| Author |
Message |
Ceek
Guest
|
 Posted:
Fri Sep 30, 2005 4:43 am Post subject:
Ageing valves |
|
|
I have an an audio research VT100, and suspect the valves are starting
to go. The signs are very subtle though, and I don't want to spend
loads of cash repacing valves that are OK if the problems are elsewhere
in the system. The main symptom I am noticing is an increase in warmth,
and bloom in the mid bass. Does this effect sound familiar to anyone in
relation to ageing valves?
|
|
| Back to top |
|
 |
Bret Ludwig
Guest
|
| Posted:
Fri Sep 30, 2005 4:43 am Post subject:
Re: Ageing valves |
|
|
Phil Allison wrote:
| Quote: | "Ceek"
I have an an audio research VT100, and suspect the valves are starting
to go. The signs are very subtle though, and I don't want to spend
loads of cash repacing valves that are OK if the problems are elsewhere
in the system. The main symptom I am noticing is an increase in warmth,
and bloom in the mid bass. Does this effect sound familiar to anyone in
relation to ageing valves?
** Your valves are likely suffering from early onset " middle age spread ".
There will be a degree of flabbiness and lethargy in the sound.
A few aerobics DVDs played loud might help to tone them up.
|
More helpfully,
If you can afford this amplifier, a curve tracer such as SOFIA, or at
least a well calibrated tube tester would be in your budget as well, by
which you could test your tubes. You can also hook up a dummy load and
run some tests with these tubes, and with a new set, seeing if there is
any difference, and thereby determine how aging affects your particular
tubes in your particular amp. |
|
| Back to top |
|
|
Phil Allison
Guest
|
| Posted:
Fri Sep 30, 2005 4:43 am Post subject:
Re: Ageing valves |
|
|
"Ceek"
| Quote: |
I have an an audio research VT100, and suspect the valves are starting
to go. The signs are very subtle though, and I don't want to spend
loads of cash repacing valves that are OK if the problems are elsewhere
in the system. The main symptom I am noticing is an increase in warmth,
and bloom in the mid bass. Does this effect sound familiar to anyone in
relation to ageing valves?
|
** Your valves are likely suffering from early onset " middle age spread ".
There will be a degree of flabbiness and lethargy in the sound.
A few aerobics DVDs played loud might help to tone them up.
.......... Phil |
|
| Back to top |
|
|
Phil Allison
Guest
|
| Posted:
Fri Sep 30, 2005 9:49 am Post subject:
Re: Ageing valves |
|
|
"Bret Ludwig"
** The most helpful thing this complete ass could do is stick his head in a
gas oven.
........... Phil |
|
| Back to top |
|
|
Ceek
Guest
|
| Posted:
Fri Sep 30, 2005 12:48 pm Post subject:
Re: Ageing valves |
|
|
Bret Ludwig wrote:
| Quote: | Phil Allison wrote:
"Ceek"
I have an an audio research VT100, and suspect the valves are starting
to go. The signs are very subtle though, and I don't want to spend
loads of cash repacing valves that are OK if the problems are elsewhere
in the system. The main symptom I am noticing is an increase in warmth,
and bloom in the mid bass. Does this effect sound familiar to anyone in
relation to ageing valves?
** Your valves are likely suffering from early onset " middle age spread ".
There will be a degree of flabbiness and lethargy in the sound.
A few aerobics DVDs played loud might help to tone them up.
More helpfully,
If you can afford this amplifier, a curve tracer such as SOFIA, or at
least a well calibrated tube tester would be in your budget as well, by
which you could test your tubes. You can also hook up a dummy load and
run some tests with these tubes, and with a new set, seeing if there is
any difference, and thereby determine how aging affects your particular
tubes in your particular amp. |
|
|
| Back to top |
|
|
Ceek
Guest
|
| Posted:
Fri Sep 30, 2005 12:51 pm Post subject:
Re: Ageing valves |
|
|
| Thanks for your help Brett, I'll look into the curve tracer option. |
|
| Back to top |
|
|
Phil Allison
Guest
|
| Posted:
Fri Sep 30, 2005 12:54 pm Post subject:
Re: Ageing valves |
|
|
"Ceek"
| Quote: |
Thanks for your help Brett, I'll look into the curve tracer option.
|
** ROTFL .....
........... Phil |
|
| Back to top |
|
|
Patrick Turner
Guest
|
| Posted:
Fri Sep 30, 2005 3:36 pm Post subject:
Re: Ageing valves |
|
|
Ceek wrote:
| Quote: | I have an an audio research VT100, and suspect the valves are starting
to go. The signs are very subtle though, and I don't want to spend
loads of cash repacing valves that are OK if the problems are elsewhere
in the system. The main symptom I am noticing is an increase in warmth,
and bloom in the mid bass. Does this effect sound familiar to anyone in
relation to ageing valves?
|
Unfortunately, you don't have a second VT100 with brand new tubes to
compare with the
one you say has old tubes.
ARC amps have lots of NFB and I doubt you would hear
the tiny extra amount of thd/imd as a result of ageing.
I have an old amp in my workshop which has been there for 8 years, and
which has
a pair of 6CA7 Sylvanias which are about -15% emission since they were
pulls
8 years ago from an amp which had them in it since around 1970 maybe.
