[[email protected][_2_]]

How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!), which is the
value of the plate supply feeding the 12AX7 stages before the EL34.
The amp is very simple: 2 typical 12AX7 gain stages, one 12AX7 cathode-
coupled phase splitter, then the EL34s in push-pull, transformer
coupled output. I'm wondering whether it's a bad tube, or I'm
exceeding maximum values.

Thanks!

Sean B

[Mike Rivers]

On Jan 11, 10:11 am, "
wrote:
How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!),

That's not good. It could be that the tube has an internal short and
needs to be replace. But you should get out your multimeter and
schematic and start doing some troubleshooting, or else take it to a
shop. Nobody can tell you what's wrong based on that general symptom.

[david gourley]

" put forth the
notion in...news:7c13b613-21be-4990-a378-cbee13ba8cc9
@r15g2000prh.googlegroups.com:

How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!), which is the
value of the plate supply feeding the 12AX7 stages before the EL34.
The amp is very simple: 2 typical 12AX7 gain stages, one 12AX7 cathode-
coupled phase splitter, then the EL34s in push-pull, transformer
coupled output. I'm wondering whether it's a bad tube, or I'm
exceeding maximum values.

Thanks!

Sean B

At first glance, this sounds like a leaking coupling cap from the previous
stage - I've had lots of guitar amps (mostly older) with that problem. You
could always substitute the tube, but my bet would be on the coupling cap.

You may also want to try over at alt.guitar.amps if it's available to you.

david

[Scott Dorsey]

In article ,
wrote:
How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!), which is the
value of the plate supply feeding the 12AX7 stages before the EL34.
The amp is very simple: 2 typical 12AX7 gain stages, one 12AX7 cathode-
coupled phase splitter, then the EL34s in push-pull, transformer
coupled output. I'm wondering whether it's a bad tube, or I'm
exceeding maximum values.

WHY is the control grid being driven that high? The control grid should
NEVER go positive, it should always be at least a little bit negative.

I would look for a leaky cap between the splitter and the output stage
if you have measured the control grid that high. I'd also look for an
open leak resistor. The meter will tell you what you need to know.

It does not take much at all to make an EL34 arc over.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Paul Stamler[_2_]]

wrote in message
...
How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!), which is the
value of the plate supply feeding the 12AX7 stages before the EL34.
The amp is very simple: 2 typical 12AX7 gain stages, one 12AX7 cathode-
coupled phase splitter, then the EL34s in push-pull, transformer
coupled output. I'm wondering whether it's a bad tube, or I'm
exceeding maximum values.

If you're getting 450V on the output tubes' control grids, that means the
coupling capacitors from the previous stage's plates are bad. Replace them.

Peace,
Paul

[Engineer]

"david gourley" wrote in message
...
" put forth
the
notion in...news:7c13b613-21be-4990-a378-cbee13ba8cc9
@r15g2000prh.googlegroups.com:

How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!), which is the
value of the plate supply feeding the 12AX7 stages before the EL34.
The amp is very simple: 2 typical 12AX7 gain stages, one 12AX7 cathode-
coupled phase splitter, then the EL34s in push-pull, transformer
coupled output. I'm wondering whether it's a bad tube, or I'm
exceeding maximum values.

Thanks!

Sean B

At first glance, this sounds like a leaking coupling cap from the previous
stage - I've had lots of guitar amps (mostly older) with that problem.
You
could always substitute the tube, but my bet would be on the coupling cap.

I'll bet on the coupling caps, too. But never replace a tube doing that
until you have diagnosed why or you'll just get more bad tubes!
Cheers,
Roger

(snip)

[Chris Hornbeck]

On Sun, 11 Jan 2009 07:11:06 -0800 (PST),
" wrote:

How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!),

Might? Why do you think so?

which is the
value of the plate supply feeding the 12AX7 stages before the EL34.
The amp is very simple: 2 typical 12AX7 gain stages, one 12AX7 cathode-
coupled phase splitter, then the EL34s in push-pull, transformer
coupled output. I'm wondering whether it's a bad tube, or I'm
exceeding maximum values.

