Vacuum Rectifiers and SS Diode Emulators

[Merlinb]

Which means that almost 70% of the sag is coming from the PT alone.

Exactly. Rectifier sag is almost an old wives' tale peddled by review magazines, they see a tube and immediately gush about sag. But the fact is sag is much rarer than they would have you believe. You don't need a tube recto to get sag, and not all amps with tube rectos sag.

[Tom Phillips]

...Rectifier sag is almost an old wives' tale peddled by review magazines, they see a tube and immediately gush about sag...

Good point. Such thinking is prevalent in the musical instrument discussions and I believe that it's because of all the internet lore that's passed around. Anyway, I present the attached measurements that I made regarding a couple of Fender Deluxe Reverb amps that use the GZ-34 rectifier. You will see that the data shows the cold DC resistance of the high voltage winding and the resulting sag in the HV winding VAC as well as the B+ sag which includes the drop due to all the parts of the circuit. The power line was maintained at 120VAC during the measurements. The data shown are actual measurements use complete refurbish amplifiers.



The 1963 Deluxe Reverb PT used a significantly smaller core that the 1965 and later units. The '63 ran much hotter.

[Helmholtz]

The effective winding resistance is secondary DCR + reflected primary DCR per side.
Even with a SS rectifier the B+ sags more than the winding voltage as increased load current discharges the reservoir cap deeper.

[g1]

So are we at the point now where to more accurately mimic a tube rectifier, a SS replacement should use zeners rather than resistors to achieve the voltage drop? Or 70% zeners, 30% resistor?

[Tom Phillips]

The effective winding resistance is secondary DCR + reflected primary DCR per side.
Even with a SS rectifier the B+ sags more than the winding voltage as increased load current discharges the reservoir cap deeper.

I edited my post to make it clear that the data I sited were actual measurements and the sag listed was due to all the parts of the circuit.

[Helmholtz]

So are we at the point now where to more accurately mimic a tube rectifier, a SS replacement should use zeners rather than resistors to achieve the voltage drop? Or 70% zeners, 30% resistor?

I you're only looking at sag (and not at details of ripple harmonics), a GZ34 can be replaced by SS diodes + one 68R resistor on the DC side. No zener required.
The 68R value follows from the GZ34 voltage drop of 17V at a current of 250mA.
A current 250mA is about the full power current of a 40W to 45W amp.

[loudthud]

See post 10 in this thread: https://music-electronics-forum.com/...upply-question

While you can see the peaks of the ripple at the first cap drop, the bottom of the ripple drops way down. The Screen Voltage really takes a dive and the choke filters out almost all the ripple. I think this is what makes the output stage compress, which is what people without an oscilloscope call "sag".

[Helmholtz]

Regarding the influence of the effective HT winding resistance (per leg):

The reservoir cap and the total source resistance constitute a low pass filter. It is this single RC filter which attenuates the ripple voltage harmonics at the reservoir cap.
The total source resistance is the sum of the winding resistance, the effective rectifier resistance and any additional resistors before the cap.
As with a series circuit the current through all components is the same, it makes no difference if an additional resistor is wired before or after the rectifier. A larger winding resistance has the same effect as a resistor after the rectifier.
So with SS diodes+68R resistor, the resistor can be placed either on the ACV or the DCV side of the diode for the same result.
But as with a full wave rectifier diode current only flows during one half-cycle, each winding leg would need its own 68R resistor with half the power rating of a single common 68R resistor after the rectifier.

[R.G.]

I stumbled into this rabbit hole accidentally, got interested, and couldn't look away.
Given how much information has come out of really very little study and simulation, I'm baffled that this field of study wasn't completely plowed decades ago. I think you're right about the internet myth-machine amplifying sag to legendary proportions. I'm really interested in seeing what deliberately tinkering with screen supply as a source of sag does to amps. Given this thread, it may be much more important than generic rectifier types.

The more I look at this, the more I get my original opinion reinforced: SS diodes plus a series resistance gets my amps close enough to the finest of vintage glass bottles.

Tom, I really appreciate the actual measurements. I don't run a repair shop nor have a big batch of amps to measure, so I'm working on estimates. I hate to ask, but would it be possible for you to add to that batch of measurements the primary winding resistances and maybe, if you can, the actual DC current into the CT of the output transformer under no-signal idle and full(ish) power output? It's a big ask, I know,

@ others: Anybody got time to measure transformers similar to what Tom has done?

[Merlinb]

would it be possible for you to add to that batch of measurements the primary winding resistances

Hammond datasheets list all the winding resistances and off-load and full-load voltages, so there's a library of information there.

[R.G.]

Thanks, Merlin. Hammond does have superior documentation. I used one of their transformers as the basis for an intro to transformers not too long ago.

I've experienced a lot of internet mythos whenever I try to do anything based on measurements, and my expectation is that using Hammond transformer data for any real conclusions from transformers will lead to a trail of comments about "well, those are just replacements; the REAL XYZ126325 transformers used in Fender/Marshall/vox/boutique#1 are made with plutonium dioxide and have more/better [something or other].

So any real measurements would be very helpful indeed, if only to filter out some of the inevitable cra.. . er, lesser informed comments.

By the way, I think I've isolated some of the spectra differences. I had some fast/soft rectifier models in the first stuff I did. Those have a huge effect - at least for the ones I have native in the library. Still sorting that out.

[Tom Phillips]

...Tom, I really appreciate the actual measurements. I don't run a repair shop nor have a big batch of amps to measure, so I'm working on estimates. I hate to ask, but would it be possible for you to add to that batch of measurements the primary winding resistances and maybe, if you can, the actual DC current into the CT of the output transformer under no-signal idle and full(ish) power output? It's a big ask, I know,...

Hi R.G.,
I pulled the data from my archives so I don't have the amps available to make additional measurements. However, I did have the primary DCR measurements in my data base so I updated the table in post #47 of this thread. Unfortunately, I did not record the OT currents. This discussion has got me thinking about recording screen data too. Speaking about going down rabbit holes, collecting this type of data can send me down a deep hole. It's easy to spend much more than the actual amp repair time to collect the extra data. I do it out of curiosity because I can't help myself and I don't want to miss the opportunity when I have a cool vintage amp on my bench.
Cheers,
Tom

[R.G.]

Thank you, Tom. No problem.
I suspect that someone else may be able to fill in more. I trust the folks like you on this forum for the straight stuff.

[Helmholtz]

As noted earlier, a good source for HT winding resistance is the rectifier datasheet specifying the minimum resistance depending on winding voltage.
E.g. minimum recommended Rt for a GZ34 is 75R per leg at a winding voltage of 2x 350Vrms, 250mA DC load current and 60µ reservoir cap.
The PT of my original 1966 JTM50 (using a GZ34 and EL34s) has an effective HT winding resistance of 2x 65R. Reservoir cap is 32µ.
DC current at full output before clipping (39W) is around 210mA.

[R.G.]

Thanks, Helmholtz!