Question for the Double E's in the group

[skyboltone]

Even if you're not an engineer you are encouraged to chime in of course.

There are lots of "Bedroom" amps on the market. I've kinda studied the possibilities of one of these but I'm thinking there is a fundamental problem with this approach. When a 6L6 class tube is biased for AB 1/2 operation we put 40-50 VDC on the grids and a plate voltage to keep the tube within plate dissipation ratings. Then we build a preamp that will swing that voltage. The effect of that is that when we pick real light the pre puts a couple of volts on the PA grids and when we pick hard it puts 35-40 volts on the grids. Because there is lots of room between those two extremes there is lots of headroom.

When we choose a 6C4/12AU7 class tube, or even worse, something like a 6CA5 our pre amp will be limited to just a few volts swing and can only produce a highly compressed sounding amp.

Is this more or less correct so far?

If we use one of the little tubes and then put something like a PI network (the attenuator thingy not the phase inverter thingy) ahead of the phase inverter we will still get that compressed sound with no dynamics etc.

So my thinking here is, in spite of it's limitations we are probably best off with a speaker simulator. I'm going to start a thread about those too in a minute but first I'd like to know what the general experience has been with little amps.

Dan

[Bob-I]

.....compressed sound with no dynamics etc.

Dynamic range is the variation between the softest possible sound and the loudest. Typically an instrument, say a sax, might have a dynamic range of 30-40Db between the softest note that a good player can produce to all out. Electric guitar can have a dynamic range far exceding that of a wind instrument, like 90Db or even more for a 100 watt amp.

If you're really looking for "bedroom" volumes you simply will not have dynamics by definition. Low volumes at 6-10Db are about as quiet as we can hear, and at 40Db you're going to be pretty loud. So that limits your dynamic range to about 30Db. No matter what, you're going to have to compress the signal.

So my thinking here is, in spite of it's limitations we are probably best off with a speaker simulator. I'm going to start a thread about those too in a minute but first I'd like to know what the general experience has been with little amps.

Dan

I've built a few small amps, a Gilmore Jr and a Firefly both using a dual triode as a PP power section. In general the bottom end tends to be farts and the dynamic range really suffers. I'm in the process of converting the Gilmore to a 2xEL84 amp using the same preamp.

For really low volume playing nothing beats the PodXT IMO. I use it for late night practice and I can get reasonable tones using headphones, where the dynamic range needs to be very small.

Interesting thread, but I'm not sure it's really for EE's. I'm an EE and I don't know that any of this was ever covered in skool, then again, it was in the 70's....

[gearhead]

Same here; I was getting my EE late 70s to early 80s. They didn't cover anything on tubes.

I joined the college radio station (on the technical staff and eventually a DJ) which had just gotten a donated 5KW tube transmitter. The head tech had to explain to me how a tube worked. When I remarked how much a tube worked like a transistor, he just gave me one of those looks.

For those who don't know, the transistor's first use was to replace tubes (lower power, heat, cost). Doh.

[skyboltone]

Same here; I was getting my EE late 70s to early 80s. They didn't cover anything on tubes.

I joined the college radio station (on the technical staff and eventually a DJ) which had just gotten a donated 5KW tube transmitter. The head tech had to explain to me how a tube worked. When I remarked how much a tube worked like a transistor, he just gave me one of those looks.

For those who don't know, the transistor's first use was to replace tubes (lower power, heat, cost). Doh.

Well I'm not an engineer and I don't play one on TV but I have worked a bunch on big AM broadcast transmitters. When stuff goes wrong with one of those babys its always spectacular and nearly always related to the power supply. You can usually trouble shoot by looking for black goo or smoke. I sorta know how they work. Great fun too I might add. Kinda like juggleing chainsaws or cleaning crude oil off a sea otter. You have to go in there fully aware you might get hurt.

My question though is how does headroom relate to PA bias voltage?

Dan

[Bob-I]

Same here; I was getting my EE late 70s to early 80s. They didn't cover anything on tubes.

Kids. I'm hanging with kids.

We didn't even talk transistors for the first year. All tubes, but none of it helped me with guitar amps, even then.

For those who don't know, the transistor's first use was to replace tubes (lower power, heat, cost). Doh.

Still do.


BTW, I heard that there have been great strides in making the transistor sound as warm and rich as tubes...... That was in 1966... I'm still waiting.

[Normster]

I think I understand where you're going with this, but I think the problem is even more fundamental. While you might be able to compensate for voltage swings by matching the correct transformer to the output, it's nearly impossible to compensate for SPL. For example, the Dumble circuit sounds OK at bedroom levels and provides a pretty good dynamic range, but crank that puppy up a few notches and it comes alive. Much better touch responsiveness! Or is that what you're talking about?

