# Tube Amp output..contributing factors ?



## greco (Jul 15, 2007)

*What the various factors that contribute to the total output/power (in watts) of a tube amp?*

Obviously power tubes are a key factor, but what about tube rectification vs SS rectification?

How much of a role do the transformers have in output?

What about the overall design of the circuit? 

Do Reverb and Tremelo (for example) reduce the amp's potential total power output? 

As for power tubes, what total output can one expect from various tubes (in singles, pairs, etc) such as: 6L6, 6V6, EL34, EL84, etc?

Thanks for your comments.

Cheers

Dave


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## dtsaudio (Apr 15, 2009)

The type of power tube does have a bearing on how much power, but it's based on how much voltage is on the tubes. For example, if your amp is EL34 based, you won't increase power by putting in KT88's. However a higher power amp can be built with KT88's because they can take higher plate voltage and can withstand more current.
The type of rectification doesn't matter in as much as the voltages needed can be achieved either way. However the differences lie in that ss rectifiers are more efficient, can generally take more current and more filter capacitance can be used. Tube power supplies also tend to sag more. That is under heavy load the voltages will dip more, giving less power. 
The circuit topology is also important. Single ended will give substantially less power power than push-pull designs. Then there's the bias type - cathode or fixed.

Reverb, tremolo, tone controls, don't affect overall power. They do affect distortion though. 

This is the short answer.


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## greco (Jul 15, 2007)

Thanks...much appreciated. 

The reason I asked was as a result of a conversation I had with a friend today. He had read about an amp (advertised on the internet) that has 2 x 6L6's in a Princeton Reverb type circuit and the output is 35 watts.

Cheers

Dave


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## nonreverb (Sep 19, 2006)

greco said:


> Thanks...much appreciated.
> 
> The reason I asked was as a result of a conversation I had with a friend today. He had read about an amp (advertised on the internet) that has 2 x 6L6's in a Princeton Reverb type circuit and the output is 35 watts.
> 
> ...


Transformers would be a huge part of the equation Dave. 
Not much chance of getting 35 watts out of a Princeton circuit without beefing up both the power and output transformers as well as the 6L6's.

Cheers,

R


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## greco (Jul 15, 2007)

Thanks nonreverb...Some day I hope to be able to build a much better understanding of all this stuff from a more "collective" /"gestalt" perspective.

Certainly is slow going at the moment....LOL

Cheers

Dave


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## Swervin55 (Oct 30, 2009)

I also appreciate the comments above. I have a Badcat Cub II which is an advertised 15W. Unfortunately unusable in a live setting (with a drummer) as it does not have the juice. Still I have experienced many other amps in the same watt range that have more than ample power. My thoughts were that it is in the transformer, never considering the overall design of the circuitry. Then again, I have read several threads on the same amp and other owners rave about the amp's capabilities in a live setting.?


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## greco (Jul 15, 2007)

Swervin55 said:


> I also appreciate the comments above. I have a Badcat Cub II which is an advertised 15W. Unfortunately unusable in a live setting (with a drummer) as it does not have the juice. Still I have experienced many other amps in the same watt range that have more than ample power. My thoughts were that it is in the transformer, never considering the overall design of the circuitry. Then again, I have read several threads on the same amp and other owners rave about the amp's capabilities in a live setting.?


Maybe the efficiency of your speaker(s) is/ are part of problem also....just a thought.

Cheers

Dave


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## dtsaudio (Apr 15, 2009)

> Maybe the efficiency of your speaker(s) is/ are part of problem also....just a thought.


And a good thought at that. Loudness is more a function of speaker efficiency than it is of power. You need ten times the power with a given speaker to double the volume. A 50 watt amp will only be slightly louder than a 15 watt if they have the same speaker. However speaker efficiencies vary a great deal. They are measured in db typically at 1watt input at 1 meter away. The higher the db the louder the speaker (or less power needed). However it is not necessarily a measure of quality.


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## Swervin55 (Oct 30, 2009)

*re: speaker*

This is a combo with the factory speaker, but point taken. In a previous attempt to "fix" this thing I completely re-tubed with no success. It is now relegated to my basement (townhouse) where it can't do any damage. It works for what I use it for, just a little disappointed I can't take it out. Thanks for the reponses. 

