5 Watt Tube Amp for Theremin

Posted: 2/23/2011 4:46:19 PM

Joined: 2/23/2011

There are a number of inexpensive 5W guitar tube amps on the market these days (Kustom Defender 5H, Epiphone Valve Junior, VHT Special 6, Vox AC4TVH, etc). Has anyone used these for theremin? I tried my Etherwave through a friend's old Fender Champ recently and liked the way it sounded.
Posted: 2/23/2011 10:34:48 PM
Chainsaw Willie!

From: Just a short walk away from Nike Missile Site S-13/14

Joined: 1/28/2011

I just assumed it wouldn't sound good, so I never even thought to try it until I read your post. So tonite I ran my Burns B3 into my Valve Junior and my home-brew tweed Princeton.

It works OK. The theremin definitely drives the amp into clipping pretty easy. It sounds a lot more "reedy". It is a neat different voice.

The Valve Junior has been modified for more of an old Marshal type voice (well kinda-sorta, as much as you can with only two gain stages and a fundamentally different architecture) and a tweed type tone control. It sounded well at all settings.

The tweed Princeton had a few settings that went "BLAAaaAAT", and one setting that would create a resounding chassis buzz at a certain bass note. But overall it sounds good as long as the volume is not cranked.

Yeah, the little tube amps sound pretty cool - as long as you like a overdriven/clipped voice. Definitely a different voice than when I play clean through my solid state amp.
Posted: 2/24/2011 4:47:22 AM

From: Colmar, France

Joined: 12/31/2007

I had just a look on the schematic of the valve junior. Its volume pot is sitting between the first and the second amplifier stage. That means that the first stage may be overdriven even when you turn the volume very low. So it would give better results if the input voltage was lowered before the 1st amp stage.

You could use a simple passive volume pedal between the B3 and the amp (doesn't require soldering) or modify the junior's input in order to adapt it to the theremin's output level.

In that case you'd to exchange one or two resistors (depending on the modding level already present in your junior and assuming that the mods that you can find on the net are developed for better adaptation to guitar pickup signals, not for theremins):
- R2 should be 390k (might actually be 22k or 68k) and
- R1 should be 68k (might actually be 68k or 1M). This will only affect the signal level and give you additional clipping-free headroom without interfering with other mods which modify the timbre.
Posted: 2/24/2011 7:59:01 PM
Chainsaw Willie!

From: Just a short walk away from Nike Missile Site S-13/14

Joined: 1/28/2011

Most all guitar amps with a 12AX7 pre-amp are built like that, with the volume pot downstream of the first gain stage. So this will be the problem with almost any guitar amp. It is very easy to overdrive that first triode.

The Valve Junior is a little different in most due to the two 68K resistors forming a voltage divider between the input and the first grid. This lowers the input by 1/2. This probably makes the Valve Junior the more tolerant of the Theremin's line out voltage than most other guitar amps. Most others have R1 at 1meg, so it is not much of an attenuator. Some amps have the 1m BEFORE the 68K (on the input jack side instead of the 12AX7 grid side) so it is not a voltage divider at all. This is probably why my 5F2A tweed princeton clone had a "audio hernia" sometimes, not enough attenuation.

For distorted guitar, one of the first Valve Junior mods is to change these resistors at the input to reduce the attenuation and overdrive the first triode intentionally. Typically R1 is 1M and R2 is 33K. Sometimes for serius gain freaks R2 is replaced with a jumper, zero ohms.

For Theremin this is not good. Guitars have a much lower output voltage. Guitar players are often looking to boost the signal to intentionally overdrive the amp into clipping and get that saxophone type tone with silly amounts of sustain.

The theremin has a much larger output that overdrives the input to the guitar amp by accident. That is probalby where my "Blaaaaat" noise came from, too much clipping changing the waveform into something beyond a pleasant musical sound. The typical theremin player may or may not want to overdrive the amp into creating a different voice. Even if they do, the theremin may distort too much if you don't knock that signal level down some.

So just as Thierry states, one needs to lower the signal BEFORE the first gain stage in order to keep the signal clean. Even if one wants distortion, an attenuator is a good idea so you only get as much distortion as you really want without going way over the top.

Yes, a simple passive volume pedal is probably the best solution. I imagine a cheapskate could build up his own box with a input jack (signal from theremin) going to the outer terminals of a 500K pot and the output (to the guitar amp) attached to the wiper and the low side of the pot.

I have seen a document for Moog modifications that described how to change a resistor to lower the Moog output to a level more suitable for a guitar amp. And of course, one could change the input of the guitar amp to allow it to tolerate a greater input voltage.

I think an attenuator (the volume pedal, or the cheapskate pot-in-a-box) in line between the theremin and the guitar amp is the better way to go, as you could use it on any amp and any theremin, and does not involve diddling with either to change them from their normal usage.

5 watt guitar amps are great. I have a couple. But even amongst guitar amplifiers they are not known for having a lot of clean head room. This is WHY they are great for distorted guitar tones. It is easy to get that powerfull overdriven guitar tone and sustain with them. Also they do it at much lower volume levels than a 50 or 100 watt amp.
Posted: 4/25/2011 12:53:29 AM
Chainsaw Willie!

