Looking for a custom made 100W power transformer . . . .

[RickyLee]

Seems that my simple plan to find a power transformer for this Mitchell Pro 100 I am working on is turning out to be not so simple after all. This amp runs 510V on the plates and screens of the 6L6's.

The power transformer I have in the amp now is a Mesa laydown style and is exactly the size of a MK V or Dual Rectifier model PT. These 100W transformers use the center tap configuration (help me with learning the proper names of the circuits and terminology) with the higher HT secondary voltage like you would see in a 50W JCM800 2204. A center tapped secondary 350 - 0 - 350 which gives you 700VAC measured across the winding. That would be the half wave rectifier? which uses two diodes in series on each leg for the rectification. 700VAC X .707 = 494V B+

Classic Tone has this PT but still does not solve my problem of a high B+. I am trying to find a PT that has a lower B+ option or multiple HT taps so I can get my plate and screen voltages lower. At least well under 450VDC.

http://www.classictone.net/40-18068.pdf

Metroamp has a laydown style PT with multiple HT taps which would be perfect for the lower B+ option I want. But this PT is the other type that takes the full wave bridge rectifier. Is that what is referred to as the voltage doubler circuit?

http://forum.metroamp.com/viewtopic.php?f=27&t=10971&start=0

Now what has me confused a bit more is this Metro PT has the center tap I need. But it is the lower voltage HT. What is that center tap used for?

Does anyone make Mesa type power transformers with dual HT taps so you have a lower B+ option? Or just a Marshall style laydown custom order would probably work. I could actually make a stand up type work as well. But it has to be center tapped secondary config. Basically like a 50W JCM800 Drake but with the higher current specs on the HT AND the heater winding.

 

[ibmorjamn]

I believe it is to reduce hum.
http://www.valvewizard.co.uk/heater.html

 

[RickyLee]

You are referring to the heater winding center tap.

Closest PT I can find is a Twin Reverb type. This one has dual HT taps but I would like to have an option to go a bit lower than 640VAC which gives about 452VDC on the plates an screens.

http://www.classictone.net/40-18042.pdf

Might have to throw in the towel and run KT77's . . . .

 

[mickeydg5]

Are you trying to confuse people?

A center tap is a center tap.
A full wave uses the center tap and a bridge does not, typically. A center tap type will have higher voltages between outer outputs and a bridge type will be lower voltages.

What is the problem with KT77?

 

[RickyLee]

I had been thinking that these transformers that have the 40-XXXXX model numbers were made by Classic Tone. This one is mentioned as a New Sensor product:

http://www.ebay.com/itm/New-Sensor-...57f99a1&pid=100033&rk=4&rkt=4&sd=111855726116

So what I can't find is where you would purchase a replacement Mesa type power transformer. Been googling and coming up empty. Classic Tone has no Boogie type transformers. Mercury Magnetics has some, but insane prices and they do not show specs or diagrams on any of the transformer offerings.

 

[bluesaxe]

Some options I might try ->

Add an extra RC filter stage before the OT tap off the power rail to reduce the B+ to the power tube plates.

Run a 5U4 rectifier if you have the right heater tap. 5U4 should give you about the right voltage drop, plus some cool spongey sag.

Switch to Cathode Bias?

I personally would like all of the above options over running 500+ VDC on the plates on a fixed bias amp. At high plate voltages, you are pretty much forced to bias the tubes cold which always sounds sterile to me.

 

[RickyLee]

Are you trying to confuse people?

A center tap is a center tap.
A full wave uses the center tap and a bridge does not, typically. A center tap type will have higher voltages between outer outputs and a bridge type will be lower voltages.

What is the problem with KT77?

Only trying to confuse myself on this one.



Crap, look what time it was when I posted that LOL. I was wracking my brain trying to find a transformer to match up. Besides, I am sure I am the only one that is getting confused here on this. That is why I would like some clarification on proper terminology and such. I just seen your post and will get back to this as I am heading out to get some dinner . . . .

 

[mickeydg5]

I thought Boogie amplifiers like that were setup to be around 460V B+. Why is your Mitchell at 510V? Is it because of the wall voltage there?

 

[Woods]

Why are you so obsessed with 510 volts? How is this the magic number that is going to make the world come crashing down upon you? The magic number for face melting tone? The resonant frequency of wet farts passed through a cinder block wall?


I say play the damn thing! Goddamn!

 

[mickeydg5]

Whaaat?

I believe he is worried about power tube screen circuits.

 

[Woods]

Firstly, there's no need to create problems that don't exist. Every piece of electronics has a tolerance, it either falls into those specs or it doesn't. I know everybody wants to deliver some laser sharp cure-all to show their total dominance in the subject; but I've got to ask, why is this all of a sudden a problem when it wasn't before?

 

[mickeydg5]

I am not quite sure. Notice my question about the KT77.

 

[RickyLee]

I thought Boogie amplifiers like that were setup to be around 460V B+. Why is your Mitchell at 510V? Is it because of the wall voltage there?

The wall voltage has a bit to do with it as it hovers around 125VAC and will get over that, and have had 126.5VAC. Bummer that. Give it to the electric companies as that just makes us consume more power and add to the bill over the long run.

As for this Mitchell, not much is known about these amps. It is just a cheap approximate copy of a Boogie MK 1. Missing the bells and whistles as well. I have not had this amp for but a few weeks. I do have a few other Mitchell amps and it is rearing its head again as this is a repeat of what I ran into on the last Mitchell.

