Low Power DR ?

[olslowhand]

Hi - I'm looking for some feedback and "how-to's" regarding making or modifying my amp into a low power amp - like a PR (10-12 watts maybe even less !!!).

I have both a PR and a DR. For some of the blues gigs I do, a DR is too loud by the time it hits it's sweet spot. In fact, sometimes even the PR is too much. I am doing a 3 pc blues gig in some smaller rooms that only hold 40-80 people.

I would love to belive it's as simple as changing the transformer, but I'm sure it's not, right?

Any ideas?

Thanks - JM

[chipaudette]

So, the obvious answer is to buy a power attenuator (THD Hot Plate or something). I have one. It works pretty well...though it only works in discrete steps: -4dB, -8dB, -12dB, and then continuous variation from -16dB down. I think that it sounds pretty darned good on the -4,-8, and -12 dB steps. The only downside is that you're quieter ;)

If you don't want to spend the cash for the attenuator, the second option is to build a single-setting power attenuator into your amp using a couple of power resistors. You'd put one 8 ohm resistor in parallel with your speaker AND you'd put one 4 ohm resistor in series before or after the resistor/speaker combination.

The trouble is that you need resistors with high power ratings. If the amp is puting out 20W, 10W will go to the 4ohm resistor, 5 W will go to the 8ohm resistor, and 5W will go to the speaker. Since you'd like some factor of safety, I'd say you need a 20+W tolerance on the 4 ohm guy and a 10+W tolerance on the 8 ohm guy. You could use multiple resistors in parallel to get the overall power rating.

If you drive the amp hard, they're gonna get hot. You might consider putting them in a box outside your amp...just like a commerically-available power attenuator sits between your amp and your speaker.


The third option is to put in a master volume prior to the power amp. I know, it sounds like heresey, but have you actually tried it? Maybe the sound of pre-amp distortion is OK to you. The simplest master volume approach is to put in a pot-to-ground just prior to the phase inverter. That's the classic approach and sound good to a lot of people.

But, by cutting down the signal prior to the phase inverter, you end up missing out on the distortion in the phase inverter as well as the distortion in the power tubes. Some people, therefore, like to put their master volume AFTER the phase inverter. That's a little more tricky, so if that intrigues you, ask for more detail.

A fourth option is to drop the voltage level available to your power tubes. That involves putting zener diodes or power resistors in your B+ line. Again, you'd need high power ratings for the diodes or resistors. I don't know of anyone who does this, but it's an option.

The final option is to rebuild the power section of your amp. The deluxe reverb has two 6V6s in class AB. You could rebias it to make it run class A (though still push-pull) and swap out the output transformer to something that could handle all that quiescent current. Running class A will lower the power thereby making it quieter. If that's not quiet enough, you could go a step further and really re-wire the power section and make it run single-ended class A. That's a major re-working of the amp, though. It would also sound totally different.

So, I recommend buying a power attenuator. If that's too rich, I'd try a master volume. If that sounds crappy to you, I'd think about building my own attenuator. If that doesn't work either, I'd go out and buy a lower-power amp.

Good luck. I'm curious if the others here have any additional suggestions. Let us know what you try.

Chip

[Enzo]

Do you realize that cutting the output power in half only reduces the volume by 3 decibels? To put that in perspective the classic Fender two input jacks where one is louder than the other is a 6db difference between jacks. In fact, if your amp has the two jacks, plug into the softer one for a try.

A factor of 2 on loudness means a factor of 10 on power. 2 watts is half as loud as 20 watts.


The transformer doesn't make power, it only, well, transforms it. Using a smaller transformer won;t reduce your power, although if you exceed the transformer ratings you might end up with less sound. Power is determined by the voltages in the circuit and the signal gain. The transformer converts the high voltage low current of the power tubes to the low voltage high current of the speaker.

Ever think about turning the amp to face a wall or even sideways? Set a road case or something a foot in front of it to block the direct sound? Tilt it up at you mon itor style so it shoots over the heads of the audience?

[olslowhand]

Thanks for your replies and ideas. Maybe I should mention that I'm not really worried about cost. I own many vintage Fender amps (TR, SU, VR, DR, PR). I am lucky enough to be able to afford them, as well as use them playing out.

I have tried & heard different attenuators, but IMHO, they don't sound the same as an amp that is driven to it's sweet spot. To my ears, they made the amp sound compressed and or nasal-ly.

Neither do the master volume amps I've played thru sound right. (I have a 70's Music Man 120).

I do realize the watts vs volume rule. But I also know my PR breaks up at a lower volume than the DR. (BTW... I have Weber 25 watt speakers in both).

Pointing the amp up will get me in trouble with the lead vocalist... who happens to be my wife... can;t go there ;)

Anyway, I was thinking it would be cool to have a DR that had PR output, maybe even less.

