Driver to drive?

[bitrex]

On 11/8/19 3:31 PM, piglet wrote:

On 08/11/2019 7:05 pm, bitrex wrote:
On 11/7/19 8:34 PM, Phil Allison wrote:

--------------------

Question 2:

The same amp has a shameful design problem with the output stage.

Wilful stupidity.

Bet nobody gets it.



....  Phil




The feedback around the output transformer back to IC4 is DC coupled,
the line out jack is DC coupled too, and if the line out jack is
connected and there's a DC offset/ground potential difference on
whatever you plug it into you get a big DC offset at the output of
both the IC-4 drivers

DC offset at IC4 outputs won't couple through the capacitors to do harm.

No, but it will cause the amp to clip asymmetrically when the output
stage is driven hard (in the way guitarists like to do) and sound like shit

> How are you going to get DC offset across the transformer exactly?

It doesn't have to, it goes up from the line out thru the shorting
"external speaker" jack, thru the transformer secondary and onto the
shielded cable and R43 feeding the inverting input of IC4

I think a problem is the shorted output transformer when no speaker
plugs are in will also short out the feedback so if there is any input
the amplifier will try its very hardest to amplify it.

piglet

 

[Phil Allison]

jrwal...@gmail.com wrote:

--------------------------

Phil Allison wrote:

The output transformer appears to be shorted when neither of
the speaker outputs are connected.


** A common safety feature in most tube instrument amps.

Helps prevent damage to the output stage.

Interestingly, Marshall tube amps do NOT have it and have the highest output stage failure rate in the business.


....... Pil

 

[Phil Allison]

piglet wrote:

----------------

I think a problem is the shorted output transformer when no speaker
plugs are in will also short out the feedback so if there is any input
the amplifier will try its very hardest to amplify it.

** Musos will not play for long when there is NO sound coming from the speaker.

Tubes can survive the abuse for a surprising amount of time.

Plate dissipation goes high ( may glow dull red) but that is not a cause of immediate failure.


...... Phil

 

[Phil Allison]

piglet wrote:

-------------

** Musos will not play for long when there is NO sound coming from the speaker.

Tubes can survive the abuse for a surprising amount of time.

Plate dissipation goes high ( may glow dull red) but that is not a cause of immediate failure.



Ok, thanks. I am stumped now. Last deperate try: is it something like
the grid +30V rising up too quickly at switchon doing something bad
before the cathodes are hot?

** Nope.

See what you can find out about Q3/Q4, SJE1692s - non listed "specials".


...... Phil
...... Phil

 

[piglet]

On 08/11/2019 9:32 pm, piglet wrote:

On 08/11/2019 9:10 pm, Phil Allison wrote:
piglet wrote:

----------------


I think a problem is the shorted output transformer when no speaker
plugs are in will also short out the feedback so if there is any input
the amplifier will try its very hardest to amplify it.



** Musos will not play for long when there is NO sound coming from the
speaker.

  Tubes can survive the abuse for a surprising amount of time.

  Plate dissipation goes high ( may glow dull red) but that is not a
cause of immediate failure.


.....   Phil


Ok, thanks. I am stumped now. Last deperate try: is it something like
the grid +30V rising up too quickly at switchon doing something bad
before the cathodes are hot?

piglet

The more I look at it the more worried I get about those transistors in
the cathode. I'd be happier if they were biased from a power rail rather
than self biased because it looks messy starting up as the cathodes
heat. What is the Vceo rating of those transistors?

piglet

 

[bitrex]

On 11/8/19 4:45 PM, bitrex wrote:

On 11/8/19 4:30 PM, Phil Allison wrote:
bitrex wrote:

------------------

DC offset at IC4 outputs won't couple through the capacitors to do
harm.

No, but it will cause the amp to clip asymmetrically when the output
stage is driven hard (in the way guitarists like to do) and sound
like shit


** No it doesn't.

The full symmetrical cathode drive circuit you see replaces an earlier
tube drive circuit that had exactly that issue.

It was however a disastrous step, helping to put the brand off the
market.


.....  Phil


It looks like a difficult feedback network to stabilize so I think I
understand why they didn't AC couple the feedback network they had
trouble getting a response over the required bandwidth that wasn't
peaked if they did.

Lots of phase shifts going on including the op amp dominant pole.

 

[Michael Terrell]

On Friday, November 8, 2019 at 4:44:49 PM UTC-5, Phil Allison wrote:

Michael Terrell is Nasty & Bonkers wrote:

----------------------------------------

Phil Allison wrote:

SJE = special type.

No, it isn't


** Yes it is.


Really?


** Yep.

FFS "special" = non stock in some way.


That isn't what Motorola told me at a trade show.


