The Amp Garage

According to the datasheet for the Toshiba chip, the output power is about 0.5W. (Not sure why you thought it was a 3W amp?). It also states that the amp chip is dissipating about 0.75W. Assuming it is running from a 9V battery, these figures imply a current from the battery of (0.5+0.75)/9 = 139mA ...

Yes, Phil_S is right, we need to know how much current is being drawn before we can estimate the resistance needed.

When a battery goes flat it gives a lower voltage and has a higher internal resistance. You could simulate that by putting a resistor in series with a good battery. You could also have a switch bypassing the resistor to return to normal. I would aim for about 1 Volt drop (but experiment might be nee...

Would be nice if they included an ‘age’ knob too. Could range from ‘brand new’ to ’70 years old’ and would model low emission tubes, leaky coupling caps, drifted resistor values, etc.

I’m talking about the age of the amp by the way, not the player!

Looks like great value.

I love the way the advertising says the 12ax7 is ‘hand selected’. I guess that means the assembly worker has a box of tubes under the bench and puts their hand in to select one!

Good point. According to Merlin again (this time his new ‘Designing High Fidelity Valve Preamps’ pp 66 and 198), resistors have two types of noise. Johnson (thermal) noise which is unavoidable, and excess noise caused by spontaneous fluctuations in conductivity, which is proportional to current. Wir...

Just measured the DC resistance at the output jack of a Gibson with one humbucking pickup switched on. With the guitar volume at zero, there was 70 ohms (should be zero – but perhaps the pot is not perfect). With guitar volume on full, I read 7k3. Finding the position of the guitar volume that maxim...

The high gain amps with no grid stopper are just single input amps. Sorry I can’t be specific about which ones I mean, but I seem to remember coming across it. Edit: Ah. Maybe I misunderstood your point. When you have the guitar plugged in with its volume up, you still get the same hiss from the gri...

If I correctly understand thermal noise (which is questionable...) the lower the resistor's value, the less thermal noise it generates. Since the resistor in question is at the input to the first stage, whatever thermal noise it generates is subject to the highest gain the amp is capable of. It mak...

You could try reducing the first grid stopper to 10k or so. The trade off here is that a lower grid stopper can reduce the hiss, but at some point you risk radio interference getting through.

I would think that would be OK. I’m not familiar with the 5879, but the cathode follower has gain of less than 1 of course.
What value of grid stopper do you have on V1a? That is usually the main source of hiss.

Yes, I should have written ' ... compare that -50dB feedback signal ... '. It's not a precise stability criterion (would need to allow for phase shift and use Nyquist or Bode etc. for that) but if we can make the overall loop gain less than 1 (i.e. less than 0dB) at any frequency, then we would avoi...

Yes, looking at the thread again it includes a lot to digest! I think the gist of it is that the signal 'leaking' into the 'other' triode of a 12ax7 is down by about 50dB at audio frequencies. This would create feedback from the higher signal voltage stage to the earlier stage. One way to look at it...

I think the issue would be the level of cross-talk between triodes in the same envelope - which I believe is capacitive.

There was an interesting series of investigations and discussion here:

http://www.diyaudio.com/forums/tubes-va ... tubes.html

Yup. If you do buy that amp, just set it to clean and use a pedal for overdrive!

Electro-Harmonix does a good one called the 'Crayon' for about $65.