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    SCA 35 Can you replace preamp section with vta preamp section?

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    Andy Jersey

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    SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Tue Jan 08, 2013 10:29 pm

    I understand Dynaco used what they called lean circuitry when making the SCA 35. I was wondering whether it would be a simple operation to remove the preamp board and install one of the tubes4hifi preamp boards, perhaps even a vta driver section upgrade. Anybody have any ideas?

    peterh

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by peterh on Wed Jan 09, 2013 8:26 am

    I guess that the simplest solution that damages your antique sca35 the least is an external phono preamp.
    There has been a small series built in sweden using nuvistors but i guess that you find simular "high quality phono-amps" in us.

    That way you may keep your phonoamp even if you replace sca35 in the future.


    tubes4hifi
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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by tubes4hifi on Wed Jan 09, 2013 2:20 pm

    there are no replacement circuits for the SCA35 that I'm aware of,
    anyone who is capable can make any change they want (I always like to say "anything is possible",
    but I wouldn't call replacing one circuit type with a totally different circuit type a simple change.
    You've got physical board size, power transformer voltages, and several other issues to deal with.

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Sat Jan 12, 2013 7:54 pm

    Going on the premise that i have to work with what it is, or go on a journey a may not want to take right now, I have found kit boards to replace the originals. One set uses 7199s like the originals, the other uses 6GH8As. I understand that 7199s are growing extinct quickly. I did notice some Russian versions available,though i haven't heard anybody singing praises of them, so i'm guessing they don't compare with NOS. Does anybody know how the 6GH8As sound? ( i realize that's a very open ended question) Let me narrow that to , from what i've read it is a electrical equivalent so, is it more or less a sonic equivalent, and are some particular brands, types, vintages more musical in these applications?

    peterh

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by peterh on Sun Jan 13, 2013 8:51 am

    Andy Jersey wrote:Going on the premise that i have to work with what it is, or go on a journey a may not want to take right now, I have found kit boards to replace the originals. One set uses 7199s like the originals, the other uses 6GH8As. I understand that 7199s are growing extinct quickly. I did notice some Russian versions available,though i haven't heard anybody singing praises of them, so i'm guessing they don't compare with NOS. Does anybody know how the 6GH8As sound? ( i realize that's a very open ended question) Let me narrow that to , from what i've read it is a electrical equivalent so, is it more or less a sonic equivalent, and are some particular brands, types, vintages more musical in these applications?
    Forget the russion 7199, they sound bad and the amp is unstable with them. Bye 2 NOS, they will last
    very long. And there is no shortage of 7199, just an increasing price.


    Flyquail56

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Flyquail56 on Mon Jan 14, 2013 4:54 pm

    I have two nearly identical ST-70s, one with 7199 input tubes and the other re-wired to use 6GH8As. The difference in sound is minimal. I believe that you would find the same to be true with the SCA-35. The 7199 was s tiny bit better, but not nearly enough to keep me from using the 6GH8A.

    Going back to your original question about a preamp board. If I understand your question, I take it you are referring to a line stage preamp board rather than a phono preamp. The SCA-35 has a phono preamp board, but no line stage at all. In today's jargon, it would be referred to as a passive line stage. Dyna used the 7199 to make a high-gain voltage amp stage in the power amp section. This extra gain was to compensate for the approximately 20 db loss imposed by the tone controls. And of course line level inputs such as CD or tuner can drive the amp to full output without the need for a traditional line stage.

    There are a few possibilities for improving the SCA-35 circuit. First, if you can live without tone controls, change the power amp boards to those of the ST-35. It is essentially the same circuit except it uses the 12DW7 dual triode input tube with lower gain than the 7199. The lower gain is offset by the elimination of the tone controls, so still no line stage needed, and the sound quality IMO is better than either the 7199 or 6GH8A circuit. Boards are a drop-in replacement.

    Adding an active line stage is only relevant if you make the change to an ST-35 power amp section as mentioned. Opinions about passive vs. active line stages are as varied as those of which capacitor is best for a given application. Though I personally don't see the need for it, it is possible to reconfigure the phono board as a line stage, followed by a good bit of rewiring of the selector switch. Implementing something such as the PAS PC-5 circuitry (sans tone controls) is possible with the existing board. However, it might not meet your requirement of a simple operation Very Happy Also, I would consider the original PC-5 circuit to be good but not great. There are better circuits out there, but no boards that would drop in the limited space of the SCA-35 as far as I know.

