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    Starting first restore

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    ttownscott

    Posts : 27
    Join date : 2018-07-31
    Age : 54
    Location : Tacoma, WA

    Re: Starting first restore

    Post by ttownscott on Thu Sep 13, 2018 1:43 am

    This evening I went through the Dynaco assembly instructions all over again looking for anything I missed. I found one thing. There is a wire from switch 3 of the Stereo/Mono switch to the center of the right input jack. I missed that on the build.

    But I don't think that was the hum problem. I brought my bookshelf speakers in from my listening room to my work area and tested the amp by playing an mp3 through my smart phone to the amp input. No noise. Maybe what I have heard as described as power transformer noise if I really really listen, but basically quiet. So I set the bias and listened to a few songs to see if any issues crop up. None did.

    Feeling confident that I had the hum issue resolved, I put the amp back in my stereo.

    The hum came back.

    Hmm, what is different.
    1. Plugged in to a different plug via power stip
    2. different speaker cable
    3. different input cable.

    The easiest to test was the input cable, so I tried the phone to RCA input to the amp and it was quiet again. Evidently either my preamp (Acurus LS-11) or input cable is the cause of the noise. The Acurus has just been re-built and the system was dead silent before, so I don't think that is the cause. The cables are a new buy from eBay. They have right angle RCA jacks which I thought would be good for the front of the Dynaco.

    Tomorrow I'll try another set of RCA cables. My original ones are too short to reach the front of the Dynaco.

    Anyway, I'm pretty sure that's it. I'll let you all know tomorrow.
    Very Happy
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    Peter W.

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    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Starting first restore

    Post by Peter W. on Thu Sep 13, 2018 7:24 am

    Cables are inscrutable. Meaning that whereas boutique cables are (in my opinion) generally a waste of money, the key to any cable is how well it is made. After the most basic electrical requirements are met, for the most part, what they are made from does not matter. As an extreme example, if the run is short enough, coat hanger wire would make perfectly acceptable speaker wire - assuming clean ends and tight connections.
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    peterh

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    Location : gothenburg, sweden

    Re: Starting first restore

    Post by peterh on Thu Sep 13, 2018 8:39 am

    Is the preamp AND st70 both grounded via 3 prong cables ?
    An additional cable, connected in cover screws could be tested.
    Grounding stuff with the signal cables only is a way to introduce hum.
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    Peter W.

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    Location : Melrose Park, PA

    Re: Starting first restore

    Post by Peter W. on Thu Sep 13, 2018 9:00 am

    peterh wrote:Is the preamp AND st70 both grounded via 3 prong cables ?
    An additional cable, connected in cover screws could be tested.
    Grounding stuff with the signal cables only is a way to introduce hum.

    Without being the slightest bit snarky, I am of the opinion that Europeans, what with IEC mandated grounded line-cords simply do not understand the potential (pun intended) issues with US grounded receptacles.

    May I suggest, with all due respect, that you not opine on US systems unless/until you have a better understanding of these issues. For the most part, multiple equipment line-cord grounds, unless very carefully managed and *designed into* the equipment are a bad idea, at least in the US.

    https://en.wikipedia.org/wiki/File:TT-earthing.svg is what we use here in the US.   Neutral and ground are bonded in the panel and so there is very nearly always some potential to-ground for obvious reasons. And a bad neutral, while not ideal, is covered by the local ground. But, this is why grounds must be so carefully managed.

    From my understanding, you use (mostly) this:    https://en.wikipedia.org/wiki/File:TN-C-S-earthing.svg Please correct me if this is a false impression.

    Accordingly, local differences in potential are less of an issue for you, whereas a bad neutral is not good at all. So, there are very real functional differences in how things work.


    Last edited by Peter W. on Thu Sep 13, 2018 9:06 am; edited 1 time in total (Reason for editing : Get the links to work - and grammar.)
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    peterh

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    Location : gothenburg, sweden

    Re: Starting first restore

    Post by peterh on Thu Sep 13, 2018 12:00 pm

    Peter W. wrote:
    peterh wrote:Is the preamp AND st70 both grounded via 3 prong cables ?
    An additional cable, connected in cover screws could be tested.
    Grounding stuff with the signal cables only is a way to introduce hum.

    Without being the slightest bit snarky, I am of the opinion that Europeans, what with IEC mandated grounded line-cords simply do not understand the potential (pun intended) issues with US grounded receptacles.