Without knowing the age or model of amplifier it's hard to be
specific, but for older amplifiers the most likely cause of
motorboating is aged decoupling electrolytics.

If it's really motorboating, the other (and spectacular)
symptoms are the results of the motorboating. A pretty good
test is to pull the driver tube out and turn the amplifier
on. Does it still "motorboat"?

This test also maximally stresses coupling caps. Let us know how
it goes.

All the best fortune,
Chris Hornbeck

[Paul P[_2_]]

wrote:

How much peak voltage can an EL34's control grid take? I've got this
weird motorboating going on in my guitar amp, then I realized the
output tubes were clicking, and arcing was visible inside one. The
EL34 control grids might be getting as much as 450v (!), which is the
value of the plate supply feeding the 12AX7 stages before the EL34.
The amp is very simple: 2 typical 12AX7 gain stages, one 12AX7 cathode-
coupled phase splitter, then the EL34s in push-pull, transformer
coupled output. I'm wondering whether it's a bad tube, or I'm
exceeding maximum values.

You sound like you know a bit about tube amps but in case
it's not that much be very careful with the voltages inside.
They're really not good for your health. Keep one hand in
your pocket or behind your back and use a chopstick to screw
around inside while it's alive. And discharge the filter
capacitors (the big ones, may be hidden under a "doghouse")
before you touch anything with your hands, even with the power
off.

It would be good to know the model and age of you amp. If
you want real help I'd go over to the Music Electronics Forum
(ex Ampage) :

http://music-electronics-forum.com/home.php

where there a people that'll walk with you through your problem.

Paul P

[Mike Rivers]

On Jan 11, 9:33 pm, Chris Hornbeck
wrote:

EL34 control grids might be getting as much as 450v (!),
Might? Why do you think so?

This is why I didn't jump on the coupling capacitor bandwagon. I think
we may be dealing with someone who has a "theory" but on information
to back it up yet. He probably knows that the plate voltage is 450V,
not unreasonable for an EL34, but certainly more than what would come
from the plate of the driver/phase inverter stage.

He needs real data, not a look inside and a guess.

[[email protected]]

x-no-archive:
On Jan 12, 9:29*am, Mike Rivers wrote:
On Jan 11, 9:33 pm, Chris Hornbeck
wrote:

EL34 control grids might be getting as much as 450v (!),
Might? Why do you think so?

This is why I didn't jump on the coupling capacitor bandwagon. I think
we may be dealing with someone who has a "theory" but on information
to back it up yet. He probably knows that the plate voltage is 450V,
not unreasonable for an EL34, but certainly more than what would come
from the plate of the driver/phase inverter stage.

He needs real data, not a look inside and a guess.
to the op
pull the EL34.
If there is still 450 Volts on the grid pin its prob a coupling cap
problem..
If the 450 V goes away, it's prob a tube problem, try swapping tubes
around..
In a normal circuit there should be 0 to some nominal negative voltage
on the control grid. Assumming we are talking about the contrl grid
here. Maybe you are mixed up with the screen grid.... in which case A
high voltage is normal , but 450 still seems on the high side. Get a
copy of the "RCA reveiving tube manual". That's the bible here.

I wish they put transistors and ICs in sockets like they did tubes...

Mark

[Scott Dorsey]

wrote:

I wish they put transistors and ICs in sockets like they did tubes...

Lots of folks still do. I spent a couple hours this weekend tracking
down what turned out to be a bad transistor in a Tektronix waveform monitor,
and I was very, very happy that they did.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected][_2_]]

Thanks for the replies. The coupling caps are new, so that's not the
problem. The problem is, I think, I'm just ramming the EL34s with too
much signal, and they're arcing. But inside the tube, what is arcing
to what? What's the most common scenario?

