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Sovtek 6B4G and Genalex 300B

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I've been thinking about changing my amp from Sovtek 6B4G tubes to 300B tubes by bringing filament voltage down to 5v and using a tube adapter. The key here is that the Sovtek 6B4G can be pushed much harder than a NOS 6B4G, which was actually a paralleled triode inside the envelope. The Sovtek is a single plate and has been claimed to really be a 300B in an 8 pin base and a higher filament voltage. Maybe the original 300B filament could take the higher voltage anyway without any ill effects. My amp was running with these somewhere higher than a 2A3 operating point and at the low end for a 300B.

So, as I'm looking at 300B tubes I've read that the Genalex tubes are considered a decent tube among the lower priced 300B, all being relative of course since 300B tube prices are astronomical. Then I started looking at the online stores and look at the pictures and it struck me. These tubes are the exact same construction as the Sovtek 6B4G that I already have. They are both made by New Sensor, which I kind of knew, but it hadn't occurred to me that these are really the same tube. One other thing I noted is that these tubes sometimes have OTK markings, which was believed to be a military inspection mark, but when I looked it up using my newly learned Cyrillic skill it just means "inspection station", not necessarily military. It is surprising to me how much I can understand of Russian just because I can read the script now.

So my guess here is that they are branding these to be a higher quality version of their 300B, but is it just branding? Do they select certain tubes to be higher quality, do they build them better, what exactly is it that they do? If all it is is the branding, 6B4G are much cheaper by at least half and I'll stick with them. I'd love to get a quad of the premium types like the EML or PSVane WE copy or Shuguang Treasure, but that isn't happening. $$$$.

Anybody with experience with these tubes care to comment?
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I have the Sovtek 6A3 and Sovtek 2A3.
Except for the base, the 6A3 and 6B4G are the same.
They are all mono-plate tubes.
They all have the same size glass, and the same size 15W plates.
They look identical.
The difference is:
2.5V @ 2.5A
6.3V @ 1A

The 300B has a longer filament and a larger diameter plate and longer plate.
Its filament is 5V @ 1.25A. They have larger glass.
The plate is a 40W plate.

Yes, you should be able to put a 300B in the amp, and have it work.
But be sure to adjust the filament voltage to 5V (that is a +/- 5% spec).
And, the plate resistance, rp is lower.
The 300B will try and draw more than 60mA. You need to re-adjust the bias.
The B+ probably drove the 6B4G to 60mA.

Your filament supply needs to be able to drive 1.25A, it only had to drive 1A before.
It may not like that (run hot).

I once built an amp that had a 6A3 / 300B filament switch (switched a series resistor in/out). The bias was adjusted to run both tubes.
The advantage of a 6A3 is that it plugs into the same socket as the 300B. With proper amp design, and the lower plate dissipation and output power, it worked well.
A 6B4G (old stock or Sovtek) can not do that, because the socket is different).

I recommend DC filament supply for all these tubes:
2A3, 6A3, 6B4G, and 300B. That is another discussion, but I and others have written much about that in this forum.

I have used JJ 300B, Electro Harmonic 300B, and Valve Art 300B, nothing more expensive than those. I enjoyed all of them.
The JJ were really well built, bigger and thicker glass, etc.
I also had a couple of Svetlana 300B, but one of them had the classic filament to grid shorts during warm-up operation.

Have fun listening to your 'new' power limited 300B amp.
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Thanks. The filament supply should be OK as it was meant for EL34 which is 1.5a.

I currently have the plate at 365v, with cathode bias with 550 ohms on the center tap configured as 4x2.2k/5w resistors dropping 60v for two tubes so about 55ma. I just took the temp on the resistors and it was running at 140 degrees F. (I have a new IR thermometer gun and I'm measuring everything around the house!) I could possibly get 420v out of the power supply by going with solid state instead of the 5U4 in there now, or with a 5AR4. Output transformer is 4.3k Push Pull. I don't really know how to calculate or set the current, the online tutorials always get me confused, and they almost never reference push pull. I'm a smart guy but this doesn't ever stick in my brain.

The amp was an ST-70 and uses the stock transformers, so I'm probably pushing it a little but it's been running this way for 10 years without problem. I've burnt out a few 6B4Gs in this time, but they are also not known for longevity. One pair though is still humming through all the years.

It being an ST-70 I also have a negative bias tap to consider using if I want more volts too, correct? I can't remember why I went cathode bias in the first place. I think that bias measuring resistor is still in there, I didn't take anything out, just bypassed the wiring. It's kind of a mess inside.
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I can confirm that they are NOT the same tube even if the construction is similar. They are bigger by a fair amount, and heavier too.

Now all I need are dropping resistors to at least get started with them in my amp. They are on the way, .27/5w, one on each leg. 1.3v/2.4a = .5416666/2 = .27. 1.3v*2.4a = 3.12w
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As 6A3Summer said. I have played with most of the 300b 'budget' valves, and in my own experience JJ are stand out better, and reliable.

The EH valves have not been reliable in my personal experience, I have tested quite a few but never seen a proper used set, whatever people have asked me to test there's always one (or more) which has given up and been replaced, hence the testing, and I'm not talking old worn out bottles here.

Of the pricey options my fave is AVVT but they are bonkers money.
So I plan on using tube adapters and using the empty pins of the 6B4G for the resistors. I'll put the .27R between 1&2 and 7&8 since 1 and 8 aren't used, then wire the adapters so that the filament for the 300B is from 1&8. I can then use the amp with either tube.

