I always wondered why different valve types sound so differently in guitar amplifiers - not in the preamp, that I can understand, since the sound is created by multiple distorting stages.
I wonder about the power stage - in modern guitar amplifiers, the output stage is far from clipping and compression, e.g. Mesa Boogie Dual Rectifier or Laney GH100L.
The two amplifiers mentioned above have the option to use either EL34 or 6L6 tubes, use fixed bias and a bias switch and there are many others that have this option as well, I only own these two.
Both amplifiers have a totally different sound with the two tube types. EL34 tone would be best described as midrange heavy and the 6L6 sound would be more like fullrange or even mid-recessed compared to EL34. All this is most apparent with distorted guitar (think heavy metal and more), where the signal is closer to a square than to a sine.
My theory was, that this is due to the phase inverter being optimalized for one type of tube will interact differently with the other tube type and that the distortion profile and frequency response is therefore different for the whole power stage. But there is certainly more to it, I would welcome any ideas on that.
I also have another amplifier, with cathode biased PP EL84 power stage - and it has a pentode/triode mode switch. The triode mode sounds more fuzzy and compressed than the pentode mode, again with distorted guitar sound. I wonder if that is because different frequency response of triode vs pentode or because of the different distortion added - it is again even at low volumes far away from any clipping in the end stage. Ideas welcome here as well
With HiFi and music type signals, I hardly notice any differences between valve types and amplifier topologies - there are some, but not as pronounced as with the guitar amps.
I wonder about the power stage - in modern guitar amplifiers, the output stage is far from clipping and compression, e.g. Mesa Boogie Dual Rectifier or Laney GH100L.
The two amplifiers mentioned above have the option to use either EL34 or 6L6 tubes, use fixed bias and a bias switch and there are many others that have this option as well, I only own these two.
Both amplifiers have a totally different sound with the two tube types. EL34 tone would be best described as midrange heavy and the 6L6 sound would be more like fullrange or even mid-recessed compared to EL34. All this is most apparent with distorted guitar (think heavy metal and more), where the signal is closer to a square than to a sine.
My theory was, that this is due to the phase inverter being optimalized for one type of tube will interact differently with the other tube type and that the distortion profile and frequency response is therefore different for the whole power stage. But there is certainly more to it, I would welcome any ideas on that.
I also have another amplifier, with cathode biased PP EL84 power stage - and it has a pentode/triode mode switch. The triode mode sounds more fuzzy and compressed than the pentode mode, again with distorted guitar sound. I wonder if that is because different frequency response of triode vs pentode or because of the different distortion added - it is again even at low volumes far away from any clipping in the end stage. Ideas welcome here as well
With HiFi and music type signals, I hardly notice any differences between valve types and amplifier topologies - there are some, but not as pronounced as with the guitar amps.
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Are all these separate stand alone amplifiers?
Or, do any of them have a loudspeaker in the amplifier cabinet?
Do you always use the same loudspeaker to listen to all of these amplifiers?
Broad Sweeping Generalizations about amplifiers are just that, Generalizations.
For each amplifier, the particular circuit, parts, tubes, and methods used for switching from UL to Triode, to Pentode/Beam Power modes, affects the sound.
And changing from an EL34 to a 6L6 without changing other parameters (not just changing bias), changes more than you can imagine, depending on the other circuit and parts details.
Just another fact . . . EL34 and EL84 are true Pentodes. 6L6 is a true Beam Power tube. Pentodes use true Suppressor Screens. Beam Power tubes use Beam Formers. That feature is completely different, even if they get the curves to be quite close.
The KT77 Beam Power tube is probably the closest relative to the EL34 that I know of.
Or, do any of them have a loudspeaker in the amplifier cabinet?
Do you always use the same loudspeaker to listen to all of these amplifiers?
Broad Sweeping Generalizations about amplifiers are just that, Generalizations.
For each amplifier, the particular circuit, parts, tubes, and methods used for switching from UL to Triode, to Pentode/Beam Power modes, affects the sound.
And changing from an EL34 to a 6L6 without changing other parameters (not just changing bias), changes more than you can imagine, depending on the other circuit and parts details.
Just another fact . . . EL34 and EL84 are true Pentodes. 6L6 is a true Beam Power tube. Pentodes use true Suppressor Screens. Beam Power tubes use Beam Formers. That feature is completely different, even if they get the curves to be quite close.
