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Old 30th May 2017, 08:48 PM   #11
kevinkr is offline kevinkr  United States
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Trying to use Pete Millet's screen voltage regulator in the SSE
Sounds like the right way to go. Securely attach the heat sink to the board, and use that to secure the mosfet. Once the mosfet is bolted to the sink the leads will not move around.

Make sure you place the gate stopper resistor very close to the gate to prevent parasitic oscillations. (Often in the VHF region where you won't see them easily)

Years ago I designed a small regulator similar to this with an IRF821 or similar and it oscillated, the mosfet fried, gate shorted to the channel and the zeners died. (shorted) A very small resistor or bead on the source may improve stability a bit.
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Old 30th May 2017, 09:23 PM   #12
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by kevinkr View Post
Securely attach the heat sink to the board, and use that to secure the mosfet.

A very small resistor or bead on the source may improve stability a bit.
After the heat sink is attached to the board how do you get the bolt hole on the mosfet to align with the one on the sink ?

In Pete Millet's circuit there's already a 4.7k 5W resistor connected to the source, do you mean another small resistor in addition to this ?
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Old 6th June 2017, 08:30 PM   #13
w5jag is offline w5jag  United States
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I just push the part through the perf and spread the leads a bit to hold it in place while I wire.

I think Kevin is talking about placing a small resistor, 100 ohms or so, as close to the FET as possible. When you are trying to break oscillation, you want the suppression parts as close as possible to the oscillating device.

Looking at the schematic, the datasheet for that part, and your pictures, I think you have the connections to the drain and source reversed on that FET. Double check me by all means, but it looks that way to me. There may be other errors, but that needs to be double checked.

Win W5JAG

Last edited by w5jag; 6th June 2017 at 08:34 PM. Reason: added; "connections to the" for clarification
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Old 6th June 2017, 11:47 PM   #14
kevinkr is offline kevinkr  United States
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Trying to use Pete Millet's screen voltage regulator in the SSE
Backwards connected mosfets are often just a diode, depending on other circuit factors the gate insulation may fail..(In normal operation most have a reverse biased diode between drain and source.) Candidate for replacement.

Yes I meant for the 100 ohm resistor I mentioned to placed very close to the gate indeed.
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Old 7th June 2017, 12:12 AM   #15
w5jag is offline w5jag  United States
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Well, that would explain the smoking hot R21. R21 should go to the drain ( middle pin ), not the source.

In addition to the FET and the series zener string, I would replace that 1K carbon comp in the gate lead, and get one end of it as close to the gate as you can get it. It is there to stop the FET from oscillating. The zener between the gate and source should be okay, but if you have a spare, you could replace it in an abundance of caution.

Win W5JAG
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Old 7th June 2017, 02:39 PM   #16
spendorite is offline spendorite  Canada
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Originally Posted by w5jag View Post
Well, that would explain the smoking hot R21. R21 should go to the drain ( middle pin ), not the source.

In addition to the FET and the series zener string, I would replace that 1K carbon comp in the gate lead, and get one end of it as close to the gate as you can get it. It is there to stop the FET from oscillating. The zener between the gate and source should be okay, but if you have a spare, you could replace it in an abundance of caution.

Win W5JAG
Just recovering from a severe bout of spring fever these past few days.
Thanks to all for the responses.

Never wired a mosfet before so it does appear that my understanding of the mosfet connections was
wrong and I have the connections reversed - see attachment.

So the correct connections should be -

Gate - pin-G should be connected to circuit via the R46 1k resistor as close to the pin as possible.

Drain - pin-D should be connected to circuit and B+ via R21 4.7k 5W resistor

Source - pin-S should be connected to circuit and screen via C21 47uF cap and R42 330k resistor

You recommend adding a small 100W resistor on the source between pin-S and the zener D6 ?

I have spares and will assemble a new board later when I'm feeling better..
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File Type: jpg N-Channel Mosfet.JPG (76.8 KB, 192 views)
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Old 7th June 2017, 03:38 PM   #17
w5jag is offline w5jag  United States
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No, no need for the extra 100 ohm - the 1K in the gate is plenty.

Win W5JAG
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Old 18th June 2017, 02:39 PM   #18
spendorite is offline spendorite  Canada
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So I finally got around to assembling another screen regulator board. As W5JAG pointed
out I had the mosfet wired in reverse on the first board resulting in it and the zener
diodes being fried.

I got it right this time and it's working fine wired to the SSE board.

Using 6Y6G tubes, 6087 rectifier and 680 ohm cathode resistor and checking about an hour
after power on I'm getting the following voltages....

B+ = 302v, Vp = 298v, Vk = 18.3v and screen voltage = 144v

It's playing fine in the SSE hooked up to my speakers. As others have remarked, in the SSE
pentode doesn't sound as good as either triode or UL, I find this too but the sound is still
quite acceptable to me. There is a little bit less refinement, with MY speakers there's some
brightness in the mid and high frequencies and a bit upfront in the presentation but overall
I can still enjoy listening to my music, especially jazz.

I will be using pentode mainly when using sweep tubes with a low rated screen voltage such as
6Y6, 6W6, 6K6 and 6CD6. The 6K6 6CD6 can use some higher screen voltage so I'll have to learn
how to modify the circuit to get two or three levels of voltages. In the meantime I'll install
another switch to be able to switch between pentode and triode/UL.
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Old 19th June 2017, 02:21 PM   #19
w5jag is offline w5jag  United States
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... As others have remarked, in the SSE pentode doesn't sound as good as either triode or UL, I find this too but the sound is still quite acceptable to me. There is a little bit less refinement, with MY speakers there's some brightness in the mid and high frequencies and a bit upfront in the presentation but overall I can still enjoy listening to my music, especially jazz. ...
Cathode feedback as implemented in the SSE boards does not provide sufficient negative feedback, imo, for pentode operation.

There is a simple solution. Schade local plate to plate negative feedback works very well in the SSE, and is easily implemented. It will cure all of the issues you cite above.

It is discussed about half way into the thread regarding 6146 in the SSE, and a couple of different ways of implementing it are mentioned.

Win W5JAG
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Old 20th June 2017, 06:44 PM   #20
spendorite is offline spendorite  Canada
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Originally Posted by w5jag View Post
Cathode feedback as implemented in the SSE boards does not provide sufficient negative feedback, imo, for pentode operation.

There is a simple solution. Schade local plate to plate negative feedback works very well in the SSE, and is easily implemented. It will cure all of the issues you cite above.

It is discussed about half way into the thread regarding 6146 in the SSE, and a couple of different ways of implementing it are mentioned.

Win W5JAG
I read your posts on that subject a while back, I'll have another look and see whether I am able to implement it.
I assume with the Schade local nfb in place I can revert to Triode/UL without it affecting the sound in any way ?
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