Princeton Reverb with Express Tx ?

[godi]

Hello folks

Is that possible ? I'm planning the construction of a Princeton Reverb (head only). The ''nomal'' Princeton Reverb AA1164 need 420 Volts B+.

I just build a Trainwreck Express style amp using Magnetic Components transformers, the PT is 300-0-300 VAC but with diodes rectification my Trainwreck have 420 Volts DC B+, witch is exactly the voltage a Princeton need to see at B+.

The Princeton is design for a pair of 6V6 power tubes and a ''normal'' PR have a output transformer with an impedance of 6600 Ohms so the Magnetic Component TW output transformer, again this Tx would be a perfect match as far as the impedance go.

Is that doable to use a pair of TW transformers into a Princeton Reverb, or is there someting I'm missing as I am relatively newbe in tube amp design ?

Thank for your help or suggestions, Jean

[Cliff Schecht]

Since that transformer is rated for IIRC 300mA after the rectifier and quite a bit of heater current to boot, what you might find is that the transformer is so underloaded that the voltages end up a hair on the higher side. Might cause the heater taps to run at over 10% of their voltage rating (i.e. closer to 7V than 6.3V) but this is relatively easy to fix with a bucking transformer if necessary. TBH this seems like a waste of massive power transformer though..

[godi]

Yes, but I did consider that a Deluxe (usual upgrade for Princeton amps) transformers bundle cost $113 (at Triode Electronic) and a TW set is $157, that only $44.00 more but I'm saving the cost of a tube socket and a rectifier tube. I paid my last NOS Mullard GZ34 $70.00 so I'm still saving money here using four diodes at $0.15 each.

Also, I will be able to install a pair of EL34 witch can be interresting experience or the regular 6V6.

And I'm also expecting some bigger, deeper and stronger bass with this set up witch is what I'm looking for with bigger iron.

[M Fowler]

If all your looking for is higher mA rating replacement Princeton Reverb PT and OT Allen Amps has custom Heyboer tranformers for that which you can run 6L6's or EL34's. TP-25 310-0-310 150mA

http://allenamps.com/parts.php#transformers

Mark

[godi]

Hey Cliff, I'm not arguing with you, you problably know much more than I do, but I would like to understand why a 300mA power transformer would be to powerfull and raise the voltage in a Princeton that run a pair of 6V6 and four (4) preamp tubes, while the same transformers are OK in a Trainwreck Express with a pair of 6V6 and only tree (3) preamp tubes ?

We are all discussing here to learn more and understand things.

Thank you, Jean

[Cliff Schecht]

The Express is meant to run EL34's safely. That's 3A alone for the EL34's, then 0.9A more for the preamp tubes. The transformer is meant to handle 7.5A on the 6.3V rail and 5A on the 5V rail (note: holy shit!). This is a legacy thing from Ken who liked to use massively overrated transformers, maybe for the added reliability factor or to prevent any power transformer saturation (the Express is meant to be quick and dynamic at all times).

While overrating transformers like this wasn't a problem when wall voltages were much lower, we now deal with wall voltages hitting 125V+. Putting this on a transformer wound for 120V and you get a 4% increase in heater voltage putting them at about 6.6V. Now you underload the transformer (especially ignoring that massive 5V@5A tap) which again will increase the heater voltage by another 5%, putting your heaters at 6.9-7V. This kills the heaters quickly in tubes.

Since I'm using some fairly hard to find tubes in my amps, I'm always careful to get the heater voltages right on. I prefer either a bucking transformer or a variac to get the voltages spot on.

FWIW (and maybe IMO) amps sound best when they hit the voltages they were designed for. I try to stay within 5% of the specified voltages for my not-quite-vintage correct builds, especially in tweed stuff and also with TW's where they sound best at their designed values. If your voltages are too high things sound too bright and harsh, too little voltage and everything sounds mushy..

