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TUBENEWB - Quad KT88 Monoblocks

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A

Adrculda

Member
Joined 2009
Guess I have to scrap that idea...
Antek isn't expecting anymore T400 for 3 months or so...

So, plan B...
Hammond Toroid for HV and for everything low profile board mounted in Hammond 229 Series ( 229D12 for KT88/120 filaments, 229D10 for 866A/X filaments and 229 E56 for Bias ) this way the power is supplied by 1 board :)

Also tempted to get a higher voltage HV transformer and drop voltage across a resistor, this way i can set my voltage between 550-650V to the tubes in case i decide to substitute the KT88's for the KT120's

IS this a good or bad idea ?
 
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richwalters

richwalters

Member
Joined 2003
UL operation or tetrode ? With UL, I'm against running todays KT88 above 500V....not even sure if even the TungSol 120's are happy at 600V, that is with any sensible life ahead.

My experience with TungSol 6550 New Edit left me very sceptical despite most of these tubes coming from the same Russian house.

richy
 
richwalters

richwalters

Member
Joined 2003
Tony: the issues with the screens. I'm using EH KT90's in UL p-p with 40% screen taps; the value of the screen resistors does effect linearity and distortion. A glance at the
EL34 pentode data reveals a 1K or sim value; but I'm finding optimum values depend on the make of output transformer with embedded parasitics and this varies. I'm using
parallel p-p EH KT90's with 40% UL;475V B+; 2KA-A will give around 120W with Majestic (UK) performs best with the screens heavily loaded i.e using a 68R screen resistors whereas the Sowter variants prefer 100-470R. All I can say is that there aren't any adhered values to stick to, only the full load screen rating need apply.

richy
 
A

Adrculda

Member
Joined 2009
gingertube said:
The double 6550 PP Schematic.

A reasonable late 60's?? design.
Van Scoyoc Cross Coupled Phase Splitter (see Phase Splitters).

There are better ways to do some of this today - IF you are prepared to accept a bit of Solid State to assist the tubes.

Cheers,
Ian
Click to expand...

I'd like too keep it as "Classic" as possible...

Also that one transformer voltage doesn't look right...
2 x 2000V @ 300-500mA
 
M

mullardel34

Member
Joined 2005
A true classic high-power, vacuum-tube power-amplifier is the conrad-johnson Premier 1, a 200-watts-per-channel "beast" of an amplifier. The Premier 1 used a sextet of KT88's, front-ended by cascaded pair of differential LTP's to generate the necessary drive voltage for the output stage. The Premier one was recently rated by The Absolute Sound as one of the ten most significant ampliers of all time:The Premier 1 schematic is available for download at the following URL:I constructed a pair of monoblock power-amplifiers about a year ago based on this circuit, using the Hammond P-T1650W 280W OPT's, and this power-amplifier is a "beast" in terms of perceived power and musical dynamics! When my budget permits, I intend to experiment with quartets of the new KT120's instead of the current sextet of KT88's to see what capabilities result; crank it to 11!!! :D
 
richwalters

richwalters

Member
Joined 2003
I've currently got a parallel p-p amp in build progress using 4x EH KT90's with 200W per ch upped on the bench.The Conrad Johnson thd spec is poor considering the sextet o/p stage and the volumus power supply used. I'm using a selectable B+ system for economical listening as well as B+ droop for MI. Alot has come foreward the past 20 yrs, and running 6550's at 500V is pushing them.
Quartets of KT120's is an overkill....massive expense. The Hammond P T1650 o/p trannies with tapped secondaries could restrict the full performance.

richy
 
A

Adrculda

Member
Joined 2009
That's the route im taking...
Quad KT120 with the Hammond P-T1650 OPT

I always start with the power supply...
i like the look of that PP6550 even though has like a million tubes and could be more simple but i had my buddy that's helping me with my build ( hes been fixing guitar amps for the better part of the last 30 years ) and he gave me the thumbs up on it.

Also, i think my front end will be a lot simpler as im running a Balanced signal from a preamp to the amp
 
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M

mullardel34

Member
Joined 2005
Yes, the conrad-johnson Premier 1 mono-block clones are about 100 pounds in weight (30 lbs for the Hammond 1650W OPT, another 30 lbs for the Antek AS-4T475 toroidal power transformer, another 20lbs for the reinforced Par-Metal 10-19201x chassis (they're next-door-neighbors to Antek Inc., according to GoogleMaps)), with the balance of the weight going to the power-supply capacitors. I did install a second set of rack-mount handles on the rear of the chassis (had to reinforce the rear panel to distribute the weight to the handles without buckling the rear chassis panel) to make it a bit easier to lug the monsters around... :cool:
 
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A

Adrculda

Member
Joined 2009
Hardest part is finding caps rated for 1kV or higher in 470+uF

My enclosures will most likely be 1" baltic birch ply wood, sanded and varnished.
2 x 2' cubes will be made for each channel with the iron housed inside an the tubes on top mounted on a recessed 1/8 - 3/16" polished aluminum plate ;)

At the back of the cube there will be an 8cm fan pushing air in and expelled through went in the top plate and bottom of the cube to aid cooling.
 
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M

mullardel34

Member
Joined 2005
Yes, the HV power-supply components are fairly expensive; unfortunately, "you've gotta pay, if you want to play..." :D

Looking back over the construction notes for my conrad-johnson Premier 1 mono-block clones, I used two totem-poled pairs of Cornell-Dubilier 560uF/500VDC capacitors (Digi-Key - 338-2005-ND (Manufacturer - 382LX561M500B052VS) approximately $13/each) for the post-rectifier capacitor bank (effectively a 560uF/1000VDC capacitor), then through a Hammond 193L 5H/500mA/800VDC inductor (Antique Electronic Supply search for P-T193L; approximately $40/each) feeding to the final post-regulator 475VDC capacitor bank, composed of four paralleled Cornell-Dubilier 560uF/500VDC capacitors (Digi-Key - 338-2005-ND (Manufacturer - 382LX561M500B052VS) approximately $13/each). So, about $150/channel for the HV power-supply components (excluding the HV power-transformer, for which I used the Antek 2x475VAC/420mA AS-4T475 (Antek - AS-4T475 ); about $60/each). While the Antek power-transformer also provides two 6.3VAC secondaries at 4A/each, I had to augment that with a separate 6VAC/8.2A filament-heater power-transformer (Antek - AS-4T475; about $22/each) to cover the remaining filament heater requirements for my output stage... :(

I'm pushing the Hammond 500mA choke pretty hard (the standing bias current for the sextet of KT88's is about 400mA), but the 193L seems to be standing-up to the demands pretty well (no excessive noise, no extreme heat, etc.). Bottom-line: classic vacuum-tube power from classic vacuum-tube power-supply parts... :cool:
 
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