# Tube vs Solid State Rectifier - Explanation and Opinion



## GuitarsCanada

Looking for a tech opinion on tube vs ss rectifier. Obviously the tube variety were the first and are used almost exclusivley in old tube amps, but whats the skinny on this topic?


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## nonreverb

You pretty well hit the nail on the head with your statement. They were the first type of rectifier used on any amp. Why? Because that's all there was! Solid state technology only made it's entrance in the late '50's and they were expensive at first. That's probably one good reason why tube rectifiers stayed around for some time. The change to solid state took place when amp builder began making high power units. Tube rectifiers like the 5U4 and 5AR4 won't handle an amp with 100 watts output...but silicon rectifiers sure can.
As for differences in "sound" well it can be a tad subjective. Tube rectifiers do two things that can affect the way an amp performs and therefore, sounds. First, they drop voltage across themselves. This means that the 450 volts you would see on your ss rectified 6L6 or EL34 would be reduced to around 420 for a 5U4 and around 430 for a 5AR4. This will drop the output wattage slightly which can have a noticeable effect. The second thing it does is sag. There's alot of debate as to how much sag plays in the sound of a given amp. Basically it's the reactance lag of the rectifier when loaded with something like a big E chord on a guitar. Since a tube rectifier cannot tolerate large filter capacitance, the capacitor filtering it's rectified voltage dumps fairly quickly when loaded and the rectifier cannot act fast enough to fill the void. This creates a momentary dip in the plate voltage which causes the output tubes to lose some power which translates into what some would describe as a compressed or gritty sound. Lastly a tube rectifier has a relatively small current capability. The 5U4 which is one of the heavy duty types can only handle around 1/4 amp of continuous current.
The solid state diode does not suffer any of these problems. With a voltage drop across it of only .7 volts, it's electronically transparent. Plus it sufferes no sag whatsoever and voltage and current ratings are no constraint as there are types that will handle any voltage and current. The other advantage is that the rectifier capacitor can be massive compared to tube rectified circuits which works well in high powered amps requiring lots of reserve for clean tight bass...an SVT is a perfect example.
I hope this wordy explanation helps.


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## Wild Bill

Here's the scoop, Scott!

A tube rectifier is perhaps the first tube ever invented. A hot filament will boil off electrons. These will hang out in a big cloud around the filament but if you put a plate near the cloud and allow it to have a positive charge the electrons will keep boiling off the filament and head for the plate. The plate will have been made part of a complete circuit so the electrons eventually return through that circuit to the filament, which is also called a cathode.

All you have to understand is that this puts a continual shortage of electrons at that cathode, since they all keep heading for the plate, or anode! This makes the voltage at the cathode read positive, 'cuz electrons are naturally negative and there's a shortage, which looks positive. If we had put an alternating current into the system the effect is that only the positive pulses appear at the cathode. That's because when the polarity of the the AC supply voltage reverses the plate is negative with respect to those electrons. Free electrons are negative in charge anyway so they won't travel to the plate. The current flow will only happen for half of the AC wave, that time when the plate can attract those electrons.

So that's how they work! At least, deep enough for rock and roll! There are a couple of practical details that are important for guitar amps. First off, tube rectifiers have a voltage drop across them that increases as the current heads for a maximum. The more you drive the amp louder the bigger the voltage drop. That means the voltage on your output tubes starts to drop on volume peaks, like when you've turned up the amp and hit a chord or a note very strongly. Less voltage on the plates of the output tubes means the gain of the tube drops somewhat.

When the gain drops, the tube doesn't stop conducting the energy of the note and clip it off, making a harsh distortion. Rather, the top of the signal wave just "rounds off", since the voltage is no longer high enough to let the tube reproduce the big spike. This rounding off adds some compression to the note, making the tone "warmer" and "thicker". 

Different tube rectifiers can handle different amounts of power. A 5Y3 is usually good for a 20 watt amp. A 5U4 is good for at least 50 watts.

They also have different amounts of voltage drop. A 5U4 might drop a maximum of 50 volts. A GZ34/5AR4 only 20. So the GZ34 is said to be "stiffer". That voltage drop increase on powerful spikes is called "sag" and that's what gives an amp that vintage, "rockin' blues" tone. The 5U4 would have lots of sag!

Solid state diodes have a tiny, fixed voltage drop. The usual power supply diodes drop around 1 volt! You'll never hear any sag at all with that! They came in around the Fender BlackFace era and were largely responsible for how they sounded "tighter" and "quicker" and following fast picking. If a really big spike comes along and there's not enough energy in the power supply circuit to let the tubes reproduce it fully the top of the signal WILL clip off, sounding harsh. Of course, we're talking pretty loud for this to happen!

You can pick whichever sound you like but manufacturers try to stick to solid state diodes for one big reason, money! A tube rectifier with socket and an extra transformer winding to heat up its filaments might add $10-$15 to the cost. A solid state diode might cost a nickel in production quantities!

I could also mention that tube rectifiers take some seconds to warm up and start delivering power, just like the rest of the tubes in your amp. This is a bit easier on things, having a warmup time. The solid state diodes are "instant on" which means the rest of the tubes see full high voltage before they even start to warm up. This doesn't hurt the tubes badly but it DOES hurt them a little bit, each time the amp is turned on. It takes a bit off the lifetime of the tubes. That's one of the reasons why it's a good idea with higher powered amps to use a standby switch instead of constantly turning the amp on and off.

