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PP KT-88 w/866A MV HT Supply
PP KT-88 w/866A MV HT Supply
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Old 7th May 2012, 03:04 AM   #1
Demonkleaner is online now Demonkleaner  United States
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Default PP KT-88 w/866A MV HT Supply

This is my amp which started life as a Bob Latino ST-120. I took the power supply components from a '50's era Motorola Transmitter and built the HT supply using the original 866A MV valves. The amp is dead quiet and performs very nicely. I'm new to the valve audio world but have some high powered RF background. It has regulated filament supplies for the KT-88's and the three driver tubes. I'm driving the amp with my Salas designed 6V6 pre-amp to whom I'm grateful. I've got 100+ hours on this system and it just keeps sounding better.
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Old 25th December 2017, 06:20 AM   #2
rmb is offline rmb  United States
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Nice to hear someone still uses the mercury-vapor rectifiers. The nice thing about the M-V tubes is the constant 15 volt drop regardless of current... which is nice with AB1 type amplifiers. For most medium power audio stuff, a pair of 866A's will deliver 250 ma continuously. The 872 (or 8008) is really for transmitting, the 575A, 266B, and the 857B is in the big leagues. If you worked on transmitters, you know about the preheat delay required before applying HT to mercury vapor tubes...

The KT88 is a nice tube, especially the older ones. The 7027A was also nice..the old RCA tube manuals had a schematic for a 50 w monoblock using them. So is the 807.

BTW the 211, 805, 810, 845, and 833A make nice audio output tubes, depending on the power level. (Many 1 kw AM transmitters used two 833A's with 2500 v on the anode as modulator for two 833A RF finals...)
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Old 25th December 2017, 09:19 AM   #3
Eli Duttman is offline Eli Duttman  United States
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No Hg vapor for me. Sooner or later, an envelope will break. That's bad enough if the bottle is cold. If it's hot, highly toxic Hg vapor is released. If you absolutely insist on using Hg vapor rectifiers, keep a tightly closed container of powdered sulfur on hand. Sprinkle the sulfur anywhere Hg has gotten loose. The metallic Hg is converted into mercuric sulfide (HgS), which is about the most insoluble stuff there is. Boiling concentrated nitric acid will not attack HgS.

You can come darned close to Hg vapor performance and avoid toxic risk by employing damper diodes. A pair of 6AU4s easily takes care of 250 mA. If more current is needed, use 6CJ3s.
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Last edited by Eli Duttman; 25th December 2017 at 09:21 AM. Reason: toxic risk
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Old 25th December 2017, 11:57 AM   #4
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Quote:
Originally Posted by rmb View Post
Nice to hear someone still uses the mercury-vapor rectifiers. The nice thing about the M-V tubes is the constant 15 volt drop regardless of current... which is nice with AB1 type amplifiers. For most medium power audio stuff, a pair of 866A's will deliver 250 ma continuously. The 872 (or 8008) is really for transmitting, the 575A, 266B, and the 857B is in the big leagues. If you worked on transmitters, you know about the preheat delay required before applying HT to mercury vapor tubes...
250mA for 5/10KV, for audio amp think 500mA /2KV.........
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Old 25th December 2017, 04:09 PM   #5
wicked1 is offline wicked1  United States
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I was very very close to building a big 'over the top' amp w/ MV rectifiers and thoriated filament tubes.. Preamp had very pretty thyratrons, and amp had 866's.
It was complete on my breadboard.
Then my wife had a baby and that project got put in the closet for about 4 years. Now I'm getting back to it, and spent the past week trying to decide if I'm going to use the MV rectifiers..

I think I've decided against it.. I would still take the risk on my own, but the thought of that breaking w/ a kid in the house is horrible. Plus the likelihood of it getting broken w/ a kid in the house is much higher.

I'm sad about it.. But it's the right decision.
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Old 25th December 2017, 06:35 PM   #6
claudiomas is offline claudiomas  France
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i use two 816 near max average current (220mA/1KV) for more of six months
max values 250mA/2400V
just for the beatyfull blue light
never any problem , not even hot
now its back to 866A, but just replace cap...and swap
but if you have children so you need ...aquarium
and tube amp inside, sorry for the fish
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Old 25th December 2017, 06:57 PM   #7
hpeter is offline hpeter  Slovakia
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Quote:
Originally Posted by Eli Duttman View Post
No Hg vapor for me. Sooner or later, an envelope will break.
with high mechanical pressure, it is possible
otherwise, i do not believe. this is not a high pressure streetlamp.
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Old 25th December 2017, 09:34 PM   #8
Demonkleaner is online now Demonkleaner  United States
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Quote:
Originally Posted by hpeter View Post
with high mechanical pressure, it is possible
otherwise, i do not believe. this is not a high pressure streetlamp.
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Old 25th December 2017, 10:48 PM   #9
wicked1 is offline wicked1  United States
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I think he means due to life.. Things happen.. I have broken a couple of tubes over the years. (not MV)
Chances are very slim, but it's possible. Someone bumps into the table. Wife tries flinging the vacuum cleaner cord over the top of it. Anything can happen.

Mercury vapors are very bad. I recently watched a video about chemists who have died from mercury poisoning. They lost their minds and died slowly for months or years after exposure. Solid elemental mercury isn't bad to touch so it's easy to be dismissive about it. A single high level exposure to the vapors can kill you. Relatively low levels will cause permanent developmental problems in children.

It's a risk I was willing to take before I had a kid... So, I totally understand people using them. But the reality is, if it breaks while running, your home becomes a toxic waste site. If you're in the room with it when it happens, you will be poisoned to some degree.
If it breaks while it's not running, it's not such a big deal. Although if you don't find the metal mercury that spills out, it will slowly evaporate giving you a nice long chronic exposure.
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