Mono Power Amps
 

Taylor T-55 tested by TV-7 tube tester

 

T-40 Power Amps

 

More Watts per Dollar

Discussing and planning SE triode amps a radical design change occured. Listening to a power amp with transmitting triodes type 812 at my Lowther horns I decided to go a similar way. It is a very lovely way to drive those horns with a powerful triode tube, especially if the tube is running without any stress. So I had a look around to find the right tube type.

On the web site of Triode Electronics I found the complete Taylor Tube catalog from 1945 with a tube which fits in a design of an AF amplifier: the zero bias Triode T-40. It is possible to run this power tube with 0V grid and drive it with a small power tube. 

Taylor produced the tube in 2 versions: the T-40 and the zero bias version TZ-40. In fact the tubes were handled in amateur radio equipment as interchangeable, but the T-40 has lower µ and lower plate resistance. There were different manufactorer and military codes for this tube:

TZ40 = Z40 = 3B/351 = HY40Z = SW40H = DA41 = VT76 = CV1076

also Eimac's 35T is a 'near equivalent' for TZ40

Main difference between T-40 and TZ-40 is the amplification factor and the plate resistance. I decided now to go for T-40. Interesting enough I found the same set-up can be used for the larger Taylor T-55 tube.

Next step was to ask my GE tube's characteristics manual for a sophisticated driver solution. And right, there are tubes with 2 triode systems, containing a high µ voltage amplifier and a low µ power amplifier. A fantastique solution, a pure triode system. 6EA7/6EM7 were build for television purposes and are octal based. One can get them NOS for a few bucks. So a 2 tube 3 triode system started to become designed.

This are the specs of the tubes: 
 

  6EA7

T-40

max values

 1st system 2nd system  
heater voltage 6.3V indirect 7.5V direct
heater current
 1.05 A
3.0 A
plate voltage
 350V 550V
1.250 V
plate current
 2 mA
40 mA
125 mA
plate resistance 30.000 Ohms 920 Ohms 8.700 Ohms
grid voltage
 - 2 V
- 45 V
- 250 V
amplification factor
 66 µ 5.5 µ

25 µ

plate dissipation

 1 W 10 W
 40 W
socket type
 Octal

UX4 bayonet


Because I had already a pair of power transformers with a 500V secondary I did experiments with a single-wave rectified power supply. Of course I wanted to run the amps with rectifier tubes. Because of the high voltage drop vacuum rectifiers like 5R4 are not the right solution. I've voted for a small mercury rectifier, type 816, because of the small voltage drop. With 500V AC from the power transformer I have approximative 585V Ub+ DC. Like all mercury filled tubes 816 needs a heat-up time of 30 seconds before plate voltage applies. But this was absolutely not satisfying. Due to the circumstance the main transformer in single-wave mode is only working on one phase I had a lot of hum and buzz from the transformer.

I changed my mind and actually found a new solution using 2 TV-damper diodes 12D4 in combination with 2 Si-diodes, building a full wave Graetz rectifier bridge.

 

Taylor T-40

GE 6EM7/6EA7

RCA 12D4 

I made a webpage with some tube rectifier data: Rectifier_data

Output transfomers are made by Rainer Baule Trafo-Baule . Based on El130a cores they have 10k impedance on the primaries. The air gap of the EI-Core is 0,6 mm. In fact Rainer performed a 'transformer bring-in service' and made a visit to bring the transformers to me, that's really a great service!

To cover the output transformers and protect against influence from the main transformer I've asked Automatic Electric Europe to build customized hoods.

With the full wave bridge rectification I'm building now a C-L-C filtered supply, with one new element: instead of real choke I'm using an electronical suppression circuit, a so-called Gyrator.

The line stage is build by the 1st system of the 6EA7, which is directly DC-coupled to the 2nd system. Coupling to the grid of the T-40 will be done by an interstage transformer, a Lundahl LL1635, gapped for 20mA DC

T-40 curves incl. my operating points are here: T-40 Curves

Transformer core specs

Main transformer: custom build by Trafo-Service-Baule

M102b

500V 150 mA

8V 4A

6.3V 4A
Main transformer: unknown from a Surplus EI60/20 0-12-24V 1A
Output transfomer: custom build by Trafo-Service-Baule EI130a PRI 10k - SEC 8R - air-gap 0,6 mm
Interstage transformer: Lundahl LL1635/20mA C-Core ratio 1+1 : 1+1, DC resistance 500R



The chassis' were build from 2mm alu with the dimensions 35 cm x 20 cm x 8 cm, large enough for those small mono blocks. They have a frame made of Teak wood around. Used Weston panel meters for 100 mA give a nice gadget to show the plate current of the T-40ies. Sockets for the T-40 are Johnson #210.

The Teak frames are varnished with Teak glaze and polished. The chassis are punched and drilled and therefore ready. After that I used also steel wool type 000 to polish the alu and then I've clearcoated it. I've build a construction consisting of 2 braces per chassis to mount the chokes, caps etc. internally.

This are the auxiliary supplies and the Gyrator circuit:

On the left side: the positive grid supply with 12V transformer, 4 * SB160 Schottky, LM317T regulator

In the mid: DC-filament supply for the T-40, with 4 * SB540 Schottky, Common mode choke

On the right side: Power Supply Gyrator with BUL58D power NPN transistor

 

1st picture of one of the amps:

 

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Last update 19/October/2003