Even though this amp is unlikely to ever leave my house, I'll mark those adapters so that the next person understands that this isn't just a straight wire. I could also wire up 6A3 adapters but why bother, the Sovtek 6A3 is an identical tube to the 6B4G.

The one thing I won't be able to do is put the original EL34 in it. I can't now either, but it wouldn't take much. The bias measure resistors are still there, I would need to change back to fixed bias, and all the wiring is still there. With this change I'm removing the wiring.

I'd have done it all today except the steam heat decided to quit in my house. Bad enough that we're mostly stuck inside...
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The EH valves have not been reliable in my personal experience, I have tested quite a few but never seen a proper used set, whatever people have asked me to test there's always one (or more) which has given up and been replaced, hence the testing, and I'm not talking old worn out bottles here. <cut>

My experience is exactly the same. The short life of the EHs made me move to the EML 300B (standard type). I found that they are better value than the EH, even though they are expensive: they last many times longer. The build quality gives confidence, and the sound is much better too.
All I need now is some crazy glue to hold the adapters together, the rest is done. I'm not going into a store for just that though. Maybe the pharmacy has some and I'll pick it up with prescriptions. In the meantime, I've put the dropping resistors in the amp but they remain open even while I use the amp with the original tubes. That was the beauty of this build, I can use either Sovtek 6B4G or any 300B.
I finally tried it out today with 300B, but the heater voltage was way too high, close to 6v. I didn't want to find out how it would work that way. I put the 6b4G in, dialed the variac so that the filament came down to 6.3 volts, then retried the 300B and it came down to 5.2v. A little high but within a 5% spec. But with the heater voltage that low, my B+ came down too, a little too low I think. It was 357v, and the cathode voltage was 57v. That would mean just over 50ma to each tube since I have 550R cathode resistors. It worked, it played music, but not as loud as with the 6B4G. I'm still surprised this amp was working all these years at 6.8v on the filaments.

I'm going to start looking at adding another .27R to the heater series resistance to get it down to 5v from 6.8. That's all I have in terms of resistors on hand. Let's see, 1.8v drop, 2.4a current draw = .75R, I'll have .81 with what I have. Let's see what I get. Not today though. B+ would be more like 375-380V without the variac dialing it down. I don't have any big cathode resistors on hand to play with that though. I also have the 5AR4 option to give me another 20 or so volts from the rectifier, but don't have a working 5AR4 either. The power supply was designed for the 5AR4 so it shouldn't hurt anything.

And if anybody has any recommendations on cathode resistors and bypass cap, I'd appreciate the advice. I don't really know what I'm doing, but hey, I made music with 300B push pull without spending a fortune. That alone is an accomplishment to me.
I've done this so I don’t need a switch. I have the 8 pin socket wired with 2&7 directly from the tranny and the the resistors between 2&1 and 7&8, .27r at each so the legs are equal. I then have 8 pin to 4 pin adapters wired so that the filament on the 4 pin 300b gets the proper voltage from pins 1&8. Got that? I can change from 6b4g to 300b just by using the adapter. With a 6B4G in place the added resistors are just open. And of course I can't put the wrong tube in the wrong socket. All of that works. Just don't plug a pentode into that 8 pin socket!

My problem is that .54r total resistance isn’t enough in spite of ohms law. 1.3 volts drop at 2.4 amps should be .54. But reality is I need to drop more like 1.8v. Although, now that I think about it, maybe there should be a dropping resistor on the 6.3v side too since it is too high. Maybe if I can drop .5v to get to 6.3v, then what is already in place will get me to 5v or at least 5.2v like I got earlier? I'll try it tomorrow. I just have to figure out where to mount that resistor, I've run out of usable posts. In all of this I haven't checked the filament voltage on the drivers, running off the same winding. It should be the same but I'd better check.

Another lightning bolt! Pin 6 is open on a 6B4G. I can use that socket pin as a mounting point. Some day I'll clean all this up and build a real chassis, but some day isn't coming anytime soon.
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I've been running 6B4G PP in it for 10 years now without the bypass cap, I was just wondering if it would be "better" with one. But I guess not.

With an additional .27R in series, I get 6v where I should have 6.3v and 5.1v on the 5v side. Close enough for now. Isn't it like a +/- 5% spec?

I have 4x2200 5w resistors in it now. That gives me 550R. I get 56 volts on the combined cathode now, so I'm right at 50ma on current per tube. This whole cathode bias confuses me though. If I'm limited to 370v on the plate with what I have, how do I change the cathode resistors to increase current? Or can I? I did just read somewhere that one of the disadvantages of cathode bias is you can't push the tube as much as with fixed bias. So maybe 50% is all I can get this way? I have a negative bias tap, standard ST-70 voltage I'd imagine, it just isn't used in this amp.

As for the sound, I've only tried 300B in one channel so far, and on a speaker that I don't like. It sounded great, but couldn't say I'd compare them. When I was measuring the other side last week I managed to short the filament supply and smoke a resistor. I need to put new ones in. At least it still worked without it. I'm waiting for the tubes to cool as I write this, so maybe later today I'll have a good answer.
Tomorrow came and went too...

I was playing the amp for a while today with the 300B on one side and the 6b4G on the other, and using the balance control to even them out. The 300B side was lower, but I expected it to be higher. But after evening them out, I would say that the 300B side sounded distortion free compared to the other side.

I would love to compare the tubes one after the other but given how hot they get you can't just quickly swap them. My guess though is that the 6B4G will be the everyday tubes and the 300B will be for serious listening.
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