Peavey Classic 60 questions

[Trevor Wilson]

I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the valve. The
plate on one valve appears to have suffered somewhat, but I don't expect
any serious problems apart from that. However, my main questions are:

* Should I match the output valves (more accurately, buy a matched quad
from Evatco)? Cost = AUS$100.00/set. Tung-Sol, Genalex (I have an
aversion to Chinese valves, even though they are cheaper),
Electro-Harmonix, JJ, etc.
* My other supplier has no matched sets (and I don't want to buy a
crap-load so I can match them), but sells two brands: Sovtek $13.75 each
or Electro-Harmonix $17.05.

Although cost is not an over-riding concern, the time to ship the Evatco
valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.

--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Phil Allison]

Trevor Wilson wrote:

I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the valve. The
plate on one valve appears to have suffered somewhat, but I don't expect
any serious problems apart from that. However, my main questions are:

* Should I match the output valves (more accurately, buy a matched quad
from Evatco)? Cost = AUS$100.00/set. Tung-Sol, Genalex (I have an
aversion to Chinese valves, even though they are cheaper),
Electro-Harmonix, JJ, etc.
* My other supplier has no matched sets (and I don't want to buy a
crap-load so I can match them), but sells two brands: Sovtek $13.75 each
or Electro-Harmonix $17.05.

Although cost is not an over-riding concern, the time to ship the Evatco
valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.

** Finding reliably good EL84s is a PITA - there are many duds being sold, with low current output and long warm up times, like over 5 minutes.

Your safest bet is probably JJ or else EH brand.

Matching is best done by YOU with the valves in the actual amp.

At least try to get the idle current the same on each half of the OT as many of them are intolerant of any unbalanced DC component resulting in severe distortion at low frequencies.


..... Phil

 

[Phil Allison]

Trevor Wilson wrote:

** Finding reliably good EL84s is a PITA - there are many duds being
sold, with low current output and long warm up times, like over 5
minutes.

Your safest bet is probably JJ or else EH brand.

Matching is best done by YOU with the valves in the actual amp.

At least try to get the idle current the same on each half of the OT
as many of them are intolerant of any unbalanced DC component
resulting in severe distortion at low frequencies.


**Thanks for the tips. The guys at Musiclink were quite helpful,
suggesting that the original valves were crap and anything would be an
improvement.


** That sort of self opinionated drivel is not helpful at all.

It is very easy to find EL84s that glow red, give low power or only last a short while in a PV Classic 30 - things the original Peavey brand ones did not.

Personally I would not take the slightest notice of any tech that made remarks like that cos it is a sure sign of a bullshit artist.



.... Phil

 

[Trevor Wilson]

On 13/01/2016 1:48 PM, Trevor Wilson wrote:

**OOps. That would be a Peavey Classic 30.

--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Trevor Wilson]

On 13/01/2016 3:46 PM, Phil Allison wrote:

Trevor Wilson wrote:

I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the
valve. The plate on one valve appears to have suffered somewhat,
but I don't expect any serious problems apart from that. However,
my main questions are:

* Should I match the output valves (more accurately, buy a matched
quad from Evatco)? Cost = AUS$100.00/set. Tung-Sol, Genalex (I
have an aversion to Chinese valves, even though they are cheaper),
Electro-Harmonix, JJ, etc. * My other supplier has no matched
sets (and I don't want to buy a crap-load so I can match them),
but sells two brands: Sovtek $13.75 each or Electro-Harmonix
$17.05.

Although cost is not an over-riding concern, the time to ship the
Evatco valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.


** Finding reliably good EL84s is a PITA - there are many duds being
sold, with low current output and long warm up times, like over 5
minutes.

Your safest bet is probably JJ or else EH brand.

Matching is best done by YOU with the valves in the actual amp.

At least try to get the idle current the same on each half of the OT
as many of them are intolerant of any unbalanced DC component
resulting in severe distortion at low frequencies.

**Thanks for the tips. The guys at Musiclink were quite helpful,
suggesting that the original valves were crap and anything would be an
improvement. In fact, they responded to my email with good advice AND a
full schematic, board layout and parts list (which I did not request).
Great service.


--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[John-Del]

On Wednesday, January 13, 2016 at 1:29:01 AM UTC-5, Phil Allison wrote:

Trevor Wilson wrote:



** Finding reliably good EL84s is a PITA - there are many duds being
sold, with low current output and long warm up times, like over 5
minutes.

Your safest bet is probably JJ or else EH brand.