[Gaz]

I actually disagree (heaters aside), about there being an optimal voltage for the best sound. Anything under spec (and even a little over) is fair game, and up to the designer's ear. Historically amps made by the same maker have been all over the place voltage-wise. i.e. Marshall running 2203s anywhere from 395vdc to 480vdc! I encourage experimentation within safe limits. Cliff is right that you should stay in spec if you''re trying to nail a very specific sound, but it seems like you're kind of abandoning that for economy from the get-go. He is also right about heaters, no doubt.

[godi]

I'm not trying to nail a specific sound, I'm trying to shape a sound and tone that does not exist. I want the Princeton features, one channel amp, reverb, tremolo, Princeton basic tone, my complaint against this amp, it sound a bit like a bedroom amp, somes of you may disagree, but I'm building this to please my desire of a more solid tone, solid bass, more presence with low sag. I think it's fun to experiment and create in opposition to clone.

Economy is not a criteria for my choices, but I want the best bang for the buck, that's for sure and the best quality possible, I don't cut the corners.

OK after reading your comments, I keep the idea of the Trainwreck output transformer and I go down to a 40 Watts power transformer like this :
http://www.classictone.net/40-18005.pdf
But now I will need a voltage drop from a rectifier tube to hit the target of 420 Volts B+
Is that doable or more reasonnable like this ?

Don't the modern transformers, like the Magnetic Components Trainwreck transformers are, are not adapt to today higher voltage espacialy the filaments design to give 6.3 Volts where you need 6.3 V ?
I never enconter the point of view of the transformers engineers, but I imagine that they try to build a transformer that delever 6.3 Volts in North America with today voltage ????

Thank, Jean

[M Fowler]

Well many of us have built Rocket Reverb amps with octal output tubes using TW iron.

I have also made one into a Fender Deluxe 90 1-12 cab using MC iron. I do not see any reason why you can't proceed with the ideas you have using the iron you have listed in prior posts.

Mark

[Cliff Schecht]

I agree with Mark and probably shouldn't have been so worrisome about details like I mentioned. This is usually not a problem with the 120V TW transformers and it's pretty easy to knock down the heater voltages using resistors or diodes. If a transformer is wound for 117V and you put it on 125V then you just barely get outside of the 10% range for heater voltages..

Like I said I'm using tubes that I want to last as long as possible and so hitting the spec'ed heater voltage is a way to help with this. If I'm using new manufacture glass then it's usually something I don't even consider..

Just to comment on this:

Don't the modern transformers, like the Magnetic Components Trainwreck transformers are, are not adapt to today higher voltage espacialy the filaments design to give 6.3 Volts where you need 6.3 V ?
I never enconter the point of view of the transformers engineers, but I imagine that they try to build a transformer that delever 6.3 Volts in North America with today voltage ????

Thank, Jean

They design the transformers to hit a certain voltage under a certain load. This is what I was trying to convey in my last post. If you don't hit that target loading then the voltages will be a bit higher than what's specified. Take a look here: https://www.antekinc.com/pdf/AN-2T300.pdf and note that they specify an open-load voltage and a fully loaded voltage (with a 28V difference) between the two. Depending on the manufacturer you will either hit the specified voltages under heavy loading or under normally loaded conditions (typically the latter, except for Hammond IIRC).

[M Fowler]

If you look at what David Allen (Allen Amps) has done he has already made the amp your talking about. Look at his Encore and sweet spot.

The Express was meant to use 6V6 or EL34.

Mark

[dcribbs1412]

+1 on the Allen Sweetspot
built one a while back

I want the Princeton features, one channel amp, reverb, tremolo, Princeton basic tone, my complaint against this amp, it sound a bit like a bedroom amp,

Not sure what is meant by bedroom amp...versatility,loudness?
It can get many tones (RAW control is cool) and it takes pedals well
and it can fill a room...
Mr.Allens hardware and xformers are top notch
also a great guy to deal with

my 2 cents


Darin