GZ34 vs 2 x triple solid state diodes (fender amps)

[mikepukmel]

Does anyone know if the reason Fender used solid state rectifiers vs GZ34 in similar era tube amps, e.g. the Twin used solid state, but Deluxe used a GZ34, for current rating only? I.e., the GZ34 could not provide enough current to run a Twin?

Id like to build a Deluxe AB763 type chassis, but considering using solid state rectifiers for cost and also easier build since it gets rid of another tube. Im not enough an engineering wiz to understand how this would affect all of the B+ circuit voltages to all of the tubes. Ive read that rule of thumb VAC RMS to peak which is the input to the first filter cap, is about 1.3 to 1.4. But this is for solid state, right?

Does anyone know the similar calculation for a GZ34?

[catalin gramada]

Hi. Different power supply means different amp. You have to understand that is exactly what you amplify. Other than voltages and currents found power supply dynamics is crucial in guitar amp building.

[d95err]

Voltages will be slightly higher with SS diodes, so you may want to use a power transformer with slightly lowe voltages. You could also add a resistor between the diodes and filter cap (e.g. 50-200 ohms) to simulate the voltage drop and sag of the tube rectifier. The resistor should be a power resistor, perhaps 10W.

[mikepukmel]

Thanks Everyone.

[mikepukmel]

Hi. Different power supply means different amp. You have to understand that is exactly what you amplify. Other than voltages and currents found power supply dynamics is crucial in guitar amp building.

Back and forth about this quite a bit. At first, Im thinking, Im not an engineer, if the amp ends up with the starting B+ (out of the first filter cap), 20V-25V higher than it would with a tube rectifier, than all of the voltages through pre-amp, will be very different, and I don't know what that would do to the amp.

Then I start a few email conversations with techs at 2 big transformer mfr's, and they're saying that a sample of period Fender amps, had B+ vary by 20%, and one comment was +/- 20% was 'generous'. 420v + 20% is over 500v. Which explains a *lot* about why some of those period amps just sounded a lot better than others.

Some people have pulled out the tube rectifier, plugged in one those solid state rectifier plug-in's, and the amps run fine. I don't know what effect it has had on the tube life, especially low quality 'modern' output tubes, but ... lots of people on blogs say the amps run and work fine.

Some people seem to like the compression they get with the tube rectifier,and some really like the more solid sound they get from the solid state rectifiers. Some put another circuit board in the amp, and a switch so they can shut down, flip the switch, and play either way.

OK its going to be a different amp, with solid state vs tube rectifiers. But the more I study, the more I get the feeling (tell me if Im wrong) that these tube amp circuits are far more robust to wide input voltage ranges, than any solid state circuits designed today.

The Twin Aa769 schematic has a 315v secondary transformer into solid state rectifiers, that should put out around 410 to 440 (load/no load) B+ near the power tubes. The twin AB763 has a 340vac secondary xformer, that has 460v B+ near power tubes. By the spec sheets for 6V6's, it looks like the AB763 twin style power supply would toast the output tubes, esp cheaper ones made today. But, the 'standard' e.g. Hammond 'twin' style transformer 320v, should output around 415 (loaded) range, which is not so far from the period 410v - 420v spec'ed for the Deluxe/Deluxe Reverb with tube rect.

What Im getting at, using today's 320vac sec PT, with solid state rectifiers, the amp would work, and probably would not blow up a good quality 6V6. (probably?)

Or maybe Im way off.

[catalin gramada]

At first, Im thinking, Im not an engineer,

I.m not either the voltage of PT will drop under lot of circumstances,static and dynamic. For tube rectifier vs solid state the main diference is the tubes have a lot internal resistance wich works givin the sag effect you,re talkin about. Its function is not as linear as a resistor do looking at graph caracteristics. Usualy Hammonds have 5V tap so can build SS and switch in time to valve to feel the differences if you wish.The effect is more manifest in PS wich bennefit by less capacitance like odd circuits.
Is nothing about blow some.more what sound you get. Transformer 320V AC no load means ,very general, ca. 365V DC full power using an average quality transformer SS recto PS
There is a good educational articles here:http://www.turneraudio.com.au/powersupplies.htm

[mikepukmel]

I.m not either the voltage of PT will drop under lot of circumstances,static and dynamic. For tube rectifier vs solid state the main diference is the tubes have a lot internal resistance wich works givin the sag effect you,re talkin about. Its function is not as linear as a resistor do looking at graph caracteristics. Usualy Hammonds have 5V tap so can build SS and switch in time to valve to feel the differences if you wish.The effect is more manifest in PS wich bennefit by less capacitance like odd circuits.
Is nothing about blow some.more what sound you get. Transformer 320V AC no load means ,very general, ca. 365V DC full power using an average quality transformer SS recto PS
There is a good educational articles here:powersupplies

Thanks for the article link.

