Convert Hifi Amp To Guitar Amp

[Jozer99]

For the pots, I want to use audio taper, correct? Also, the diagram says 1/2 watt resistors, does this apply to pots as well, or can I use standard (guitar) pots?

[Geo]

For the pots, I want to use audio taper, correct? Also, the diagram says 1/2 watt resistors, does this apply to pots as well, or can I use standard (guitar) pots?

Do you have the right values on hand as "guitar pots"? They should work fine, but if you're buying them, I would buy pots from an amp parts seller. Then you know you'll be in the right wattage rating. I don't know what amp pots and guitar pots are rated wattage-wise.

Yeah, use audio taper, although I think the mid pot in a Fender tone stack should be linear. When in doubt, check a schematic of a classic amp. Volume controls are definitely audio taper.

[Jozer99]

The amp came today! I plugged it in to see if it works, and was disappointed when I didn't hear any speaker hiss. I thought it was broken, even though the tubes glowed. Then I plugged my MP3 player into it and VOILA! The cleanest tube amp I have ever heard! This thing sounds amazing! It smells like burning dust when I turn it on though...

I have quite a few guitar pots sitting around, plus there is a guitar (and guitar parts) store much closer to me than the nearest electronics depot where I could get any other kind of pots. Most of the pots I have are CTS, so I imagine they are pretty good.

I'm looking forward to beginning modifications soon (after I listen to a few Radiohead albums through it... ).

Quick update:

After listening to a Radiohead album, I noticed all the tubes were glowing quite nicely except for one of the power tubes, which was dark. I assume that means I need to replace the power tubes (2x EL84's). The amp still sounds fine, even with the dead tube. Out of curiosity, does this mean that the amp is working in Class A mode?

I have heard I may need to "bias" the new tubes. Given that the tools at my disposal are a digital multimeter and some speakers, how do I go about biasing the tubes correctly?

Edited August 14, 2007 by Jozer99

[Geo]

Your amp is cathode biased. R30 and C32 (between the power tubes) automatically bias the power amp, so all you have to do is put in a new tube (no need to adjust anything). Don't worry about matching them. Mismatched tubes may make a more "musical" guitar amp.

Because the amp is cathode-biased, I believe it's "class A" whether both power tubes are working or not.

Since only one power tube was conducting, you were only hearing half of the amp's volume potential (probably less). Before you buy a new tube, though, check the heater wiring at the socket of the dark tube. The heaters should be pins 4 and 5. If the tube heaters aren't hot, the tube can't do anything.

[Mike Sulzer]

Your amp is cathode biased. R30 and C32 (between the power tubes) automatically bias the power amp, so all you have to do is put in a new tube (no need to adjust anything). Don't worry about matching them. Mismatched tubes may make a more "musical" guitar amp.

Because the amp is cathode-biased, I believe it's "class A" whether both power tubes are working or not.

Since only one power tube was conducting, you were only hearing half of the amp's volume potential (probably less). Before you buy a new tube, though, check the heater wiring at the socket of the dark tube. The heaters should be pins 4 and 5. If the tube heaters aren't hot, the tube can't do anything.

Could just try interchanging the output tubes, briefly. You can have class AB with cathode bias, but I think it is better with grid bias. The problem with cathode bias and only one tube working is that the current through the working tube is higher than normal and the extra dissiaption could shorten the life of the tube. So you should get both working.

[Jozer99]

I will give it a look, and see if the heaters are live. The tube in there didn't seem to heat up at all. First, I think I may try switching the tubes, to see if the problem lies inside one of the tubes.

I will take your expert opinion that the amp is self-biased, and/or Class A. I was fooled by the potentiometer on the amp, which I mistook to be a biasing pot. Looking at the diagram, it appears the only thing it could be is a "hum control" pot. What exactly does that fancy hum control circuit do?

Your amp is cathode biased. R30 and C32 (between the power tubes) automatically bias the power amp, so all you have to do is put in a new tube (no need to adjust anything). Don't worry about matching them. Mismatched tubes may make a more "musical" guitar amp.

Because the amp is cathode-biased, I believe it's "class A" whether both power tubes are working or not.

Since only one power tube was conducting, you were only hearing half of the amp's volume potential (probably less). Before you buy a new tube, though, check the heater wiring at the socket of the dark tube. The heaters should be pins 4 and 5. If the tube heaters aren't hot, the tube can't do anything.

[Mike Sulzer]

R32, hum control: The AC on the heaters and heater wiring can be "picked up" by other parts of the circuit. The purpose of the pot is to balance it out. The idea is that one side of the heater circuit is positive and the other part negative with a net effect near zero. The pot is a bit surprising; most amps just use a pair of fixed resistors. Further hum reduction can be achieved by connecting the "center" (the pot slider in this case) to about plus 70 volts rather than ground (although it should be AC bypassed to ground with a capacitor). This reduces the hum pickup inside the tubes.

