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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
The idea to build these amplifiers came with purchasing a set of old studio monitors, the legendary JBL 4333. These 75 Watt speakers with 15″ bass drivers needed a amplifier that could deliver some more punch than my 20 Watt EL34 amplifier.
Huben Panev here from 4hv helped me source twenty 6P45S beam power tetrodes, tv sweep tubes, the equivalent of PL519. The original plan was really to build a VTTC with 20 tubes in parallel, but balancing issues with 20 tubes with very different characteristics due to being used was a major setback for that project.
A big thanks to Steve Conner for helping me sort out various mistakes, improvements and fault finding :)
The complete article with all pictures, information etc:
DONE: Prototype microcontroller software Prototype enclosure Prototype amplifier circuit Prototype power supply Start up and test with tone generator Measurements with oscilloscope Music test Final enclosure Final amplifier PCB Final power supply PCB
TODO Measurement and shutdown hardware Measurement and shutdown software
I based the design on a rebuild of a old Easteuropean amplifier, the APX-100, Claus Byrith EL34 and a amplifier called KT88 Beastie from user Kruesti on diyaudio.com using long tailed pair for balancing the pre amplifier and phase splitter stages.
It it possible for the 6P45S to deliver approximately 120 Watt and will also have their bias adjusted for around 100 Watt, but will never be driven harder than the 50 Watt output transformers can handle.
I plan to build in a AVR microcontroller for showing voltages and currents on a LCD, making output stage adjustments/balancing easier and emergency shutdown to protect speakers automatic.
Monostage schematic
Power supply schematic (OBSOLETE, look for most recent version in latest posts, these schematics are kept as they were valid for the prototype)
The prototype is built from scrap materials and BirdNestTechnology 2.0
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
I'm really amazed at the 100W rating you mention... I always thought a pair would deliver something like 20W at its maximum, as a tube can deliver 35W max rating as in the datasheet of the PL509
I should really dig for my tubes and start my own amp... something for the holidays ^^
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Here are the wave forms from yesterdays testing.
From wave of lowest amplitude to largest it is 23V(73VA), 26V(92VA), 28V(107VA) and 30V(123VA). I used a dummy load of 7,3 Ohm.
These numbers are of course the peak voltage across the resistive load, so the power output at 123VA with a guessed efficiency of 0,85 would be 74 Watt RMS.
The HF response is not quite as good as expected and I will do some more tests with different NFB circuit values to see if I can lessen the amount of rounding on the rising edge.
Before you rework the NFB you may want to do the music test. This is an audio amp after all, the HF components in a 20kHz square wave should be above human hearing. If there isn't truly significant dampening of a 24kHz sine then I'd call it good. Just my two pence.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Sigurthr wrote ...
Before you rework the NFB you may want to do the music test. This is an audio amp after all, the HF components in a 20kHz square wave should be above human hearing. If there isn't truly significant dampening of a 24kHz sine then I'd call it good. Just my two pence.
You are right, I talked with Steve Conner about this last night and looking at the wave forms there was only a drop from 28,4 Volt at 1kHz to 27,2 Volt at 20kHz, so it is only -0.375dB.
Today I will do some tests on -3dB of clipping power and find the -3dB lower and upper frequency. Then I will decide weather to experiment further with NFB values.
Edit:
I tested the above with my 7R3 resistive dummy load and a 8R speaker. It seems there i absolutely no reason to change anything on the amplifier. The square wave tests showed some rounding that should be expected on 10kHz as the transformer can not pass the 100kHz harmonics. The sine wave tests shows great linear band width.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Second performance measurements after NFB capacitor is changed from 1nF (41kHz cutoff) to 0.47nF(87kHz cutoff), the bandwidth of the amplifier was increased and clipping occurs at higher power than earlier measurements.
The prototype is now considered done as it has proved itself capable of outputting 210 Watt peak before clipping. At 100 Watt peak it have its -3dB points at 11 Hz and 72 kHz.
I forgot to set the V/div correct, so all voltages should be multiplied by 100 or just read them with one decimal like 39.2mV is 39.2V
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100W 6P45S mono block tube amplifier