From: Just a short walk away from Nike Missile Site S-13/14

Joined: 1/28/2011

I fooled around with this some more and made two interesting discoveries.

1. The volume knob on the Burns B3 seems to not tune the response of the volume antenna, but instead it appears to be a regular volume adjustment like you would find on a guitar or a radio. It doesn't significantly change how theremin responds to different hand proximity to the volume antenna. Instead the knob just sets the overall loudness of the theremin.

So, with the B3 no attenuator or volume pedal is nessisary before a guitar amp. You can just lower the volume knob setting so as to not overdrive the amp.

2. The ugly Blaaahaaat voice I got and some ghost notes occur when I overdrive the amp so much that an odd harmonic appears.

Used my iPod touch with a scope / spectrum analyzer called
Audio Kit. Due to my being arithmetically challenged I chose 1k Hz to make the math easy.

I set the theremin output volume low and moved my hand to
the pitch antenna until the spectrum analyzer showed 1k Hz. The trace showed a single peak.

I increased the volume. The voice got more "reedy". The trace showed a lower peak forming at 2k Hz. As I increased more volume the 2k peak got taller and a 4k peak began forming. So there ya go, it started producing some even order harmonics with moderate volume overdrive. These were voices that sounded good and musical to me. Perhaps others would not prefer them, but they were definitely musical tones.

Rolling up more overdrive caused a peak to appear at 3k Hz, an odd harmonic of the primary 1k. I think most anyone would agree that this was not a musical voice. This was the Blaaahat noise. Increasing volume just a little bit from this point started producing a large number of spikes in a wide range of frequencies above 1k. The trace really got messy and the voice really got unpleasant. More volume and the 3k odd harmonic got taller than the primary 1k tone. Very ugly and un-musical.

I also noticed that at certain frequencies that some mid level overdrive sounded fine over most of the frequency range of the theremin but that a weird ghost note would appear at a few small spots as I swept my pitch hand from bass to treble. I did not have the spectrum analyzer app on, but I could hear it loud and clear and weird. It did not show up at a lot of notes, but it was enough that it would ruin a song.

So, slightly to moderatly overdriving a tube amp works well and may produce a nice theremin tone to some folks depending on their taste. But it is easily overdone into something that sounds pretty bad.
Posted: 4/25/2011 4:54:35 PM

Joined: 2/23/2011

I picked up a VHT Special 6 amp recently. These are inexpensive 6 watt amps that have been getting very good reviews from guitarists. I got the combo version because it was only $20 more than the head ($190 including shipping from Instrument Alley). I've been playing both theremin and guitar through it and I'm happy with the results.

One attractive feature for theremin is that it has both high and low inputs, so it handles the theremin's output better than the Epiphone Valve Junior or Kustom Defender 5H. I've only played theremin through the low input so far. Additionally, the amp has a high/low output switch and volume boost pedal. It also has a tone knob.

I've played it both through the amp's internal 16 ohm speaker and through an external 8 ohm speaker. Played through the amp's speaker, theremin mostly sounds good, although at one point I think I hit the resonant frequency of the cabinet. That sounds bad, probably something like the Blaaaht sound Willie describes above.

Playing through my external speaker is an entirely different story. My external speaker is a leslie that was salvaged from an old console organ. Through this combination, theremin sounds great, even when the leslie's baffle is stationary. The speaker in the leslie is more midrange-y than the VHT, which gives the theremin an appropriate old radio sound, and combines well with the warm tube-y sound of the amp. I can hit the same note that gave me the bad sound through the amp's speaker without any undesirable overtones. With the baffle rotating, it sounds like theremin going through a leslie, which, as mentioned before by other people on this board, sounds freaking cool.

Guitar, on the other hand, sounds better through the amp's internal speaker.

I play through an MXR 10-band graphic equalizer, to roll off the high end. I also play with a number of effects (delay, phase shifter, fuzz, etc). The amp doesn't have an effects line out, but handles the effects I use very well anyway. The fuzz sound is especially improved compared to my old solid-state amp.
Posted: 6/13/2011 12:49:14 PM

Joined: 2/23/2011

Final update: I changed the speaker in my VHT combo to a Jensen C10Q, which got rid of the ugly sound I mentioned above, and generally improved the sound of the amp, making it sound very similar to an old Fender. I still prefer the way theremin sounds through the leslie cabinet, but now the amp is usable for theremin and sounds better for guitar.

I played the amp with the leslie at a show about a month ago, at a performance space in a bookstore. Outside of my apartment, I finally got to play it with the high output (still using the low input). It sounded good. It was loud enough to fill the space, with an audience of about 40 people, but not too loud to offend anyone in the rest of the store; at least no one complained.
Posted: 6/16/2011 12:50:40 PM

From: Tucson, AZ USA

Joined: 2/26/2011

Chainsaw Willie! said:

[i] So, slightly too moderately overdriving a tube amp works well and may produce a nice theremin tone to some folks depending on their taste. But it is easily overdone into something that sounds pretty bad.[/i]

This is a very good point for those that falsely believe a tube/valve amp on the final output of a theremin voice will significantly enhance the sound. The character of the voice must develop at an earlier stage.

It is the same logic as I have learned this past week that a high end tube mic-preamp will not add to the sound, rather it optimizes in a pleasing way the audio signal that is presented.

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