Matching is best done by YOU with the valves in the actual amp.

At least try to get the idle current the same on each half of the OT
as many of them are intolerant of any unbalanced DC component
resulting in severe distortion at low frequencies.


**Thanks for the tips. The guys at Musiclink were quite helpful,
suggesting that the original valves were crap and anything would be an
improvement.


** That sort of self opinionated drivel is not helpful at all.

It is very easy to find EL84s that glow red, give low power or only last a short while in a PV Classic 30 - things the original Peavey brand ones did not.

Personally I would not take the slightest notice of any tech that made remarks like that cos it is a sure sign of a bullshit artist.



... Phil

Unless they meant that originals that are in the amp right now are crap and any brand of new ones would be an improvement. (?)

 

[Trevor Wilson]

On 14/01/2016 2:30 AM, John-Del wrote:

On Wednesday, January 13, 2016 at 1:29:01 AM UTC-5, Phil Allison
wrote:
Trevor Wilson wrote:



** Finding reliably good EL84s is a PITA - there are many duds
being sold, with low current output and long warm up times,
like over 5 minutes.

Your safest bet is probably JJ or else EH brand.

Matching is best done by YOU with the valves in the actual
amp.

At least try to get the idle current the same on each half of
the OT as many of them are intolerant of any unbalanced DC
component resulting in severe distortion at low frequencies.


**Thanks for the tips. The guys at Musiclink were quite helpful,
suggesting that the original valves were crap and anything would
be an improvement.


** That sort of self opinionated drivel is not helpful at all.

It is very easy to find EL84s that glow red, give low power or only
last a short while in a PV Classic 30 - things the original Peavey
brand ones did not.

Personally I would not take the slightest notice of any tech that
made remarks like that cos it is a sure sign of a bullshit artist.



... Phil

Unless they meant that originals that are in the amp right now are
crap and any brand of new ones would be an improvement. (?)

**That is precisely what the tech suggested. He was clearly unhappy with
the Chinese valves chosen by Peavey in this model.



--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Gareth Magennis]

"Trevor Wilson" wrote in message news:dflsdrFnqpnU1@mid.individual.net...

I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the valve. The
plate on one valve appears to have suffered somewhat, but I don't expect
any serious problems apart from that. However, my main questions are:

* Should I match the output valves (more accurately, buy a matched quad
from Evatco)? Cost = AUS$100.00/set. Tung-Sol, Genalex (I have an
aversion to Chinese valves, even though they are cheaper),
Electro-Harmonix, JJ, etc.
* My other supplier has no matched sets (and I don't want to buy a
crap-load so I can match them), but sells two brands: Sovtek $13.75 each
or Electro-Harmonix $17.05.

Although cost is not an over-riding concern, the time to ship the Evatco
valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.

--
Trevor Wilson
www.rageaudio.com.au




I have a thought.
The Peavey Classic 30 is a rather nice sounding amp IMHO.

(Considering it's a Peavey)


If I was a proper guitarist, I'd have one of those - but I'm not, I just
fix the things.



Gareth.

 

[Phil Allison]

Trevor Wilson wrote:

** That sort of self opinionated drivel is not helpful at all.

It is very easy to find EL84s that glow red, give low power or only
last a short while in a PV Classic 30 - things the original Peavey
brand ones did not.

Personally I would not take the slightest notice of any tech that
made remarks like that cos it is a sure sign of a bullshit artist.


Unless they meant that originals that are in the amp right now are
crap and any brand of new ones would be an improvement. (?)


**That is precisely what the tech suggested. He was clearly unhappy with
the Chinese valves chosen by Peavey in this model.

** The PV Classic30 dates from 1994, but new examples are still on sale.
If the EL84s in TW's PV are original and Chinese, they a likely very old. PV have been using Russian made EL84s since about 2008, supplied to them by Sovtek.

Letting air in ( or the vacuum out ) is a sign of overheating - or a drink spill. Sometimes you can find a tiny crack in the glass between the pins.

Truth is, the tech at Musiclink gave TW no useful advice at all, cos he had none to give. But TW considers he got a useful answer, cos it let him do whatever he felt like.


..... Phil

 

[John Heath]

On Tuesday, January 12, 2016 at 9:51:45 PM UTC-5, Trevor Wilson wrote:

I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the valve. The
plate on one valve appears to have suffered somewhat, but I don't expect
any serious problems apart from that. However, my main questions are:

* Should I match the output valves (more accurately, buy a matched quad
from Evatco)? Cost = AUS$100.00/set. Tung-Sol, Genalex (I have an
aversion to Chinese valves, even though they are cheaper),
Electro-Harmonix, JJ, etc.
* My other supplier has no matched sets (and I don't want to buy a
crap-load so I can match them), but sells two brands: Sovtek $13.75 each
or Electro-Harmonix $17.05.

