Tuesday, February 12, 2008

YBA-1A Re-Cap

At some point during rehearsal my amp started blowing fuses. So i decided to use it as an excuse to replace the 40 year old caps that were inside and learn a bit about tube amps.

One thing with this amp is that there is no bias pot and the bias voltage is set to -46. This seems to have possibly caused a problem when the capacitors started to dry out. When electrolytics dry out they begin to have resistive quanlities as well as a capacitance. A less negative bias results in a greater current flowing through the tubes. Which could burn them out prematurely, or overload the plate and cause it to glow bright red (seriously shortening the tubes lifespan if not ending it right there).

This is the schematic for my amp. Mine is either a later or earlier revision and has a few less parts plus runs a higher voltage to the preamp tubes.

Basically the main B++ power supply caps were replaced as well as a few others. A bias pot was added as well for a bit more versatility.

Heres a close up of the power supply section. C15, C17, C18, C19 and C20 were all swapped out for 100uF 450V nichicon electrolytics.

But one thing at a time. Going back to the bias problem. When I measured the bias had sagged to -41v.

Looking back at the schematic, if C14 had even a somewhat large resistance, it would be in parallel with R31, effectively lowering its resistance and lowering the voltage drop across it. since the -46v comes from the voltage divider setup by R32 and R31, C14 developing a resistance would cause the bias voltage to become less negative. So i replaced both C14 and C15. C14 with a 10uf and then changed the circuit a bit to have a bias pot that can vary from about -44v to - 55v. Essentially R32 is replaced by a 5k resistor in series with a 10k pot. The wiper of the pot is connected to a 47uf cap and is considered the output voltage. The 3rd leg of the pot goes to R31 which remains unchanged.

Here is a sketch of the old and new bias sections

Here is some math showing the effect of the resistive qualities of a bad cap on the bias voltage

Heres the bias section modified with the pot. Also you can see the replacements for C15 and C19.

Heres C20, C16, C17 replaced. The holes are where the can caps used to be.

I also replaced a few resistors that were floating out of spec. And I changed R16 from 10k to 11k. The reason is because on my version the voltage at c20 is exactly 450 volts (if everything is ok) which is the caps rated limit. Although this is not likely a problem I bumped R16 up to 11k to reduce that voltage just slightly to around 442 volts.

And finally I also replaced C21 with a 10uf 450V xicon electrolytic. While doing this I melted a bit of one of the filter caps being careless with the iron. So I replaced it with a metal film cap of the same value (to the left of the yellow capacitor in the picture below).

Saturday, February 2, 2008

Scrambler Clone

This is a clone of the ampeg scrambler pedal that you may or may not have heard of. Its a pretty high gain sounding fuzz box that can also do an octave up buzz. One knob controls the wave shape and one blends the new sound with the clean sound.

The PCB was made from the tonepad layout. However better bypass circuitry was added.

I perf boarded this pedal about 4 years ago and it worked but the output was so low. I asked about this on a forum and the situation was left unresolved.

the thread can be seen here.

Anyways years later here we are making another one and guess what? same deal. The output is about .6 volts. When you blend to the clean side its going to be at a volume level equal to whatever is coming in. Which in the case of any instrument I have is at least 2 volts and in terms of guitar more like 3 (courtesy of a dimarzio X2N). This works out to around a 12dB drop when the effect is switched on.

Quick searches of the tonepad comments and the DIYstompbox forum revealed that other people have noticed as well.

here and here

If you had low output pickups you might not notice. square waves sound louder than sine waves so the distorted sound will sound louder then a clean guitar signal with the same amplitude. (Rms of a sine wave is approx .707 * Amplitude, Rms of a perfect square wave = Amplitude) .

A lot of people are mentioning building a booster in the same box to bring the volume up. This is poor solution for a few reasons. This circuit calls for 3 high gain darlington transistors and has 2 gain stages and a buffer section. I shouldn't need to add anything to bring the level up.

The problem can be resolved by modifying the final output stage comprised of Q3 and Q4. They are setup as a differential amplifier with a single ended output. Without doing to much math to calculate A (gain) = Rc / re'. Where Rc is the 1K resistor coming from +9v to the collector of Q4.

To calculate re' you must find the DC emitter current and so on. This is not necessary at this point as we can see from the equation in the last paragraph that Rc and A are directly proportional so increasing Rc will increase the gain. It will also however decrease the current through the transistor but this is of no consequence for our purposes.

I put mine up to 39k which gave me about 2.0 - 2.4 v out depending on the wave shape from a 3 v sine. (.707 * 3v = 2.12V so it should "sound" like about the same level). Since the clean part of the blend does not pass through this final differential gain stage its volume is unaffected. It would seem the problem is solved. I would suggest for anyone doing this to use a trim pot to find quickly they're preferred output level, 50k or 100k should be fine.

A millennium bypass was implemented after the board was made so thee are no traces for it.


Heres a video of the effect on a sine wave. The top trace is the output and the bottom is the input measured straight off the function generator. It gives a good idea of how the octave up is generated. First the effect is off. Its switched on when you see the square wave. Then the tone knob is twirled, then blend, then a bit of one and the other.