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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: KT88 OT Question
PostPosted: 23 Jul 2020, 12:38 

Joined: 23 Jul 2020, 12:20
Posts: 1
Hi all,

I want to build theKT88 SE Amp from Alex Gendrano. According to his Schematics a 5k Output Transformer is needed.
I have here 2 pairs of OT for KT88 Tubes, but 1 pair is 2.5k Impendance and the other pair is 3.4k Impendance. Both pairs have UL Tabs at 35% and 65%
I am worried, using either one of them is not a good Idea? I have built a few Guitar Amps so far but never a HiFi Amp.
Please be nice and i would appreciate any Help. I am not a newbie in Tubeamps but this goes a little deeper. However i am keen to learn new stuff.

Can my OT's work? With less Power? (I don't mind) Or am i going to face trouble?
Thank you so much for your Help


 Post subject: Re: KT88 OT Question
PostPosted: 05 Aug 2020, 04:20 

Joined: 26 Nov 2019, 02:09
Posts: 13
Hello Pero,

As rules of thumb:

In triode mode, you will get the highest output power when the load is about 2 x Ra (= internal resistance). With higher loads you will get less distortion, but at the cost of some output power.

In pentode mode, the value of the load is much more critical. It should be (close to) Vak / Ia (so the voltage between cathode and anode, divided by the anode current). Note that the amplifier of Alex Gendrano follows this rule of thumb. Ik = Vk / Rk = 39 / 560 = 69.6 mA. This includes the screen current but that will be something like 2 mA at no signal, so Ia is about 68 mA. Vak is about 360 V (actually a little bit less, because of the DC resistance of the primary of the OPT). So: Load = Vak / Ia = 360 / 0,068 = 5294 Ohm (actually a little bit less because of, again, the DC resistance of the primary of the OPT).

For ultra-linear mode, I don't have a rule of thumb but I guess that what's OK in pentode mode, will also do in ultra-linear mode (like in the amplifier of Alex Gendrano).

The KT88 in triode mode has an Ra of 670 Ohm so in triode mode your OPT's would do fine.

If you want to use your OPT of 3.4 K (or even the OPT of 2.5 K) in pentode mode, you should choose a working point where Ia / Vak = 3400 (or 2500) Ohm. In the working point you choose, values like the maximum anode and screen dissipation and the maximum cathode current should off course not be violated.


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