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Old 3rd January 2014, 07:35 PM   #161
hpeter is offline hpeter  Europe
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offtopic a bit

is a simple way how to calculate-estimate g2 triode parameters /g1 tie to c/, from tetrode datasheet ?
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Old 3rd January 2014, 08:15 PM   #162
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Originally Posted by jhstewart9 View Post
The 4BQ7 serves a specific purpose. Its heater runs on the 5V winding of the PT, so there are no problems with H-K insulation.
That makes sense now. The earlier schemo didn't include the PS.

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I dont worry much about which tube or that may sound better. IMO, an amplifier should & can be a piece of wire with gain. Any imperfection will be reduced provided it is inside a stable NFB loop. Microphonics are a problem that can be avoided. But they may add to the sound of a rock musicians performance!
That's pretty much how I design. However, more linear (and better sounding) VTs reduce the errors that need correcting. NFB should improve a good sounding open loop design, not cover up for a poor one.

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A while back in one of my projects I did objective A vs B tests on the BQ7 family when compared to the rather expensive 6SL7 / 6SN7 family. The THD & IMD results would surprise many! But I didnt have any problem with microphonic BQ7s.
Good loadlines aren't so easy to find for 6BQ7s, but you can find ones that predict better performance than some of the triodes you see frequently in audio designs. I get good performance with them, and you definitely don't see 'em as often as you should. Maybe that's because the spec sheet doesn't mention audio at all? The cascode LTP splitter used in the Le Renard design measured quite clean.

All the 6BQ7s that have series connected heaters are nastily microphonic. You can recognize these by the thin bit of filament running between cathodes. Those with parallel connected heaters haven't given me any problems with microphonics.
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Old 3rd January 2014, 08:41 PM   #163
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Originally Posted by smoking-amp View Post
Bootstrapped 6SN7 driver plate loads. I would guess those UL taps are providing more voltage swing than required however, giving some positive Fdbk.

It might be interesting to replace the 6SN7 drivers with some pentodes, using the same cross UL bootstrapped plate load idea. But add UL tap Neg. feedbacks to the driver g2's as well to provide some local Neg. Fdbk. A UL driver stage if you will. Likely have to use some R dividers to attenuate the Neg Fdbks and reduce the DC voltage for the driver screens.
A while back I used what is left of my math skills to come up with a solution to the +ve FB question. All attached here. Actual measurements showed it to be never more than 1.5 to 2 db.

I tried OPTs with both 20% & 43%. For the 6AS7/6080 version the 43% solution was needed. The 20% worked but not nearly as well. The 20% solution would be OK for a pair of the 2A3 family.

The next version I built avoided the UL OPT entirely. See attached the original hand drawn version setup for 2A3s. And another easier to visualize done with Electronic Workbench software & a 6BQ7 diff amp front end. This one wastes more of the output power in the bootstrapping connexion.

When dealing with the 6AS7/6080 one needs to be careful with the biasing. Each grid needs to be biased separately by cathode biasing. Otherwise thermal runaway is a sure thing. I found out the hard way. Otherwise a bias servo for each grid.

The THD result on the previous post had a pair of 6080’s as the OP. Running thru a special wound Hammond 1600 Series of 2150 ohms. I used a Pico Technology ADC-100 to get that result.

Just realized I wasn’t with R&S at that point but rather Navair Div of Field Aviation. So was pushing IFR (now Aeroflex). Aside from cell site test, spec A’s & so on not much tubes or hifi! I had managed to avoid both for about 40 years.

Cheers to all, John L Stewart
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Old 3rd January 2014, 09:21 PM   #164
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Originally Posted by tubelab.com View Post
Isn't this thread about TV tubes? The most popular TV tube is our friend the horizontal (line) output tube.....they like plenty of voltage too.



As mentioned tube cascodes were used in RF. Common applications were TV and FM radio tuners and Tektronix scopes. The 6BQ7 was popular in TV and early Tek scopes. Tek later switched to the 6DJ8. The cascade was popular to approximate pentode operation with low capacitance triodes.

The TEK 543 I used at the Graduate Lab at U of T Physics had something like 14 tubes in the final vertical PP amp. A travelling wave amplifier, delay lines & all. And got to only 30 MHz. But got a lot done, anyway. Today I use something that is 250 MHz, 2-channel, fits in the palm of your hand & runs into the laptop. Big change in 50 years

I also have a very similar amp design under construction using pentodes. The cross coupled local feedback goes to the driver tube screens. The cathode followers driving the output tubes are replaced with mosfets. Output tubes are TV sweep tubes.....not decided on which ones yet....or just which grids are being driven.

Pearl in Calgary did some of those NFB to input screens back about 20 years ago. See attachment. His stuff was top of the line. Same guy that marketed the tube coolers. The Cascode front end I posted is easily converted to a pair of pentodes. But would have to take into screen loading of the NFB network, both AC & DC. The cascode avoids that.

Class B Zero Bias, Screen driven pentodes were common with amateurs as modulators. 807s were plentiful. A few tried direct coupling. That lifts the grids off ground so there is less distortion. But the added B+ current was not popular for mobile work.



They are on my short list. I want at least 100 WPC which the 13GB5's can easily do. I have 3300 ohm OPT's and the B+ will be in the 500 to 600 volt range.



