5w SE guitar Amp

[N9VOC]

Hello!

I recently completed a 5 watt tube type guitar amplifier, love it and wanted to share the schematic here.

I used a power transformer from an old HP audio generator, It's High Voltage winding was almost twice as high as needed for a "Champ" style amplifier, and had two 6.3V heater windings. I simply hooked it up as if for 240 volt operation, and ran it under 120V.

The five volt winding was now at 2.5 volts, so, I used solid state rectification (1n2007).

VERY similar to the famous "Champ", but with an additional control to limit amount going to power tube -- can get preamp tube crunch if I want, without causing my wife to yell (excessively).

Was an education in the value and placement of "Grid Stop" resistors, Originally I had place the 1 Meg input resistor directly on the grid of the first triode, and came through the 68K to it -- Oscillation city!!

Simply moving the 1 Meg to across the input, and then through the 68k to the first grid, solved the problem!

Just thought I'd share this with you-all.

Cheers!

[bob91343]

I would suggest some minor changes. Move the standby switch to the center tap of the power transformer to reduce the stress on the input filter capacitor and eliminate arcing when switching into standby. Increase the LED resistor to maybe 100k or more to make it last longer. C6 and C2 are redundant. A small capacitor across R9 might make it more stable.

These are minor but if it works, don't fix it.

[Nigel Goodwin]

Originally Posted by bob91343

I would suggest some minor changes. Move the standby switch to the center tap of the power transformer to reduce the stress on the input filter capacitor and eliminate arcing when switching into standby.

I would disagree, moving the standy switch to the centre tap would INCREASE the surge - where it is the choke reduces the surge when you close the switch.

[bob91343]

The inductive transient will make the switch arc when opened.

The stress on the input capacitor to which I was referring is the rather high voltage to which it is charged with the switch open.

[N9VOC]

Thanks for viewing, and the suggestions!

I decidedly like the idea of putting the switch in the CT, hadn't thought of that. Do you think the inductive transient of the Power Tranny will be less than that of the choke?

The power supply caps are also ex HP "test equipment grade cans", "reformed" with 450VDC at least 72 hours prior to first use -- the no-load voltage on the cap with standby open is only 425 volts - no problem for this type of capacitor, IMHO. I also placed the first filter capacitor in it's position to serve as a "surge current" sink for the inrush currents that do occur when the HV switch is closed - reducing the peak current level on the diodes, as well as working with the choke to stop diode switching transients.

Word of explanation on C6 and C2 - true, they are redundant from a DC isolation standpoint. However, when either control is at maximum, the combined value of the two 0.047 caps in series approaches that of the 0.022 uF value used in classic "Champ" amps, and experience shows that I get a VERY champ-like tone from this configuration. When not at "full crank" the higher capacitance value seems to give me a "darker"/"warmer" tone that I wanted with this amp.

Increasing the negative feedback on the higher frequencies with a capacitor is an idea - one I would definately use in a bass amp!

Thanks for the food for thought! As with many other items in amplifier design, each layout has its plusses and minusses, one must choose intelligently where compromises must be made! I won't be messing with this amp anymore though - it works exactly as I like and I agree "If it ain't broke, don't fix it!"

Cheers!

[N9VOC]

Almost forgot - with the LED resistor, I used a 20 mA LED. With the 68K resistor, current is approxmately 4.7 mA, 1.6W dissipation. With a 100K resistor, the current would be 3.3 mA, 1.1 W dissipation. I had a 68K 3 watter on hand, and was fresh out of 100K 2 or more watters in my box at the time.

Cheers

[ChrisA]

DOn't break the ground connection with a standby switch. You want a broken wire that shorts to the chassis to always blow a fuse. I'm not 100% sure that every broken wire would blow the fuse if the PT where ungrounded. You'd have to work that out and would need to think about pathological cases too.

Your 10 ohm resister in series with input cap will handle any surge problem. But a in-rash termister on the mains side would prevent it too.

Your standby switch opens the ciruit, one thing you might change is to switch in a high value resister rather then open. Currently you have an infinate value resister, just lower that value a little

Yes the two caps on either side of the pots are respondent if the goal is only to block DC but I wonder if they don't form a kind of tone control? When the pot is at 0 ohms your effective coupling cap is 1/2 the value of when the pot is at the other end of it's scale. You are cutting bass when you crank the volume. Smart idea if that is what you want.

[N9VOC]

Hmm, you may indeed be right about the potential hazards of the CT not being to ground and a shorted wire to chassis not blowing the fuse.
Gotta think on that one.
Putting a large value resistor which is in line when in "standby" and shorted by the switch is another idea I hadn't thought of!

[flood]

i'm sold on using a DPDT standby to switch the secondaries BEFORE the rectifier.

[SemperFi]

Nice amp and great way to recycle old parts! Many guitarists have discovered that low powered amps are more fun, and in 99% of the time powerful enough since they are mikked into the PA anyways.

No doubt this sounds great and not that you should change anything, but for the sake of discussion...(that's why we post here isn't it).

I love the simple no-nonsense with just a gain and volume control.

The powersupply is huge in terms of capacitance. No sag here. That is great for dynamic playing, where you want light touch to be mellow, and hard to be loud. Some guitarists love that, they can actually play the amp, not just the guitar. But for old-school sag, minimum capacitance helps as well as some resistance in series with the rectifiers.

Standby switches are not needed. Just unplug at the input, or turn the guitar volume down during breaks.

The two gain stages can often benefit from some isolation from each other with a resistor in series with the b+ supply, each stage bypassed seperately.

Your volume/partial tone control idea is cool, but 22nF is still a bit high when driving a 220k resistance. Many go down to 1nF into <100kohms for rauchy blues crunch. Anything in between your value and this is fun to play with. In my amps I have a 'blue/jazz' switch that changes the resistance from 60kohms (blues) to 220kohms (jazz) and the coupling cap is 4.7nF. All these RC values are often played around with depending on my mood when assembling the amp
This can easily be put in parallel with the Gain pot, and C1 could be experimented with. (Yeah I know, it messe up VR1's log tracking, but who cares).

I'd remove the loop negative feedback.

C5 is fun to play with as well. From 1UF and up...

I love instrument amps, since the 'HiFi rules' can get the shaft, and the design takes on a new fun dimension.

You have pictures of the amp?