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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 14th April 2018, 08:16 PM   #11
riven67 is offline riven67  United States
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Originally Posted by Tubelab_com View Post
I'm using a phone so l can't go into the details now but this tube will likely blow the cathode bypass cap and possibly the resistor. The 6BQ6 and 6DQ6 also share the same pinout.The 6BQ6 can be made to work, but no so well. The 6DQ6 blew my board. This tube looks more like a 6DQ6.
So I'm guessing it not as easy as replacing a few parts for this tube to work? Ah well as I said in my previous post perhaps using a circuit designed with this tube in mind is the way to go, in order to experiment. It would make me learn a bit more about design of tube gear by doing it this way, I just wish I had someone close to me that I could learn from as I'm better at learning from seeing and doing than reading and trying to do on my own. I do have a vtvm and an oscilloscope but very little experience using them! Lol

Well any insight would still be appreciated. Thanks
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Old 15th April 2018, 02:00 AM   #12
Tubelab_com is offline Tubelab_com  United States
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So I'm guessing it not as easy as replacing a few parts for this tube to work?
I am just guessing here, since I have never seen this type of tube, much less experimented with it, but I can say that it MAY be possible to get this tube to make some music in an SSE. It likely will not work as well as a 6L6GC, and the possibility of success without some more knowledge and test capability is near zero. How do I know this?

I started out tinkering with tube stuff in the 1960's. I had an electric guitar and my amplifier was al old mono Magnavox console HiFi that I got when my parents upgraded to stereo. I wanted to make a better guitar amp, but had zero budget. In the 60's you could go to the trash dump and pick through old TV sets, radios and Hifi's for the tubes, transformers, and other parts.

I traced out the schematic of an old Fender Champ borrowed from a friend, and managed to make my own from trash parts. What comes next? Well I wanted to make it bigger and louder, so I tried stuffing in bigger tubes. Anything with a similar pinout got tested. Sticking a 6L6 into the 6V6 socket made it a little louder, but not what I had expected. Bigger tubes like the previously mentioned 6BQ6 and 6DQ6 actually lost volume, and the 6DQ6 made my output transformer start smoking. It didn't make sense to me why a bigger tube was less loud than the smaller tubes, but I kept on experimenting, and blew up quite a few parts along the way. I was a 10 year old kid with no clue about bias, maximum voltage ratings, current, impedance, load lines, and other important concepts, and there was no internet to look stuff up on. I would pick up bits and pieces from books and talking to a local ham radio guy, but I really didn't understand the basics until I got into a technical high school electronics program a few years later.

Fast forward about 40 years and I'm tinkering around with an SSE, and I remembered those old days.....OK, now that I understand, I'm going to stick that 6BQ6 and 6DQ6 into my SSE (same SE output stage as that guitar amp from 1963) and make them work, or blow something up trying.

Pentode and beam tetrode vacuum tubes have two main grids to regulate the current flow inside the tube. There is also a third grid for suppression of some unwanted characteristics. The control grid (G1) is used for primary current control and usually operated with a more negative voltage than the cathode. The screen grid (G2) acts as a shield between the control grid and the plate and serves to accelerate the electron stream towards the plate. Some tubes have a very sensitive screen grid and it usually operates at a low voltage in the 100 to 300 volt range. Some tubes have a less sensitive screen grid which usually operates at a voltage near or the same as the plate voltage. These tube types are usually NOT interchangeable, and bad things will happen if you stick a sensitive G2 type into a design made for a insensitive G2 tube.

The SSE, amplifier was designed to work with common audio power output tubes that have a rather insensitive screen grid. This allows the tubes to operate in triode or UL mode since both of these run G2 at or near the plate voltage.

Your 12E1's have a maximum screen voltage rating of 300 volts. Your B+ is probably in the range of 420 to 450 volts. The 12E1 isn't going to like this.

The SSE uses cathode bias, sometimes called automatic bias. G1 is grounded. There is a resistor and a bypass cap from the cathode to ground. Current through the tube goes through this cathode resistor causing its voltage to be several volts positive (25 to 45 volts in an SSE). This makes G1 more negative than the cathode. If the current through the tube rises, the cathode voltage rises causing the apparent G1 to cathode voltage to be even more negative, reducing the current through the tube.

