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Vertical deflection tubes for audio output?

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#21
Looks fine to me (maybe the picture was corrected...), I would add two small resistors (10 ohms say) unbypassed in each cathode to create an easy pair of test points for balance adjustment (just test for 0V between them). Gain would fall a little bit but linearity would improve by the same amount.
 
#23
Sch3mat1c said:
Could even add AC balance (well okay, not easily, unless you say, changed the preamp to LTP), then you could use completely unmatched tubes!
Click to expand...

Do you really think I'll need AC balance--wouldn’t the NFB help me balancing the signal?
On the other hand, nobody around here is selling matched pairs of 6LU8, and I don't know how much they differ in the first place.
I'm not very familiar with the 6LU8 at all, so what I’m planning is more like an experiment. :xeye:
 
#24
ilimzn said:
I would add two small resistors (10 ohms say) unbypassed in each cathode to create an easy pair of test points for balance adjustment (just test for 0V between them).
Click to expand...

You're right, no point in adjusting the bias if I cant measure it, so your 10R suggestion is appropriate. Another way reading the bias is the plate voltage, especially if the primary RDC of the OPT is equal for each side. (well, no problem even if the RDC differ, It's just more job..)
 
#25
Yes, NFB will reduce imbalance some, but reduce, not null. It may be better to chug through a few sets of tubes to see which have the lowest 2nd H (or most symmetrical clipping, although that is more due to perveance than Gm), rather than trying to add balance to a cathodyne. Empirical matching.

For biasing you could also go for two cathode resistors (and two bypass caps) and a wirewound pot of say 250 ohms; subtract 125 ohms then from each resistor (which let's see, you marked 330 ohms for two so 660 ohms per minus 125 is 525 ohms, close to the standard value 510 ohms).

Tim
 
#27
Sch3mat1c said:
For biasing you could also go for two cathode resistors (and two bypass caps) and a wirewound pot of say 250 ohms; subtract 125 ohms then from each resistor (which let's see, you marked 330 ohms for two so 660 ohms per minus 125 is 525 ohms, close to the standard value 510 ohms).
Tim
Click to expand...

In that case, setting a trimmer in parallel with an increased value cathode resistor would be a better idea as you may get away with a standard cermet pot.
For instance, you could take 680+47 ohm per tube, connect a pot of 2.5k each end to one of the cathodes, and wiper in series with about 3.3k to ground. At midpoint your effective cathode resistors are cca 665 ohms. The dissipation of the pot should stay quite low as it only sees a fraction of the current. At the end points, the effective resistance of one cathode resistor is close to 600 ohm, and the other close to 720 ohm so you gat about +- 10% variation.
 
#28
Sweep AMP

The finale schematics, Sweep AMP:

An externally hosted image should be here but it was not working when we last tested it.


As you see, I had to "starve" the first triode a little bit to get enough gain, around 40x. This, with 20dB NFB (I don't know if I need that much), will give about 2.5v rms input for full drive.

An externally hosted image should be here but it was not working when we last tested it.
 
#29
alternatives for a PCL82?

Hi everybody,

are there any tube (possibly a dissimilar double triode...) whose power triode is similar (for what regards mu and Ro) to the (triode connected) output pentode of the 6BM8/ECL82/PCL82 ?

(...and if its input section also has medium-mu it would be perfect! 8🙂

Thanks!
 
#32
With all this experience around: I once looked at vertical def. tubes and sweepers for audio output, but was advised that many of them had widely varying gms between tubes, that not being a very important parameter.

With no experience on my side re TV tubes, can anyone please comment?
 
#34
The EF184 was another veteran which was popular in 1950's TV if stages and some audio amp manufacturers used this tube in pairs
as a long tailed phasesplitter. Although the transfer graphs are remarkably "straight", one ought to recon that low distortion was on the cards....it needs a fair amount of currect....but many amps using this tube sounded "lifeless"........only my point of view that longtail phasesplitters sound worse than other configurations.

richj
 
#35
alternatives for a PCL82?

6DE7
9 pin small button
9HF Basing
Used for Vert. deflection amp & osc.

Section 1:
µ=17.5
330V max plate
1.5W pd

Section 2:
µ=6.0
275V max plate
7.0W pd

Once used one of these and a 6EM7 for a series regulated PSU for a linestage about 20 years ago! Swithced back to a triode connected 7591. That experiment was filed away long time ago. That tube may be hard to find where you're at.
You wanted a medium µ input section (very sim. to a 12AU7), that's what made me think of it.

Wayne

Edit: The 6EW7 would be even better. 330V plate voltage, 10W pd. The 6FM7, 13FM7 compactrons are worth looking at if the higher µ (66) of the first stage is no problem. 550V max plate and 10W plate diss.
 
#36
I would also recommend the 6EM7 / 6EA7. I built a real simple amp with two of these (one per channel) and it sounded pretty good. I don't buy the "like a 2A3" stuff. I have seen that mentioned for a couple of years. These tubes are nothing like a 2A3, but they put out about the same power and run on the same voltages.

I also tried the 6DN7 (Single Ended eXperimenter's kit) which sounded similar, but I only had a few of them. There are several other disssimilar triodes thet were designed for vertical output use.

The popular 6BL7 and the 6BX7 were also vertical output tubes. I have even seen the 6SN7 used in vertical output use in an old 10 inch TV with a round picture tube.

Most vertical deflection tubes make good audio amps, since that is what a vertical output circuit is, a 2 to 10 watt (depending on the TV size and deflection angle) amp that is optimized for one frequency. They had to be linear or round things weren't round on thr TV screen. See what vertical output tubes that you can get for a good price, then figure out how to use them.
 
#37
Originally posted by: tubelab.com
They had to be linear or round things weren't round on thr TV screen.
Click to expand...

Yeah! So don't listen to someone who diss a TV toob!
Very few tubes where designed with audio specific in mind. They were designed originally for Radio and then TV. That's why we here in Ameericka (and Texass) call'em Receiving Tubes.
Some people think of a 12AU7, 12BH7, and such as audio specific toobes. Nope! They were designed for a combined vertical osc. and amplifier and then they were adopted into the audio world!
I've eyeballed and used a lot of compactrons and "TV" tubes for audio use. RF tubes like 6BQ7's for instance, one of my favorites. Zillions of them and dirt cheap. 6DJ8/ECC88, 12AT7/ECC81, 6AQ8/ECC85... not so dirt cheap anymore; RF Radio & TV tubes!

Wayne
 
#39
Well, then I am finished with the sweep amp!
Seems to be Christmas this year too. 😀

Some data and measurements:
2x12W rms, 2x25W peak
NFB: 14dB
16Hz-30kHz (-1dB)
THD: less than 1%, 12 watt output.

Thanks to Birger, the flight mechanic that made the chassis.

The inside:
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Outside:
An externally hosted image should be here but it was not working when we last tested it.


Front:
An externally hosted image should be here but it was not working when we last tested it.


Final schematic:
An externally hosted image should be here but it was not working when we last tested it.
 
#40
Interesting design......That 120V zener in the g2 supply: In a sim circuit I worked on I found them very noisy which was breaking into ajacent wiring. Perhaps I had another type but I was forced to use tube stabiliser.
Anyone using H.V zeners in circuits .. Which makes can be recommended ?

richj
 
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