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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Post by jeffjmr Sat Dec 03, 2022 4:18 pm

    Time to get serious about new tubes.

    My TriodeUSA ST-70 has all JJ tubes; EF34Ls behind two EF806 and a ECC82 or sometimes I swap it out for a RCA 12AU7A, and a 5AR4.

    It is important to note that I have a powered subwoofer system, so the ST70 is asked to only drive my extremely efficient Altec A5Xs from 80hz and up.

    My major complaint is a lack of dynamic range. My other system is a solid state amp running a set of B&W DM605S2s. High dynamic range source knocks me over with that one. My tube setup sounds compressed across the range in comparison.

    I am likely to replace all 8 tubes, but I’d like to get some advice as to whether a lack of punch is more likely to be determined by the driver tubes, phase splitter, or power tubes, or even the rectifier.

    At this point I am leaning towards KT88s, hoping the reputed additional headroom will get me that punch I’m looking for but thought I’d run this by more experienced tubers before I take the dive.

    Thanks for any advice.
    Jeff
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    Post by peterh Sun Dec 04, 2022 3:43 am

    jeffjmr wrote:Time to get serious about new tubes.

    My TriodeUSA ST-70 has all JJ tubes; EF34Ls behind two EF806 and a ECC82 or sometimes I swap it out for a RCA 12AU7A, and a 5AR4.

    It is important to note that I have a powered subwoofer system, so the ST70 is asked to only drive my extremely efficient Altec A5Xs from 80hz and up.

    My major complaint is a lack of dynamic range. My other system is a solid state amp running a set of B&W DM605S2s. High dynamic range source knocks me over with that one. My tube setup sounds compressed across the range in comparison.

    I am likely to replace all 8 tubes, but I’d like to get some advice as to whether a lack of punch is more likely to be determined by the driver tubes, phase splitter, or power tubes, or even the rectifier.

    At this point I am leaning towards KT88s, hoping the reputed additional headroom will get me that punch I’m looking for but thought I’d run this by more experienced tubers before I take the dive.

    Thanks for any advice.
    Jeff

    You are comparing different speakers. Try your st70 on the BW speakers.

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    Post by jeffjmr Wed Dec 07, 2022 12:06 am

    Thanks Peter. That was a good idea, though the results are inconclusive.

    Subjectively of course, the sense of compression is not as readily apparent with the B&Ws. But the rooms are so different that an A/B comparison is impossible. Moving the B&Ws is plausible but not easy. Moving the Altecs is not an option.

    The original questions, even if just for my education, remain.  Can I expect to experience a dynamic range change with tube replacement? The Tubes are of unknown age as I bought the amp used, and I have put the amp on my bench and see clear distortion beginning at 22W so I think new tubes are indicated anyway.

    I was just curious how various gain tubes in the driver positions may affect dynamic range, and what changes I can expect by moving from EL34s to KT88s. And how will additional headroom, a supposed advantage to moving to the KT88s, affect dynamic range, if any?

    Thanks,
    Jeff

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    Post by peterh Wed Dec 07, 2022 6:16 am

    jeffjmr wrote:Thanks Peter. That was a good idea, though the results are inconclusive.

    Subjectively of course, the sense of compression is not as readily apparent with the B&Ws. But the rooms are so different that an A/B comparison is impossible. Moving the B&Ws is plausible but not easy. Moving the Altecs is not an option.

    The original questions, even if just for my education, remain.  Can I expect to experience a dynamic range change with tube replacement? The Tubes are of unknown age as I bought the amp used, and I have put the amp on my bench and see clear distortion beginning at 22W so I think new tubes are indicated anyway.

    I was just curious how various gain tubes in the driver positions may affect dynamic range, and what changes I can expect by moving from EL34s to KT88s. And how will additional headroom, a supposed advantage to moving to the KT88s, affect dynamic range, if any?

    Thanks,
    Jeff

    If the amp only makes 22w then the EL34 is worn out. Replace. There is no point in
    KT88 ( or 6550 ) as the B+ and output impedance is not changed, it will only cost more.
    Avoid KT77 as they are on the limit as regards to g1 resistor, use EL34.