    May I suggest, with all due respect, that you not opine on US systems unless/until you have a better understanding of these issues. For the most part, multiple equipment line-cord grounds, unless very carefully managed and *designed into* the equipment are a bad idea, at least in the US.

    https://en.wikipedia.org/wiki/File:TT-earthing.svg is what we use here in the US.   Neutral and ground are bonded in the panel and so there is very nearly always some potential to-ground for obvious reasons. And a bad neutral, while not ideal, is covered by the local ground. But, this is why grounds must be so carefully managed.

    From my understanding, you use (mostly) this:    https://en.wikipedia.org/wiki/File:TN-C-S-earthing.svg   Please correct me if this is a false impression.

    Accordingly, local differences in potential are less of an issue for you, whereas a bad neutral is not good at all. So, there are very real functional differences in how things work.

    Equipment in europe has both incoming AC lines isolated from ground, all AC is
    "floating" as regards to chassies and or signal ground. No special treatment
    of "live" or "neutral" is done, all wires are considered "live" for security purposes.
    The ground cable in the outlet that has them is not connected to either
    wire but to a ground post in the electric distribution center.

    As for the equipments, signal ground and chassie ground may be separated
    by some amount, but neither is allowed to be connected to any mains AC line.

    Interconnecting equipment by the groundline is common practice and will
    normally prevent hum issues due to leakage of AC internally. Additional
    interconnects of equipment done in industrial racking or with external
    "grounding cables" helps to prevent ground loops.

    I am not aware of US equipment or AC distribution methods. But apart
    from "death caps" used in some older stuff i do think AC lines are
    floating relative chassies and signal grounds.

    Leaving units floating with no ground whatsoever will leave them humming
    at half mains voltage, some units may leak more to one ac-conductor
    then the other, creating a weak but real hum-loop. This is easily observed
    by slight touching with a dry finger, any "humming" will be an indication
    of a floating amp.

    As we also have a lethal AC in europe ( 230V ) it's even more importent to
    make sure that chassies are not only isolated but also grounded to prevent
    accidents even if faults occurs in the equipments.

    Do interconnect your amps chassies. This will normally decrease hum, any
    residual hum is due to other reasons. And its easy to implement ( or remove)

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    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Starting first restore

    Post by Peter W. on Thu Sep 13, 2018 12:30 pm

    Some snippage, some interpolations.

    peterh wrote:
    Equipment in europe has both incoming AC lines isolated from ground, all AC is
    "floating" as regards to chassies and or signal ground.  No special treatment
    of "live" or "neutral" is done, all wires are considered "live" for security purposes.
    The ground cable in the outlet that has them is not connected to either
    wire but to a ground post in the electric distribution center.

    Understood. In such a case, a bad neutral will lead to havoc with the entire live circuit above-ground thereby. On this side of the pond, ground and neutral are bonded in the panel, and local grounds are required.

    As for the equipments, signal ground and chassie ground  may be separated
    by some amount, but neither is allowed to be connected to any mains AC line.

    Also understood. But, component failure could lead to mains-potential on the signal lines given that Euro systems have what amounts to an isolated ground system - not a local ground.

    Interconnecting equipment by the groundline is common practice and will
    normally prevent hum issues due to leakage of AC internally. Additional
    interconnects of equipment done in industrial racking or with external
    "grounding cables" helps to prevent ground loops.

    Yes, and here is the biggest difference - under the system you describe, there is, ultimately, a single ground option which, perforce is at a single potential. So, loop grounding vs. star grounding is less of an issue than here. In the US, no two receptacles will have exactly the same ground potential, nor will the neutral necessarily be at the same potential as the ground. The difference may be in millivolts, but it is real enough to cause all sorts of hum issues THAT ARE NOT DUE TO LEAKAGE.

    I am not aware of US equipment or AC distribution methods. But apart
    from "death caps" used in some older stuff i do think AC lines are
    floating relative chassies and signal grounds.

    Death Caps is an apt descriptive, but largely replaced these days by properly rated caps. Only the hot is isolated, as noted before. Just as an example: We have a central ground-rod our incoming service. We also ground to the water utility line (cast iron, 2 meters underground, 100' to a ductile-iron main. Our service (overhead triplex aluminum) consists of two hots (120/240) and a neutral. The transformer is isolated but the shell is grounded. and it is fed by 13,200 VAC single-phase power. The neutral is also overhead, and grounded at each transformer, bonded to ground in our panel, and bonded to the local ground rod and water-service as well.