    Finally, if you plan to use a turntable, I can direct you to a step-by-step to convert the existing phono circuit to a modified PAS PC-6 circuit that is a very worthwhile improvement. Hope this helps.

    Mike


    Last edited by Flyquail56 on Mon Jan 14, 2013 11:12 pm; edited 1 time in total (Reason for editing : Clarification as to when active line stage is applicable.)

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Mon Jan 14, 2013 9:52 pm

    Tremendous reply. I'm blown away by the thoroughness. Just worked a 12 hour shift and spent 4 hrs commuting. I'm too spent to absorb it all right now. I'll reread when i'm a little more focused. Thanks so much for the post.

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Tue Jan 15, 2013 6:45 pm

    Hey Mike, after reading article after article (it's amazing how much copy there is on these amps, i'm curious how many were actually made), what i think i should do is change to to the 6gh8a boards Make the EFB mod. add a new cap power suppy board (i might just replace rectifiers and upgrade the cans). I think i'd be interested in the phono mod (modified pas 6)you just wrote about. One guy had a post on AK about bypassing tone controls through a switch (they could be in or out if i understand it correctly). Is there a link to look at the mod or do you want to write about it? AR used to make a power suppy board with the EFB incorporated but they shut down. My understanding is Classic Valve is negotiating to make one.

    Flyquail56

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Flyquail56 on Tue Jan 15, 2013 11:37 pm

    That sounds like a good plan.

    The following is an article that I wrote about six years ago and posted on Audio Asylum. It is a mildly tweaked PAS phono stage adapted to the SCA-35. (Tape head input is deleted in the process) If you don't have a schematic, it is available in the Sticky section here in this forum as well as a couple other sites online.


    Values given are for left channel, all resistors are ½ watt.

    R1 – 1.2M, no change unless PEC is removed from phono input jacks, then change to
    47K
    R2 – 150K, no change
    R3 – 5.6K, change to 47K
    R4 – 120 ohm, change to 1K 5% carbon film or 1% metal film
    R5 – 120 ohm, change to 1K 5% carbon film or 1% metal film
    R6 – 1.2M, no change
    R7 – 47K, change to 100K
    R8 – 390 ohm, change to 1K 5% carbon film or 1% metal film
    R9 – 18K, replace with jumper wire
    R10 – 100K, remove, do not replace
    R11 – 47K, replace with jumper wire
    C1 – 0.1 mfd, replace with 10K resistor
    C2 – 0.022/400V, replace with same value new cap
    C3 – 0.1 mfd/400, change to jumper wire
    C4 – 330 pF, remove, do not replace
    Replace wire from eyelet 11 on circuit board to lug 2 on selector switch with 0.1/400V capacitor.
    Remove wire from eyelet 3 to lug 4 on selector switch.
    Make up a new feedback network consisting of 2 resistors and 2 caps. Solder a 95K resistor in series with a 2M resistor, both 1% metal film. Then solder an 820 pF/500V silver mica or polystyrene cap in parallel with the 95K resistor, solder a 2700 pF/500V cap in parallel with the 2M resistor. Solder the 95K end of this network to eyelet 11, and the 2M end to eyelet 3. Be aware that space is really at a premium. I soldered these networks on the bottom of the board, but in doing so, you must ensure that there is no chance of shorting against any other wiring.

    That’s it! You are now ready to enjoy a much better sounding phono stage. My concerns about hum were unfounded, even though the unit uses AC on the heaters. No hum with the volume fully up when using LS3/5a speakers. If you’re using very high-efficiency speakers, the Sovtek 12AX7LPS might be a worthwhile investment. But I have used 70’s Mullards and new production Chinese 12AX7s with excellent results. Gain is reduced by about 10db from the original, but is still adequate.

    Beyond what I have already mentioned specifically, resistor and capacitor types are very subjective, so I’ll leave that issue alone other than to point out again that space is at a premium. I have used Illinois MWR metallized polyester and CDE metallized polyester coupling caps with good results, and they don’t take up a lot of space.

    Any further questions, please feel free to ask.