I don't here any increase in warmth over a new set of tubes.
Its sounds very nice compared to many other amps I may test
during repair work.
In some amps which have very aged tubes the max po at clipping
may fall to about 1/2 what it should be and **then** you sure hear a
generally
poor sound.
Patrick Turner. |
|
| Back to top |
|
|
Patrick Turner
Guest
|
| Posted:
Fri Sep 30, 2005 4:01 pm Post subject:
Re: Ageing valves |
|
|
Ceek wrote:
| Quote: | Thanks for your help Brett, I'll look into the curve tracer option.
|
The tube tester curve tracer option is OK if you have the gear
available.
Most ppl don't, since nearly all electronics techs don't service tubed
gear anymore.
If you know what you are doing, testing tubes for
age is a little simpler.
First take a look at the gettering when the tubes are cold.
If it has become dull with browned edges, the tube is old...
After warm up most old tubes will have a +ve DC voltage across their
grid bias resistor.
This means that if the tube is biased at -35V, then the voltage at the
grid may be -34V,
and the grid is more positive than the bias supply voltage.
If the bias R = 220k, then up to about 2V across the 220k is ok but any
more and
it indicates the tube is gassy or there is leakage from the HV anode and
screen
to the grid.
Grids should measure slight negative with respect to bias voltage when
the tube is new.
They go +ve when older.
As they go +ve, they cause more Ia to flow, and its not good in a fixed
bias amp where
there is no cathode resistor to regulate the Ik flow.
So 2V of +ve g1 voltage means perhaps 20 mA extra will flow in the tube,
so the tube runs hotter, and even morev =ve voltage appears at g1, and
so on.
So check the grid voltages of your tubes with respect to idle current.
If there is separate bias adjustment for biasing multiple output tubes,
all applied Eg should be within 15% of each other
for the same idle currents.
If there is just one applied bias voltage for all tubes, then all the
idle currents should be within
15% of each other and the Ea x Ia product should always be lower than
the rated
anode dissipation for the tube in the dtata sheets.
And check the +ve grid voltages after 10minutes, and then after 1/2 an
hour, and if there has been a large increase
so that there is say more than 2V across a 220k bias R it may be time to
buy new tubes.
Also one should run the amp up to 1/2 full clipping voltage,
which is 14Vrms in the case of an amp capable of 100W into 8 ohms,
and then measure all the gains of all the tubes;
ie, measure the Va, and divide that by Vg-k.
Tubes each side of the PP circuit should be withing 5%.
Other input or driver tubes should measure correct DCV at anodes, and
correct voltage gains for the loads involved.
If thd can be measured, each channel should be within 6dB of each other,
and no more than 6dB over the spec at any level.
The thd should be mainly 3H without any large amount of 2H, since all PP
amps have
mainly 3H thd at all levels. Having mainly 2H means the balance of
power contribution by the output tubes is poor, ie, the tubes have
become unmatched.
Tubes can be swapped around to improve balance.
Maximum po should be within 90% of max quoted po for clipping as per the
maker's specs
if they are honest.
Both channels should be within 90% of spec.
Patrick Turner. |
|
| Back to top |
|
|
Gilbert Bates
Guest
|
| Posted:
Fri Sep 30, 2005 9:29 pm Post subject:
Re: Ageing valves |
|
|
On 29 Sep 2005 17:11:06 -0700, "Ceek" <c.keating@optusnet.com.au>
wrote:
| Quote: | I have an an audio research VT100, and suspect the valves are starting
to go. The signs are very subtle though, and I don't want to spend
loads of cash repacing valves that are OK if the problems are elsewhere
in the system. The main symptom I am noticing is an increase in warmth,
and bloom in the mid bass. Does this effect sound familiar to anyone in
relation to ageing valves?
|
Keeping it simple...
I don't know anything about the VT100.
I might suspect a little bit of bias drift and is common in aging
tubes. Generally speaking tubes sound warmer as their idle (bias)
current goes up and equates to the max plate diisipation of the tube
used for the given plate voltage. IOW the hotter the tube runs the
warmer it will sound. Bass bloom kind of goes along hand in hand with
the warmth. Guitar amp guys refer to the bass bloom as flabbiness and
lacking definition in the bass.
If the VT100 has the facility to check bias by the user, you might
want to check the bias when the amp is fully warmed up. |
|
| Back to top |
|
|
Ceek
Guest
|
| Posted:
Sat Oct 01, 2005 4:42 am Post subject:
Re: Ageing valves |
|
|
Thanks guys, but unfortunately, although I am a music enthusiast,
electronics is not my forte, in fact, I must confess to a complete lack
of understanding in this area. Basically, I just listen to the stuff.
And although I thank you for your comments, I am geting in a little
over my head with much of this technical information.
I would really rather leave activities such as bias checking to a
competent tech, which your comments have motivated me to seek out.
Regarding your comments on age Trevor, I did buy the amp second hand,
so the valves could be much older than my original estimate (anything
up to 4-5 years really). |
|
| Back to top |
|
|
|
|
|
|
Profile
Log in to check your private messages
Log in