New Build Issues

[dorrisant]

I have a 6V6 ODS build that I'm scratching my head over. One problem that seems intermittent is that one of the (two) power tube plates is higher than the center tap and other plate. I am wondering if I have a bad OT.

The blue and brown OT wires had to be swapped for phase squeal when I fired it up. I have built several of these amps with the same TX set and never had to swap the plate leads before... Weird.

Then, with the amp warmed up and biased (JJ6V6S) at about 70% of 19 watts (each tube), everything seemed fine. Played and sounded fine. Then there was a slight pop. After that, I noticed a trailing fizzy distortion coming up, low volume, as held notes fade out. Happens with OD engaged and without. When I pulled the chassis, I measured all again and found the brown plate wire at 510v while the blue plate wire is at 478v. CT is at about 480v. WTF?!?

How does that happen... More voltage from a plate tap than from the supply?

Oddly, the bias for each tube had been set for roughly 28mA metered across each 1 Ohm resistor. Even with the one plate voltage elevated, both tubes still show a 28mA draw. I would have expected to see the tube at 510v to show less mA draw.

What am I missing here? Have any of you experienced this?

EDITED to clarify with actual numbers from startup notes.

[sluckey]

Probably some hi-freq oscillation going on only when you touch your meter probe to the plate pin. The oscillation fools the meter. When this happens just measure B+ at the center tap and use that number for plate voltages.

[bepone]

I have a 6V6 ODS

Then, with the amp warmed up and biased (JJ6V6S) at about 19 watts,

6V6 tube is small tube with 12W anode dissipation, nowadays i have seen they are labeling it 14W. 19W bias is double than normal. also 470V supply is too much for this tube..remove the power transformer and rebuild it with new PT targeting 400VDC max..

[dorrisant]

Thanks... I was doing that and thought I should post it to verify. The current draw not changing had me thinking the 510v was false somehow.

The oscillations I've battled before we're more gain dependant... Turn down the gain to a certain level and the osc would stop. Does the fizzy distortion as a note trails off relate to the oscillations?

[bepone]

i think you are pushing too much this tube..is possible that you have flashing or arcing inside?
rebias it low, and if the tube is loosing the "control" then you can try to reduce grid leak to some low values, 50-100k and see..

[dorrisant]

I have a 6V6 ODS

Then, with the amp warmed up and biased (JJ6V6S) at about 19 watts,

6V6 tube is small tube with 12W anode dissipation, nowadays i have seen they are labeling it 14W. 19W bias is double than normal. also 470V supply is too much for this tube..remove the power transformer and rebuild it with new PT targeting 400VDC max..

Not a normal 6V6... These are JJ6V6S. Think of a 5881 with a different impedance requirement. They are proven and tested to handle much higher plate voltage and bias. I've read where they have been pushed past 30 watts with no signs of trouble. Also can handle at least 800v for long periods of use. Google can help fill in the details. I have ran them this way (450-470v and biasing as a 25-28 watt tube) for a few years now on a quite a few amps. I have only had one failure from dozens of tubes ran this way. The only failure was one of a quad in a Jim Kelly clone. This happened a few months after the amp was built. That amp is gigged regularly for the last couple of years and the plates are around 440-450v, the bias is higher than a 6V6 should be, but I can't recall exactly what they were set at. I hear what you are saying, but I have no trouble believing that it is not the tubes. Also, since I have built the exact circuit with exactly the same parts (tubes and TX set), so I don't think it is the circuit or the parts selection.

[wpaulvogel]

Actually they are 6v6 but they do tend to handle more voltage. They don’t have the ability to pass the current of a 5881 and aren’t built to do that. This is why they are referred to as 6v6. You’ll notice the cathode tube is much smaller.

[roberto]

Concerning the voltage, it is clearly an oscillation that fakes the instrument.

Is it this tube?
https://www.jj-electronic.com/images/st ... f/6v6s.pdf

It clearly says 14 W as maximum plate dissipation. Even if has been proven that some of them can do more, I wouldn't push them at 140% consistently, also considering that, if you use a low Raa or a speaker with low impedance in the midrange, they will be pushed even further while playing.

