pwr tube cathode question

[daz]

I hope. Hammond seems not to post complete specs, but it is one of the newer guitar line for JCM 800's and is 50W with a 3.2k primary. So i guess at 50W it should be fine unless there is another spec thats important.

[pdf64]

Sorry I was under a misapprehension, el34 are 25watt tubes
http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf
There are some suggestions in there for cathode resistor values under various operating conditions, in range 440 to 500 ohms for 430V B+. Peter.

[daz]

Thanks. If i'm reading that right, tho i may well not be, it would seem i'm very close with my 450R reistors, but it says the 1k R is "common to both valves". Should i be using 500R instead of 1k or make one of the 1k's i have common to both tubes and remove the other?

[pdf64]

I think you're looking at the Rg2 (screen grid resistor ) value (table at top of p3), rather than the Rk value (which is 465ohms per el34 in that table)?
The reassuring thing is that you're close to the manufacturer's recommended values, and they wouldn't want tubes to fail within warranty in their own circuits. Peter.

[daz]

I was looking at Rk as the cathode R and figuring Rg2 was the screen because theres an asterick next to Rg2 which below says "screen grid resistor common to both valves".
Anyways, it seems i'm biased correctly so i'll just leave it. I swear, i've never heard a gain tone out of any amp that i wouldn't want to improve....till now 1 i'm serious, and i never thought that was possible. The clean however is not as good as i'd like, but i don't care,(because it plenty good enough) nor do i think it's possible w/o making a second switchable gain structure. ya wanna know what this amp does that really amazes me? it's the only amp i have ever played in my life where the harmonic complexity and squishy feel literally doesn't change from the highest gain setting to almost clean. For years that has been a pet peeve of mine. I always used more gain than i wanted in every amp i've owned because if i turned the gain down to the amount i wanted the tone would love all it's harmonic complexity. thats one thing you find with for example a cranked fender.....you can have a fairly low amount of drive in the tone, yet the squishy feel and richness is there along with a super strong punchiness, and i could never find an amp that would do that using preamp distortion instead of output distortion. Till now that is ! I really think i got stupid lucky after all the months i've spent on this thing.

[tubeswell]

Hi Daz

If the screen grid resistor is a single resistor common to both screens then you need to halve the calculated current draw measured at the resistor to get the screen current for each valve. So where I calculated ~8mA before (I was assuming you had 1k on each screen), it becomes ~4mA (which sounds more realistic BTW), and therefore the plate current is ~59mA (not 55mA). So you dissipation per tube would be about 399V x .059 = 23.5W, which is getting pretty close to 25W, so I wouldn't bother upping it any further if you like it.

[daz]

No, i wasn't saying i have one screen R. I have two. I was saying that in the data sheet Peter linked in his last post showed the conditions with one common screen R. But good to know that anyways, tho i don't know if there is any reason to use just one.

[Steve Conner]

Hi guys,

Re the dissipation issue, the Mullard 5-20 ran its tubes at 400V and 62.5mA, which is 25W exactly, and I've seen one guy who cloned it complain that it eats new manufactured EL34s like they were Pringles.

I just finished restoring an old Crown stereo amp, which I converted to EL34s as part of the process. I was lucky enough to find a set of lightly used Mullard tubes for it (well, two- the other channel has Matsushitas ) and to start with I biased them up to 50mA, at which point the B+ was around 420V. That's only 21W per tube, but even so they got as hot as hell, and smelled a bit.

I ended up backing it off to 45mA (about 18w) because the PTs were getting too hot to touch after it had run all day.

And yes, it's been said before, the idle dissipation has nothing to do with the power output! You can push a pair of EL34s to 100W output with 800V B+, but in that circuit they don't dissipate 50W each at idle.

[Robert M. Martinelli]

As a matter of fact it's becoming more and more difficult to find new power tubes capable to cope with the original specs; as a matter of fact I find myself always biasing current-production EL34s around 17-18W to ensure them a long and trouble-free life ( I wish I could bias myself a little bit lower too )

Regards

Bob

[daz]

Ok, so you guys are saying at 22 watts i will be eating my winged C's regularly? I don't get that because my voltages are right in the range of those similar cathode biased amps i mentioned and they're both using a pair or 270 ohm resistors while i'm using a pair of 450R. granted the transformers aren't the exact same, but still how is this possible. am i going to have to go to 1k cathode R's, because thats what it looks like it will take to get down to 18w.

Steve, was that amp you restored fixed bias or cathode? And wasn't it you that mentioned self adjusting as CB is, it should be fine pretty close to 25w? Or do you think i still need more resistance than 450 per side which would be like 225 shared?

[Bruce / Mission Amps]

As a matter of fact it's becoming more and more difficult to find new power tubes capable to cope with the original specs; as a matter of fact I find myself always biasing current-production EL34s around 17-18W to ensure them a long and trouble-free life ( I wish I could bias myself a little bit lower too )

Regards

Bob

Me too... the EL34 is the one power tube that I do think sounds better running pretty hot idle current. Here is where that 70%-75% idle current rule of thumb seems to make a real positive, tonal difference.

But, with respect to the higher and higher AC mains voltage found here in the USA.. (it used to be 115v-117v but now the averages are 122v-126vac).... I've refurbished many, and I mean many, old VOX EL34 amps and found that the magic number for great VOX EL34 tone vs max tube life is exactly that, 16-18 watts each.
Running those cheaply made imported EL34s higher then that, with high plate voltages developed in vintage AC50/AC100 VOX amps, while running the mains switch or PT in 110v position at 126vac, just seems to wipe out the power tubes in about 30 days of real use.

[daz]

Then what would you do Bruce? Would you go even much higher with the resistors or mess with voltages or what? i want this thing to be reliable 1st and foremost.

[Bruce / Mission Amps]

Ok, so you guys are saying at 22 watts i will be eating my winged C's regularly? I don't get that because my voltages are right in the range of those similar cathode biased amps i mentioned and they're both using a pair or 270 ohm resistors while i'm using a pair of 450R. granted the transformers aren't the exact same, but still how is this possible. am i going to have to go to 1k cathode R's, because thats what it looks like it will take to get down to 18w.

Steve, was that amp you restored fixed bias or cathode? And wasn't it you that mentioned self adjusting as CB is, it should be fine pretty close to 25w? Or do you think i still need more resistance than 450 per side which would be like 225 shared?

No. I think you are OK right where you are because the amp is cathode biased and you stated the actual plate to cathode voltage is under 400vdc.
That is actually a good spot to be with these Svet tubes set up the way you are.
What I'm talking about is fixed bias amps with large grid signal swings and a higher B+ where the plates and screens are seeing about 470vdc-490vdc... or maybe higher in some cases.

[daz]

Thanks. By the way, it is right about at 400 plate to cathode. Usually reads 399.x, so it's right there.