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

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    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Sun Jan 27, 2013 7:26 pm

    Ok I'm looking for some guidance. I wasn't planning on replacing every piece of wire during the upgrade. I did anticipate finding some things that could be cleaned up along the way as necessary. Having never taken apart one of these (or building one for that matter), I was making myself diagrams and notes and photos to make sure things go back where they were. I did download the instruction manual, so if all else fails i can refer back to schematics and instructions. Two things came to light during this operation. The output transformer on one channel had reversed leads blue white lead on 7 green white on 9 (the amp was working) i'm thinking it might put amp out of phase, i'm not sure. I'll set it right during rebuild. I also noticed xfmr leads very short in one spot (not even twisted) they're so short it might not be necessary to twist but if i lose another 1/4" inch it might be tough to reattach. Any thoughts on neatly extending leads? Should i just solder on a couple of longer wires and heat shrink. Its such a short lead and the entire run is all of less than 3 " from bottom of xmfr to tube socket. I don't think its going to look neat like that. Maybe if i heat shrunk the whole lead, at least it would look uniform. I'm open to suggestions. Thanks

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Fri Feb 01, 2013 9:52 pm

    Ok boys and girls, I need to pick your knowledge base. I'm utilizing Triode store power suppy cap board (SDS Audio Labs). I hope some of you are familiar with it for sake of question. It appears to mimick factory supply in electrical layout and part #s (which i didn't realize at first and that was confusing me). That being said , they seem to split the Cathode bias circuit into two halves each with its own cap and resistor C24 d and e (1800uf) and R42a and b (200 ohms which in parallel gives you 100 which is close to the factory 95). Ok here's where it get's interesting. Because i want to make EFB mod and AR is gone I have to do it point to point. My understanding would be to remove both r42s, leave both caps and then continue with mod like before (at first i was thinking i had to tie ungrounded end of point where r42 were removed, it doesn't seem like it makes any difference once they're removed (maybe the caps have to be tied together but i'm not seeing why). Can anybody shed any light on this (hopfully more articulately than my ramblings). Oh PS I have to give Kudos to the triode store. They shorted me a cap. I realized last night at around 8pm. I wrote them an email around then (according to the website they're closed then) when i woke up this morning, i saw i got and email around 12:30am this morning apologizing for this missing cap saying it would go out today express mail and i'd have it tomorrow. That's pretty impressive support to me!

    Flyquail56

    Posts : 24
    Join date : 2009-05-04

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

    Post by Flyquail56 on Sat Feb 02, 2013 5:57 pm

    Looks like you got the information you needed from the best source. Dave is very good about helping with the EFB circuit. Good luck with the project!

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Sat Feb 02, 2013 6:22 pm

    Thanks for the follow up

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Sat Feb 09, 2013 5:47 pm

    Got amp back together and putting through its paces. I'm shy 2 caps for the phono mod (my screw up, they're ordered) . Running through some Polk monitor 60s for checking and running old tubes, (at least until i see i didn't screw anything up so badly. I posted pics on AK if you're curious. Sounds pretty sweet right now!

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Tue Feb 19, 2013 4:00 pm

    Gave up first idea and refurbed amp
    Very pleased with outcome, but i had a what if moment. Could you hybrid in a vta driver board without an engineering degree, utilizing existing power supply, intercepting key signal points?

    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

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

    Post by stewdan on Wed Feb 20, 2013 12:19 pm

    Andy -- It "could" probably be done using a Mk3 or Mk4 VTA PCB, plus some "interface mods" for the SCA-35. Not sure what type of Hybrid you would end up with though? Would it sound like an improved SCA-35 or a Mk3 or 4??

    How you would mount the replacement boards is unclear to me since two Mk3 or Mk4 boards take up more space then the two original SCA35 PC10 Boards. I don't know if they would fit inside the SCA's Chassis and Cover? Maybe on stand-offs above the chassis?

    Maybe you could use the VTA70 PCB where the L and R Channel Circuitry would replace the two PC10 Boards?? Enough Mounting Space???? I don't know.

