Tube rectification in bass amps?

[agedhorse]

Sloppy, saggy, squishy sound really good on paper and in marketing materials, but IME a little goes a long way with regard to the real world for bass guitar.

 

[Fab Superlative]

Thanks folks, this is a very informative discussion!

Well, you started it. So we should be thanking you too.

 

[BogeyBass]

Thanks for the detailed explanation BogeyBass.

As per your comment below, I was not trying to say tube rectification would improve sound over SS, but in my Decware amps (www.decware.com), different rectifier tubes give different signature sounds. One of the amps, for instance, is compatible with 5U4, 5Y3, GZ32, GZ34, 274B, Type 80, and quite a few more rectifier tubes. They all sound different given all else constant.

Ahh I see. Usually with a 3 or 6 watt amplifier you wouldnt notice a big change with a Rectifier Tube change.

Ill assume these are single ended or Class A amplifiers.
If so the big change heard would be change in available current
or swapping the tube and not adjusting bias.
Idle current is substantially higher with class A.
The amplifier operating point is changed so distortion may increase.
You could possible bias out some changes with certain Rec tubes.
Since quick swapping rec tubes might shift the bias to a more distorted sound.
Which could possibly be adjusted so tube swaps might not be so obvious.
Changing a tube Rec in Class A/B in theory wouldn't have much change with
a 3 to 6 watt amplifier.

A quick view of various data sheets, find the highest current rec tube.
set the bias and that would most likely give cleanest performance.
Most would assume around 30 volt voltage drop for a tube rec.
Some can shoot up to 50 volts. So it might be another audible
change with quick swapping rec tubes.

With your amplifier it might be good to know the current capability
of the 5 volt Heater winding in the power transformer.

Some rectifiers pull around 1.8 amps for 5 volt heaters.
Some can pull up to 3 amps for 5 volt heaters.

Say swapping to a GZ34 is about 2 amps
or a 5U4G is about 3 amps.

So if by chance your power transformer has only 2 amp 5 volt winding.
A 3 amp Rec tube would possibly cause a voltage drop to heater voltage.
Since its pulling more current.
Another scenario would be it does have a 3 amp heater Winding
and lower current Heaters are now getting higher voltage.

Sometimes its better to design the heater windings to a specific rec tube.
So you know to use a 2 or 3 amp 5 volt winding.
Often if 2 amps is all that is needed. No reason to eat up space in
the power transformer core for a larger 3 amp 5 volt winding

 

[lowphatbass]

I thought about a plug-in rectifier for my Sunn 200S when I first got it. After playing it a bit I decided to leave it alone, the amp sounds great. I came to the realization that it’s the 2x15 JBL cab that really much makes it a true “gig-worthy” amp...it’s a stout 60w but it’s still only 60w.

 

[FronTowardEnemy]

Interesting. Always wondered about this. My Fender Super Bassman has a SS rectifier.

 

[Alper Yilmaz]

I found a graph like the attachment.

By the way, the amps I am talking about are self-biasing, yet, I am not sure how precise the self-biasing mechanism is. So there might still be minute differences in how the output tubes are fed.

 

[Alper Yilmaz]

FronT, I have not looked into lots of bass amplifiers, but the ones that used to use tube rectification are relatively older designs like Ampeg B15, which are 25-30 watts, which is consistent with the explanations above. I understand that your amp is 300 watts. So, not a contender!

By the way, interestingly, all of my hi-fi gear at home is tube, yet as a sell-out bass player, all my bass amps are solid-state! But everytime I plug my Precision with TI flats into an SVT feeding a fridge, I say "holy poopie!" I can see why that is one of the benchmark sounds that most amp designers are trying to reach at!

Interesting. Always wondered about this. My Fender Super Bassman has a SS rectifier.

 

[Wasnex]

I found a graph like the attachment.

By the way, the amps I am talking about are self-biasing, yet, I am not sure how precise the self-biasing mechanism is. So there might still be minute differences in how the output tubes are fed.

In simple terms, the higher the plate voltage the harder the tubes will conduct. The cathode resistor, which is what self biases the amp, will need to be selected to work with the highest plate voltage. Then as you select rectifiers that produce less voltage, the resistor will bias the tubes progressively colder.

