Hiwatt power tubes question

[Randall]

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

[Leo_Gnardo]

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!

[g1]

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):

[Randall]

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.

[Leo_Gnardo]

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!

[Chuck H]

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

[Randall]

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.

[Leo_Gnardo]

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.

[Chuck H]

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.

[g1]

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.

[Randall]

So, is what I am hearing here, stick with KT77s because of the higher plate voltages?

[g1]

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.

[Randall]

OK, no KT66s for you.

[Justin Thomas]

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

Justin

[pdf64]

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