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Old 9th October 2012, 11:59 PM   #11
Simon B is offline Simon B  United Kingdom
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There is a bit of a conflict between wanting soft clipping and max headroom......

At least a cheap amp is less to worry about experimenting with than an expensive one if you're just starting out on this stuff.

If you put an inductor in you'll have to do some maths, otherwise you'll find you've created something that rings.

If you really want to try a regulator, use a series one, with a mosfet as the pass element, as tubelab suggests, something like the one shown below. The ordinary diodes are simply to protect the mosfet in case you manage to discharge the upstream cap before the downstream one. The zener diode sets the output voltage. Sorry it's not a drop in board.....
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Old 10th October 2012, 02:44 PM   #12
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Hi all!

Tube Amp Mods - - QS-LV - Quiet Supply for Low Voltage Tube Amps

It just goes to show me, if I look long enough, I will find it!!! Neat site, lots of stuff I would like to try out!!! Thanks again!
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Old 10th October 2012, 03:21 PM   #13
Cassiel is offline Cassiel  Libya
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It doesn't say how much voltage drops. Keep in mind that you may lose a few watts of output power.
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Old 10th October 2012, 06:09 PM   #14
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Quote:
Originally Posted by Simon B View Post
But hang on...
I had to read it twice. He doesn't want it to clip.
A "clean" guitar sound still clips on pick attacks, especially on cord strums. If you turn the gain down to prevent clipping on pick attacks, you will have very little volume. You would be surprised how little volume you get from a Fender Twin Reverb if you turn it down far enough to prevent any clipping. A guitar with a hot pickup can clip the input stage before it hits the volume contol where you can turn it down. You need an oscilloscope to reveal the clipping, you rarely hear it when the amp starts to clip.
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Old 10th October 2012, 07:53 PM   #15
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Hi All!
Thanks for replys!
Simon - thanks for your research and experience. Probably going to do a drop in board for now, and may revisit this later. Yes, after rethinking it, I probably do want clipping.

Cassiel - As I understand, Valve Jr. is already biased very hot on the EL-84, many tweakers are turning down the voltages there for tube life. Worst case scenario, it won't work and I'll have to incorporate something else. Valve Jr. is a big compromise, I understand, but right now it's what I have, and I can't afford much else. So it's tweak time with the limited resources I have.

Loudthud - Yes, I stand corrected on clipping. Ahhh, I haven't got an osciliscope nor the budget for one. Still trying to justify a variac purchase I want to make. Anyway, thanks for the tips from all of you.

Have a blessed day!!!
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Old 11th October 2012, 01:16 AM   #16
Simon B is offline Simon B  United Kingdom
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Originally Posted by Loudthud View Post
A "clean" guitar sound still clips on pick attacks, especially on cord strums.......
Good point Loudthud, dry guitar signal does show big picking transients, probably less noticeable even on a Fender Twin Reverb at low volume than through full range speakers.

Whose tweeters will of course blow much sooner than the unwary might like them to......
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Old 11th October 2012, 02:10 AM   #17
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Quote:
use a series one, with a mosfet as the pass element, as tubelab suggests, something like the one shown below
That circuit is pretty much what I use. You can wire a pot across the zener, connect the wiper to the mosfet gate and you have a VVR. VVR's are the hot ticket these days, but they are intended for getting dirty at low volumes by reducing the B+.

Quote:
I probably do want clipping.
You probably already have some clipping from the initial pick attack transient. This is why guitar amps have smallish coupling caps compared to HiFi amps. The pick attack moves the guitar string away from the pickup pole and then lets it fly across the pole's face before it starts to vibrate. This generates a large low frequency transient. If the transient reaches the OPT it will saturate causing a large increase in tube current and clipping even before there is any sound.

I started making tube guitar amps in the early 60's because tubes were free out of dead TV's at the local trash dump. They worked pretty good for a teenage kid.

I made my first solid state guitar amp in high school. I used circuitry from HiFi amps since there was no schematics available then. I got a 15 inch field coil speaker from a Hammond organ. The amp made a loud pop every time you plucked a string and destroyed the speaker in a week. Frequency response down to 10 Hz is not a good thing in a guitar amp.
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Old 12th October 2012, 09:17 PM   #18
Struth is offline Struth  Canada
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Hi Guys

Like all economy SE amps, the VJr is limited in what can be done in the context of the circuit. The PT is the ultimate limiter of power output and effective headroom. The OT is skimpy and restricts bandwidth when pushed hard. A larger OT - one with hifi bandwidth instead of just mids - will provide cleaner output without tone change all the way up to clipping. A replacement designed for Champ, VJr, etc will not provide any more bandwidth or cleaner tone than the original.

You have to be careful throwing in different output tubes. In a Champ, just because a 6L6 fits the 6V6 "hole" does not mean the PT will be happy supplying the extra 450mA of heater current required - that's a 63% overload on the heater winding!

In a VJr, the heater load is just over 1A, so installing an octal socket for alternate tubes present possibilities of quite different tones. Pulling the EL-84 and adding 6V6 gives a creamier/muddier tone with reduced heater draw. Plugging in 6L6 instead will provide cleaner tone but no power increase with just a 13% overload on the heater winding.

Note that tube life is easily extended by adding a proper value screen resistor - 1k-1W flameproof is the absolute minimum for safety. EL-84s have sensitive screens, so a 2k2-5W is better. You don't have to change Rk if you add a screen resistor. TUT has advocated this since 1994.

VVR is a poor copy of an early version of Power Scaling. Real Power Scaling is applicable to any tube amp of any power level and has a control range of "unusably loud down to unusably quiet". his allows the player to select his own control limits.

One point not addressed is speakers. The biggest improvement you can make will be to bypass the tone crippling combo speaker and plug the electronics into a full size detuned cabinet or two. See SPKR for construction details.

Have fun
Kevin O'Connor
London power.com
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Old 22nd October 2012, 08:28 PM   #19
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Hi All!

Yes, I do understand the limited nature of the Valve Jr., in terms of PT and OT. I have been considering an octal instead of 9 pin for output tube. I do have a new OT for it.

Hey Kevin! Glad you had that B+ board available! Looking at getting a few books from you to understand the circuit. Also looking to get your Power scaling version. Which do you recommend?
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Old 24th October 2012, 04:30 PM   #20
Struth is offline Struth  Canada
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Hi Guys

In a Champ, VJr et al, you can go two ways to Power Scale:

Use SV2 for a Full-PS solution, taking care with heat management, or wire it for PS-TT with less concern about heat.

Use SV84 for a single-node Full-PS solution where you have either added another RC or two to the Va filter, or added QS-LV for a much quieter amp. SV84 can also be wired for PS-TT operation with less attention to heat management.

If you are installing QS-LV anyway, then the QS-LV + SV84 solution is much more compact with only two mosfets to mount, compared to QS-LV + SV2 with a total of three mosfets and a larger foot print.

The kit notes for SV2 and SV84 show both wirings. See the articles on our site to learn a bit about Full-PS and PS-TT. TUT7 will be out early 2013 and has a chapter about PS-TT - it is the corrected power control approach everyone else tries and gets wrong.

As far as voltage drops across QS-LV: this will be essentially just the gate-to-source threshold voltage plus a small drop across the current-sense resistor.

Note that the regulator circuit shown in a post above will have a strong tendency to kill mosfets at turn-on. People afraid of solid-state circuits seem to think it is better to remove vital current clamps to make the circuit simpler and have less silicon. For the sake of one R and one Q you can remove all worry about replacing the expensive mosfet.

Have fun
Kevin O'Connor
londonpower.com
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