TW Express clone with a Post PI Master Vol...

[geetarpicker]

I think the main thing you loose on a wreck with a master volume is the compression effects of the power supply. If you aren't really pushing your output stage you won't get nearly as much power supply sag when you push the amp. The audible difference is that when you play a cranked Express and run the amp into overdrive the B+ sags considerably, lowering your output level quite like a brick wall limiter would do in the studio limiting the volume. The sag will also contribute to a slightly sweetening/darkening caused by the lower B+. Then when you back off your guitar the B+ ramps up giving you more apparent headroom, a clean that is almost as loud as the prior more compressed dirty signal, and even a slight extra shimmer in the highs added back due to the B+ being back up under the lighter load. The net result is an almost even volume level between "clean and mean" as wreck folks tend to call it.

When you run a master you won't have as even a volume between the clean and dirty, and if anything your clean won't seem loud enough to keep up with the dirty in a band situation. Also the dirty tone may seem more harsh.

Other folks can chime in on if the sag effect of the power supply is still apparent with a VVR or not. I only have limited experience playing wrecks with VVRs, not enough to truly comment there. I would assume the power transformer doesn't sag as much, but perhaps the VVR can be designed to sag under load even with the PT cruising along with little load.

A quality attenuator is probably best for retaining the optimum dynamics of a wreck IMHO.

But still even with a master in a wreck can make for a usable amp, but will probably be a bit more of a one trick pony and probably not as controlable from the guitar.

[Jackie Treehorn]

Something to keep in mind is that the power tubes do not clip in the vast majority of guitar amps with ac coupled long tail pairs. It's a phase inverter distortion reached when the ac swing exceeds the bias voltage, 0 volts on the power tube grid, "diode clamping" of the signal, etc. So, it's a sort of signal limiting more so than compression. I actually don't think there's a single aspect of the express that fully explains the clean to mean capacity, but I think a huge portion of it is that you're pretty much always in this operating range.

I think there's a general misconception that this distortion is the most dynamic, and since it occurs at the amp's maximum output level, you've really got the full dynamic range. The problem is, that when you start using products/mods to bring the db level where the diode clamping occurs lower, you start to hear that this type of distortion really doesn't work well musically at low volumes. I've noticed that with a 30-50 watt amp, once you go down more than 10 db or so, you've lost a lot of musical usefulness of the amp. Maybe that's why Ken called it bedroom level. It's interesting that everyone tries to make a better, improved attenuator but very few try to make a more dynamic power amp. My point being is that at a certain point, master volumes/pedals, running a distorted signal into a power amp with some headroom works better.

I am hoping to do a master volume express in the near future. But, I don't think it's something that can be done well by just tacking a dual pot somewhere in the circuit. It's true that non master volume amps sound better, but it's not because the power tubes are adding anything aside from crossover distortion, bias shifts, etc., so the corollary is that the post phase inverter masters are causing something to sound bad. I think the spirit of the 3 common post pi masters is that they are a quick, cheap, reversible, mod to add to an existing amp, but perhaps not the perfect solution for a scratch build. I think the best approach would also not be one where turned up the circuit is stock.

[solderstain]

I've got a LarMar Type-II in my Express clone, and while I agree that no MV sounds 'great', I AM happy with this one in THIS build. (There are some amps that it absolutely DOESN'T 'work' in - like a black-face Fender style circuit...)

I use it to 'take the edge off' - meaning dial the volume back just a little bit so I don't have to wear ear plugs when I play.

I mostly play this amp through a 4x12, and have a set of custom-fit Sensaphonic ear plugs with 15dB filters. Sometimes I just don't wanna put them in. The Type-II MV can dial back just a little bit of the volume so it's tolerable, and yes, some of the immediacy goes with it, but most of the 'good stuff' is still there.

Now, if I try to dial it back to 'bedroom' levels, it really does suck, but... why would someone try to do that with an Express in the first place? If you really need to play at bedroom levels, a POD will do...

All attenuation devices are a compromise - physical attenuators, or MVs... I sometimes scratch my head at people who complain about it - what the heck are some people expecting???? Attenuation WILL and DOES suck some of 'the good stuff' out. Some more than others. "Pick your poison" is one of my mottoes...

[jjman]

Attenuators preserve power tube clipping but many claim they alter tone. I never tried one.

VVRs lower PS voltage and you have to choose what section(s) to regulate. I never tried one.

A Larmar consists of one dual pot and perhaps 2 safety resistors. It is invisible on a schematic when set at max. Turning it down does not change the "load" on the signal. It prevents power tube distortion but allows PI distortion and stage 3 distortion, if desired. I would rarely be able to use amp distortion on my build w/o the MV. When the sacrilege overcomes you, just turn it to max and you will be healed. And leave it maxed when you turn the amp off to keep the taboo out.

[billyz]

If you are only looking for a modest amount of attenuation I do think an Air Brake is as good as it gets. You can also use fewer and even inefficient speakers to control the volume some what. Perhaps a combination of all the above is really needed. VVR, Power attenuators, less efficient speakers and even a MV.
I found that using the VVR on the entire circuit worked best, but had it's limits at around 100v on the power tube plates and I limit mine on the low side to that amount.

[ampdoc1]

I'm going to mention this again, cause I'd like someone else to try it! The first TW I built (2004-5) was just crazy good,.. great tone, response to picking, etc. But it was SO loud!!!.

