Is there a dual HV switch option?

[jordan86]

Loving my new BF Princeton Reverb that I just built. I  built it with a Hammond P290ax which comes with dual HV Secondary options: 325-0-325v or 275-0-275v.

Currently using the 325v option but would like to try the 275v option soon, for curiosity and learning sake.

Is there a way to wire both options into a switch and be able to select between them? Rather than pulling out the soldering iron every time?

Wondering if there is a switch   I know Dr Z has something “like” this on his Z wreck. At least I believe that switch toggles between two different plate voltages.

[Latole]

Yes with a DPDT toggle switch.

One transfo secondary output voltage connected to 2 outside terminals
Other transfo.    "               "      "                  "      "  other 2 outsides terminals
2 center terminal ( is the output ) to rectifier.

All ground are connnected to chassis or where you want

[shooter]

I would spend some time making sure my switch was RATED for high voltage, otherwise you might find out if YOU are 

[Latole]

Or use a relay.

[jjasilli]

You will also want, maybe need, to simultaneously SW bias voltage. I don't understand Latole's SW.  Seems to me: the 325V HT lead should go to one SW outer lug; the 275V HT lead should go to the other SW lug, same side.  Then the desired HT V will exit the center SW lug > the amp's B+ supply line.

Meanwhile, on the other side of the DPDT SW, a similar arrangement can be made for bias V.  There are a number of ways to implement 2 bias voltages.  The best way might be determined by the amount of bias V needed for ea HT voltage.

SW'ing between HT voltages on the fly may result in loud popping.  Simplest solutions may be to SW HT only when the amp is Off or in Standby mode using a separate, quiet Standby SW.

[jordan86]

Thanks all. I do have an adjustable bias pot installed. Although, on second thought, I guess isn't that terribly useful if inside the chassis :)

[shooter]

my2C
hard -wire it each way, play enough that you can tell OR not.  re-measure ALL voltages each way, including bias, making sure you're not "outside" normal operating conditions.

switching HV is NOT something that should be done without a lot of reading and understanding.  and a relay is WAY better than a toggle switch rated for possible HV spikes into the KV range

[jordan86]

That sounds like a very valuable .02 shooter :)

[jjasilli]

If you prefer, the DPDT SW can be a relay.


And then there's KISS (not the band).  I, among others, don't seem to actually bother much with options when actually using an amp, despite how appealing these options are in theory.  YMMV.  You may wish to try the amp both ways, and see if there's a tonal difference, or amount of headroom you prefer, or even notice; and just hardwire the amp that way.  These are personal, subjective choices; no right or wrong.

[pdf64]

My finding is that a switch arrangement as per post #2 works fine.
With the proviso not to actuate the switch when the amp is powered up.
The bias sorts itself out, as it gets derived from the same Vac as the HT, so the ratio between them stays the same. Kinda like VVR.

[shooter]

With the proviso not to actuate the switch when the amp is powered up.

 

I have yet to meet a guitar player that could resist turning a knob or flipping a switch, no matter what you tell them 

[PRR]

A big chunky toggle switch will be fine switching a couple 6V6 worth of HV occasionally.

Most affordable relays do not have as much internal spacing as a $3 thunk-switch. Real HV-rated relays cost more.

Since I agree you will flip it a few dozen times at most, switch meets the K.I.S.S. and least-trouble goals. If you learn you will flip it a dozen times a day for decades, like an air-conditioner, I would look for an A/C "contactor" (fat relay). These are usually 24VAC coil, but 115VAC if you dig in the catalog.

[pdf64]

It’s worth bearing in mind that the switch types used for HT standby aren’t adequately rated for that application 
And this application is pretty similar to that.
Also I generally suggest that standby isn’t actuated whilst the amp is powered up 

[66Strat]

I think that this one will work.

https://www.mouser.com/ProductDetail/691-2BL62-73

It's a two position DPDT switch, on-none-on. It is rated 6A/125 volts, 3A/250 volts, AC or DC. It has a 1 kilovolt dielectric rating. My thoughts are that it should be fine switching 350 volts of pulsing DC at 250mA or so. I have one in my cart at Mouser for this purpose.

[jordan86]

Love it. Thanks 66. I have an extra carling "ground switch" but its not DPDT. 

The knock off Z Wreck just came to my mind, made by Ceriatone. Looks like they do the switch on their C-Wreck. Schem attached. It's the "feel switch" (fast/warm). Still not sure what model switch it is.

Lots of chassis photos here, down the page:
https://www.flickr.com/photos/137947485@N03/albums/72157671753347556/with/28661586246/

[PRR]

> aren’t adequately rated for that application 

If it were the breath-power switch on my hospital ventilator, I would fret more about switch/relay ratings. (Though I wonder if the polio Iron Lungs used those same Carling switches?)

For a DIY guitar amp, used by the DIYer, the risk of failure is low. The amp may quit working.

[jjasilli]

. . . The bias sorts itself out, as it gets derived from the same Vac as the HT, so the ratio between them stays the same. Kinda like VVR.

W/o a schematic for this amp build, we don't know for sure where the bias is sourced from.  If it is sourced form the HT, and not a separate bias tap off the PT secondary, then bias voltage should vary OK with the HT switching.  Though maybe not optimal.

[nandrewjackson]

Jordan,
It looks like that ceriatone schematic is the way to go, in terms of your original question about switching between 2 different power transformer HT windings.


That's exactly what latole showed with the DPDT diagram, with the addition of a full amplifier schematic around it for reference. 

[Latole]

I would spend some time making sure my switch was RATED for high voltage, otherwise you might find out if YOU are 

Some Marshall stanby switches are rated 250 volts. We know Marshall voltage are more than 250 volts . 500 ?
Same on most amp I fix. Current load is more important.

https://www.tubesandmore.com/products/switch-marshall-toggle-dpst-chrome-tip

[jordan86]

Grabbing this switch and just had a thought. Wanted to see if I am thinking correctly.

If I were to grab a 3 position DPDT in an On-Off-On scenario, could that double as both a HV selector and a standby switch???

Up - 275v option.
Center - no connection to HV
Down - 325v option

Thinking a standby may come in handy if I ever wanted to run 5881’s and a Webber copper cap in my Princeton. Would give me a slow warmup option still. 

[sluckey]

yes

[jordan86]

One last Q....Any issue with a standby switch (breaking the hv) BEFORE the rectifier???

I'd essentially be taking the HV leads from each tap straight off the PT secondary into the DPDT switch, then into pins 4 and 6 of rec socket, or both OFF in the center position. Seems like most Standby switches break the B+ after pin 8 of the rectifier tube.

Just not sure if there are practical differences between the pre-rec/post-rec location with a tube or webber copper cap. The only difference I see is that voltage would be flowing to the tube and bias circuit in a traditional standby approach, and with this switch, the bias circuit would not see HV until standby is taken off. Meaning my power tube grids would not see negative voltage correct?

I know the Princeton doesn't NEED a standby but it's the possibility of a SS rectifier with a 5881/6L6 arrangement that I am trying to leave room for down the road. I guess I could skip the OFF position switch if its a problem. Just wondering if I could kill two birds here without killing myself or my amp. :)

[mresistor]

IMO  the Hammond 290AX is a little skimpy for running 5881/6L6. Isn't that what killed most PR power transformers in the past? Granted you have 30 more ma to play with. Running higher wattage power tubes is  why David Allen spec'd out his TP25 transfomer.

[jordan86]

Yup, I almost got his TP25 for that very reason. Actually, I almost just built the Sweet Spot kit but saved some big money doing the mojotone with mods.

And yes, the 290AX does supply 100ma, which is a LITTLE better than stock ~70-75ma. Maybe enough to try some 5881's with a SS plugin rectifier? I don't necessarily NEED more power. It's pretty dang loud as is, and runs clean up to about 7.  Just thinking if I did go that route one day to try it, a standby switch would be nice.

[sluckey]

One last Q....Any issue with a standby switch (breaking the hv) BEFORE the rectifier???

No. Actually, there would be less stress on the switch versus switching the DC.

[mresistor]

 But if using 5881s I'd be a little concerned with the 6.3VAC heaters; 290AX is only 2.25 Amps.