[Marsilio]

First time poster here. I recently purchased an entry-level OTL tube amp (an Xduoo TA-26s) along with some upgraded tubes, and I'm looking forward to giving them a try. I'm having some trouble, however, finding clear answers to some basic questions regarding "best practices" when using an OTL tube amp -- specifically, regarding the proper methods for powering up the amp and burning in the tubes. I realize these are frequently discussed topics, but there is a lot of muddled and contradictory info out there. I've done my best to wade through it all, but I'd appreciate any advice anyone might have in connection with the following questions:

(1) When powering on an OTL tube amp, is it best to turn on the amp first and then plug in the headphones, or vice versa? My understanding from what I've read is that with OTL tube amps it's best to turn on the amp first and then to plug in the headphones due to concerns about DC offset when the amp powers on. I've also heard that with transformer-coupled tube amps, the opposite is true, i.e., that it's best to plug in headphones before turning on the amp, because transformer-coupled tube amps (unlike OTL amps) need a load from the headphones on their circuit before they are turned on. Do I have all this right?

(2) What is the best way to burn in tubes? I've seen some people who seem to know what they're talking about say that it's best just to begin listening to music using new tubes and to burn them in naturally as you listen, but there also seem to be a lot of equally knowledgable people who think you should intentionally set aside time (5+ hours, sometimes considerably more) to burn in the tubes before starting to listen to music through them. Complicating matters further, I've seen some posts to the effect that it may be a bad idea to have a tube amp on continuously for more than 5 hours or so. Also, for those who set aside time to burn in tubes, do you have headphones plugged into your amp during this process, or do you just leave on the amp without headphones plugged in? (Maybe the answer to this question depends on the type of amp used, as above?)

(3) One last, unrelated question: is it okay to plug an OTL tube amp into a surge protector? I've previously only used a JDS Labs Atom+ solid-state amp, and the guys at JDS Labs said it wouldn't be a problem to use a surge protector with their amps. I'm curious if the same logic applies to tube amps, or whether there might be additional concerns.

I'd be grateful for any thoughts on some or all of these issues. I realize I've asked a lot of questions here, so feel free to address the part that interests you / that you know the most about.

 

[msing539]

I can only share what I do, and it only addresses some of your questions.

Before listening, I let my Kenzie warm up for 20 minutes before plugging any headphones in. And I unplug the headphones before turning it off, which I do every night.

For burning in tubes, I listen to music while doing it but at moderate volume.

And last, I never touch the glass bare handed. If handling, I'll either use gloves or a microfiber.

 

[AnamCeoil]

I have also been looking for similar answers, particularly to question 1. I came across this response from the owner of Woo Audio in another, very old post:

1) What's the proper way to turn on WA6 with headphones? J: It is recommended to switch on the amp with a headphone on. This would avoid the amp being open circuit - not a good thing for any tube amps. You will not harm the amp at all if you leave an open circuit for a short period of time....A proper way to use the amp is 1) plug in your headphone, 2) mute the volume or stop your source 3) switch on the amp and let it warm up for at least 10 seconds, and that's all.

The WA6 is a SET, rather than OTL so he also said the following: "Just to clarify, the problem of open circuit in an OTL tube amp is not as serious as output transformer (OT) coupled tube amp. In an OTL amp as the WA2, there is a pseudo load on the headphone output, which means you can switch on the amp without the headphone plugged in"

I personally leave mine (also a TA-26) plugged in when switching on, but I only have it a few weeks so looking forward to hearing from more experienced hands as well. Cheers.

 

[carlman14]

For 1, I personally turn the amp on and let it warm up before plugging in headphones for both OTL and OTC amps. You're correct in saying that an OTC amp should always have a load. Any competently-designed OTC amp will have a dummy load present in the case that a headphone or speakers are not plugged in. This is done either by having a relatively large parallel resistor permanently across the output transformer secondary, or having a "switched" headphone jack that automatically switches in a dummy load once headphones are unplugged. The 2nd option is how I design my amps. So I don't worry about turning them on without a load, because the protective/dummy load is always there. I do this in OTL amps as well even though there are no output transformers to protect. IMO if an OTC amp doesn't have this protection, it's a massive oversight that costs 20 cents to fix to protect output transformers that could cost hundreds if not thousands of dollars.

