Rectifier efficiency?

[dehughes]

Hey all,

So, somewhere, sometime, I came across a listing of the various efficiencies of different rectifiers (diodes, 5AR4, 5Y3, etc..).

Can anyone point me in the direction of such a chart? I'm trying to figure out what B+ rating I need on my PT in order to get voltages in the ballpark for my 6V6 pair. Specifically, I'm looking to get 365-ish volts on the plates of my 6V6 with a 5AR4 recto...and most every PT I've put in seems to be kicking out standard "Deluxe" voltages (410-420v) on the plates.

Thanks!

[dartanion]

Hey David,

The 5AR4 is the same thing as a GZ34. It's not going to drop much voltage, but you can figure out what you need fairly easily.

A GZ34/5AR4 will drop about 20V due to internal resistance. Of course this is an estimate and real world will vary. So, if you are looking to get about 365V on the plates, here is a good range to start with:

240-0-240
through
270-0-270

As well, if you need to tweak the plate voltage, you can add zeners between the HV CT and ground to limit the voltage. It works well for testing purposes and leaving in the amp for good.

What are the specs of the PTs that you have used so far?

[dehughes]

Thanks. I've used everything from 320-0-320 to 380-0-380. I've had 4 PTs in this amp now and 3 out of 4 landed in the 409-419v-ish range. Odd...especially as the first one was a 335-0-335 FBDP from MM which gave me 360v on the plates. However, that PT ended up failing, so I wager the low voltage readings were part of the failing process.

Does the ma rating of a PT change what voltages end up on the power tubes? That is, if the PT is rated at a high voltage but low ma, will I see lower voltage readings on the tube plates? Just curious...

Basically, I want low-ish voltages for the 6V6s...somewhere in the 370v range (and I say "low" in relation to Fender's penchant for driving those little tubes way beyond their specs...).

I did find this on my hard drive...don't know where I found it. What do you all think of it as a method to determine B+ ratings for a given plate voltage goal?

_____________________________________
How to determine proper PT and OT sizes

PT 



1. determine your desired plate voltage for the power tubes

2. determine if you want tube or SS rectification (for tube add 20-50V) 

3. divide by 0.9 for losses 

4. divide by 0.7 for conversion to AC 

5. divide by two for centre tap 



e.g.
B+ 378V 

before tube rect: 428V 

before losses: 475V 

before rectification: 679V 

centre tap voltage: 339V 



So I need a 340-0-340 V PT 





From the tube's datasheets: 
- for current on HT add up all your preamp and power amp tubes 
- for heater currernt, simply add up all the heaters current 
- and if you use tube rect, get a 5V winding 



OT 



Much harder to decide, because the OT impedance depends on the power stage topology, class rating and tone. If you are designing a power stage from scratch, look at the tone lizard SE and PP pages, this is how to design the output stage and determine all the components sizes. If you are taking from an existing design just use what the design asks for. 

Look at the output tube data sheet (our friend). For example for a 6L6GC in PP class A, with a B+ of 250V and cathode bias, we use a 5k OT. 




[dartanion]

That's way too much voltage for a pair of 6V6s.

The conversion from RMS voltage to DC post rectifier is pretty simple.

For a 335 -0- 335 PT, you would have a theoretical B+ of:

335 x 1.404 = 470V

This assumes SS recto as there is virtually no voltage drop.

Assuming a 20V drop due to internal resistance of the 5AR4 and you're at 450V. Again, this is in theory.

In practice, you have fudge factors like those in your last post. Depending on how much current the PT can handle, your voltages will drop in varying amounts by how much the PT is taxed. My Vapour 20 checks in around 400V B+ with SS recto and a pair of 6V6s using a 275 - 0 - 275 PT rated at 170 mA. I bias these fairly hot, so an overrated PT helps keep things in check. Also, it's rated at 275V, but in reality it's closer to 285-290V per side.

Anyway, I think you are using too high of HT voltage on your PTs for that application.

[dehughes]

Thanks dartanion! That helps a lot. I'll look for a smaller rated PT then...I think my initial low values were kinda flukey with that one PT (that failed).

[tonestack]

Rectifier efficiency is only part of the equation. The transformer needs to be rated to handle the full current draw or the amp's B+ rail is going to drop. If the transformer being used can source enough current to keep the secondary AC voltage from dropping, a quick and dirty calculation for 5AR4 rectified voltage is ~1.3 x secondary AC voltage. However, the voltage drop on a tube rectifier is not perfectly linear; therefore, the multiplication factor drops slightly as more current is drawn through the tube.

With the above said, as long as the transformer secondary rated for at least 100ma, a 280-0-280 transformer will put you in the 365VDC ballpark with a 2x6V6 power section. If you use a significantly overrated transformer, you can drop down to using one with a 275-0-275 secondary.

Finally, one can use Duncan Monroe's power supply design software if one wants to play around with different transformer parameters (http://www.duncanamps.com/psud2/download.html). It is a nice tool that will get one into the ballpark parameter wise.

[tonestack]

Also, it's rated at 275V, but in reality it's closer to 285-290V per side.

That's because your are using an overrated transformer. A transformer's secondary voltage is dependent on current draw. If one draws more current than the secondary rating, and the secondary voltage will drop. Conversely, if one draws significantly less current than secondary the rating, the secondary voltage will rise.

