full wave rectifier vs rectifier as a time delay

[tonyg88]

Just to add another interesting "twist" to the class A power topic, as it's a "stable" range, then the question becomes how is there any power output to the speakers if the power through the PS is steady and unchanged as music power is sent out to the speakers and we also use "X" watts of idle current on the output tube plates?

What you said earlier appears to be true because i've heard PP amp still make music with only one output tube per channel with low volume (i am guessing the volume only works until its raised to switch to AB mode instead of A only mode)

 

[Tom Bavis]

Just to add another interesting "twist" to the class A power topic, as it's a "stable" range, then the question becomes how is there any power output to the speakers if the power through the PS is steady and unchanged as music power is sent out to the speakers and we also use "X" watts of idle current on the output tube plates?

Ideal class A has constant input power, so dissipation in output tubes goes down as output power goes up. In class AB, dissipation drops, then increases with output, reaching a maximum at about 2/3 power, then drops above that - kind of an 'S' curve. Input power is constant at first, then increases with output. Of course no components are "ideal" but it's a close approximation.

 

[maxhifi]

In hind sight, it's hard to argue with the Hafler/Laurent/Tucker logic. They designed arguably the most popular stereo basic power amplifier in the world, that was hardly known for eating rectifier tubes. Their call to use but one rectifier -- based on the use patterns they envisioned their purchasers as having -- kept the price of the kit at the intended price point, which no doubt helped to rocket its sales. But for my own purposes, I absolutely agree with this comment: Coming from a band background in the 60's, this is a design approach I like to call "Band Rated". Like a Timex watch, it can take a lickin' and keep on tickin'. From a durability standpoint, the approach serves any design concept very well.

Dave

Yep, that's why the McIntosh Mc60 has two 5U4G rectifiers for a single 60W amplifier. Same with RCA commercial amps, the Marantz 2, and other similar products. Using a tube at half its rating so it will last basically forever is the conservative approach, and the best equipment did it this way.

 

[gadget73]

Another datapoint that may or may not mean anything

I put it back completely stock, no solid state diodes, just built as Pilot intended with the pair of 5U4GB installed
unloaded voltage 406.5v
22w voltage 377.5
7.6% difference

so removing the solid state diodes makes the regulation basically identical to paralleled plates on the rectifier, and voltage ends up right between with the diodes and with the plates paralleled.

 

[kward]

That’s also interesting. The saying “If it ain’t broke don’t fix it” comes to mind.

Of course now the next question will be what will it do with silicon rectification and both 5u4s or both 5AR4 installed with the rectifier plates tied together. Prob will see the same but a percent or so better regulation. Or were the above tests with both rectifiers already?

 

[gadget73]

All tests were done with both rectifiers installed.

 

[trobbins]

A cautionary note relates to the option of bridging the valve rectifier anodes.

In the original configuration the anode of each diode is heated each alternate mains half-cycle when it conducts current, and the peak current rating is related (in some manner) to that presumption.

In the bridged configuration each anode conducts current every mains half-cycle (ie. twice as often). If the energy absorbed by the anode during each half-cycle is more than half of what was absorbed in the original configuration then the valve anodes would operate at a higher temperature - even though the peak current level may be lower than the max datasheet rating. As such, it may not be easy to determine what max peak current rating is applicable with this option - which may be something to consider if an aim with this option was to increase the load current level and/or increase the first filter capacitor size.

 

[Tom Bavis]

The peak current may be higher with plates paralleled, but it's always split between two plates, so each will see a lower peak current. The average current is the same (load current), now spread across al the AC half cycles, not alternating ones. Lower voltage drop means lower dissipation in the rectifier.

 

[Tubeman777]

L

I can't speak for how a VTA120's power claim is figured. But I know for the ST-70 I have no worries about my 5AR4 being able to handle it. I just keep in mind that to get 3dB of sound power you have to double the wattage so when you wring your hands over a few % difference in B+ by paralleling the plates, .... I ain't bovered. And I think there's more to it than is being considered here, such as taking the cathode into consideraton, and if you lower the internal resistance which lowers the voltage drop and makes it easier for current to flow through it, what happens if you increase the 1st filter cap and make it pull more to charge it with a plate that is in constant conduction instead of having 1/2 cycle off. Will that also increase the risk of a hot start arc-over?

I totally understand that, but I like equipment which can put out full power "all day long" without adverse effect. I think it's poor practice to do otherwise. Once I used a Dynaco ST-70 in mono with an audio oscillator and a step up transformer to supply 50Hz power to a European record player. It consumed about 50W and didn't stress the Dynaco after hours of operation.

 

[Tubeman777]

After reading this thread I think I can see why Bob retired. It appears he was building circuits that he had no clue about. He should have fixed this rectifier problem before retiring thou.

I had aVTA120 built with some customization. I eliminated the ugly quad can cap, and designed my own unique power supply. I used 2 Emission lab 274b mesh plates , wired in parallel. I used an 8.7. UF first cap, and 20uF for 2nd cap. I added 2 dual section caps 16uf+ 32uf. And a 50uf +50uf dual section cap, kept the resistors that are supplied with the VTA 120. Finally, I ran 2 chokes in parallel ( the stock c24 chokes). A matched quad of KT88 Mullard reissues. Sylvania (5953 gold brand in the 2 phase inverters,)and aGenelex 12au7 input. The parallel 274b’s can supply about 180mA each( over 310 macombined) my B+ is close to 500 vDC. Parallelrectifiers cuts impedance by 50%.The stock single GZ34 is almost at 100% of its maximum and is inadequate!( even in st70 clone it is over matched with EL34 or 6CA7 power tubes.