Although cost is not an over-riding concern, the time to ship the Evatco
valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.

--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

The plate on one valve appears to have suffered somewhat !!

Here is the schematic

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

You could have a leaky grid capacitor C32 or C35 .0022 uf at 400 volts if the plate appears to be over heated. They are cheap so just change them both.. Also tube amps are still popular so a music sort with a repair shop will usually have a tube tester. He can match up the output tubes for gain , emission and check if gassy. If you feel uncomfortable repairing this amp a local tech in a music store can do all this for you. Do not go to the TV repair man. He is qualified but it is unlikely he will have a tube tester. You want the repair tech in a music store as he will have a tube tester and has been there done that many times and will know how to match output tubes and change coupling condensers if necessary.

 

[John Heath]

On Saturday, January 16, 2016 at 2:05:19 PM UTC-5, Trevor Wilson wrote:

On 15/01/2016 1:52 PM, John Heath wrote:
On Tuesday, January 12, 2016 at 9:51:45 PM UTC-5, Trevor Wilson
wrote:
I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the valve.
The plate on one valve appears to have suffered somewhat, but I
don't expect any serious problems apart from that. However, my main
questions are:

* Should I match the output valves (more accurately, buy a matched
quad from Evatco)? Cost = AUS$100.00/set. Tung-Sol, Genalex (I have
an aversion to Chinese valves, even though they are cheaper),
Electro-Harmonix, JJ, etc. * My other supplier has no matched sets
(and I don't want to buy a crap-load so I can match them), but
sells two brands: Sovtek $13.75 each or Electro-Harmonix $17.05.

Although cost is not an over-riding concern, the time to ship the
Evatco valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.

-- Trevor Wilson www.rageaudio.com.au

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

The plate on one valve appears to have suffered somewhat !!

Here is the schematic

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

You could have a leaky grid capacitor C32 or C35 .0022 uf at 400
volts if the plate appears to be over heated. They are cheap so just
change them both. Also tube amps are still popular so a music sort
with a repair shop will usually have a tube tester. He can match up
the output tubes for gain , emission and check if gassy. If you feel
uncomfortable repairing this amp a local tech in a music store can do
all this for you. Do not go to the TV repair man. He is qualified but
it is unlikely he will have a tube tester. You want the repair tech
in a music store as he will have a tube tester and has been there
done that many times and will know how to match output tubes and
change coupling condensers if necessary.


**Thanks for the tips, but the job was completed a couple of days ago. A
few points:

* I have a valve tester (AVO VCM163).
* The initial fault (AFAICT) was due to the owner transporting the foot
pedal inside the enclosure. At some time, the foot pedal made contact
with the nipple on one EL84, thus allowing the ingress of air. Amp was
switched on and two other valves failed soon after.
* I am quite comfortable servicing valve amps, but, due to the output
stage arrangement, I was curious about the importance of matched output
valves.
* Control grid Voltages were within spec, as were all other Voltages
(though plates were a little higher than the schematic indicated, due to
higher local mains supply (240VAC).
* The local Peavey agent supplied a schematic and board layout.
* The screen supply had also failed.

--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

I do not see feedback from output tubes to split phase tube as can be seen on the diagram. With solid state amps there is feedback from output to input so matching gain is a non issue. However in your case of a tube amp without the benefit of feedback to correct for this leaves you vulnerable to the gain of the output tubes. Then again soft distortion from the limits of tube amplifiers is the charm of having tube amps and the reason they are still sold today. Play and enjoy as perfection is not the goal of music.

Just check now and then that the output plated are not glowing red hot. If this is the case then coupling condensers C32 and C35 are suspect and could smoke those expensive output tubes. Do not mean to be pessimistic or rain on your parade but when you are in the service business all electronic equipment is junk as I only see it when it is broken not working. Your own experience in service will confirm this. Eye ball the tubes now and then to make sure the plates are not red hot in the same way you check the oil now and then in your car. Speaking of this I need to get off the net and buy some antifreeze .

