Request: Circuit examples with EL34's pin 1 disconnected?

[Wilder Amplification]

Jon: Is this common? More common than a grid to heater short? Can you see where I'm going with this? Sorry to be a smart arse but I think you may be going a little overboard here.

Who cares how common it is!? The fact that it's POSSIBLE (no matter how common or uncommon of an occurance it may be) should be a reason not to do it. Do all the so called trend analysis you want...there is absolutely no way one can predict the failure mode that a valve will undergo at the end of its service life.

This is why we design things for "worst case scenario".

[g1]

This is why we design things for "worst case scenario".

But where do we stop? Practicality and economy come into play somewhere. Here's an analogy: in a push pull amp with fixed bias, one tube failure killed the bias and I lost 3 other perfectly good tubes. This was due to the fact that the amp didn't have independent bias for each output tube. I have yet to see a guitar amp with such a bias arrangement yet these failures seem fairly common.
I don't think it would be fair to me to say things like "I'm not sure whose bright idea it was to use common bias supplies for fixed bias amps" or "just because Fender did it doesn't make it right" or "bottom line... it's a dumb idea".
Is this so different from what you are saying?
(if your amps have don't use a common bias circuit please ignore the above http://music-electronics-forum.com/i...es/biggrin.png).

[kg]

i find it extraordinarily unlikely that g3 will short out to H in a tube failure, but that's just me.

g3 is really floating in a rather large space between screen and plate. because it's voltage is around ground, power dissipation is low, making overheating and melting/deforming pretty unlikely.

now, g2 is a different story altogether, and may dislodge enough to touch g3 (or g1 for that matter).

[jmaf]

i find it extraordinarily unlikely that g3 will short out to H in a tube failure, but that's just me.

g3 is really floating in a rather large space between screen and plate. because it's voltage is around ground, power dissipation is low, making overheating and melting/deforming pretty unlikely.

now, g2 is a different story altogether, and may dislodge enough to touch g3 (or g1 for that matter).

Is g3 a plate-like structure in true pentodes as well? Or is it a grid? Because I've taken some beam tetrodes apart and g3 isn't a grid at all, unless i'm missing something, it's made of graphite-like material and offers 2 windows from cathode/g1/g2 to the plate.

Right next to the plate structure on this photo I took of a Tung-Sol 6L6STR

[Merlinb]

IF you decide to try B- on the suppressor grids, DO NOT USE THE BIAS SUPPLY AS THE B- SOURCE!!! Derive a 2nd negative voltage source that is dedicated to the suppressor grids.

Or... just connect g3 to the bias supply via a resistor. That way the bias supply can't be shorted out.


Connecting g3 to the bias supply is kinda silly anyway, as it just reduces anode current but increases screen current, which seems cruel on the screens. Considering the same results are acheived simply by biasing the tube a bit colder in the first place, I don't see the point. I suspect this is more a 'trendy' mod than a rational one.

[Merlinb]

Is g3 a plate-like structure in true pentodes as well? Or is it a grid? Because I've taken some beam tetrodes apart and g3 isn't a grid at all, unless i'm missing something, it's made of graphite-like material and offers 2 windows from cathode/g1/g2 to the plate.

g3 is a grid in pentodes, and a plate structure in beam tetrodes. That is, in fact, the fundamental difference between the two types of valve.

(Perhaps it is worth mentioning that in the US many valves that were advertised as pentodes on the data sheet were actually tetrodes under the hood)

[Merlinb]

i find it extraordinarily unlikely that g3 will short out to H in a tube failure, but that's just me.

The short usually occurs on the tube socket itself when it is dirty, rather than inside the tube. If there's B- on g3 then the voltage between those two pins is obviously greater than normal.

[nsubulysses]

I had one of my amps on the bench that was putting out low power. When I looked at it on the scope, I would see a "jump" or "step" in the the output signal level when I reached a certain point in the volume level. It turned out pin 1 on one of the output tubes was accidentally left floating, and when I soldered it, the problem was fixed. Apparently, some charge builds up on the floating element and causes strange behavior. Don't leave it disconnected.

Ah thanks for posting this. I was a little dumbfounded by a V4 I have on my bench now that has EL34s. All power supply voltages checked fine, screen grid, control grid and plate resistors check fine, and all voltages in the phase inverter are normal but it was still only putting out about 75W. Bypassing the preamp with the power amp in jack yielded the same results. I was starting to wonder if the OT was somehow damaged. then I noticed pin 1 was not tied to pin 8 on any of the tube sockets. Now I have them tied together and it puts out 104W before clipping which is way more normal.

1K screen grid resistors were toasty. One was just sitting in the hole in the PCB because the solder has been vaporized......I replaced with 1.5K and jumpered pin 1 to 8 and amp is back to normal. Now I gotta re-bias.......damn.

[Leo_Gnardo]

I noticed pin 1 was not tied to pin 8 on any of the tube sockets. Now I have them tied together and it puts out 104W before clipping which is way more normal.

1K screen grid resistors were toasty. One was just sitting in the hole in the PCB because the solder has been vaporized......I replaced with 1.5K and jumpered pin 1 to 8 and amp is back to normal. Now I gotta re-bias.......damn.

Good moves all around. You caught that pin 1 mistake in time - the EL34's are still working. With G3 left floating the tubes can quit working after a very short life. I found out the hard way.

Maybe you answered this somewhere else but I'm left wondering, maybe Enzo too, are you a submariner, a Cream fan, or both?

[g1]

Maybe you answered this somewhere else but I'm left wondering, maybe Enzo too, are you a submariner, a Cream fan, or both?

See here there: http://music-electronics-forum.com/t36989/#post352065

[nsubulysses]

Good moves all around. You caught that pin 1 mistake in time - the EL34's are still working. With G3 left floating the tubes can quit working after a very short life. I found out the hard way.

I have no idea how long these JJ E34Ls were in the amp. The amp was brought to me by a friend who had the unfortunate tour horror story experience of rolling in an SUV on the highway. Amazingly, all tubes were fine except one 12AX7. The 6FQ7 was missing. No power tubes were broken or shorted although the pin to line up the tube in the socket was broken off on 2. I definitely had to re-tension some tube sockets, but other than that the bad EL34 conversion was the main problem! Rugged old Ampeg.

[g1]

The rubber mount/shock mount chassis system they use is both simple and amazing. All amps should be built that way, not just for reliability, as in this case, but also for microphonics.

[Ronsonic]

Zombie Thread Warning!

I came across this thread because there's an old Carvin VTR 2800 on my bench that was built with G3 floating right from the factory. Sure it seems to have worked but I don't like it. The main problem with this amp is that someone got very confused rebuilding the power supply section years ago.

Another curiosity one variant had a 1/2 power switch that killed the heater supply to two of the power tubes.

[Enzo]

Schematics in owner manual here:
http://www.carvinmuseum.com/pdf/amps/VTR-2800.pdf