DC on cathode follower output

[apa]

I have two tube pres with unbalanced outputs taken from cathode
followers - (a Peavey VMP-2 and a modified Ampex 601). When testing
the output of the Ampex I noticed the scope trace bounce at power on
so I set my Fluke to measure MAX DC and connected it across the output
during power up. The voltage rose steadily and until it showed
"OL" (overload). That doesn't quite make sense to me since the
coupling cap should be blocking DC. And even if it were not, the
entire B+ voltage isn't enough overload the Fluke. So I don't know how
to interpret what it's telling me. I took a look at the unbalanced
outputs on the Peavey VMP-2 and noticed the same thing.

After some searching around I found some people recommending that a
cathode follower output should not be connected to another device
until it is warmed up for just this reason. That makes sense to me
intuitively if there is truth to my measurements but I can't imagine
that Peavey would sell a product that was in danger of damaging other
equipment if it was powered up while connected.

Can someone set me straight and tell me if I should be worrying about
this or not?

Thanks, Andy

[John Williamson]

apa wrote:
> I have two tube pres with unbalanced outputs taken from cathode
> followers - (a Peavey VMP-2 and a modified Ampex 601). When testing
> the output of the Ampex I noticed the scope trace bounce at power on
> so I set my Fluke to measure MAX DC and connected it across the output
> during power up. The voltage rose steadily and until it showed
> "OL" (overload). That doesn't quite make sense to me since the
> coupling cap should be blocking DC. And even if it were not, the
> entire B+ voltage isn't enough overload the Fluke. So I don't know how
> to interpret what it's telling me. I took a look at the unbalanced
> outputs on the Peavey VMP-2 and noticed the same thing.
>
> After some searching around I found some people recommending that a
> cathode follower output should not be connected to another device
> until it is warmed up for just this reason. That makes sense to me
> intuitively if there is truth to my measurements but I can't imagine
> that Peavey would sell a product that was in danger of damaging other
> equipment if it was powered up while connected.
>
> Can someone set me straight and tell me if I should be worrying about
> this or not?
>
Check it with a resistive load attached. Many (Especially older..)
capacitors leak enough to show a voltage using high impedance test
equipment which disappears under normal load conditions.

The scope trace bounce at power up is to be expected as the output
capacitor charges through the load impedance. If the capacitor cold end
is grounded via an internal resistor,this may be high or open circuit.

--
Tciao for Now!

John.

[Arny Krueger]

"apa" > wrote in message


> I have two tube pres with unbalanced outputs taken from
> cathode followers - (a Peavey VMP-2 and a modified Ampex
> 601). When testing the output of the Ampex I noticed the
> scope trace bounce at power on so I set my Fluke to
> measure MAX DC and connected it across the output during
> power up. The voltage rose steadily and until it showed
> "OL" (overload). That doesn't quite make sense to me
> since the coupling cap should be blocking DC. And even if
> it were not, the entire B+ voltage isn't enough overload
> the Fluke. So I don't know how to interpret what it's
> telling me. I took a look at the unbalanced outputs on
> the Peavey VMP-2 and noticed the same thing.

The advice to add resistive loads is good. Here's what is happening.

The DC voltage at the cathodes of the cathode followers starts out at zero,
and ramps up to a few dozen volts when voltages stabilize after power is
turned on.

A capacitor followed by a resistor (input to next stage or input of
voltmater) is an electrical differentiator. It turns the ramp into a pulse
with ramps at both ends of it.

> After some searching around I found some people
> recommending that a cathode follower output should not be
> connected to another device until it is warmed up for
> just this reason. That makes sense to me intuitively if
> there is truth to my measurements but I can't imagine
> that Peavey would sell a product that was in danger of
> damaging other equipment if it was powered up while
> connected.

Well-designed equipment should be able to tolerate this sort of thing/

> Can someone set me straight and tell me if I should be
> worrying about this or not?

You might want to think about why people like Peavy and Ampex would put
dangerous equipment on the market. They would be liable for any damage.

