4x6V6 output transformer

martin manning · Post by **martin manning** » Sat Mar 28, 2015 5:40 pm

If you are measuring one tube's current at a time the waveform there is something like half-wave rectified AC, which your meter may not handle very well.

turbo5speed · Post by **turbo5speed** » Sat Mar 28, 2015 7:56 pm

martin manning wrote:If you are measuring one tube's current at a time the waveform there is something like half-wave rectified AC, which your meter may not handle very well.

Not sure i understood it very well... are you talking about ripple?

I am aware that the meter can give me a false reading on a half rectified wave but this is not the case. I have the habit of looking at the wave form in my scope and read my meter at the same time on same test point.

Hepcat · Post by **Hepcat** » Sat Mar 28, 2015 10:05 pm

Hmm...if you have 1 ohm 1% 2 watt resistors in series with the cathode to ground, you can measure the resting (no signal) plate + Screen current through the tube. use your digital multimeter on the millivolt scale (in a range to read less than 100 mV). Read the voltage across the 1 ohm resistor and you should see anywhere from 20 - about 50 mv on the meter. It's DC so no ripple to worry about. If your plate voltage is around 410 V DC, 60 to 70 % of the rated plate dissipation is perfectly safe. I run about 440 volts (like a Bassman) and set the bias at about 35 mV which corresponds to 35 mA (ohms law makes this really convenient). that's about 60% of the maximum plate dissipation of a typical JJ 6L6. Tubes last a long time and sound good.

Hepcat · Post by **Hepcat** » Sat Mar 28, 2015 10:27 pm

However, if I've misread your question, you are trying to measure the plate current through each tube. 6V6s at about 420 volts should have the resting current at about 20 mA at about 60% of the plate dissipation. Reading each tube with an RMS type of meter will give you the most accurate (if that's possible) current reading. A trusty old Simpson 260 is good for that. Most digital voltmeters that aren't made by Fluke use a fudged method of measuring RMS current and voltage. My 20 year old Beckman is pretty accurate compared to my friend's much more expensive Fluke. In the end, close is probably good enough. If you are within the safe specs in resting plate dissipation spec'd by the manufacturer, you should be good.

pdf64 · Post by **pdf64** » Sat Mar 28, 2015 11:13 pm

I measured the cathode current at full power in pentode mode (using the 1ohm bias resistors) and i see 66mA with 388Vdc in the plates... this gives me 25.6Watts plate dissipation Shocked what am i missing here again?

The 25.6 watts, if accurate, may be the sum of the plate dissipation and the power delivered to the load.

martin manning · Post by **martin manning** » Sat Mar 28, 2015 11:50 pm

Can you describe exactly what you measured and how you measured it, and how you calculated the plate dissipation?

Hepcat · Post by **Hepcat** » Sun Mar 29, 2015 3:03 am

pdf64, I think you are right.

turbo5speed · Post by **turbo5speed** » Sun Mar 29, 2015 2:12 pm

Everybodys effort to help me is really appreciated.

I've made some more measurements.

Everything was measured using my digital multimeter and with an eye on the waveforms looking for ripple, clipping and what not.

The tubes are biased at 9.3Watts. With the amp idle, i've got 433V on the plates and i read 21,5mV on the cathode resistors.

I then used a signal generator @1Khz and applied a 200mV peak-to-peak to the input of the amp. With a 15ohm dummy load connected to the 16 ohm tap (i know this causes a slight impedance mismatch) i adjusted gain and volumes until the amp starts to clip and then backed up a bit just to clean the waveform. There isn't a trace of crossover distortion or other types of wave distortion. I then measured 395V on the plates, 64mA on the cathodes and i measured the voltage drop on the screen resistors, applied ohms law and came up with 10mA screen current.

Grid 1 has no dc current or is neglectable. If i take out the screen current from the reading on the cathodes i get the anode current wich is 54mA, still very high.

I've been reading everywhere that these are not really 6V6's but relabeled or designed like KT77's. I don't see any red plating just that "blue aura" around the plates.

The amp sounds great and thats a fact but i really want to be sure about this before i wrap this thing up.

pdf64 · Post by **pdf64** » Sun Mar 29, 2015 3:55 pm

i get the anode current wich is 54mA, still very high

Why do you think it is very high (ie what are you comparing it to)?
It is normal for power tubes in class AB to pass more current at full output than under static conditions.
The load on your amp is getting hotter at max output, then cools back down to ambient under static conditions?
The current to do that heating has to come from somewhere.

I've been reading everywhere that these are not really 6V6's but relabeled or designed like KT77

There's a great deal of unfounded nonsense/ opinion stated as fact on the net; it's best not to take 99% too seriously.