Applying 430 volts to a screen grid designed for 300 volts maximum will cause a lot of current through the tube. This will develop a lot of voltage across the cathode resistor in an attempt to reduce this current. If this voltage is high enough, the cathode resistor, it's bypass cap, or both will die! Simply sticking in bigger parts may subject your tubes and transformers to more current than they were designed for causing damage...... this can get expensive quick.

Quote:
Any time you go out of the parameters the designer intended, you have to be prepared to experiment and live with the consequences, good or bad. Some work. Some don't, spectacularly.
I ALWAYS experiment with expendable parts (especially transformers) until I am satisfied that stuff won't blow up. I use adjustable lab power supplies on everything so I can find out where I can ignore some ratings, and where I can't. I have been doing this for over 50 years, and I still blow stuff up!

Back to the SSE with 6BQ6's. Those sensitive screen grids (175 volts max) really didn't like 450 volts. The tubes glowed like light bulbs....for a few minutes. I connected a variable power supply to the screen grid, and fed it 150 volts, and the tubes did work, but they needed more drive than the usual audio tubes, so you couldn't get full power from a CD player.

The bigger 6DQ6 caused the cathode voltage to rise into the 80 volt range leading to the death of the 63 volt bypass cap.

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Perhaps it would be easier to experiment using a design that was made for this tube instead of the sse? I found one but being new to reading schematics i'm not certain its an audio amp cause it does use the crt from a oscilloscope in the design,
This IS an audio amp design. It also includes some type of crude oscilloscope to display something resembling audio. It however can't work right, if at all as it is drawn. A quick look shows two major mistakes. The connections from the amp to the oscilloscope have about 400 volts on them since they come from the plates of the output tubes. They connect to the wipers of the input pots on the "scope." Turning the input all the way down will short out the B+ really frying some parts. The design also puts the tone control INSIDE the feedback loop. This is a good way to make a very loud oscillator. It also violates the 300 volt screen grid rating of the 12E1. I doubt that this was ever built.

I agree that the 12E1's would be best used in some sort of push pull amplifier. They would probably make well over 50 watts per pair too.
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Old 16th April 2018, 02:01 AM   #13
riven67 is offline riven67  United States
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Ok thanks George for that great explanation, I think I'll pass on trying to stuff this tube into the sse. I did find a bunch of other designs after searching for 12e1 schematics so perhaps I'll look for one of those to play around with.

Yeah I'm old enough to remember tubes being mundane items in just about all households but too young to have been able to play with them until now. Trouble is I really need to find someone locally I can learn from. The internet is a great resource and I have purchased some books but without experience trying to design or play with unknown tubes is difficult if not impossible.

Well thanks again I'll look for another design I can work with and let you all know how it goes.From reading online these tubes are supposed to sound sweet, but it seems that to is a matter of taste.
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Old 16th April 2018, 01:11 PM   #14
Tubelab_com is offline Tubelab_com  United States
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If I was to look for a ready made design for those tubes I would look at Pete Millett's Engineers Amp. It was designed for tubes with a sensitive screen grid.

I got one of those boards and tried several dozen tubes in it, with good success. The board is laid out for 12 pin TV tubes, but different sockets can be rigged into it.

If you want to learn a bit, try reading the whole thread.....It's long and goes off in several directions including several of my experiments that squeezed as many as 525 watts out of Pete's 18 WPC design. Plenty of good information there.

Posted new P-P power amp design
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Old 16th April 2018, 03:38 PM   #15
riven67 is offline riven67  United States
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Quote:
Originally Posted by Tubelab_com View Post
If I was to look for a ready made design for those tubes I would look at Pete Millett's Engineers Amp. It was designed for tubes with a sensitive screen grid.

I got one of those boards and tried several dozen tubes in it, with good success. The board is laid out for 12 pin TV tubes, but different sockets can be rigged into it.

If you want to learn a bit, try reading the whole thread.....It's long and goes off in several directions including several of my experiments that squeezed as many as 525 watts out of Pete's 18 WPC design. Plenty of good information there.

Posted new P-P power amp design
Thanks George i'll give it a read. Thats actually funny because that engineers amp was on my list before i bought your boards, i think it came down to overall cost that made me not buy one at the time.
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