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Thread: VHT/Fryette power amps

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    VHT/Fryette power amps

    Hi,

    Just wondering if anybody tried to reverse engineer vht power amps like 2150, 2902, etc. There's just something that's been bugging me.
    I know they all tend to use buffered cathodyne PIs. I hear a lot of claims that say 2150 and 2100 use the second tube (12at7) as AC coupled CFs, but numbers just don't add up.
    The manual claims sensitivity up to -10dbV, which is 0.316V rms or 0.447V amplitude.
    Now, KT88 would probably need what, around 75V bias, give or take? That means the previous stages need to provide gain close to 168, and that is in a closed loop (i.e. after gain reduction due to nfb).
    So, the first 12ax7 is likely a gain stage followed by cathodyne. Considering the most usual place for nfb is the cathode of the gain stage, it can't have its cathode resistor bypassed. This means the stage is unlikely to have gain more than 50 (at a push). Then cathodyne gain is just under unity, and so is CF's. Clearly there's not enough drive to give all the swing required, and this is even before one looks at nfb.
    Now, manuals for 2902 and 2502 mention that their second tubes (12au7) are "voltage amp/driver", which would suggest they are common cathode gain stages, rather than cathode followers. So, does this mean that in 2150 the second tube is actually a pair of post-PI gain stages, rather than CFs, as people lead to believe?
    Or am I missing something?

    Thanks,
    Konstantin

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    Last edited by kostic2000; 11-15-2019 at 02:23 PM.

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    The Deliverance 60 is a gainstage > cathodyne > power tubes. The 120 has a CF driver after the cathodyne.

    The schem here is the D60
    Layout is the 120 with the CF buffer/driver

    Click image for larger version. 

Name:	D60_all_on_one.png 
Views:	37 
Size:	1.34 MB 
ID:	55956

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    Quote Originally Posted by Zozobra View Post
    The Deliverance 60 is a gainstage > cathodyne > power tubes. The 120 has a CF driver after the cathodyne.

    The schem here is the D60
    Layout is the 120 with the CF buffer/driver

    Click image for larger version. 

Name:	D60_all_on_one.png 
Views:	37 
Size:	1.34 MB 
ID:	55956
    Well, these are not standalone power amps, they have the preamp section to get the signal to the right level

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    Well yes, but it at least shows the power amp topology. The retrofitted FX loops for the Deliverance were opamp based and probably not a great deal different to the ones used in the various Marshall reissues.
    There are a bunch of posts on sloclone about VHT/Fryette stuff and some guesses at the standalone power amp topologies.

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    True. I have already read most of stuff in sloclone about this. In fact, this is where quite a few people suggest that 2150 uses CFs. I have seen D120 schematics before too, and I wonder if this is from where people extrapolated the design onto 2150. But I just don't see how CFs would physically be possible in 2150.

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    not exactly a new thing ,Marshall major had the cathodyne and the gain stages after to 4 KT88s.

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    Yes, indeed. So I wonder if this is what they did too? It seems that way, but I have never come across one to verify, hence asking here if anybody tried to reverse engineer

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    this site has most of the good practices in tube amps
    http://education.lenardaudio.com/en/...ve_amps_4.html

    For education purpose i find that any old book by RCA GEC Radiotron etc has most of the solutions you can possibly need,once you start looking at modern amps,soon you recognize a lot of the techniques explored in the golden years and some of these gurus even patented or claimed as their own invention.

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    Quote Originally Posted by alexradium View Post
    this site has most of the good practices in tube amps
    http://education.lenardaudio.com/en/...ve_amps_4.html

    For education purpose i find that any old book by RCA GEC Radiotron etc has most of the solutions you can possibly need,once you start looking at modern amps,soon you recognize a lot of the techniques explored in the golden years and some of these gurus even patented or claimed as their own invention.
    Thanks. That link is really interesting.

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