Tube stereo sounds -smaller- after being on

You will have to evaluate each of the many components of the system in order to figure this one out, but the one thing that could affect all is your AC supply.  Is your line voltage constant?  Otherwise, this is a DIY question.  Got a meter?  Check AC voltage out of that step down transformer as it heats up.

I myself have been Wed to Tube Amplification for nearly 30 Years.

I don't delve into the Circuitry as all Items in use are Bespoke Built, so I leave this to the EE's whom I have trusted. 

I have done and do Tube Roll, I am supported with my Valve Purchasing and have a Testing Service at Hand, all my chosen Valves as a result of Tube Rolling are Matched and Optimal in the Test Measurements, some pairs are even above Manufacturers Spec'.

Certain Tubes are now discovered for their ideal placement in the circuitry and there are a few tubes, I would very much like to play with as a Journey of discovery but have not, as Tube Rolling will be quite some expense.

During Tube Rolling, I have had in the Line Up, Vintage, NOS and Modern Versions.

My finding with Certain Tubes is that there has been a initial impact that has been quite attractive and certainly a comparative attraction to another Tube that made a good impression.

I have used Tubes that have initially impressed, and have improved as the Tube has a extension of use. This good impression has been followed by a need to remove the Tube with immediacy, there has been a change to the sonic after  a certain period of being heated that has caused a detrimental effect to the SQ. In a few cases to the point, it was a very concerning sound being produced, filled with unwanted distortion.

I can't help narrow down your own Trouble Shooting requirements, as there is not too much info' on offer to explain how the approach to fault finding has been undertaken.

My experiences has led me to Swapping Tubes across Channels, it is one of a few  commonly seen methods.

In your case, with Two Systems, swapping Tubes across systems might also be an option to see if the unwanted condition manifests with Tubes that are satisfying in another device. 

On the plus side you are fortunate not to be dependent on one system only, music can be replayed at your leisure and time is available to tinker around with the Second System. 

A couple of suggestions.  I'd first swap out the tubes and see if that made a difference.  Tubes heat up and generally the changes are good but not always.  Not saying this is it but an obvious thing to try.  

On your power converter and electrical system feel it and see if anything is getting warmer than you would expect.  As copper heats up its resistance goes up putting it into a positive feedback loop.  So it could be something in your power circuit is undersized.   

I have a 240V tube amp made in Europe.  I run it on 240 from my home panel.  You could consider installing a 240V outlet for it and eliminating the power converter.  I did experiment with my amp and ran it successfully from a transformer style power converter.  It was hard to find one that didn't hum.  I also oversized by a factor of 5.  If your amp is 200W, buy at least a 1000W rated transformer. 

Jerry

First you have to figure out which piece of equipment is causing the problem. Fortunately you have two systems. You take each piece of equipment in the bad system, one at a time, over to the good system. When the problem tracks to the good system you have narrowed down to one piece. Then you can change tubes etc. and ID the exact problem. I would start with the amp.

The only time I've experienced what you describe is when I was using my vintage Pilot 232 (1959/1960 PP EL84 power amp) 15+ years ago.

After 3-4 hours of play the soundstage would gradually shrink/compress and the power transformer would become extremely hot by then.

The amp was all original (lytic caps reformed to spec) with only the ceramic resistor feeding the 5ar4 rectifier being replaced with a like unit.

When I powered it off a variac set to 110 volts the problem did not reoccur and the PT did not become overly hot.

Didn't check the A/C line voltage @ the time, but assume that it must have been fairly high.

DeKay

Thanks everyone for your thoughts. I only thought about the line voltage being an issue when I wrote the above post. The amp came with a Chinese made voltage regulator that doesn’t seem to be adjustable and shows 217 volt output on the led screen. I always thought that was pretty weird. But since it came with the amp from China I didn’t question it. As for other possible issues, the mixer, being Australian made, could also be in some voltage netherworld. It comes with a separate power supply set up for the US which gets very hot. I can take that out of the loop easy enough and see if that’s the issue. Plus I probably should just buy a different regulator for the amp. 

@yaluaka Sorry when you say Mixer what component are you referring to? A mixer as in pro sound? And you're using it as a preamp?

The mixer is a great sounding one hand built in Australia called a Condesa Lucia.

If you are "adding" EQ with your preamp/mixer it's possible that doing so is overtaxing the 3 watt amplifier and/or the step-up TF you are using.

What speakers are you using, or are you using headphones (I know ZIP about headphones)?

Just throwing this out there for thought/discussion.

Interesting LM amp which I was not aware of (like the tube compliment and the soft start feature).