Martin Manning's work indicates that the JJ 6L6S, whilst differing a little from some 6V6GT characteristics, is still far closer to 6V6 than other tube types, see http://ampgarage.com/forum/viewtopic.ph ... c&start=60

I then measured 395V on the plates, 64mA on the cathodes and i measured the voltage drop on the screen resistors, applied ohms law and came up with 10mA screen current

It often takes a good meter to give correct readings when there's Vac superimposed on Vdc, eg under signal conditions, it can be better / more reliable to measure the Vdc at the OT CT, rather than the Vp.

martin manning · Post by **martin manning** » Sun Mar 29, 2015 4:13 pm

I think there are a couple of problems with trying to measure dynamic plate dissipation this way: The plate current and plate voltage are 90 degrees out of phase, and the plate current is cut off over part of the cycle, making dissipation zero. Instantaneous dissipation looks something like half of a sine wave with the peaks caved-in. To get an average over time you'd have to integrate the product of Ia and Va-k.

Leo_Gnardo · Post by **Leo_Gnardo** » Sun Mar 29, 2015 4:20 pm

turbo5speed wrote:btw... i have JJ's 6V6S with 422V on the plates. The JJ's are 500V max so i'm ok. Other manufacturers have much lower limits.

Anyone ever tried other brands of 6V6's with such high (or even higher) plate voltages with good results?

JJ 6V6 "tougher than the rest" for now. About 2 years ago I asked Jim Kelley - he's a member here - what he recommended in his amps that run 4x6V6 with 500V - NOS RCA and Sylvania was his answer. I have run used RCA 6V6A's in amps that had high B+ voltages and they work surprisingly well. Lucky I have a box of pulls, can't imagine what sellers want now for NOS RCA's.

From observation the plate power on JJ 6V6 seems to be around 18-19 watts compared to the usual 12-15W for other 6V6. Perhaps this is the source of rumors that it's more 6L6-like, more likely 5881*. So far no customers have complained about JJ 6V6 tone. One customer who thrashes his Deluxe RI with volume set typically at 9 (!) took 4 years to wear out his JJ's. They looked as-new when I removed them but power jumped up about 30% with fresh ones, and he heard his tone improve.

*Just got delivery of a pair of new JJ 5881, same envelope size as their 6V6 but a larger plate inside, so it isn't a re-labeled 6V6S that's for sure.

Post by **Colossal** » Sun Mar 29, 2015 4:31 pm

Leo_Gnardo wrote:JJ 6V6 "tougher than the rest" for now.

I have run JJs at 440-460VDC, no problem. Screens lower of course.

Leo_Gnardo wrote:About 2 years ago I asked Jim Kelley - he's a member here - what he recommended in his amps that run 4x6V6 with 500V - NOS RCA and Sylvania was his answer. I have run used RCA 6V6A's in amps that had high B+ voltages and they work surprisingly well. Lucky I have a box of pulls, can't imagine what sellers want now for NOS RCA's.

Do you know what the screen voltages were under these conditions?

Leo_Gnardo wrote:From observation the plate power on JJ 6V6 seems to be around 18-19 watts compared to the usual 12-15W for other 6V6. Perhaps this is the source of rumors that it's more 6L6-like, more likely 5881*. So far no customers have complained about JJ 6V6 tone. One customer who thrashes his Deluxe RI with volume set typically at 9 (!) took 4 years to wear out his JJ's. They looked as-new when I removed them but power jumped up about 30% with fresh ones, and he heard his tone improve.

Same here. Tubes lost their sparkle (you know they were never the same), but did not fail.

martin manning · Post by **martin manning** » Sun Mar 29, 2015 4:38 pm

Colossal wrote:Tubes lost their sparkle (you know they were never the same), but did not fail.

Kept on truckin' eh?

Post by **Colossal** » Sun Mar 29, 2015 4:40 pm

martin manning wrote:
Colossal wrote:Tubes lost their sparkle (you know they were never the same), but did not fail.
Kept on truckin' eh?

They did indeed. All a friend could say was "ain't it a shame?".

turbo5speed · Post by **turbo5speed** » Sun Mar 29, 2015 6:57 pm

So we all agree that under these conditions the JJ's will perform well but won't last longer than other tubes with more, let's say, "friendly conditions" right?

The truth is that JJ does not provide any usable data about this tube so we can be more sure of ourselves when trying to develop an amp.

I will probably have more questions but so far the amp is sounding great.

https://plus.google.com/107178527420257 ... evLHekTT2o

Bad timing in the playing i know... this was done in a hurry

The Amp Garage

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