    Fresh EL34 will give you the power back, no need to replace driver tubes.

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    Post by Brap Thu Dec 08, 2022 9:37 am

    See my pervious post about tube replacement. I did that on my set-up and detail came back immediately. From a cost standpoint, I agree with jeffjmr -- that is your best option. Going to KT-88's may also require internal changes. I would leve he driver tubes alone at first after you change the power tube and see what come back. Then you can start rolling the driver tubes to get additional improvements. If the rectifier is working ok, no sense in changing that right away.

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    Post by jeffjmr Thu Dec 08, 2022 11:18 am

    Thank you both for your feedback.

    I am aware and capable of making the internal changes necessary to move to KT88s, bias circuit mostly, and a phase splitter change from 12AU7 to 12BH7 according to the manufacturer. Your points are well taken, given the EL34s are not only weak but one channel’s pair are not well matched either. My drivers are also not well matched, so I do believe they need to go as well.

    Since you both seem to have a lot of experience on the subject, is there any consensus on brands? Any advantage to the EL34L? Remembering I don’t need low end, which the EL34s supposedly excel at, given my powered subwoofers.

    Jeff
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    Post by peterh Thu Dec 08, 2022 11:24 am

    jeffjmr wrote:Thank you both for your feedback.

    I am aware and capable of making the internal changes necessary to move to KT88s, bias circuit mostly, and a phase splitter change from 12AU7 to 12BH7 according to the manufacturer. Your points are well taken, given the EL34s are not only weak but one channel’s pair are not well matched either. My drivers are also not well matched, so I do believe they need to go as well.

    Since you both seem to have a lot of experience on the subject, is there any consensus on brands? Any advantage to the EL34L? Remembering I don’t need low end, which the EL34s supposedly excel at, given my powered subwoofers.

    Jeff

    12AU7 ??? This is not a dynaco we are discussing. Possibly a tubes4hifi VTA70 ?
    If so the phasesplitter should be a matched triods to reduce dist.
    As for EL34 any brand that follows spec will do, the important thing is that they are matched.
    Also source may be important if any problems occur, serious vendors sells with warranty. Check
    this before purchase.


    Last edited by peterh on Thu Dec 08, 2022 11:25 am; edited 1 time in total (Reason for editing : Spelling)
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    Post by Brap Thu Dec 08, 2022 11:45 am

    Good catch on the 12AU7's peter.
    Regarding my choices for driver tubes, I use Amperex (Holland) vintage 12AU7 for center driver and vintage RCA clear tops for the two sides. I do roll the center and have tried NOS Mulard and Telefunken black diamond. For output tubes, I am a firm believer in Gold Lions. They are my flavor of choice. Recommend using vacuumtubevalues.com as a source. Excellent supplier.
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    Post by Dale Stevens Thu Dec 08, 2022 6:23 pm

    Jeff, I goto Jim Mcshane for tubes. Jim is a member here and super helpful. Mcshanedesign.net
    What is your preamp? The pre is your first stepping stone. Dale

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    Post by New2Tubez Thu Dec 08, 2022 8:38 pm

    I've purchased from McShane on a couple occasions. He's a good guy to deal with.

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    Post by jeffjmr Sat Dec 10, 2022 12:58 am

    Apparently I failed to mention this is a TriodeUSA ST70, likely factory built as the wiring and soldering are very professional looking.

    A 12AU7 is spec for the phase splitter and EF86 for the drivers. Mine came with a JJ ECC83S that was not particularly well balanced so I replaced it with a perfectly balanced (mhos, tested before I had plate current measuring capability) RCA cleartop 12AU7A. The manual calls for the 12AU7 not the 12AX7 (ECC83S), so I made the change but there was not a significant audible difference. The only other meaningful changes I made was the yellow paper diode mod to the rectifier, and an inrush current limiter.

    Hope that makes it a bit clearer.

    Jeff



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    Post by peterh Sat Dec 10, 2022 4:00 am

    jeffjmr wrote:Apparently I failed to mention this is a TriodeUSA ST70, likely factory built as the wiring and soldering are very professional looking.