    For point of info, there is some related info at the Tubes4hifi website (under VTA 70 customer comments/reviews) where someone took 2 VTA70 Boards, populated 1/2 of each board and used them in two Heathkit MonoBlock Amps (W-7's) which are rated at 60 Watts each and just hooked up the driver portion of each board to the existing Heath Power Supply (Upgraded of course) and used the original Heath Biasing circuitry. But, I believe that the person who merged the VTA70 boards with the Heath W-7's was very knowledgeable and the Heath chassis is much larger than the SCA35.

    Take a look at what I am referring to in the customer comments/reviews.

    Stew


    stewdan

    Posts : 161
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

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

    Post by stewdan on Wed Feb 20, 2013 2:50 pm

    Andy -- the more I think of it, just IGNORE what I said about replacing the PC10 Boards with VTA boards. It won't work, because the output tubes are on the PC10 Boards and the VTA boards only contain the driver circuitry for the output tubes.

    The customer review/comments info is interesting and probably is somewhat valid for what you asked about.

    Stew


    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Wed Feb 20, 2013 4:11 pm

    Ok i appreciate the thought. Without looking at schematics and dimensions, i was thinking 7199 driver board mimicked 7199s of sca. Again without investigating i thought if you pulled 7199s intercepted signal path at right point. Used filament power from 7199s either go to 6v tube or pick up 12v from phono board and mount on standoffs over 7199 side of pc 10 boards. Wouldn't need bias circuit for bq5s. Maybe a misdirected fantasy. I love the sound of my vta st 120. I really like sound of the bq5 amp
    I'm assuming and perhaps wrongly , that lack of air and space was limitation in passive pre section. Really unfair comparison. I'll investigate schematic and dimensions further. Most likely i'll backburner idea. Yeah, now i'm thinking 7199s not just drivers ala st 70. Also pre. I'll still look but probably an unrealistic

    iguring cost, effort, and result

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Wed Feb 20, 2013 4:12 pm

    Making post on phone a little difficult. I just wanted to thank Stew for his time and effort

    DarthBubba

    Posts : 81
    Join date : 2012-05-05

    Another possible SCA-35 driver tube

    Post by DarthBubba on Thu Feb 28, 2013 3:13 am

    What about subbing the 6AN8a for the 7199? Dynaco used it in several mono amps to good effect. You'd need to do a bit of "trace surgery" on the PCB and change a resistor or two. Perhaps one of the group has already given this some thought, and made notes. Smile

    peterh

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    Join date : 2012-12-25
    Location : gothenburg, sweden

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

    Post by peterh on Thu Feb 28, 2013 3:59 am

    DarthBubba wrote:What about subbing the 6AN8a for the 7199? Dynaco used it in several mono amps to good effect. You'd need to do a bit of "trace surgery" on the PCB and change a resistor or two. Perhaps one of the group has already given this some thought, and made notes. Smile

    Yes we have, get 2 NOS-7199 , replace and forget the problem for another 20 years.

    tubes4hifi just announced in this very fora that that had such tubes.

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Thu Feb 28, 2013 6:51 am

    I accumulated a half dozen nos 7199s. Unless somebody has a proved change i think I'll sit tight for now. Thanks
    Still interested in ideas. I just not looking to get in too over my head..

