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    SCA-35 Questions..

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    boeingpilot

    Posts : 9
    Join date : 2015-06-20

    SCA-35 Questions..

    Post by boeingpilot on Fri May 06, 2016 1:52 pm

    A little background first.  I have two PAS / ST-70 systems.  My primary pair is a ST-70 with a VTA-70 board, triode electronics power transformer and Cucio Audio Capacitor board.  It's paired PAS is a PAS-3X which has tube Nirvana's re-pops of the original PCB's installed, as well as the tubeNirvana rectifier / capacitor board.  

    The 2nd one is an original ST-70 with the original driver board replaced with a Chinese re-pop of the original (still driving 7199's), a Dynakitparts replacement power capacitor and a Dynakitparts bias kit. installed.  It's paired PAS is a PAS-2 with chinese re-pop's of the original boards installed (this whole rig was an experiment with the Chinese boards, and to be honest, I'm quite pleased with how they all sound). Since the PAS-2 cosmetics were awful, I topped the rebuild off with a black anodized front panel and repaint of the case .

    Anyway, on to my SCA-35 questions / comments.  Both of the above systems have been used in the past with my Boston Acoustics A60's at one time or another.  Not the greatest speaker, but a good solid bookshelf.  When driving them with either the ST-70's sound level was quite good, never needing more than 1/3 deflection of volume on the PAS to get them to a comfortably loud level.

    This brings me to the SCA-35.  I recently acquired a factory wired SCA-35.  It was sold working, and looking inside prior to power-up, nothing outstanding looked to prevent giving the unit a test run.  Tubes installed were JJ EL-84's, Dynaco Branded 7199's and dynaco branded 12AX7's on the preamp board.  

    So, I threw caution to the wind, hooked the SCA up to the BA's and connected my iPhone via the 'Spare' input to the amp.  Power up was uneventful (as in no smoke leaked out, and no strange sounds).  Sound is clean, though not as 'transparent' as either of my PAS / ST-70 combos.  My biggest concern and question is sound level.  In order to get what I would consider a nice sound level, I had to max out the volume control.  So.....

    1 - yes I realize the SCA-35 is rated at 1/2 the power of the ST-70, and I have no idea how the internal preamp gain compares to the gain of a PAS, but would the sound level really be that much lower?

    2 - if no to 1, then where to start looking? (I have no way of checking the 7199's, but do have a good set on the shelf from the ST-70 that was converted to a VTA-70 to try)

    3 - Seeing as this thing is all original, what upgrades should be considered, or is there more value in conserving it as a factory example?  My thoughts are to go with Dave's SCA-35 power supply board, and perhaps his amplifier (PC-10A) boards or a set of Chinese re-pops.  Or, should I conserve the original and just replace the can capacitor?  (I assuming that cap is original and hence a ticking time bomb)

    In some respects wish I had gotten one in worse shape, then there would be no question on whether to update or not.  Don't want to lower the value of the unit should I decide not to keep it.  

    The group collective thought is appreciated.

    daveshel

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    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: SCA-35 Questions..

    Post by daveshel on Fri May 06, 2016 3:00 pm

    All other things being equal, the output level of a 17.5 watt/ch would be 3db lower than that of a 35watt/ch amp. 3db is by definition the smallest increment of sound the human ear can distinguish. So I'd say something else is wrong.

    Maybe the power supply caps are so worn out you're not getting enough power. That would be my starting point - that was a weak point of the original design and Dave's boards address the deficiencies nicely.

    After you get the power supply in order, then you can evaluate the audio boards. I've always kind of distrusted that amp: it takes some serious shortcuts in supposedly combining a ST-35 with a PAS. And no other Dynaco product needed that hum adjust.

    boeingpilot

    Posts : 9
    Join date : 2015-06-20

    Re: SCA-35 Questions..