On top of that, they need a very low 100 kOhm grid leak in fixed bias, have you considered it?
Something you don't find in modern datasheet and often not even in old ones, is that the higher the bias, the lower the grid leak resistor must be to avoid thermal runuway of the tube. With your setup I'd dc couple the grid to the previous stage to be on the safe side. The other advantage would be that you can run them in AB2, as they are designed to do so (see the curves with g1 above zero).

[bepone]

Not a normal 6V6... These are JJ6V6S. Think of a 5881 with a different impedance requirement. They are proven and tested to handle much higher plate voltage and bias.

this is 6V6 tube from modern production, makes it even more fragile..JJ octals are random quality..
for every builder it is better to be on conservative side for durability , than searching the maximums and then asking "why this happen" .

if you want to explode the tube and find breakdowns ok, nice to read and know
if you want reliable amp then you must be on the safe limits, way below published data.. so i would use JJ 6V6S max to 400V and static biased to 10W max, or lower, cca 9w

[R.G.]

sluckey's guess is probably right - some kind of local oscillation on the "higher voltage" tube. This is where an oscilloscope comes in really handy, and would nail down that issue.

Do you have grid stopper resistors on the power tube grids? A grid stopper as close to the grid pin as you can get it is often effective. This kind of thing is so common with MOSFETs that gate stoppers are pretty much mandatory for any linear applications.

[LOUDthud]

The original V.T.V.M. (Vacuum Tube Volt Meter) had a 10Meg input impedance. They caused so many oscillation problems that a 1Meg resistor was added to the tip of the probe and then calibrated out to retain accuracy. You needed a different probe when you needed to measure resistance.

[dorrisant]

I will look at the output on my scope (down at the moment). Thanks for that direction.

I hate to say it, but some of you have your head in the sand...

https://ampgarage.com/forum/viewtopic.php?t=20883
https://www.thetubestore.com/6v6-tube-r ... goto=jj6v6
https://www.thegearpage.net/board/index ... -1.443601/
https://www.tdpri.com/threads/jj6v6s-an ... le.940383/
https://el34world.com/Forum/index.php?topic=436.0

If you haven't experimented like this with this tube, you can't call it bunk. I have been testing these tubes in crazy situation for several years, https://ampgarage.com/forum/viewtopic.php?f=6&t=29580 .

As you can see from the above, quite a few people, including some trusted members here, have seen some amazing things with these JJ6V6S tubes and thus been motivated to push them beyond the datasheet limits in some cases. I have set them up as 19-20 watt tubes for a long time. Are all of us coincidentally out of our minds?

That being said... I am nearly 99.9% sure this is not a tube or "how this tube is biased" issue. I thank you for your concern, kindly. I will get this amp back and bet that the problem still exists, even with a new set of JJs and the bias set as a 14w tube. Trust me when I type it, I have built this exact circuit enough to know it. It's not the power tubes or the bias.

I am not convinced that it might not be an OT issue. I have never had to swap the plate leads due to feedback squeal... same TX set (MM), same circuit, same tubes. I am not trying to piss anyone off, just sharing what I know and why it may be relevant.

[dorrisant]

sluckey's guess is probably right - some kind of local oscillation on the "higher voltage" tube. This is where an oscilloscope comes in really handy, and would nail down that issue.

Do you have grid stopper resistors on the power tube grids? A grid stopper as close to the grid pin as you can get it is often effective. This kind of thing is so common with MOSFETs that gate stoppers are pretty much mandatory for any linear applications.

3k3 grid stoppers, installed between pin 1 & 5 at each power tube socket.

[roberto]

Hi dorrisand,

I didn't correlate the issue to the bias, I talked about oscillation in the first line of my first post.

Then I just said what I'd suggest to do with that tube, and underlined an aspect that is no more written in any datasheet.
Before being offensive, be sure to have understood what others have written, please.

[bepone]

i'm on roberto's side.. dorrisant you are pushing too much the tube, this is not 6L6 .. when you are pushing so much the tube you need to know mechanisms how to compensate..if anode voltge is high, g2 voltage must be low. so in your case try to kill 100V on g2 with R-C. but usually better is to have another winding for this. or small transformer and rectifier for g2.

allready is in the topic, you need also to lower your Rg. grid leaks to 50-100k. from anodes you need to make divider to the ground , 1M+1M+100k, and then you can measure voltage on 100k. multiply by ratio.

with the scope is the same method because you will burn the probe measured amp on anode directly (peak voltage in action VDC+VAC= 800-900V).

good luck