I mentioned the 450 volt figure with the idea that if a 12AX7 cuts off
suddenly, you could have a full rail valued pulse (with a source
impedance of the plate resistor) being applied to the next grid. I
was speaking in AC terms. In this case, the 12AX7s are fed from a 450
volt rail.

With tube output stages, where is a good place to look with a scope
for RF oscillations (without destroying the scope)?

Thanks!

Sean B

[Scott Dorsey]

wrote:
Thanks for the replies. The coupling caps are new, so that's not the
problem. The problem is, I think, I'm just ramming the EL34s with too
much signal, and they're arcing. But inside the tube, what is arcing
to what? What's the most common scenario?

Just about anything can arc to just about anything. Don't overdrive
the tube. If you run the grid anywhere near zero, bad things will happen.
If you run it too negative, bad things will happen in the other direction.

I mentioned the 450 volt figure with the idea that if a 12AX7 cuts off
suddenly, you could have a full rail valued pulse (with a source
impedance of the plate resistor) being applied to the next grid. I
was speaking in AC terms. In this case, the 12AX7s are fed from a 450
volt rail.

No, it doesn't work that way. Ignore that. Use a scope and stop guessing.

With tube output stages, where is a good place to look with a scope
for RF oscillations (without destroying the scope)?

You probably don't have an RF oscillation issue. If you were looking for
one, you would use a scope on the plate and all the grids of the final.
More likely you have a motorboating issue. What model amplifier is this
and how new are the supply bypass caps?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected][_2_]]

Scott Dorsey wrote:

Just about anything can arc to just about anything. Don't overdrive
the tube. If you run the grid anywhere near zero, bad things will happen.
If you run it too negative, bad things will happen in the other direction.

What are "bad things"?

I mentioned the 450 volt figure with the idea that if a 12AX7 cuts off
suddenly, you could have a full rail valued pulse (with a source
impedance of the plate resistor) being applied to the next grid. I
was speaking in AC terms. In this case, the 12AX7s are fed from a 450
volt rail.

No, it doesn't work that way. Ignore that. Use a scope and stop guessing.

Yes, a resistively loaded tube does work this way. Minimum conduction
equals highest (most positive) output voltage. I mention this because
I have more than 60dB of voltage gain before the phase splitter, so
approaching the rail is very likely with a guitar able to put out
several volts peak.

More likely you have a motorboating issue. What model amplifier is this
and how new are the supply bypass caps?

It's a Mesa/Boogie Mark III that I am "Marshallizing." Supply caps
could be as old as 1986. Will an old cap measure lower on a
capacitance meter? Besides a short or an open, how do you detect a
bad electrolytic?

Thanks,

SB

[Scott Dorsey]

wrote:
Scott Dorsey wrote:

Just about anything can arc to just about anything. Don't overdrive
the tube. If you run the grid anywhere near zero, bad things will happen.
If you run it too negative, bad things will happen in the other direction.

What are "bad things"?

Clipping. Possibly arcing, but you'll have clipping way beforehand.

I mentioned the 450 volt figure with the idea that if a 12AX7 cuts off
suddenly, you could have a full rail valued pulse (with a source
impedance of the plate resistor) being applied to the next grid. I
was speaking in AC terms. In this case, the 12AX7s are fed from a 450
volt rail.

No, it doesn't work that way. Ignore that. Use a scope and stop guessing.

Yes, a resistively loaded tube does work this way. Minimum conduction
equals highest (most positive) output voltage. I mention this because
I have more than 60dB of voltage gain before the phase splitter, so
approaching the rail is very likely with a guitar able to put out
several volts peak.

What does this have to do with anything? Notice that there is a capacitor
between stages.... and notice that the plate of the driver stage is
probably around half of the B+. But the real key is that capacitor, which
takes time to charge and discharge.

More likely you have a motorboating issue. What model amplifier is this
and how new are the supply bypass caps?