The KT77 Beam Power tube is probably the closest relative to the EL34 that I know of.
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Perfect. Let us narrow down to one model, the Laney GH100L, the schematic diagram can be downloaded for example here: Laney Schematics - Tube amp Schematics
The comparison was done with the same knob settings with the same guitar in the same room with the same 4x12" Marshall 1960BV cabinet. The volume did not change too much, but the sound changed as described. From metal guitar sound point of view, it was a night and day.
That particular test is always on my mind when I talk to someone about EL34 and 6L6 differences. And I would narrow down the topic to two questions I was never able to answer to myself.
1) what could be the reasonds to sound signature change
2) can EL34 be made to sound as close as possible to 6L6 or is that next to impossible due to tube differences?
I used to own "clean" guitar rack amps - both 6L6 and EL34 based - and the difference between the sound of the two was close to none. So I think the influence of the component values is higher than the difference caused by the tube parameter difference. Can that be true?
The comparison was done with the same knob settings with the same guitar in the same room with the same 4x12" Marshall 1960BV cabinet. The volume did not change too much, but the sound changed as described. From metal guitar sound point of view, it was a night and day.
That particular test is always on my mind when I talk to someone about EL34 and 6L6 differences. And I would narrow down the topic to two questions I was never able to answer to myself.
1) what could be the reasonds to sound signature change
2) can EL34 be made to sound as close as possible to 6L6 or is that next to impossible due to tube differences?
I used to own "clean" guitar rack amps - both 6L6 and EL34 based - and the difference between the sound of the two was close to none. So I think the influence of the component values is higher than the difference caused by the tube parameter difference. Can that be true?
Part of the EL34/6CA7's are not true power pentodes but beam power tubes (Philips ECG, Sylvania, Zenith, EI, and probably one or more manufacturers in current times).
On this site the history of the EL34 is described (in German): Die EL-34-Story
Have extensively tested this and it all boils down to tube current capacity
Look at a typical Guitar speaker (Legend 1258) *impedance* curve:
Couple that to the common practice of loading Guitar amp power tubes with as low an impedance (plate to plate) as possible, with the end to extract down to the last watt and them some ..... with the end result of pushing them to the limit, specially when overdriven.
Under same conditions (as in: you switch tubes on the same amp, only change bias but keep voltages, impedance, etc. all the same), EL34 have significantly higher current capability than 6L6 , so when speaker impedance dips, typically in the 200 to 400 Hz range (check the curve), 6L6 simply can´t cope and so voltage at those frequencies drops ... with the end result of attenuating that band by a few dB , pulling low mids down.
EL34 can supply the extra current when pushed (again, under exact same conditions, same amp), so *by comparison* overdriven sound is "middier".
As simple as that; no need to delve into minute screen to beam comparison, plate structure, etc. , just look at the end result.
As a side note: transistor amps typically sound muddy when overdriven because they can always supply as much current as needed; I experimented with setting current limiters so, say, they could *barely* drive nominal 8 ohm speakers when overdriven and definitely clipping earlier (so "wasting" watts
) when driving real world 8 ohm speakers and I could get the unusual result of having a transistor amp keeping "bite" when overdriven.
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I believe the manufacturing of Beam Power tubes, and Pentode tubes became
Very Blurred in modern times.
The original difference in Pentode Tubes and Beam Power tubes was very real.
The reason Was the Patent Laws.
Pentode and Beam Power are separate patents.
Decades later, either the patents ran out; or the manufacturers did not care because over the decades Lawyers became more and more expensive to hire.
Does anybody have a better reason?
Take a look at 6L6 tubes. I bet nobody ever designed a Real Pentode 6L6. Please find me one, it is Bogus.
But you can find a 6BQ5 Beam Power, and you can find a 6BQ5 Pentode.
You can find an EL84 Pentode.
In later years it became very blurred:
You can find a tube marked 6BQ5/EL84 or a tube marked EL84/6BQ5.
I am pretty sure it was not so in the beginning (unless in the beginning of those two tube types, the patents had all run out).
Very Blurred in modern times.
The original difference in Pentode Tubes and Beam Power tubes was very real.
The reason Was the Patent Laws.
Pentode and Beam Power are separate patents.
Decades later, either the patents ran out; or the manufacturers did not care because over the decades Lawyers became more and more expensive to hire.
Does anybody have a better reason?
Take a look at 6L6 tubes. I bet nobody ever designed a Real Pentode 6L6. Please find me one, it is Bogus.