** Every "house number" power BJT I have come across WAS special.

Selected on test for Vbe, Hfe or Vce breakdown, mostly so they could be used in parallel groups with close matching.

Service info from famous makers like like Crown and Peavey warned to only use the same in repairs.



The 'S' prefix is how Motorola marked house numbered parts for customers.

** Yep, after putting them through a selection process as needed for a particular purpose. The stock number in this case was quite obscure.


Only if the customer requested it, and paid dearly for it.

** It was not very expensive, Motorola returned parts that did not meet customer spec back to stock.

Yawn. Custom marked parts were not 'returned to stock. Your tiny world does not match reality.

SJE1692s were matched for close tolerance on Vbe and Hfe.

All examples were damn near the same.



Sigh. I said, May or may not be tested for the customer.


** You must be nuts - there is NO dispute here.

FFS Selected = special !!!!




In another thread you made a comment about ';an old HP Curve Tracer..



** Was not mine and I never saw the damn thing.

Could have been Tek575 or 576, not HP.

So fucking what?

In other words, you were caught in another lie


** FSS, it was nearly 40 years ago, not mine and never seen by me.

You are one VILE FUCKING NUT CASE

Get cancer and fucking die.

Take your own advice. I lost one parent to cancer and the other, survived cancer so stuff it. You are to be pittied, but I'm running low right now. I just spent a month without my medications for Diabetes because someone as ignorant as you deleted all my prescriptions from the system. You wish death on people all the time. Tr it for yourself, and let us know how it turns out. I haven't had a pain free day in two decades, so a flea like you won't get to me. I had a VA nurse tell me that it was a level 3 pain on a scale of one to ten when I told her that if my chainsaw was working that I was ready to cut off my legs that morning. You are absolutely useless.

 

[Phil Allison]

piglet wrote:

----------------

Do guitarists test the amp is working by touching the plug tip to get a
mains hum/buzz from the amp - that invites static discharge?

** You gotta pick up the plug first, so the metal frame discharges you.

The 2000 odd pF capacitance of the cable cuts any spike voltages right down.

Every similar amp would fail in a week if you were right.

So you are not.


..... Phil

 

[bitrex]

On 11/8/19 4:57 PM, Phil Allison wrote:

piglet wrote:

-------------



** Musos will not play for long when there is NO sound coming from the speaker.

Tubes can survive the abuse for a surprising amount of time.

Plate dissipation goes high ( may glow dull red) but that is not a cause of immediate failure.



Ok, thanks. I am stumped now. Last deperate try: is it something like
the grid +30V rising up too quickly at switchon doing something bad
before the cathodes are hot?


** Nope.

See what you can find out about Q3/Q4, SJE1692s - non listed "specials".


..... Phil
..... Phil

Tube has to be conducting appreciable current thru the cathode before
the cathode voltage drops to normal operating value, only way
appreciable current can flow thru the cathodes is those transistors,
only way those transistors can turn on is via the bias resistor from
their collector/tube cathode junction. So at startup the transistors CE
junction get momentarily walloped with the full B+ which they aren't
rated for

 

[Phil Allison]

Some Fucking Psycho called Michael Terrell wrote:
--------------------------------------------------

Only if the customer requested it, and paid dearly for it.

** It was not very expensive, Motorola returned parts that did not meet customer spec back to stock.

Yawn. Custom marked parts were not 'returned to stock.

** FFS that is NOT what I wrote.

Obviously, parts were inked AFTER passing the test.

You fucking nut case moron !!!!

In other words, you were caught in another lie


** FSS, it was nearly 40 years ago, not mine and never seen by me.

You are one VILE FUCKING NUT CASE

Get cancer and fucking die.

Take your own advice. I lost one parent to cancer and the other, survived cancer so stuff it. You are to be pittied, but I'm running low right now. I just spent a month without my medications for Diabetes because someone as ignorant as you deleted all my prescriptions from the system. You wish death on people all the time. Tr it for yourself, and let us know how it turns out. I haven't had a pain free day in two decades, so a flea like you won't get to me. I had a VA nurse tell me that it was a level 3 pain on a scale of one to ten when I told her that if my chainsaw was working that I was ready to cut off my legs that morning. You are absolutely useless.

** Get off the NG.

NOW !!!

You are in no fit state to be here.

 

[piglet]

On 08/11/2019 21:44, piglet wrote:

On 08/11/2019 2:01 am, Phil Allison wrote:
On Friday, November 8, 2019 at 12:57:25 PM UTC+11, bitrex wrote:
On 11/7/19 8:34 PM, Phil Allison wrote:

Hi to all my loyal followers,

-----------------------------

The term "bounce" is applied when an item that has been recently
repaired is brought back for a similar or repeat failure.

Mostly it's a false alarm, a new problem or just once in a while the
same failure involving the new parts. This is the worst scenario and
for me very rare.