    A 12AU7 is spec for the phase splitter and EF86 for the drivers. Mine came with a JJ ECC83S that was not particularly well balanced so I replaced it with a perfectly balanced (mhos, tested before I had plate current measuring capability) RCA cleartop 12AU7A. The manual calls for the 12AU7 not the 12AX7 (ECC83S), so I made the change but there was not a significant audible difference. The only other meaningful changes I made was the yellow paper diode mod to the rectifier, and an inrush current limiter.

    Hope that makes it a bit clearer.

    Jeff



    Schematics please ! Otherwise we only guess.
    Replacing 12AU7 with a 12AX7 will cause the tubes working point be totally wrong. In fact it might
    be the cause of missing power in this case ( just guessing)
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    Post by jeffjmr Sat Dec 10, 2022 2:18 pm

    A link to the schematic of a slightly older version than mine is below. Some resistor and capacitor values had changed.

    This model has many options for power tubes as well as driver board tubes, with detailed instructions for required bias circuit changes, etc. Also triode operation is an option.

    Jeff

    https://jeffjmr.smugmug.com/Other/Misc/i-Ssqc5Dz
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    Post by jeffjmr Sat Dec 10, 2022 2:31 pm

    Dale Stevens wrote:Jeff, I goto Jim Mcshane for tubes. Jim is a member here and super helpful.  Mcshanedesign.net
          What is your preamp? The pre is your first stepping stone.  Dale

    Thanks, Dale.

    I'm sorta forced to use my Yamaha RX-V685 as a preamp. It connects to a NHT crossover which feeds two sub amps and subs, and high passes to the ST-70. Can you tell I like to feel my bass? The Altecs roll off sharply below 50-60hz.

    I tried the RX-V685 on the Altecs, which have 12 ohm dividing networks, and it could not provide a reasonable listening level. That led me to the ST-70.

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    Post by tubes4hifi Sat Dec 10, 2022 8:09 pm

    The EF86 tube is an amplifier, the 12AU7s are the phase splitters.
    Don't sub 12AX7 for the driver tubes, the 12AU7 is a better driver tube and the circuit parameters were designed for 12AU7.
    Using RCA clear tops for the driver tubes should give a nice sound.

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    Post by jeffjmr Sat Dec 10, 2022 11:00 pm

    tubes4hifi wrote:The EF86 tube is an amplifier, the 12AU7s are the phase splitters.
    Don't sub 12AX7 for the driver tubes, the 12AU7 is a better driver tube and the circuit parameters were designed for 12AU7.
    Using RCA clear tops for the driver tubes should give a nice sound.

    I’m not sure we are talking about the same amp. My ST-70 uses 2 EF86 tubes, and one 12AU7. Am I confusing the roles of these tubes? I assumed the EF86s were the driver tubes and the 12AU7 the phase splitter.

    Other users of this model ST-70 have used a 12AX7, citing higher gain. In fact, my unit came with an ECC83 that had noticeably (on my scope) output imbalance which led me to the 12AU7. Other users reported better results with the lower gain 12AU7. I didn’t need the gain, but I’m still looking for answers about what affect, if any, different tubes and headroom have on dynamic range.

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    Post by tubes4hifi Sat Dec 10, 2022 11:23 pm

    yes, that's correct, two EF86 amplifiers, and one shared 12AU7 for the phase splitters, according to your schematic
    You may want to try 12BH7 for the phase splitter although I think the RCA clear tops sound better
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    Post by peterh Sun Dec 11, 2022 4:47 am

    I agree with 12AU7 as the correct tube here.

    BTW it seems according to the schematics that g3 on the EF86 is left unconnected, something that
    will increase dist. Check the board, it should be connected to cathode ( pin 8 to pin 3 )

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    Post by jeffjmr Sun Dec 11, 2022 10:40 pm

    peterh wrote:I agree with 12AU7 as the correct tube here.

    BTW it seems according to the schematics that g3 on the EF86 is left unconnected, something that
    will increase dist. Check the board, it should be connected to cathode ( pin 8 to pin 3 )

    There are some differences between the finished amp and that schematic, but the G3 on EF86s is not one of them. Mine are not connected, and I looked at the driver board assembly manual and the driver board PCB diagram shows pin 8 as having no connection.

