Tube Amp Archive

You can do it, but you'd have to change the values appropriately. I attached a resistor calculator i made in excel. the first page shows the stock airbrake values and lets you plug in your amp wattage and speaker impedance to see what values a stock airbrake would present to your amp in steps 2-5 of...

Well I just built mine, although it's suited for 8 ohms. I just realized after building mine that the wattage rating for the adjustable resistors goes down proportionally when you put the tap at a lower value. Mine has been okay so far since I used 100W resistors and I have 6v6's in my express, but ...

Actually, I usually hear the OT singing when cranked and hooked to a load. The windings actually vibrate slightly trying to push the core back and forth. --mark That's probably what it is. I couldn't tell if it was coming from the tubes or the OT. Good to hear that it's not the sound of things brak...

I just got an oscilloscope and wanted to trace some signals. I don't have a proper high power dummy load yet, but I temporarily hooked up my 18Watt to a 20W 8ohm resistor, sent a sine wave through the input, and scoped the output. The thing is, when I started turning the volume up I could hear the s...

I just picked up a 465 with the DM44 voltmeter attached to it. I haven't attached the probes to any high dc voltages yet, but so far it seems to work great for viewing an output waveform. I still have to get some higher wattage resistors to make a dummy load for the bigger amps, but I briefly hooked...

from what I've seen, the 453 is a dual channel scope, so if you get two probes, you can trace two signals at once and have them both displayed on the screen. This can be useful to see what's going on before and after any part of the circuit. I don't actually own a scope, so there's probably alot mor...

I really like this approach with the two adjustable resistors. :D I think I might build one like this too. It won't react like an actual speaker load which will change impedance depending on frequency, but at least it presents a more consistent load to the amp than a standard airbrake. Let us know w...

well, what's happening is that the two ends of the heater winding are alternating wrt each other at 6.3V AC, but they're isolated from any DC voltage until you attach part of the winding to a voltage reference point. Remember that ground is always relative. The ground plug in your outlet could be at...

I hadn't seen that L-pad calculator before. That looks useful too, although the connection used there is different from the Airbrake. On the Duncan Load Calculator, use the "3 s+p(B)" tab to figure out the total impedance presented by the speaker and attenuator where: Speaker A is your speaker or ca...

I guess I'm not sure whether you're asking if you can get rid of just the presence control or if you're asking if an express will sound right without any NFB. I can't answer the second question with any sort of authority, but my initial thought is that the express is on the verge of instability and ...

To make it work wrt 4ohm, like the current design works wrt 8ohm just cut each resistor value in half. The only thing wattage matters for is to make sure that the attenuator can handle the amp you plug into it, so keeping everything the same wattage for a bassman would be safe. The trouble is that y...

The hum goes away when the Volume is down, so I'm pretty sure it's the heaters injecting hum into V1. With V1 removed, the amp is very quiet other than a very slight hiss that is only controlled by the presence. I can also dial in a buzz with the balance pot, but this is controlled by the presence t...

if you always keep the presence at the same position, you could just measure what value you have it set to and wire in some fixed resistors in it's place. Of course, this would only free up space on the front panel, not inside the chassis.

Don't forget to add a filter cap to ground from the junction of the voltage divider. It will help keep AC off of your center tap.

I just might wind up cloning yours. mines 'quieter', but not 'quiet' yet. I still have a ways to go. baby steps. but thanks for the confidence boost. :D I'm still down for comparing, or even if you want to come over and play loud for a while. I used to live above an older couple that would get angr...