P2P Redd 47 - a few questions

[earthsled]

I remember Abbey commenting on another thread about toroidal PTs...

Now the main problem with off-the-shelf pwr toroidals is that they are designed for maximum power rating in a specific form factor, i.e. they are operating near maximum flux, which means a lot of stray field.
Most manufacturers use custom-made mains xfmrs, spec'd with lower flux. Operating a xfmr at 30% less flux than an off-the-shelf makes it 10 to 20dB quieter.
This is not recent practice. Leo Fender used to do that from the beginning.
Distance is also a good and cheap ingredient to quiet a xfmr.
Now you can try using floobydust to quiet a standard toroid, like those silicon-steel ribbons that you wrap the xfmr in...
And if I had to choose an off-the-shelf, I would look at the one with the lowest rating in the biggest form factor.

... so bigger is better when you want to reduce flux from the PT -- thought this might be helpful to the discussion.

 

[abbey road d enfer]

Just to make sure that I'm not misunderstood.
You have to find a transformer with a low magnetizing current.
Most off-the-shelf xfmrs are designed for operating at the core's max recommended induction (typically about 1.7T), because customers generally want the highest power in the smallest form factor (and smallest wallet depletion). Due to these considerations, the largest the xfmr, the largest the magnetizing power and thus the largest flux. So the bigger xfmr will probably result in the noisiest product.
The idea I exposed is that some xfmr manufacturers, conscious of the particular requirements of audio applications, design their xfmrs for 10-20% less induction than max recommended, which typically results in using a core one size larger than needed (typically a 20VA xfmr in a 30VA core or a 30VA in a 50VA core).
The problem is that it's difficult to know the design induction. Some manufacturers claim their units to be very-low induction, but in fact they compare to a standard industrial xfmr.
When I didn't have the resources to have custom-wound toroids, I just tested a number of off-the-shelf parts until I found the one that gave me the best results.

 

[letterbeacon]

Very interesting.  How about non-toriodal PTs that are designed for audio - like the Hammond 'Classic' range you see in a lot of tube gear?

http://www.hammondmfg.com/300series.htm

Are these wound with low induction in mind do you think?  Would they be any less noisy than a standard toroid?

 

[earthsled]

Sorry Abbey -- I think I was misinterpreting you originally. So, lower induction and larger cores generally result in reduced flux, but it's best to test off-the-shelf models to find the best one -- correct?

Like Letterbeacon, I'm also curious about how this applies to the regular "EI" type transformers. You mentioned Leo Fender had low-flux transformers built for him -- Can I suppose that a quality Fender amp replacement pwr xfmr might be a good choice for low flux?

 

[abbey road d enfer]

Sorry Abbey -- I think I was misinterpreting you originally. So, lower induction and larger cores generally result in reduced flux, but it's best to test off-the-shelf models to find the best one -- correct?

Yes, correct.

Like Letterbeacon, I'm also curious about how this applies to the regular "EI" type transformers. You mentioned Leo Fender had low-flux transformers built for him -- Can I suppose that a quality Fender amp replacement pwr xfmr might be a good choice for low flux?

You would have to find a low power PT. The smallest are the stand-alone Reverb, the Deluxe Reverb and the Harvard. The first two deliver only ca. 330V dc, which is below the 380 recommended for the REDD47. The latter would deliver the right DC voltage. Any PT for the larger amps would deliver the voltage, but would be too bulky.

 

[earthsled]

The latter would deliver the right DC voltage.

Interesting. I have two Harvard schematics -- model 5F10 and 6G10. The 5F10 shows a B+ of 305V while the 6G10 shows 360V, so I'll assume it's the 6G10's power transformer we're after (model 125P1A). Mercury Magnetics makes replacements for the 125P1A, the FBPP or FBPP/240 with international voltage taps on the primary. Unfortunately, I don't think this model has a bias tap, so my plans for phantom power would have to change.

Any PT for the larger amps would deliver the voltage, but would be too bulky.

The Weber model PT I mentioned a while ago is certainly bulky. Are you suggesting this factor will increase the flux?

 

[abbey road d enfer]

The latter would deliver the right DC voltage.

Interesting. I have two Harvard schematics -- model 5F10 and 6G10. The 5F10 shows a B+ of 305V while the 6G10 shows 360V, so I'll assume it's the 6G10's power transformer we're after (model 125P1A). Mercury Magnetics makes replacements for the 125P1A, the FBPP or FBPP/240 with international voltage taps on the primary. Unfortunately, I don't think this model has a bias tap, so my plans for phantom power would have to change.

