Master volume, power soaks, and tube wear

[mbenigni]

(Please go easy on me: I'm sure everything I'm about to type will reveal how ignorant I am about guitar amp design and maintenance.)

So... there was that whole kerfuffle where I bought a bunch of DSL100HRs and they all had dodgy, noisy power transformers (I think). I finally found a used DSL100H that was nice and quiet (mechanically), and I'm still having a blast with it. However, unlike the DSL100HR, the DSL100H doesn't have a master volume. This means if I'm running in 4cm, the only attenuation happening anywhere in my signal chain is the FX loop block in my effects processor. Similar concern if I'm just running a modeled amp direct into the DSL's FX return. Personally, I find having a (fallible) audio processor running direct into a 100W power section a little unnerving.

@stonge was kind enough to lend me a Bugera PS1 Power Soak, and it finally occurred to me that I could use that, essentially, like a master volume. That's my safety net if the modeled amp I'm feeding into the DSL's power amp decides to go "poof" for any reason. At first I was concerned that this would increase tube wear. Then I started to wonder how much of a difference it really makes. Without the power soak, I'm attenuating volume in the DSL's preamp. The power section is wired up the same way regardless, but it sees much lower input levels, resulting in lower output levels. Versus the power soak configuration, where the preamp and power amp output signals are quite high, and the power soak burns most of this off before it hits the speakers.

OK, so that's the setup. The question is: where and how would a master volume perform its attenuation, and would it be more like the power soak or the preamp volume controls in terms of power tube wear? Should I be thinking about adding a master volume to this (really inexpensive) amplifier, or is the power soak a good (essentially equivalent?) workaround?

 

[Frodebro]

Master volumes are always part of the preamp circuit and regulate the signal the power amp sees. Attenuators come after the power amp, so they regulate the volume no matter how hard the power section is being pushed.

Now here’s the part people seem to forget: Not all amps need the power section pushed hard to get great sounds. Pretty much every amp with a master volume (or channel volumes independent of gain controls) get the bulk of their tone from the preamp section. Pushing the power tubes hard is not necessarily going to have a huge impact. Amps like Boogies are designed to run the power amp very clean, even at volume levels that would sterilize a water buffalo.

 

[JiveTurkey]

I'd be more afraid of the Bugera thing dying than the preamp suddenly causing the tube head to run at full 100 watts. Is this something that has happened to you? I'd just continue using the QC/whatever modeler/preamp level control for attentuation.

 

[Devin]

I've ran amps always-on attenuators for well over a decade and never had a single tube fail FWIW

 

[Chocol8]

Temperature cycles and other things influence tube life, but running them hard is also going to be a factor. Not one I have ever worried about, but I am not running $1,000 a piece rare NOS KT66's or anything like that. I have lost more power tubes to a thrown beer bottle than to an attenuator or load box.

Personally I would not worry about either approach.

 

[mbenigni]

Good feedback - thanks, all.

@JiveTurkey: Yes, my HX Stomp abruptly melted down in the FX loop of my TA-30. That's "only" 40 watts, but they're Mesa watts. It felt like a black hole was forming in the center of my skull. I like seeing a big volume knob as far "downstream" as possible.

 

[laxu]

where and how would a master volume perform its attenuation,

Between the preamp and poweramp. There are two master volume types:

• The standard pre-phase inverter found on most master volume amps. Your DSL100H channel volumes are effectively this.
• The less common post-phase inverter, which comes in several circuit variants which don't concern the end user. Post phase inverter allows the phase inverter tube to distort, which will add extra gain to low gain preamps.

Your DSL100H does have pre-phase inverter channel master volumes, but not a global master volume. Ma If you want to add one, you can use e.g the JHS Little Amp Box in the fx loop. It's nothing but two jacks and a 100K pot in a box so you can build your own if you want.

and would it be more like the power soak or the preamp volume controls in terms of power tube wear? Should I be thinking about adding a master volume to this (really inexpensive) amplifier, or is the power soak a good (essentially equivalent?) workaround?

It would be exactly like the channel volume controls. The only difference is that it will control the volume of all channels.

If we look at the DSL100HR which has two switchable masters, it's basically gain -> channel master -> global master 1 or 2 for both channels.

For reliability, I'd be far more worried about the Bugera. It's cheap and purely resistive load so it will mess with your tone/feel. It's also under spec for your 100W amp. You'd ideally want above 100W power handling from the attenuator. It will work fine, but it can get quite hot which might lead to potential failure. I wouldn't crank your amp through it for hours on end at least.

 

[EOengineer]

Master volumes are always part of the preamp circuit and regulate the signal the power amp sees.

This is probably useless semantics, but the phase inverter is technically considered the power amp stage in traditional tube circuit design, so something like a post phase inverter master would probably be considered to live at the power amp stage. At least this is how I understand it as a layman.

To your point though, if someone said their amp had a master volume I’d assume it to be the more common pre phase inverter master we see on most Marshalls, Mesas, etc.

 

[JiveTurkey]

Good feedback - thanks, all.

@JiveTurkey: Yes, my HX Stomp abruptly melted down in the FX loop of my TA-30. That's "only" 30 watts, but they're Mesa watts. It felt like a black hole was forming in the center of my skull. I like seeing a big volume knob as far "downstream" as possible.

What caused the Stomp to di this? You could put an EQ pedal in place and use the gain slider to control volume if you're worried about the QC somehow having the same issue.

 

[mbenigni]

What caused the Stomp to di this? You could put an EQ pedal in place and use the gain slider to control volume if you're worried about the QC somehow having the same issue.

I never got any feedback from Line 6 as to what might have caused the damage. It was running on battery power at the time, but that had never caused any trouble before. That doesn't rule out the possibility of the battery having caused a problem... or the amp... or the HX Stomp itself. I wrote Line 6 hoping they might be able to speculate based on which specific parts had failed, but I never heard back. (So from that point on I had to mistrust D. All of the above. The Stomp is gone and the battery never gets used in its absence, but the TA-30 gets a lot of side-eye LOL.)

As for an EQ pedal, or any other attenuator between the QC and the power amp... Yes, that's probably more sensible. I have to imagine the power tubes would see less wear if I attenuate in front of them. Now that I think about it, stonge dropped off a GE9 the same day he brought that power soak.

 

[ChordInversion]

volume levels that would sterilize a water buffalo.

Can confirm. Don't ask.

 

[laxu]

Can confirm. Don't ask.

Sounds like you no longer have a problem with your water buffalo population growth.