• 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.

Girlfriend Wants to Build One

Hi

Hy Girlfriend had decided she wants her own hifi (possible the Wima caps incident - i dont know) and she is starting with the Amp.

So - taking into account my nice new $10k hospital bill - what would be a good, simple, low cost choice for her to start with?

My assumptions are as follows:

Single Ended
Stereo chassis not mono block or twin stereo

We have some fairly sensitive speakers so she is not needing lots of power.

I thought about the GU50 as it is readily available over here, apparently it sounds far nicer in Ultra Linear than Triode so we can get about 6/7W out of it. It does not run at stupid voltage AND it is an unusual looking valve!

But, seriously over here is really well regarded and many prefer it to the 300B.

What do people think and what other options would you people recommend??
 
If you are willing to go PP, instead of SE, an inexpensive "El Cheapo" meets your needs. ;)

Any 6V6 family tube will work as "finals", with zero parts value changes. :D As Bulgaria was a Warsaw Pact country, the Russian 6П1П (6p1p) is an obvious possibility.
 

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Hy Girlfriend had decided she wants her own hifi (possible the Wima caps incident - i dont know) and she is starting with the Amp.

That's cool!

I thought about the GU50 as it is readily available over here, apparently it sounds far nicer in Ultra Linear than Triode so we can get about 6/7W out of it. It does not run at stupid voltage AND it is an unusual looking valve!

I think you should use this tube - you've made the choice already, you don't need folk on the forum to validate it for you - go for it man!

I would send a PM to 'Wavebourn' - I trust his judgment on most thins to do with tube amps and I believe he knows this tube very well.
 
GU50 is a powerful honker for the money. I think Wavebourn has said that it's best used in pentode, not in ultralinear, as its screen grid can't take very high voltage. Maybe use plate-grid feedback local to the output tube?

What kind of transformers can you get? That's going to be the most expensive part of the project, so I'd choose those first.

Russian 6P14P (western EL84) is a good tube for use with less expensive transformers. If you don't need a lot of power, maybe you could use a pair triode-wired in push-pull for about 5W per channel. The plain old 'concertina' phase splitter (try 6N23P or 6N1P for that) into a push-pull pair of 6P14P is a classic.

Eli D's El Cheapo circuit would work with 6P1P outputs (Russian 9-pin version of 6AQ5).

You have a lot of choices...
 

Shoog

Member
2002-08-15 10:16 pm
Eire
How sensitive are your speakers ?
Try this for size if you can live with a very sweet 3w. The E55L is quite expensive but the toroidal PT used for inputs and outputs only need to be 20VA so are cheap as chips.
(Tune the circuit by substituting a 470K pot for the feedback resistor and dialing in whats best for you, when you find the right value sub in a fixed resistor).

Shoog
 

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Shoog

Member
2002-08-15 10:16 pm
Eire
If you need 7w try this for size. Sounds just like a SE amp and uses ultra-cheap Russian tetrodes. the only expensive component is the 12L8GT used in the front end. Makes for a compact 3 valve stereo PP amplifier.

Shoog
 

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Hi - I really appreciate all the advice - im not set on the GU50's but it has the advantage of being powerful and cheep.

We should be lucky on the transformers - one of the "respected" manufacturers of transformers (who make transformers for lots of companies that CLAIM to make transformers) actually manufacture there teroidal (sp?) transformers in Bulgaria - they are hand wound by freelance "piece" workers in the villages and mountains. Needles to say a lot of these dont make it back to the manufacturer. They get paid about $5 for hand winding a large transformer so apparently a bottle of Vodka will easily acquire you a Teroidal output transformer.

This would account for the many valve amplifiers for sale in Bulgaria with ABSOLOULTY COLOSSAL power supplies :)

The guys in there houses making tube amplifiers in Bulgaria REALLY do not do a shoddy job - have a look at this GU50 amp that I nearly bought (in not trying to replace this is is simply that this brought the GU50 to my attention)

From memory that power supply weighed 45kg.........!
 

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gyro

Member
2004-07-26 3:43 pm
Definitely go with the GU50s - I switched to them when the price of 6550 / KT88s became too silly. They're very pretty if you take the metal lids off too!

I use them in P-P Triode (because of the G2 voltage limitation), very cheap and sound far better than the above tubes. Don't forget that the heaters are 12V.

As others have said, go with P-P, much cheaper for the output transformer.

6n2p-ev and 6n1p-ev are excellent tubes for input / drivers and should be easy and cheap there too. (Voskhod tend to be lower distortion).
 
Paul Joppa has provided us with a useful rule of thumb for matching speakers and amps. Joppa's Rule states that in a listening space of "average" volume an amp/speaker combo should be capable of producing 102 dB. SPL at a 1 meter distance. Applying the rule to 91 dB. sensitive speakers yields upwards of 15 WPC, as the power needed.

The Russian 6П14П-ЕВ (6p14p-ev), AKA EL84M, is a genuine 7189 equivalent, with good sonics, that's tougher than old rubber boots. Sherwood, in their S5000, squeezed 20 good sounding W. out of PP 7189s. Something along the lines of the S5000 power section circuitry should do quite nicely.
 

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I have never used the GU50 but I am under the impression they are copies of the Telefunken LS50.

http://www.mif.pg.gda.pl/homepages/frank/sheets/043/l/LS50.pdf

On the last page, they show triode connected curves and it doesn't look pretty to me. Anybody out there with a curve tracer take any plots with a GU50 triode connected? I am curious if they look as bad.
 

gyro

Member
2004-07-26 3:43 pm
Mine seem happiest at around 70-75mA with a 380V B+, cathode biased (45V across 620R cathode resistors), with a 6k6 A-A output transformer. G2 strapped to anode via 100R.

I'm not after high power (sensitive speakers), more low distortion and long life so not pushing them at all hard - Pa around 24W.