Operating Points for GU50 Triode

Here are some operating point options for 2k5, 3k5 and 5k loads.

6n6p Tube

• The pin connection is the same as the 6DJ8, and also has a screen on pin 9.
• Service life of 2,000 hours.
• Heater 6.3V at 750 mA.
• Gain of 22.
• Curves look very linear.
• Was the pre-cursor to the 6h30p tube.
• Still very readily available and cheap.
• http://www.klausmobile.narod.ru/testerfiles/6n6p.htm
  

6c19p SE Design Idea

Here is a quick idea for a simple 6c19p SE design.

• Operating point at 210V at 45 mA with -80V at grid.
• Say 300V at B+
• Swing from -5V @ 40V @ 70mA to -160V @ 380V @ 25mA.
• Loadline at 5k or 7k5 both have similar outcome.
• Require a grid signal of around 50Vrms to drive to full power, with a bit of headroom to avoid A2 operation.
• Assuming 2Vrms input, require a driver stage with a gain of around 25 minimum.
• CSS loaded input stage could use C3g, 6n6p, 6DJ8/family, 6/12SN7, etc, a lot of tube choices in the medium mu grouping. Given the B+ of around 300V, operating the input tube at around 200V would be ideal.
• Normal cathode biasing and capacitor coupling.

The great thing is that I have most of the parts lying around, with the exception of the power transformer. This can make good use of the Lundahl choke rated at 150mA, the CSS kit from K and K Audio and a few Jensen and Cerafine electrolytic capacitors. As usual, my enemy is time in getting around to hooking it all up. This would make a nice stereo amp. Maybe a schematic to follow.

This could even work for swapping out the output tube for a 45, by changing the bias capacitor and heater supplies. With B+ at 300V, the 45 could be 250V at 50V bias. Could work as a great convertible experimenter's amp.

Key Facts of the 6C33C Tube

Here are some key facts and information regarding this tube. The information below is gleamed from tube datasheets, websites, forums and playing with the tube itself (but not in an electrical circuit yet).

• In Cryllic, it is 6C33C. In transliterated English it is 6S33S.
• Comes in two flavours, the vanilla (6C33C/6S33S) and the rugged version (6C33C-B/6S33S-V).
• The main difference between the B and the normal version are the 'B' has lower Internal resistance and can withstand much more g forces in impacts and loads.
• Rate life is around 1,000 hours, but in a non-military/extreme environment and operated well below maximums, this could extend out to 10,000 hours as suggested by some.
• The tube is actually two triodes in one envelope. The cathode, plates and grid are internally connected in parallel. The heater is left separate, which can then be used singularly or both in a parallel or series connection.
• The tube is designed to run with lower voltage (150-250V range) and higher currents (150-250mA range).
• Very low internal resistance and a good candidate for OTL.
• The heater takes a lot of heater power; either 12.6V @ 3.3A or 6.3V @ 6.6A. That is 41.58W.
• The heater needs to turn on at least 2 minutes prior to B+ voltage. The longer the better.
• The tube itself feels very rugged, sturdy and well built.
• It was made in a number of different factories; Ullyanov, Svetlana and Poljaron.
• It was first discovered in the West via a MiG-25 from a defector's plane.
• Uses a special socket and has been suggested that it needs to be ceramic because of the high heat in the operation of the tube.
• The pins of the socket generally need to be well ventilated and clean to ensure good cooling and electrical contact.
• The tube runs hot and cooling and placement are critical to reliability and long life. Consider forced cooling, and considered passive cooling.
• A link from Good Sound Club with lots of advice on the tube. Rather biased opinions, but nonetheless valid observations from someone who has used this tube extensively.

Bottlehead Crack OTL

Just purchases the Bottlehead's new OTL headphone amplifier, the Crack . Seems like an interesting addition to my headphone system. It is relatively cheap, and looks to have plenty of room for upgrades/tweaks and expansion. Never used an OTL amp before, so it'd be interesting to build and compare this to the other headphone amps I have running.

The tube complement is very standard and readily available. The NOS prices on the two tubes, 12AU7 and 6AS7 are quite reasonable, moving up to the very expensive. But since it only uses one tube each, tube rolling may be an easy reality. Upgrades such as capacitor replacement (it looks 5 electrolytic can be replaced), CCS to the driver tube, resistor upgrades, sockets upgrades, etc. are possible.

Bottlehead seems to imply up to 4 weeks wait time, which is fine, as I am in no hurry to build. Over the last few months, I have come to the conclusion that headphone audiophile will be the main method of music enjoyment for myself. Mainly due to work and family arrangements. So, in the meantime, using my HD650, I will endeavour to build and listen to a wide a range of headphone amps as possible!

For reference, the following tubes may be suitable as substitutes.

12AU7 - ECC82, CV491, 5814, 6189, 5963, 6680, ECC802, E80CC and B749

6AS7/6080 - 7236, ECC230, 5998, WE421A, CV2523, 6336, 6520, 6h5c/6n5s, 6h13c/6n13s

6C33 Tube

Recently gained interest in Russian military surplus tubes, and inevitably the power tubes that tops the list include: GM70, 6C33 and GU50, etc. Here are some info on the 6C33 tube.

• http://digilander.libero.it/paeng/russian_numbering_system.htm
• http://www.tubes.ru/techinfo/AmateurRadio/6c33c-b.html
• http://www.jogis-roehrenbude.de/Russian/6C33C.htm

It appears there are two kinds of the 6C33; 6C33C and 6C33C-B. From reading various tech info, the 'B' designation seems to imply 'vibration proof, high reliability'. On the website above, the operation time (presume rated life) is shorter for the 'B' version at 750h rather than 1000h. The 'B' version seems to have lower internal resistance does have better acceleration and impact ratings. The curves for the two appear to be near identical.