[Structo]

That's way too much voltage for a pair of 6V6s.

Unless you use JJ 6V6's!

[dartanion]

That's way too much voltage for a pair of 6V6s.

Unless you use JJ 6V6's!

Which aren't really a 6V6. The innards look more like a 7591 to me.

[dehughes]

Very cool! Thanks so much guys.....exactly the info I needed. Here is what I'm thinking I'll have Heyboer make for me:

PT with the mounting dimensions/style of the 125P17A (e.g., laydown mount, centers being 2 1/4" x 2 13/16").

Rating for AC 120v

Hi Volt: 270-0-270 @120ma

5V: rated @ 3A

6.3v: Center tapped (6.3-0-6.3), rated @ 5A

I figure this should be enough to comfortably run a 6V6 pair, a 5AR4 recto, two 12AX7s and one 6SN7, while keeping plate voltage for the cathode biased 6V6s (300 ohm Rk) in the 320-ish range (I like low-ish voltages, I've found...). Thoughts?

[dartanion]

Also, it's rated at 275V, but in reality it's closer to 285-290V per side.

That's because your are using an overrated transformer. A transformer's secondary voltage is dependent on current draw. If one draws more current than the secondary rating, and the secondary voltage will drop. Conversely, if one draws significantly less current than secondary the rating, the secondary voltage will rise.

Thanks, but I understand this already There's a few good reasons why I use an oversized PT for this amp.

[tonestack]

Very cool! Thanks so much guys.....exactly the info I needed. Here is what I'm thinking I'll have Heyboer make for me:

PT with the mounting dimensions/style of the 125P17A (e.g., laydown mount, centers being 2 1/4" x 2 13/16").

Rating for AC 120v

Hi Volt: 270-0-270 @120ma

5V: rated @ 3A

6.3v: Center tapped (6.3-0-6.3), rated @ 5A

I figure this should be enough to comfortably run a 6V6 pair, a 5AR4 recto, two 12AX7s and one 6SN7, while keeping plate voltage for the cathode biased 6V6s (300 ohm Rk) in the 320-ish range (I like low-ish voltages, I've found...). Thoughts?

That transformer is going to put your in the 350 +/- 5VDC range with a 5AR4 rectifier. You will need to drop down to using a transformer with a 250-0-250 120ma secondary to hit 320VDC.

[dehughes]

Very cool! Thanks so much guys.....exactly the info I needed. Here is what I'm thinking I'll have Heyboer make for me:

PT with the mounting dimensions/style of the 125P17A (e.g., laydown mount, centers being 2 1/4" x 2 13/16").

Rating for AC 120v

Hi Volt: 270-0-270 @120ma

5V: rated @ 3A

6.3v: Center tapped (6.3-0-6.3), rated @ 5A

I figure this should be enough to comfortably run a 6V6 pair, a 5AR4 recto, two 12AX7s and one 6SN7, while keeping plate voltage for the cathode biased 6V6s (300 ohm Rk) in the 320-ish range (I like low-ish voltages, I've found...). Thoughts?

That transformer is going to put your in the 350 +/- 5VDC range with a 5AR4 rectifier. You will need to drop down to using a transformer with a 250-0-250 120ma secondary to hit 320VDC.

How are you figuring that? With your 1.3 multiplication factor for a 5AR4, 250-0-250 gets about 305v after a 20v drop at the rectifier.

[dartanion]

Very cool! Thanks so much guys.....exactly the info I needed. Here is what I'm thinking I'll have Heyboer make for me:

PT with the mounting dimensions/style of the 125P17A (e.g., laydown mount, centers being 2 1/4" x 2 13/16").

Rating for AC 120v

Hi Volt: 270-0-270 @120ma

5V: rated @ 3A

6.3v: Center tapped (6.3-0-6.3), rated @ 5A

I figure this should be enough to comfortably run a 6V6 pair, a 5AR4 recto, two 12AX7s and one 6SN7, while keeping plate voltage for the cathode biased 6V6s (300 ohm Rk) in the 320-ish range (I like low-ish voltages, I've found...). Thoughts?

That transformer is going to put your in the 350 +/- 5VDC range with a 5AR4 rectifier. You will need to drop down to using a transformer with a 250-0-250 120ma secondary to hit 320VDC.

How are you figuring that? With your 1.3 multiplication factor for a 5AR4, 250-0-250 gets about 305v after a 20v drop at the rectifier.

The voltage drop is already factored in with the 1.3 fudge factor.

[Structo]

dehughes,
I hope you don't mind me butting in, but have you checked out Magnetic Components Inc. transformers?

They seem to make a broad range of trannies.

On there vintage line, they use paper bobbins just like the old days.

They have a OT shootout on there as well.
They compare four major OT brands on a Granger 50 watt Plexi amp.

If the clips can be believed, I was surprised at how muddy the Merc Mag OT sounded compared to the others.
I liked the Weber and Mag Comp OT's the best with a slight nudge towards the Magnetic Component OT.
I suppose they could EQ the clips to favor the M.C. OT but if they are accurate, the MM OT was not nearly as clear as the others.

http://magneticcomponents.net/HearWhyBu ... romUs.html

Of course this is listening through my 5.1 computer speaker system but it keeps everything relative.

Sorry if these trannies have been discussed here, I just recently stumbled onto them.
Look and sound decent and the price is right as well.