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Getting close to finishing up a current amp project (I'll post pics soon), and am interested in my next step. I want to do a class A PP integrated amp using cathode feedback windings on the OPTs. I understand that cathode feedback requires a stronger driver than a normal class A amp.I was looking at the Softone RX-80-5 output transformer. If the specs list 8.2dB of feedback, does this mean that the driver must put out 8.2dB more to drive the tube into clipping? In other words, if it would take 40VP to drive the power stage into clipping in a normal class A amp, would it take 2.57 (8.2dB) times that driver voltage (102.8VP) to drive the cathode feedback amp into clipping? I'm trying to figure out what it would take to drive the amp into clipping.
twystd
Edits: 03/24/15Follow Ups:
Whatever AC output voltage your driver can swing is ultimately where the output voltage on the speaker terminals is coming from.
Since both the output tubes and the transformer are going to cause a reduction in this voltage, you will need to swing a lot out of the driver tube.
Something else to keep in mind- the output impedance of a cathode follower is generally lower than that of a plate circuit- it may well be that the 5K primary winding impedance of the Softone might be a tad on the high side.
"Since both the output tubes and the transformer are going to cause a reduction in this voltage, you will need to swing a lot out of the driver tube."
That was what I was trying to determine, looks like the driver will have to swing at least ~103vp to fully drive my PP power tubes (6p36s), if I understand this correctly.
"Something else to keep in mind- the output impedance of a cathode follower is generally lower than that of a plate circuit- it may well be that the 5K primary winding impedance of the Softone might be a tad on the high side."
Not really following you there. Are you saying that the cathode feedback windings turns the PP power tubes into a form of a cathode follower, and therefor lowers the effective plate resistance of the power tubes?
BTW, the driver I was thinking about is a choke loaded differential pair of 6e6p-drs. That driver pair would in turn be driven from a 6n6p parafeed line stage, that uses a custom 3:1+1 parafeed autoformer splitter on the output.
twystd
I might be confused here- I was thinking that the amp has the load entirely in the cathode circuit, which means that it has 100% cathode feedback of course. If you are building the Mac circuit than its a different matter....
The McIntosh has a 50-50 split between plate and cathode. This particular trans is about 90% plate/10% cathode...which on a global calculation yields ~6k load. Kept Class A, that leaves each final with a 3k load...quite fine IMO. The same OPT with an AB1 bias will of course only deliver the nominal a-a/2 loading 'til one phase starts getting near cut off.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I totally thought that a DIY amp was being built with all the load in the cathode... wrong tree!
The only time I tried that was for a linestage. Small, Ni, bi-filar dealie for a 5687. Worked quite well IMO.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
It is just feedback that happens to be on the cathode instead of the more common global feedback. Most push pull amps utilize global feedback that is more than 8dB so the driver section in almost any amp schematic you look at will be more than up to the voltage gain task. Your point on the driver current, more current is always better, in any amp. The MC-60 used the Gutsy 12AX7 Cathode follower to put the iron hand of fear into the cathode feedback output section.
Thanks, but I'm not the kinda guy that would use any of the classic driver stages, I think they are too colored, and unnecessarily complicated. I like the minimalist brute force approach, much more suited to my lack of education.I'm thinking of a simple 6e6p-dr (triode mode) choke loaded, differential driver stage. The minimalist parts count, high linearity, high current, low output impedance, coupled with a relatively high mu, appeal to my audio dogma. Just trying to figure if I can get there from here :-)
BTW, I was thinking of making this a 3 stage integrated amp, with a 6n6p parafeed line stage, utilizing a custom 3:1+1 parafeed autoformer/splitter (already have them). That could drive a 6e6p-dr choke loaded (already have the PP chokes) differential driver stage, cap coupled to a 6p36s ultralinear PP power stage.
Utilizing cathode feedback on the OPT for the damping factor and lower distortion is inviting. Maybe I'll try Blumlein's garters as well. Just spitballing it at this point, but I'm nutz enough to try it.
twystd
Edits: 03/26/15
LOL--Why did Mac do that, instead of, say, another 12BH7?
The main issue with the 12AX7 as a driver is its high output Z. That problem is resolved in the CF configuration. Why waste additional filament current on a more robust tube when it isn't needed?
--------------------------
Buy Chinese. Bury freedom.
Same filament draw.
CF output impedance is approximately Rp/mu.
12AX7 gives about 600-800 ohms.
12AT7 gives about 180-200 ohms.
Designers sometimes simply use what they have on hand, or what they're most familiar with. It could be as simple as that, rather than the result of some in-depth technical analysis. If they use the 'AX7, and it sounds good and measures good, what would be the point in a production environment of investigating other tubes?
--------------------------
Buy Chinese. Bury freedom.
...because they could? At first glance the 12AX7 seems an odd choice esp considering that it's operated beyond its ratings in this position. OTOH, Mac successfully used this config. from the early 50s thru the end of the 60s.
I read someplace that Mac claimed the 12AX7 driver stage was capable of 1W output when terminated into a proper load which assured low distortion drive when g1 was driven +. Curiously, Mac switched to the 12AT7/12AZ7 in the driver position for the MC75/MC275 series.
Looks about right, assuming you use the two 16R secondaries for the CFB coil...and in which case you will add 20% to the primary load.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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