    Post by boeingpilot on Fri May 06, 2016 3:08 pm

    daveshel wrote:All other things being equal, the output level of a 17.5 watt/ch would be 3db lower than that of a 35watt/ch amp. 3db is by definition the smallest increment of sound the human ear can distinguish. So I'd say something else is wrong.

    Maybe the power supply caps are so worn out you're not getting enough power. That would be my starting point - that was a weak point of the original design and Dave's boards address the deficiencies nicely.

    After you get the power supply in order, then you can evaluate the audio boards. I've always kind of distrusted that amp: it takes some serious shortcuts in supposedly combining a ST-35 with a PAS. And no other Dynaco product needed that hum adjust.

    Along the lines that I was thinking. When I was a 'young' medical electronics tech I always got yapped at to check power supplies first.

    I'll take it you don't have an issue modifying a factory wired unit.

    Yes, they probably did cut corners (it's a baby box) However, to send my son to college with a ST70/ PAS and a set of Heresy's, his bed will be in the hallway for lack of room Smile Hopefully once it's straightened out he'll have a nice system




    MontanaWay

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    Re: SCA-35 Questions..

    Post by MontanaWay on Fri May 06, 2016 3:18 pm

    one other option, is to perhaps look at 'gutting' your SCA-35, keep the OPT's, power transformer, maybe even the RCA pcb and perhaps the volume/balance controls, depending on their state, and look at our SCA-NG.
    Even though we sell that as a complete new kit, chassis and all, but we would be happy to just sell the new power supply, phono preamp and driver/power amp pcb's.
    If you want to know more about it, 'pm' me, and we'll see what we can do for you.

    peterh

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    Re: SCA-35 Questions..

    Post by peterh on Fri May 06, 2016 3:39 pm

    boeingpilot wrote:A little background first.  I have two PAS / ST-70 systems.  My primary pair is a ST-70 with a VTA-70 board, triode electronics power transformer and Cucio Audio Capacitor board.  It's paired PAS is a PAS-3X which has tube Nirvana's re-pops of the original PCB's installed, as well as the tubeNirvana rectifier / capacitor board.  

    The 2nd one is an original ST-70 with the original driver board replaced with a Chinese re-pop of the original (still driving 7199's), a Dynakitparts replacement power capacitor and a Dynakitparts bias kit. installed.  It's paired PAS is a PAS-2 with chinese re-pop's of the original boards installed (this whole rig was an experiment with the Chinese boards, and to be honest, I'm quite pleased with how they all sound).  Since the PAS-2 cosmetics were awful, I topped the rebuild off with a black anodized front panel and repaint of the case .  

    Anyway, on to my SCA-35 questions / comments.  Both of the above systems have been used in the past with my Boston Acoustics A60's at one time or another.  Not the greatest speaker, but a good solid bookshelf.  When driving them with either the ST-70's sound level was quite good, never needing more than 1/3 deflection of volume on the PAS to get them to a comfortably loud level.

    This brings me to the SCA-35.  I recently acquired a factory wired SCA-35.  It was sold working, and looking inside prior to power-up, nothing outstanding looked to prevent giving the unit a test run.  Tubes installed were JJ EL-84's, Dynaco Branded 7199's and dynaco branded 12AX7's on the preamp board.  

    So, I threw caution to the wind, hooked the SCA up to the BA's and connected my iPhone via the 'Spare' input to the amp.  Power up was uneventful (as in no smoke leaked out, and no strange sounds).  Sound is clean, though not as 'transparent' as either of my PAS / ST-70 combos.  My biggest concern and question is sound level.  In order to get what I would consider a nice sound level, I had to max out the volume control.  So.....

    1 - yes I realize the SCA-35 is rated at 1/2 the power of the ST-70, and I have no idea how the internal preamp gain compares to the gain of a PAS, but would the sound level really be that much lower?