I was thinking about using a Variac, but the idea in my mind was to lower the DC voltage in the output section. This was prior to the VVR, and I didn't have the electronic chops to be able to design such a thing.

So, in my ignorance I wondered what would happen if I dropped the heater voltage to the output tubes. Viola! It worked very well, changing little of the tonality (either clean or driven) with output down to less than a watt. My design was kludgy (using a large variable 3 ohm resistor to drop the voltage), but the results were great. I think these days the same thing could be done using the VVR as the variable control.

As I examine it in my head it makes sense. All the circuits are running at normal voltages, so any distortion generated in the pre, the PI, and the output tubes, is exactly the same. It's just limiting the number of electrons being boiled off in the output tubes, IE less volume.

Come on somebody respond! Tell me why I'm full of shit and why this is such a "outside" idea.

a'doc1

[geetarpicker]

Some folks claim of "cathode stripping" on tubes when you variac down too far. Apparently underheating a tube can hurt it in time, and it would seem if you are going down to 3v you are way under voltage. I've always wondered if one variacs things down often perhaps a periodic run at full voltage would minimize or slow the long term effects? That said it's probably a fair sacrifice as long as you are using cheaper new production tubes and not expensive NOS stuff.

I run a '68 Superbass all the time run at about 10% under voltage with an inline bucking transformer. Seems the output tubes last a couple three years, but they might have not lasted much longer in any situation.

I'd say go for it if it works, and you are using fairly cheap tubes.

[dr.barlo]

I am a 1987 user... 50W!

So can bear high volumes. But quite often, I would have to fight with the sound guy... Even going from T75's (97dB) to V30's (100dB) can upset the balance! Thus, basically ppimv for me would be used whenever I would have go down like 3-4 dB. Not 30dB... See? I am no way asking about them master volumes to get me the TW Express tone at bedroom levels. I know from using a Dr.Z Airbrake and 1987 (I also have a 1987 clone RR mod and I recently learnt from my friend who built mine that my 1987 clone already has that LarMar master volume) this (sounding great with a 30dB attenuation) simply is not possible...

Sometimes in gigs and all (where the audience is not hairsplitting tone gurus ) I would have to go down to 15-18W's... So basically, I wanna get the 6V6 level of output using EL34s... See? The reason is simply because 35W's is simply too loud.

I guess I've got my answer. So apart from additional wires carrying the signal, the LarMar appears to be transparent. So I would not even need a toggle on/off thing that I was thinking about. In the ideal setting, I would be turning that mv full on... Yet, when needed for stage and all, it may become useful. I don't think that I should go the VVR route, because my (humble) experience with Marshall's kinda shows that the tone change associated with plate voltage changes is quite important in terms of the overall tone. Maybe I am making a mistake... I dunno...

Thanks for all the answers. I really appreciate it.

B

[M Fowler]

Are we sure the Express was running with the 300-0-300 taps or was it running the lower voltage 260-0-260 to get the plate voltage down when using the 6k6 primary OT?

Mark

[ampdoc1]

The cathode stripping theory has been de-bunked by several knowledgeable amp designers. Evidently that only applies to transmitting tubes. But, I think I'll try an experiment and lower the heater voltage on one of my amps and run it a few weeks at low power to see if it measures the same before and after. Never too old to learn a lesson.

a'doc1

[Diablo1]

There is another way to cut the volume down.

http://www.fluxtone-speakers.com/

[Reeltarded]

Getting the volume down is worth.. $200.

Yikes.

The funny thing is that getting the voulme up is worth thousands. hah

[vibratoking]

Those fluxtone speakers look like a nice idea, but I think there are too many roadblocks to success. Price being at the top of the list. Losing the ability to use the 'real' speaker of choice, instead of a clone, being next.

Ampdoc - I like your idea of reducing the heater supply, if it works. I'm too busy to try it now, but I'll be putting it on my list of thins to do. It sounds like it could be a very simple and elegant solution.

[John_P_WI]

Come on somebody respond! Tell me why I'm full of shit and why this is such a "outside" idea.
a'doc1

I for one will not tell you are full of it. I remember you mentioning this years ago and thought it had great merit.... albeit I have not tried it.

The only down fall that I could foresee would be in a performance situation. KOC's "DC" power scaling - the "newer" style - allows for performance switching and is almost instantaneous in application. I wonder IF one was to try to do this with a VVR control of the filament in a performance situation, would there be a heater lag induced volume swell and decay due to the heating and cooling time? Curious.

Then again, the OP is an Express volume discussion not a multi-channel switching amp which could possibly exhibit the aforementioned volume lag. Carry on.

[ampdoc1]

John P.

There is a lag , as with the VVR. But as with any MV, it's purpose is to set the limit of the full amp acoustic output. With my "idea", you set up your presets/gain levels as desired, then dial up/down the overall volume, using the heaters as control. Changes on the fly take a few seconds, but it's not a big deal.

Literally, that is all that changes. Looking back at the measurements from my original attempt, I ran the amp into a speaker then raised/lowered the level by a few db. Same EQ, same distorion,..just not as loud. When I got the amp down to less than 1V AC output, the sound began to change because the speaker just didn't have enough drive to excite the entire cone.

Since my first post on this thread, I ran an amp for 4 weeks continually with a signal input and the output heaters @ 2-3 VDC. Afterwards, the tubes showed no problems or loss of gain. I also experimented with the VVR, it works really well to regulate the 6VDC output heaters.

a'doc1