 

[Trevor Wilson]

On 15/01/2016 1:52 PM, John Heath wrote:

On Tuesday, January 12, 2016 at 9:51:45 PM UTC-5, Trevor Wilson
wrote:
I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the valve.
The plate on one valve appears to have suffered somewhat, but I
don't expect any serious problems apart from that. However, my main
questions are:

* Should I match the output valves (more accurately, buy a matched
quad from Evatco)? Cost = AUS$100.00/set. Tung-Sol, Genalex (I have
an aversion to Chinese valves, even though they are cheaper),
Electro-Harmonix, JJ, etc. * My other supplier has no matched sets
(and I don't want to buy a crap-load so I can match them), but
sells two brands: Sovtek $13.75 each or Electro-Harmonix $17.05.

Although cost is not an over-riding concern, the time to ship the
Evatco valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.

-- Trevor Wilson www.rageaudio.com.au

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

The plate on one valve appears to have suffered somewhat !!

Here is the schematic

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

You could have a leaky grid capacitor C32 or C35 .0022 uf at 400
volts if the plate appears to be over heated. They are cheap so just
change them both. Also tube amps are still popular so a music sort
with a repair shop will usually have a tube tester. He can match up
the output tubes for gain , emission and check if gassy. If you feel
uncomfortable repairing this amp a local tech in a music store can do
all this for you. Do not go to the TV repair man. He is qualified but
it is unlikely he will have a tube tester. You want the repair tech
in a music store as he will have a tube tester and has been there
done that many times and will know how to match output tubes and
change coupling condensers if necessary.

**Thanks for the tips, but the job was completed a couple of days ago. A
few points:

* I have a valve tester (AVO VCM163).
* The initial fault (AFAICT) was due to the owner transporting the foot
pedal inside the enclosure. At some time, the foot pedal made contact
with the nipple on one EL84, thus allowing the ingress of air. Amp was
switched on and two other valves failed soon after.
* I am quite comfortable servicing valve amps, but, due to the output
stage arrangement, I was curious about the importance of matched output
valves.
* Control grid Voltages were within spec, as were all other Voltages
(though plates were a little higher than the schematic indicated, due to
higher local mains supply (240VAC).
* The local Peavey agent supplied a schematic and board layout.
* The screen supply had also failed.

--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Phil Allison]

John Heath wrote:

I do not see feedback from output tubes to split phase tube as can be seen on the diagram.

** Feedback comes from the 8ohm tap on the output transformer to the cathode of V3A. The phase splitter cannot accept feedback since it uses the "concertina" circuit rather than the more common cathode coupled pair.

Tube matching is normally refers to nominally identical tubes that show the same plate current under class A conditions. For pentode and beam power tubes, actual plate and screen voltages are critical so the best test conditions are those found in the amp itself.


.... Phil

 

[Phil Allison]

Trevor Wilson wrote:

* I have a valve tester (AVO VCM163).


** The VCM163 ( Valve Characteristic Meter) is oddball among valve testers, uses only AC voltages for ALL supplies - plates, screens and grids !!

http://www.radiomuseum.org/forum/tube_tester_avo_vcm_163_measurement_principle.html

Expensive too, working examples go on Ebay for up to $3000.

* The initial fault (AFAICT) was due to the owner transporting the foot
pedal inside the enclosure. At some time, the foot pedal made contact
with the nipple on one EL84, thus allowing the ingress of air. Amp was
switched on and two other valves failed soon after.


** Valves need a good vacuum in order to work. When air gets in, valves stop working immediately and a broken nipple lets air in FAST.

You previously said three 3 valves had let air in ??

BTW most versions of the Classic30 leave the valves fully exposed:

http://profile.ultimate-guitar.com/profile_mojo_data/3/3/6/5/336564/pics/_c32086_image_0.jpg

Later versions have a full, perforated steel cover over all the valves.


..... Phil

 

[John Heath]

On Saturday, January 16, 2016 at 10:18:09 PM UTC-5, Phil Allison wrote:

John Heath wrote:



I do not see feedback from output tubes to split phase tube as can be seen on the diagram.

** Feedback comes from the 8ohm tap on the output transformer to the cathode of V3A. The phase splitter cannot accept feedback since it uses the "concertina" circuit rather than the more common cathode coupled pair.

Tube matching is normally refers to nominally identical tubes that show the same plate current under class A conditions. For pentode and beam power tubes, actual plate and screen voltages are critical so the best test conditions are those found in the amp itself.


... Phil

Yes , now I see it. It is marked with a big sign saying FEEDBACK --> , ha. So who cares about matching tubes if it has feedback. The feedback should compensate for that yes / no ? As long as the output idling current is in a reasonable range and not increasing over time it should be okay. There is an obligation in service to look after the customer's best interest. Does he need German imported tubes with color coded matched specification or will a china cheap knockoff do just as well. I suspect the latter is in the costumer's interest.

