Sourcing transformer for a Blackvibe build

[Bretly]

Hey all.

Fairly new to the game here and currently building a 5e3 kit while sourcing parts to build either a Blackvibe from RR’s site or just a straight up vibroverb.

I see a few main options for transformers and am leaning towards mojotone and hoping I can get some validation that I am making the correct choices here.

- https://mojotone.com/products/bassman-p ... 9730908256
- https://mojotone.com/products/vibrolux- ... 9763610720
- https://mojotone.com/products/filter-ch ... 9733070944

There seem to be other options out there from mercury magnetics (classic tone?), Weber and Hammond. They are all labeled as replacements for 125P5D but all seem to have varying voltages. 320-370vac. The mojotone looks to be perfect for what is listed in the schematic 355vac.

- Do these transformers look correct in regards to what I need?
- Does the voltage difference matter that much or is it more of an upper end like resistors, caps, etc and it’s fine to go higher.
- the mojotone power is rated at 355vac 300ma. Where the Weber says 720 Vct 200ma. My understanding is this is just representing some very close numbers in different formats, but does the 200ma vs 300ma make a difference?

Thanks again. Super eager to learn and get these parts bought.

[johnnyreece]

Pacific also makes transformers domestically and internationally (the red series are from overseas and reasonably priced): https://pacificaudiomagnetics.com/collections/all

[martin manning]

My understanding is this is just representing some very close numbers in different formats, but does the 200ma vs 300ma make a difference?

Any voltage specification should have a current to go along with it. It could be zero (unloaded), at max capability, or somewhere in between. Hammond is the only manufacturer I can think of that provides loaded and unloaded voltages. Some just give a voltage without specifying the load, which is not very helpful. In fact the idle condition where people (amp techs and builders) do voltage checks is a part-load condition. In theory a "Vibroverb replacement" transformer should get you the right voltage and have adequate current capability.

[Stevem]

Let’s get down to Brass tacks just a tad.

In terms of wattage output that power is made by a combination of the DC voltage on hand and the current.

In terms of power / wattage ohms law tells us that V x I equals power.

Having more then what’s needed on hand will change the basic nature of the amp.

For example, having more current then what’s needed will keep the voltage higher at idle and at off idle usage up to a point.

While this will keep the amps level of output puch up as the amp gets played volume wise up to the point where the DC voltage starts to sag it will also mean that below that point the level of gain the amp has will be higher and thus the amps feel will be different then a original unit.

This gain factor relates mainly to the preamp section since output stages do not have anywhere near the gain of a preamp section.

On the other side of the coin if your preamp voltages are lower then expected you can easily and cheaply lower the value of the preamp power supply dropping resistors to get that voltage back up.

The only down side to this might be that more 120 hz power supply hum might be heard at idle and at times of low volume usage.

Back to the other side of the coin again if you play the amp cranked and there’s not enough current output this 120 hz noise riding on the power supply voltage causes something called modulation and what this does is cause the notes you play to be bent out of tune.

This same condition can take place if the power supply filtering at that first node at the output tubes gets weak or is not enough at that level of power output.

Keep in mind that all 50’s 60’s and early 70s amp manufacturers never dreamed that there gear would be played loud enough to distort and start to tax the power supply .

[Bretly]

Thanks for the responses. If I’m understanding everything correctly.

- More is better in the sense that, it’s cooler, more headroom, less sag. Maybe more longevity.
- Lower is better in that it’s probably closer to the original sound.

Not overly concerned with the latter and I wouldn’t know what that sounds like anyway really. This is meant to be a learning experience (an expensive one I guess).

Leaning towards the mojotone, as research says it typically produces the right b+ voltages in a build like this.

Waiting to hear back from Mercury as it seems they have an option for both depending on if you are looking to hit that exact vintage spec or if you are modding the build and pulling more voltage.

[Stevem]

Mercury’s are great and to me overkill and way over priced for a amp like this , but hey it’s your money do with it as you will.

[pdf64]

... if you play the amp cranked and there’s not enough current output this 120 hz noise riding on the power supply voltage causes something called modulation and what this does is cause the notes you play to be bent out of tune.

This same condition can take place if the power supply filtering at that first node at the output tubes gets weak or is not enough at that level of power output. ...

I don't see how poor regulation at the mains transformer could cause modulation issues?
Such modulation (between the signal and HT ripple) is all on the degree of HT ripple at the anode and screen grid supply nodes, factors affecting this include reservoir cap value and ESR, choke inductance, screen grid supply node capacitance value and ESR.

Mains transformers intended for valve rectifiers will get hot at full load, at least vintage types anyway, because their winding resistance is usually designed to be sufficiently high to prevent peak current exceeding the rectifier's limiting value.
Such winding resistance will cause poor regulation.

Modern transformer design might not include such resistance, perhaps because users think that transformers that don't get hot in use are somehow better.
Then they complain about valve rectifiers failing and the amp sounding stiff and sterile.

[imjonwain]

https://www.musicalpowersupplies.com/

He has some imported models and will also wind you basically whatever you want. Cheaper than the big names. I've used his OTs, PTs, and chokes in a couple builds and repairs.