But you can find a 6BQ5 Beam Power, and you can find a 6BQ5 Pentode.
You can find an EL84 Pentode.
In later years it became very blurred:
You can find a tube marked 6BQ5/EL84 or a tube marked EL84/6BQ5.
I am pretty sure it was not so in the beginning (unless in the beginning of those two tube types, the patents had all run out).
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I thought that the sound comparison on the amplifier was all done at lower listening levels, far below the max output of the EL34 and 6L6 on that amp.
Were the tubes 6L6GC, or were they any other 6L6 type?
Thanks for the schematic of the Laney GH100L.
What I see in the output tubes circuitry schematic is the following:
Hard Wired Pentode/Beam Power Mode
No UL Mode, and No Triode Mode
Adjustable Fixed Bias
There is global negative feedback to the driver stage, which includes the output stage, and the output transformer.
There is absolutely no way to balance any of the 4 output tube currents (you either have a perfectly matched Quad, or your guess is as good as mine
(versus the sound performance).
What would you like it to be?
In fact, I do not even see a way to measure the individual cathode currents, and no way to measure the total of the cathode currents.
That means bias is set by looking at a tube chart, with plate voltage, screen voltage, and plate current versus bias voltage (not a good way to do things).
The screen voltage is way to high for any 6L6, except a 6L6GC.
Pick 4 tubes according to the quiescent current match or mismatch:
Match 2 Push tubes, match 2 Pull tubes, and then either make the total Push and Pull currents equal, or make the total push and pull current unequal.
Or, do not match 2 Push tubes, but match 2 Pull tubes; then have the total Push and Pull currents either matched, or not.
Or, make sure there are no matches of the 2 Push, and no matches of the 2 Pull; but then either match the total Push and Pull currents, or do not match them.
But not only do we have the Quiescent current matches to worry about, we have to worry about the Transconductance matches (or mismatches).
And those have to be done at the amplifier's quiescent operating conditions. A tube tester match of Transconductance at a different plate voltage or plate current, may not be a match when installed in the amplifier at its quiescent voltage and current setting.
Now, if you have a matched Quad EL34 tubes, and you listen,
Then, when you get a matched Quad of 6L6 and you listen, do you reset the bias for equal current or not?
The output transformer primary may, or may not be optimum; the screen voltage may, or may not be optimum; the plate voltage may, or may not be optimum; and the plate current may, or may not be optimum; Versus EL34s or for 6L6s.
Optimum for what?
Hi Fi.
A Guitar amp to sound like xxx.
A Guitar amp to sound like yyy.
At first glance, I am missing the boat completely, it already pulled away from the docks.
Or, perhaps I did not understand what you were changing on the Laney GH100L when you did comparative listening tests.
You said that there was a slight difference in the sound level. That could have been due to the large gain differences of an EL34 versus a 6L6.
Lots of negative feedback would make the power amp gain equal.
But lower amounts of negative feedback would make the power amp gain slightly different for the EL34 versus the 6L6.
And lower amounts of negative feedback would have different damping factors too.
Different damping factors make the sound of a loudspeaker different as well.
Now, for some more observation:
There is lots of circuitry ahead of the driver and output stages (power amp).
What is the distortion characteristics of the power amp at low volumes?
What is the distortion characteristics of all that earlier circuitry that is ahead of the power amp?
What is the combination of those two distortions? Aiding, Cancelling, or mixed depending on the harmonic number, 2nd, 3rd, . . .?
Too many variables at this point to make a qualified solution to the cause of the sound differences.
Were the tubes 6L6GC, or were they any other 6L6 type?
Thanks for the schematic of the Laney GH100L.
What I see in the output tubes circuitry schematic is the following:
Hard Wired Pentode/Beam Power Mode
No UL Mode, and No Triode Mode
Adjustable Fixed Bias
There is global negative feedback to the driver stage, which includes the output stage, and the output transformer.
There is absolutely no way to balance any of the 4 output tube currents (you either have a perfectly matched Quad, or your guess is as good as mine
(versus the sound performance).
What would you like it to be?
In fact, I do not even see a way to measure the individual cathode currents, and no way to measure the total of the cathode currents.
That means bias is set by looking at a tube chart, with plate voltage, screen voltage, and plate current versus bias voltage (not a good way to do things).
The screen voltage is way to high for any 6L6, except a 6L6GC.