[MarkB]

You might want to start here:

http://www.amptone.com/

[krash]

Dynamic range is the variation between the softest possible sound and the loudest. Typically an instrument, say a sax, might have a dynamic range of 30-40Db between the softest note that a good player can produce to all out. Electric guitar can have a dynamic range far exceding that of a wind instrument, like 90Db or even more for a 100 watt amp.

well, only if #1 you consider a "rest" as a part of the dynamic range. in reality a clean electric guitar under normal playing circumstances (not radical volume knob changes) has maybe a 20-30dB dynamic range and an overdrive guitar has less than 20dB, more likely 10dB.

If you're really looking for "bedroom" volumes you simply will not have dynamics by definition. Low volumes at 6-10Db are about as quiet as we can hear, and at 40Db you're going to be pretty loud. So that limits your dynamic range to about 30Db. No matter what, you're going to have to compress the signal.

Well first off, no you can't hear 6-10 dB, well not at least in a normal household because it's way below the ambient noise floor. your 40dB number is more like the minimum audible volume level you can hear, like as in my air conditioner running in the house now is probably close to 40dB. A "bedroom" level guitar amp will easily make 100dB or more. I would consider about 80-90dB right in front of the amp to be "bedroom" level for a guitar amp.

FWIW my 35ish watt amps make in excess of 125 dB when cranked.

I've built a few small amps, a Gilmore Jr and a Firefly both using a dual triode as a PP power section. In general the bottom end tends to be farts and the dynamic range really suffers. I'm in the process of converting the Gilmore to a 2xEL84 amp using the same preamp.

most of that bottom end problem is not because it's a low-watt amp. It's because of the amp voicing, OT selection, and of course the Fletcher-Munsen effect (you want to hear more low end comparatively at low volumes). Not only that but you almost never crank a big amp that high, and of course Firefly and most other low-watt amps don't have negative feedback and almost any non-NFB, cathode-biased amp when cranked to absolute max will have kind of flubby bottom end but it's all part of the juiciness of a cranked amp. I think most guitarists don't really know what a cranked guitar amp sounds like because it's just too freakin' loud to endure and also listen critically. However a microphone doesn't lie and I crank amps all the time in the studio. The bottom end in my Serena is every bit as solid, if not more so, compared to my 18W.

[krash]

There are lots of "Bedroom" amps on the market. I've kinda studied the possibilities of one of these but I'm thinking there is a fundamental problem with this approach. When a 6L6 class tube is biased for AB 1/2 operation we put 40-50 VDC on the grids and a plate voltage to keep the tube within plate dissipation ratings. Then we build a preamp that will swing that voltage. The effect of that is that when we pick real light the pre puts a couple of volts on the PA grids and when we pick hard it puts 35-40 volts on the grids. Because there is lots of room between those two extremes there is lots of headroom.

As long as you are talking about a clean tone, you are correct. However once you are making an overdrive tone with power tube saturation, then the tube is clipping nearly all the time and there is very little difference peak voltage (at the output of the OT or tube plates) between soft and loud playing.

When we choose a 6C4/12AU7 class tube, or even worse, something like a 6CA5 our pre amp will be limited to just a few volts swing and can only produce a highly compressed sounding amp.

Is this more or less correct so far?

Not really, I would say "compressed" is hardly the right term since it's really "clipped", not compressed. Of course this is the whole point of a small-wattage guitar amp, is to get a distorted (clipped) tone. However if you are talking about clean tones, then you are correct and there is no point to a low-watt amp.

So my thinking here is, in spite of it's limitations we are probably best off with a speaker simulator. I'm going to start a thread about those too in a minute but first I'd like to know what the general experience has been with little amps.

The net effect is the same, only the attenuator changes the tone considerably while a tube amp using small tubes can be tuned to drive the speaker directly without the variable of the attenutor, not to mention lower cost in terms of power supply, transformers, tube count, and the attenuator.

However, once you are clipping the tube, really it doesn't matter whether it's a small tube or a big one, you are clipping and you are going to have little difference in peak amplitude between the softest you can play and the loudest, simply because the peaks are clipped. Additionally if you are getting enough current flowing then you will also get some compression and perhaps you have even designed the preamp to intentionally compress. This is the whole cranked-amp mojo and the entire point of cranking the amp to begin with.

The net result is the same type of tone just at a much lower total amplitude with a low-watt amp.

Trust me a low-watt amp can sound every bit as good, expressive, touch sensitive, and satisfying to play as a bigger amp, as long as you don't define your happiness in tone with a decibel meter such that higher SPL = better tone. The only difference is the SPL, apart from whatever differences result from the use of different tubes, transformers, and speakers dictated by creating a lower-power amp.

[krash]

BTW, even my estimates on ambient sound in your home are probably low.