    2 - if no to 1, then where to start looking? (I have no way of checking the 7199's, but do have a good set on the shelf from the ST-70 that was converted to a VTA-70 to try)

    3 - Seeing as this thing is all original, what upgrades should be considered, or is there more value in conserving it as a factory example?  My thoughts are to go with Dave's SCA-35 power supply board, and perhaps his amplifier (PC-10A) boards or a set of Chinese re-pops.  Or, should I conserve the original and just replace the can capacitor?  (I assuming that cap is original and hence a ticking time bomb)

    In some respects wish I had gotten one in worse shape, then there would be no question on whether to update or not.  Don't want to lower the value of the unit should I decide not to keep it.  

    The group collective thought is appreciated.
    It might be a quite normal behaviour.

    What you experience is a lower sensitivity ( the sca-35 needs more voltage ) to
    get full power. And as long as the 7199 works they will perform close to identically,
    there is no need to try to change them.
    Weak el84 tubes _might_ give the effect you describe ( low sensitivity)

    weak/old caps will not give the effect you describe, thus there is no need to
    change power supply parts ( not for this reason)

    If you use vinyl ( both PAS-2/3 and sca35 has simular sensitivity , i would not
    expect any dramatic differences in volume. Can you try that ?

    daveshel

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    Location : Tucson AZ USA

    Re: SCA-35 Questions..

    Post by daveshel on Fri May 06, 2016 4:09 pm

    boeingpilot wrote:

    I'll take it you don't have an issue modifying a factory wired unit.


    None. I like building/modifying/restoring. I don't play on ebay because I don't trust paypal, and I'm not thinking of starting a museum.

    daveshel

    Posts : 148
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: SCA-35 Questions..

    Post by daveshel on Fri May 06, 2016 4:12 pm

    peterh wrote:
    It might be a quite normal behaviour.


    I can't believe having to max the volume would be normal. Especially of the device behaves as expected on the other systems. But you're right about trying vinyl - other sources as well.

    peterh

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    Re: SCA-35 Questions..

    Post by peterh on Fri May 06, 2016 4:29 pm

    daveshel wrote:
    peterh wrote:
    It might be a quite normal behaviour.


    I can't believe having to max the volume would be normal. Especially of the device behaves as expected on the other systems. But you're right about trying vinyl - other sources as well.
    the reason i mentioned vinyl is that it's only the phono input that has any additional amplification. The high level inputs need 1V for full power.

    corndog71

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    Re: SCA-35 Questions..

    Post by corndog71 on Fri May 06, 2016 5:46 pm

    I believe the SCA-35 doesn't actually have a preamp stage except for the phono input. It's otherwise an amp with volume and tone controls. Someone correct me if I'm wrong.

    PeterCapo

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    Re: SCA-35 Questions..

    Post by PeterCapo on Fri May 06, 2016 6:04 pm

    Right. Power amp boards, active low level (phono) board, no active line stage - no line stage board. The line "section" is a passive arrangement of pots and switches strung together.

    boeingpilot

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    Join date : 2015-06-20

    Re: SCA-35 Questions..

    Post by boeingpilot on Fri May 06, 2016 8:06 pm

    PeterCapo wrote:Right.  Power amp boards, active low level (phono) board, no active line stage - no line stage board.  The line "section" is a passive arrangement of pots and switches strung together.

    Ahh..... that may be it.

    I guess let me actually connect my DAC to the SCA-35 and compare rather then just a line cord to the iphone.

    Also probably the better comparison would be to hook my Heresy's up to the SCA


    PeterCapo

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    Re: SCA-35 Questions..

    Post by PeterCapo on Fri May 06, 2016 10:47 pm

    Even though there is no active line section, the circuit provides for enough gain for a "standard" high level source to work.

    I'd try a different high level source and see how you do.  Even without the Heresy's, you should get a decent level of volume without having to turn it all the way up.  Try your source on a different pair of high level inputs, too.

    If you want to keep it original and make sure it is working right, you could get a copy of the manual, look for the table of voltage values and take some readings in the amp.  This could lead to discovering what might be wrong (if there is anything wrong) and then just fix only what is needed.

    boeingpilot

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    Re: SCA-35 Questions..

