Here's the desert island amp for many of us and I think after all these years I'm one to be on that island myself. A really clean 1964 Fender Deluxe Reverb.
About the amp: Transformers are original, speaker was replaced with a Celestion G12M greenback 30 watt. I'm a fan.... Amp was re-capped well enough. All of the electrolytic caps were top shelf expensive Sprague Atoms. Some of the work wasn't exactly tidy but done well enough.
But, I just didn't like the amount of hum I was still getting. I right away saw some things to improve. First is the bias supply cap:
Whoever re-capped this amp went with the original value. I mean, why wouldn't you. That's what Fender used in 1964 so....
Nah. When I worked for Jeff Bober in 1994 he just told me to upgrade it to 100@100v rather than 25@50v. I've been doing it ever since. Better regulation and less hum. I do not detect any change to the sound of the amp and I have well trained ears. And for what it's worth Fender upped it later to 70uf then 100uf cause they could. Caps got a little better. So just do it. If you think that's blasphemy try it anyway and put the old value back in if it freaks you out.
Yup, being dramatic here but I'll share a story. I once had a client who paid me to convert an AB165 Bassman to an AA864. That's no small task. Big job. I made one mistake and one executive decision. He took the amp home and inspected the work under a microscope. The next day I had an irate customer who brought his amp back and he (understandably) expressed his trust had been burnt. So I gladly changed the one pot value that I missed to the correct one. But he was really upset about my little executive choice: using a 100@100v for the bias supply. I explained to him why I did this: with a 25@50v cap it will hum. He didn't believe me one bit and I could tell I was losing his business.
So we gave it a listen, I then replaced it with a 25uf cap.... HUM! And not a teeny tiny difference. It was significant enough that he heard it and looked a bit embarrassed. He asked me if it would make a tonal difference and I said no but if he thinks it does bring it back in a week. I put my little 100uf cap back in, fired it up and he was happy with the quiet. I never saw him again. All is well that ends well!
So here ya go. Use a pretty blue Philips Vishay cap if you will! I like them cause they look nice and European:
But wait! We're not done yet. Amp was much improved but I still wasn't 100% satisfied. This next part involves the signal caps.
If you have been inside as many old Fender and other high quality amps you'll notice even the signal caps have a direction. No they're technically not polarized like an electrolytic cap is, but there is a direction you want them to go in. The have an inside foil and an outside foil. Some old caps like my favorite Ajax blue capacitors you find in Fender amps even tell you which side is the outside foil.
Rule of thumb: always have the outside foil closer to ground. In other words, if one side of the cap is on the 100k plate load resistor and the other is connected to the 1Meg volume pot, outside foil goes on the 100k resistor. Fender almost always got that right! I've only seen a few exceptions.
Another example is the tone stack. The .1 bass cap goes to a 250k pot and the .047 mid cap goes to a 6.8k resistor. On the other side is the 100k slope resistor then the 100k plate load resistor for a total of 200k. So the bass cap should have the outside foil attached to that slope resistor and the mid cap should be going the opposite direction with the outside foil connected to the 6.8k resistor on the bass cap. Like this:
The phase inverter has three .1 caps. Two are connected to the plates of the 12AT7 while the third is connected to the grid. On the plate load side you have a 100k and an 82k resistor going to each plate. On the other side you have two 220k resistors going to the bias supply. So the outside foil goes to the plates, inside to those 220k resistors. The third cap however is connected to a 1meg resistor on one side and a 47 ohm resistor going right to ground on the other side. So that cap goes in the opposite direction. In this case the previous owner used my least favorite cap, the Sprague 715P. No indication for inside or outside foil so you just read it from left to right. It will look like this:
Notice how the print is going in the opposite direction.........
Here's the dry channel tone stack. I had a spare old polyester orange drop cap that has a stripe on it for outside foil. The mid cap you'll notice reads left to right with the left going to the 6.8k resistor. If you are experienced you'll notice the .047 600v blue Ajax cap for the final stage. Sadly entire channel was orange dropped. But I have a bag of .047 Fender "Death Caps" from the AC balance. I never throw these out when I ground an amp. I save them for classy amps like this one!
One cap backwards will not make all that much of a difference but in this case I counted 5. It adds up. In the hundreds of old Gibson and Fender guitars I've seen over the years, the tone caps are almost always done in this arrangement: outside foil to ground. If the good people at those companies were sticklers for such a detail on the treble bleed cap, there's something to it.
So there you have it. The amp behaves like my own '69 model now. Quiet! Of course it's not possible to eliminate all hum and noise but it is possible to reduce it using these little steps. And the less noise the more music dig?!
Thanks for reading, hope this was helpful and feel free to leave a comment of ask a question below! J
