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Question about building an amp without a preamp stage
Question about building an amp without a preamp stage
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Old 9th June 2018, 10:31 PM   #41
H713 is offline H713  United States
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Join Date: Nov 2017
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I see Fisher 400 and 500C/800 transformers on eBay pretty regularly... I assume that the 5K primary impedance would be okay for an 807. Might be a little on the low side (7591 is a 5k impedance, correct?). Thoughts? I know the ideal impedance would be a 6.6K for an 807/6L6.
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Old 10th June 2018, 12:02 AM   #42
kward is offline kward  United States
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Fisher 400 outputs won't offer quite enough power handling capacity. If the Fisher 500/800 outputs are 5KΩ, they won't really offer a very good impedance match with 475V B+.

Really do need primaries between 6KΩ and 8KΩ to work the best with 475V B+ with the 807 tube (this assumes pentode mode connected). You might consider AA-100 outputs. I think their stated spec is 25 watts even though their cores are maybe 20% bigger than the Fisher 400's. Their primaries are 7.6KΩ so I think these will work well if you reduce the screen voltage maybe to 290V, but that configuration will probably still overpower them just a bit. Still, it's an option I'd leave in the consider column.
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Old 10th June 2018, 12:24 AM   #43
H713 is offline H713  United States
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Alright, I was a little off here. According to the Transcendar website, the early fisher 400 transformer was of 6.4K primary impedance and the late fisher 400 is 10K. All the fisher 500 and 800 transformers are 6.4k. I have no idea why I thought all the 7591 receivers used a 5K-ish primary, as that's clearly not the case.

Also, I'm fine with taking that B+ down a bit using resistors- I don't need a crap ton of power. My speakers are 87 DB (average) @4 ohms (Okay, maybe they're more like 3.5 ohms... I know, they're probably not the best mate for tube gear...). I'm fine with 25 watts. Hell, I'd probably be fine with 20 since my listening room is not particularly large.

By the way, speaking of Transcendar, does anyone know if they're taking orders? I know they weren't last year when I checked, but does anyone know what's the status with them? I have heard a lot of good things about the quality of their iron.

BTW, am I correct that if I use global negative feedback I want to have some magnetic overhead?
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Old 15th July 2018, 06:03 AM   #44
thorpej is offline thorpej
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Quote:
Originally Posted by H713 View Post
So I'm starting my Push-Pull 6L6 build (transformers FINALLY arrived... its like christmas ). I am using a design from Jason Thorpe on AudioKarma (thread can be found here Looking for schematic for nice clean push pull 6l6 monoblock )

I would like to omit the 12AX7 preamp stage as the last thing I need is more gain in my system. My 12AU7 preamp (eventually I'll post a thread about how that turned out) can drive just about anything.
Hey, sorry that I missed this thread starting -- I haven't had much time for audio hacking lately, and am playing catch-up on forum posts.

If you want to make the input less sensitive, you could change the feedback resistor from 6.8K to 2K, which would result in -19.9dB of feedback. On paper, that works out to input sensitivity of 1.31Vrms rather than the stock 440mVrms. You would need to adjust the cap in the feedback network, but that is left as an exercise to the reader.

I built it with the more sensitive input because I wanted the ability to drive it directly from a line source. But I ended up not using it that way and might one day adjust the feedback network as I describe here (I'm restoring the cabinets of some KG-4s, and when they're done, I plan to swap those into my main system, so some reduction in output impedance would be helpful).

Quote:
Originally Posted by kodabmx View Post
You could just build it with more gNFB to reduce the gain, or rework it to use another 6SN7 as the input. Also that 33k grid stopper seems large for a hifi amp. It might cause HF rolloff. I usually use 300r to 1k in that position.
At a glance, that 33K grid stopper does seem like a lot. But it works out due to the gNFB's reduction of input stage's gain, and thus the Miller effect. When selecting the value for the resistor, I did the math and it was fine on paper, and it was fine in testing.
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Old 15th July 2018, 08:11 AM   #45
thorpej is offline thorpej
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Quote:
Originally Posted by PRR View Post
_I_ would *first* just build it as-drawn. Don't muck-up a working plan until you try it. Just turn-down the input pot. (Note that it is Audio taper- someone expected large turn-down.)
To be honest, I used an audio taper pot there out of habit and to make it feel "normal" when adjusting it. In fact, I always intended the put to be mostly turned up all the way, i.e. attenuating the signal almost none at all. If I had intended a large turn-down, I would have used a much smaller grid stopper.

In any case, yes, I would suggest "build as-drawn". I use mine with a Parasound pre-amp that has 12dB of gain, and I am not attenuating the signal with the input pot at all in my setup.

Quote:
Originally Posted by H713 View Post
I'm not opposed to a preamp stage. I just want to avoid the issue I have with my Heathkit amp (AA-121) where RF noise is a real issue and I have my preamp at like 1/4 volume...

And it would be fairly easy to design a preamp stage with a 12A_7, whichever seems fit.
What sort of RF noise are you worried about? Dreaded "cell phone chatter"? You needn't worry about that with that amp as-drawn; that large grid stopper on the 12AX7 keeps it at bay.

As far as changing the front end to use a different 12A_7... it might require more changes than you think because of being direct-coupled to the LTP inverter/driver stage. Not impossible, of course, but it can turn into a real yak-shaving exercise.
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