The 274b can easily handle over 300 mA@ 500+ DC volts are easily handled as well. The tube lag is almost non-existent, as the parallel rectifiers have Much less impedance (50% less voltage drop!) You gain 30-40 volts in B+ and the response is quick like solid state rectifier ( butmaintains tube sound) . And the mesh plates are slow warming, so they protect the power tubes,Dont try PSVANE 274b’s; they are “instant on” Direct heated, and slam the power tubes with 500voltz while the kt88 Are still cold. I tried the PS Vanes; they work, but are not comparable to the EMLs. The EML mesh plates are easy on power tubes. And the EMLz use 2 amps heater current; the PSVanThey also are dead quiet, no static, sparks, no noise. And no red plates. I had a russian gz34 almost light on fire! It red plated and burned to a crisp! It cant handle the stock setup. It even failed with el34 powder tubes. The sound of the amp is awesome! I get about 65 watts per channel.
The elimination of the quad can cap was no problem. EML told me I could use 8uf first cap wifh parallel rectifiers. The 274b are much less stressed than GZ34. The EML mesh plate are costly, but the circuit is easy on the 274bs. I have run the same pair for almost 15 years, and they still test very strong. They are very well made, and will outperform any 5AR5 ir GZ34. They only need 2 amps each for heater. And they look great! No hum at All. And you can even run stronger outputs than kt88! Kt90 are possible! You can replace the two

Last edited: Today at 8:32 PM

 

[Tubeman777]

I don't have a VTA dog in this fight. Your data sheet example may not be anything like the VTA120 is set up for. Look at the bias voltage in your example ~59v. It's clear that that setup is deep into AB. Does the VTA 120 put that much signal on the outputs? I don't really care if it does or not. Someone should post the biasing values for kicks... they don't seem to be available on the site. But the original post here is about the ST-70 ish type loads.

I wonder why it is designed that way, it clearly exceeds the ratings of the GZ34. Actually, even the original ST-70 pushed it to the limit, two in parallel would have been better. Maybe the designer counts on the fact the amp is seldom called on to deliver full power. This connection seems like a quick fix for those amplifiers who ask too much from a single rectifier.

 

[Tubeman777]

I had aVTA120 built with some customization. I eliminated the ugly quad can cap, and designed my own unique power supply. I used 2 Emission lab 274b mesh plates , wired in parallel. I used an 8.7. UF first cap, and 20uF for 2nd cap. I added 2 dual section caps 16uf+ 32uf. And a 50uf +50uf dual section cap, kept the resistors that are supplied with the VTA 120. Finally, I ran 2 chokes in parallel ( the stock c24 chokes). A matched quad of KT88 Mullard reissues. Sylvania (5953 gold brand in the 2 phase inverters,)and aGenelex 12au7 input. The parallel 274b’s can supply about 180mA each( over 310 macombined) my B+ is close to 500 vDC. Parallelrectifiers cuts impedance by 50%.The stock single GZ34 is almost at 100% of its maximum and is inadequate!( even in st70 clone it is over matched with EL34 or 6CA7 power tubes.

The 274b can easily handle over 300 mA@ 500+ DC volts are easily handled as well. The tube lag is almost non-existent, as the parallel rectifiers have Much less impedance (50% less voltage drop!) You gain 30-40 volts in B+ and the response is quick like solid state rectifier ( butmaintains tube sound) . And the mesh plates are slow warming, so they protect the power tubes,Dont try PSVANE 274b’s; they are “instant on” Direct heated, and slam the power tubes with 500voltz while the kt88 Are still cold. I tried the PS Vanes; they work, but are not comparable to the EMLs. The EML mesh plates are easy on power tubes. And the EMLz use 2 amps heater current; the PSVanThey also are dead quiet, no static, sparks, no noise. And no red plates. I had a russian gz34 almost light on fire! It red plated and burned to a crisp! It cant handle the stock setup. It even failed with el34 powder tubes. The sound of the amp is awesome! I get about 65 watts per channel.
The elimination of the quad can cap was no problem. EML told me I could use 8uf first cap wifh parallel rectifiers. The 274b are much less stressed than GZ34. The EML mesh plate are costly, but the circuit is easy on the 274bs. I have run the same pair for almost 15 years, and they still test very strong. They are very well made, and will outperform any 5AR5 ir GZ34. They only need 2 amps each for heater. And they look great! No hum at All. And you can even run stronger outputs than kt88! Kt90 are possible! You can replace the two

Last edited: Today at 8:32 PM

 

[Tubeman777]

I can't speak for how a VTA120's power claim is figured. But I know for the ST-70 I have no worries about my 5AR4 being able to handle it. I just keep in mind that to get 3dB of sound power you have to double the wattage so when you wring your hands over a few % difference in B+ by paralleling the plates, .... I ain't bovered. And I think there's more to it than is being considered here, such as taking the cathode into consideraton, and if you lower the internal resistance which lowers the voltage drop and makes it easier for current to flow through it, what happens if you increase the 1st filter cap and make it pull more to charge it with a plate that is in constant conduction instead of having 1/2 cycle off. Will that also increase the risk of a hot start arc-over?