new lite with 1/4 power switch

PepeJara · Post by **PepeJara** » Mon 02/09/09 7:45 pm

Ok, it´s done and working, I have separate the two right tubes from the left tubes, installed two resistor (double the value) and halved the caps, it´s working fine...I think in reduction mode there is a little less treble, could it be caused by the cap values?

phsyconoodler · Post by **phsyconoodler** » Thu 02/12/09 10:55 pm

No,it's because the mod injects a little negative feedback and that's why it rolls off the highs slightly.

ontariomaximus · Post by **ontariomaximus** » Thu 05/07/09 8:55 am

I finally got around to building an amp with this 1/4 power feature. Here are some measurements:
EL34 PP, cathode bias, 455VDC B+, 470 ohm cathode resistors, 47uf bypass caps switched in and out.
The output voltage was exactly 1/4 with the caps not grounded (same input level).
However that is only part of the story.
Power output (visual rounding of the waveform):
27 watts with the caps ungrounded
36 watts with the caps grounded.

However above that the ungrounded cap setup had an overall nice rounding of the waveforms above that level
The 'normal' grounded setup had increasing crossover distortion

Max power was 45w grounded and 39w ungrounded. As the output stage has far less gain, the difference in power output could be input limitations.

Post by **zaphod_phil** » Thu 05/07/09 10:00 am

ontariomaximus wrote:The 'normal' grounded setup had increasing crossover distortion.

However, crossover distortion adds some nice harmonic richness to the tone of a guitar amp, and is considered by some to be a major factor in the signature Marshall 18W tone. So it could be questioned how useful this trick is with a regular 18W type of amp. With your EL34 amp it could well be a different story....

ontariomaximus · Post by **ontariomaximus** » Thu 05/07/09 11:48 am

I have to spend some time listening to it in both modes. Judging from how it looks, it's 2 very different amplifiers, which could be useful. What should be gleaned though is this:
It's not a power reducer.
The wattage output drops to 1/16th with the same input level (that's squaring 1/4 voltage)
Increasing the input would result in the same max power output, but your LTPI may be clipping well before the output stage.

klipp · Post by **klipp** » Sat 10/09/10 12:21 pm

Hi,

I just installed this in my Minimalist/LiteIIb.
I expected something different. Maybe I understood something wrong...
I installed the 2k linear pot version without the switch.

I can turn down the volume but the amp just gets cleaner, not less loud. It's almost the same as turning down the volume-knob.
I thought that this mod would allow me to have power amp distortion at lower volumes.

Is this as it should be?

Thanks,
Karsten

phsyconoodler · Post by **phsyconoodler** » Tue 10/12/10 10:43 am

I have no idea what you are talking about.The 20k linear pot is not part of this mod at all.
The low power switch simply removes the cathode bypass caps from any ground reference.What mod are you talking about?

Post by **colossal** » Tue 10/12/10 10:58 am

phsyconoodler said to Klipp: I have no idea what you are talking about.The 20k linear pot is not part of this mod at all.
The low power switch simply removes the cathode bypass caps from any ground reference.What mod are you talking about?

psychonoodler originally wrote on Page 1: Run two cathode resistors instead of one shared one.You have to double the size to about 250 ohms a piece.Then use two cathode bypass caps about HALF the size you are using now.Connect the positive ends ONLY.Then connect the negative ends of the bypass caps to an SPST switch.One for each contact.DO NOT GROUND THE BYPASS CAPS!!!
Then simply use the switch.Full power is connected and 1/4 power is open.
This works 100% and there are no side effects at all.If you want it to be variable,simply add a 2k linear pot inbetween the negative sides of the cathode bypass caps. NO GROUND.

I don't need the pot so I just used the SPST switch.
The boys over at Hoffman amps forum developed this one and it's flawless.

