Power Transformer Selection - DSL401

[diddymix]

Hello Folks

I was hoping I may be able to get some advice and help with regard to selecting a power transformer for an old amp of mine I've fully upgraded.. it sounds really nice now but since Ive invested in the upgrades and time, I feel I may as well go the whole hog and replace the power transformer.. the amps 15 years old now, and the thing does have low level hum and the quality is very questionable on the stock one.
I don;t want to spend the money on a Mercury replacement as theyre over $200 before shipping to UK here.. way too much. However there are a few Classictone models here I spotted in their 'project transformer' page... I was hoping one of these could just drop in. Ive included the schematics of the 2 models + my Marshall on this post.
My question is I guess really, can this be done? My circuit knowledge is limited and with transformers and all the taps and current/voltage etc... My concerns are really if these models I've found can supply the bias for my amp... On my schematic I can see the VBias circuit as having +/- 15V... On these transformers, one of them says it has a bias supply of 50V.. I don't know whether this is too much?? Or would my amp circuit just use that available AC safely and still produce the correct -15V bias?
My other question is if one can use the higher voltage output on the B+, so that my EL84 plates have a higher plate voltage... could this be done without mods and just drop in?

Thanks so much for your time on reading, and please any pointers most appreciated!

Dave

[martin manning]

The PT's in the data sheets you linked will not work in your amp. The winding that supports the bipolar supply for the op amps and the negative bias supply for the EL84's needs to be around 12VAC. You could measure that to find out exactly what it is.

Have you replaced all of the electrolytics in the power supply? If not, I would start there. They are a common source of hum as they age, and at 15 years they would definitely be past their prime.

[diddymix]

The PT's in the data sheets you linked will not work in your amp. The winding that supports the bipolar supply for the op amps and the negative bias supply for the EL84's needs to be around 12VAC. You could measure that to find out exactly what it is.

Have you replaced all of the electrolytics in the power supply? If not, I would start there. They are a common source of hum as they age, and at 15 years they would definitely be past their prime.

Thanks Martin! Ok well that does seem to make sense, I uploaded the datasheet here for the mercury DSL replacement and another model Ive just found by a european company., mercury states 27V, and TT states 24V for their model... Do you think this is the 12V that you mention x 2 either side (+/-)??
To be honest I could take a punt with the Tube Town model its quite a bit cheaper.. and it is a drop in replacement... I just wanted a classictone model because Ive been so impressed with the output transformer from them I used in this amp.. it really opened everything up.
Do you think that the models I mentioned simply wont work and would even possibly damage my bias section..? I may measure that secondary on the transformer tomorow and see what I find..

I did replace all the caps, hum is still there but its only noticable with the amp dimed which is very loud. Although you can hear it acoustically coming from the chassis... Ive a feeling it may be mismatched NOS power tubes.. of which I bought two seperate matched pairs.. do you think this could cause a hum on the output?.

Thanks again MArtin and any pointers you got always appreciated

[martin manning]

Yes miss-matched tubes can cause hum in the output, but the volume setting shouldn't affect that. How miss-matched are they? Can you swap them around and equalize the current on each side of the OT?

Another thing to check: is the heater voltage centered around ground? You could experiment with replacing R34 and R35 with a pot and see if you can dial the hum down.

I'm surprised by the bias/op amp supply voltage on those replacement PT's. I would think that R23 and R24 are running pretty hot.

[diddymix]

Ah right! Very interesting.. a problem I came across when removing the post PI master volume, and moving it pre PI.... was that the hum became quite loud... and yes independent of master volume, just one static volume buzz. So I did what Sluckey said and isolated the problem by not adding a master volume, and just removing the Post PI master... I of course added the fixed bias supply via 2 x 220k resistors. Well what I found is the buzz thats there, is the same buzz with the master volume dimed (i mentioned in the last post), only much louder.
I think this must be the NOS tubes, but thanks Im going to check the heater thing too! This may be one for a technician Im a little lost with it, and really want to have that pre PI master, the whole thing sounds and feels much better with it before PI!

[diddymix]

Hey Martin,

I bought the TT transformer in the end, and am just trying to install it... I was hoping I could get your advice on this.. Basically on the datasheet attached.. the secondaries are in 3 pairs and each pair is the same colour so for example -

red - 300V B+
red - 0V

... Just finding this confusing... err how do I figure out which one is the 300V out of the two?! Do I have to measure the two cables AC while plugged into the PBC??

Thanks so much

[sluckey]

There is 300v BETWEEN the two red wires. Doesn't matter which is called what.

[diddymix]

There is 300v BETWEEN the two red wires. Doesn't matter which is called what.

Oh shit! Sluckey thanks man, do you mean whichever I hookup to my B+ and the other to the ground node... all will be well? Would this apply to the 230V blue and the 0V brown in the primaries too?? I could just swop them? I'd kinda like to use the brown wire for my live feed from the IEC socket... since brown is usually live!

[sluckey]

About the red wires... Neither wire connects to B+ and neither wire connects to ground. The red wires connect to a FWB. Be sure you understand this. Helps to look at the schematic.

The blue and brown wires can be swapped. Makes no difference which which. Just be sure to connect the gray and black wires together.

[pompeiisneaks]

There is 300v BETWEEN the two red wires. Doesn't matter which is called what.

Oh shit! Sluckey thanks man, do you mean whichever I hookup to my B+ and the other to the ground node... all will be well? Would this apply to the 230V blue and the 0V brown in the primaries too?? I could just swop them? I'd kinda like to use the brown wire for my live feed from the IEC socket... since brown is usually live!

The way ac works is that the voltage mentioned alternates in a sine wave between the two leads. So as he said, it doesn't matter which you use. They both will go between 0 and 150V alternating over time at the frequency of the mains on each lead. (frequency of 60Hz in the US and Canada for sure and 50Hz in europe and many other places). therefore between the two it's 300 volts ac between them.

~Phil

[diddymix]

Guys!
Thanks so much. So its equal between the two, great. So.. just curious... why does the datasheet label one wire xxV and the other 0V? Is it a 'theoretical' thing in terms of the application being DC?

[martin manning]

No, it's just convention. Windings with no center tap are xxx-0, center tapped windings are labeled xxx-0-xxx, and it's understood that these are AC volts.