Pick 4 tubes according to the quiescent current match or mismatch:
Match 2 Push tubes, match 2 Pull tubes, and then either make the total Push and Pull currents equal, or make the total push and pull current unequal.
Or, do not match 2 Push tubes, but match 2 Pull tubes; then have the total Push and Pull currents either matched, or not.
Or, make sure there are no matches of the 2 Push, and no matches of the 2 Pull; but then either match the total Push and Pull currents, or do not match them.
But not only do we have the Quiescent current matches to worry about, we have to worry about the Transconductance matches (or mismatches).
And those have to be done at the amplifier's quiescent operating conditions. A tube tester match of Transconductance at a different plate voltage or plate current, may not be a match when installed in the amplifier at its quiescent voltage and current setting.
Now, if you have a matched Quad EL34 tubes, and you listen,
Then, when you get a matched Quad of 6L6 and you listen, do you reset the bias for equal current or not?
The output transformer primary may, or may not be optimum; the screen voltage may, or may not be optimum; the plate voltage may, or may not be optimum; and the plate current may, or may not be optimum; Versus EL34s or for 6L6s.
Optimum for what?
Hi Fi.
A Guitar amp to sound like xxx.
A Guitar amp to sound like yyy.
At first glance, I am missing the boat completely, it already pulled away from the docks.
Or, perhaps I did not understand what you were changing on the Laney GH100L when you did comparative listening tests.
You said that there was a slight difference in the sound level. That could have been due to the large gain differences of an EL34 versus a 6L6.
Lots of negative feedback would make the power amp gain equal.
But lower amounts of negative feedback would make the power amp gain slightly different for the EL34 versus the 6L6.
And lower amounts of negative feedback would have different damping factors too.
Different damping factors make the sound of a loudspeaker different as well.
Now, for some more observation:
There is lots of circuitry ahead of the driver and output stages (power amp).
What is the distortion characteristics of the power amp at low volumes?
What is the distortion characteristics of all that earlier circuitry that is ahead of the power amp?
What is the combination of those two distortions? Aiding, Cancelling, or mixed depending on the harmonic number, 2nd, 3rd, . . .?
Too many variables at this point to make a qualified solution to the cause of the sound differences.
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That's the real problem today: fake parts. A 6BQ5 is a pentode.
That you can find beam tetrodes labelled "6BQ5" doesn't surprise me but it's still a fake.
(one place otherwise similar tubes differ between a BT and a Pent is in screen current draw, especially as the plate voltage drops)
I used matched quads for quiescent current +- 0.1 mA both for 6L6GC and EL34. I see the preamp as harmonics generator. Now I understand it is a very complex problem. So I just narrow it down again to a single question - does exchanging the power tubes change frequency response or distortion characteristics? Am I correct that in general, both things happen at the same time.
Generalization,
1. In Push Pull, Beam Power/Pentode mode, and with global negative feedback from the output transformer secondary to the driver or splitter, versus no negative feedback.
The gain of the output tubes changes according to the Transconductance of the output tube. EL34 ~ 11000uMhos; 6L6 ~ 6000uMhos.
The gain is almost 2:1 when the negative feedback is disconnected.
And these tubes have very high plate resistance, rp.
So the frequency response is controlled by the output transformer impedance versus frequency, and the loudspeaker impedance versus frequency.
And the push pull circuit tends to cancel most of the 2nd harmonic distortion.
So the 3rd harmonic distortion is dominant.
But with moderate to large negative feedback, the feedback controls the gain.
And it improves the distortion, the frequency response, and the damping factor.
As I sometimes say, global negative feedback tends to reduce (cover over) the
faults of the circuit and tubes (but as some say tending to close in the sound).
2. When you compared the 6L6 and the EL34 tubes, did you set the bias so that the Total current of the Quad tubes was the same.
If you did that, then the bias voltage was almost certainly different for the 6L6 versus the EL34.
That is important.
The transconductance different, so the drive level of the earlier stage (driver/phase splitter) changes to drive the output to the same sound level.
That changes the distortion of the driver/phase splitter.
Negative feedback makes up for part of the driver/phase splitter, but not all of the difference.
3. The distortion of a 6L6 versus an EL34 in the same circuit is different.
Negative feedback covers up some of that, but not all.
1. In Push Pull, Beam Power/Pentode mode, and with global negative feedback from the output transformer secondary to the driver or splitter, versus no negative feedback.