Hi 'noodler,

I too am very interested in this mod and I tried emailing you some months ago but maybe it died in your spam filter. Not wanting to speak for Klipp, but I am guessing he is talking about the mod as quoted from your original post.

I did a lot of digging on EL34World about this but still haven't tried it and was sort of unconvinced that it might be the way to go (but still revisit the notion from time to time).

I think the most important and outstanding question I (and perhaps Klipp too) have is the ability of the mod to maintain the distortion quality at low volume. To this end, you originally said you double the amp's cathode resistor value and add one to EACH tube. Then cathode bypass caps are placed on the resistors but terminate on either side of a 2k Linear pot that does NOT connect to ground. So this creates a virtual ground of sorts.

When we use a cathode resistor only and no bypass, there will of course be lower output and local negative/degenerative feedback which will clean up the output stage. I read all the discussion on EL34 about Carr's use of a similar mod but it seemed that Geezer's, tubenit's, and your versions might be variations on this theme.

So for me, if the variable mod as outlined above using a 2kB pot between bypass caps with individually biased output tubes (at 250R each instead of a single 125R) has volume reduction but it still sounds like a cranked 18, then to me that would be a lot less hassle than adding a VVR system (which is pretty easy). That's why I'm trying to understand its true nature.

Glad you checked in!

phsyconoodler · Post by **phsyconoodler** » Tue 10/12/10 11:14 am

Ok,forgot about the 2K pot.It definitely doesn't make the amp cleaner in low power mode.You may have done something wrong in your implementation of the mod.
It does not work like VVR at all,being more like having a lower power version of the 18 watt and not like a scaling system.
What exactly did you need to know?
I use this on 6L6 amps with great success.But you have to realize that when the amp is cranked to 10,the effect is less pronounced.So it's not for guys that crank their amps to 10 all the time.If you use anything less than a cranked amp,it's very effective.

Post by **colossal** » Tue 10/12/10 11:31 am

phsyconoodler responded: It does not work like VVR at all,being more like having a lower power version of the 18 watt and not like a scaling system.

What exactly did you need to know?
I use this on 6L6 amps with great success. But you have to realize that when the amp is cranked to 10,the effect is less pronounced. So it's not for guys that crank their amps to 10 all the time. If you use anything less than a cranked amp,it's very effective.

Normally, I'm a crank-to-10 kind of guy because that's where the tone is (!) but alas, the battle between tone and volume rages ever onward.

No, I understand that a voltage scaling system vs. this technique are worlds apart, but I am trying to understand how the 1/4 power/infinitely variable method compares to how a VVR sounds at low(er) volumes. A VVR does a great job of preserving the distortion tone albeit at lower volume. So, if this mod creates essentially the same effect without a perceived loss of low end or the tone thinning out *beyond* what the loss of air movement causes, then it sounds like a killer mod. I have VVR'd amps before and it does work quite well. My interest is just being able to take the ear-ringing edge off the sheer volume of a cranked amp, not go down to 10pm-with-neighbors-next-door volume levels. 18W amps are more particular in that anything you do to mess with the formula typically results in a loss of the magic. So, for instance, if you add a PPIMV (called a Drive control here) and turn it down half way (lowering the grid reference resistance to around 235k/235k) then you are not hammering the output stage in typical classic 18W fashion and the tone will be different. The trick is to be able to get that cranked tone and being satisfied while not coming out of the practice room telling yourself you have got to stop playing so loud (LOL).

Of course it's easy to test the mod but I guess I was looking for someone who has experienced both this 1/4 power mod and the VVR to say, yeah, they perform similarly or no they are really different by such-and-such.

Thanks.

phsyconoodler · Post by **phsyconoodler** » Tue 10/12/10 11:54 am

It would be better for you to test drive the mod rather than have me explain it to you.It basically makes the amp squishy vs focused.It does make the amp distort earlier which is what most guys are after.
It makes the amp sound more single ended,sort of.Lo-fi is a way of describing it too.Kind of like a marshall vs a Silvertone.