    Leaving units floating with no ground whatsoever will leave them humming
    at half mains voltage, some units may leak more to one ac-conductor
    then the other, creating a weak but real hum-loop. This is easily observed
    by slight touching with a dry finger, any "humming" will be an indication
    of a floating amp.

    Only if there is leakage. But, in the US, as there are multiple ground options, and as grounding is also local, picking a *SINGLE* ground to bleed off any leakage will effectively eliminate ground loops and the associated hum. having two separate ground options will create a loop. Why it is that Star-Grounding or single-ended series grounding is typical here. If you look at most US designs, the signal ground and the chassis ground are one-and-the-same. Exceptions include head and phono amps, but even then not in every case. This puts _ALL_ equipment grounds at equal potential within the system if properly laid out. Where things go bad is when the cable shields doing the grounding from A to B are usurped by an extraneous ground. Or these shields are damaged, or not making good contact, or any of a dozen other ways they could fail. But the idea is to have them NOT fail, not to add more grounds.

    As  we also have a lethal AC in europe ( 230V ) it's even more importent to
    make sure that chassies are not only isolated but also grounded to prevent
    accidents even if faults occurs in the equipments.

    Do interconnect your amps  chassies. This will normally decrease hum, any
    residual hum is due to other reasons. And its easy to implement ( or remove)

    Each component in my systems are all grounded to a single ground in a single-ended series-ground configuration. The exceptions to this are the turntables, where the chassis grounds are sent to the left phono headshell. I do not have hum problems.

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    peterh

    Posts : 1054
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Starting first restore

    Post by peterh on Thu Sep 13, 2018 12:46 pm

    Peter W. wrote:Some snippage, some interpolations.

    peterh wrote:
    Equipment in europe has both incoming AC lines isolated from ground, all AC is
    "floating" as regards to chassies and or signal ground.  No special treatment
    of "live" or "neutral" is done, all wires are considered "live" for security purposes.
    The ground cable in the outlet that has them is not connected to either
    wire but to a ground post in the electric distribution center.

    Understood. In such a case, a bad neutral will lead to havoc with the entire live circuit above-ground thereby. On this side of the pond, ground and neutral are bonded in the panel, and local grounds are required.

    As for the equipments, signal ground and chassie ground  may be separated
    by some amount, but neither is allowed to be connected to any mains AC line.

    Also understood. But, component failure could lead to mains-potential on the signal lines given that Euro systems have what amounts to an isolated ground system - not a local ground.

    Interconnecting equipment by the groundline is common practice and will
    normally prevent hum issues due to leakage of AC internally. Additional
    interconnects of equipment done in industrial racking or with external
    "grounding cables" helps to prevent ground loops.

    Yes, and here is the biggest difference - under the system you describe, there is, ultimately, a single ground option which, perforce is at a single potential. So, loop grounding vs. star grounding is less of an issue than here. In the US, no two receptacles will  have exactly the same ground potential, nor will the neutral necessarily be at the same potential as the ground. The difference may be in millivolts, but it is real enough to cause all sorts of hum issues THAT ARE NOT DUE TO LEAKAGE.

    I am not aware of US equipment or AC distribution methods. But apart
    from "death caps" used in some older stuff i do think AC lines are
    floating relative chassies and signal grounds.

    Death Caps is an apt descriptive, but largely replaced these days by properly rated caps. Only the hot is isolated, as noted before. Just as an example: We have a central ground-rod our incoming service. We also ground to the water utility line (cast iron, 2 meters underground, 100' to a ductile-iron main. Our service (overhead triplex aluminum) consists of two hots (120/240) and a neutral. The transformer is isolated but the shell is grounded. and it is fed by 13,200 VAC single-phase power. The neutral is also overhead, and grounded at each transformer, bonded to ground in our panel, and bonded to the local ground rod and water-service as well.  

    Leaving units floating with no ground whatsoever will leave them humming
    at half mains voltage, some units may leak more to one ac-conductor
    then the other, creating a weak but real hum-loop. This is easily observed
    by slight touching with a dry finger, any "humming" will be an indication
    of a floating amp.

