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Circling back to the OP

136.175.75.118

In Reply to: RE: From a Acro pamphlet on UL operation showing different tap values posted by Tre' on September 14, 2023 at 15:22:58

It's interesting to hear about the UL debate and see some curves showing UL effects in push-pull amps. However, as the original poster, I'm looking for advice for a driver for my SE KT88, as well as looking at the debate between fixed-power-supply G2 mode vs "triode" mode. This is partly Because my existing transformers have no UL tap, and I have no time/money/ or interest in experimenting with UL.

So, trying to just redirect this thread to the two original questions asked, which were:

1. Favorite drivers/circuits/tubes for driving SE (including fixed-tetrode or triode) KT88

And

2. Debate on fixed-voltage g2 vs "triode" mode G2 using fixed value resistor from Anode to G2

Thank you in advance for your wisdom !

Follow Ups:

RE: Circling back to the OP - Paul Joppa 22:06:29 09/20/23 (1)
tl;dr -- the choice of a driver depends on the kind of feedback used - the requirements are quite different.

My view is that feedback is feedback, and UL is a form of feedback. Therefor other forms of feedback can be substituted for the UL connection. There is nothing magic about UL.

My current interest is in plate-to-grid feedback, a.k.a. Schade or anode follower or transimpedance. This is mostly theoretical; I've only designed two such amps that have been built and listened to, and the jury is still out - there are too many variables for a couple of samples to judge by. But the theoretical advantage is that the feedback is confined to the output tube and does not involve the driver or the output transformer, thus the loop can be quite stable.

If the feedback is large, the driver tube sees a low impedance load - the output tube becomes a current-controlled voltage source, i.e. a transimpedance. To get a large amount of feedback, you need to have a high source (plate) impedance in the driver or a low-resistance feedback resistor (which would waste a lot of power as heat). This is the opposite of the usual rule for a driver, which calls for a high load impedance and a low plate impedance (such as a 5687).

Most plate-to-grid feedback amps designs I've seen on the web use a (relatively) low-rp driver so the amount of output-tube feedback is usually small. The RH-88 is a widely recognized example. The feedback is sometimes augmented with an ultralinear connection, which partially brings the output transformer into the loop.
- RH-88 circuit (Open in New Window)
- RE: Circling back to the OP - PakProtector 09:01:18 11/20/23 (0)
  mmmmmhhhh...plate to grid. I recall one of Gary Pimm's pictures of doing that sort of thing to a MOSFET. Resulted in vertical, evenly spaced gate lines of drain current. IMO, it is slightly easier to use pentodes as getting the heat out is slightly easier... :)
  
  Douglas
  
  Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Try the Russian 6E6P or perhaps the 6E5P - coronadope 14:41:58 09/18/23 (2)
Several years ago, under the guidance of an OEM, I ran the 6E6P to a direct coupled KT88/KT 150.

EBB of about 390VDC, Plate resistor 18K, cathode resistor 500R. About 205 VDC 'cross the tube at 9.55MA. (Some may say that this is tube abuse and will claim max P K to be 150VDC.) My friend has been running the 6E6P at these values and has even goosed things up a bit and reports zero problems.)

Here's my comments from May 14 2020 when I breadboarded this design:. Explosive! Dynamic! Clean! Clear! Holographic! Detailed! Drive! Build it!
- RE: Try the Russian 6E6P or perhaps the 6E5P - coronadope 14:50:36 09/18/23 (1)
  I should have added that the 6E6P was triode connected.
  - RE: Try the Russian 6E6P or perhaps the 6E5P - Tre' 17:18:43 09/18/23 (0)
    .
    Have Fun and Enjoy the Music
    "Still Working the Problem"
    - https://www.bartola.co.uk/valves/tag/6e6p-e/ (Open in New Window)
RE: Circling back to the OP - Tre' 13:50:14 09/18/23 (0)
The KT88 in pentode mode is very easy to drive. Depending on how much gain you need, I would just use your favorite signal triode. If you have a preamp and if your speakers are reasonably efficient one triode stage (per output tube) should be all you need.
Hooked up as triodes KT88 are harder to drive but they require less feedback.
Edit, I think I am wrong here. Tetrodes have high plate resistance and a lot of distortion and they need a lot of feedback for both of those things. I think you would need a lot of gain so you can apply a lot of feedback.
Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"

Edits: 09/21/23

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