old 3rd July 2009, 11:29 AM   #13
Join Date: Dec 2001
Location: Schaffhausen Switzerland
You wrote several rather amazing things in your post above, and I'm wondering if you are willing to expand on your ideas?

>>Sorry, I just saw this post, maybe a little late ?

What I would do with this amp is decrease the screens voltage to 450 or 500V max, preferably 450V This way, the primary reflected impedance by the OT will stay good with the circuit. If you use a triode connection, it will be not very optimal ...<<

Not my experience. I have converted many pentode amps to triode, and find the OPT works just fine.

>Secondly I'd put out the 10 ohms cathode resistor, and put the cathode at ground potential !! If you actually have the g3 at ground, which I suppose is the case, it's a very bad thing<<

Why is it "a very bad thing" to have the cathode a few hundred millivolts more positive than g3?

>> because even if the positive voltage is very small on the cathodes, it's positive relative to g3, and it's really not good for the tube, especially at high voltages ...<<

Again, why?

>This mod will make biasing a little longer<<

A little longer, would actually make it almost impossible for a regualr enthusiast with a multimeter but nothing else.

> but personnaly I bias with a scope and my ears, so it doesn't change anything<

Very interested in knowing how to bias an amp with a scope - sure doesn't tell you how close you are getting to the tube's maximums IMO.

>>And third I'd put out the g3 from ground, which is the case I suppose ? and connect the g3 to the bias supply (before the bias resistors) <<

So while it's dangerous to have few millivolts of negative on g3, it's OK to have 50-100V on it, with respect to the cathode...please explain why?

>>And finally rebuild the bias supply<<


>>This way is I think the safest one to use EL34 on high voltages, and you will be surprised how robust the amp will become, the tubes too !!<<

I'm not knocking your statement here, it's just that I've been working with tube amps for 40 years now and these are very different ideas to what I know works.

Regards, Allen (vacuum State)
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Old 3rd July 2009, 11:57 AM   #14
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Any recommendations for the "toughest" current production KT88 and what about those which are supposedly just re-labelled 6550?

thanks, sp
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Old 3rd July 2009, 12:36 PM   #15
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Join Date: Nov 2002
Location: Australia
600 volt screens on EL34 whoa! That is just nuts.

Apart from the long term life of the valves. What about the drive requirement of the EL34 with such a high screen voltage?!? So in one stroke you remove one of the major advantages of pentode outputs stages and run you valves into the ground.

Pentode output amps get lots of bad press - some justified some not - one of the reasons is the screens are often run at plate voltage or some case higher then the plate voltage (that is the plate is lower due to the resistance of the output transformer.) The screens should be not just be lower then the plate but in some cases much lower - in short picking the right screen voltage you can get lower distortion and also low drive requirements.

600 volt screens is just awful lazy design work. God the average DIYer could do better.

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Old 3rd July 2009, 01:51 PM   #16
Join Date: Dec 2001
Location: Schaffhausen Switzerland
>>600 volt screens is just awful lazy design work.<<

Oh, no, no. Luke Manley himself told me it was perfectly fine, and how could he be wrong? He's certainly sold a lot more amps than all of us on this board put together.

You just have to use special VTL EL34's only obtainable from the factory in California...

Like as if myself in Europe, or you in Oz have instant access to such special devices. Plug in a set of JJ's and you'd most likely have a fire on your hands!

Regards, Allen (Vacuum State)
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Old 3rd July 2009, 02:36 PM   #17
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Location: Australia
"and how could he be wrong?"

I'm all for making your own mark in amp design and not following fashion. So normally an amp design that upsets the status quo I would be all for... but running tubes like this makes little to no sense. This is an amp meant to be sold to people who are not DIYers...nuts.

A friend had for many years a pair of VTL Tiny Tridoes and must say they sounded really nice, not perfect but they had many great qualities.....but two things always struck me about the amps; 20 watts out of a 4 EL84 in triode mode means they must be running really heavy into class AB but also the amps ran really hot which made a joke out of the Class AB bit of things. (They ran so hot you really couldn't touch them.) The transformers were so small it made almost no sense that this was a 20 watt hifi amp that cost serious money.