    Post by boeingpilot on Fri May 06, 2016 11:04 pm

    PeterCapo wrote:Even though there is no active line section, the circuit provides for enough gain for a "standard" high level source to work.

    I'd try a different high level source and see how you do.  Even without the Heresy's, you should get a decent level of volume without having to turn it all the way up.  Try your source on a different pair of high level inputs, too.

    If you want to keep it original and make sure it is working right, you could get a copy of the manual, look for the table of voltage values and take some readings in the amp.  This could lead to discovering what might be wrong (if there is anything wrong) and then just fix only what is needed.

    Some great suggestions! Here's what I found fooling around this evening.

    -- hooked up my DAC to the input instead of the iPhone output. Better, but I could still max the volume and not say it's loud. Connected it to my Heresy's. I can truly understand why people pair low power amps with them. Still not painful volume, but much better.

    -- I think in the short term I will order the SCA EFB power supply board. At the very least I have no idea how old or what condition the quad cap is in, and I think this makes a very logical upgrade. The sound does feel a little muddy and definitely without the head room of the PAS / ST-70 combo. But before I replaced the boards (and ancient components on them) with new, the PAS's sounded a bit like this two.

    -- While I truly like the tubeNirvana stuff, I don't think I can keep the treble and bass controls, and want to keep the operation as stock. So, if I just don't repopulate the original PC-10's, I'll probably just buy the PC-10A's from Dave as well down the line.

    -- Yes time to not be lazy, break out the manual and the DVM and start checking voltages just to make sure it's all the way it should be.

    Thanks to the collective for your input


    PeterCapo

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    Re: SCA-35 Questions..

    Post by PeterCapo on Fri May 06, 2016 11:14 pm

    If you're having a problem with some of the tubes, nothing else really matters.

    If it is all original, it won't sound as clear as your other Dynas having all fresh parts.

    IIRC, the EFB power supply board has its own capacitors, eliminating the original multi-section-style caps.

    When you check the voltages, remember to take into account what your AC mains measures at the wall socket as this will affect the other readings.

    evoroadster

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    Re: SCA-35 Questions..

    Post by evoroadster on Sun May 08, 2016 4:42 pm

    I have an SCA-35 with all new boards via Dave' store where I am not using the PC11 board nor the original selector switch. Hooked up to a pair of Klipsch Forte IIs anything past 1/2 to 2/3 volume is way too loud.

    daveshel

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    Re: SCA-35 Questions..

    Post by daveshel on Sun May 08, 2016 4:53 pm

    evoroadster wrote:I have an SCA-35 with all new boards via Dave' store where I am not using the PC11 board nor the original selector switch.  Hooked up to a pair of Klipsch Forte IIs anything past 1/2 to 2/3 volume is way too loud.

    More what I'd expect. I can't say I've had any hands-on experience with an original SCA-35 since the early 70s, but I've just never seen anything designed so that the volume control has to be at maximum rotation.

    davidpg

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    Re: SCA-35 Questions..

    Post by davidpg on Sun May 08, 2016 10:38 pm

    A few comments:

    1. A frequent complaint about the SCA-35 is that the volume knob has to be turned up higher than expected.  But, this usually means that it has to be turned to about 12:00, and what you are seeing is definitely not normal.

    2. As others have noted, the SCA-35 does not have a line-stage preamplifier, and the volume and tone-controls are "passive", which means that they effectively reduce the signal amplitude.  The power amplifier boards, however, are designed to have more gain than in typical power amplifiers, including the related ST-35, to make up for the lost gain in the control stages.

    3. Something that you might try is to connect the input to the tape-out RCA jacks.  This will send the signal directly to the volume and tone controls, without passing through the old and very complicated selector switch.

    4. Some contact cleaner might help.

    5. Dave Gillespie's EFB modification is an excellent upgrade, and the board he sells is a very good way to implement it.  But, first you should probably figure out the volume problem.