    Only if there is leakage. But, in the US, as there are multiple ground options, and as grounding is also local, picking a *SINGLE* ground to bleed off any leakage will effectively eliminate ground loops and the associated hum. having two separate ground options will create a loop. Why it is that Star-Grounding or single-ended series grounding is typical here. If you look at most US designs, the signal ground and the chassis ground are one-and-the-same. Exceptions include head and phono amps, but even then not in every case. This puts _ALL_ equipment grounds at equal potential within the system if properly laid out. Where things go bad is when the cable shields doing the grounding from A to B are usurped by an extraneous ground. Or these shields are damaged, or not making good contact, or any of a dozen other ways they could fail. But the idea is to have them NOT fail, not to add more grounds.  

    As  we also have a lethal AC in europe ( 230V ) it's even more importent to
    make sure that chassies are not only isolated but also grounded to prevent
    accidents even if faults occurs in the equipments.

    Do interconnect your amps  chassies. This will normally decrease hum, any
    residual hum is due to other reasons. And its easy to implement ( or remove)

    Each component in my systems are all grounded to a single ground in a single-ended series-ground configuration. The exceptions to this are the turntables, where the chassis grounds are sent to the left phono headshell. I do not have hum problems.

    I do not see any discrepancy in our interconnects.
    As for the turntable, it consists om wood, only the tonearm is metal, and grounded
    via the signal cable.

    I assume that if in US several boxes are grounded via 3-prong from different outlets
    there might be hum-problems ?
    Isn't the solution here to use a distribution panel connected to a single outlet ?

    We also have hum in the form of electro-magnetic fields in our homes, they
    might be strong enough to cause audiable hum, especially if the AC
    is deformed by  phase-regulated thyristor equipments. Grounding won't help here.


    Last edited by peterh on Thu Sep 13, 2018 1:47 pm; edited 1 time in total (Reason for editing : Added middle section of message.)
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    ttownscott

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    Location : Tacoma, WA

    Re: Starting first restore

    Post by ttownscott on Fri Sep 14, 2018 12:41 am

    To follow up, I did have another pair of interconnect cables. Monoprice cables.
    First I tried them with some right angle adapters and some hum. Less than before, but more than in my work area.
    Then I tried them without the right angle adapters thinking that maybe there is a magnetic effect having both right angle adapters so close to each other. This reduced the hum. I can still hear it with my ear to the amp but it is less. I think at this time it is just a cable effect. The original cable and the Monoprice cable are both pretty big thick cables. The one I tested from the headphone jack was thinner. The cable I used with the Hafler was also thinner, but too short to reach the front of the Dynaco.

    In my house, an electric guitar has hum on the 2nd floor where my listening room is, but is quiet on the first floor. Same guitar, cable and amp.

    Regarding plugs, the Dynaco is two prong. The preamp is a grounded plug. That did not seem to be an issue with the Hafler though. It also has a two prong plug.
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Fri Sep 14, 2018 11:14 pm

    I got it working without hum in my main system.
    I tried lifting the ground on my pre-amp using an 3 to 2 prong adapter. That made no change.

    What made a change was finding an old monster cable RCA cable in a box and using it. No noise.

    Here she is Hum free and Rocking away:
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    dmagazz

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    Re: Starting first restore

    Post by dmagazz on Sat Sep 15, 2018 6:08 pm

    so it was a bad cable?
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Sun Sep 16, 2018 2:35 pm

    Yes. It is quiet now that I have changed out the cable.
    After the one in the picture, I picked up a shielded cable on Amazon that two people on another forum said eliminated their hum. It further reduced the hum on my amp as well. It is totally gone now.

    https://www.amazon.com/gp/product/B016QVZEZM/

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    peterh

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    Location : gothenburg, sweden

    Re: Starting first restore

    Post by peterh on Sun Sep 16, 2018 2:46 pm

    ttownscott wrote:Yes. It is quiet now that I have changed out the cable.
    After the one in the picture, I picked up a shielded cable on Amazon that two people on another forum said eliminated their hum. It further reduced the hum on my amp as well. It is totally gone now.

    https://www.amazon.com/gp/product/B016QVZEZM/

    Probably less resistance in the shield, interconnecting the amps grounds more efficient.