Swervin


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## Doctordog (Jun 9, 2010)

Hello,
If the amp is really able to put out 15 watts, that should be more than enough to run a 4x12 cab at very loud volume. But if you are just running it into a single 6" or 8" speaker, even a single 10" you are going to get much less volume than your amp is capable of putting out.

JB


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## GuitarG. (Apr 1, 2010)

greco said:


> *What the various factors that contribute to the total output/power (in watts) of a tube amp?*
> 
> Obviously power tubes are a key factor, but what about tube rectification vs SS rectification?
> 
> ...


Some amps can take changes in output tubes. EL34-6550-KT88 etc. can be interchanged without altering much on an amp other than 2 or 3 resistors usually. If the amp has good transformers this should'nt be a problem. Marshalls in Britain during the 70's and 80's were equipped with EL34 tubes while Marshalls exported to the US came equipped with 6550 (higher handling output tube than the EL34). There were no differences in the transformers. The voltage is set, what you are adjusting is the power (watts), the bias current which you can increase or decrease the output tubes power or wattage. A 6550 equipped Marshall or even a KT88 can have its idle current set higher than an EL34 for better power handling when the amp volume is really pushed, however the output transformer will also work harder with the more beefier tubes if you're looking to push the volume. Someone on here like Wildbill and mabe a few others could go into exact technical details on this. 
The tubes mentioned can usually be interchanged because they can share the same pinouts in most amps. What you don't want to do is swap EL34's with 6L6's in these older circuits. Make sure you know what tubes can be swapped and which ones can't. This can be done in some amps but usually with more re-wiring and changes in the circuit. One amp I've wondered about is the THD univalve and Bi-valve which the builder advertises can take any tube from 6V6-EL34-6L6 too KT90's without any modification. I'm wondering if it still puts the amp at any risk.
SS rectification will allow more power to the output tubes and give you quicker response than the same amp with tube rectification. Some amps with tube rectification (the Super Reverb comes too mind) will exhibit more sag. As you initially pluck a string the rectifier tube will draw more power somewhat cutting or starving the output tubes giving it a softer feel or attack. As the note decays and the power comes down in the rectifier tube it allows more power too be fed to the output tubes so the signal remains reletively the same even though the string is decaying. this gives the appearance of sustain or compression.
As far as the Reverb and Tremelo, it depends on the cirucuit. I know that in Blackface and Silverface amps some signal gets leaked too ground whether the tremolo is turned on or off. If you don't use tremolo in these amps, you can disconnect the tremolo from ground which will disable the tremolo. This will put more signal into the rest of the amp circuit giving a bit more volume and richer midrange.
And that Princeton you mentioned would have had to have been heavily modified too the point it's a different amp in a princeton chassis if it is infact pushing 35 watts.


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## greco (Jul 15, 2007)

Thanks GuitarG...very helpful....especially what you wrote about rectification, tremelo and reverb.

Cheers

Dave


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## nonreverb (Sep 19, 2006)

GuitarG. said:


> Some amps can take changes in output tubes. EL34-6550-KT88 etc. can be interchanged without altering much on an amp other than 2 or 3 resistors usually. If the amp has good transformers this should'nt be a problem. Marshalls in Britain during the 70's and 80's were equipped with EL34 tubes while Marshalls exported to the US came equipped with 6550 (higher handling output tube than the EL34). There were no differences in the transformers. The voltage is set, what you are adjusting is the power (watts), the bias current which you can increase or decrease the output tubes power or wattage. A 6550 equipped Marshall or even a KT88 can have its idle current set higher than an EL34 for better power handling when the amp volume is really pushed, however the output transformer will also work harder with the more beefier tubes if you're looking to push the volume. Someone on here like Wildbill and mabe a few others could go into exact technical details on this.
> The tubes mentioned can usually be interchanged because they can share the same pinouts in most amps. What you don't want to do is swap EL34's with 6L6's in these older circuits. Make sure you know what tubes can be swapped and which ones can't. This can be done in some amps but usually with more re-wiring and changes in the circuit. One amp I've wondered about is the THD univalve and Bi-valve which the builder advertises can take any tube from 6V6-EL34-6L6 too KT90's without any modification. I'm wondering if it still puts the amp at any risk.
> SS rectification will allow more power to the output tubes and give you quicker response than the same amp with tube rectification. Some amps with tube rectification (the Super Reverb comes too mind) will exhibit more sag. As you initially pluck a string the rectifier tube will draw more power somewhat cutting or starving the output tubes giving it a softer feel or attack. As the note decays and the power comes down in the rectifier tube it allows more power too be fed to the output tubes so the signal remains reletively the same even though the string is decaying. this gives the appearance of sustain or compression.
> As far as the Reverb and Tremelo, it depends on the cirucuit. I know that in Blackface and Silverface amps some signal gets leaked too ground whether the tremolo is turned on or off. If you don't use tremolo in these amps, you can disconnect the tremolo from ground which will disable the tremolo. This will put more signal into the rest of the amp circuit giving a bit more volume and richer midrange.
> And that Princeton you mentioned would have had to have been heavily modified too the point it's a different amp in a princeton chassis.