One that comes to mind was a MusicMan hybrid guitar amp - the RD50.
The circuit is all solid state except for a pair of 6L6GC tubes for
output.

https://www.thetubestore.com/lib/thetubestore/schematics/MusicMan/Musicman-RD50A-Schematic.pdf


One came to me with the input op-amp ( IC1) having it's output stuck
to a DC rail. Fitted a new TL072 and away it went, good as new.
Unusual failure, but shit happens sometimes.

It was back in a week with the new IC blown, just like before.

Another TL072 fixed it again, but I smelt a big fat rat.

Since the op-amp interfaced with the external world of musicians and
their assorted paraphernalia - it must had failed due to abuse, twice.

But what exactly ?  To stop the nonsense, I fitted 1N4148 diodes
from pin 3 to each rail as a precaution. Walter C. Jung's "IC Op-Amp
Cookbook " gave support for fact that such inputs were vulnerable.

The customer was not happy, he insisted on speaking with me and was
very aggressive. I had understood he was a school teacher but it
turned out he was in fact a *Taekwando*  instructor !!

He expected the new repair to be covered by warranty, but I was not
budging.

He insisted that guitar amps were built very tough to take all sorts
of abuse.

I assured him he was mistaken: "I make my living because that's not
true" - I stated firmly.

After extracting a few details from him, it all became clear.

Any guess what seemingly innocent act he had performed done, twice ?


--------------------

Question 2:

The same amp has a shameful design problem with the output stage.

Wilful stupidity.

Bet nobody gets it.





He was running line-level signal from a mixer into the input, had the
line out from the amp connected to the same mixer, and accidentally
routed the output directly to the input


** Very devious thinking there.

The actual scenario is quite simple and what many musos do all the time.

Guitarists that is ...


..... Phil





Do guitarists test the amp is working by touching the plug tip to get a
mains hum/buzz from the amp - that invites static discharge?

piglet

I mean touching with their *finger*

piglet

 

[bitrex]

On 11/8/19 4:30 PM, Phil Allison wrote:

bitrex wrote:

------------------

DC offset at IC4 outputs won't couple through the capacitors to do harm.

No, but it will cause the amp to clip asymmetrically when the output
stage is driven hard (in the way guitarists like to do) and sound like shit


** No it doesn't.

The full symmetrical cathode drive circuit you see replaces an earlier tube drive circuit that had exactly that issue.

It was however a disastrous step, helping to put the brand off the market.


..... Phil

It looks like a difficult feedback network to stabilize so I think I
understand why they didn't AC couple the feedback network they had
trouble getting a response over the required bandwidth that wasn't
peaked if they did.

 

[Phil Allison]

piglet wrote:

---------------

The more I look at it the more worried I get about those transistors in
the cathode. I'd be happier if they were biased from a power rail rather
than self biased because it looks messy starting up as the cathodes
heat.

** FYI:

Q3 & Q4 effectively current drive the cathodes of the output tubes.

This significantly linearises the transfer curve so the bias setting can be unusually low and THD is reduced.

Bias current is SET by the two transistors, not the tubes, so matching is non critical and different tube brands and conditions barely matter.

Note the use of split plate and screen voltages, 600V and 300V.

All MusicMan amps have this unusual feature.

EL34 tubes can plug in replace 6L6s.



...... Phil

 

[Phil Allison]

bitrex wrote:

----------------

** You are clutching at imaginary straws.

Tube has to be conducting appreciable current thru the cathode before
the cathode voltage drops to normal operating value,

** Nope.

Initially after powering on, there is no tube current so no voltage on Q3/4.

Then some current starts and the voltage rises on each collector up to the bias value - quicker than most amps cos of the current drive.

So at startup the transistors CE junction get momentarily walloped
with the full B+ ...

** LOL - every amp would blow ay switch on if that were true.

There is no standby switch, B+ does not rise instantly.



..... Phil

 

[Lasse Langwadt Christense]

fredag den 8. november 2019 kl. 23.37.20 UTC+1 skrev Phil Allison:

piglet wrote:

----------------


Do guitarists test the amp is working by touching the plug tip to get a
mains hum/buzz from the amp - that invites static discharge?



** You gotta pick up the plug first, so the metal frame discharges you.

The 2000 odd pF capacitance of the cable cuts any spike voltages right down.

isn't typical instrument cable something like <100pF/m

 

[bitrex]

On 11/8/19 5:47 PM, Phil Allison wrote:

bitrex wrote:

----------------

** You are clutching at imaginary straws.


Tube has to be conducting appreciable current thru the cathode before
the cathode voltage drops to normal operating value,


** Nope.

Initially after powering on, there is no tube current so no voltage on Q3/4.

