Some small amp questions

Stevo Bambino

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My small practice amp died not to long ago, so I thought it might be time to "fix" it. It is a little solid state 1 or 2 watt piece of garbage that someone gave me when I was in high school. I only used it because I live in an apartment with thin walls. Anyway, after looking for options, I found two schematics I liked. The first is the PRR Third Watt over at EL34world. I think it will fit in the chassis and cabinet of the old practice amp. I had everything to build this except the power transformer, tubes and sockets. I ordered what I needed and then I looked at my old practice amp and discovered it has a 6 ohm speaker and the circuit uses an 8 ohm reverb driver for an output transformer.
1- Will it be fine driving a 6 ohm speaker? Would there be any side effects, tonally or in the way the tube operates?
2- Could I just add a 2ohm resister inline and be fine? ( I know, DC resistance and Inductance are not the same thing, but I am trying to build this on the cheap)
3- If neither of the above work, what should I change?

The other circuit I liked was Bob's Fat Mama (http://ephaseaudio.com/audio/Tube amp projects.htm) The only problem is his schematic is really hard to read. I attached a picture on the best I could guess. The circled parts are what I could not read well and the whole line out part, which I am not sure I am going to include anyway. I have all the parts for this except the output transformer, which is only $15 if I remember correctly. Could anyone look it over and see if it needs any corrections?

Thanks
 

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2L man

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6 ohm speaker vs. 8 ohn speaker increase power stage (and power supply) load ~33% and in theory operative voltage should be lowered. It should be OK but only after good testing it is possible to say was it :)

If circuit was optimized for 8 ohms, using 6 ohms it will produce less max power and possibly more distortion already on lower power which both should be good for your use.
 
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Len058

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Why wouldn't you just upgrade the speaker? You're going to put in a lot of effort, building an amp, so it's a small step.
There's a good chance you'll be very happy with the result. Cheap amps have very crappy speakers. If you put in a Celestion, Jensen or something like that, you might really enjoy all the tube deliciousness.

A resistor would work but it sucks tone.
 

middy

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The speaker is one of the most important parts of your sound. Don’t skimp there.
 

Stevo Bambino

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I did check out Rob Robinette's Micro amps. I almost went with one of them, but I prefer pentode distortion over triode distortion. Also, this practice amp is tiny. It has a 6 inch speaker, the cabinet is only 8"x10" and the chassis is only 6 3/4"x4"x 1 1/2". This "Third Watt" amp is simple enough and the components, especially the transformers, are small enough that I think I can get it to fit. The speaker actually is decent, not jaw dropping awesome, but much better than good enough for a practice amp. Also, I have maybe $40 into this, buying a different speaker is going to more than double that. I did briefly look at different 6 inch speakers and the least expensive 8ohm one I found was an MOD from Antique Electronic Supply for $30, which will bring the cost over the budget I allowed for this project. As far as the time and effort, I like doing this kind of stuff, so I don't care at all about that.

What does the 100KL pot connected to the screen on V2 do? I need to be able to label it on my chassis but I have never seen that in a design before.

Has anyone looked at the Fat Mama circuit and seen anything wrong there? That will not fit in my little practice amp but I was thinking of using it with an old AlNiCo CTS 12 speaker I have.
 

Stevo Bambino

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Another question I just thought of: I want to elevate the heaters. what voltage should I aim for? I was thinking of using a 250k - 50k voltage divider off the 150v B+ to get 25 volts. would that be good? Too low or too high? Could I also use a humdinger for a virtual center tap if I still have hum?

I think I will try the 6 ohm speaker first. If it does not sound good, I can try it with an 8 ohm speaker and if that works better, then I will in vest in an 8 ohm 6 inch speaker. I found PRR's comment on speaker impeadance for this circuit and he says "To "make the most" of the 6AU6, we want somewhere in the 12K to 50K range." for the primary OT. I did the math and a 4 ohm speaker will give me a 14.5k primary, a 6 ohm speaker will give 21.75k, an 8 ohm speaker will give 29k, and a 16 ohm speaker will give 58k.
 
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printer2

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From input to output, 200 pF, 47 mF, 0.57V, 13.5V and 180 ohms. The 100k pot reduces the screen voltage, acts as a sort of master volume. 25-50V is fine for an elevated voltage. I think you would want a virtual center tap first and foremost. Give it a try with the 6", the difference between 6 ohms and 8 ohms may not be all that much in practice. I am guessing the power from the 6AU6 will only manage to give you a dirty sound, not much for clean.
 

Stevo Bambino

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It took forever to get my power transformer from Mouser(Hammond 262E6). I just finished building this practice amp today. My first amp. I built PRR's 1/3 watt with some mods. I added a simple tweed Princeton style tone control, an LED pilot light and a virtual center tap for the heaters. The tiny chassis did not help either. All future builds WILL be in a bigger chassis. What a pain in the butt. After going through RobRob's startup procedures, I started playing on it. First, it is really quiet with out anything plugged in. Cranked there is just a tiny bit of hum. Second, I put in the optional switch for disabling the 25uf bypass cap on V1. It sounds far better with the cap in circuit. I might make the switch switch between two different bypass caps. Any suggestions for the value of the second cap? Third, I noticed the higher I turn up the screen voltage on V2, the more clean it gets. I have not been able to fully explore this yet. Anything higher than 1/3 of the way up is too loud for my apartment and the neighbors are not going anywhere. It is surprising how loud 1/3 of a watt is. It will never hang with a drummer, but it will definitely piss off the neighbors. Fourth, I am surprised at how similar the distortion sounds like my old MXR Distortion Plus pedal. I was expecting it to have a little smoother breakup but it goes from clean to shredding with almost nothing in between. Fifth, the tone control was probably unnecessary. I have it turned all the way clockwise to get good tone. Sixth, my B+ is 195 volts and I was expecting 150-160 volts. Also, my heaters were at 7.5 volts. I put two anti parallel 1n4003 diodes on one leg of the heaters before the virtual center tap and that brought those down to 6.4 volts. My power at the wall is 124.4 volts, which is typical for here. Is my B+ too high? Will that affect the bias?

Pics to follow.
 




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