The triode strapped PT15 AB2 Tube Amplifiers
Here's the temporary setup of my two tube amplifiers together with a
Philips CD104 CD player and a passive volume control. These amplifiers use PT15 direct heated power
pentodes strapped as triode . The PT15, also know as
VT104(RAF), was used in transmitters on board the RAF's AVRO Lancaster bombers during
WWII.
There is no fixed point in time when I started this project, it kind of happened gradually. In about a year I acquired most of the parts for this project. At the beginning of 1998 I came across the power tubes. I bought two of them to do some measurements and experiments.
Here's a picture of a PT15 next to an ECC83.
(kindly provided by Duncan Munro)
On a Saturday afternoon I build a test amplifier using two triode strapped PT15's and two output transformers, actually I used one of them as input transformer, phase splitter. The input transformer was driven by an old transistor amplifier. The plate voltage was fixed at 650V because I only had one set of identical power transformers. At 650V the PT15's were biased at 50mA each requiring a Vg1 of about -85V.
Here's the test amplifier.
Just behind the amplifier you can see the power transformer, rectifier and a
C-L-C filter
using a 8H 0.46A choke and two sets of 160uF capacitors connected in series.
I did some experiments with different plate loads to determine the maximum output power at 650V. With the optimum load this amplifier puts out about 90Watts (AB2). The output transformers I was going to use had a Raa of about 10K. With this load the amplifier still managed to put out 50Watts but it had to be operated in class AB2 to reach it. Next question how much power in AB1? Well about 25Watt which in most cases is enough. After these experiments I was convinced I could build a set of good amplifiers using these tubes.
After the initial experiments I spend a several weeks thinking about
how to drive these low gain (gain=4) triode strapped PT15 tubes. One of the
driver options I considered was a high gain amplifier with a lot of feedback (local
feedback). The feedback was introduced to reduce distortion in the driver
stage so the
overall sonic performance was dictated by the power tubes (no global feedback). An added
benefit of local feedback is a lower output resistance of the driver amplifier. This fact
started me exploring the possibility of a driver suitable for class AB2 operation. I added
a small power tube with a high transconductance as cathode follower to the high gain
section and applied feedback as before. A quick calculation showed me that this driver
configuration could have an output resistance so low that distortions due to grid current
are negligible. Even when I put in a tube with a medium transconductance the output
resistance was still low enough.
To fully drive the PT15
tubes the drivers should be able to drive 380Vpp at a current of about 80mA. Because the
driver power tube is a cathode follower the cathode-filament voltage is a big issue. I
selected the EL86
as the driver tube because this tube can handle up to 200V between cathode and filament
and can handle up to 100mA. Because g1 of the PT15's are typically biased
at -85V the EL86 filaments have to be biased at about -85V as well. The gain stages in the
drivers are build using ECC83's and the
input stage and the phase splitter are build using a single ECC82.
Around june 1998 all design work was finished and I started to build the first of the two amplifiers. The first amp was finished at the end of july.
The inside view of this amplifier
After I had checked all wiring I put in all the tubes except the power tubes, switched on the amplifier, and checked if everything worked as I should, it did. Then I put in the power tubes, hooked up a speaker, fired up the amplifier again and biased the output tubes. First a hum check, I stuck my ear in the bass speaker.... dead silent, mid tone speaker... dead silent. Did I forget to turn on the high voltage??.. No. When I listened to the tweeter I could hear a faint hiss. Amazing this amplifier has no hum at all. Then I put on some music and then slowly turned up the volume. The sound was absolutely clean right up to the point of clipping (50Watt!!). After a few more tests I was quite impressed with the results. Then I started to do some frequency and phase response measurements. The frequency response was okay, nearly flat from 30Hz to 20KHz, but I wasn't happy with the phase response. It turned out that the Cag in the first driver stage introduced a phase shift. Adding a phase correction circuit solved this problem.
Here the finished amplifier undergoing its final
tests.
Satisfied with the results I started building the second amplifier. Early november 1998 the second amplifier was finished. This one worked fine from the start.
I have been listening to these amplifiers for over a year now and I'm very happy with them.
Corne Janssen.
e-mail: cjan@oce.nl