Post by **colossal** » Tue 10/12/10 12:21 pm

phsyconoodler said: It would be better for you to test drive the mod rather than have me explain it to you.

Yeah, that might be the best way. I had originally wanted to just build a nice pair of 18W to run in stereo with a complete front and rear panel and all the aesthetics but I realized I have too many unanswered questions so have resigned to build a prototype/test platform instead. So I was being lazy and looking to be spoonfed

Partly too since we are all looking for new and innovative means of reducing volume without losing tone I was very interested in this technique when I unearthed it in a search.

phsyconoodler went on to say: It basically makes the amp squishy vs focused. It does make the amp distort earlier which is what most guys are after. It makes the amp sound more single ended, sort of. Lo-fi is a way of describing it too. Kind of like a marshall vs a Silvertone.

Ok, this is interesting. I would expect there to be some added compression vs. focus/tightness do to the degenerative feedback at the cathode. That's cool! Most of my interest is so that I can run a stereo pair of amps and just take the edge off the combined loudness, not run it down to what most guys consider bedroom levels. I kind of equate this to listening to a great stereo that is loud but full and articulate but at no point ever making you wince or fatigue.

Sounds like I need to give it a shot.

One more thing: when you say squish vs. focus, is there a perceived reduction in volume that goes along with it?

phsyconoodler · Post by **phsyconoodler** » Tue 10/12/10 12:32 pm

Yes,the volume definitely is reduced when you hit the switch.It's much more pronounced at less than full volume,because as we all know,the EL84 has it's own gain which is unlike other types of power tubes which don't add gain to the equation.
That's why it's much more effective on non-EL84 amps at full volume.

scole · Post by **scole** » Wed 10/13/10 9:49 am

im not getting the attachments with the sketches, could you post the again
thanks

Post by **colossal** » Wed 10/13/10 3:52 pm

Scole said: im not getting the attachments with the sketches, could you post the again
thanks

Scole,

All of the attachments appear to have been destroyed with all of the site-hacking 18watt.com has experienced over the last year. I am working on a drawing and will post it in a few hours.

Thanks.

pjgeerlings · Post by **pjgeerlings** » Thu 10/14/10 3:27 pm

... I am working on a drawing and will post it in a few hours...

Has it been a few hours yet?

I'm kind'a curious myself

peace y'all
pj

Phil_H · Post by **Phil_H** » Thu 10/14/10 6:12 pm

That drawing would be great!

I've been looking at this mod for a while, but the lack of schems made me draw my own...

I'm guessing both resistors go to ground and the caps have their "-" ends tied to an SPST. With the switch on, the bypass caps form a 50u cap between the cathodes. Don't know if I missed this, but would I be able to use a DPDT and separate the two cathodes, connecting two 50uF electros to ground as in the original schematic? Planning on putting the switch on the back of my chassis, it's not something I'd play with with the amp on.

Also, seeing how the power is dissipated over two resistors, would I be able to use 3W Metal Oxides instead of 5W wirewounds?

Thanks for the mod!

57chevy · Post by **57chevy** » Thu 10/14/10 6:42 pm

Post by **colossal** » Thu 10/14/10 7:17 pm

pjgeerlings asked: Has it been a few hours yet?

Sorry guys, work had the nerve to get in the way of my drawing time. 57chevy's drawing is from the original thread and allows for varying degrees of cathode bypass. I'll post one with the pot in the center for the continuous variable version. But essentially it's the same thing EXCEPT that the caps are virtually grounded via the pot but not actually grounded to earth.

Post by **colossal** » Thu 10/14/10 9:53 pm

Here is a link to the drawing for the continuously variable method. I showed it switched so it can be full power or variable.

http://dl.dropbox.com/u/7561678/18W/18W ... Option.pdf

EDIT: Drawing updated slightly (only for housekeeping).

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new lite with 1/4 power switch