Still open to more ideas...

[Enzo]

As to tilting, you could set it in front of you facing up, unless you both stand in the same place...

I recall a gig many years ago our guitarist threw a throw rug over his speaker cab covering the front.

There are a number of small low power amps now on the market, and you could certainly build a small micropower amp - something in the 1-5 watt range.

[pdf64]

Ask Ted Weber if he could recommend a speaker with a similar tone but lower sensitivity.
A lot of players use plexiglass screens in front of their cabs to lower the SPL, Joe Bonamassa gives details in a tonequest report interview I've seen.
Try a different recifier (ie 5Y3) to lower the B+ (need rebias, once rebiased would required rebias again if put the GZ34 back in, maybe lose 33% power.
Try cathode bias to drop maybe 25% power. Try a sag resistor in the B+ to drop another 25%. Both these could be switchable.
How about a Champ? Or build a Champ output section onto a DR pre amp for more flexibility?

Chip, I think changing a class AB output stage to class A would require the B+ to be lowered (ie replace PT) as well as uprate the OT primary current rating. Reason being that the tubes would burn up through being several times over their max plate dissipation if you try to bias them to conduct over 360 degress prior to onset of clipping. Or the PT would burn out the B+ winding due to excessive current demand. That's why a Champs B+ (360V) is lower than a Deluxe or Princeton (420V). Peter.

[chipaudette]

So why don't you use the princeton reverb? Is there something about the DR that is missing from the PR?

If you can put it in words, maybe we can suggest ways to mod the PR to make it sound more like the DR.

Chip

[tubeswell]

FWIW I think the PR 8k primary on the OT compared to 6k6ish for the DR, together with the fact that the PR has a cathodyne PI, has something to do with the overall differences in the way the two amps perform as well. In these two aspects, the PR driver and output stage has more in common with a tweed deluxe than a DR does (except that the PR and DR are fixed bias).

On what pdf64 said about a switchable fixed/cathode bias and using a 5Y3GT to lower the B+, you could also zener down the B+ with reverse-biased 5W zeners on the CT of the HT winding. Mount a string of one or more 9V-12V zener(s) on a tag strip (so the tags help to disburse some of the heat) and wire this up so that you can switch the zeners in and out. This will help lower the B+ when you switch to cathode bias mode (so that you can arrange things to get a low plate voltage and a hotter bias in cathode bias), which could potentially sound really sweet.

[MWJB]

"That's why a Champs B+ (360V) is lower than a Deluxe or Princeton (420V)." I've never seen a Fender built Champ that ran 360v, plate to ground. SF champs run 410-430v. Only then, with a 470ohm cathode resistor, are they aproaching ideal bias for Class A (halfway betwen cutoff & saturation).

[MWJB]

To get the DR to break up earlier the 5Y3 trick is a good one, also replace PI tube with 12AX7 (cuts headroom). Go up on the preamp dropping resistor (in the power supply) to get the plate voltages at V1/V2 down to 170-180vdc (tames highs, drops headroom, makes the amp perceived as quieter).

[walkman]

...cutting the output power in half only reduces the volume by 3 decibels? ..... Fender two input jacks where one is louder than the other is a 6db difference between jacks........ A factor of 2 on loudness means a factor of 10 on power. 2 watts is half as loud as 20 watts.

So for SF twin reverb 100w , on the low (2nd input) would that be equivalent to a 25w output ?? ( reduction of 6db ~ 1/4 power ?? ) and if speakers were swapped to speakers 3db less efficient that existing speakers the power would be like 12 watts output equivalent ??? ....... resulting that the amp would be about half as loud ?? Does that make sense or did i miss something??

Thanks

[MWJB]

"So for SF twin reverb 100w , on the low (2nd input) would that be equivalent to a 25w output ??" in theory yes, in reality no - because tone would drop out/go clean, dynamics would be lost...player would just turn the amp's volume up higher to compensate.

You get to a point with low efficiancy peakers where you lose fidelity, you might want to play quieter, but you still want people to hear the subtleties of what you are doing.

Human ears are easily confused when it comes to power ratings (but they're all we have to listen with), if you dial out high end/headroom (whilst retaining a degree of fidelity) the amp sounds less aggressive & drops back in the mix.

If I had a Twin that I wanted to make significantly quieter I'd try a pair of EH/JJ 6V6s (biased accordingly), wire the speakers for 16ohms total load, maybe try a 12AX7 in the PI, maybe drop preamp voltage to under 200vdc...would it then be percieved as half as loud? Probably not quite, you'd have a 2x12 Deluxe Reverb with big trannies - still man enough for the job at medium sized venues. At that point, if not quiet enough, I'd just get a more suitable amp.

