• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hammond 1650W

GK

Disabled Account
2006-01-08 4:35 am
I’m in the process of procuring parts for my next OTT power amplifier project.
I’m currently considering using the Hammond 1650W output transformer with three pairs of ultra-linear connected KT88’s with an idle current of 50mA each (class AB1) and a plate voltage of 500V.
Bias will be fixed, controlled by a PICc uC (adjusted at each power up) and the plate supply will be electronically (linear) regulated. A linear electronically regulated constant current supply will power the heaters (all connected in series).
The input / driver circuits will be entirely solid state (although I haven’t completely made my mind up on that yet.)
Oh, and it will offcourse have an inbuilt signal monitoring oscilloscope as well.

Anyone here have (positive?) experience(s) with the Hammond 1650W? It’s not exactly a cheap bit of iron.
 

tubewade

Member
2007-06-03 8:54 pm
Sorry, no word for you on the 1650W as I've never used that one yet, but I have used the 1650P (60 watt UL version of 1650) and I am 100 percent satisfied. I was first a little concerned as to whether I would be pleased or be looking for another transformer but I am pleased. I am using them Ultra-Linear with KT66 valves at 500 volts and cathode bias for about 25 or 30 watts of output.
 
Not that I'm against leviathan projects, but What makes you go for it ?

I am building one because I can, and it is different. I have breadboarded the typical push pull design using 6 X EH 6550. That is now "plan C". I got more power and punchier sound using 4 X 6LW6, now "plan B". Power output is in the 300 watt range.

The power supply is not too hard. The plate supply for the output tubes is a 480 volt to 120 volt 1 KVA industrial control transformer connected in reverse, feeding a SS bridge and CLC filtering. The choke is in the ground leg to avoid the need for a HV rated choke. Output is over 600 volts at 1 amp. Building a voltage regulator for this supply would be a serious experiment, but I am not convinced it is necessary.

I am currently experimenting with cathode follower output stages using 6LW6's or possibly 6336A's. This allows lower output impedance and distortion. I haven't got it all working right yet, so I may choose plan B or C at a later date.

I bought some Plitron toroidal OPT's that are rated for 400 watts at 20 Hz from their "surplus transformers" page last year for less money than the big Hammonds. Excellent transformers but the DC imbalance must be essentially zero, so I am using a microprocessor controller of my own design.
 

GK

Disabled Account
2006-01-08 4:35 am
ErikdeBest said:
A bit off topic, but as you touched the subject of biasing. Could you give us some more information on the valve biasing with the microcontroller?

Many thanks, Erik


I haven't decided on a scheme here yet. I would like to keep it all DC coupled (avoiding capacitive coupling to the KT88 grids and the asssociated over-drive problems), but then I will need an individual driver stage with a programable DC operating point for each KT88.
A simpler alternative would be a driver stage with a clamped/"soft" clipped output swing to prevent rectification at the KT88 grids.



richwalters said:
Not that I'm against leviathan projects, but What makes you go for it ?
I presume you are expecting 250W output poke ?

Iq = 300mA
Idc f load = appr 1A + headroom
I'm interested to the psu concept.

richj


Same as the guy above - because I can ;)
250W is about right. The PSU is pretty straight forward, just a series pass regulator with a 700V collector (drain) supply and a big heatsink.
 

GK

Disabled Account
2006-01-08 4:35 am
G.Kleinschmidt said:
The PSU is pretty straight forward, just a series pass regulator with a 700V collector (drain) supply and a big heatsink.


Hmmm...... actually, this would be a rather good way to use up some of the 12E1's I've got laying around - I wouldn't need the heatsinks and it would look way cooler that a bunch of boring power MOSFETs or high voltage BJT's....................
 

tubewade

Member
2007-06-03 8:54 pm
I admire you guys for building giant amplifiers like these, and I especially appreciate the reason "Just because I can!" If I ever decided to go ahead with my plans to build such an amplifier it would be for exactly the same reason. As for what I would do with it afterwards... I have no idea. I already have amplifiers that are too big to be of practical use in the living room.
 
I already have amplifiers that are too big to be of practical use in the living room.

I am the nut case that actually prototyped and considered building a 200 watt SET guitar amp. ( http://www.tubelab.com/833SE.htm ) I may build it some day but it is very low on my list of priorities since I know that I will never use it.

A 250 WPC vacuum tube stereo amp would be useful, and I already have the parts. I have been experimenting with all sorts of efficiency improving techniques for tube amps and I will eventually apply some of these techniques to the "Megatron". My goal is to build a 250 WPC tube amp that weighs about 50 pounds (the OPT's total 35) and fits in a 9 inch tall rack mount box. The long term power supply must obviously be an SMPS.

just a series pass regulator with a 700V collector (drain) supply and a big heatsink.

The regulator design is easier if the pass device is is series with the negative side of the rectifier bridge (or transformer CT). The cathode (source) is grounded and its drive is ground referenced. My experiments used an IRFPG40 N channel mosfet rated at 1000 volts and 4 amps of drain current.

but then I will need an individual driver stage with a programable DC operating point for each KT88.

I used a modified PowerDrive circuit ( http://www.tubelab.com/powerdrive.htm )

The driver in my case is a vacuum tube (low Mu section of a 6EM7) I used 3 seperate coupling caps each feeding its own mosfet follower, which is coupled to its own output tube. Each follower has its own bias adjustment to allow individual adjustment of each output tube.

A bit off topic, but as you touched the subject of biasing. Could you give us some more information on the valve biasing with the microcontroller?

