18watt for keyboards?

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18watt for keyboards?

Postby amt » Sat Nov 13, 2010 11:44 pm

I just acquired a donor H/K Allegro mono amp that uses El84/12ax7s and would like to use the iron for a keyboard amp. I found several schematics and thought the "minimalist 18watt" would be perfect. But I have a couple of questions. I tried to locate the 18watt.com forum but its defunct.

http://mhuss.com/18watt/schematics/18minwatt.gif

First, since this is a guitar amp design, Im assuming it needs more gain than a keyboard so can the first 12ax7 be eliminated or is it still needed. I believe it is if using tone controls though.

Lastly, Id like to use the SS rectifier for less heat and better bottom end. Any reason I should use the ez81 thats used in the less minimalist version?

Thx amt
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Postby nyazzip » Sun Nov 14, 2010 12:00 am

i believe the 18watt site is down due to tech difficulties, and will resume again in the ensuing weeks. i built an "18 watt Lite IIb", more or less, and for guitar, it prettymuch needs to be dimed to keep up with a drummer. and dimed equals heavy clipping. in short, i would say 18 watts is way underpowerd for keyboards in a band situation, unless heavy distortion is the sound you want
just my unqualified/amateur 2 cents
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Postby EWBrown » Sun Nov 14, 2010 1:49 pm

For keyboards, you'd be better off with something along the line fo a MarkIII or any PP amp with 60 watts to 120 watts. And you want to avoid the high gain and distortion which will make a guitar would good, but can make a keyboard sound really nasty (666) (sick)

Keyboards generally have a line level outout (1 or 2V RMS) where a, electric guitar, bass or microphone is on the order of 10 to 100 millivolts, and needs the extra amplification.

Some more modern guitars and basses may have a built-in battery operated preamplifier and effects, , though I have no experience with these, my guitar experiences go back to Fenders and Gibsons from the 1960s. Friend of mine has an amplified acoustic guitar, with built in preamp, and it has an XLR balanced output for use with "pro" sound systems, in addition to the usual 1/4 inch plug.

HTH

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Postby amt » Sun Nov 14, 2010 2:31 pm

Well, I should have been more specific in my description of the amp I want. Basically, this is a practice size amp - hopefully a higher quality alternative to the 8"/10" 20 watt SS amps sold at the big box music stores and way better than the little built-in speakers on the keyboard. Ive been told that a few less, soft clipping valve watts are preferable to more, cheap SS watts. This comes from the seasoned guys I know.

In addition, I could always mod it for guitar use in the future.

Still seem viable?

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Postby EWBrown » Mon Nov 15, 2010 5:31 pm

For practice and "small venue" purposes, it should be fully viable. You may want to reduce the gain of the input stage, but that is just a matter of using a lower mu tube and changing the cathode / [late resistor valuse.

I have been considering putting together a minimalist 18 watter for guitar usage (practice, experimentation) and keepint it as simople as possible, volume controls only no tone stack, etc. I can then just use the Dedluxe Clone's speaker and cab, until I get around to making something more permanent.

Anything with tubes will be better than those small cheap SS amps. (sick)

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Postby dhuebert » Tue Nov 16, 2010 8:45 am

Be careful with those low mu tubes, I had no end of headaches with one of my amps due to a microphonic 12AT7. I tried a half dozen brands new and old and they were all completely unsuitable for the front end of an instrument amp. I ended up puting an attenuator on the output of a 12AX7 which did the job nicely. With a little switching it became a nice overdrive option.

I built an amp for a friend who needed one for his fiddle. Same basic requirements as keyboard amp but with a little more gain in the front end. I used the transformer set from a Diytube st-35 that I had left over from a previous build and 4 6BQ5 for the output. It makes 35 watts of nice clean sound he loves it. No global negative feedback and ultralinear output with RCA/Baxandal tone controls.

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Postby amt » Thu Nov 18, 2010 1:42 pm

Since I dont need the extra gain, can I just remove the first 12ax7 from the circuit? Just the input into a pot and then the one 12ax7.

My cabinet is going to be ported and not open backed. Im also wondering about the heat. Im thinking making the back panel gapped at the top and bottom to form two thin, slotted type ports. Hopefully convection with put the heat out of the thing. This is another reason for the SS rectification.
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Postby EWBrown » Fri Nov 19, 2010 2:04 pm

Just replace the input 1 Meg resistor witn another volume pot, 100K should be fine for this purpose. THis will allow you to attenuate the input signal so that it won't go into distoertion in the first stage. Then use the main volume pot to set the actual output level.

This way, there are no major circuit changes, to a proven design.

The SS FWB rectification is better for your application, as the usual tube rectifier PSU "sag" would be deleterious for a keyboard / organ amp.

It would be good to use as much capacitance in the PSU as you want, the built-up stiored electrical energy in the bigger cap(s) will handle the volume peaks a lot better. 33uF is quitesmall, especially for the first "A" cap, which supplies the B+ to the OPT and EL84s, and can cause unwanted B+ sag. . I'd use at least 100 uF here, and 220, 330 or 470 uF would be fine. Use 450V rated caps, 300V or 350V would be cutting it close, especially before the tubes are warmed up and drawing current.