Some research suggests that typical "quiet" ambient sound level in a home or even in a library is about 65dB, 75dB is about what you would experience in a noisier environment or outdoors in a public place, and 100dB is about like your lawn mower.

That said, if 65dB is as quiet as you likely can get your house with the refrigerator running and the computer on, the AC on, and noise leaking in the windows and whatever from outdoors, then a guitar amp with 30 dB of dynamic range is enough to go from inaudible to 95 dB. In reality the quietest sound you likely can hear in a 65dB noise-floor environment is closer to 70 dB so 30 dB of guitar dynamic range goes from "just audible" to near 100 dB. FWIW at 2 meter listening distance a 1W guitar amp will produce right at 95-100dB into an efficient speaker, so your "in the room" dynamic range is about 25 dB.

[Bob-I]

Looks like I have to get out the DB meter, my guesses are way off.

I recently had my hearing checked and was told that I can hear the entire tested range at between 3-6Db, which is where I got my numbers. Apparently I'm way off as to ambient noise though. Gotta rethink that.

As for dynamic range, yes the guitar is limited witht he volume cranked, I was assuming using the volume control too for variety of tones.

As for my little amps, I've done a ton of mods and used much more iron than required. The Gilmore output xformer can handle 2 x EL84 and I'm using a 6N1P with a LTP PI. I still can't seem to get the bottom end tightened up. I've been successful with several amps in the 5-8 watt range but below that I just can't seem to push the bottom needed for guitar.

[BobW]

If we use one of the little tubes and then put something like a PI network (the attenuator thingy not the phase inverter thingy) ahead of the phase inverter we will still get that compressed sound with no dynamics etc.

So my thinking here is, in spite of it's limitations we are probably best off with a speaker simulator.

I'll admit, I've been an EE since the mid 70s, but based on what you've stated, I have no idea what you are seeking, compresssion or more headroom at a lower bedroom volume?

[krash]

Well, coupling caps and cathode bypass caps can have a huge effect on the bottom end (which is what I meant by "voicing" by and large). I have a ~10W rated transformer in my Serena and it's obviously only making maybe 1-2 watts max so it's certainly over-rated. In fact I think the OT in my Serena is about like the one in my Princeton Reverb.

I am sure you have probably already tried this, but you might consider some cap values like this (presuming you have two gain stages starting with V1A):

V1A cathode bypass: .1uF-1uF
V1A-V1B coupling: .001-.0033uF
V1B cathode unbypassed
V1B-PI coupling: .01uF
PI-power-tube coupling: .01uF
power tube cathode: 1000uF

If the amp is too bright for your tastes, then maybe then bypass (shunt) the V1B plate resistor with .001-.01uF cap. IMHO choice of capacitor here (value, as well as the type/vendor/etc) is very critical to tone. If at this point there's not enough bottom end, then I suggest whipping up a bandwidth limited line out with a voltage divider and a capacitor and plug it into a powered subwoofer to make up for the fletcher-munsen effect.

[Bob-I]

Well, coupling caps and cathode bypass caps can have a huge effect on the bottom end (which is what I meant by "voicing" by and large).

The issue isn't that I'm not getting enough bottom, but that it's not tight enough. It get flabby and farty sounding when the amp starts to break up.

Yes, I tried all the tricks with bypass, coupling, shunt etc. My conclusion was that small output tubes get farty. I was about to revisit this amp when this thread started. Good timing. Maybe next week I can tear into it again.

If at this point there's not enough bottom end, then I suggest whipping up a bandwidth limited line out with a voltage divider and a capacitor and plug it into a powered subwoofer to make up for the fletcher-munsen effect.

That'll be worth a try, but I'm not sure that's the right way to go. The sub will be for 150Hz or so down which I don't think is the problem area.

[krash]

The issue isn't that I'm not getting enough bottom, but that it's not tight enough. It get flabby and farty sounding when the amp starts to break up.

Yeah I think the issue is more generally too much bottom end, so that the bottom end distorts earlier than the midrange and the top end. So in some way it may be a matter of tone shaping into gain stages. That's why i suggested rolling off some bottom end throughout the whole circuit, and keep rolling it off until it sounds right in the bottom end. Then deal with high-end shaping which is actually quite easy with cut control or parallel cap+res across the plates of the PI, etc.

Could be some negative feedback would fix you up. I guess I haven't heard your amp or really what you are after. However one thing's for sure, the tone of a small amp cranked is very different from the tone of a big amp not-cranked, with a lot of preamp distortion. particularly in the bottom end.

That'll be worth a try, but I'm not sure that's the right way to go. The sub will be for 150Hz or so down which I don't think is the problem area.

You may be right. I haven't tried it with a subwoofer. I really don't think that's necessary but of course, I am rather biased. Clearly I'm happy with the tone of my Serena.