Single Ended EL-34 build

[henry4841]

Finally satisfied with the tubes in my dedicated 6L6GC amplifier. I have a number of 6N3C tubes that are Russian made that were bought years ago and never used. It appears that the ones I have are made by two different companies. Of the 3 pair I have 2 pair still have hot spots on the plates with 18W dissipation. Some of the 6L6 tubes are only rated for 18 watts but the 6L6GC ones are rated nominally at 30 watts. I have a third pair being sold as Sovtek that have a different looking plate than the other 2 pairs and were sold as 6L6GC. Obviously rebranded by Sovtek with the only manufacturing mark of 6N3C just like the other 2 pair but they are working perfectly biased at 18w with no red spots on the plates and sounding much better than the other two pair. This was using a 750ohm cathode resistor. I am measuring 3.1w of power at the speaker with 18W of dissipation. Plenty enough using Klipsch speakers so I see no need in increasing the 18w I am now biasing the tube at. I am satisfied with the performance and sound of these Sovtek tubes. When purchasing 6N3C old production tubes one must be aware of the large range of performance in those old tubes. They may not work in the amp you currently own so be careful of the cheap price they are being offered at on Ebay and other sources. They may not work correctly in your amp. The Sovtek 6L6GC seems to perform as expected. I have a Chinese made pair of 6P3P tubes and according to Franks tube data they are rated at 20 watts dissipation so I may try them and see what they sound like later on. That and see if they have any red spots on the plates. 

 

I have a box of tubes I have bought over the years and never used. Fun just looking through all of them just to see what I have and possible can use to build an amp around.  

 

Now I can get back to finishing the chassis for my new EL-34 amplifier. Making progress working a few hours every day on it. Just a few ventilation holes to drill in the top plate and in a few more days may start on the electronic part of the build. Life is good. 

[henry4841]

Not perfect but I would say presentable. 

 

 

[billybob]

No, look really nice...

[henry4841]

I have done some more searching or you might say researching on the net about the 6N3C Russian tubes and it is hard to find out exactly what one may have when buying these tubes. I found out that some of them are only rated for 18w and that would explain the problem I was having biasing the ones I have for this amp. Two pair I own have plates that had red spots at 18w so I am to assume they are rated for 18w and should be biased for 15w or so. The Sovtek, with the same designation from the factory of 6N3C and that only, are rebranded and sold as 6L6gc tubes with a rating of 30w. Different manufacturer from appearance being the plates are different looking in the Sovtek tubes. They are working great with no red spots and sound much better than the other two pair. I plan on providing some numbers of my testing of these tubes for this amp and will post the results since many only this forum like numbers. Such as K to P voltage and current flow through the tubes along with some other numbers when I find time. 

 

Brought my new chassis inside to begin the component placement and electronic wiring yesterday. More to follow.  

[henry4841]

Here are the numbers I am using for the 6N3C, sold by Sovtek as 6L6GC. Cathode to plate voltage of 377V with 46ma of current. If math is correct I am running a 30W tube at 17.3W of dissipation. Both tubes are so close as to be twins. Sounds really good and the tubes should last a long time. 

[henry4841]

Almost ready to start building circuits. 

 

 

[henry4841]

It is working. Details forthcoming but for now here are some pictures. I built this one for either SS or tube rectification. Right now I have it wired SS but it can easily be changed to tube rectification. 

[henry4841]

[Shakeydeal]

Looks nice. Your bird in the hand looks better than my one in the bush.........🤨

[henry4841]

I am still in the process of biasing this amplifier for the KT-77 tubes. My first choice of tubes for this build were some EL-34's I bought off of Ebay as being used but measuring excellent. Or so the seller said. No matter the measurement they did not sound that good. Loose flabby bass and no sparkle to the top end. I would call the tubes tired. Completely change the sound installing a pair of Gold Lion KT-77's I have had but not used that much for a number years. Really sounding good after a few biasing attempts. I now running 47ma of current with 428V from cathode to plate with roughly 20 watts of dissipation on a tube that is rated for 25 watts. There may still be a tiny spot of red platting on one of the tubes running that much so I am going to lower the dissipation for something like 17 or 18W  giving the tubes long life before I call the amp completed. I listened to the amp for a few hours last night and am really satisfied with what I am hearing. The tiny red spot I may be seeing on one of the tubes could possible be a reflection from the heater but it is hard to tell being the possible red spot is on the backside of the tube where it is placed now. In a dark corner on the floor. Could possible be nothing but I would still like to run the tubes closer to 18 watts. I measured a touch more than 7 watts of power available for the speakers and that is more than I actually need for Klipsch speakers. Five or so watts was what I was building this amp for. Plenty for horns or for that matter most everyone with speakers rated for 90db or better at 1 watt. 