It's a Mesa/Boogie Mark III that I am "Marshallizing." Supply caps
could be as old as 1986. Will an old cap measure lower on a
capacitance meter? Besides a short or an open, how do you detect a
bad electrolytic?

Was it motorboating before you started to change things?

Supply caps, if they are bad, will test bad on an ESR meter. Or you can
bridge the cap with a known good one, and if the combination is fine, the
total thing is fine.

If you have been hacking the amp up, another possibility is that you have
moved all the poles and zeros around and made an oscillator. You can
disconnect the global feedback and see if the problem persists. If it
goes away with the global feedback removed, start looking at the delay at
the oscillating frequency through the electronics. If you have changed
coupling capacitor or feedback network capacitor values, this is very
possible.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Chris Hornbeck]

On Mon, 12 Jan 2009 02:33:23 GMT, Chris Hornbeck
wrote:

If it's really motorboating, the other (and spectacular)
symptoms are the results of the motorboating. A pretty good
test is to pull the driver tube out and turn the amplifier
on. Does it still "motorboat"?

This test also maximally stresses coupling caps. Let us know how
it goes.

Haven't heard back about this, so to clarify: if you
pull the paraphase driver tube, and turn the amplifier
back on, you'll do two things:

You'll break the usual motorboating paths (there's still
a couple of unusual ones, but that's for later). And, you'll
have stressed the coupling cap's to the max. Measure DC
voltage on the output tubes' pin(s) 5. Should (must!) be
about -40 volts, give-er-take.

Any signs of motorboating? Any AC signal on the grids?
DVM numbers are fine. What are the DC voltages?

It's unclear how well instrumented your shack is at present.
These kinds of gigs are a whole lot more fun with an O'scope,
but not everyone is born with one, so it's hard to make blanket
recommendations about where to look next, so all us r.a.p folks are
shooting all around like Dick Cheney, trying their best to hit a
lawyer.

Given the age of the machine (old enough to vote) and the
ongoing modifications, there's a further possible gremlin, which
can *look* to the un-Oscilloscoped-eye just like a motorboat:

It's called (by old geezers) "squeeging" and is a lower-frequency
repetitive choking as the amplifier's DC average bias ramps
up and down during a higher-frequency parasitic oscillation.
An old ARRL manual would be a good source for discussion of
this if no easier answer falls out.

All the best fortune,
Chris Hornbeck

[[email protected][_2_]]

Grid-stoppers on every stage did the trick. I had forgotten to fit
them. Real important if you are running things in overload condition
for extended periods. I do think the amp was "squeeging," because I
remember a very high but audible squeal on the front end of the
motorboat pulses when the treble control was set at maximum.

It seems that if you're going to batter a power tube with tons of
level, it's a good idea to have a grid stopper not only to prevent
oscillation, but also to current limit pulses from the previous
stage's coupling cap. Arc prevention.

Thanks everyone!

Sean

[Scott Dorsey]

wrote:
Grid-stoppers on every stage did the trick. I had forgotten to fit
them. Real important if you are running things in overload condition
for extended periods. I do think the amp was "squeeging," because I
remember a very high but audible squeal on the front end of the
motorboat pulses when the treble control was set at maximum.

Why was the amp not having the oscillation before you modified it,
though? The grid-stopper resistors reduce a sort of local feedback that
can cause oscillation... you shouldn't need to add them to a circuit that
works right unless you have increased the bandwidth somehow.

It seems that if you're going to batter a power tube with tons of
level, it's a good idea to have a grid stopper not only to prevent
oscillation, but also to current limit pulses from the previous
stage's coupling cap. Arc prevention.

Okay, you have oscillation which is causing the supply to sag which is
causing the output stage to clip which is causing the arc. The problem
isn't the arc, the problem is the oscillation (although it would still
seem like a good idea to beef the supply up if you can get it into a state
like this with the oscillation). The scope is your friend.

The resistor is really providing no current-limiting since there is so
little current there anyway. You could replace it with an inductor and
it would work just as well, maybe better.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."