The output tube current is sampled with a resistor in the cathode leg. This is fed to the A/D inputs on the processor. The processor controls some external D/A converters via the SPI bus. The D/A converters output a 0 to 5 volt control voltage under processor command. An opamp - mosfet level shifter circuit converts this to a 0 to -150 volt signal that is used for bias. Tube current is adjusted at start up and every time the button is pressed. Software for automatic adjustment during periods of quiet is being developed.

I developed the high efficiency tube circuits and the microprocessor bias circuitry for entry into a microprocessor design contest, where a very unique vacuum tube amplifier won one of the prizes in a sea of microprocessor based gizmos. The 50 page contest entry including schematics is here:
http://www.circuitcellar.com/microchip2007/winners/MT2209.html

I am in the process of writing an article for their print magazine that explains the operation in further detail. I will eventually put all of the details on my web site. I plan to offer the bias controller, the SMPS and the modulated buck regulator as stand alone PC boards at a later date.
 
The only reason I am not building one right now is becase I can't...

Thats OK. You (and most of the readers on this forum) can let us figure out what works and doesn't work, and blow up a few (OK lots of) parts. The working stuff gets posted and some of the circuits will make their way into stuff that you can build.

Seriously, I can remember someone giving me some monster Heathkit OPT's for KT88's back in the 1970's. I set out to design a state of the art direct coupled P-P power amp. After blowing up several 6550's and KT88's, I gave up and passed the transformers on to a friend who used them in a guitar amp. I would love to have those OPT's today. I now have 30+ more years of electronics experience and a couple of college degrees, and I still blow stuff up, especially when playing with SMPS's fed from the power lines.

I doubt that any 250 WPC tube amp will ever be reduced to the simplicity and with the success rate of a SimpleSE, but the lessons learned with the big stuff can be applied to smaller tube amps to improve them.
 

GK

Disabled Account
2006-01-08 4:35 am
tubelab.com said:

The regulator design is easier if the pass device is is series with the negative side of the rectifier bridge (or transformer CT). The cathode (source) is grounded and its drive is ground referenced. My experiments used an IRFPG40 N channel mosfet rated at 1000 volts and 4 amps of drain current.


Maybe a little easier, but the output impedance and ripple rejection is much worse.
The power supply chassis the the first part of the amp I'm going to build (I have nearly all the parts already). I'll get stuck into it this weekend (have nothing else planned) and post up the schematic. I've done this before, so the PSU circuit is already 90% drawn in my head ;)

The pass element will consist of five parallel 12E1's, each cascoding a BD681 darlington emitter follower - grid connected to the emitter, cathode to the collector, anode to +700V (this is for just one channel, supplies will be independent).
This way you get the low output impedance of a biploar emitter follower while the valve dissipates (nearly) all the heat.
 

GK

Disabled Account
2006-01-08 4:35 am
Got an email back from Hammond - List price of the 1650W is $389.58 AUD each plus (10%) gst.

Turns out that I don't live far from Hammonds Australian warehouse, so atleast I won't have to foot the freight bill to my front door (they're 28 Lbs each !)
 
tubewade said:
I admire you guys for building giant amplifiers like these, ..... I already have amplifiers that are too big to be of practical use in the living room.


I'm the 2nd nut who has built several monster amps and somehow get to listen to them......strong man to lift'em too with extra bracing in loft floor.
Series pass tubes in psu wouldn't hold up without consuming more heater power. Chassis big enough as it is.
My cue to the psu would consider pfc boost converter, esp when uprocessor would be on board. Complexity ?

richj
 

Brett

Member
2002-01-07 6:02 pm
I regularly contemplate hi-po tube amps as bass heads for my rig. Then sense takes over and I leave it alone. I don't want to have to hump it in and out of gigs along with my mudmagnet speakers. Contemplating a very small tube amp on the end of the preamps to give a similar O/D effect.

For hifi, I fail to see he point except for the 'because I can'.
 

GK

Disabled Account
2006-01-08 4:35 am
I started out working / building stuff in/for chemistry and nuclear physics/fusion research labs (495kW RF generators with 10kW drivers, ROTOMAK's, racks/rooms full of charged 16kV capacitors, 5V to 100,000V variable regulated power supplies, 8W lasers, etc, etc.

This amp I'm building now is neither complex nor big (well from my perspective anyway – especially compared to some of the other things being built up in my shed)!

Attached below are a few crappy conceptual drawings of how I’m going to construct this amp. (No dimensions added yet and the scribbles are not really to scale, but they should give an idea.)
The amp will stand upright, like a tower, probably 1 to 1.5m tall. It will be constructed around a TIG welded frame made from 50mm by 50mm aluminium angle 5mm thick. This frame will support everything, including the transformers.
Two varnished solid pine bits (15 or 20mm thick) will form the side panels and the legs. The front panel will be cut from 5mm thick aluminium sheet either painted or powder coated gloss black. The lower section of the front panel will consist of a big Conrad heatsink for the heater supply regulators, behind which the numerous power supply toroidal transformers will live.
Nearly all the electronic circuitry and all the valve sockets will mount on various sub panels attached to the main frame, about an inch from and parallel to the inside surface of the front panel. All tubes will mount horizontally, poking out the front panel.
The front panel won’t actually provide structural support for anything, being easily unscrewed for servicing access. The rear panel will be cut from a sheet of pegboard, with flyscreen wire glued to the inside surface, over all the holes. This will ensure adequate internal ventillation. As for the top panel, that will just be another pine sheet.
The aluminium angle frame will be fitted out with pneumatically riveted Nutserts for screw mounting of all the panels.

BTW, ~250W per channel for HiFi use driving not-so efficient speakers with high dynamic range material at reasonable listening levels (but still avoiding clipping) is much less over the top than many people here might imagine.
 

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