The gain of the output tubes changes according to the Transconductance of the output tube. EL34 ~ 11000uMhos; 6L6 ~ 6000uMhos.
The gain is almost 2:1 when the negative feedback is disconnected.
And these tubes have very high plate resistance, rp.
So the frequency response is controlled by the output transformer impedance versus frequency, and the loudspeaker impedance versus frequency.
And the push pull circuit tends to cancel most of the 2nd harmonic distortion.
So the 3rd harmonic distortion is dominant.
But with moderate to large negative feedback, the feedback controls the gain.
And it improves the distortion, the frequency response, and the damping factor.
As I sometimes say, global negative feedback tends to reduce (cover over) the
faults of the circuit and tubes (but as some say tending to close in the sound).
2. When you compared the 6L6 and the EL34 tubes, did you set the bias so that the Total current of the Quad tubes was the same.
If you did that, then the bias voltage was almost certainly different for the 6L6 versus the EL34.
That is important.
The transconductance different, so the drive level of the earlier stage (driver/phase splitter) changes to drive the output to the same sound level.
That changes the distortion of the driver/phase splitter.
Negative feedback makes up for part of the driver/phase splitter, but not all of the difference.
3. The distortion of a 6L6 versus an EL34 in the same circuit is different.
Negative feedback covers up some of that, but not all.
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Pelanj,
You have too many variables to make a proper determination of the causes of the sound differences.
The GH100L is first of all a Guitar Amp, it is not a Hi Fi amp.
It was designed for EL34 and 5881.
A 6L6 (and various letters after the last 6), most 6L6 types are not a 5881.
Different plate dissipation, plate voltage, screen dissipation, and screen voltage.
The construction of the plate, screen, and perhaps the Beam Formers can and will be different.
Well, there are Dual Marked tubes, but that is frankly wrong to do for most of the 6L6 names.
An EL34 uses a Real Suppressor Grid, not Beam Formers.
You will notice that there is a bias switch for EL34 and for 5881.
But there also is a Bias Adjust Pot.
Question:
Did you adjust the bias setting dead on: exactly 35V for the EL34, and exactly for 45V for the 6L6 (5881 setting) ?
Question:
Were the Quad EL34 tubes matched at 460V plate, 457V screen, and -35V bias ?
If not, then the quiescent current to the output primary may have been un-balanced . . .
Early Saturation of the laminations is possible.
Depending on the current, the Plate and/or Screen may have been over its dissipation limit.
A Guitar Amp designer does not care (and some Hi Fi designers do not care either).
Hot screens, and Hot Plates can disturb the performance of a tube.
Question:
Were the Quad 6L6 tubes matched at 460V plate, 457V screen, and -35V bias ?
(you have to use a 6L6GC model)
If not, then the quiescent current to the output primary may have been un-balanced . . .
Early Saturation is possible.
And any version of 6l6, other than 6L6GC in that amp is beyond the maximum specs for the tube.
A Guitar Amp designer does not care (and some Hi Fi designers do not care either).
Hot screens, and Hot Plates can disturb the performance of a tube.
When EL34 or 6L6 tubes are matched at different plate, screen, and bias voltages than the amp provides, then the tubes might no longer be matched properly in the amp.
What plate voltage, what screen voltage, and what bias voltage(s) were the EL34 Quad, and the 6L6 Quads tested to match ?
The amp designer either was to lazy or too cheap; to make measurements of the individual cathode currents possible, make make individual adjustments of the individual cathode currents; . . . or to use individual Self Bias that would make individual measurements possible, and not well matched tubes to be intrinsically fairly well matched in the amp.
Or, the amp designer did not care.
Quad matched tubes that are Properly matched for the exact amp parameters are typically very expensive. The amp is "cheap" but the requirement for the Properly matched Quad will not be "cheap".
Again, this is a Guitar amp, so care may have been thrown out the window.
I would bet that 2 of those identical amps that had improperly matched Quads sets of the same tube type, might very likely sound different from each other.
Question:
What is the reason to make a Guitar amp have the ability to use EL34 or 5881 ?
Sound differences in that particular amp circuit ?
Desire of Marketing to increase sales ?
Price of tubes and Quad matched tubes ?
Boutique look ?
The phase inverter has un-equal plate load resistors.
But they have common bias, and the schematic shows 205V on both plates.
That is not possible, unless a weaker triode is selected for the tube that has the higher plate load resistor.
