Custom transformers for Super Reverb build

[Smradek]

I'm making a Super Reverb head and have sourced all components except the transformers.
I can locally get excellent custom made transformers for cheaper than ordering them from the usual suspects.

The output transformer would be 4.2kΩ to 2Ω, 4Ω and 8Ω. On the web they sell anything from 4kΩ to 4.5kΩ, is 4.2kΩ the best choice?

As for the power transformer, for the heaters there are the standard secondaries 6.3V 5A with center tap and 5V 3A, but what about the HV secondary?

I find 200mA, 300mA, 360mA, 370mA and even 500mA ratings. Is 370mA enough or too much?

Some have the 50V for bias as a extra tap on the HV secondary, some as a separate secondary.
What would be the pros or cons of each choice?

And as for the HV voltages, most have 355V, I go with 360V as per the original schematic.
Since I can get extra taps for the HV, I was thinking of getting taps for maybe 2 or 3 lower voltages for earlier breakup / lower volume.
What lower values would make sense?

Thank you all in advance for comments and suggestions.

[Merlin]

The output transformer would be 4.2kΩ to 2Ω, 4Ω and 8Ω. On the web they sell anything from 4kΩ to 4.5kΩ, is 4.2kΩ the best choice?

Splitting hairs, 4.2k is fine. But wouldn't 8 and 16 ohm secondary be more useful, as they are the most common impedances for guitar speakers? Who sells 2 ohms cabs anymore?

I find 200mA, 300mA, 360mA, 370mA and even 500mA ratings. Is 370mA enough or too much?

It's a 50W amp so you'll need ballpark 100W from the B+ supply, or about 250mA dc. That amounts to about 200VA in the transformer. EDIT: More like 150VA for a centre-tapped transformer and tube recto.

Some have the 50V for bias as a extra tap on the HV secondary, some as a separate secondary.
What would be the pros or cons of each choice?

If you want switchable B+ voltages then you'll want to take the bias supply from the HV, not a tap or separate winding. That way, the bias will track the B+.

Since I can get extra taps for the HV, I was thinking of getting taps for maybe 2 or 3 lower voltages for earlier breakup / lower volume.
What lower values would make sense?

A half-voltage option would be nice (quarter power), so 180V. But you could go even lower!  The preamp can always be powered from the 360V supply.

[Latole]

Too many ma 300, 400....than the circuit need , High voltage will be higher.

Fender Super reverb need 325-0-325 200ma, 50 volts tap , 6,3 4A and 5 V 3A

[Smradek]

Thank you very much Merlin and Latole

Splitting hairs, 4.2k is fine. But wouldn't 8 and 16 ohm secondary be more useful, as they are the most common impedances for guitar speakers? Who sells 2 ohms cabs anymore?

Yes, that makes perfect sense.

It's a 50W amp so you'll need ballpark 100W from the B+ supply, or about 250mA dc. That amounts to about 200VA in the transformer.

Afraid to ask, but I have to . So, how much mA AC? 250mA?

If you want switchable B+ voltages then you'll want to take the bias supply from the HV, not a tap or separate winding. That way, the bias will track the B+.

OK, i get that. So could I use extra taps for the bios voltage too, like 50V, 30V and 25V for bias and 360V, 216V and 180V for B+?

Fender Super reverb need 325-0-325 200ma, 50 volts tap , 6,3 4A and 5 V 3A

I looked up the original schematic for the Super Reverb from Fender, it shows 360V AC...

[Merlin]

Afraid to ask, but I have to . So, how much mA AC? 250mA?

I = VA / V
I'm probably being too conservative in assuming 200VA. Probably 150VA is enough, so:
150VA / 720Vac = 208mA ac

[mresistor]

When I built my SR head I wanted versatility, since it wasn't going to be in a 4x10 cabinet, so I went with a classictone 40-18001 with 4,8,16 ohm taps. I figured if I ever do want to install it into a SR 4x10 cabinet the 4 ohm tap would be OK. Might not sound exactly like a Fender SR though, but close enough.

[Willabe]

I looked up the original schematic for the Super Reverb from Fender, it shows 360V AC...

The exact B+ winds acv depends on the transformers self regulation.

A transformers secondary acv will drop/sag when loaded. The more it's loaded, the more it will drop/sag. Some sag/drop more than others. Every wind on a PT's secondary has resistance. The more current you pull/draw through a resistance, the more the voltage will drop/sag. It's Ohms law working.

And modern PT's seem to have a different resistance on secondary winds because of how their made these days compared to how they used to make them. Different wire gauge on a different iron core material.   

So you can buy a PT that has a 360-0-360 B+ wind and buy 1 that has a 325-0-325 B+ wind and when both are loaded with the Super Reverb's circuit, the 325-0-325 will end up being higher than the 360-0-360 PT. Because the 360-0-360 PT sagged/dropped more than the other PT.

And you never really know how much a PT's secondary voltage will drop untill it's fully loaded for what you want it for.