    That’s a big change counter to the manufacturer’s design so please pardon my caution. What are the potential risks to making this mod?

    Jeff
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    Post by peterh Mon Dec 12, 2022 3:40 am

    jeffjmr wrote:
    peterh wrote:I agree with 12AU7 as the correct tube here.

    BTW it seems according to the schematics that g3 on the EF86 is left unconnected, something that
    will increase dist. Check the board, it should be connected to cathode ( pin 8 to pin 3 )

    There are some differences between the finished amp and that schematic, but the G3 on EF86s is not one of them. Mine are not connected, and I looked at the driver board assembly manual and the driver board PCB diagram shows pin 8 as having no connection.

    That’s a big change counter to the manufacturer’s design so please pardon my caution. What are the potential risks to making this mod?

    Jeff
    Is there no connection between pin 3 and pin 8 on the board ?
    Lower dist is one of the risks of modifying. Without this connection you have a tetrode , with the
    connection it is a pentode. Tetrodes has a kink where it can oscillate during low plate voltage.
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    Post by lorne Mon Dec 12, 2022 9:34 am

    I dunno if I am adding to his conversation or not ... but I'll comment anyway. Back in the late 80's - early 90's I had a stock ST-70 stuffed with 7199's and fat Sylvania EL34's. It had been converted to SS rectification. I was driving big Magna Planars — perhaps ill-advised.


    I don't remember any "compression". I say I had "bloom".

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    Post by jeffjmr Mon Dec 12, 2022 12:40 pm

    [/quote]
    Is there no connection between pin 3 and pin 8 on the board ?
    Lower dist is one of the risks of modifying. Without this connection you have a tetrode , with the
    connection it is a pentode. Tetrodes has a kink where it can oscillate during low plate voltage.
    [/quote]

    There is no connection between pins 3 and 8 on the driver tubes. PCB diagram below, viewed from the top. It appears reversed to indicate markings on bottom side. As you can see, pin 8 has no trace.

    Seems like a major oversight on the manufacturers part. As a matter of academic interest, my unit actually has JJ EF806S tubes in those positions, functional equivalents to EF86.

    https://jeffjmr.smugmug.com/Other/Misc/i-c8N7KJW/A

    Jeff
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    Post by peterh Mon Dec 12, 2022 1:56 pm

    Is there no connection between pin 3 and pin 8 on the board ?
    Lower dist is one of the risks of modifying. Without this connection you have a tetrode , with the
    connection it is a pentode. Tetrodes has a kink where it can oscillate during low plate voltage.
    [/quote]

    There is no connection between pins 3 and 8 on the driver tubes. PCB diagram below, viewed from the top. It appears reversed to indicate markings on bottom side. As you can see, pin 8 has no trace.

    Seems like a major oversight on the manufacturers part. As a matter of academic interest, my unit actually has JJ EF806S tubes in those positions, functional equivalents to EF86.

    https://jeffjmr.smugmug.com/Other/Misc/i-c8N7KJW/A

    Jeff[/quote]
    Then make a wire bridge 3 <> 8
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    Post by jeffjmr Wed Dec 14, 2022 9:08 pm

    Mod made and confirmed. 1.4vdc on both pins 3 and 8.

    Can’t say I noticed an audible change, nor expected, but it didn’t blow up! 🤣

    I haven’t measured the distortion power point in a while so it will be interesting to see if the 22W limit I experienced before has improved, though I’d be surprised if that wasn’t more a function of the power tubes.

    Thanks for the tip, Peter.

    Followup.

    First a correction. The distortion I mentioned occurring at 22W was clipping, not distortion.

    I scoped the amp tonight and confirmed via an A/B comparison a measurable drop in distortion as reported by my K2015THD with the jumper installed between the driver tubes pins 3 and 8. Thank you again Peter. I mention this again in case other TriodeUSA ST70 owners are skeptical about making the mod.

    Clipping now showing at 25W so will be ordering some tubes soon after the holidays.

    Happy Holidays all!
    Jeff
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    Post by MechEngVic Sun Jan 01, 2023 3:30 am

    Nice catch Peter, as always! Jeff keep us updated!

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