Where to mount additional 6v power transformer on Revibe build?

[itsGiusto]

For various reasons, I want to add some relay switching to my Revibe build, and to power the relays, I'll need to have an additional power transformer mounted, Dumble-style. But unlike Dumble, there's no room inside the very small chassis, so I think I need to mount it outside, with the rest of the transformers. I'm generally uncertain what are the rules I should follow for mounting transformers.

Here's a shot of the current transformer layout, with me also holding up an additional 6v transformer that I want to mount for relay power.

For what's currently mounted, from left to right, we have the the existing PT, the choke, then the OT to drive the reverb tank.

Here's a shot showing more detail around the PT area.

Do I need to mount the new PT in some particular orientation compared to the other transformers, for less interference?

I could maybe move the choke to the right, and then mount the new PT in its place. But would that put the choke too close to the OT? Also, then would it be important for me to splice extensions onto the new PT's primary leads, so I can send them through the same grommet as the original PT?

Alternatively, I could move the choke over, but then also drill a new hole with a grommet for both the secondary leads from the new and old PT below where the original one was, and kinda mount the new 6v PT over the old hole for the secondaries. That would allow me to get the PT very close to the other PT.

Generally, any advice is appreciated! Thanks!

 

[SerpentRuss]

Have you checked the coil loading of the relays to see if your heater circuit might handle the extra load, or is does that cause some sort of noise? I've never had a design that needed relays. Wherever you place it, you're going to have to feed 120 VAC to it. I would think you'd like to keep it very close to the PT for that reason.

 

[itsGiusto]

Have you checked the coil loading of the relays to see if your heater circuit might handle the extra load, or is does that cause some sort of noise? I've never had a design that needed relays. Wherever you place it, you're going to have to feed 120 VAC to it. I would think you'd like to keep it very close to the PT for that reason.

Hmm, that's a good question. I always assumed that you don't put the relays on the heater circuit due to noise, or excessive current draw or something, since it's so standard in the Dumbles that you always add an extra 6v transformer. But I don't truly know the reason why.

Edit: Sounds like it's debatable, but some people in this thread indicate that it adds extra noise and can be unreliable to take from the filament tap:

powering 5v relays with the filament supply. - The Amp Garage

 

[2L man]

I think it is better to have all transformers outside of the Chassis! Yes they do have electromagnetic interaction and there is "headphone method" to try to avoid hum what transfers to OT but I have not used it. I mostly use toroidal power transformers because their magnetic stray field is better controllable when it is strongest srtaight out of holes and weak to the sides so mounging them flat is fine.

So install it close the Choke as far from OT as possible and turn 90 decrees and then it should interact OT least.

 

[sds1]

I've never had trouble with using filament windings for relay switching, no noise or nothing like that. I've used both shared 6.3VAC secondary as well as unused 5VAC secondary to power relays.

My understanding is that the separate transformer is for safety isolation; ie, if a tube shorts to filament your low voltage DC circuit is protected.

 

[2L man]

When filsment CT is elevated or connected to Common the relay coil and its wiring must not have Common connection.

I would use DC for relay coil circuit if it controlled by a foot switch!

 

[itsGiusto]

I would use DC for relay coil circuit if it controlled by a foot switch!

Can you elaborate? I definitely plan on using a rectifier (either bridge or center tapped) to get DC for the relay (I don't know of another way to do it). Do you mean, you'd use DC off the filament tap, or from a dedicated transformer?

When filsment CT is elevated or connected to Common the relay coil and its wiring must not have Common connection.

Thanks, that's useful info, I had heard of that before, but it's good to hear again so I can make sure. If I did decide to use the filament tap for the relays, I'll have to double-check to make sure I can keep the entire rectified DC relay portion lifted from ground.

Edit: actually, if I use a two-diode center-tap rectifier instead of a bridge rectifier, would I need to make sure to keep the ground lifted, if I used the heater tap?

 

[2L man]

If you rectify around CT the DC voltage comes about half what a bridge rectifier does.

