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Saturday, July 29, 2017

Tube of the Month : The 7B4


This months tube is a high mu triode with Loktal base, the 7B4.

This is the 4th Loktal tube covered in this series. For some more background on Loktal tubes see the posts about the 1LE37F7 and 7N7.

The 7B4 contains a single triode with an amplification factor of 100, internal plate resistance of 66kOhms and a transconductance of 1500 micromhos. For all technical data please see the Sylvania data sheet. The 7B4 is electrically equivalent to the 6SF5 and could replace it in the Octal Phono Preamplifier. Like other Loktal tubes the 7B4 is still available in good quantities and at modest prices. Loktals are still largely overlooked by amplifier builders and are a good alternative to Octal tubes. As always let's start with a look at the plate curves. This set of curves is from the data sheet:

And these curves are taken with a 7B4 on the curve tracer:

Since most of the Loktal tubes were produced by Sylvania and just a few by some other manufacturers, there is not a lot of variety of tubes to show.

All the tubes I have in stock appear to be made by Sylvania.

Like all Loktal tubes the 7B4 has the characteristic metal base.

Two of the three unused pins are omitted.

Most of the glass is covered by getter.

Philco branded 7B4:

Another one with red printing on the glass:

These are all the 7B4 varieties I have to show.

Since the getter covers the internals, let's break one open to see more of the construction details.

With the glass off we see the plate structure which is perfectly cylindrical.

And the holder for the getter at the top.

A view of the top:

Side view:

The bottom side with the heater wires:

The plate removed:

Nice circular cross section.

The grid and cathode:

Close Up:

Some more views of grid and cathode:

The cathode removed:

The grid on a ruler to get a feel of the tiny spacings of the grid wires:

With all the getter inside, there is not much glow visible when the tube is in operation:

That's all for the 7B4. Stay tuned for another Loktal tube in next months post.

Best regards


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 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