I forgot about that one. I got serial #001 way back when they cost $100. It was in an SSE until I stole the OPT's for another project. It's around here somewhere. It would need a small boost transformer to generate 600 volts.
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Old 20th March 2014, 01:58 AM   #165
AJT is offline AJT  Philippines
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Hi,
i built a 12W6 pp amp, G2 regulated using 0A2(150v) and working at plate B+ of 300 volts dc..

the amp is a mullard 5-20 implementation, used the 5693 as LTP, and 12at7 inputs...

the amp is now with me to fix some issues...

gain is way too high so took care of that,
the 0A2 regulator blinks, a sign that is it is
running out of current on music peaks,
took care of that too..
noticed that one sylvania 12W6 red plates at 15 volts bias,
so i replaced the entire set with RCA 12EN6's,
no more red plates....

note that even if the RCA 12EN6 is rated for 7 watts compared to 12W6's 10 watts, no difference was noted in actual use...
i still have the GE 12W6 cleartop quads to try out...

the current setup now is, 12EN6 output tubes, 5814 long tail pair phase inverter,
and 12AV7 as input tube....although the JJ 12at7 and siblings had more gain and decent sound,
i found out that 12AV7 is even better in that spot, perhaps because the 12AV7 has lower rp than the
12at7.....so i am working the amp real hard now.....

Click the image to open in full size.
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Old 14th April 2014, 08:03 PM   #166
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You know, it's a shame that tube R&D has basically died out. The frame grid was such a great innovation! I've thinking about how to improve the frame grid even further. For example, instead of wires stretched over a frame, you could use a metal foil that has many thousand micro perforations. You could use perforated graphene as well! I'm sure you could make many very precise holes in the foil with lasers. Just some ideas I have kicking around.
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Old 14th April 2014, 08:29 PM   #167
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Yeah, but hey what do you need a tube for in the world of cheap sillicon?
The only advantages of tubes are its immunity to EMP pulses and that they are very forgiving of overloads. That led to their continued use into the 90s in soviet electronics.

Anyway, tubes are likely going to be around for a few decades, that is for transmitting and industrial heating applications. The only bottleneck I see is that there are no people receiving education on how to build and design whit them.
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Old 19th April 2014, 10:47 AM   #168
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Quote:
Originally Posted by v4lve lover View Post
Anyway, tubes are likely going to be around for a few decades, that is for transmitting and industrial heating applications.
Maybe not so ...
AM transmitters - so they still exist - are already completely solid state nowadays. A 600kW unit consists of 12 cabinets filled with 600 identical switching LDMOS power modules, each card complete with SMPS, modulator, oscillator, driver, outputs, filter.
Similar for FM, up to 100kW.
Even UHF transmitters have gone solid state at least up to 40kW, where the traditional inductive output tube (IOT or klystrode) is being replaced by MOS modules. Introduction of DTV / DVB was a good opportunity to replace tube equipment there.
So what rests ... satellite uplink and maybe the magnetron for industrial due to its simplicity.
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Old 19th April 2014, 07:39 PM   #169
jerrys is offline jerrys  United States
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Originally Posted by payloadde View Post
Maybe not so ...
AM transmitters - so they still exist - are already completely solid state nowadays. A 600kW unit consists of 12 cabinets filled with 600 identical switching LDMOS power modules, each card complete with SMPS, modulator, oscillator, driver, outputs, filter.
Similar for FM, up to 100kW.
Even UHF transmitters have gone solid state at least up to 40kW, where the traditional inductive output tube (IOT or klystrode) is being replaced by MOS modules. Introduction of DTV / DVB was a good opportunity to replace tube equipment there.
So what rests ... satellite uplink and maybe the magnetron for industrial due to its simplicity.
For some reason, this makes me feel the same way I felt when they decommissioned the USS Enterprise. I hated every minute I served on that ship, but when I heard she was being torn apart for scrap, I almost felt like writing my congressman and saying she belongs as a museum piece. A world without tubes.... Although I appreciate modern technology, I wonder if it hasn't been more of a curse than a boon. I enjoy the fact that tubes hold their own sonically even though they are "obsolete."
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Old 20th April 2014, 12:49 AM   #170
kimbal is offline kimbal  Australia
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Originally Posted by v4lve lover View Post
Yeah, but hey what do you need a tube for in the world of cheap sillicon?
The only advantages of tubes are its immunity to EMP pulses and that they are very forgiving of overloads. That led to their continued use into the 90s in soviet electronics.

Anyway, tubes are likely going to be around for a few decades, that is for transmitting and industrial heating applications. The only bottleneck I see is that there are no people receiving education on how to build and design whit them.
For the little info it's worth - I was on the last class of students that learn't about tubes at Royal Melbourne Institute Of Technology ( RMIT as it was known back then in Melbourne, Australia) in 1977. The radio school which also trained our military guys was dumping the tube subjects after our lot graduated. I recall seeing the old 833a 3XY AM Radio Transmitter in their Museum and the massive Mercury Tube Power Supply to run it. Since then I've always wanted to build an 833 audio amp. Fortunately for me, I had a reasonable knowledge of such technology before I did my class studies as an apprentice back in the 70's. Their level of tube training at that time was very good, but still basic.
Today students in electronics only know about PICs, Micros, IT and Software design ( which in my opinion is not true Electronics, but more IT. Many have never heard of a transformer either - let alone used one.

( I now have an Electronics Engineer at work who is much younger than me and ironically my Boss. He is fresh out of Uni and grapples to use basic hand tools or a soldering-iron. Most practical jobs he delegates to me - despite me being over loaded with other repair work as it is as know else knows how to do the work. He comes to me every day with basic questions and for me to check his work before submitting it to his Boss - yet he is paid twice my salary because he has piece of paper - yet I carry 40+ years experience. So what does that tell you about the so called new comers in the Electronics - sorry I meant I.T - Industry ! )
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