[Willabe]

What I'm saying is that buying a PT that has a 360acv B+ wind, because the Fender schematic shows a 360acv B+, will not automatically guarantee you'll get the same voltages as shown on the Fender schematic.

And part of it is the mA rating, how much current it can deliver. Fender does not say what they had the PT's B+ spec'd at for current. We can give an educated guess because we know how many/what tubes the amp uses. But they could have been built with barely enough or they could have beefed them up a little. Because adding extra current costs more, they probably were on the just enough side without burning up. Excess current draw causes excess heat.

So how close will you get is the question.

Even companies that make PT's and list them as for certain amps, they can be pretty far off when loaded with the circuit.

[Smradek]

So the Voltage drops depending on the load, and drops (more or less) more if the mA rating of the transformer is lower?
Could I then use 200mA AC rated transformer with 340V, 255V, 204V and 170V taps for HV and 50V, 37.5V, 30V and 25V taps for bias?

[Willabe]

So the Voltage drops depending on the load, and drops (more or less) more if the mA rating of the transformer is lower?

Yes, and;

Depending on how the PT is made. When they build a transformer they can use thicker or thinner gauge wire along with different grades of steel for the iron core lamination's. This causes less internal resistance or more internal resistance. More internal resistance will cause more acv drop/sag. The transformer engineers have to balance all of this.

We have guys over the years post fairly often about they built an amp it works fine but the dcv are high and they want to know how to lower them. 

Could I then use 200mA AC rated transformer with 340V, 255V, 204V and 170V taps for HV and 50V, 37.5V, 30V and 25V taps for bias?

Again, that won't guarantee that you'll hit the dcv's you want. It's just not that easy.

And 4 high acv tap's is a lot to deal with, will take up extra space/be messy and will cost most.

Some guys will get a PT that has 2 B+ taps and will use those along with swapping rectifier tubes to get close(r) to what they want.

You kind of did this backwards, you source the iron set, PT/OT/chock 1st, along with a chassis. Because the iron set (and chassis) can be the hardest to get.

You know what amp you want to build, so search here and on other forums to see who built a SR amp where their PT nailed the dcv's. A Fender SR PT clone is a very popular choice.

Somebody probably makes 1 that gets the voltages right or very close.

You don't need all those bias taps/winds, only need 1. Unless your going to build an amp with a 1/2 power switch. And that would be a more complex build to nail down if your new to building amps. Unless you found a proven design.

[mresistor]

Here's the Hammond export offering  https://www.hammfg.com/files/parts/pdf/290D2EX.pdf?v=1697661948

[Willabe]

Here's the Hammond export offering  https://www.hammfg.com/files/parts/pdf/290D2EX.pdf?v=1697661948

That's 300mA, only need 200mA to 250mA.

[Willabe]

Find someone here or on another amp forum who's built a SR clone that the PT's voltages came out right.

Then you'll know what to buy.

[Smradek]

Thank you all for your help.

Hammond recommends the mentioned 290D2EX with 355V 300mA, Mojotone has also 355V 300mA, Tubeampdoctor has 365V 270mA.
Mercury magnetics offers the ToneClone with 375V and one with lower B+ of 344V.

But as Willabe said, even companies that make PT's and list them as for certain amps, they can be pretty far off when loaded with the circuit, and it probably depends on the individual amp build, so it's a hit or miss either way.

The guy that makes the transformers suggested to just make additional taps for the primary winding of the power transformer so it has the default 230V taps and taps for 220V and 240V. I ordered a 50W output transformer, and a 200VA 340V power transformer which gives about 325V and 355V with the additional taps.

The price for both the power and output transformer together is 80€, local pickup, and if the B+ turns out to be to far off I simply order another power transformer with the corrected Voltage and still pay less than anything I could find on-line. And the guy has a excellent reputation for his transformers, the build quality is first class, audio electronics seems to be his passion and he makes them because he likes to make them.

[mresistor]

That sounds great!   $84-$85 is very reasonable considering the others you mention are well over that cost.

[Willabe]

Will the OT be wound interleaved with a paper bobbin?

You don't want a plastic bobbin.

[Merlin]

You don't want a plastic bobbin.

I don't see why not?

[Willabe]

You don't want a plastic bobbin.

I don't see why not?

On an audio OT?

[Merlin]

On an audio OT?

OTs have been built that way for donkeys' years 
 

[Willabe]

Yes, that's true.

But there's a lot of debate on paper verses plastic OT bobbins which sounds better. 

[Smradek]

It's plastic. But psst... Don't tell anyone 

Seriously, I think the mojo is in the fingers of the winder, not in the bobbin.

[Willabe]

Seriously, I think the mojo is in the fingers of the winder, not in the bobbin.

That's for pick ups, they have wire as fine as a human hair. Then it matters.

The wire on transformers is large. 

Paper and plastic bobbins will be of different thickness, and will effect/react to flux magnetic fields differently. 

