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Old 18th October 2009, 02:23 PM   #31
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Quote:
Originally Posted by JandG View Post
That would greatly be appreciated. I have built 26 pre before & know to keep PS far away from signal chassis, so I wouldn't be bugging you on WHY it hums...*s*
I am on the down hill portion of a PSEP amp build that will be switchable via flick of a switch between integrated mode & pure amp. It is a 2 stage 5687-IT-807 with seperate PS's for everything including screens. I really don't build other pre's as I have heard what a 26 can do so time to build my one & only last one & a good one. I have the life time stash of 26 allready.
Joe
Hi Joe,

I built mine (image/schematic at the beg. of the thread) in single chassis (signal+psu) with no hum issue what so ever.

Kanishka
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Old 18th October 2009, 02:30 PM   #32
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Originally Posted by bequerel View Post
Looking at the 220nF coupling cap at the output; isnít that generally too small?
In normal (not DHT) linestages this is usually 2-4uF. Are there special considerations with DHTs?

A very nice build btw., coolzero.

I am confident that a well constructed 26 linestage is one of the best linestages money can buy... regardless of price.
I have not jumped on the DHT linestage wagon myself yet, but I have a lot of 26 tubes.
Think I will try a 27 linestage first. I have collected a small truckload of Arcturus Blue 27 (tipped).
I changed the output cap/resistor to 680nf/220K and best sound I heard out of this pre so far.
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Old 18th October 2009, 03:06 PM   #33
JandG is offline JandG  United States
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I have built your original posted signal schemetic exactly w/a glow tube regulated 150v. I like the 135v 5.5ma fixed version you show also. i am heavily intrested in active loading of the plate, at least to try it. easy enough to go back to tranny loading or resistor which still sounds excellent. I see you got away with PS in same chassis, this is something i have never been able to do, your layout is most likely much better than i ever did with single chassis try's. I will play it safe & go with 2 chassis. battery bias also works well with 26 w/ NiMH bieng what i used.
Joe
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Old 18th October 2009, 08:32 PM   #34
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Quote:
Originally Posted by JandG View Post
I have built your original posted signal schemetic exactly w/a glow tube regulated 150v. I like the 135v 5.5ma fixed version you show also. i am heavily intrested in active loading of the plate, at least to try it. easy enough to go back to tranny loading or resistor which still sounds excellent. I see you got away with PS in same chassis, this is something i have never been able to do, your layout is most likely much better than i ever did with single chassis try's. I will play it safe & go with 2 chassis. battery bias also works well with 26 w/ NiMH bieng what i used.
Joe
I tried active load with CCS as well. Sound was perfect but channel balance was an issue. Kevin advised me not to use NIMH claiming those sound worst. I used alkaline he had recommended which sounds great btw.
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Old 18th October 2009, 09:15 PM   #35
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I'm getting interested in these 26 preamp designs! I'm just building another to try out some things. Two chassis - seems the most predictable way of eliminating hum. Instead of the 156C choke I'm going to try an IXYS active load. Need to do some calculations - need a bit more volts out of the glow tube than 150v, so maybe two 105v glow tubes to give me 210v. I'll start with four red LEDs for 6.8v on the cathode and see how that works out - shoot for 130v on the anode and 7mA as before.

andy
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Old 19th October 2009, 06:48 PM   #36
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Next experiment - mixing LED bias and cathode resistor. Replaced 4 LEDs with one LED and a 1k cathode resistor. Doesn't seem to sound any worse. I'm trying to get rid of the glare of the LEDs and figured one might be better than 4. Will have to listen some more.

I also built a another preamp where I used IXYS active loads (cascode). Right now it has a pair of VT-67 (30 special) at 3mA, with four LEDs for cathode bias. 6.8v on cathode, 114 volts on the anode and 150v supply from the glow tube. Sounds pretty similar overall to the 26 - but slightly flatter and not quite so nuanced, so the 26 wins still, though it's more hungry on filaments! Will go on to try 26s in this new one and then 10Y.

Since I'm now using a separate power supply it's much quicker to build just the preamp chassis.

andy
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Old 19th October 2009, 09:38 PM   #37
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Well - some conclusions. took out the red LEDs and went back to cathode bias with wirewound resistors. Best sound so far. Nice and clean, and the glare of the LEDs is gone. With a wirewound cathode resistor the VT-67 preamp sounds really good with its IXYS 10M45 cascode active load. Very nice! 114v on the plate, 2.4k cathode bias for 3mA. IXYS sense resistor is 1k. Fed by a 150v glow tube.

26 preamp now has 1.2k wirewound resistor for 7mA and still the 156C choke load, for 130v on plate, again fed by a 150v glow tube.

andy
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Old 19th October 2009, 10:17 PM   #38
Zen Mod is offline Zen Mod  Serbia
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use IXYS instead of series resistor preceding gas tube .
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Old 19th October 2009, 10:43 PM   #39
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I found with valve pre amps care is needed with the heater and B+ supplies.
I ended up using a DC heater supply.
I also used a CRC on the B+ supply to kill most of the ripple.
A C on its own is just not good enough even if it is large.
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Old 19th October 2009, 10:56 PM   #40
robazar is offline robazar  United States
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Andy,
How does your 26 pre compare to your 10Y pre
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