I will give it a look, and see if the heaters are live. The tube in there didn't seem to heat up at all. First, I think I may try switching the tubes, to see if the problem lies inside one of the tubes.

I will take your expert opinion that the amp is self-biased, and/or Class A. I was fooled by the potentiometer on the amp, which I mistook to be a biasing pot. Looking at the diagram, it appears the only thing it could be is a "hum control" pot. What exactly does that fancy hum control circuit do?

Your amp is cathode biased. R30 and C32 (between the power tubes) automatically bias the power amp, so all you have to do is put in a new tube (no need to adjust anything). Don't worry about matching them. Mismatched tubes may make a more "musical" guitar amp.

Because the amp is cathode-biased, I believe it's "class A" whether both power tubes are working or not.

Since only one power tube was conducting, you were only hearing half of the amp's volume potential (probably less). Before you buy a new tube, though, check the heater wiring at the socket of the dark tube. The heaters should be pins 4 and 5. If the tube heaters aren't hot, the tube can't do anything.

[curtisa]

Because the amp is cathode-biased, I believe it's "class A" whether both power tubes are working or not.

Since only one power tube was conducting, you were only hearing half of the amp's volume potential (probably less). Before you buy a new tube, though, check the heater wiring at the socket of the dark tube. The heaters should be pins 4 and 5. If the tube heaters aren't hot, the tube can't do anything.

Be careful here - just because the amp is cathode biased doesn't automatically mean it's operating in class A. In fact given the cathode resistor in your schematic and where it's biased at (25V thru 210 ohms gives 120mA, each tube shares this at 60mA), I believe your amp is running class AB1. Given that the amp supposedly belts out "14W Hifi" it's highly unlikely to be running class A. The class of operation is a function of how "on" the tubes are under no signal conditions, not the fact that it's cathode biased. Check the datasheets for the EL84 for more info.

However Geo is right - it is self biasing and you won't need to worry about tweaking it when changing tubes.

You may find that your amp will stay pretty clean sounding right up to the point of clipping. The power stage is not likely to clip nicely as it is configured ultra-linear (pin 9 of each EL84 is connected to an extra tap on the output transformer). Most guitar amps run the output stage in regular pentode or triode-connected pentodes, ultra-linear output stages are pretty uncommon and are more a Hifi thing. This may be what you're after, but you could also consider disconnecting the ultra-linear taps and either running the EL84's as triodes (connect pin 9 of each EL84 directly to pin 7), or as regular pentodes (pin 9 is connected to the plate supply via a dropping resistor, say 100R-1K 5W - The schematic for the Vox AC30 shows the placement and value of a suitable screen resistor).

Some of these tweaks may make massive changes to the way your amp sounds, especially under high volume, so experimentation is recommended!

[Mike Sulzer]

But remember if you connect the output tubes as triodes, you will lose a lot of power. The pentode connection connection using a resistor to the screen grid as described here seems to be the favorite way for a guitar amp.

Because the amp is cathode-biased, I believe it's "class A" whether both power tubes are working or not.

Since only one power tube was conducting, you were only hearing half of the amp's volume potential (probably less). Before you buy a new tube, though, check the heater wiring at the socket of the dark tube. The heaters should be pins 4 and 5. If the tube heaters aren't hot, the tube can't do anything.

Be careful here - just because the amp is cathode biased doesn't automatically mean it's operating in class A. In fact given the cathode resistor in your schematic and where it's biased at (25V thru 210 ohms gives 120mA, each tube shares this at 60mA), I believe your amp is running class AB1. Given that the amp supposedly belts out "14W Hifi" it's highly unlikely to be running class A. The class of operation is a function of how "on" the tubes are under no signal conditions, not the fact that it's cathode biased. Check the datasheets for the EL84 for more info.

However Geo is right - it is self biasing and you won't need to worry about tweaking it when changing tubes.

You may find that your amp will stay pretty clean sounding right up to the point of clipping. The power stage is not likely to clip nicely as it is configured ultra-linear (pin 9 of each EL84 is connected to an extra tap on the output transformer). Most guitar amps run the output stage in regular pentode or triode-connected pentodes, ultra-linear output stages are pretty uncommon and are more a Hifi thing. This may be what you're after, but you could also consider disconnecting the ultra-linear taps and either running the EL84's as triodes (connect pin 9 of each EL84 directly to pin 7), or as regular pentodes (pin 9 is connected to the plate supply via a dropping resistor, say 100R-1K 5W - The schematic for the Vox AC30 shows the placement and value of a suitable screen resistor).

Some of these tweaks may make massive changes to the way your amp sounds, especially under high volume, so experimentation is recommended!