[[email protected]]

On Tue, 17 Aug 2010 13:26:13 -0700 (PDT), apa > wrote:

>I have two tube pres with unbalanced outputs taken from cathode
>followers - (a Peavey VMP-2 and a modified Ampex 601). When testing
>the output of the Ampex I noticed the scope trace bounce at power on
>so I set my Fluke to measure MAX DC and connected it across the output
>during power up. The voltage rose steadily and until it showed
>"OL" (overload). That doesn't quite make sense to me since the
>coupling cap should be blocking DC. And even if it were not, the
>entire B+ voltage isn't enough overload the Fluke. So I don't know how
>to interpret what it's telling me. I took a look at the unbalanced
>outputs on the Peavey VMP-2 and noticed the same thing.
>
>After some searching around I found some people recommending that a
>cathode follower output should not be connected to another device
>until it is warmed up for just this reason. That makes sense to me
>intuitively if there is truth to my measurements but I can't imagine
>that Peavey would sell a product that was in danger of damaging other
>equipment if it was powered up while connected.
>
>Can someone set me straight and tell me if I should be worrying about
>this or not?
>
>Thanks, Andy

First, you need to check the voltage with a load, in other words with the
equipment hooked up. If you can't manage this, you need to get a resistor and
use that, around 10,000 ohms will do. Your meter is more like 10,000,000 ohms
and will pick up leakage current.

If you find anything over 0.5 volt or so, the coupling cap in the pre is
probably bad. You need it repaired.

Another thing, a fault in the pre amp circuit (open cathode resistor or tube
shorted) could cause a too-high output voltage, but you would hear this as
gross distortion. This is rare so don't worry about it! It's probably just the
cap.

One other fault you could have is oscillation in the circuit, this would
certainly cause your Fluke to freak out! Try changing your Fluke to AC and see
what you get. Also, please report ACTUAL voltages and settings - I can't trouble
shoot by guessing at everything...

As for your other equipment, most small signal transistors are rated between 25
and 50 volts, and most chips are rated around 30 volts. They usually have some
kind of cap and/or resistor input and should resist normal surges.

The turn on surge from your pre is rather slow since the tube is not passing
current until it is warmed up, so actually, connecting to an already hot tube
would give you more of a surge! Think of the tube as a slow-turn-on device. The
voltage on the cap starts at zero and the tube ramps it up to half the supply
voltage in about 5 seconds. That could be 100v once the tube is hot!

Ignore the people recommending waiting for the voltage to rise before
connecting!

[PStamler]

An Ampex 601 isn't a tube mic preamp, it's the tubed electronics
section from an Ampex tape recorder, which incidentally includes a
(not very good) mic preamp section. Its unbalanced output really
prefers to drive something like 100k, not 10k.

The Peavey, on the other hand, will be reasonably happy driving a 10k
load from its unbalanced output (although it'll be happier driving a
higher-Z load). In fact, in some ways it's happier driving from the
unbalanced output than the balanced, at least according to the
measurements I did when I reviewed it several years ago.

Peace,
Paul

[Mike Rivers]

PStamler wrote:
> An Ampex 601 isn't a tube mic preamp, it's the tubed electronics
> section from an Ampex tape recorder, which incidentally includes a
> (not very good) mic preamp section.

But it has two very important things - tubes, and a big VU
meter.
With bling like that, who cares what it sounds like? It
LOOKS like it sounds great.


--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson

[Frank Stearns]

Mike Rivers > writes:

>PStamler wrote:
>> An Ampex 601 isn't a tube mic preamp, it's the tubed electronics
>> section from an Ampex tape recorder, which incidentally includes a
>> (not very good) mic preamp section.

>But it has two very important things - tubes, and a big VU
>meter.
>With bling like that, who cares what it sounds like? It
>LOOKS like it sounds great.

Guffaw. Funny one, Mike.