The two other 33 uF PSU filter caps are OK, and a J/J 32 + 32 uF / 500V dual section cap coule be used for these, or two separate caps, your choice. These both could also be increased to 100 uF / 450V.

One caveat - I ran into this particular issue on a build about three years ago, and found that the Hammond 261M6 power transformer throws a very strong 60Hz magnetic (H) field, and any open frame SE or PP OPT can, and will, pick up the 60Hz induced field up to nearly two feet away! =:o

Note: this was 60Hz AC hum, and not 120Hz PSU ripple (which is the usual "demon" in DIY tube amps)

I used 125ESEs in my testing, and orienting them orthoganally at 90 degrees only reduced, but did not eliminate, the magnetic coupling. THis shows up as an "incurable" hum problem, even if the power tubes are pulled from their sockets.


If you use a steel chassis, and mount the OPT inside the chassis, and the 261M6 PT outside, or on top, of the chassis, then the steel should provide sufficient shielding, though a steel bottom plate may be required for total enclosure. Aluminum does not stop or contain H-Field (magnetic) radiation / coupling.

See my next posting for a reprint of this testing


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Last edited by EWBrown on Fri Nov 19, 2010 2:25 pm, edited 4 times in total.
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The Incurable Hum Problem

Postby EWBrown » Fri Nov 19, 2010 2:07 pm

Originally posted by me on January 7, 2008:

Over the weekend, I did some experiments, using an open frame (unshrouded) Hammond 261M6 power trannie (215 V @ 269 mA; 6.3VAC @ 4 ma). With 123VDC line voltage, the outputs are more like 230 & 7VAC). The purpose was to determine just how far out the AC induced stray magnetic fields can reach out and couple into various OPTs.

Later on I built up (literally) on a bread board a DC supply using the same trannie, and it delivers 268VDC @ 184 mA (a somewhat overstressed Philips 230V 40W lightbulb). This DC voltage is near the "perfect" level for any 6EM7 design, as it allows for the best plate to cathode voltage and current combination to get the best sound out of these tubes (along with 13EM7, 10EM7, 6GF7, 6FD7, etc).

FWIW, the trannie runs only moderately warm with 50 watts of loading on the B+ (filament not loaded at the time of the test, that would add another 25.2 watts, for a total of 75 watts consumptu\ion).


I started out with a 230V / 40W light bulb load on the (230VAC) winding, for a 175mA current load that a possible PSE 6EM7 amp design.

Where did I manage to find 230V lightbulbs?

My sister and her hubby were over in Europe last fall, and I asked them to bring me back some "souvenirs" in the form of 230V / 40W lightbulbs.

For the test OPTs, I used a couple of Hammond 125ESEs with the 8 ohms output connected to a couple of small speakers, to measure the "safe" separation" between power and output trannies. Primaries were left "open".

With the above combinations, the 60 Hz AC was audible in the speakers all the way out to 18 inches separation :o :o :o Even laying the OPTs flat in order to create a 90 degress axis separation didn't help all that much. I didn't achieve acceptably low levels of hum until after two feet of separation. :o

Since these open-frame trannies and OPTs normally all orient the core in the same vertical axis, the mutual coupling really reaches out.

I tried the same power trannie / load combination with a pair of Transcendar 3K, 10 W OPTs, and the "kill range" was about six inches, with best oriantation.

Next test was a couple of Z565 clones, these weren't nearly as magnetically "sensitive" but they still needed careful orientation in order to keep the 60Hz AC magnetic fiend coupling inaudible.

I repeated all the tests with the DC supply, and had nearly identical "AC DX" results. :o

So, the lesson is: if you use open-frame type power trannies, test before committing to a chassis layout and design. Or use a separate PSU and amp chassis... This includes using "back to back" filament trannies, isolation / step-up trannies, etc. I suspect that some of my "incurable" low-level hum issues with the K502 amps is caused by the opn frame power trannie, and original open framce OPTs - the Hammond 1608 / 1609s are better, but still need more separation than I used....

FWIW, toroids (either shielded or open) are the best at self-containing their stray magnetic fields, and I would suspect that the use of the toroidal PS trannie in my 13EM7 PF SET amp is the reason that it is so unbelievably quiet, with zero detectable hum.

/ed B in NH
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Postby amt » Mon Nov 22, 2010 12:54 pm

Hey, thanks for the help with the changes. Im going to order parts today and hopefully get something done over the holiday. Im using this guys build guide and layout since it is well thought out and very complete.

http://www.silvatone.bravepages.com/18% ... /Build.htm

I will watch out for the hum problem although the transformers are both
enclosed and pretty heavily built (H/K)

I will be using clear pine and just tung oil finish with chrome pieces/parts. Not really for throwing in and out of a van but should look nice in the house and when thoughtfully transported.

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