I think this all brings up a problem with valves - which is also wonderful - they are tough beasts that can be mistreated and still sound good....well when compared to sand not treated with kid gloves.

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Old 4th July 2009, 03:58 AM   #18
bozole is offline bozole  France
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Join Date: Jun 2009
Hi Allen Wright

Sorry to disturb you with my post, I'm going to explain my views, in the same order as you asked.
We speak about EL34 tubes, just EL34's

When you use a pentode as a triode, the optimal load impedance, considering the datasheet, is not the same in triode mode with same voltages as in pentode mode. It's not a big difference, but it is some, that's why I said it was not optimal. But it will work, you're right

For the cathode a few hundred milivolts more positive than g3, I don't think it's a problem with all tubes in all voltage conditions. But I think it can be one with actually poor EL34's used at high voltages (the old one were more robust ...), because this tube is more fragile in that way. The advantage of a negative voltage on g3 is that that way the g3's effectiveness is increased, efficiency is improved (in term of plate current vs grid voltage), linearity too, and finally reliability too : if one day you have a bias failure, the negative g3 will limit the uncontrolled anode current to sufficient safe levels ... I learned that from Kevin O'Connors writes, and others Internet sites, and in my experience it's perfectly right ...

For the bias setting, yes I use the scope and my ears, it's my preference, sorry for that. And after that I always verify that I'm not out of safe settings for the tubes using the shunt method, and I'm never out of safe settings, so how more bad is the scope method that another ?
That way, I find some advantages : first of all, I see the amp in action, at big power, and it may be very instructive to find some problems on the amp when it's in use condition (oscillations, bad coupling cap causing distorsion when the drive signal increases etc ...)
What do you do ? you use the 70% common rule of tube plate power dissipation to bias your class AB amp ? It's not always the spot point, sorry !! sometimes higher, sometimes less, depending on "how much" you go in class B regarding how much you stay in class A (depending on the OT's reflected primary impedance : if you change it, don't you think you will must change the 70% rule for biasing the amp ?).

And finally rebuild the bias supply (if necessary) because when you tie the g3 to bias supply, the tube is "more efficient", in that sense that at same bias voltage, you'll have more plate current, so you must be able to put the bias voltage a little more negative ...

Sorry not to have the same opinions as your's, that's life
Best regards
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Old 4th July 2009, 07:14 AM   #19
kegger is offline kegger  United States
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Join Date: May 2004
Location: michigan

I think Allen was looking at the amp from the standpoint of customer and tech.
(while also wondering about your suggestions)

As techs we should think in terms of what is in the best interest of the owner here.
(quite often means less major changes the better, and keep things simple)

The amp was brought in with problems, obviously means the owner does not know
much about amps or how to fix them, making radical changes outside the "norm" to
where the owner won't understand them or be able to check there own amp wouldn't
be advisable, if they have problems in the future it makes much more difficult to fix an
to explain to someone why and what was done.

Some of your suggestions may very well have merit, but in this context not what most
would suggest as being "proper" for the case at hand.

If this was a Diy/Scratch built amp by the designer then anything is fair game, but one
that is being repaired for someone usually it is best to stick with that more "standard"
approach/methods and hopefully improve the amp at the same time.
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Old 4th July 2009, 10:00 AM   #20
bozole is offline bozole  France
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Join Date: Jun 2009
Hi kegger

I understand his point of view, no problem, really !! And your's too, believe me.

But what I suggested here in this case is what I personnaly would have suggested to the customer (even if it implies mods, but I don't think really more than triode operation, lower the scrrens voltage etc ...), just because I think it's the mod that would keep his amp sound the most similar to the way it sounded originally (if your customer likes this amp), and fix definitively the bad reliability and the tubes consommation ...
Putting the stage in triode mode will make a real difference in sound, putting KT88's too, but that's just my opinion.

Have a good day, best regards
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