Nobody selects dual triodes that way.
Question:
The global negative feedback is from the 4 Ohm tap.
But what tap was the speaker connected to ?
Then there is the setting of the Resonance switch?
Then there is the setting of the Presence switch?
That effects the global negative feedback
But . . .
The open loop to closed loop feedback ratio, is totally different for the EL34 than it is for the 6L6. Because of the different open loop gain due to the wide differences in those 2 tube types transconductance.
Hi Fi amps are typically domesticated.
Guitar amps are typically a beast.
You have to nail down all the variables, except 1.
Scientific study, and getting
You have too many variables to make a proper determination of the causes of the sound differences.
The GH100L is first of all a Guitar Amp, it is not a Hi Fi amp.
It was designed for EL34 and 5881.
A 6L6 (and various letters after the last 6), most 6L6 types are not a 5881.
Different plate dissipation, plate voltage, screen dissipation, and screen voltage.
The construction of the plate, screen, and perhaps the Beam Formers can and will be different.
Well, there are Dual Marked tubes, but that is frankly wrong to do for most of the 6L6 names.
An EL34 uses a Real Suppressor Grid, not Beam Formers.
You will notice that there is a bias switch for EL34 and for 5881.
But there also is a Bias Adjust Pot.
Question:
Did you adjust the bias setting dead on: exactly 35V for the EL34, and exactly for 45V for the 6L6 (5881 setting) ?
Question:
Were the Quad EL34 tubes matched at 460V plate, 457V screen, and -35V bias ?
If not, then the quiescent current to the output primary may have been un-balanced . . .
Early Saturation of the laminations is possible.
Depending on the current, the Plate and/or Screen may have been over its dissipation limit.
A Guitar Amp designer does not care (and some Hi Fi designers do not care either).
Hot screens, and Hot Plates can disturb the performance of a tube.
Question:
Were the Quad 6L6 tubes matched at 460V plate, 457V screen, and -35V bias ?
(you have to use a 6L6GC model)
If not, then the quiescent current to the output primary may have been un-balanced . . .
Early Saturation is possible.
And any version of 6l6, other than 6L6GC in that amp is beyond the maximum specs for the tube.
A Guitar Amp designer does not care (and some Hi Fi designers do not care either).
Hot screens, and Hot Plates can disturb the performance of a tube.
When EL34 or 6L6 tubes are matched at different plate, screen, and bias voltages than the amp provides, then the tubes might no longer be matched properly in the amp.
What plate voltage, what screen voltage, and what bias voltage(s) were the EL34 Quad, and the 6L6 Quads tested to match ?
The amp designer either was to lazy or too cheap; to make measurements of the individual cathode currents possible, make make individual adjustments of the individual cathode currents; . . . or to use individual Self Bias that would make individual measurements possible, and not well matched tubes to be intrinsically fairly well matched in the amp.
Or, the amp designer did not care.
Quad matched tubes that are Properly matched for the exact amp parameters are typically very expensive. The amp is "cheap" but the requirement for the Properly matched Quad will not be "cheap".
Again, this is a Guitar amp, so care may have been thrown out the window.
I would bet that 2 of those identical amps that had improperly matched Quads sets of the same tube type, might very likely sound different from each other.
Question:
What is the reason to make a Guitar amp have the ability to use EL34 or 5881 ?
Sound differences in that particular amp circuit ?
Desire of Marketing to increase sales ?
Price of tubes and Quad matched tubes ?
Boutique look ?
The phase inverter has un-equal plate load resistors.
But they have common bias, and the schematic shows 205V on both plates.
That is not possible, unless a weaker triode is selected for the tube that has the higher plate load resistor.
Nobody selects dual triodes that way.
Question:
The global negative feedback is from the 4 Ohm tap.
But what tap was the speaker connected to ?
Then there is the setting of the Resonance switch?
Then there is the setting of the Presence switch?
That effects the global negative feedback
But . . .
The open loop to closed loop feedback ratio, is totally different for the EL34 than it is for the 6L6. Because of the different open loop gain due to the wide differences in those 2 tube types transconductance.
Hi Fi amps are typically domesticated.
Guitar amps are typically a beast.
You have to nail down all the variables, except 1.
Scientific study, and getting
answers requires that.
That is why I usually do Not get involved with questions and answers on Guitar amps.
And, that is why I Do get involved with questions and answers on Hi Fi amps.
