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Thread: Hiwatt power tubes question

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    Hiwatt power tubes question

    A rather rare and unmolested '83 Hiwatt DR504 OL head came in today for a go over from a guy who literally just got it. First thing I found was both fuses were wrong, a 5 amp int the power supply, and a 5 amp in the HT supply. Then I noticed it has the original 1979 British made Gold Lion KT77s in it. I have not had any experience with KT77s. I tested them on my TV-3B/U on the EL34 setting (it doesn't have a KT77 setting), and they both tesyed about the same, just above acceptable conductance. Then I measured dissipation with a Hoffman bias probe, which measures cathode current, and I got 63mA and 59mA at 503v. By my math, that is 31.7 and 39.7 watts. I am unable to find a data sheet on the British made KT77s, but I did find the Russian specs, which probably they copied here:

    https://www.thetubestore.com/lib/the...nalex-kt77.pdf

    Customer wants to know if he should change the tubes or not, and I am unsure of what to tell him. When I measure cathode current x plate voltage, in reference to this datasheet, does that mean 'anode and screen dissapation?' (I swear these sheets with all their nominclature give me fits!) And if so, should I replace these tubes based on what I am seeing? I tend to think these Hiwatts run pretty hot, and I am not sure how these tubes ran when they were new. BTW, a pair of NOS matched made in England tubes fetch ~$700!

    I tried a matched set of Russian Svetlana EL34s in it and got 52/52mA at 506v for 26.3 watts, so they are running stupid hot as well.

    PS, fixed bias, but no adjustment.

    https://hiwatt.org/Schematics/DR504OL.gif

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    Old Timer Leo_Gnardo's Avatar
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    There's almost always a way to adjust bias voltage, even if it means changing out a resistor. Or put a 20K trimpot in series with that 27K resistor. In any case I've used JJ KT77 and had no problems, ordinary wear & tear over the course of years. And New Sensor offers their Gold Lion version if you want to pay a lot more. Push comes to shove, I have an original pair of unused British Genelex Gold Lions, I'd love to see $700 for. Ka-ching!

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    I'm pretty sure those particular gold lion tubes have been re-issued by someone, these are definitely original?

    The Hi-watt bias circuit is weak, and many add a doubler to get the idle current under control.
    This is from Mark Huss' website (https://hiwatt.org):


    Click image for larger version. 

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    While I realize a bias mod could be made, this thing is dead stock and signed inside by Harry Joyce. I was really asking whether these tubes' time is up, or if this amp is designed to run them so hard, and these original tubes have lasted this long.... He has gigged it once, and did not complain about anything other than the bright channel cutting out a couple of times, that I cannot reproduce ( I suspect a cord or something)

    And no, these are not the russian re-issues, these are date coded 7916, and Made in England.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    While I realize a bias mod could be made, this thing is dead stock and signed inside by Harry Joyce. I was really asking whether these tubes' time is up, or if this amp is designed to run them so hard, and these original tubes have lasted this long.... He has gigged it once, and did not complain about anything other than the bright channel cutting out a couple of times, that I cannot reproduce ( I suspect a cord or something)
    What kind of power are you getting? If the amp's obviously lagging in the power department then fresh output tubes will likely put it back to normal. Or if it's sludgy sounding... old tube syndrome. If the tubes are rattling or making other noises, time's up.

    Crustomer will be complaining how much it costs him to play a gig! Others have noted Hiwatt set 'em up hot hot hot. If you're a deep pocket rock star, so what? Kind of like race cars, you wear out tires, engines, transmissions, meh all part of the soup. If you want to save $$$$ and run it sensibly, better put in some bias adjustment. Then you can pick any competent tube including EL34. KT77, though the glass is narrow like a '34, are tetrodes not pentodes. There's a sound difference to be found, especially if operated past the clip point.

    If crustomer is all worried about "it's a collector's item" then he should sell it to a collector, buy an amp that works for him and he'll have plenty of change left over to tip you for your sage advice. I hope!