    I hope that some of this is helpful.

    David

    tubes4hifi
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    Re: SCA-35 Questions..

    Post by tubes4hifi on Wed May 18, 2016 6:34 pm

    3 things (all mentioned, I'm just recapping)
    1 - the SCA35 has low sensitivity, and needs a higher input level than other amps
    2 - get the Gillespie EFB board, and the PC10A board (and a pair of 6GH8 tubes)
    3 - the PC11 seems like just a phono board, but it also does some amount of pre-amplification to line input sources, which will make your low output sources like you iPhone, louder

    boeingpilot

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    Re: SCA-35 Questions..

    Post by boeingpilot on Sun May 22, 2016 10:52 pm

    Recap --

    Purchased the SCA35 EFB board. Except for some bill of material issues (obsolete p/n's, substitutions of out of stock items), instructions were outstanding. Not since Heathkit have I seen such clear and direct instructions. If you can solder, you can get this in!

    With the Boston Acoustics (90db-Wm sensitivity) still not good. Maybe a little better. On the Heresy I's (99db-Wm) much much better. Sonics seem much improved. Less muddiness. Still seems to be missing the headroom of the PAS / ST70 combo, but then again I still have the original PC-10 boards in. Still a very listenable combination.

    I did try using the phono in. Volume better, but a fair bit of hum.

    Overall, satisfied (ok, I'm not going to do heavy metal with this setup!). Once I get time and funds, will probably replace the PC-10 boards.

    In any case, comfortable with the fact that the unit now has new capacitors, so I don't have to worry about 35-45 year power caps blowing. Good chance this is going to go with my son to college

    daveshel

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    Re: SCA-35 Questions..

    Post by daveshel on Sun May 22, 2016 11:53 pm

    Maybe to replace the volume pot as well - could be at some point it was replaced with one of higher resistance.

    tubes4hifi
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    Re: SCA-35 Questions..

    Post by tubes4hifi on Mon May 23, 2016 10:20 am

    boeing,
    likely your 7199 tubes are totally worn out, they never were good tubes to start with, and the replacement tubes are worst.
    Buy a new PC10-A board that uses 6GH8A tubes instead (from Gillispie or on ebay)

    daveshel

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    Re: SCA-35 Questions..

    Post by daveshel on Mon May 23, 2016 10:38 am


    boeingpilot

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    Join date : 2015-06-20

    Re: SCA-35 Questions..

    Post by boeingpilot on Mon May 23, 2016 9:13 pm

    tubes4hifi wrote:boeing,
    likely your 7199 tubes are totally worn out, they never were good tubes to start with, and the replacement tubes are worst.
    Buy a new PC10-A board that uses 6GH8A tubes instead (from Gillispie or on ebay)

    I doubt it..... Though a definite possibility seeing as they were marked Dynaco so were probably original (the EL34's are JJ's)

    That said, I did swap for two good Dumont 7199's I had on the shelf. Did not see any appreciable change.

    peterh

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    Re: SCA-35 Questions..

    Post by peterh on Tue May 24, 2016 3:50 am

    boeingpilot wrote:
    tubes4hifi wrote:boeing,
    likely your 7199 tubes are totally worn out, they never were good tubes to start with, and the replacement tubes are worst.
    Buy a new PC10-A board that uses 6GH8A tubes instead (from Gillispie or on ebay)

    I doubt it.....  Though a definite possibility seeing as they were marked Dynaco so were probably original (the EL34's are JJ's)

    That said, I did swap for two good Dumont 7199's I had on the shelf.  Did not see any appreciable change.

    I am quite sure that there is no EL34 in a sca35, instead there is 4 el84.

    boeingpilot

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    Re: SCA-35 Questions..

    Post by boeingpilot on Tue May 24, 2016 6:56 am

    I stand corrected. Late evening posting will incur typos.

    Scott

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