I've probably forgot sometihing, Scott. I haven't had a coffee yet! Hopefully, I've given you enough for what you need.


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## nonreverb

I have heard tell that some amp manufacturers actually put a series resistor in ss rectified amps to mimick the sag of a tube rectifier...


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## GuitarsCanada

Excellent tech explanation there boy's. That gives me a very good idea as to what is going on in terms of the use and reasoning behind them. I wonder just how much he whole "sound" or "tone" aspect comes into play. If there is no significant difference, then it would appear that the new technology makes perfect sense from a cost and maintainance standpoint.


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## Ripper

There is a difference in tone/sound. It is subtle but it is there...tube rectifiers tend to have a more compressed and a more airy or open sound when being driven hard. It also differs between rectifiers as well.


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## Rugburn

I think for certain styles, a tube rectified amp is best. However, I really like a quicker "feel" for country pickin'/rockabilly/surf type things that diodes provide.


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## GuitarsCanada

What about these solid state plug-ins that you can buy for tube amps? Any experience with these or is it just not worth the effort?


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## Wild Bill

GuitarsCanada said:


> What about these solid state plug-ins that you can buy for tube amps? Any experience with these or is it just not worth the effort?


Once again, be careful! Most of these are just some diodes in a tube base. They don't allow for the greater voltage drop of a tube. So when the circuit has been designed for a certain plate voltage and you drop one of these SS suckers into the socket, suddenly all the voltages increase by that voltage drop, which can be 20 volts with a 5AR4/GZ34 or maybe 40 volts with a 5Y3.

Since most amps already run voltages pushed beyond the comfortable specs this can start burning things out At the very least, the bias should be reset on the output tubes!

Weber's "Copper Cap" tube rectifier replacements are the only ones I know that preserve the tube voltage drop.


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## GuitarsCanada

Your talking about these things here, Bill? Very good


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## nonreverb

It's a common mod on Fender amps. Fenders like Traynors seem to be the the "lego" amps of the past. People modded the crap out of them. I've had Fenders on the bench with diode mods that had obviously been there for years. They seem to take it OK. I'm not a fan of the way they change the sound though. For the extra couple of watts you might reclaim from the extra voltage, the tone tends to be a little harder. Way back when I got my first old Deluxe Reverb, I did this mod with a plug-in type rectifier as seen above. It didn't last long though and I reverted back to the tube...it just sounds better.
If reliability is the issue, then your best bet is the Weber copper caps. I use them in all my Hammond rental units as they are far more reliable than the delicate 6X4's they replace.


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## WCGill

Another difference between SS and tube rectifiers is the amount of capacitance you can use immediately after the rectifier. With SS, there is no limit but each tube rectifier has it's own maximum allowable value. Some are as low as 8uf, some like the GZ34 list a maximum of 60uf. Too high a value will stress the rectifier on initial charge and may result in a shorted tube. This is a very important design consideration.


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## Lincoln

Is it true that when you replace a tube rectifier with SS diodes in a circuit designed for a tube rectifier.......that first cap will take a beating because it's suddenly able to "load" so quick/hard/fast??
could a resistor be used to slow down that "rush" into the cap?

Looking at schematics, it appears to me that tube filter caps are often in the 20 to 40uf range while diode filter caps are in the 60 to 80uf range. Is there a relationship there?


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## Lincoln

just noticed I bumped a thread so old it was pre-amp repair forum & it ended up in the amp section


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## Jim DaddyO

Lincoln said:


> just noticed I bumped a thread so old it was pre-amp repair forum & it ended up in the amp section



Thanks, it was worth reading.


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## loudtubeamps

nonreverb said:


> You pretty well hit the nail on the head with your statement. They were the first type of rectifier used on any amp. Why? Because that's all there was! Solid state technology only made it's entrance in the late '50's and they were expensive at first. That's probably one good reason why tube rectifiers stayed around for some time. The change to solid state took place when amp builder began making high power units. Tube rectifiers like the 5U4 and 5AR4 won't handle an amp with 100 watts output...but silicon rectifiers sure can.


Question..... Limitations of output stage with tube rectifiers?
I'm a wee bit confused, not a big user of tube rectifiers, so not alot of practical experience with them. I see lots of 4 valve amps out there with tube rectifiers, singles and ganged (GZ 34's, 5U 4's, 5C 3S), not sure what the correlation is between rectification and output other than demand on the rectifier?? 
Any info here would be appreciated, cheers, D


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## Beach Bob

Jim DaddyO said:


> Thanks, it was worth reading.


Ditto.. (just chiming in so I get notifications on new posts).


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## mhammer

Always good to understand how things work.

FWIW, I have a little SS plug-in rectifier that I swap with the 5Y3 on my tweed Princeton now and then. You can hear the difference. Not night and day, but plainly audible.

One of the things to remember is that there are a great many amps, by companies we all know and love, that use essentially the same tube complement, yet have a slightly different feel and tone. Often, the difference will be the result of the power supply used for amp X vs amp Y. This one gets 410V on the plates, where that one gets 395 or 415.

When you switch from tube to solid-state rectifier, or the other way around, you are not only changing the manner in which current is supplied, but you are also often changing the plate voltage that results.


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