Replace stock transformers in MK3

I am not necessarily interested in throwing money away but I wanted to know if I should replace the power and/or output transformers on my vintage MK3's? they don't show any signs of problems but I have read about deterioration especially concerning the power transformer. Are the vintage transformers something that should absolutely be replaced (like old caps), or are they considered robust enough and leave them until they fail?

Also, I know there are re-issued transformers out there, but are there any toroidal's that will work?

Thanks bob. When ever I do a modification I always keep the stock parts. I still have the original 100% stock driver boards for example. If I were to replace the power transformers I would definitely keep them. I also try and keep things in original condition too. This comes from growing up with my dad who is into antique cars so I have learned about the value of "all original" (he restored a 1949 Buick Super 8 convertible to the point of even having the tires cast from the original molds to keep the car as "factory" as possible). For day to day use of my MK3's they might benefit from new power transformers. I am also a little concerned about safety in that old transformers can fail in sometimes spectacular and dangerous ways. Usually a winding will break and fry and that will cut power but sometimes they can short out and arc and cause a fire. I can probably get a pair of toroids that will work for $100 each so its tempting.

plus shipping to canada plus potentially 13% tax and $? brokerage fees. I emailed dynakit for shipping prices. Toroids are generally quieter, cooler and generate less EM. so taking price and quality into consideration I would more likely move to toroids for the PSU - I don't care if it doesn't look stock in day to day use. Plus I think a well integrated toroid could update the look for the MK3 if we are at all concerned about aesthetics. YYMV. will post my final decision on this.

Plexus,
I see no reason to replace your MK III power transformers if they are working
correctly for you. The amperage measurements you posted look fine.

Regards,

Kevin

Go for it...do one power transfourmer at a time.... tell us with all stock Mk3s if you think it better in any way?...then do the output tranfourmers..give us with MK3s a headsup.....You wont no if you dont go...thanks

Thanks foe the site Bob..

DynakitParts wrote:
Plexus,
I see no reason to replace your MK III power transformers if they are working
correctly for you. The amperage measurements you posted look fine.

Regards,

Kevin

Kevin, thanks for the reassurance. I was mainly concerned with longevity. There is an early post on this forum by Bob showing a Dynaco PSU xformer that caught fire and the description around it seems to be geared at scaring the sh*t out of us so we buy new transformers.

I have to run my MK3's 24/7 for 17 days in order to burn in these V-Caps (400 hours). this had me thinking about how warm the stock PSU transformers get (55-60C with a moderate level signal input). I was also thinking if the stock PSU xformers do deteriorate, why not replace them with say toroidal (I can get 2 for $180 shipped new) and then the stock ones will still work in the event I ever want to sell the amp.

So are you saying that if the stock transformers work, they will likely keep working and don't pose the malfunction / fire threat that was posted earlier in the forum?

It would be nice to get some accurate information and not a sale pitch.

Brett

i've been running 3 sets of mark III's, all with original trannys for years without any issues at all with the trannys. i've done various upgrades as they were needed to all of them but i would never replace a perfectly good transformer for a new one. the old cloth lead trannys in the mark III's can not be improved upon. same with the st-70's trannys. why don't you just sell me the mark III's and buy something new. there's no reason to believe that a new transformer will not fail, catch fire, and burn your house down either.

wedg714 wrote:i've been running 3 sets of mark III's, all with original trannys for years without any issues at all with the trannys. i've done various upgrades as they were needed to all of them but i would never replace a perfectly good transformer for a new one. the old cloth lead trannys in the mark III's can not be improved upon. same with the st-70's trannys. why don't you just sell me the mark III's and buy something new. there's no reason to believe that a new transformer will not fail, catch fire, and burn your house down either.

Well, when you see something like this forum post about dynaco transformers not designed to last 50 years and you have no other experience to go by, it makes me think about switching them out for new ones. But the OP to that thread, and yourself, have advised opposite of what was posted there. ok, no problem, glad i asked and thanks for the advice.

wedg714 wrote:there's no reason to believe that a new transformer will not fail, catch fire, and burn your house down either.

To that end, I don't think I'd be sleeping very well at night knowing I had something with the potential to fail in a very spectacular way powered and unattended in my house.

jjones3318 wrote:

wedg714 wrote:there's no reason to believe that a new transformer will not fail, catch fire, and burn your house down either.

To that end, I don't think I'd be sleeping very well at night knowing I had something with the potential to fail in a very spectacular way powered and unattended in my house.