[catalin gramada]

You,re welcome.
so.if you have a transformer rated at 320v no load that means 320vx1.4142 ca.450v peak. your caps shoud be able to keep this voltage (better 500v) but the RMS in respect with filter cap, voltage drop by diodes will be arround 420v (voltage measured) that will be in no load condition. You can expect at idle to drop arround 400v(means low current ) but when draw current at full nominal power the voltage will drop arround 15% for an average quality PT. You can expect less loses for a very good quality transformer or more current rating than you circuit needs (double or more).Usually I use double VA rated and get 10% regulation with Hammonds. But this is not a rule. A transformer can be stiff or soft(means capacity to keep voltage "topped up" till a certain curent level then suddenly drop, or drop in a progresive way) characteristic in respect of how many turns per volt was build. And a warn-don.t play with primary windings to adjust the voltage in secondary side, All the time choose right primary for mains voltage you have. otherwise the transformer will overheat and can fail. If you use a Hammond you can get a technical data from their site. Usualy they put th.data which include voltages in both condition: no load and full nominal curent means you allready have a info how much those transformer drop at specified current. This is a general picture. More particular infos you need (including decouple caps) to determine dynamics of power supply, including peaks and transients.Those article is a good point to start. There is a good simulator (power supply designer) on Duncan,s page can use , and is free: http://www.duncanamps.com/software.html
Cheers

[catalin gramada]

The Twin Aa769 schematic has a 315v secondary transformer into solid state rectifiers, that should put out around 410 to 440 (load/no load) B+ near the power tubes. The twin AB763 has a 340vac secondary xformer, that has 460v B+ near power tubes. By the spec sheets for 6V6's, it looks like the AB763 twin style power supply would toast the output tubes, esp cheaper ones made today. But, the 'standard' e.g. Hammond 'twin' style transformer 320v, should output around 415 (loaded) range, which is not so far from the period 410v - 420v spec'ed for the Deluxe/Deluxe Reverb with tube rect.

I just take a look at Hammond 290/291FX which is direct Twin replacement you,re talking about. From the spec they mentioned: 665V (332.5) no load/ 640V (320) at 518mA wich means under 4% regulation, seems to be sort of joke I expect more like 640V (320) arround for half current draw in a best case and arround 290V for nominal power (nominal rated current) of PT

[mikepukmel]

I just take a look at Hammond 290/291FX which is direct Twin replacement you,re talking about. From the spec they mentioned: 665V (332.5) no load/ 640V (320) at 518mA wich means under 4% regulation, seems to be sort of joke I expect more like 640V (320) around for half current draw in a best case and around 290V for nominal power (nominal rated current) of PT

Wow that is a huge drop, at least it sounds like it to me with what little I know 330 to 290. no load/load. What effects would that be on the sound of the amp, when its more heavily loaded?

[catalin gramada]

Hi. This was a discussion by principle and not related by your particular circuit. Means maybe your circuit demand less resources than transformer is able to provide. That means better regularisation or less voltage drop. Also your power supply contains some capacitors which decouple the stages but also act like some reservoirs to compensate peaks. What I talked about was how I think those transformer works on its maximum nominal limit
Edit; you can insert a small value power resistor in series and determine how much current you amp circuit draw in different conditions. By power a AB class have around 30-35% efficiency but we need to consider all; preamps. heaters etc.
I have doubts your tweed 769 draw 518mA even full blast distorted (maybe 250-300mA arround) so you can expect the transformer to drop less

[Mick Bailey]

For 6V6 tubes in amps that run higher plate voltages, I always use JJ 6V6S tubes. They're very robust and the plate assembly is similar in size to that of a 6L6.

[R.G.]

re: tube rectifiers vs solid state rectifiers

Solid state diodes essentially don't conduct until the voltage across then gets up to something like half a volt. From there on, they conduct dramatically more for each fraction of a volt until at 0.7V or so they are as "on" as they can get. With ever more current through them, they act like resistors of a few ohms.

Tube rectifiers also start conducting at miniscule voltages compared to the normal voltages on tubes, but they have a much, much higher internal resistance. If you think about it this way, it takes a lot of voltage to drag electrons across that vacuum gap, and much of the energy the electron had in being dragged across the gap is expended on the plate when the electron socks into it.

It is possible to concoct a mashup of solid state diodes and resistors and perhaps other parts that acts like a given tube rectifier to any degree of accuracy you'd like. This is what the solid state rectifier modules that plug into a tube rectifier socket do. the "copper top" versions from Weber do what many people think is a very good job of faking the tube rectifier that they're intended to fake. They contain solid state diodes, a power resistor or two, and usually a thermistor to fake the warm up actions of a tube to some degree.

The big difference in just a tube rectifier and just solid state diodes seems to be that the solid state diodes make a much "harder" power supply, one that sags less under loading, other things being equal, and a nominally 20-50V higher DC voltage, since the SS diodes don't have that big voltage drop across them that tubes do.

It is possible to do a SS rectifier setup, include your own power resistor to "soften" the power so it both is lower DC to start with and also sags more, and fake a tube rectifier setup. There are commercial amps that do exactly this, some even switching the resistor in and out. If you want to also have the same DC voltage when switching to SS diodes, you will need to also switch in something to "remove" those excess volts. This is both feasible and common; many people have done similar things in their amp. If you don't want to switch back and forth, and want a similar DC voltage, you typically need to somehow get a lower AC voltage from your power transformer; there are ways to do this that are common as well.

I realize this is getting to sound vague. That's because you're at one of those branching points where the answer to one question leads not to a result but to another set of decisions to be made.

[Justin Thomas]

Does anyone know if the reason Fender used solid state rectifiers vs GZ34 in similar era tube amps, e.g. the Twin used solid state, but Deluxe used a GZ34...

Id like to build a Deluxe AB763 type chassis, but considering using solid state rectifiers for cost and also easier build since it gets rid of another tube.

The second part is the answer why Fender did it... the fact the amps still sounded good either way, and that diodes were ".ore reliable," is a bonus. Leo wasn't a musician, he was an engineer/inventor & a businessman. He built the best amps he could, as cheaply as he could get away with, for as much profit as possible. All other later "discoveries" and opinions of vintage Fenders being so awesome and durable is just coincidence, and a by-product of a different mindset of a different age.

As to the technical side, I'll leave that to others.

Justin

[catalin gramada]

c'mon Thomas. They didn.t give up, they still made cheap amplifiers