DeKay

@yaluaka , potentiometers are the death of high fidelity. Get rid of that awful device. It does not belong in a high fidelity system. I am not even sure what kind of system it belongs in!

It’s obviously not the mixer of the sound is initially good with the mixer in the line up. I was eying that mixer a few years back - it’s top notch among rotary mixers.

I would set up a few experiments with the equipment running on 220 and the overheating transformers - then if the problem persists I’d look into tube swapping. If the mixer has a tape loop you may be able to connect the loop out to another input. After a few hours switch to the loop input to see if the problem persists. 

@mijostyn 

+1

There is a reason high end equipment eschews extra functions adding extra potentiometers. 

@yaluaka I don't see any reference to your OP or follow up that suggests you are seeking route to HiFi Nirvana.

I only see a request for ideas on a solution to resolve a issue with a sonic that develops that is unattractive and wanted to be overcome.

I see no reason for you to get rid of any of your equipment unless you feel the need is necessary. 

Your Systems, Your Home, Your 20 Years spent building a Tube Based Set Up.

I'm sure there is plenty close at hand to meet your needs perfectly, it does not have to be anybody else's ideas that are needed to be followed.  ,

Ok a couple of posts to unpack here. Pindec not to open a can of worms, but I’m more in the musical nirvana side of things as opposed to high fi nirvana. The system has a fault somewhere, so you’re right the equipment doesn’t need to be changed, I don’t think, as it does sound good sometimes, just the fault found. 
 

As for Mijostyn, this is exactly the style of thinking I run away from. This didactic belief that something like tone controls or no tone controls has any real meaning at all. So many times people post their vague notions instead of actual knowledge. I don’t really think that helps anyone. We all have bias’ but there’s no need to think our bias’ is a cogent fact. -  You know every record you listen to went through multiple sets of tone controls. 

@yaluaka I am fortunate like yourself to have a real love for the music.

I got to a place with Source (30 years ), Power Amplification (20 years) and Speakers (20+years) that I have been loyal to for the years bracketed.

These are the foundation to delivering to me a musical encounter just how I like it.

The Mechanical and Electronics of the Vinyl Source has evolved over this period but the Source Material is collected since the early 80's.

I love to see creativity around HiFI especially the Vinyl Replay and get caught up in a variety of not too commonly undertaken experiences. This could easily be interpreted as I am a 'equipmentphile', but no, this is just an extension of my enjoyment of musical replays and is also a social activity as it takes getting out and meeting with others and in some cases witness the end product of an individuals skill set.

I was sure and am now quite sure you are a individual, who is without snobbery towards how a musical encounter is to be created. I would have wager on, there being the odd component in a Device you are using, that is known for its sonic character, and the idea of parting from it will be a real bitter pill to swallow.

For me the component is Valves, but I am not a under the hood delver, so other parts are out of bounds unless an EE is at hand as a guide.

@yaluaka , I'm all for tone controls, digital EQ to be specific. That piece of equipment does not belong in a home HiFi system. You are obviously enamored by old stuff and perhaps old commercial stuff. Technology moves on. 

I am not the only one who thinks this way. I am just more vocal about it. I am all about the most accurate sound. I am afraid the equipment you have is not going to get you there or even remotely close. You are entitled to be all about whatever it is you are looking for so don't listen to me. I am the resident  PITA.

Hey Mijostyn, everybody has their mindsets. Some people think the world is flat or the US never landed on the moon. I’ve been making records for the last 40 years, before cd and digital. I was around when cd’s were going to solve everything, and digital recordings were going to be more ‘accurate’ than analog. I’ve worked on all sorts of pieces of equipment. I’m not saying this because I think I’m an expert, far from it. I have so much more to learn. I’m just saying this as a background for the concept, that in a recording studio and mastering lab, analog eq’s are held in very high regard. And a lot of vintage equipment has great monetary value and is even replicated (to some extent) as plug ins because the qualities of that gear is treasured. So you know what you know, but I don’t think you should discount the idea, because you feel you know something, that it’s true. Not to sound like Donald Rumsfeld here. 😂

Have you tried testing and/or replacing the rectifier tube with one tested to new spec?

The 2 hour time period/demise in sound is what should be telling you/us something, but what is that?

DeKay

Hi DeKay, I've been in Mississippi last week, so have done nothing. Someone suggested the rectifier tube which is a good suggestion. I don't think I have a spare. My EMT turntable however, needs a power converter so I can take that out of the system and use that converter which is a high quality one and switch out the Chinese made one that came with my amp. I will do that and report back when I have the time. Thanks again everyone.