I hope we have all learned something here.
Without some other Major Revelation about this amplifier, and the things you did that got different sounds, I am done with this thread.
That is why I usually do Not get involved with questions and answers on Guitar amps.
And, that is why I Do get involved with questions and answers on Hi Fi amps.
I hope we have all learned something here.
Without some other Major Revelation about this amplifier, and the things you did that got different sounds, I am done with this thread.
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Thank you again for the detailed insight. It would need some serious research as you suggest to find the cause.
The manufacturer has implemented the dual tube operation because the guitar amplifier sonic signature depends on which tubes are used - and all the "wrong" design decisions are a part of creating the signature. Your point about the resonance switch (and presence control) which are in the feedback path is very valid and I have not realized that.
I must say I subjectively preferred 6L6 family type tubes in any guitar amplifier I had as they seem to add the last bit of good sound to my ear. The amplifier is a part of the tone generation chain, from pick/string selection to the speakers and box (and room/recording mic). Now I at least have an idea about the factors involved in tube type exchange in a guitar amplifier, your posts contained very useful information for me and food for thought on this topic.
The manufacturer has implemented the dual tube operation because the guitar amplifier sonic signature depends on which tubes are used - and all the "wrong" design decisions are a part of creating the signature. Your point about the resonance switch (and presence control) which are in the feedback path is very valid and I have not realized that.
I must say I subjectively preferred 6L6 family type tubes in any guitar amplifier I had as they seem to add the last bit of good sound to my ear. The amplifier is a part of the tone generation chain, from pick/string selection to the speakers and box (and room/recording mic). Now I at least have an idea about the factors involved in tube type exchange in a guitar amplifier, your posts contained very useful information for me and food for thought on this topic.
Really? Here's an example of an EL84 BPT (courtesy Wikipedia) that was made by German manufacturer Lorenz quite some time when faking tubes became popular:
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An externally hosted image should be here but it was not working when we last tested it.
Best regards!
Now I said 6BQ5 not EL84 and my RCA data sheet (normally canonical) says pentode. However, it shows a 9CV basing which has supressor == anode => beam tetrode. And my GE manual says "beam tetrode" .
So I'm wrong. <==
It's the 6GK6 which is the pentode version, with a 9GK basing.
The EL84 definition on the other hand (specifically the "L") allows for either pentode (strapped as a tetrode) or a tetrode.
The catch is you won't get 6BQ5 curves from an EL84 which is based on a pentode (unless you're really lucky).
Mind you, back in the day, a pentode-based EL84 would have run foul of the same patent that resulted in the commercialisation of the beam pentode.
These days, all bets are off. Which is your point. (I think)
Exactly. Even in those good old days you couldn't be sure if you've got a tetrode or a pentode with your »L« coded tube. An example: Telefunken first made their EL11 power tube as a real pentode (maybe also their EL12), but later switched to tetrodes with an increased screen toplate distance. Maybe the had learned from their ECL11 tube which always contained a tetrode (with the same data as the EL11).
Best regards!
Best regards!
How to determine if the tube you have in your hands is a Pentode,
or if the tube you have in your hands is a Beam Power Tetrode:
Do a simple visual inspection . . .
A Pentode has 3 pairs of vertical grid posts; g1, g2, and g3 (each grid is wound around its pair of posts).
A Beam Power Tetrode has 2 pairs of vertical grid posts; g1 and g2 (each grid is wound around its pair of posts). And . . . There are Beam Formers made of Sheet Metal (you can see sheet metal penetrating the Mica insulators, in the same area where a Pentode's g3 vertical grid posts would have penetrated the Mica insulators.
Hint: Do not confuse the sheet metal of the Plate that also penetrates the Mica insulators, versus the Beam Former sheet metal.
or if the tube you have in your hands is a Beam Power Tetrode:
Do a simple visual inspection . . .
A Pentode has 3 pairs of vertical grid posts; g1, g2, and g3 (each grid is wound around its pair of posts).
A Beam Power Tetrode has 2 pairs of vertical grid posts; g1 and g2 (each grid is wound around its pair of posts). And . . . There are Beam Formers made of Sheet Metal (you can see sheet metal penetrating the Mica insulators, in the same area where a Pentode's g3 vertical grid posts would have penetrated the Mica insulators.
Hint: Do not confuse the sheet metal of the Plate that also penetrates the Mica insulators, versus the Beam Former sheet metal.
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