    sailor

    Posts : 269
    Join date : 2011-04-04

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

    Post by sailor on Thu Feb 28, 2013 5:49 pm

    Ok everybody I am about to commit a grave sin in the eyes of the purest. I modified the left channel this morning and once the new resistors burned in, about a half hour, the sound was much cleaner and more detailed. To put it simply, if you were listening to a classical guitar. The original sound puts you in the 25th row of seating the change puts you in the first row. A much more detailed and fuller sound. You will need to adjust the volume a little so they are putting out the same db. sound level.
    Here is the problem: I have said in the past that the positive feedback muddies up the sound, causes the whole amp to be on the verge of unstable, and unbalances the signal to the 2 output tubes.The change I made is really old school. You will loose about 3 to 6db of gain in the driver stage, this change should make the Sovtec 7199 and other substitutions operate stable.
    OK here it is but do at your own risk as I am not there to make sure you followed my instruction exactly.
    1. Remove the 120K resistor it will not be replaced. [positive feedback resistor].
    2. Match 2- 22K/1watt Resistors buy 5 or 6 and find 2 that measure exactly the same. I prefer metal film. Replace R32 and R34 with these resistors. This will bring the drive signal to the output tubes to less than one db. of each other.
    3. Now we need power to the screen at pin 3. We do that by soldering a 1.5Meg./1/2 watt resistor to the B+ power supply side of the new R32 [the side closest to the back] and the other end to the hole where the 120K resistor was removed that is the hole closest to the back of the amp. The 1.5 Meg resistor will have to straddle Resistor R30. Put it exactly like you want it before soldering to the circuit board. Do not move this resistor once soldered as it could damage the circuit board trace.
    4. The 1uf cap. stays where it's at, no change.
    The positive feedback is removed and the pentode screen at pin 3 now powered direct from the B+ supply.
    That's it, I will make more changes in the future.

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Thu Feb 28, 2013 8:27 pm

    You're a brave soul. I'm going to let you play with yours. You are definately in uncharted territory. Let me pick your brain a sec. There is a thread somewhere here for bypassing tone controls using filter switch. I did a little experiment today direct connecting the middle of the volume pots to the middle of the balance pots (which if i'm not wrong is basically that (feel free to correct me anyone). I have to say i think amp opened up a lot. I don't just think it was the the added gain (which i felt was a good thing also). I was pming someone way smarter than me asking about it. He said in order to truly bypass tone controls you need to completely isolate the components and you would need a 4 pole double throw switch (or relay) to accomplish this. I was wondering if anyone had a link to this mod or would be able to explain it referencing the schematic. Any help greatly appreciated. Sailor, keep up the good work. I'm not as brave as you.

    sailor

    Posts : 269
    Join date : 2011-04-04

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

    Post by sailor on Thu Feb 28, 2013 10:51 pm

    Hi Andy,Not so brave as knowing what is happening in the circuit. All I did was remove the positive feedback resistor which was also supplying DC to the pentode screen pin 3. The 1.5 Meg resistor was to replace the lost DC to that screen. The change in resistor values was because removing the 120K resistor changed the overall resistance on the cathode side of the splitter. Normally the 2 resistors are the same value so the output is the same which is why I changed them. My wife gets home in an hour and she will get to blind test the changes. Dynaco put in the positive feedback to increase the output by 6 db. not for sound quality reasons.
    As far as your question about the controls I took a quick glance and it looks like it would be easiest to cut out everything after the volume control and back in before it goes to the driver stage. That would cut out the Bass. Treble, and balance control. The switch could only be flipped when off or at least turned all the way down as you would get a bad pop if it was on. I would have to look at it a little closer. Do you need more gain to the driver stage? A while back I bypassed the whole front and went to a stepped attenuator I was surprised that it made as little difference in the overall sound quality. Yes a little but not much.
    You mentioned the tubes for hifi board. I was thinking of building an EL84 amp. from scratch using that board. But It would take a chassis twice the size of the SCA35 to build it and a whole different power supply circuit and power transformer.

    sailor

    Posts : 269
    Join date : 2011-04-04

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

    Post by sailor on Fri Mar 01, 2013 1:42 pm

    Andy, You would need an on-on toggle switch which has 3 rows of 4 poles. the center row would have the outgoing wire from each attenuator and the 2 in going signal wires to the amp. One side would have The in going wires to the tone controls and the outgoing wires to the balance control. The other side would have 2 wires soldered between poles so when the switch was flipped to connect that side the attenuator would be directly connected to the amp. incoming wire of each channel. Probably not clear but to short on time to draw it out right now.
    A note to anyone who wants to bypass everything and use just a volume control. Make sure you use a 250K audio pot because the 250k pot and the 4.7 meg input resistor in the amp. are in parallel with each other and give the amp. about 237K input impedance. Don't believe me? Take your trusty volt/ohms meter and check the ohms of the 4.7Meg. resistor. And you thought the amp had an input impedance of 4.7Meg.? nope.