    Glad your problems are solved.
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    deepee99

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    Re: Starting first restore

    Post by deepee99 on Thu Sep 20, 2018 9:31 pm

    Interconnect (RCA-type) cable ends and the input jacks should be cleaned at least once a year (or weekly on some Hafler gear, which used cheap and corrosive aluminum for the jacks) if you want to stay ahead of hum as well.
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Sat Oct 13, 2018 4:40 pm

    Now that I have been living with it for a month, I'm wondering about what would be a good upgrade.
    I have two pairs of 12AU7 type tubes, so I was of course thinking to pick up the VTA board probably with a cap upgrade.

    Not sure if upgrading the PA060 and possibly the choke would buy me any performance increase.

    another thought is to see if it sells for say $800 and build the vta-70. It would be fresh, but I would have to buy output tubes and maybe a cage.

    As it is now, I am happy with the chassis and the nice but vintage look to my cage. Also the triode swich is not a big appeal to me. Trying KT88's after the EL34's wear out sounds like something I would try.
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    corndog71

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    Re: Starting first restore

    Post by corndog71 on Tue Oct 16, 2018 11:29 am

    ttownscott wrote:Now that I have been living with it for a month, I'm wondering about what would be a good upgrade.
    I have two pairs of 12AU7 type tubes, so I was of course thinking to pick up the VTA board probably with a cap upgrade.

    Not sure if upgrading the PA060 and possibly the choke would buy me any performance increase.

    another thought is to see if it sells for say $800 and build the vta-70. It would be fresh, but I would have to buy output tubes and maybe a cage.

    As it is now, I am happy with the chassis and the nice but vintage look to my cage. Also the triode swich is not a big appeal to me. Trying KT88's after the EL34's wear out sounds like something I would try.

    You need the upgraded power transformer if you plan to use KT88's. The upgraded choke might not be necessary but it does give breathing room and it's not too costly. The only other upgrade option besides tubes would be the output caps.
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    Peter W.

    Posts : 965
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    Location : Melrose Park, PA

    Re: Starting first restore

    Post by Peter W. on Tue Oct 16, 2018 12:03 pm

    ttownscott wrote:Now that I have been living with it for a month, I'm wondering about what would be a good upgrade.
    I have two pairs of 12AU7 type tubes, so I was of course thinking to pick up the VTA board probably with a cap upgrade.

    Not sure if upgrading the PA060 and possibly the choke would buy me any performance increase.

    another thought is to see if it sells for say $800 and build the vta-70. It would be fresh, but I would have to buy output tubes and maybe a cage.

    As it is now, I am happy with the chassis and the nice but vintage look to my cage. Also the triode swich is not a big appeal to me. Trying KT88's after the EL34's wear out sounds like something I would try.

    Some basics:

    a) The OEM 70 is not the proper platform for anything bigger than the 6CA7/KT77 family of tubes. For that, go to the VTA. or a pair of monoblock M125s.
    b) With that in mind, there is no need to upgrade the OEM transformer if your restoration is effective and the amp is operating properly.
    c) Given that tube amps clip softly, unless you have extremely inefficient speakers, the difference between the stock 35 watts and 75 watts is perhaps 4 dB in any case.

    This comes down to horses for courses. The OEM 70 platform is a good start for many mods and many mods have been developed for it over the years. Writing only for myself, and with a box full of 7199 tubes, I find the OEM design perfectly acceptable. Others do not, and it is their privilege and pleasure to roll tubes and designs as they wish. But substantially up-sizing (I am *NOT* writing "up-grading") the output tubes is not a good choice.

    You might experiment with the 6L6/KT66/5881 family of tubes and see how that works for you. There are those that swear by (or at) the 5881.
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    PeterCapo

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    Re: Starting first restore

    Post by PeterCapo on Tue Oct 16, 2018 12:31 pm

    Peter W. wrote:a) The OEM 70 is not the proper platform for anything bigger than the 6CA7/KT77 family of tubes. For that, go to the VTA. or a pair of monoblock M125s.
    I'll second that.  The original Stereo 70 was not intended for use with 6550/KT88.  

    ttownscott wrote:Now that I have been living with it for a month, I'm wondering about what would be a good upgrade.

    For a change of pace, try the EH 6CA7 that has a solid reputation and that is said to sound less like a typical EL34.  The Gold Lion KT77 also has a different presentation and is highly regarded.  Changing to a different brand of driver tube could also be worthwhile.