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    Bent Member Chuck H's Avatar
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    Just a suggestion... Pull the original tubes and replace them with reissue counterparts and save the originals while they're still operational. Consider it protection for the investment. This way you know you are giving your customer a good working amp AND adding value to it by preserving originality (and sold value) should he ever decide to roll it. Makes everyone's life easier if he's willing to spend the $$$ on new tubes. I won't shop at thetubestore (for my own reasons), but this is the first seller that came up. A hundy and your customer can save the old tubes for future sale and YOU don't need to wonder about the suitability of the old tubes.

    https://www.thetubestore.com/genalex...y=2&custcol2=1

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    I'm going to try to sell him on the above bias mod. Chuck makes a good point for pulling the originals to be kept for future sale. Even tho it has KT77s in it, the schematic calls for EL34s. I might try a good used pair of Gold Lion reissue KT66s that I have on hand, tho I don't have much experience with those either.

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    Quote Originally Posted by Chuck H View Post
    Just a suggestion... Pull the original tubes and replace them with reissue counterparts and save the originals while they're still operational. Consider it protection for the investment. This way you know you are giving your customer a good working amp AND adding value to it by preserving originality (and sold value) should he ever decide to roll it. Makes everyone's life easier if he's willing to spend the $$$ on new tubes. I won't shop at thetubestore (for my own reasons), but this is the first seller that came up. A hundy and your customer can save the old tubes for future sale and YOU don't need to wonder about the suitability of the old tubes.
    ^^^ what Chuck said. I do my tube shopping at CE. I'm sure there are other reliable sources, for instance Doug's Tubes on Long Island, showing JJ KT77. And Jim McShane in Chicago - he will probably have the Russian Gold Lions, well tested & matched, he doesn't handle JJ's. Got burnt by some bad ones early in the millenium, & he's still upset. Also, EuroTubes in Portland Oregon, nothing but JJ.

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    Bent Member Chuck H's Avatar
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    The kt66 is the smaller, more frail cousin to the kt77. You'll be taxing them hard with over 500Vp. I've had trouble with any new el34 at those voltages too. Crank it up to clipping and watch the light show.

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    Part of the problem with the bias is that the amp is no longer running on 'period correct' line voltages of that time. That is something that we often overlook when saying 'but it was designed this way!'. Sort of, but we're not usually providing the same parameters as when it was designed (lower line voltage). All operating parameters are affected (plate, bias, heater etc. voltages).
    So good if you can persuade the owner to at least mod the bias to protect his investment.

    Also at the time, EL34 and KT77 were used somewhat interchangeably (along with original 6CA7 skinny bottle type).
    KT66 was used somewhat interchangeably with 6L6 and is more like that, not a good sub here.

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    Supporting Member Randall's Avatar
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    So, is what I am hearing here, stick with KT77s because of the higher plate voltages?

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    If we're talking modern production, I don't believe there would be much difference for voltage rating of KT77, EL34, or 6CA7, spec. wise.
    Others may have recommendations based on failure rates, but I think specs are pretty much the same.
    I would say definitely not KT66 though.

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    OK, no KT66s for you.

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    If you'd still like original KT66 & KT77 data sheets, tubedata.org has them for GEC.No New Sensor branding at top either.

    Justin

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    My preferred tube info search page https://bms.isjtr.ro/search.html
    Don’t even need to hit ‘return’!

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    Quote Originally Posted by pdf64 View Post
    My preferred tube info search page https://bms.isjtr.ro/search.html
    Don’t even need to hit ‘return’!
    I like it

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    The KT66 is described as a "beam pentode" with a max Vp of 500 and a max diss of 25W.

    The kt77 is described as a "beam tetrode" with a max Vp of 800 and a max diss of 28W.

    You have 503Vp at what is already significant current that should be reduced a little (by doing the voltage doubler bias mod). As g1 indicates, this is a common solution to an old amp operating in new conditions. It's a common addendum to many HW circuits because they don't work right otherwise.