That being said, I turn the amp on first for a reason not mentioned here yet. In my experience, if a tube is going to arc over, it usually does so in the first couple minutes of being turned on. I've had a few tubes arc on me in the past. One time it blew out a driver in a pricey pair of headphones. So for me, considering there's no real danger leaving headphones unplugged in either OTL or OTC amps, I'd rather take that extra bit of precaution to save my expensive headphones in case the tubes try any funny business while warming up.

For 3, I don't see what the downsides would be. If there's a surge, I'd rather fry my surge protector than my gear.

 

[AnamCeoil]

For 1, I personally turn the amp on and let it warm up before plugging in headphones for both OTL and OTC amps. You're correct in saying that an OTC amp should always have a load. Any competently-designed OTC amp will have a dummy load present in the case that a headphone or speakers are not plugged in. This is done either by having a relatively large parallel resistor permanently across the output transformer secondary, or having a "switched" headphone jack that automatically switches in a dummy load once headphones are unplugged. The 2nd option is how I design my amps. So I don't worry about turning them on without a load, because the protective/dummy load is always there. I do this in OTL amps as well even though there are no output transformers to protect. IMO if an OTC amp doesn't have this protection, it's a massive oversight that costs 20 cents to fix to protect output transformers that could cost hundreds if not thousands of dollars.

That being said, I turn the amp on first for a reason not mentioned here yet. In my experience, if a tube is going to arc over, it usually does so in the first couple minutes of being turned on. I've had a few tubes arc on me in the past. One time it blew out a driver in a pricey pair of headphones. So for me, considering there's no real danger leaving headphones unplugged in either OTL or OTC amps, I'd rather take that extra bit of precaution to save my expensive headphones in case the tubes try any funny business while warming up.

For 3, I don't see what the downsides would be. If there's a surge, I'd rather fry my surge protector than my gear.

Not OP, but thanks a million for your insight regarding timing and arcing. Much appreciated!

 

[Marsilio]

Yeah, this is all really helpful. Thanks to all of you who have replied!

I also came across that thread with the info from Jack from Woo Audio and agree that it sheds some light on things. I also liked this thread and this thread from the Bottlehead Crack forum, in which Dan Schmalle offers thoughts on OTL amps that seem to accord with what carlman14 has said here. It sounds to me like there are good reasons to plug in headphones after powering up an OTL amp, rather than before, and that there's a somewhat more complex picture regarding transformer-coupled amps, since they need to have a load and may or may not be designed in a way that allows them to function properly without headphones plugged in.

Any thoughts on how this applies to burning in tubes, i.e., would you burn in tubes with your headphones plugged in or just with the amp on but no headphones plugged in? In general, do you think of burn-in as something that you do before you start using the tubes to listen to music by leaving the amp on for a while, or as something that you let happen organically in the first few weeks of listening to music?

 

[carlman14]

Any thoughts on how this applies to burning in tubes, i.e., would you burn in tubes with your headphones plugged in or just with the amp on but no headphones plugged in? In general, do you think of burn-in as something that you do before you start using the tubes to listen to music by leaving the amp on for a while, or as something that you let happen organically in the first few weeks of listening to music?

Honestly, I wouldn't overthink it. Tubes don't have an infinite lifespan. I'm not sure I even believe in tube burn in, but if I did, I wouldn't spend some of the tube life worrying about if it is burned in or not. I just plug them in and enjoy. And if you're someone who notices differences with tube burn in, they'll get burned in eventually anyway. Might as well listen to music while it's happening.

 

[kumar402]

I plug in the headphone first and keep the volume to zero and don’t play any music when I switch on the amp. Mine is SE OTL amp and the chances of shorting is higher while plugging in the headphone in live amp.

 

[Marsilio]

I plug in the headphone first and keep the volume to zero and don’t play any music when I switch on the amp. Mine is SE OTL amp and the chances of shorting is higher while plugging in the headphone in live amp.

Thanks for this! I'm interested in what you say in part because it sounds like you take the opposite approach from that described by some other commenters here (i.e., plugging headphones in first then powering on your OTL tube amp, as opposed to powering on the amp and then plugging in your headphones).

How do you weigh the risk of shorting that may come with plugging headphones into a live amp against the risks others have described that may be linked to powering on an amp that already has headphones plugged in (e.g., exposing headphones to DC offset or to the effects of an arcing tube)?

Does the fact that your OTL amp is single-ended play a role in your thinking? (I'm not familiar with this subset of OTL amps.)