BTW, can't remember what was being sold -- hair or makeup products, perhaps -- but
in some recent, mindless tv spot I saw in the background what sure looked like an
Ampex 350-8, all polished and with all the VUs lit. This appeared to be in the 440-8
configuration, with eight channels above the transport, but I always thought that
with the larger (3U?) high 350 electronics modules the transport had to sit to the
side of the electronics stack (42 inches vs. 28 inches). It'd be too top-heavy
otherwise.

A tall 350-8 polished to mirror-perfection and lit with over-bright bulbs... Now
THAT'S bling!

Frank
Mobile Audio
--

[[email protected]]

On Wed, 18 Aug 2010 13:28:58 -0700 (PDT), PStamler > wrote:

>An Ampex 601 isn't a tube mic preamp, it's the tubed electronics
>section from an Ampex tape recorder, which incidentally includes a
>(not very good) mic preamp section. Its unbalanced output really
>prefers to drive something like 100k, not 10k.


The use of the 10k resistor was to test the coupling cap, not run the equipment.


>The Peavey, on the other hand, will be reasonably happy driving a 10k
>load from its unbalanced output (although it'll be happier driving a
>higher-Z load). In fact, in some ways it's happier driving from the
>unbalanced output than the balanced, at least according to the
>measurements I did when I reviewed it several years ago.
>
>Peace,
>Paul

[PStamler]

On Aug 18, 9:25*pm, wrote:
> On Wed, 18 Aug 2010 13:28:58 -0700 (PDT), PStamler > wrote:
> >An Ampex 601 isn't a tube mic preamp, it's the tubed electronics
> >section from an Ampex tape recorder, which incidentally includes a
> >(not very good) mic preamp section. Its unbalanced output really
> >prefers to drive something like 100k, not 10k.
>
> The use of the 10k resistor was to test the coupling cap, not run the equipment.

Right, but the input impedance of your average sound card is 10k. If
it's balanced, that's 5k per leg.

Peace,
Paul

[apa]

Thanks for the responses, everyone. Loading the stage in parallel
with the meter helped. The other problem was that the Fluke meter was
autoranging and for some reason it was fooled by the cap charging and
displayed "overload". With a manual range the voltage rises to about
2 volts during warm up. Everything seems OK on both units now.

Best, Andy

[apa]

On Aug 18, 5:08*pm, Mike Rivers > wrote:
> PStamler wrote:
> > An Ampex 601 isn't a tube mic preamp, it's the tubed electronics
> > section from an Ampex tape recorder, which incidentally includes a
> > (not very good) mic preamp section.
>
> But it has two very important things - tubes, and a big VU
> meter.
> With bling like that, who cares what it sounds like? * It
> LOOKS like it sounds great.
>
> --
> "Today's production equipment is IT based and cannot be
> operated without a passing knowledge of computing, although
> it seems that it can be operated without a passing knowledge
> of audio." - John Watkinson

Yeah, I'm sure the reason came to me was that it looked so cool (I'm
rebuilding it for a friend of mine). It's not a stock 601 - it's got a
Jensen 115-K-E feeding a slightly modified EF86 stage which in turn
feeds a cathode follower output. The other electronics have been
converted to a second identical channel.

[apa]

On Aug 19, 9:46*am, apa > wrote:
> Thanks for the responses, everyone. *Loading the stage in parallel
> with the meter helped. The other problem was that the Fluke meter was
> autoranging and for some reason it was fooled by the cap charging and
> displayed "overload". *With a manual range the voltage rises to about
> 2 volts during warm up. *Everything seems OK on both units now.
>
> Best, Andy

Just a clarification - that 2 Volts is only registered on the Fluke's
Min/Max/Avg voltage tracking. It happens to fast to see it with the
meter tracking real time.

-Andy

[Mike Rivers]

apa wrote:

> Yeah, I'm sure the reason came to me was that it looked so cool (I'm
> rebuilding it for a friend of mine). It's not a stock 601 - it's got a
> Jensen 115-K-E feeding a slightly modified EF86 stage which in turn
> feeds a cathode follower output. The other electronics have been
> converted to a second identical channel.

OK, so you have a custom tube mic preamp built on a 601
chassis.

--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson