# DPDT for Switching OT Primaries?



## bcmatt (Aug 25, 2007)

Hi,
I'm presently building a Trainwreck Liverpool Amp and sourced al my parts from all over the place, including Toneslut Transformers.

The OT gives an option of Primary imdedances. There are a pair of 5.2K taps and 6.6K taps. Now, I realize I could just wire them up one at a time and find the one that I like the sound of the most; but I am wondering if I could install a DPDT switch somewhere along the wires' path and be able to switch between them. Is there a potential problem with doing this?

250VAC 10A/125VAC 15A DPDT Toggle


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## Adicted to Tubes (Mar 5, 2006)

When you switch impedances like that,it increases the current draw from the PT.It should not be an issue with quality transformers,but it changes the load ratio to the output.
So say a trainwreck uses apair of EL34's,fixed bias.When you alter the load the tube sees,it translates into increased output power.The lower the load the more output power you can get.
The speaker load relationship changes also.Where a 5.2k load likes to see an 8 ohm speaker,the 6.6k can use a 4 ohm.
The only real-time change is likely to be power output.It is unlikely the 'tone' will change a whole lot.
The original Trainwreck exploited the JTM 45 6.6k/EL34 or KT66 load relationship.A normal EL34 marshall uses a 3.4k loading to get the most output power from a pair of EL34's.
Making it switchable would not be worth the effort in my opinion.You can accomplish load changes by simply changing speaker impedances.
Be wary of the increased temperature of PT's when you lower the load.
The difference from 5.2k to 6.6k is not worth mentioning.

Edit:it is the same basic realtionship with 4-EL84's like the liverpool uses.

www.claramps.com


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## Geek (Jun 5, 2007)

Let's do some math here.

Assuming a 40W peak, Vrms = SQRT(W*R) = 513.8 Vrms = 726.5 Vpk = 1453 Vpk across the anodes (hence peak seen by the switch).

Not a chance.

It might not fail right away, bit it will.....

You'll need a proper HV relay in there to be controlled by a switch and keep those anode and transformer leads as short as possible.

Long leads to/from a switch can cause all sorts of grief, like feedback or oscillations.

Or a HV switch as close to the OPT as you can get.

Cheers!


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## Wild Bill (May 3, 2006)

Just don't do it!

There are too many problems for a trivial if even noticeable difference in tone. There's a good reason why NO ONE else has ever tried this!

Were they ALL just dumb?

You'll be sorrrrrrrry!

:food-smiley-004:


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## bcmatt (Aug 25, 2007)

Ok, good enough. Thanks guys. I'll just wire up the one set. Then maybe one day I'll try the other.
Cheers.


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## Emohawk (Feb 3, 2006)

Isn't this what the optional "crunch/munch" switch does on Trinity's TC-15? Switches between the 5K & 8K primaries on the OT doesn't it?


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## bcmatt (Aug 25, 2007)

Emohawk said:


> Isn't this what the optional "crunch/munch" switch does on Trinity's TC-15? Switches between the 5K & 8K primaries on the OT doesn't it?


Ya, it does. Which is confusing because it's just a little mini-toggle. Is it because it is only a 15 Watter?


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## WCGill (Mar 27, 2009)

An ordinary DPDT will fail, don't ask me how I know. You need a relay controlled by a switch. I'm not familiar with the Trinity setup, but I assume this is what's happening here. Worth it? probably not.


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## bcmatt (Aug 25, 2007)

WCGill said:


> An ordinary DPDT will fail, don't ask me how I know. You need a relay controlled by a switch. I'm not familiar with the Trinity setup, but I assume this is what's happening here. Worth it? probably not.


Maybe not worth it but they do it with no relays:








It's between the speaker jack and the power tube sockets. Perhaps I should tell this friend to choose a primary preference and I'll hardwire it to that and remove the toggle for him...


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## WCGill (Mar 27, 2009)

Wow, messier than my amps!  If this is the way Trinity wires it, then I'd assume it's ok to leave as is, but as I said earlier I'm not familiar with the setup.


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## Wild Bill (May 3, 2006)

Addicted to Tubes is quite right - it's really not worth it! Any tone change will be trivial. Changing to a 4 ohm speaker instead of an 8 ohm will give more change and even then not enough to be "slap you in the face" obvious. The only reason the transformer has the extra taps is so that it can match a different set of output tubes.

You will likely fry any switch you can find. A high voltage relay would do the job but it would take up a huge amount of real estate. It will also need a dc coil with another dc power supply, since an AC coil relay would likely add hum.

Looks like Trinity wired a switch to short out the extra turns. Normally the higher impedance is running and if you throw the switch it shorts out the taps so that the lower impedance is in play. That will likely be ok as long as no one gets in the habit of throwing the switch while the amp is wailing a loud series of power chords.

This is one of those ideas that seems simple only when you don't know all the things involved to make it work! When you're finished, who's going to care? The switch feature will not give you a great tone option and your amp will look all clunky and unnecessarily complicated. Nobody's going to give you the extra money if you want to sell the amp, either.

I'd put the money and effort into something more worthwhile, if it was me. This strikes me as in the " gimmick " category rather than a useful feature.

WB


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## WCGill (Mar 27, 2009)

Getting your OPT wound with several secondaries or fractions thereof and switching them to manipulate the primary negates the need for a HV relay. Elegant solution.


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## bcmatt (Aug 25, 2007)

Good talk! Nice to know why it's generally not done.

For my Liverpool, I did swap the primaries yesterday to 6k6 from the usual 5k2. It did make a significant change to the character of the amp, but I really didn't like it, so I changed it back after a few hours. There had been rumors that some very early Liverpools used the 6k6 and a few popular builders today are getting requests for Liverpools with the 6k6 primaries. Gives less headroom and a bluesier tone, all that I thought would suit me. However, I felt robbed of my usual attack and bite that I like to get with P-90s. Sort of changed it from an amp that I thought suited all sorts of guitars, to one that was difficult to dial in unless you are using a Tele or a Strat only. It's true though, that if I ever want that effect, I can just pop the impedance to 4 ohms on the amp while running an 8 ohm cab. The amp is plenty versatile enough though with plenty gain and the Variable Voltage Regulator. Clean to mean with the guitar volume knob.

As for that friend with the TC-15, I won't worry too much about him, but I will suggest that once he settles on his primary impedance choice that I can pop in another speaker jack in place of that switch.


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