Exactly. I might just go for the toroids. it would be <$100 each amp and then the stock transformers are out of the equation but preserved for future restoration or sale.

I'll have to agree with the majority here, if the power transformers are working, keep them.
If you want to be safe, just make sure the primary power fuse is the correct size, I didn't check the actual spec
but I would use a 3 amp slo-blo. You could also add a fuse to the secondary, that's cheap insurance, the two ouput tubes
are using up to 140ma of current total. The driver circuit uses 10-15ma. A 1/4 amp fast blo gives you all the protection you would ever need.
I'm rebuilding an amp now that I'm putting 80ma slo-blo fuses in series with the cathode resistors. That saves the expensive output tubes.
Very cheap insurance.

plexus wrote:

jjones3318 wrote:

wedg714 wrote:there's no reason to believe that a new transformer will not fail, catch fire, and burn your house down either.

To that end, I don't think I'd be sleeping very well at night knowing I had something with the potential to fail in a very spectacular way powered and unattended in my house.

Exactly. I might just go for the toroids. it would be <$100 each amp and then the stock transformers are out of the equation but preserved for future restoration or sale.

I wasn't saying that - I wouldn't leave tube gear on unattended regardless of what type of transformer it has. IMHO, that's just begging for trouble.

jjones3318 wrote:

plexus wrote:

jjones3318 wrote:

wedg714 wrote:there's no reason to believe that a new transformer will not fail, catch fire, and burn your house down either.

To that end, I don't think I'd be sleeping very well at night knowing I had something with the potential to fail in a very spectacular way powered and unattended in my house.

Exactly. I might just go for the toroids. it would be <$100 each amp and then the stock transformers are out of the equation but preserved for future restoration or sale.

I wasn't saying that - I wouldn't leave tube gear on unattended regardless of what type of transformer it has. IMHO, that's just begging for trouble.

That's life sometimes.

on a mark III you should be using a 3 amp slo-blo fuse. anything higher and you're asking for trouble. i have all of my system running through a variac so i know that there is only 120 volts going to it. unstable and high house current can cause trouble with vintage gear. high voltages can fry a transformer. when i turn my system off, i also turn off the variac. better safe than sorry

Here's a radical solution to your transformer dilemma. I can just about guarantee this will not cause a house fire: http://victor-victrola.com/ Sorry, I couldn't resist!!!

Best Regards,
Charlie

cute.

I can opt to burn these bloody caps in through listening but listening 4 hours a day will require 4 months to burn them in. so I am opting to burn them in-situ in the amp, running, into a dummy 8 ohm load. with trepidation. hence the research on the PSU xformer. I found that post by Bob which showed a burned out P782 and stating "Dynaco transformers were not design to last 50 years". heeding the expert's advice, I posted here only to find that everyone is suggesting just leave the stock transformers. so a little confusing, ya know?

I might opt to get the toroids and use those instead. keep the stock vintage P782's aside and in a still-working state. the toroid might run cooler as well and be more energy efficient. I can get a pair for $180 shipped as I mentioned, so $90 each amp. not a bad price i guess.

Still deciding what to do...

Brett,
A sales pitch? I believe I suggested you keep the original transformers. The
P-782 photos you refer to were of one of my customers transformers. I suspect that
the leads were very brittle and one or more of the secondary leads shorted to the
casing and/or to each other. A visual inspection would have revealed this prior to
applying full voltage. Another Ebay plug & play tube amplifier.

The life expectancy of a Dynaco power transformer?

50 years or 100 years...who can really say? Too many variables.

In the end it is your decision regardless of what I or others may say.

Kevin

I think it is part of the charm of old tube gear. Hendrix banging out the Star Spangled Banner and in the middle of the song an amp explodes. How cool would that be? I am probably perverse in my thoughts though. I just bought some MkIII's, so getting anxious to fiddle with them.
I was looking at the cloth transformer leads coming into the chassis and they are fairly tight around the bare chassis hole with no bushing around the metal. I could see how, with a little vibration over the years, they could short out. My first step will to remove the transformers and install plastic bushings from Dynakit. I figure at $1.50 per amp it's one less thing that could go wrong. Not an expert, but this will be my first step and maybe yours already has them installed.

I've seen way too many old Dynaco's where the cloth leads had turned brittle and actually flaked off, exposing wire.
Some people open them up and splice in new high temp modern wire leads on them

I was also thinking about shrink tubing over the cloth on each lead while it was out. Is that a bad idea? Maybe clear teflon.