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Friday, April 3, 2020

The New 45 Drives 45 Mono Amplifiers


It's been a while since I built 45 power amps with 45 as driver tube.

Here a pair of newly finished amplifiers in that configuration.

About to be shipped to their new owner in the US.

The amplifiers use a 45 in the single ended output stage, delivering about 2W.

The output tube is driven by another 45 through an interstage transformer.

The input transformer offers the possibility to have a balanced XLR input.

The amps have been constructed in the tower style chassis using all the latest construction techniques which evolved over the years.

A few photos showing the construction process, starting with the raw top plate.

Sockets and meter mounted:

Some components are mounted underside of the top plate via threaded blind holes:

Initial wiring:

Adding the capacitors which are closest in the signal path:

Interstage and output transformers reside on a separate sub assembly along with more capacitors and chokes:

The power supply section:

LCL filtered filament supplies for zero noise operation even on the most efficient speakers.

Adding the sub assemblies layer by layer, with additional plates in between for mechanical ruggedness:

The completely assembled amp, without enclosure, ready for testing:

Some more pics

Best regards


Monday, March 30, 2020

Silver 10Y Line Preamplifier


Here some photos of a recently built 10Y linestage with silver transformers.

The preamp is finished with silver anodised metal plates in smoked eucalyptus wood and with graphite grey transformer covers and capacitors.

Here shown with ELROG ER801A which can replace old production 10Y, VT25, VT25A, 801A or VT62.

This linestage was specified with silver output transformers and transformer volume controls, 3 SE inputs and 2 balanced inputs, one set of which uses silver input transformers.

The power supply, this time without on/off indicator LED.

Best regards


Tuesday, March 24, 2020

Tube of the Month : The 25DK3


Today I am presenting a TV Damper tube with series string heater : The 25DK3.

The 25DK3 was designed for damping diode in the horizontal deflection unit of color TVs. Like other TV damper tubes it can be used as rectifier in power supplies.

The 25DK3 has a 9-Pin Novar base which is not to be confused with the much more common 9-Pin Miniature base which has the pins arranged in a smaller diameter circle. The Novar base is very similar to the Magnoval base as used on the EY500 but the latter has slightly thicker pins. The cathode connection is wired to a small cap on the top of the tube. As mentioned above the 25DK3 was intended to be heated in series with other tubes with the same heater current of 450mA. If that current is applied the heater drops 25V. There is also a 6.3V parallel heater version, the 6DK3. Like the EY500 the 25DK3 and 6DK3 have very large current handling capability. It can deliver up to 400mA per tube and can handle current peaks of 1.2A. That is coupled with an insane peak inverse voltage rating of 6.5kV and a very small voltage drop between plate and cathode. Detailed technical specs are available in the General Electric data sheet. With these specs a pair of the tubes can deliver up to 800mA.

An ideal candidate for high current power supplies. Like other TV dampers the 25DK3 has a very slow warm up which causes the output voltage to rise gently at start up. Especially series heater tubes can be found at low prices. The heater can be operated from a suitable transformer. Readily available 24V transformers will do the job.

I only have 25DK3s made by Philips ECG. So let's have a close look at these.

The plate is made of copper:

Some more details:

The base:

The base pins in comparison to a EY500:

Quite an impressive little device!

Now lets open one up:

Some close up shots:

Removing the heater:

The spiral around the heater wire ensures the high voltage insulation against the cathode.

The cathode:


And lastly some photos with the heater lit up:

I hope you enjoyed the presentation of the 25DK3!

Best regards