6CB5 in PP?

[Blair]

I bought 8 of these on a wild hair after reading on DIYAUDIO about sweeps. I was going to use these with the Sowter outputs I purchased, but I'm having some hesitations. I will probably go with a more conventional tube for the Sowter OPTs.

I am interested in the screen drive operation of this type of tube. Has anyone done anything with this tube? TubeLab over there says he got 125W into 3.3K with a pair in screen drive. I'm curious if maybe a 3.5-4k 100W Edcor output transformer with since screen drive circuit sounds about right for 100W or so. Also, has anyone ever compared distortion figures of "hifi" tubes Vazquez sweeps in audio amplification?

Thanks!

Blair

[Blair]

Looking at various sweeps, I see that they have very limited screen voltages. I found the old Acro catalog, and see how they did their 6146B schematic based on the TO-350 output transformer:

http://www.clarisonus.com/Archives/Trans/Acro55.pdf

I inquired with Edcor and they said they could do something similar like I drew here:



I see the TO-350 is 6.6K, so I assume this is because the 6146 plates only see the outer part of the winding. It could be nice to have a pair if these made to play around with using the 6CB5 or similar.

Blair

[kheper]

It would be great to get outputs with tertiary taps at a reasonable price. I would not count on it, however.

On another forum, a guy modded the MK III for 6146 tubes, using a well-regulated screen supply. He says they sound great. Maybe the same approach could be taken with the 6CB5, if Edcor can't deliver at a decent price.

[Blair]

Decor quoted me the $70 design fee and $98 each to split the windings at the 43% taps.

Blair

[EWBrown]

Another method for NFB, is to use the secondary winding for cathode feedback, connect the 4 ohm tap to ground, and the cathodes to the "0" and 16 ohm taps,
this works best with fixed bias, as the fed-back audio doesn't have to travel through wire-wound resistors and electrolytic caps.

The screen grids, as previously mentioned, could be fed with a simple regulated supply, which could be designed around a 6EM7 (or a 6MF8, as I tried).

/ed B

[Impmon]

I bought 8 of these on a wild hair after reading on DIYAUDIO about sweeps. I was going to use these with the Sowter outputs I purchased, but I'm having some hesitations. I will probably go with a more conventional tube for the Sowter OPTs.

I am interested in the screen drive operation of this type of tube. Has anyone done anything with this tube? TubeLab over there says he got 125W into 3.3K with a pair in screen drive. I'm curious if maybe a 3.5-4k 100W Edcor output transformer with since screen drive circuit sounds about right for 100W or so. Also, has anyone ever compared distortion figures of "hifi" tubes Vazquez sweeps in audio amplification?

Thanks!

Blair

Haven't done anything with 6CB5s specifically. I did, however, put some 6BQ6GAs and 6BQ6GTBs to good use. Same deal: TV HD power finals designed for wide screen B & W TVs. The 6BQ6GA gives excellent sonic performance, making mostly H3, and very little of the nastier, higher order harmonics. In that regard, they're like 6V6s that can put out 40W. All you really need is some gNFB to take the edge off and get the woofers under control.

As for local NFB, the 6BQ6GA/GTB doesn't require it. They're plenty linear as it is, and don't make the nastier harmonics. I wouldn't bother with OPTs that include Ultralinear tertiaries. Save on that unnecessary expense.

The 6CB5 is probably similar though having a much higher Pd rating. TV screens got bigger, with sharper deflection angles, and demands on horizontal deflection subsystems kept increasing. Color TV produced some real beasts of HD finals. These types are easy loads, and can produce lotsawatts at reasonable rail voltages, and lowish load resistances. That will definitely mean custom OPTs (The 6BQ6 project used an off-the-shelf OPT originally intended for Class A 6L6s, and so comes in at 30W, not the 40W the 6BQ6s could pump into it. That OPT did include Ultralinear tertiaries that I'm not using.)

I have a project in mind for some 36LW6s that came my way recently. These being of the last generation of octal HD types for big screen color TVs, and they are beasts indeed. 95W with grid drive easily, and north of 200W with screen drive, and even here, screen drive is a good deal easier than driving audio triodes for a fraction of those watts. MOSFET drive is certainly up to the job.

"TubeLab over there says he got 125W into 3.3K with a pair in screen drive".

But for how long? Tubelab likes to drive 'em right to the poof-point, and often beyond. I'd try to run 'em a bit more conservatively, but you could still do ~100W or so.

[Geek]

Blair,

If you do get Edcor to do that transformer, can you let us know the model number they assign it so we can order it too?

[Blair]

Hi Gregg,

The transformer is available per Edcor, but everyone I talk to online claims that the TO-350 is a different beast despite their schematic diagram. Everyone claims that there is a full "pentode" style winding with a tertiary winding for the screens. I'm OK with that and dropped the inquiry, but it really should have been represented differently on the original Acrosound data pages. I do, however think there is some merit to my idea. I am just not wealthy enough to commit:(

Blair

[Blair]

Regarding the 6CB5, I do agree that I would never take a tube where George does. He likes to watch things melt. I am surprised that he does not just buy a welder. It is cheaper;)

Gregg,

Here is the dilemma. I read the Acro catalog as I represented it in my diagram in the original post. Everyone days it is a tertiary winding and not part of the main winding. The issue I have with that is that it is not how it is represented in the schematic for the transformer. If it was, wouldn't it look like the second one in this image?

[battradio]

Use it as a cathode winding

[battradio]

Use it as a cathode winding

[Impmon]

A difference without a distinction. The schemo on the left was probably drawn that way to indicate the possible interleaving of the main primary and the tertiary. The one on the right just indicates that there are three sets of coils: primary, secondary, and that tertiary. Otherwise, makes no difference.

As for how it works, that depends on the actual care they did with the design and construction. Poorly made OPTs with UL connections will usually sound "better" if the UL isn't connected, as bad, out-of-balance UL will make the performance worse, not better. It's also easier to fuxxor an OPT with a tertiary as opposed to UL primary tap-offs intended for use with VTs that run the plate and screen at the same DC voltage.

[Blair]

I actually drew both of those to try to figure out what the difference is between what Acrosound did and what I asked Edcor to do.

[EWBrown]

Some transformer manufacturers did have split primary windings, just connect the two "B+" leads together, and it is the same as the normal center-tap primary winding. This was more likely to be found on bifilar-wound trannies, rather than the more common "solenoid" wound primaries.
If both sides of the primary winding have the same DC resistance from plate to B+, then it is most likely a bifilar winding. Solenoid-wound primaries will have differing DCRs, normally between 10 to 20% difference, due to the progressively greater length of the wire, in each succeeding layer.

HTH

/edB

[Blair]

Let me explain further what I'm referring to.

http://www.clarisonus.com/Archives/Trans/Acro55.pdf

In the link, there is a 6146B amp using the TO-350. Look at the diagram, and it just looks like a broken UL tap. If you look at a MC-240 (just reference. Not TO-350), it clearly is represented as a tertiary winding. My point is that I asked Edcor to essentially split the output transformer at the 43% taps to allow separate B+ levels to be used for plates and screens.

Is there any merit to this, or am I just wasting $$ if I buy them?

Thanks!

Blair