• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

50 Type Tube

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Interesting topic. I thought about a 50 amplifier as well. The new production SG50 is a really fine tube ans just round ~50$ (Shuguang SG-50 new Hi-Fi vacuum tube)

Problem is...sadly...the max. grid resistor is only 10k. So forget about about a nice old globe triode driving it directly. Either interstage or Cathode/Source/Emitter follower (=Power Drive).
A good high-gm pentode, a mosfet and a 50. All directly coupled. Would yield a nice, unusual amplifier. Far away from the mainstream "Sakuma Style".
Thought I would share this with you since you were kind enough to reply.:D


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From what I understand, the types 45 and 50 present music rather differently. To generalise at a very gross level: 45 = direct yet natural; 50 = tone monster yet natural.

Both are relatively easy to drive, if you can supply the voltage. NOS 50s have significant constraints regarding the amplifier's grid circuitry (max grid resistor 10k, transformer or grid choke coupling), but new production do not have this issue. Frankly, I'd not bother with unreliable and expensive NOS 50s when new tubes do not have the grid circuitry constraints, Shuggies are available for cheap and EML does a reportedly very nice (nicest?) 50 for about the same cost as the NOS.

I'd like to try a 50 someday.
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The problem with Shuguang SG50 is quality control. I bought 2 pairs and found one of them had bad filament after 50-60 hours of usage. Another pair has been good for many years (4 already), however.

I've never compared with old 50 but it sounds really good to me. I use 5K opt which I can switch between 50 and 300B. I like them both.
I have just finished two stereo SE 50 amps, the second of which will get first power today. Here's a link to a quick cell phone photo of the first one playing (sorry for the poor quality, I'll get some porn shots up soon). http://Imgur.com/hGoWzHD

It has a Newcomb TR-100 or TR-91 input transformer, type 56 triode, Lundahl LL1660 IST, 50 triode and Hashimoto OPT. it has twin 5U4 rectifiers and oil and motor run caps. I have a pair of EML50s as well as some pairs of vintage 50s. The amp sounds amazing. I don't ever want to stop listening:) I can't hear a difference between the EMLs and vintage tubes. The EMLs are extremely well-built, like all of their products.

To me, the sound is similar to a 45 but with lots more oomph.
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Here it is. Please excuse me for hand drawing. I have not learnt to draw on my computer....

Each filament supply has its own transformer. lc supply where the l is lundahl 2733: This choke can be made with gap for different currents .

The Coleman regulator is well known, the Coleman bias supply
is quite new , very promising, highly recommended.
Same goes for Daves Irons: excellent stuff!

I am about to build a new version of this amp in a larger chassis.
So far the psu is a shunt regulated supply
borrowed from Stephie :
Tube Based Voltage Regulators - Part 5
the one with multiple controll tubes.

I am rather curious about psus, and I am making a choke input supply for comparission with the shunt regulated
(Actually it will be lc-lc-lc for output tube, and lc-lc-lc for driver input tube)

The currents in each rail allows the use of plate chokes which have rather high inductance. I am curious to see how this kind of psu behaves.

I would also like to say that Rod C, Dave S and Thomas M (delivered chokes and transformers for the psu)
are reliable and very nice to deal with: their contributions to diy'ers cannot be praised enough!

I am also about to investigate other drivers for the 50.


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