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Fri Mar 01, 2013 5:32 pm

    Dave , would it be too much to ask you to draw out circuit when you get a free moment. I'm not looking to bypass volume or balance. Thanks

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Sat Mar 02, 2013 3:18 pm

    Ok Dave, somebody sent me a decription, i had to draw points and a fragmented schematic before the connections made sense to me(i couldn't visualize what was being descibed. If you still want to post for anyone else feel free (also i'll double check my own understanding against.) Thank you






    sailor

    Posts : 269
    Join date : 2011-04-04

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

    Post by sailor on Sun Mar 03, 2013 12:53 pm

    Before I installed the switch I would pick the easiest channel to work with and unsolder the volume out wire and the signal in wire at the matching driver board. Then solder a wire direct between these 2 connections. This would have the same affect as flipping the switch to connect the right row to the center. You now can AB the difference before you go to all of the trouble of buying and installing the switch and a lot less work. To AB say a CD player plug the incoming RCA plug in different, adjacent high levels sources and turn the selector switch between them. Remember to lower the volume because the direct connection will increase the volume by as much as 17 db.
    The center row is always on and pairs with the left row or the right row. The left row puts the connections back the way they were. The right row bypasses the controls. The items in the center row can be in any order as long as the other 2 row match that row. Use a on-on switch, not an on-off-on. I guess you could change the treble for the balance but unless you need the balance I would bypass it.



    Left row Center row Right row
    1 left Bass in 1 Left volume out 1-2 solder wire between 1 and 2
    2 left balance out 2 Left amp in
    3 Right Bass in 3 Right volume out 3-4 solder wire between 3 and 4
    4 Right balance out 4 Right amp in

    sailor

    Posts : 269
    Join date : 2011-04-04

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

    Post by sailor on Sun Mar 03, 2013 12:57 pm

    I had left, center and right rows in nice neat rows but when I posted it it removed the spacing an ran it all together. Not sure how to correct it. Sorry

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Sun Mar 03, 2013 8:01 pm

    Information i got was nearly the same except they were picking up signal at the wiper of the volume control instead of the bass in, maybe one point earlier in the circuit.

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Sun Mar 03, 2013 8:03 pm

    Taking Peco out of circuit entirely

    Andy Jersey

    Posts : 69
    Join date : 2012-09-29

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

    Post by Andy Jersey on Sun Mar 03, 2013 8:06 pm

    My only tweak when i do build (my switch is Back ordered until the 11th) is to use resistors instead of jumpers to attentuate gain to approx original. I imagine it'll take a little trial and error. I'm thinking between 2 and 5 meg.

    sailor

    Posts : 269
    Join date : 2011-04-04

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

    Post by sailor on Sun Mar 03, 2013 9:06 pm

    If you can, please post the drawing you have. Volume out is the wiper. Bass in [the wire removed from the wiper], is the beginning of what has been removed.
    You don't need resistors Do only one channel. Just remove and tape off the wire to one wiper and remove and tape off the matching driver board signal in. Do not remove any ground wires. Then run a new wire 20 or 22 gauge from the wiper to the board same locations where the wires were removed. If you did it right you will have sound in both channels. One will now be a lot louder.
    Having said that I have kept one channel original and the second has all my alterations. I wired as stated above. The modified channel went from 1 o'clock to 9 o'clock for the same output. The all original was at 12 o'clock to match the sound level.
    First a correction, I said I had removed the bass, treble and balance once before and it made little difference to the sound. I was wrong. With the changes I made on Thursday to the driver board [see earlier posts] and the removal of the tone and balance controls, Well, It simply blows the doors off the original. In fact it's reaching the sound of a very high quality amp. Up till now the only thing I would use this amp. for was Blue ray movies because it had great Bass and not much else. There are still a few more improvements to go. I want to improve the noise level of the power supply. Plus a few other tweaks.

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