[Smradek]

I meant in the hands and brain of the winder. If it's properly calculated, designed and made without cutting corners… HV Secondary with enough winds per Volt and windings in 4 sections  - 1,4,3,2, - both sides then have the same resistance and inductivity to the center tap so the quiescent currents and dissipation are low, voltage drop under load is low and there is no unnecessary overheating. I think that the material of the bobbin sure does somehow influence the transformer’s properties, but not so much as the core and winding materials.

Besides, I spent all my cork sniffing money a long time ago. It'll be fine.

[Smradek]

The transformer guy just messaged me, he rewinds his own transformers if needed, so if the HV would turn out to be wrong he will rewind the power transformer

[pdf64]

Bear in mind that if the mains transformer windings don't incorporate sufficient resistance feeding the anodes to fulfill the GZ34 minimum requirements, the designer (in this case you :)) is expected to add resistors.
https://frank.pocnet.net/sheets/010/g/GZ34.pdf
Or is your transformer builder aware of this, and designing the transformer accordingly?

[Willabe]

I meant in the hands and brain of the winder.

You said "the mojo is in the fingers of the winder" now it's changed to hands and brain. 

If it's properly calculated, designed and made without cutting corners…

A plastic bobbin is cutting corners for audio guitar amps. It's faster, that's why they started using them.

HV Secondary with enough winds per Volt and windings in 4 sections  - 1,4,3,2, - both sides then have the same resistance and inductivity to the center tap so the quiescent currents and dissipation are low, voltage drop under load is low and there is no unnecessary overheating.

No, they don't have the same resistance and inductivity to the center tap because of how they're wound. As they wind each layer, the wire is longer because the wind the next layer gets wound over is larger. So the side that's wound past the CT will always have more resistance. Measure any transformer with a center tapped winding from CT to each wind end and the resistance is different.

The only way to get the CT centered and the resistance the same CT to the wind ends, is to use a bifiliar wind. But they cost more so they don't get used in guitar amps. And you don't need them in guitar amps anyway, they use them in stereo's. 

I think that the material of the bobbin sure does somehow influence the transformer’s properties, but not so much as the core and winding materials.

And your basing that on what? All 3 things matter, they play off of each other.

Besides, I spent all my cork sniffing money a long time ago. It'll be fine.

If you do enough research, you'll find it's not cork sniffing at all. The iron set used is the heart of the amp! There is a difference in the way they sound, and it's a pretty large difference. And you could have said the same thing basically without getting snarky. I don't think it's necessary.

You started this thread and I pointed out a number of things you were missing, not understanding about guitar iron sets for amp builds. Including not sourcing the iron last after all the other parts.

Buy what ever iron you want, it's up to you. I was trying to help you make a more fully educated purchase so you would get close to the dcv's that amp needs and that it would sound right, that includes paper bobbins, especially for the OT.

[Merlin]

Plastic vs. paper is just about the least important thing about an OT, I don't know why it's upsetting you so much. Heck, 5mA of bias offset in the valves will have a thousand times more effect on the sound we get from the OT than the bobbin material does, and nobody gives that a second thought to a little offset. Let the man buy his cheap transformers already.

[Smradek]

Ah, Willabe, I didn't mean to come across as disrespectful or offensive and I really do appreciate your effort to help, sorry.

Regarding me thinking that material of the bobbin does influence the transformer’s properties less than the core and winding materials, it says me thinking, not me knowing...
I do believe that It could sound better with transformers 3 times the price, but not that much better. I'm sure the amp will be a mighty fine amp, for me.

The cork sniffing comment was self-criticism, I'm no more chasing the 'best' parts like I used to chase them. And I really did chase them.

[Smradek]

Bear in mind that if the mains transformer windings don't incorporate sufficient resistance feeding the anodes to fulfill the GZ34 minimum requirements, the designer (in this case you :)) is expected to add resistors.
https://frank.pocnet.net/sheets/010/g/GZ34.pdf
Or is your transformer builder aware of this, and designing the transformer accordingly?

I keep that in mind, thank you pdf64

[HotBluePlates]

I'm making a Super Reverb head ...

The output transformer would be ... 2Ω, 4Ω and 8Ω. ...

... wouldn't 8 and 16 ohm secondary be more useful, as they are the most common impedances for guitar speakers? Who sells 2 ohms cabs anymore? ...

The following is just a perspective to consider:

By the time Fender got to the blackface amps, they'd developed a single amp-circuit for all "with Reverb" models.  So everything from Deluxe Reverb to Twin Reverb is essentially the same preamp circuit, and models are differentiated by making certain changes.

   - Number & type of outputs tubes are changed to change output power (which also implies some power supply changes).
   - Only a few models get "Middle" controls in their tone stack (those being the Super Reverb one one channel, and Twin Reverb & Showman amps getting it on both channels).

   - Number and Size of speakers were the other big change.  To the point it might be the primary difference between models.


The biggest element that differentiates a Pro Reverb from a Super Reverb is that the former is a 2x12 combo, while the latter is a 4x10 combo.  There is a single cap-change in the tone stack, and one channel gets a Middle control, but fans will say "it's not a Super Reverb" unless there are 4x10 speakers.


Only food for thought.