    Best regards,
    Mike


    sailor

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by sailor on Wed Jan 16, 2013 6:24 pm

    First let me say I have got a lot of good info in the past on SCA35 from Flyquail56, Thank you.
    I have a SCA35 and here are a couple of things I found. First I am using the Russian 7199 which are not true 7199's but a slightly more powerful tube in disguise. I think they are quiet, have better bass but are a little less musical and they can squeal at start up before the sound starts. But overall I like them. As far as 7199's most are either used sold as new or noisy rejects. Because of the SCA35 high gain it will not tolerate a noisy 7199. Even one that will work just fine in a stereo70. I know this because I have bought a lot from dealers and had to return them all because 80% were to noisy for the SCA.
    I also noticed something I think is very important. This may be why the passive stage works so well in the SCA an will not work well with other amps. The tone controls act like a parallel resistor with the input load resistor. Although the load resistor I think is 1.5 Meg. it measure around 243K. If you remove the tone controls I would use a volume control of 250K.

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Wed Jan 16, 2013 9:17 pm

    Thanks for the mod info and the thoughts on the tubes. I anticipated putting in the 6gh8a boards. Just seems like 7199 are getting scarcer and more expensive. I hope to have amp within the next couple of days. then i can start playing. I'll give updates. Thanks again

    peterh

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by peterh on Thu Jan 17, 2013 5:40 am

    Andy Jersey wrote:Thanks for the mod info and the thoughts on the tubes. I anticipated putting in the 6gh8a boards. Just seems like 7199 are getting scarcer and more expensive. I hope to have amp within the next couple of days. then i can start playing. I'll give updates. Thanks again

    Would it not be both cheaper and simpler to buy 2 7199 (about us 20$ - 30$ )
    then to purchase 6gh8a boards ?

    sailor

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by sailor on Thu Jan 17, 2013 3:13 pm

    Again, the SCA35 pushes the 7199 to its limits. If you can find someone with a good cash of 7199 you had better order 8 to 10 tubes so you can get 2 quiet enough to use in an SCA35. Make sure the tube seller will take the tubes back. I know of no dealers that have that many and none who will let you cherry pick his tubes to get 2 good ones. And I have found no one who test for noise. If you know of a source for good, quiet [tested], 7199 please let me know.
    This would have been the best place to buy a 6GH8 board and cap board, but he has shut down sales until the economy gets better.
    http://www.audioregenesis.com/products/SCA-35/index.html

    peterh

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by peterh on Thu Jan 17, 2013 3:37 pm

    sailor wrote:Again, the SCA35 pushes the 7199 to its limits. If you can find someone with a good cash of 7199 you had better order 8 to 10 tubes so you can get 2 quiet enough to use in an SCA35. Make sure the tube seller will take the tubes back. I know of no dealers that have that many and none who will let you cherry pick his tubes to get 2 good ones. And I have found no one who test for noise. If you know of a source for good, quiet [tested], 7199 please let me know.
    This would have been the best place to buy a 6GH8 board and cap board, but he has shut down sales until the economy gets better.
    http://www.audioregenesis.com/products/SCA-35/index.html
    I bought 10 of them ( NOS jan 7199 ) from england 2 years ago. 4 was transferred to a friend, 2
    into my sca35 , the rest is sleeping in my drawer. At that time i think i payed 20$ each.
    Each one was was working ok. I also have the 2 russian 7199 that i tried, but i found them
    not acceptable. So, unless it's really that different i do not think it's impossible to get 7199 today.

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Thu Jan 17, 2013 8:21 pm

    The NOS 7199 I'm seeing on websites are running 60$ and up.most of the factory dynaco boards i see are burnt looking and i've read prone to failure. Tha clone boards are cheap and the 6**8s are cheap and plentiful i'm not a collector and being factory original isn't important to me. I want it to sound good and i want it to be reliable and if a tube goes bad, i don't want to pay out the nose. I'm not saying any of you are wrong and i appreciate the opinions and advice. I haven't seen convincing enough arguments to sway my choice right now. I am willing to hear out anyone else.

    Andy Jersey

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    Join date : 2012-09-29

    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Sun Jan 20, 2013 10:50 am

    Came across a reasonable set of NOS RCA 7199 (5) so i'm going to go with the 7199 board. If i could pick up one more i'm confident my supply will outlive me.