Seems these power transformers could be possibly under spec'ed on the heater taps. There is no ID numbers at all on this power transformer. It runs four 6L6 and has four 12AX7's. Even with a high wall voltage the heaters are running under 6VAC. If I run all four 6L6 compared to only running with two 6L6 pulled there is a 1 volt difference (heaters drop to under 5VAC). So this issue as well as the high plate voltage is why I figured it would be best to replace it. You might remember me bringing up that last Mitchell as it had a problem maintaining the heater supply up past 5VAC LOL.

I have been playing it safe running it in half power mode 50W with two 6L6's and using the VariAC set about 115V. The heaters run just over 5VAC and keeps the plate and screens down to just over 470V.

So in regards to this power transformer and what Mesa Boogie used in the 70's/80's, I do not know. I have an early 90's Mesa DC-10 that runs about the usual 470V on the plates with my high voltage.

 

[RickyLee]

Why are you so obsessed with 510 volts? How is this the magic number that is going to make the world come crashing down upon you? The magic number for face melting tone? The resonant frequency of wet farts passed through a cinder block wall?


I say play the damn thing! Goddamn!

First off, why are you the one getting all pissed off in my thread about my amp having problens and pissing me off?



No MaGiK ToNeZ Chasing going on here. I am not obsessing over and digging having 510V LOL. 510V on the screens of these modern day tubes/valves is asking for trouble. And can get a bit pricey burning through them prematurely.
510V on my screens is part of the issue I am going to solve. Also, read my post up above just before this as it explains what else is going on. Have only had this amp a few weeks. Had to revive it back to life you could say.

After replacing filter caps, I loaded in one of my favorite sets of valves/tubes, a pair of old Saratov Russian KT66's. One of the tubes had a nice orange glow on the plate. And that pissed me off a bit. Nothing cool about running that high a voltage if it can be helped.

Firstly, there's no need to create problems that don't exist. Every piece of electronics has a tolerance, it either falls into those specs or it doesn't. I know everybody wants to deliver some laser sharp cure-all to show their total dominance in the subject; but I've got to ask, why is this all of a sudden a problem when it wasn't before?

Again, I am not here trying to show off my lack of electronics skill and knowledge.



ADD: KT77's would be the safer valve to run. Just do no have a set at this time to use. I do have some in a couple other amps that are buried in the storage room.

 

[Woods]

Excellent I hope you come to a solution that's cheap and easy.

 

[RickyLee]

Some options I might try ->

Add an extra RC filter stage before the OT tap off the power rail to reduce the B+ to the power tube plates.

Run a 5U4 rectifier if you have the right heater tap. 5U4 should give you about the right voltage drop, plus some cool spongey sag.

Switch to Cathode Bias?

I personally would like all of the above options over running 500+ VDC on the plates on a fixed bias amp. At high plate voltages, you are pretty much forced to bias the tubes cold which always sounds sterile to me.

I like your idea of adding in the front end dropper resistor. How come know one does this instead of the circuit with the zeners dropping the voltage?

Also, this PT in this amp does not have a 5V tap either. So no tube rectifier option there.

But I need to replace this power transformer regardless. It has a weak heater supply for some reason, and drops to under 5VAC if I run all four 6L6's. I can't figure out what make and model this power transformer is as it has no numbers or ID on it at all.

I was hoping to find a new Boogie replacement PT, but can not find where you would buy one.

I do have an old Mesa transformer in my stash I am going to dig out and see if it will work out. I think it might be from a very early Dual Rec. But it might have a somewhat high B+ voltage as well. Was just holding out hope I could find a new one that had multiple secondary tap voltage options similar to the awesome Classic Tone one I put in my Plexi build. That one gives you three different B+ options.

At this time, I am just running it in half power mode with two 6L6's. And then my idea of the KT77's is good for the high screen voltage issue. But then even worse for this weak heater supply issue as KT77's draw quite a bit more current than 6L6.

 

[RickyLee]

Whadda Ya Know . .

Found this:

http://www.tubeampdoctor.com/en/shop_Mains_transformers_Mains_Transf_for_Mesa_Boogie/

I'm figuring the shipping will be a bit pricey. Could be an option. Still looking into some special or custom order possibilities, so I can have the B+ options I would like to have.

 

[mickeydg5]

All you need is robust transformers for 100 watt use.


Does your amplifier have the 100 ohm resistors in the heater circuit? If so did you check them and hopefully they are not cutting voltage down.


If it is just the transformer with the heaters that low along with the HT being high you should definitely look into another.

 

[RickyLee]

All you need is robust transformers for 100 watt use.


Does your amplifier have the 100 ohm resistors in the heater circuit? If so did you check them and hopefully they are not cutting voltage down.


If it is just the transformer with the heaters that low along with the HT being high you should definitely look into another.

When I got this amp, the original false center tap resistors on heater winding were Chicharones. Could not even tell what they originally were. So obviously the heater tap took a good hit there. I put new 100R in place. BUT, this PT is doing the exact same thing as that last Mitchell. Unknown variables. I am figuring better safe than sorry later. PT has no center tap for the heater winding otherwise.

Today I will measure the AC on this power transformer secondary HT (unless it is in my notes LOL) and then dig out my stash and check that old Dual Rec PT to see what it puts out.

If I wanted to rework the power supply to the non center tap full wave type, I could possibly fit that Metro 100W Plexi PT in. But I am saving that for my next project build.

ADD: Another question I just thought of. Should the bias supply circuit have more than one capacitor? This one just has the one reservoir cap. I added in a few parts/trim pot to make it adjustable. Wondering if I need another cap at the tail end of the supply?

I put a new reservoir cap of 47uF 100V value.

 

[Woods]

How about a Mesa dual rectifier take out? Should be cheap enough. You get that rectifier tap and what did they say? 470 plate volt?

http://www.thegearpage.net/board/in...il-mesa-boogie-rectifier-transformers.287141/