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Old 10th June 2013, 08:28 PM   #21
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Something like this maybe?

Click the image to open in full size.

No values because I don't know if these are the tubes I will use yet. I added the gain stage on the input in case the first triode of the 6GF7A is not sufficient. Should those stages be cap coupled?

Thanks!

Blair
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Old 11th June 2013, 10:22 PM   #22
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Does the 6GF7A I am proposing to use have enough voltage swing to drive a pair of 6CB5As to full output, or is a gain stage necessary to accomplish full output.

Something like a high powered "El Cheapo" with only three tubes.

Thanks!

Blair
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Old 12th June 2013, 02:12 PM   #23
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I have little fingers around the house, so I'm going to try to keep the top caps under the hood. There is adequate venting above and below the bulbs in this old chassis from a Proceed preamp. The Ecdors will fit in here as well as long as I stick with the 60W outputs.

You think I will need a "whisper fan" to circulate around the output tubes?

Is there enough room to build an amp in the chassis without too much crowding? This is one of the reasons I want to use the 6GF7A as a single tube driver.

Click the image to open in full size.

Thanks!

Blair
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Old 15th June 2013, 04:52 AM   #24
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How much voltage swing is necessary to drive a pair of these 6CB5s? The 6GF7A is essentially a 13EM7 in a novar bottle. First section gain is 64 and second section is 5.4.

Will I need an additional gain section in front of these 6GF7As to get full drive voltage?

Thanks!

Blair
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Old 20th June 2013, 03:02 AM   #25
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Quote:
Originally Posted by Miles Prower View Post
Forch, I saved all the original notes. Loadline (attached) Do keep in mind that this particular project never got built, so it's very much a preliminary design.

With an Rl= 635R/phase, you'll need ~2.54K (P-2-P) A 3K3, 60W Edcor should be just fine here, and the somewhat lighter plate load will drop the output a bit, and maybe the distortion as well (though it doesn't look like you're gonna get very much of that anyway).

Even though these H3 estimates are rough, and high-side errors, they do correlate somewhat to sonic performance. However, in the final analysis, you still have to spend some time listening to the open loop performance to see how that distortion sounds, and what's likely to be necessary to correct for those sonic defects. If the 6CB5 is like the 6BQ6, all it'll need is enough gNFB to take the "edge" off. There shouldn't be any of that "pentode nastiness".

I may have found a pair of 1650T Hammonds which are 1.9K and 120W. Is that way too low for a pair of the 6CB5 tubes?

Thanks,

Blair
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Old 20th June 2013, 03:54 AM   #26
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I thought Miles already answered all your questions on the drive requirement and the plate load.
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Old 20th June 2013, 04:02 AM   #27
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Hi,

He did by stating that the optimal load would be 2.54K, but 3.3K would suffice at the cost of a little power. 1.9K is less difference to 2.54K than 3.3K, so I am asking if it would be an OK plate load to use 1.9K vs. 2.54K.

Will the 1.9K load cause excessive distortion?

So, yes, miles did answer the question.
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Old 20th June 2013, 04:14 AM   #28
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Quote:
Originally Posted by deicide67 View Post
Hi,
Will the 1.9K load cause excessive distortion?
No, the distortion would be a little bit higher but not what you would call excessive, especially at low-medium playback volume, fully crank'd, you will hear it - or not, as it would be so darn loud, you may have to cover your ears
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Old 20th June 2013, 04:21 AM   #29
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Thank you. That's what I'm asking more less. Assuming I can get the output iron at a good price.

If not, I will suck it up and wait 6 weeks for Edcor
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Old 20th June 2013, 04:44 AM   #30
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Just curious, what speakers are you using, do they require all that power?
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