Can a power transformer be TOO BIG for a build?

bastardbus · Post by **bastardbus** » Fri May 23, 2014 5:17 pm

Here is a question I was wondering about and have not had luck getting a solid answer. I come across all kind of old tube amp stuff in my collecting and have PTs of every shape and size. Sometimes I come across PTs from consoles that run both the amplifier and a tuner (radio) full of tubes. These usually are p-p 6v6 power output but with a dozen or more other tubes. The PT HV line is perfect for 6V6 set up but the current ratings can be very high due to the large number of tubes it runs.

I am interested using these PTs for guitar amp builds but I really like doing smaller bedroom sized builds like champ-princenton-AX84 type stuff. These PTs will physically fit just fine in these amps but I was wondering if there was some reason these were too much output for a smaller build?

I have done a bit of searching for answers but get a wide array of opinions on the matter. Some guys will say a PT can be too big but can't give me a reason why or where the "cut off" for too big and just right is. Others have told me there is no such thing as too big...the circuit will draw what it needs and the PT will run fine.

Just curious...I have never really got a good answer from someone with facts to back it up.

Thanks
T

Bob S · Post by **Bob S** » Fri May 23, 2014 5:35 pm

Big transformers in small amps have a couple of drawbacks.
One being the inrush at power up means you may have to up the rating of the time delay fuse in the supply. Fuse the HV correctly & it won't be a problem.
Secondly - big transformers are HEAVY. This is more a consideration for us older guys...

YouTube · Post by **xtian** » Fri May 23, 2014 5:42 pm

Hmm...I've got this massive Fender Tone Master iron sitting around. How about a 60 pound, 1 watt Firefly build?

Phil_S · Post by **Phil_S** » Fri May 23, 2014 5:43 pm

If otherwise in-spec for your build, the only thing I don't care for is the weight. If it was running 12 tubes, it will have a monster filament winding. You may want/need to reinforce the chassis so it doesn't flex. When using a 9# PT, you'll find you don't want to move it around.

cbass · Post by **cbass** » Fri May 23, 2014 5:50 pm

What about voltage drop under demand?I think I read somewhere that the only reason some old Marshalls didnt meltdown was because the PT's sagged so much under load

martin manning · Post by **martin manning** » Fri May 23, 2014 6:03 pm

Phil_S wrote:If it was running 12 tubes, it will have a monster filament winding...

Which means the filament voltage will run high when only a few are connected.

cbass wrote:What about voltage drop under demand?I think I read somewhere that the only reason some old Marshalls didnt meltdown was because the PT's sagged so much under load

True, but only because of the chosen combination of voltage and load.

bastardbus · Post by **bastardbus** » Fri May 23, 2014 6:31 pm

Here is a couple examples of what I speak.

SHF-3
Secondary
720vct@.110a
5v@ 2a
6.3v@5.2a

SHC-6
Secondary
580vct@.130a
5v@3a
6.3v@5.6a

Phil_S · Post by **Phil_S** » Fri May 23, 2014 8:14 pm

Those 5A windings are robust but not monster. A pair of 6V6 and 3 preamps is going to want to see almost 1.8A. 2x EL84 + 3 preamps wants almost 2.8A. I don't imagine the jump to 5A is quite so huge. I was thinking in the range of 9A. I've got one I keep on the bench with 6.3V @ 9.5A. I think it weighs in at almost 10#.

If you have trouble with high filament voltage, a big wirewound resistor (10W?; ~0.5 Ohm) on one leg between the transformer and the first tube should get you close to what you need.

The SHC-6 is perfect for an 18W build and the SHF-3 looks good for a Tweed Deluxe! Many other possibilities. What are you waiting for?

bastardbus · Post by **bastardbus** » Sat May 24, 2014 12:28 am

I have a couple guys interested in amps from the AX84 site. One wants a P1 extreme, it is basically a hot rod fender champ with more gain. I built one with a 6V6 and everyone around here is goo-goo over it. Another guy wants a high octane which is similar but with a single ended EL-84.

As you can see they are pretty small amps with little demand PT wise. Those two console PTs are not HUGE but quite a bit for a Champ-ish build. They will fit nicely in the chassis and not that much heavier then the Hammond 269EX recommended but I was concerned about them being too much for these builds. Seems most folks buy new iron designated for their designs, I am building amps from old `50s hi-fi left overs and toss outs. Guys around here seem to love the idea and the "steam punk" vibe these things have besides they sound absolutely wonderful. The only issue I found is in situations like this, figuring out what works and the limits you can go to using this old iron.

Phil_S · Post by **Phil_S** » Sat May 24, 2014 12:43 am

You can't beat it for the price. Go for it. If not right, you can always make a change.

Regan · Post by **Regan** » Sat May 24, 2014 7:02 am

I would go for it,, you could always jump up to a bigger power tube that will use some of that extra capacity. Maybe kt88 single ended in both?
Regan

Jack Hester · Post by **Jack Hester** » Sat May 24, 2014 9:11 am

Bob S wrote: One being the inrush at power up means you may have to up the rating of the time delay fuse in the supply. Fuse the HV correctly & it won't be a problem.

I stocked up on thermistors to handle this chore:

http://www.mouser.com/Search/ProductDet ... 22-12103-B

Of course, you will need to take into account the current demands of the PT, to size your thermistor.

Jack

bastardbus · Post by **bastardbus** » Sat May 24, 2014 4:56 pm

Regan wrote:I would go for it,, you could always jump up to a bigger power tube that will use some of that extra capacity. Maybe kt88 single ended in both?
Regan

Thought of that too but seems most of these run 6V6s and the larger tubes usually want higher voltages then a 6V6 PT puts out.

rp · Post by rp » Wed May 28, 2014 10:49 pm

No one mentioned this but a too big PT will sound and play very different. Faster and punchier, a bit lit like switching to ss rectifier. Something you may or may not want. For reasons of sophmore ignorance I put a 500mA PT in my 5F6A/JTM45 clone. Definitely snappier than a Tweed or JTM. Local teacher musician here, great modern shredder style player loved it. I had a SS rectifier plug in at the time, amp had that old style dynamics, bloom and break up but with a fast modern edge. To me it was kind of annoying but with him it was great.

A 500mA PT will cause your amp to hang very lopsided when you carry it

billc · Post by **billc** » Thu May 29, 2014 5:35 am

Interesting thread and I recently got educated on this myself when I converted an old PA amp from 6L6's to 6V6's...the lighter load made the B+ and filament supply voltages go too high so I changed the cathode resistor back to a 6L6 value and installed new 5881's this evening and the voltages look much better....so a good lesson for me and I'd now say a PT can definitely be too big, before this did not realize that changing the secondary loads from the design values would make such a large difference.

The Amp Garage

Can a power transformer be TOO BIG for a build?

Can a power transformer be TOO BIG for a build?

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