And I would add that the rumor that China products are of less quality is just sour grapes from an industry that can not compete. 90 percent of all desktop , laptop , flat TVs and cell phones come from China. These are not the easy but the hardest electronic products to make and they are all coming from the shores of China. The winds of change are in the air. I can see I am rambling off topic. Why is it when we get older there is a need to talk all the time? It is as if life as filled the brain with stuff so you need to talk to vent some of it off.

 

[Phil Allison]

John Heath wrote:

** Feedback comes from the 8ohm tap on the output transformer to the cathode of V3A. The phase splitter cannot accept feedback since it uses the "concertina" circuit rather than the more common cathode coupled pair.

Tube matching is normally refers to nominally identical tubes that show the same plate current under class A conditions. For pentode and beam power tubes, actual plate and screen voltages are critical so the best test conditions are those found in the amp itself.



Yes , now I see it. It is marked with a big sign saying FEEDBACK --> , ha.. So who cares about matching tubes if it has feedback. The feedback should compensate for that yes / no ?

** Feedback is used to mainly reduce the 3rd harmonic distortion of a push-pull valve power stage - particularly class AB ones as used in most guitar amps.

What it does not do however, is adjust the bias current balance so there is no net magnetisation in the output transformer. Few amps have an adjustment for this so the valves need to be matched, at least in pairs fitted to each side.

Using poorly matched valves often results in a large increase in 2nd harmonic (ie bad waveform symmetry ) at low frequencies, since the iron core of the transformer goes into saturation much earlier in one polarity.


BTW:

With transistor amps, the same idle current always flows in both output devices cos they are connected in series.



..... Phil

 

[Trevor Wilson]

On 17/01/2016 3:04 PM, Phil Allison wrote:

Trevor Wilson wrote:



* I have a valve tester (AVO VCM163).


** The VCM163 ( Valve Characteristic Meter) is oddball among valve
testers, uses only AC voltages for ALL supplies - plates, screens and
grids !!

http://www.radiomuseum.org/forum/tube_tester_avo_vcm_163_measurement_principle.html

Expensive too, working examples go on Ebay for up to $3000.


* The initial fault (AFAICT) was due to the owner transporting the
foot pedal inside the enclosure. At some time, the foot pedal made
contact with the nipple on one EL84, thus allowing the ingress of
air. Amp was switched on and two other valves failed soon after.


** Valves need a good vacuum in order to work. When air gets in,
valves stop working immediately and a broken nipple lets air in
FAST.

You previously said three 3 valves had let air in ??

BTW most versions of the Classic30 leave the valves fully exposed:

http://profile.ultimate-guitar.com/profile_mojo_data/3/3/6/5/336564/pics/_c32086_image_0.jpg

Later versions have a full, perforated steel cover over all the
valves.

**Two of the other output valves had very small cracks near the pins. I
can only surmise that the owner had been rough in removing the valves
(she admitted to doing so) and cracked the glass.


--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Trevor Wilson]

On 17/01/2016 7:02 AM, John Heath wrote:

On Saturday, January 16, 2016 at 2:05:19 PM UTC-5, Trevor Wilson
wrote:
On 15/01/2016 1:52 PM, John Heath wrote:
On Tuesday, January 12, 2016 at 9:51:45 PM UTC-5, Trevor Wilson
wrote:
I have one of the above-mentioned amps on the bench. Pretty
straightforward - 3 X EL84 output valves have air inside the
valve. The plate on one valve appears to have suffered
somewhat, but I don't expect any serious problems apart from
that. However, my main questions are:

* Should I match the output valves (more accurately, buy a
matched quad from Evatco)? Cost = AUS$100.00/set. Tung-Sol,
Genalex (I have an aversion to Chinese valves, even though they
are cheaper), Electro-Harmonix, JJ, etc. * My other supplier
has no matched sets (and I don't want to buy a crap-load so I
can match them), but sells two brands: Sovtek $13.75 each or
Electro-Harmonix $17.05.

Although cost is not an over-riding concern, the time to ship
the Evatco valves could be. My other supplier can ship today.

Thoughts appreciated. Particularly from PA.

-- Trevor Wilson www.rageaudio.com.au

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

The plate on one valve appears to have suffered somewhat !!

