Guitar amp idea

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Naterbater

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I picked up an old GE Porta fi satellite speaker the other day. It was one of the first attempts at wireless home speakers. I believe these must have worked with a transmitter that broadcast on a specific frequency.

Upon opening it up I saw that it
 

Naterbater

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Well I guess I will have to finish this post later because the fucking ads keep popping up and if they happen to pop up when your finger is in the wrong place then you hit the post button before you are ready.

I will fill in more later, but the general idea is that I want to use parts from the GE speaker cab to make a guitar amp. Nothing serious, really more of a challenge for myself to see if I can do it. In the end I just want something unique that I can claim as my own design.

It is a 12.6v filament series running a bunch of 7 pin miniature tubes. I will be salvaging the transformers and tube sockets and things like that, but I will be designing and implementing my own circuit based on the salvaged parts.

It had a selenium rectifier, but I will be ditching that in favor of either a diode bridge or a tube rectifier. I will have an extra socket left from the salvage. Would a 12x4 rectifier tube provide enough power for a couple watts of output?

For the preamp I am thinking of using two gain stages both consisting of a 12av6 tube. There was one in the speaker cab originally and I had a second one on hand.

For the power output it came with a 12cu5 and I am going to use it.

Let me know if you think this absolutely won't work or if it sounds interesting. Thanks - Nate
 

Bo Deadly

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Well I guess I will have to finish this post later because the fucking ads keep popping up and if they happen to pop up when your finger is in the wrong place then you hit the post button before you are ready.
Note that you can disable ads in settings. It's an open secret.
 

Naterbater

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Noted and tended to. Thanks for the heads up.

I should also mention that there was a 12au6 and a 12ba6 in the speaker cab. Don't know if either of those are better suited for audio amplification than the 12av6 .
 
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Tubetec

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Heres a schematic , doesnt seem to be indentical to what you have , but maybe not too far off either .
Looks like a reasonable base for a small guitar amp project although you wont need any of the Rf stages .
Looks like a decent mains transformer , you might even be able to run a push pull amp with the extra capacity ,although single ended is ok too if your happy with a few watts.


First thing to do is check the primary winding on your output transformer , leaky caps at the o/p stage grid can cause these to burn out .
 

Naterbater

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Thanks for the link! I plan on wiring it all point to point, but I will reference the schematic. Wish I could find one for my model so that the values of the components would be applicable.

The power transformer is good. I'm getting 170vac on high voltage winding and 14.8vac on the heater.

Do you think a single 12X4 rectifier will have enough amperage to drive the whole amp?
 

musipol

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Just a thought, but why bother with a rectifier tube on this? If you are looking for PS sag, a resistor in series after the diodes will provide for that.
 

Naterbater

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I haven't decided on rectification yet. I could build a diode rectifier or I could use a 12x4 on one of the original 7 pin sockets.

I believe that the 12av6's have a pair of diodes in each tube. Could they be used for rectification?

Unfortunately I don't have a center tap on the output transformer so I can't do a push pull build, but it's good to know that the transformer is up to the task.
 

Brian Roth

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The diodes in a 12AV6/6AV6 are rated for a very low current, and were used in radios to "detect" an incoming AM broadcast signal.

Bri
 

abbey road d enfer

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12CU5 has a max anode voltage of 150V so kind of relies on the rectifier and xfmr to sag if starting with 170Vac.
It should be capable of producing about 2W, but it takes an input signal of nearly 3Vrms, so you definitely need to have a preamp tube. 12AU6 would be ideal for that, more than 12BA6, which is not exactly an audio tube.
Regarding the use of a 12X4, does the secondary have a center-tap? If not, you can't have full-wave rectification. Well, the 12X4 could be used, but not to full potential.
 

Naterbater

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It's interesting that you say that. One of the amps that I am referencing is the Vox student amplifier v5. It uses a 12au6 as a preamp tube and feeds into a 50c5 which is a similar tube to the 12cu5. Do you think the 12au6 is better suited for preamp duty than the 12av6.

I don't have a center tap on the power transformer either. Would I be better off using a diode rectifier?
 

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Naterbater

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Here is an amp with two 12av6's in the preamp. Unfortunately it's a no name Japanese model so there's no schematic to look at.

 

abbey road d enfer

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It's interesting that you say that. One of the amps that I am referencing is the Vox student amplifier v5. It uses a 12au6 as a preamp tube and feeds into a 50c5 which is a similar tube to the 12cu5. Do you think the 12au6 is better suited for preamp duty than the 12av6.
12AV6 is a triode+dual diode. It would have less gain and teh diodes would be unused.
12AU6 is a pentode, has more gain and a slightly different flavour. Read difference between AC15 (pentode) and AC30 (triode).
I don't have a center tap on the power transformer either. Would I be better off using a diode rectifier?
If you use a SS rectifier, I'm afraid teh voltage may be too high. You would have to drop some into a power resistor.
 

Naterbater

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I'm sure I can figure out how to bias the tube so that it gets the correct amount of voltage. I could even trace the circuit on the board to see what values GE used. With that being said, I am not set on using the 12cu5. If you have a suggestion, I am completely open to it.

I was thinking that cascading ttwo 12av6's in the preamp would offer enough gain. I must be confused about how they are rated. The 12av6 has an amplification factor of 100 while the 12au6 only has an amplification factor of 36. Should I be looking at something else?
 

abbey road d enfer

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The 12av6 has an amplification factor of 100 while the 12au6 only has an amplification factor of 36.
36 is the gain in triode mode, not the amplification factor. Actually you can see the gain ranges from 120 to 350.
The amplification factor is not the gain. It's a performance index that represents the max gain if the circuit was infinitely optimized and had no load.
In the 12AU6 datasheet it's not mentioned but you can calcultae it, it's the product of the transconductance by the internal resistance, so for the low voltage case (100V), you have
Gm=3.9mmho (mA/V) and Rp=500kohm, which results in K=1950
 

Naterbater

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I bread boarded a solid state diode rectifier using some 1n4007 diodes. The rectified dc voltage is 167vdc. If I strap a 10k resistor on it with an led the voltage at the rectifier drops to 152vdc. With a .1uf smoothing cap strapped across the output I get 241vdc with no load, but still 152vdc when I add the resistor and the led. I guess that looks pretty good for the 12cu5 output tube. After some filtering it should be well within the safety range.

At this point my thought is that I'll have two gain stages. Gain 1 will be the 12av6, Gain 2 will be the 12au6, and the output will be the 12cu5. The way I see it is that these tubes have have lived in the same house since the mid 60's they ought to know each other well enough to work together by now.
 
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abbey road d enfer

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I bread boarded a solid state diode rectifier using some 1n4007 diodes. The rectified dc voltage is 167vdc. If I strap a 10k resistor on it with an led the voltage at the rectifier drops to 152vdc.
I believe you measured it with an ordinary average-value meter, not an RMS voltmeter. That suggest an RMS open voltage of about 170V. Avg reading is about 0.9x rms reading.
With a .1uf smoothing cap strapped across the output I get 241vdc with no load,
0.1uF is not enough to smooth the rectified voltage. The 10k resistor draws about 15mA. You need at least 10uF for decent smoothing.
but still 152vdc when I add the resistor and the led. I guess that looks pretty good for the 12cu5 output tube.
Do you think it will operate at only 15mA? I would think about 50 mA would be more correct. That would suggest at least 30uF or a CLC arrangement.
 
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