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Old 31st October 2010, 05:00 AM   #1
Ian444 is offline Ian444  Australia
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Default EL34 PP acoustic guitar amp

I want to build this acoustic guitar amp, I have 95% of the components and the chassis has been cut out and mostly drilled and powder-coated, the schematic...well...wondering if anyone would care to comment if they can see obvious bad things, or suggest improvements. The amp is for a niece, her acoustic guitar has a piezo pickup with an inbuilt 9V battery-powered preamp with volume and tone controls. The transformers are from Chinese hifi amps, just using up what I have. I chose the 6GK5 since I have some and I think only a single gain stage is required here. Could be wrong though. Open to any suggestions before construction commences. Thanks.

Edit: Of course, the design is not original, there are bits and pieces from contributors on this forum all tacked together somehow...
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Last edited by Ian444; 31st October 2010 at 05:02 AM.
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Old 1st November 2010, 12:12 PM   #2
DF96 is offline DF96  England
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Add a 1M resistor from the volume pot slider to ground. This means the valve still gets bias when the slider lifts off the track. Better still, add a coupling capacitor too so grid current does not make the pot noisy. The bias circuit should also ensure that bias still happens when sliders lift off tracks.

Why is there a 1M resistor feeding the phase splitter? Is this to add an HF roll-off?

With a stabilised screen supply you will also need a stabilised bias supply. Alternatively and more simply, you could have both unstabilised but then you would need to ensure that they are stable with signal - fat capacitors!
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Old 1st November 2010, 04:06 PM   #3
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Looks nice. I remember Regent-30 German amp, it sounded gorgeous. It had EL34 outputs, class A biased. Weak point was hum.
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Old 3rd November 2010, 10:40 AM   #4
Ian444 is offline Ian444  Australia
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The 1M resistor feeding the phase splitter prevents undesirable effects when the cathodyne is overdriven, and lowers the grid voltage a little due to quiescent grid current. The Miller effect is not a problem due to the high input impedance, like in a cathode follower. Well that is how I interpret what I have read from Merlin Blencowe's book on guitar preamps. Maybe a 470K would do better in this position for this amp. Not sure if the neon will strike with a 1M either.

I was thinking unstabilized bias supply since if the mains voltage rises, the bias will become larger negative?

The screen supply is straight from Pete Millett's DCPP amp, it looks simple and the voltage can be adjustable if required.

Thanks for the German schem Wavebourn, it all helps.
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Old 3rd November 2010, 01:34 PM   #5
DF96 is offline DF96  England
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I think the 1M is there to protect the PS grid from excessive current if the previous stage cuts off. It will introduce a little distortion due to normal grid current, but this might not be noticeable in a guitar amp. An alternative is a smaller resistor (or none at all, as the 100K anode load in the previous stage has the same effect) with the neon replaced by a silicon diode, such a 1N4148.

The general rule with grid bias supplies is that the control grid and screen grid supplies should both be stabilised, or both unstabilised. This is because they have opposite effects so tend to cancel provided they roughly track each other. Stabilising just one of them can make anode current variation worse than having both unstabilised.
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Old 3rd November 2010, 03:22 PM   #6
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Quote:
Originally Posted by DF96 View Post
I think the 1M is there to protect the PS grid from excessive current if the previous stage cuts off. It will introduce a little distortion due to normal grid current, but this might not be noticeable in a guitar amp. An alternative is a smaller resistor (or none at all, as the 100K anode load in the previous stage has the same effect) with the neon replaced by a silicon diode, such a 1N4148.

I have 470K/470K voltage divider in my Pyramid amps; both to protect grid from destructive current, and to allow higher anode voltage for the 1'st tube.

Neon lamps in such low current places are used as low voltage spark gaps.
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Old 3rd November 2010, 03:41 PM   #7
DF96 is offline DF96  England
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The neon won't do anything, apart from protect the grid during warm-up. The forward grid current when hot will drop enough voltage across the resistor that the neon will never reach its firing voltage of 60-70V.
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Old 3rd November 2010, 04:09 PM   #8
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Exactly. It limits negative grid-cathode voltage.
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Old 4th November 2010, 08:43 AM   #9
Ian444 is offline Ian444  Australia
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Quote:
Originally Posted by DF96 View Post
The general rule with grid bias supplies is that the control grid and screen grid supplies should both be stabilised, or both unstabilised. This is because they have opposite effects so tend to cancel provided they roughly track each other. Stabilising just one of them can make anode current variation worse than having both unstabilised.
Got it, I understand. Ok I will use zener or zeners on the bias supply as well. Won't be too hard to do that. I wanted the screen voltage regulator to keep the power down a bit with the high B+, I only need about 30W.

One other question I have is, should I have a capacitor somewhere as a low pass filter to prevent frequncies over 10 to 20KHz from reaching the OPT? Or is it that, since there is no negative global feedback, this is not required?
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Old 4th November 2010, 02:00 PM   #10
DF96 is offline DF96  England
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With no global NFB you won't have the stability problems which affect hi-fi amps. Whether you want to limit high frequencies is a matter of voicing, which is outside my experience. I guess an OPT intended for a guitar amp is quite likely to have a lower HF resonance frequency than a hi-fi OPT, so it may be best to keep signals away from it.
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