[Enzo]

What he said.

In theory, yes, that would equate to 12 watts or whatever. When I wrote all that, my point was not that it was a practical means to get to the desired end, I was merely pointing out that if someone wanted to tone down a loud amp, cutting the power in half may sound like a great way, but really it doesn't make as much difference as one might think - only 3db in this case.

But yes, in terms of loudness, dropping in speakers with 3db less efficiency will drop the loudness the same as dropping power to half. That only discusses loudness, not tone or anything else.

And of course the example assume everything else remains unchanged. If you reduce power or speaker efficiency or circuit gain, and then turn the knobs up further, then the loudness won't reflect those changes as much.

[olslowhand]

Lots of good info here... I have to ponder and absorb all of this. I'll get back to you and let you know what I end up doing.

BTW... My PR was modded to have the adj bias as the DR, if that helps any. We also used a push-pull pot to activate the tremolo. And a mid tone control was added in the extension speaker hole.

Some asked the question why not use the PR instead od the DR. To my ears the tone in the DR is just nicer sounding.

It's funny how time (and age) changes one's requirements. 10 years ago I used my '64 Super Reverb exclusively, in spite of the complaints over my volume. Now the DR & PR are considered too loud... go figure... The only times I get to use the 'ol Super are outdoor gigs / festivals or large rooms (which are getting harder and harder to come by in the blues scene).

[jamez]

Hi,
I dont know how powerfull your amp is (Watts), but if its less than 50 then you might like to try this:

Tube Amplifier Attenuation (http://www.geocities.com/ge0ne0/)

I just posted this in another thread, but it possibly will help you out too. Course its a mod so you would need a soldering iron ! I did it to my JTM with amazing results.
Good luck:)

[thunderfoot]

Hi - I'm looking for some feedback and "how-to's" regarding making or modifying my amp into a low power amp - like a PR (10-12 watts maybe even less !!!).

I have both a PR and a DR. For some of the blues gigs I do, a DR is too loud by the time it hits it's sweet spot. In fact, sometimes even the PR is too much. I am doing a 3 pc blues gig in some smaller rooms that only hold 40-80 people.

I would love to belive it's as simple as changing the transformer, but I'm sure it's not, right?

Any ideas?
www.torresengineering.com,half power switch,12ay7, 12au7,5751 for V2 or PI will suck the loudness right outta that D.Half power-or pentode/triode-plus 5751 for V2,12ay7 for PI worked great on my '73 pro.I have an '80's Princeton Reverb II that is hard to play with because there's no bottom end at low volume,but the pro's trannies keep the sound nice and fat with a nasty,greeezy break-up around 3-4 with treble on 1,bass on 3,bright switch on.With the tube swap,ya never hafta open the chassis which has worked fine on what I've seen on stage.The substi-tube or even yellow jackets adapter for EL84's may even be a big help(preamp tube swap is still way cheaper)

[Chris / CMW amps]

Post PI MVs will help for sure, I do prefer them above (most) soaks.

(or changing the 1M resistor in front of the splitter to a 1M volume-pot-type-circuit in the PrincetonReverb, DR: just a dual 250k A pot io. the two biassplitters).

[Fretts]

Enzo knows a lot about this sort of thing, maybe he will chime in here...
O'Connor has a practice of increasing the screen resistance to influence the behavior of the power tubes...
I haven't tried these things myself so I can't attest to what they do to the character of the amp's final sound, but it looks easy enough.

[Aphid]

I'll just chip in and mention that I have THD hotplates on a Vox AC30, a Fender Twin and a Marshall JTM45.

Ultimately the attenuated never sounds as good as the raw, but then thats not really surprising when you think about it... when those amps are cranked flat out you don't just hear them, you FEEL them!!!

I've tried a number of amps with master volume and I've always been dissapointed on account that they don't even come close to what they should sound like. Having said that, it's less of an issue on the Fender or Vox where I'm looking for clean tones and not pushing the output tubes. Looking for crunch on any Marshall amp with a master volume though is an excercise in futility (unless the MV is at 10 of course).

In my humble opinion the hotplate attenuated sound is the best way to get pretty damn close to the cranked sound at a sensible volume. Versatility is also a big factor, you can safely play at 4am and even the dog will sleep through it... gig in a small venue - no problem, just knock 8db off... gig in a hall? Just unplug the attenuator altogether!

Ultimately if you absoultely must have that flat out sound then you are going to have to live with the volume. Probebly worth noting that when I crank the marshall I can clearly hear the speakers breaking up - you are only EVER going to get that sound when the speakers are flat out.

Anyway, thats my peace. At the end of the day I think we should all be aspiring to make that $20 battery powered transistor micro-amp sing like a bird! :)