I don't see a problem when filaments are elevated or "grounded" and you use its voltage to power relays. Just use insulated jack and foot switch potential comes what it comes. Depending relay its coil insulation can be very high but so called "circuit board relays" have sufficient. Do not use "signal relays"

I have not yet used relays on my amps but there is lots of net-info of their use. I have few power transformers which have additional output for relays or SS circuits so I will use them. If not I will install a small transformer which I have many. Low VA circuit board transformers cost about max $5.

6,3mm stereo jack/plug allow only two relay operations but if you use only one relay you can still use stereo cable and connect its shield to amp shield/safety earth/chassis. I can't say does it improve something?

 

[itsGiusto]

Depending relay its coil insulation can be very high but so called "circuit board relays" have sufficient. Do not use "signal relays"

I don't understand, what does "coil insulation" mean in this context? The coils are insulated from what? And what are examples of circuit board vs signal relays? I couldn't find info on the difference by searching Google.

If you rectify around CT the DC voltage comes about half what a bridge rectifier does.

Interesting. You mean if I use the heater tap and bridge rectify then it'll be 6v, and if I center-tap full-wave rectify then it'll be 3v? That shouldn't be a problem because I have 3v relays and a 3v regulator.

 

[2L man]

It is a safety feature for circuit and user when insulations are specified and builder use relay which meet them. If you elevate the filament it is max about 70V where relay coil elevate and so does the switch in a foot switch. Sds1 said that in tube amps there are high voltages and possibility for HV to "jump" to filament

"Circuit boar relays" are usually soldered to circuit boards and there are few different ratings. I just bought a kit for two relays and it was about 11 euros ~$11) including circuit board, two jacks, two relays and few required components.

Some are for low voltages but there are for 250VAC and actual switch contact can handle it but also voltage won't "jump" to relay coil and destroy other circuits or cause danger to schock the user, so cheaper and smaller switches or drive circuits can be used and thinner = cheaper wires.

Signal relays are also soldered to circuit board but they are small and rating is lower. Next bigger size after curcuit board relays are used in sockets and among them there are many sizes and ratings.

 

[2L man]

Interesting. You mean if I use the heater tap and bridge rectify then it'll be 6v, and if I center-tap full-wave rectify then it'll be 3v? That shouldn't be a problem because I have 3v relays and a 3v regulator.

If you use "schottky" diodes, which waste less voltage and use them to build a bridge rectifier and filter the 6,3VAC it turns even 7,8VDC. I think most requlators need 2,2V higher input that they output but I don't know are modern regulators advanced there?

 

[itsGiusto]

If you use "schottky" diodes, which waste less voltage and use them to build a bridge rectifier and filter the 6,3VAC it turns even 7,8VDC. I think most requlators need 2,2V higher input that they output but I don't know are modern regulators advanced there?

In my past Dumble build, I've gotten away with not having 2.2v higher for the regulator. See this thread:

Transformers for Ceriatone Overtone Special (Dumble Overdrive Special clone)

Ah, I'm suddenly worrying if the PT I got is good for this... Unfortunately I'm uncertain what the b+ current draw of this amp will be. But the JTM45 PT I got says that it's only rated for 150ma current draw on the main winding: http://www.classictone.net/40-18054.pdf Unfortunately, I chose it...

www.tdpri.com

But whatever I do, I'll be sure to rig it up beforehand and make sure it works satisfactorily.

 

[itsGiusto]

So install it close the Choke as far from OT as possible and turn 90 decrees and then it should interact OT least.

I never really know how to tell if the transformer is turned 90 degrees or not. But if I'm not mistaken, I find it weird that in the stock layout of the Revibe, it seems that the OT and PT are not turned 90 degrees from each other (see my original picture). I don't know why they would have made the layout like that.

Maybe I can turn the PT 90 degrees, and in that way, both reduce the interference, and also make more room for me to mount the 6v transformer right next to it.

 

[itsGiusto]

To elaborate on my idea in my last post, this is what I was thinking of doing:

Does this look like a good idea? This will both turn the main PT 90 degrees, which I THINK will reduce interference with the OT, and it'll also provide room for my 6v PT.

Outstanding questions I have:
1. Is it bad for the PT to be moved slightly closer to the tubes in this way? I could maybe instead try to move the main PT more towards the other side of the chassis, towards the knobs
2. Should I also rotate the choke 90 degrees as well, to match the PT, so it also will interfere less with the OT?