Then some current starts and the voltage rises on each collector up to the bias value - quicker than most amps cos of the current drive.


So at startup the transistors CE junction get momentarily walloped
with the full B+ ...


** LOL - every amp would blow ay switch on if that were true.

There is no standby switch, B+ does not rise instantly.



.... Phil

Ok, I was assuming the output tube filaments were already hot, how
they're connected doesn't appear to be labeled on the schematic.

If they were with +30 on the grid and 250 on the screens I thought it'd
briefly behave more-or-less like a vacuum diode

 

[Phil Allison]

Lasse Langwadt Christensen wrote:

---------------------------------

** You gotta pick up the plug first, so the metal frame discharges you.

The 2000 odd pF capacitance of the cable cuts any spike voltages right down.


isn't typical instrument cable something like <100pF/m

** Nope. That would be RF co-ax.

Guitar cables have thin insulation over the conductor and a layer of conductive plastic over that - then copper braid.

Low handling ( tribolectric) noise but high capacitance.

http://209.3.61.214/low-noise-triboelectric-cable.html

Audio is soooo simple....


..... Phil

 

[Lasse Langwadt Christense]

lørdag den 9. november 2019 kl. 00.18.43 UTC+1 skrev Phil Allison:

Lasse Langwadt Christensen wrote:

---------------------------------

** You gotta pick up the plug first, so the metal frame discharges you.

The 2000 odd pF capacitance of the cable cuts any spike voltages right down.


isn't typical instrument cable something like <100pF/m


** Nope. That would be RF co-ax.

Guitar cables have thin insulation over the conductor and a layer of conductive plastic over that - then copper braid.

Low handling ( tribolectric) noise but high capacitance.

http://209.3.61.214/low-noise-triboelectric-cable.html

Audio is soooo simple....

so this list is all outliers?

http://www.shootoutguitarcables.com/guitar-cables-explained/capacitance-chart.html

 

[bitrex]

On 11/8/19 7:00 PM, bitrex wrote:

Smashed his guitar on the amp on stage of course trying to be Jimi
Hendrix

The guitarist follows this ritual first, before the smashup:

1. power on the amp
2. turn the amp gain all the way up
3. turn the guitar gain all the way up
4. stand in front of the speaker and face it
5. plug the guitar into the live amp
6. turn the gain down a smidge to stop the shrieking feedback
7. if U1's still alive tune the guitar otherwise exit the ritual
8. strum the A chord
9. strum the D chord
10. strum the E chord
11. play an ascending arpeggio
12. play a descending arpeggio
13. pick through the first few bars of "Stairway to Heaven" a few times
14. Go back to standing in front of the amp and face it to create the
     shrieking feedback necessary for a Hendrix state of mind.
15. play a rough rendition of the "Star Spangled Banner" once

Thank you, 73,


the first few bars of Stairway are acoustic, Van Halen's "Ain't Talkin'
'Bout Love" are more appropriate if you're going to play the first few
bars of something

Van Halen 1 is a weird album it's like whomever mixed it decided that
the guitar should live mostly in the left channel, and the reverb on the
guitar should live in the right channel and that's just how it is

 

[bitrex]

On 11/8/19 8:46 AM, Don Kuenz, KB7RPU wrote:

boB <boB@k7iq.com> wrote:
On Thu, 7 Nov 2019 19:40:03 -0800 (PST), Phil Allison
pallison49@gmail.com> wrote:

bitrex wrote:

-------------


** Very devious thinking there.

The actual scenario is quite simple and what many musos do all the time.

Guitarists that is ...


..... Phil



Related to some kind of distortion or effect box before the input, like
maybe one of those tube compressor/limiters that have a transformer
coupled output and put out a hot signal. Or running the guitar into a
mic preamp first and using similar output stage.



** Normal use, no unusual devices and something he had done many times.



..... Phil



Smashed his guitar on the amp on stage of course trying to be Jimi
Hendrix

The guitarist follows this ritual first, before the smashup:

1. power on the amp
2. turn the amp gain all the way up
3. turn the guitar gain all the way up
4. stand in front of the speaker and face it
5. plug the guitar into the live amp
6. turn the gain down a smidge to stop the shrieking feedback
7. if U1's still alive tune the guitar otherwise exit the ritual
8. strum the A chord
9. strum the D chord
10. strum the E chord
11. play an ascending arpeggio
12. play a descending arpeggio
13. pick through the first few bars of "Stairway to Heaven" a few times
14. Go back to standing in front of the amp and face it to create the
shrieking feedback necessary for a Hendrix state of mind.
15. play a rough rendition of the "Star Spangled Banner" once

Thank you, 73,

the first few bars of Stairway are acoustic, Van Halen's "Ain't Talkin'
'Bout Love" are more appropriate if you're going to play the first few
bars of something