It's quite common in DIY using several xfmrs as substitutes for a custom-built one. That also gives th eopportunity to mount them in humbucking mode (the flux of one cancelling that of the other).

Any PT for the larger amps would deliver the voltage, but would be too bulky.

The Weber model PT I mentioned a while ago is certainly bulky. Are you suggesting this factor will increase the flux?

Considering two xfmrs using the same design rules (in particular induction), the most powerful will create more flux.

 

[earthsled]

It's quite common in DIY using several xfmrs as substitutes for a custom-built one. That also gives the opportunity to mount them in humbucking mode (the flux of one cancelling that of the other).

Cool! This is something I'd like to try. How would you arrange the two power transformers for best humbucking? Does humbucking also work for toroids?

Considering two xfmrs using the same design rules (in particular induction), the most powerful will create more flux.

So, essentially, using an over-rated transformer may not be a good idea because this could have the effect of increasing unwanted flux.


Thanks!

 

[abbey road d enfer]

It's quite common in DIY using several xfmrs as substitutes for a custom-built one. That also gives the opportunity to mount them in humbucking mode (the flux of one cancelling that of the other).

Cool! This is something I'd like to try. How would you arrange the two power transformers for best humbucking?

While one of the xfmr is in is supposed final position, move the other one and rotate it until you find best cancellation. No need to menttion that you need to be extremely careful while doing that. I recommend wrapping the xfmr in insulating material and wearing gloves.

Does humbucking also work for toroids?

It works extremely well with toroids, because the stray flux lines are much more evenly distributed.

 

[earthsled]

I'd like to revisit the idea of generating DC heaters from the 6.3V plus 5V taps on a typical valve power transformer. Will it work to combine these voltages to achieve an 11.3V AC source? Two channels of REDD.47 would require 1A minimum for the heaters, but I'd like to overshoot a bit so that the remaining power could be used for LEDs and such (perhaps 2A total).

Abbey mentioned that the DC heater supply must be well designed so that there will be no additional noise introduced. I estimate the regulator may need a massive heatsink for this to work. The LM350 or LM338 seem like reasonable choices. Has anyone worked out a good circuit for this already?

Thanks!

 

[Jean Clochet]

Post removed.

 

[earthsled]

Are the ratings of both your windings (6V3 & 5V) similar in volt amps?

Well, no. The transformer I currently have is rated for 6.3V at 5A and 5V at 3A. So, thats 31.5VA and 15VA respectively.

Not sure if this helps but the 6.3V winding does have a CT. Perhaps it would be better to use half the 6.3V for 3.15V plus the 5V tap and a total of 8.15V? Does the current rating remain the same when using the CT? If so, then it would be 15.75VA which is close.

Attached is a schematic from Morgan Jones' book, Valve Amplifiers. The circuit specifies a 9V input, but maybe 8.15V will do? The circuit is rated for 1.5A output which should be enough.

Anyway, I've done this myself a couple of times.  I'm not near where my computer documents are filed but I can post what I ended up doing later if you like.

That would be great. Thanks for your help!

 

[Jean Clochet]

Post removed.

 

[chrispsound]

In the power section there are three 8uF caps in parallel, isn't this the same as a single 24uF cap or is there a benefit other than cost running them that way?  Thanks ChrisP

 

[abbey road d enfer]

In the power section there are three 8uF caps in parallel, isn't this the same as a single 24uF cap or is there a benefit other than cost running them that way?  Thanks ChrisP

Different times...You'd be better off today with a single 22uF cap.

 

[JayDubrek]

Apologies if I'm being a dumbass...so the REDD47 needs 380VDC- the hand drawn layout on page one shows a power transformer at 540VAC with CT being used- would this be the correct voltage secondary to use if following the original schematic?
Thanks
Jay

 

[abbey road d enfer]

540V CT = 2x270V
Once rectified and smoothed 270Vac give 270x1.414= 381.78Vdc, so this is correct.

 

[earthsled]

On the same topic here with the 540V CT transformer -- does the center tap get tied to ground?

Thanks!

 

[abbey road d enfer]

On the same topic here with the 540V CT transformer -- does the center tap get tied to ground?

Thanks!

Yes; more correctly, it must be tied to the negative side of the smoothing cap, which happens, very often, to be tied with ground too. What I mean is, don't just return this center-tap to any old ground anywhere.

 

[chrispsound]

If one were to use a 1:10 Jensen input xfrmer what would  the zobel be?