Mikaels KT88SE & VTV Octal Linestage - diyAudio
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Old 21st May 2008, 12:59 PM   #1
AKlein is offline AKlein  United Kingdom
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Question Mikaels KT88SE & VTV Octal Linestage

Hello all at diyaudio,

My first post so please excuse me should you think my thread a little stupid

Just waiting for some bespoke transformers from Sowter, then I can finish Mikaels KT88SE amp; to which I have a couple of queries if anyone can help. (?)

1) The amp schematic includes a pot. I would like to remove this and have a power amp only (To go with my VTV linestage). Would this be OK? and if so would I need to omit the pot and add in a 100k resister across the input?

2) Regarding the Linestage (Issue 11 of VTV), the article states that the B+ is 370v. The same editions goes further in testing a number of 6sn7's (GT's, GTA's, GTB's, CV181's and VT231's) using this linestage. I understand that the GT's, CV181's and VT231's have a lower max plate voltage (around 250v - 330v). The reason I ask is that at present I am using EH gold pins but would like to use a couple of Ken-Rad VT231's that I have, but i'm a little worried about catastrophic explosions or arching.

Strictly hobbiest only i'm afraid, self educated. But I have built a WAD KEL84, a number of speakers and also Upgraded Quad 303's (I like) and 405's (Not so much).

Would welcome some comments and opinions on the VTV & KT88, any good?

Thanking you all in anticipation.

Regards

Adam
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Old 21st May 2008, 02:15 PM   #2
alexg is offline alexg  Philippines
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100K resistor across the input should work in lieu of the 100K pot. Mikael's SE KT88 is one of the best amp I have ever built.

I built a similar linestage to the VTV octal (is this the grounded cathode into a cathode follower linestage). Am using 12sn7 and the plate voltage is around 230V. Don't forget to lift the ground by about 50V.

HTH.
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Old 21st May 2008, 04:12 PM   #3
AKlein is offline AKlein  United Kingdom
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Hi Alex,

thanks for the reply.

VTV linestage is as:-

http://www.geocities.com/northbirdten/line_stage.htm

Sorry, what do you mean when you say dont forget to lift the ground by 50v ?
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Old 21st May 2008, 04:16 PM   #4
smbrown is offline smbrown  United States
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I think he meant lift the heaters by 50v. You don't want to lift the ground. The heaters are lifted by virtue of the voltage divider that connects to the heater string shown in the schematic. It helps reduce hum.
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Old 21st May 2008, 04:18 PM   #5
smbrown is offline smbrown  United States
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By the way, if you want to lower the B+ a bit, you can change the 5AR4 rectifier for something like a 5U4, provided your rectifier heater winding (the 5v one) can handle a 3 amp draw. That will knock things down 20v or so. I don't think the 6SN7's your using, however, should not have a problem.
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Old 22nd May 2008, 02:54 AM   #6
alexg is offline alexg  Philippines
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Quote:
Originally posted by smbrown
I think he meant lift the heaters by 50v. You don't want to lift the ground. The heaters are lifted by virtue of the voltage divider that connects to the heater string shown in the schematic. It helps reduce hum.

Thanks, lifting heaters is what I meant.

The linestage that I built was very similar, but B+ is around 300V and I reverted to AC heaters, somehow, I did not like the DC heaters for the 6sn7 (actually am using 12sn7).
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Old 22nd May 2008, 10:58 AM   #7
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Default 6SN7

The maximum voltage ratings for 6SN7 means the dc voltage from anode to cathode during steady operation, so don't worry about 370V at startup. Sometimes you see ratings for 1960s/1970s valves for the maximum supply (Vab) or maximum Cold Anode Voltage Va0 (eg Telefunken specs) or Max Voltage at Cutoff (Russian specs). These all mean the same: max volts applied when no anode current flows. If the 6SN7 had such a rating it would be about 500V.

Raising the heater voltage to 50VDC is a hum countermeasure, as noted. The 6SN7 has has a long production history, and some variants will need this, while really good ones (Like GEC, Sylvania GE, RCA) will not (Can run at heater grounded one side or 100 Ohm resistors from each side to ground). Some later Eastern kind will still hum even with raised heater voltage - because one of the grids is internally wired too close to the heater pin next door to it. If this happens, you'll need to run the heaters dc (or get an expensive 6SN7)

I see the VTV linestage has a cathode follower output. Many folks here won't agree, but I believe cathode followers are a waste of a valve, and sound poor. If you are given to experimentation, try replacing the follower with a 600V rated MOSFET source follower or a 450V BJT. Try comparing the sound.
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Old 22nd May 2008, 03:15 PM   #8
AKlein is offline AKlein  United Kingdom
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Thanks for the replies gents,

In honesty, its all a bit beyond me though.

Rod, I really like the sound of mine as it is, certainly wouldn't describe it as poor. But then again I probably haven't been exposed to really good circuits as you obviouly have.

Would you be prepared to expand on your follower replacement?
Yes, I am given to experimentation but in all honesty is usually on a 'painting by numbers' basis if you know what I mean.

Still confused with these 6sn7's though. Almost tempted just to put the Ken-rads in and see what happens. Surely, if VTV tested them in the linestage they should be OK?
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Old 22nd May 2008, 11:48 PM   #9
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The source follower replacement for a cathode follower is neatly explained in the MOSFET follies page:

http://www.geofex.com/Article_Folder...osfetfolly.htm

Don't worry about using the 6SN7 in that circuit! It will be quite safe.
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Old 23rd May 2008, 10:52 AM   #10
AKlein is offline AKlein  United Kingdom
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Thanks Rod,

Keep an eye of this post, i'll update in a week or so.

Regards

Adam
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