Bad idea?

[keving]

Or merely dumb?

Have an 5 watt Epi VJ combo. Been reading various posts, the old GN site, etc. about power tube vs pre amp distortion, attenuators, etc, in search of driving the PTs a little more without the added volume.

I read the JA's original amp "Myths" and he mentioned a remote, mic'd sound-proof chamber alternative (not a closed box, I assume heat would be the problem?) Not workable for me, but it hatched an idea. What about a cover/baffle over the front of the speaker cabinet with a small port? I realize this will accentuate some frequencies and reduce others (ala "tuned" ports in stereo speaker designs, but those of course channel air movement from the back of the speaker). I was thinking rectangular port, either across the diameter or radius of the speaker cone, reducing some air movement but not completely restricting movement of the speaker cone.

In the end it may well yield less than satisfactory(tone and/or volume) results, but my big concern is not damaging the amp or speaker while playing around. Any other opinions, physics lessons, etc are also appreciated.

[ChrisK]

The purpose of the tuned port on a bass reflex speaker enclosure is to add a tuned "filter" at a frequency lower that the natural cutoff frequency of the speaker. It also enables the peaking of the bass response in the frequency range of great pleasure to those addicted to rock music.

This comes at a cost however. The speaker becomes acoustically unloaded below this enhanced frequency and can be easily damaged from excessive cone excursion. This is why many bass speaker enclosures are sealed (acoustic suspension) systems.

While one could reduce the output level this way, it will have effect on the tone of the speaker. It will also change the impedance seen by the power tube.


Why do you want to drive the power tube harder? Since the Epi VJr is a single-ended class A amp output stage, the plate current is essentially relatively constant regardless of output power (the very definition of class A). A single-ended power tube has the same circuit structure as a single-ended pre-amp tube (asymmetrical clipping/compression). As a result, aside from the power level, the clipping/compression is similar. It's just very loudly crunchy.

You may get what you want by increasing the drive to the pre-amp stage. Put a booster between your guitar and the amp and see if this is the case.

In a push-pull class AB power output structure, plate heating from current flow IS most dependent on the output power level and is a different effect than pre-amp clipping/compression.

Also, tube rectifier "sag" is only of interest in a class AB output stage. There's no "sag" in a class A output stage since the plate current is constant.

[andy]

I think you are talking about an isolation box, which is usually just used for speakers- putting your amp in one is possible, but heat would be an issue. These boxes can vary in size, from enough to hold a 4x12, to just a single speaker mounted in the box itself. It will usually be made of ply (sealed cabinet), lined with foam, and have a mic permanently fixed inside, so all you need to do is plug in a mic cable and the amps speaker out, and your good to go. I had a quick search for pages I remember seeing on them, but mostly just came up with forum stuff- however, there are (or at least were) pages where you can look at design diagrams, and even buy them fully completed, with Celestions installed, if I remember rightly, though I forget the company who makes them. A bit of web-hunting should bring something up though.

As Chris K says, the idea behind a class A design is that it is effectively at full whack all the time anyway- if you want more drive, then boosting the front end will help. Greater volume will only reap the benefits of a speaker which is shaking around a bit more, which is usually a good thing for guitar tone. The air inside the box will restrict the movement of the speaker a little, but that is what is happening in many classic speaker cab designs anyway- most 4x12's and some combos are closed backed, making the speakers work against the sealed pocket of air behind them, and the results seem to have been quite popular, on the whole! So don't let the sealed nature of the box put you off- the one question remains as to whether you have an external speaker out on the Valve Jr.?

[ChrisK]

www.randallamplifiers.com/products/cabinets/isolation/index.asp