    Andy Jersey

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    Join date : 2012-09-29

    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Sun Jan 20, 2013 10:35 pm

    Quick update, Out of box, not bad, pretty clean. 3 dynaco 6bq5s, 1 curtis mathes hifi 6bq5 (?) 2 original dynaco 7199s, 1 dynaco 12ax7, no markings on the other one. Hooked up powered on. One channel hums. Trim pot lowered hum, still there. other channel no hum regardless of hum pot position. At first i thought the channel was dead. Played some music. Both channels even and clean. Really musical with more bass than i expected from what i've read. Really very nice sounding overall. can't wait to clean up and tweak. Oh one thing i noticed. running selector switch through positions, tape head position had a squeal (not a true squeal tough to describe) Anyway being how most of it will be gone through cleaned and changed, not going to worry about it.

    Flyquail56

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Flyquail56 on Tue Jan 22, 2013 11:18 am

    It sounds like you are off to a good start.

    There's one more thing that I forgot to mention: It's always a good idea to verify that the tone control knobs are correctly installed. The usual, centered in the shaft rotation is not correct in this case! As the manual states, the bass knob should be pointed to 6:00 when fully counterclockwise, and the treble at 6:00 when fully clockwise. Then when rotated back to 12:00 they are set to flat response. Hope you enjoy the new amp and the restoration process. Keep us posted on your progress.

    Best,
    Mike

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Tue Jan 22, 2013 10:08 pm

    Thanks Mike, Hey i have a question. what can you use to clean up the aluminum chassis?

    DynakitParts
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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by DynakitParts on Tue Jan 22, 2013 11:16 pm


    Andy,
    The chassis is steel with a cadmium plated finish. Any metal polish will do the
    job but may remove some of the silk-screening. Wear rubber gloves...cadmium is toxic.

    Kevin @ Dynakit

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Thu Jan 24, 2013 8:29 pm

    Kevin, I appreciate you watching out for me. I went out and got gloves when i got the polish. It cleans up nice. I wasn't planning on lifting trannys but i cant polish thoroughly without it. Maybe try q tips. I guess i have to live with little pit spots. Still waiting for clone and power supply boards. Ordered efb parts today with phono mod parts. DCG is apparently coming to market with an ECB which he said will be compatible with these and the st 35 (and probably others). He seem to feel about 60 days or so. I found a set of speakers to match it when its done. I'm pretty psyched

    GP49

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by GP49 on Sat Jan 26, 2013 1:37 pm

    If it's REALLY cadmium, beware: cadmium is toxic. It is a known carcinogen and should be handled with gloves. There is no known effective treatment for cadmium toxicity.

    peterh

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by peterh on Sat Jan 26, 2013 2:15 pm

    GP49 wrote:If it's REALLY cadmium, beware: cadmium is toxic. It is a known carcinogen and should be handled with gloves. There is no known effective treatment for cadmium toxicity.
    You are not supposed to EAT the amp, just drool over it !


    GP49

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by GP49 on Sat Jan 26, 2013 2:36 pm

    peterh wrote:
    You are not supposed to EAT the amp, just drool over it !

    True! According to the Centers for Disease Control, "For the general public, the primary source of exposure to cadmium is dietary." Absorption by inhalation is primarily a hazard if fumes are breathed. Transfer through the skin has had little research. Nobody seems to mention how much would be transferred if one were to polish the cadmium.

    Awareness of cadmium toxicity has only been since the mid-1950s or so (probably why the Mark III and Stereo 70 and later models were chrome-plated).

    I handled LOTS of cadmium plated stuff occupationally for many years. In my currently-active system, the two Dynaco Mark II amplifier chassis, PAS preamp chassis and the linkages underneath the Garrard 301 turntable are cadmium-plated. I don't know whether the Dynaco AF-6 chassis is; probably not.

    Andy Jersey

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    Re: SCA 35 Can you replace preamp section with vta preamp section?

    Post by Andy Jersey on Sat Jan 26, 2013 5:10 pm

    Gives new meaning to the expression "to die for". No reason not to use gloves while polishing. I am happy to be made aware of the nature of what i'm handling. I do feel safe working on the amp. Please let me know if any of them are radioactive. Thanks all again.

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