The way the bias works is a voltage drop develops across a common cathode resistor depending upon how much current is passing through the resistor. This voltage drop forces the cathode positive with respect to the control grid. In other words, the control grid is negative with respect to the cathode, which is what controls how many electrons pass from the cathode to the plate.

For example I have an old Vox AC30 which is a pseudo class A design with cathode bias. The cathodes of the four tubes are tied together and attached to an 82 ohm resistor that is tied to ground. FYI the original value of the resistor was actually 50 ohms, but it's common to use 80-82 ohms to get some life out of the tubers. The schematic shows 10mV across the 50 ohm resistor which works out to 200mA of current, and puts the tube dissipation well over 100%.

When the amp is operating, it has 330V on the plates and 11.09V is dropped across the cathode resistor. That means 135.2mA of current is flowing through the cathode resistor. (11.09/82=~0.135.2mA) The effective plate voltage is (330-11.09=~319V). If your curious the voltage on the control grids is 45mV on two tubes and 55mV on the other two. So the control grid voltage is positive, but it's considered negative with respect to the cathode.

Per this bias calculator: Tube Bias Calc
90% dissipation for EL84 at 319V effective plate is ~33.9mA per tube. The amp has four tubes, so (4x33.9=135.6mA) is the total target for 90% dissipation.

What you may not realize is tubes are biased to a certain power level, and in order to keep this power level constant as the voltage goes up, the current must go down. So if you want the amp to bias optimally at different plate voltages the value of the cathode resistor must be changed.

I run this amp on a variac set to 110V. It's fairly common to run vintage AC30 like this because if you run them at 120V on 60hz the plate voltage runs way above spec and the tubes run really hot. Although I haven't documented what the actual effective plate voltage would be, it should be about 10% higher. (319x1.1=~351V) effective plate (estimated). 90% dissipation for one tube at 351V effective plate voltage is 30.8mA; so the total for all four tubes would be (4x30.8=123.2mA).

Hopefully you see the problem here. At 319V, 90% dissipation is at 135.6mA, and at 351V, 90% dissipation is at 123.2mA. This confirms what I said earlier, as the voltage goes up the current must come down to keep the dissipation at 90%. But that's not what will happen. As the effective plate voltage increase from 319V to 351V, the tube current will also increase, so at some point dissipation will go over 100%...meaning the tubes will burn out relatively quickly.

A solution to allow this amp to run on 120V may be to swap out the GZ34 rectifier for a 5V4 rectifier. I have read the 5V4 will lower the plate voltage by 20-25V. Course there is some concern that 5V4 cannot provide enough current, so perhaps a better solution is a buck boost transformer to drop the 120V down the the 110V the amp likes. I think I will just continue to use my variac for now.

 

[Wasnex]

FronT, I have not looked into lots of bass amplifiers, but the ones that used to use tube rectification are relatively older designs like Ampeg B15, which are 25-30 watts, which is consistent with the explanations above. I understand that your amp is 300 watts. So, not a contender!

By the way, interestingly, all of my hi-fi gear at home is tube, yet as a sell-out bass player, all my bass amps are solid-state! But everytime I plug my Precision with TI flats into an SVT feeding a fridge, I say "holy poopie!" I can see why that is one of the benchmark sounds that most amp designers are trying to reach at!

The Sunn 2000S and Matchless Thunderchief are the biggest tube rectified amps I know off. The Sunn 200S is 60W and many say it sounds better than the 2000S.

 

[Downunderwonder]

What's a rectifier? Sounds like something that fixes something that was a little bit broken.

 

[Rockbobmel]

Back there, behind the PT.

 

[Rockbobmel]

 

[BassmanPaul]

A tube rectifier like the GZ34/5AR4 can supply a fifty watt amplifier. !00W needs two. 160W needs three. Two Hundred Watt four. An SVT six. On top of that you would need heater supplies for them, 5V AC at 2A per tube. so that SVT would need 12A for the heaters.

On the subject of silicon rectification it doesn't have to be any noisier than a tube rectifier. It's all in how it is used.

 

[agedhorse]

In fact a silicon rectifier can be quieter than a tube rectifier, especially if modern HF design techniques are used.