It's true that 6550's were planted in Marshalls shipped to the US in the '70's and '80's. However, there is no jump in power between them. There is a difference in the sound though. Remember, plate voltage ultimately determines the wattage rating. And since the power transformer, PI circuit and the output transformer is the same, there's no difference. 
A 50W Marshall in Britain is a 50W Marshall in the US too.
As for bias voltage, yes there is a difference as 6550's need more neg. bias.


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## GuitarG. (Apr 1, 2010)

nonreverb said:


> It's true that 6550's were planted in Marshalls shipped to the US in the '70's and '80's. However, there is no jump in power between them. There is a difference in the sound though. Remember, plate voltage ultimately determines the wattage rating. And since the power transformer, PI circuit and the output transformer is the same, there's no difference.
> A 50W Marshall in Britain is a 50W Marshall in the US too.
> As for bias voltage, yes there is a difference as 6550's need more neg. bias.


Setting the bias is going to give the output tubes more or less output wattage with a shift in current draw. As for the plate voltage B+ is not hardly going to shift. It is pretty much set high at a specific voltage usually 450-470vdc in most Marshalls with the exception of some in the early 70's with over 500 vdc. I agree that a 50 watt amp is a 50 watt amp regardless of what output tubes are used or where the amp is used. The differences are in the power ratings of the tubes, or more specifially max dissipation when the tubes are really pushed. A 6550 or KT88 is not going to break up as easily as an EL34 in the same amp since they can handle more power or wattage.


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## nonreverb (Sep 19, 2006)

GuitarG. said:


> Setting the bias is going to give the output tubes more or less output wattage with a shift in current draw. As for the plate voltage B+ is not hardly going to shift. It is pretty much set high at a specific voltage usually 450-470vdc in most Marshalls with the exception of some in the early 70's with over 500 vdc. I agree that a 50 watt amp is a 50 watt amp regardless of what output tubes are used or where the amp is used. The differences are in the power ratings of the tubes, or more specifially max dissipation when the tubes are really pushed. A 6550 or KT88 is not going to break up as easily as an EL34 in the same amp since they can handle more power or wattage.


Hence why some prefer US Marshalls and some don't. It's a taste thing. Both are good but for different reasons.
The difference that's a potential problem is the filament windings of the power transformer. That's where guys can get into trouble. Two 6550/KT88's take more filament current to light than EL34's. Put them in an amp with a low spec 6.3 and there's a chance the tranny will overheat.
As for the Marshall conversion to 6550's, the other thing typically done is move the neg. feedback wire to the 4OHM leg of the output impedance switch. Not absolutely necessary but it sounds better with 6550's IMO. There are other mods that can be done to make the conversion more reliable but that's another story...


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## GuitarG. (Apr 1, 2010)

nonreverb said:


> Hence why some prefer US Marshalls and some don't. It's a taste thing. Both are good but for different reasons.
> The difference that's a potential problem is the filament windings of the power transformer. That's where guys can get into trouble. Two 6550/KT88's take more filament current to light than EL34's. Put them in an amp with a low spec 6.3 and there's a chance the tranny will overheat.
> As for the Marshall conversion to 6550's, the other thing typically done is move the neg. feedback wire to the 4OHM leg of the output impedance switch. Not absolutely necessary but it sounds better with 6550's IMO. There are other mods that can be done to make the conversion more reliable but that's another story...


Interesting. Thanks for adding that Nonreverb.


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