[henry4841]

I think I am now happy with what I am running the tubes at. My numbers are cathode to plate 439V, running 42ma of current for a plate dissapation of 18.43w of a rated 25 watt tube. I have not checked the watts at speaker but I would expect 6 or more watts for sure being I did not lower the numbers that much. I cannot see any hot spots on the plates now. Below is a picture of the Eurotube bias tester I have. I love that thing and need to get another one. Makes checking bias much easier. I did the math using the voltage drop of the cathode resistor and ohms law and came up with a really close figure of the same numbers. 

 

The number you see on the meter is the milliamps before I made changes to the cathode resistor. Earlier picture. 

[henry4841]

This amplifier has evolved into a SE KT-88 one. Why, because to me this design sounds best biased for the Kt-88 tubes after doing a lot of testing and listening. I have been playing with different bias setting for the tubes using scope and Arta distortion analyzer software and I have found that reducing the current through the KT-88 tubes from the reference setting produces the best results as far as output power vs distortion. I've reduced the current in the output tube from 78ma to 64ma at 406v's from K-P. The dissipation of the tube now is 26 watts on a Electro-Harmonix Kt-88 rated for 44 watts. This is where the output power is much the same as when being run at 78ma but with better distortion numbers. This means the tubes should last many hours running around 1/2 of it's maximum rating. From a couple of popular SE KT-88 designs that are similar as far as V's and current being run in the output tubes the more common value for the cathode resistor is between 470r and 560r. I have found using a 680r resistor has the best results and that is what is going to stay in this amplifier. I've bought 7 12AT7's and some JJ-EL34 tubes and some JJ-Kt-88 tubes before settling on what looks and sounds best on the bench. The spares will go in stock for possible use in the future. The EL-34 tubes in this amplifier do sound really nice and really cannot be faulted it is just the KT-88 tube sounds somewhat better as far as sound stage and vocals with a much better bass response that is almost as good as a good SS amplifier. I said almost as good but that is only going to apply to horn speakers capable of using the 6 or 7 watts this amplifier produces. 

[captainbeefheart]

@henry4841 have you tried the 6AV5GA strapped as a triode yet? I know you like the 6Y6 a lot and it is a nice tube I found the 6AV5GA to sound better.

 

Another tube that is better than the 6AV5GA and probably the best sounding tube I have ever heard in my life is the 4P1L. It's a Russian directly heated pentode but of course wire it as a triode. I thought something was wrong the first time I was taking distortion measurements it's that linear. It was a tough fight between the 45 and the 4P1L in my book for best no feedback sound and I have concluded the 4P1L is better sounding.

 

I really think you would like both. I sent curiousgeorge some 4P1L's but not I don't think he has had the time to mess around with them yet. The 4P1L being directly heated filament is a little tougher to setup in that it works best with DC filaments like all directly heated triodes do. The 6AV5GA is much simpler to build with due to it being indirectly heated 6.3v heaters.

[captainbeefheart]

Here are some plots for different operating points. Readings taken at 10vrms output into 100k load.

 

I have never seen a tube with no external feedback measure .0022% THD

 

Although that's extremely low distortion there are some higher harmonics. It's not enough to sound horrible since 2H is still dominant but the second plot that's recommended Va=235 and Ia=28mA is still very low distortion at .028% THD but it's all second harmonic.

 

Of course this is for line stage applications,  the test is measured at 10vrms output, at 2vrms which most of the time is all that is needed to drive a power amp this tube is squeaky clean.

 

If you want to try some I can send a couple your way as a Christmas gift.

 

https://www.mvaudiolabs.com/tubes/tube-data-library/4p1l/

 

 

[henry4841]

14 hours ago, captainbeefheart said:

Here are some plots for different operating points. Readings taken at 10vrms output into 100k load.

 

I have never seen a tube with no external feedback measure .0022% THD

 

Although that's extremely low distortion there are some higher harmonics. It's not enough to sound horrible since 2H is still dominant but the second plot that's recommended Va=235 and Ia=28mA is still very low distortion at .028% THD but it's all second harmonic.

 

Of course this is for line stage applications,  the test is measured at 10vrms output, at 2vrms which most of the time is all that is needed to drive a power amp this tube is squeaky clean.

 

If you want to try some I can send a couple your way as a Christmas gift.

 

https://www.mvaudiolabs.com/tubes/tube-data-library/4p1l/

 

 

First let me check my stock of tubes and see if I already have some. I've bought a number of tubes over the years without ever using them. My favorite non-audio tube is the GU-50. Built a couple of SE amps with them and they sound great.