Here is the schematic

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

You could have a leaky grid capacitor C32 or C35 .0022 uf at 400
volts if the plate appears to be over heated. They are cheap so
just change them both. Also tube amps are still popular so a
music sort with a repair shop will usually have a tube tester. He
can match up the output tubes for gain , emission and check if
gassy. If you feel uncomfortable repairing this amp a local tech
in a music store can do all this for you. Do not go to the TV
repair man. He is qualified but it is unlikely he will have a
tube tester. You want the repair tech in a music store as he will
have a tube tester and has been there done that many times and
will know how to match output tubes and change coupling
condensers if necessary.


**Thanks for the tips, but the job was completed a couple of days
ago. A few points:

* I have a valve tester (AVO VCM163). * The initial fault (AFAICT)
was due to the owner transporting the foot pedal inside the
enclosure. At some time, the foot pedal made contact with the
nipple on one EL84, thus allowing the ingress of air. Amp was
switched on and two other valves failed soon after. * I am quite
comfortable servicing valve amps, but, due to the output stage
arrangement, I was curious about the importance of matched output
valves. * Control grid Voltages were within spec, as were all other
Voltages (though plates were a little higher than the schematic
indicated, due to higher local mains supply (240VAC). * The local
Peavey agent supplied a schematic and board layout. * The screen
supply had also failed.

-- Trevor Wilson www.rageaudio.com.au

--- This email has been checked for viruses by Avast antivirus
software. https://www.avast.com/antivirus

I do not see feedback from output tubes to split phase tube as can be
seen on the diagram. With solid state amps there is feedback from
output to input so matching gain is a non issue. However in your case
of a tube amp without the benefit of feedback to correct for this
leaves you vulnerable to the gain of the output tubes. Then again
soft distortion from the limits of tube amplifiers is the charm of
having tube amps and the reason they are still sold today. Play and
enjoy as perfection is not the goal of music.

Just check now and then that the output plated are not glowing red
hot. If this is the case then coupling condensers C32 and C35 are
suspect and could smoke those expensive output tubes. Do not mean to
be pessimistic or rain on your parade but when you are in the service
business all electronic equipment is junk as I only see it when it is
broken not working. Your own experience in service will confirm this.
Eye ball the tubes now and then to make sure the plates are not red
hot in the same way you check the oil now and then in your car.
Speaking of this I need to get off the net and buy some antifreeze
.

**Since the amp is used professionally, I ran it under a test condition
which was somewhat more rigorous and lengthy than normal. Following on
from PA's suggestions, I fitted some 1 Ohm cathode resistors to each
output valve and monitors the cathode current. It was within 10% for all
valves. I consider that to be fine. The major problem with this amp (as
with most of it's type) will be the owner. If she transports the foot
pedal inside the amp again, then there is a high degree of risk of
further damage.



--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

[Trevor Wilson]

On 17/01/2016 3:04 PM, Phil Allison wrote:

Trevor Wilson wrote:



* I have a valve tester (AVO VCM163).


** The VCM163 ( Valve Characteristic Meter) is oddball among valve
testers, uses only AC voltages for ALL supplies - plates, screens and
grids !!

http://www.radiomuseum.org/forum/tube_tester_avo_vcm_163_measurement_principle.html

Expensive too, working examples go on Ebay for up to $3000.

**The transformers inside the thing are amazing things. They would be
very expensive to produce today. Mind-numbing numbers of taps.


--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

 

>"With transistor amps, the same idle current always flows in both >output devices cos they are connected in series. "

I have wanted to pound a couple of things into people's heads about this for some time.

What you say is true for idle current, the bias, but if there is an offset the current through the outputs is not equal. With no load it should be no matter what.

Anyway, the sledge hammer is to beat it in that :

1. Adjust the bias first, with no load.

2. Adjust the balance/offset next, also with no load.

Do this after having warmed it up a bit, if you can't, recheck it hot.

If the manufacturer starts this shit about taking out jumpers to measure bias current, fuck all that. Just do the math and calculate it from the emitter resistors. Take my word for it.

In fact I can adjust it with no spec. II can just feed it with low level program material and watch the waveform at the collector(s) of the voltage amp(s). The |Ż|_|Ż|_ in that waveform, produced when the feedback is taking care of the crossover distortion is the prime test point. Adjust bias to flatten that out, done.

However, going farther, that is biasing it harder, does have some merit. First of all it is not that much, and it also gives the amp more low power damping factor. Believe me I know the difference. I am not talking going into class A here, I think that is stupid. If you are going to go class A go all the way and go SE. Then for the most part you only got even order distortion.