DISCLAIMER: Vacuum tube circuits work with dangerously high voltages. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

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Tuesday, July 25, 2017

Hong Kong High-End Audio Visual Show 2017


I am happy to announce the presence of ELROG tubes and my amplifiers at the Hong Kong High-End Visual Show 2017 from August 11th to 13th.

This exhibition is hosted by audiotechnique in the Hong Kong Convention and Exhibition Centre. Our participation is organised by Baoling Alternative Audio.

Come to hall 3, booth A07 to see some of my amplifiers and and listen to ELROG tubes.

Looking forward to meeting you in Hong Kong!

Best regards


Friday, July 21, 2017

The Tweeter Amplifiers, Part 2


In part 1 I showed the tweeter amplifiers half way through assembly. Here are some photos of the finished amps.

These are mono blocks each with it's power supply on board.

Output tube is the 801A, driven by a 6N7.

A 6BY5 located at the back side does the rectification.

Top view:

Both amps together:

Although designed as tweeter amps for an active set up, these could be used fully range with speakers which do not have deep bass anyways. Roll off kicks in below 60Hz

Best regards


Thursday, July 13, 2017

Summer Sale


I need to clear out some parts of which I have too many in stock and which are not planned for projects in the near future.

These Lundahl transformers are up for sale at discounted prices. The list prices per piece are given for each item. This is the net list price for export outside the EU. Inside the EU 19% sales tax is added. For a purchase of one pair subtract 20%. 25% discount when buying 2 pairs at once, 30% for 4 pairs and 40% for 5 pairs or more. A mix of any of the listed types can be ordered to apply for the discounts. The offer is valid only while supplies last. Send an email for a total quote including shipping. Payments can be received by bank transfer or PayPal.

LL2753 large interstage transformer 1:1 (list price 120,- Euros per piece)

available with different air gaps for these currents:
25, 30, 35, 40, 50 mA

LL2745 interstage or line out transformer 2.8 + 2.8 : 1 + 1 + 1 + 1 (list price 105,- Euros per piece)

available with different air gaps for these currents:
11, 18, 36 mA

LL1692A interstage or line out transformer 1.75 + 1.75 : 1 + 1 + 1 + 1 (list price 92,- Euros per piece)

available with different air gaps for these currents:
10,  36 mA

LL1689  line out transformer 9 + 9 : 1 + 1 + 1 + 1 (list price 105,- Euros per piece)

available with different air gaps for these currents:
18, 25, 36 mA

LL1694 filament choke (list price 64,- Euros per piece)

available for these currents:
1.25, 1.0, 0.75A

I also have some horns lying around unused, maybe you can bring them to good use:

Aluminium cast IWATA horns for 2 inch drivers painted in metallic grey.

1.500,- Euros for the pair.

These are quite heavy so shipping cost can be quite high. Personal pick up can be arranged.

Elliptic horns made by David Haigner.  Painted in silver/champagne metallic with a slight tint of green.
These are made of plaster and come with adapters to reduce the mouth opening to 2 inch. Holes for mounting the adapters and drivers need to be drilled by yourself.

750,- Euros for the pair.

Best regards


Tuesday, June 27, 2017

Tube of the Month : The 6FM7


I haven't presented a Compactron tube since more than two years, so I pulled the 6FM7 from my stash as this months tube.

The 6FM7 is a dissimilar duo triode. This means it contains two triodes which have different parameters.

In case of the 6FM7 this is a high mu triode as section 1 and a low mu high current triode as section 2. The pinout is shown on the left. The two triodes leave several of the 12 pins unused. The 6FM7 was developed as vertical deflection oscillator and amplifier. Of course it can be used for other purposes as well like many TV tubes. Such a combination of triodes has not been unusual and I already showed a dissimilar due triode with octal base, the 6GL7. The high mu section of the 6FM7 has an amplification factor of 66 with a plate resistance of 30 kOhms. This could be useful for applications needing a lot of gain like phono stages. Section 2 is a low mu triode with an amplification factor of only 5.5 with a low plate resistance of 920 Ohms. This could be used in line stage applications or even as output
triode for a flea power amplifier. In such a use case the high mu section can be used as driver, enabling an amplifier with a single tube per channel. Or the tube could make a nice headphone amp. Many possible uses for the ambitious amplifier builder. Since Compactron tubes are still widely ignored by audio people such tubes can be picked up at low cost. The Compactron series of tubes contains many gems which are very useable in audio applications. For example the 6GE5 which I presented in 2014. Look up that post for some background information about the Compactron series. I have not used the 6FM7 myself yet but I am sure it will work nicely if used in the right circuit. As always let's have a look at the curves of the two triode sections as shown in the General Electric data sheet. First the high mu triode.

Here a comparison to actual curves taken from a tube sample:

Nice linear curves, here another shot with smaller grid voltage steps:

The low mu section is not quite as linear, but still well useable:

And some real life curves:

An interesting tube! It would make a nice 1 to 1.5W 'spud' amp, or preamp triode.

Let's start the photo gallery of 6FM7 tubes with General Electric since they were the inventor of the Compactron series.


This tube looks suspiciously identical in construction to the GE above:

It was most likely just rebranded.

Some details.

Hoffmann branded 6FM7:

Probably a label of a store chain.

6FM7 made by Sylvania:

Some details of the tube:

Another Sylvania:

Two versions of 6FM7 made by Raytheon:

Amperex 6FM7:





And lastly RCA:

As mentioned in posts about other Compactron tubes already, one disadvantage of the 12 pin base is the force you need to apply to insert the tubes into the socket.

When not careful it is easy to bend the pins and even cause tiny cracking in the glass which can go unnoticed until you power the tube up. This creates a spectacular light show like this:

During the photo session for this post an unlucky tube fell off the table.

Since it broke, it is a good candidate for a tube dissection.

The two triodes without the glass:

The top:

This photo shows how the heaters of the two triodes are connected in parallel:

Bottom with the heater wire of the high mu section partly pulled out:

The heater wire of the low mu section:

The high mu triode removed from the rest:

The low mu triode, with the large plate for good cooling:

The two fins mounted on the rods provide cooling to the grid:

Grid and cathode of the high mu section:

Close up:

The low mu section has gold plated grid wire:

Tube manufacturing technology was very evolved when the Compactron series was introduced as these photos show.

A 6FM7 with the heaters lit up.

I hope you enjoyed the presentation of this tube. Stay tuned for many more tubes to be presented in the Tube of the Month series.

Best regards