Hammond 1650W

[GK]

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]

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.

[ErikdeBest]

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

[Koifarm]

Originally posted by ErikdeBest
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

Buy the Elektor Audio part 2 now in your stores. There is a complete design and artikel about microcontrolled biasing in it.

Ronny

[richwalters]

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

[tubelab.com]

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]

Originally posted by ErikdeBest
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.

Originally posted by richwalters
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]

Originally posted by G.Kleinschmidt
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]

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.

[tubelab.com]

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/microch...rs/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.