    You might also try a different diode for the bias supply.  Much is said of the Cree Z-Rec SiC Schottky diodes, like this one: Mouser 941-CSD01060A https://www.mouser.com/ProductDetail/Wolfspeed-Cree/CSD01060A?qs=D3QG1iK7FqhfQT7VqjsiJw%3d%3d   I used one like it in my Stereo 70.  If you decide to try one, get these, too: Mouser # 532-577002B00 and Mouser # 532-4880SG.

    You could also try polypropylene bypasses on each section of the quad cap https://www.mouser.com/ProductDetail/Panasonic/ECW-FA2J104J?qs=4aNf6TG8tj9fV9T4aVwdMg%3d%3d

    The original design was/is excellent.  Suggest exploring options for bringing out more of its potential before scrapping the driver or dumping the whole amp.
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Tue Oct 16, 2018 2:40 pm

    Thanks guys!

    The more I think about it, the more I do want to keep it closer to the original design. It's not my only amp, so I have variety. I've also got a Hafler DH200 and ST150 solid state amps and am restoring a Motorola 3 channel vibrasonic console with 6v6 tubes.

    I have two sets of speakers that I have listened to them through. One is Kef Q100 which are 86db efficient but are nice sounding. The other are towers I built and are 96db efficient.

    Maybe try other driver tubes that work within the stock ST70 design like the 6L6/KT66/5881/6C87/KT77
    The EH 6C87 sounds really interesting!

    Also, since the driver board for the 6GH8A's was pre-populated with Mallory tubular metalized polyester coupling caps, I have received the suggestion to just go back and upgrade the caps on the original board and try to find some 7199's

    One thing I did was order a bucking transformer to reduce the line voltage.
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    PeterCapo

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    Re: Starting first restore

    Post by PeterCapo on Tue Oct 16, 2018 2:47 pm

    If those are six polyester coupling caps on your PC board, changing them to polypropylene might result in an audible improvement.

    This one should do for the 0.1µF capacitors: Mouser # 75-MKP1839410404 https://www.mouser.com/ProductDetail/Vishay-Roederstein/MKP1839410404?qs=sGAEpiMZZMv1cc3ydrPrF9JX2l62Mr4rYo%2fZc%2flEIBY%3d

    For the 0.05µF, there are a variety of radial leaded 0.047µF polypropylene capacitors that should do.  If you measure the distance between the solder pads, you can see if one of the box style (like Wima) will fit.  If not, then there are others, like the Panasonic polypropylenes, with formable wire leads.

    You could also spend more on boutique style capacitors, if you like.

    If you switch to a 6L6 variant, you might not be able to bias the amp without changing the resistors in the bias supply.  Substituting a 5U4 rectifier might solve this without changing the bias supply resistors, but total power output will decrease and you'll lose the slow startup of the 5AR4.  This is what I experienced when I tried 6L6, though there are some variables that might make your experience a little different.  In any case, assuming you can get it to bias okay, it may still play loudly enough for you and you might like the sound quality with a 6L6 variant.
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    ttownscott

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    Re: Starting first restore

    Post by ttownscott on Mon Oct 29, 2018 12:11 am

    I decided to order the dynakitparts board and resistors. I was going to order the resistors separately, but it worked out the same as ordering their full kit. Ebay had a 1 day 15% off promo, so I ordered some green 500v Russion PIO caps.

    So I'm in the process of assembling the stock board with newer and better components than original.

    I also ordered an Emotiva CMX-2 that should remove dc voltage from my line. I think that will remove the last remaining mechanical hum in my transformer. If not, Amazon has a good return policy so I will send it back.

    By the way, I researched my EL34's. I think they are probably as good as they get so I want to wait and hear them with the new board using the PIO's and 7199's. They're Dec 1996. Looking through the forum, it appears that with newer (than the 50's) tubes it is better to bias low than 1.56? I set my bias at 1.3.

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    PeterCapo

    Posts : 561
    Join date : 2008-12-05

    Re: Starting first restore

    Post by PeterCapo on Mon Oct 29, 2018 11:07 am

    Your St. Petersburg EL34s were known for a terrific midrange. With those tubes and the green capacitors on the PC board, the sound may tend toward the warmer side vs. some polypropylene capacitors and brighter sounding tubes like KT77 or EH EL34.  If it is too warm or too rolled off on top for you, it may be brightened with different tubes and perhaps different coupling caps.

    Decreasing the bias will increase distortion.  The question is whether it is audible or not.  You might want to experiment.

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    Re: Starting first restore

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