    Knowing how the problematic it's been in recent history to load current production big bottles into higher voltage vintage amps, this would be all the information I needed to avoid putting kt66's into that amp. JM2C and YMMV.

    I would probably go with Ruby EL34-B tubes. These are just Ruby selected Shuguang tubes. They're the only ones that held up and sounded good in a recent build of mine with 480Vp.

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    Quote Originally Posted by Chuck H View Post
    You have 503Vp at what is already significant current that should be reduced a little (by doing the voltage doubler bias mod). As g1 indicates, this is a common solution to an old amp operating in new conditions. It's a common addendum to many HW circuits because they don't work right otherwise.
    I dīont know if I understood correctly but the DR504 does not need a voltage doubler. Itīs enough to extend the voltage divider to the good side with an adjustable resistor. It's the DR103 that needs it.
    My advice is to use low-grade EL34 to get some compression. Otherwise they are hard like a stone.
    The low grade EL34 (B-STR - Modern Shuguang) I buy them through TAD selecting lower PC number (relative plate current). I imagine that with Ruby it can also be done.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Pedro Vecino View Post
    I dīont know if I understood correctly but the DR504 does not need a voltage doubler. Itīs enough to extend the voltage divider to the good side with an adjustable resistor. It's the DR103 that needs it.
    You're absolutely right. I didn't look at the schematic and took g1's post at face value. Looking at the schematic I see that the bias supply for the dr504 has room for adjustment in the voltage division. Thank you for the clarification.

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    "I dīont know if I understood correctly but the DR504 does not need a voltage doubler. Itīs enough to extend the voltage divider to the good side with an adjustable resistor."

    Could this be explained further? I am not sure I understand your meaning here.

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    Senior Member Pedro Vecino's Avatar
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    Yes. It appears in the schematics of M. Huss.

    Click image for larger version. 

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    The voltage doubler is only essential in the DR103 (100 watts). Although it also serves the system that Marshall uses in the JCM900. The latter is simpler, with fewer components.
    I dīont have pics of the bias circuit in the DR504 because itīs very simple. In the DR103 the voltage doubler can be quite laborious if you want to do with the original style of the amp. I keep this image as a reference:

    Click image for larger version. 

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    Supporting Member Randall's Avatar
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    Why is the diode changed in the bias mod? Would a 1N4007 do the job?

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    Itīs not necessary. A 1N4007 works perfectly.

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    Customer opted for JJ KT77s, and rated at 800v, works for me. JJ datasheets sometimes get lost on me. Looking at this one, is this a max diss. 25w or 28w?

    https://www.jj-electronic.com/images...s/pdf/kt77.pdf

    I did that bias mod, and man was that a chore. I did it topside so as to not disturb the wiring.

    Click image for larger version. 

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    The plate dissipation is 25W. What the sheet means by that is that the total of plate AND screen dissipation shouldn't exceed 28W total. So any combination where Plate <25W, Screen <6W, AND Plate+Screen <28W is good.

    Justin

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    Supporting Member Randall's Avatar
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    So, when measuring with a socket type bias probe or across a 1 ohm cathode resistor, I am measuring total dissipation, right? Plate and screen?

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    Yes, both plate and screen current run through the cathode.
    If using socket type bias probe, you need to verify it is measuring cathode current, some do measure plate current only.

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    yes, this one breaks the connection at pin 8 and routes it thru your ammeter.

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    Quote Originally Posted by Randall View Post
    Customer opted for JJ KT77s, and rated at 800v, works for me. JJ datasheets sometimes get lost on me. Looking at this one, is this a max diss. 25w or 28w?

    https://www.jj-electronic.com/images...s/pdf/kt77.pdf...
    I think that the Wa,g2 limiting value is intended for UL applications; for this Hiwatt it's the Wa 'plate only' limit of 25W that's relevant.

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