Many vintage guitar amps were engineered with the tech and maintenance in mind.
Fisher 800C consistent hum. Help needed!
- Thread starter LPCOLLECT
- Start date
kvflyer
Super Member
I offer some information that can be relevant to four section electrolytic capacitors. HH Scott used a lot of them having the two cathode resistor bypass capacitors inside the can with the two B+ filter sections. I have rebuilt (or replaced with JJ Capacitors, clamp mount) them by using the B+ sections only. Then, at the tube, I have placed the cathode resistor bypass capacitor. Today, the electrolytic capacitors are tiny in comparison to the "vintage" electrolytics. These bypass capacitors are small in value and voltage, being something around 25 µFD @ 25 VDC. See the example showing "C7". Just another option. Often, the resistor, R13 in this example is located on the capacitor, going from a lug on the capacitor to a lug on the mounting ring. That resistor can also be placed at the tube.
Just a thought.
EDIT REASON: To add a before picture.
Just a thought.
EDIT REASON: To add a before picture.
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kvflyer
Super Member
This is a different amplifier but an example of the "After" capacitor and resistor installation. The electrolytic capacitor on the left is a clamp mount used for B+ only and at the tube socket, you can see an example the bypass capacitor and resistor. Plenty of room for this modification.
trainbuffton
Active Member
I've done these this way too, it's acceptable because the small electrolytic is light enough that it doesn't stress the pins of the socket, and cheap/common enough that if it does fail due to heat in 20 years, who cares, slap in a new one and party on!
its also attached to something at both ends. The ones that bug me are where you see some big chunky cap hanging from just one lead and the other has a connection with 400 volts on it simply floating in space. I've done it for quick diagnostics but its not a great way to make a permanent repair.
Did you decide to keep the carbon comp resistors in the power rail, or was that the owner's decision?
Just a query. Thanks!
kvflyer
Super Member
I left it as original as possible as long as they measured within tolerance. I (unfortunately) have found that many "purchasers" think that amplifiers loose their "sound" if you use new parts. All of these pictures are of amplifiers that were passed on to new owners.
Additional comments:
I have found that vintage Ceracaps that seem to test OK with no leakage are still suspect. When I test them for leakage, the often pass with flying colors. But when measured for value, using a vintage Cornell Dubilier (restored of course) Capacitor tester, the edges of the eye tube are not crisp. Rather they are fuzzy. An engineer that I know says that it is too much dielectric dissipation caused by the Mylar breaking down in old capacitors.
But some Ceracaps that were installed toward the end of Scott's vacuum tube era are still good. Pyramid or "Bumble Bee" capacitors are always no good. Yes, they could be left on the tone controls because of the low voltage. But that's like putting a resistor across every capacitor. When replaced, there is a noticeable difference.
Additional comments:
I have found that vintage Ceracaps that seem to test OK with no leakage are still suspect. When I test them for leakage, the often pass with flying colors. But when measured for value, using a vintage Cornell Dubilier (restored of course) Capacitor tester, the edges of the eye tube are not crisp. Rather they are fuzzy. An engineer that I know says that it is too much dielectric dissipation caused by the Mylar breaking down in old capacitors.
But some Ceracaps that were installed toward the end of Scott's vacuum tube era are still good. Pyramid or "Bumble Bee" capacitors are always no good. Yes, they could be left on the tone controls because of the low voltage. But that's like putting a resistor across every capacitor. When replaced, there is a noticeable difference.
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Yes. Floating in the ether is that "thing" about how vintage carbon comp resistors (especially Allen-Bradley) possess that Magic Juju — and replacing them will lead to the displeasure of the Sound Gods.
Today, I replace the resistors in the power supply with modern metal oxide or wire-wound ones, or I will not redo the power caps. It's a matter of liability. I have not heard anyone complain about wanting the old carbon comps back in the amp. Now, carbon comps in the signal path are an entirely different argument. Some people steadfastly refuse to change to metal oxide no matter how much the information says MOs are better, quieter, longer-lasting resistors. This is despite how out of spec the old CC resistors measure. To them, it's part of the "aging" of the amp, like the "aging" of a fine wine.
So for that owner, they stay put, with my caveat.
And I'm running out of my stash of NOS AB resistors.
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kvflyer
Super Member
You no doubt have noticed that HH Scott used 1-Watt AB resistors as plate resistors in many of their amplifiers. Many other manufacturers only used ½-Watt resistors. Just about all of those AB resistors in an HH Scott amplifier are still within tolerance.
I am fond of Allied Radio Knight Kit amplifiers as well. Maybe because I grew up with them and as a teenager could afford them. They used ½-Watt resistors and many, many are way increased in value. You can guess what happens when I come across them.
I am fond of Allied Radio Knight Kit amplifiers as well. Maybe because I grew up with them and as a teenager could afford them. They used ½-Watt resistors and many, many are way increased in value. You can guess what happens when I come across them.
Princeton? Not Showman, could be a Bandmaster…..These companies should have done what Fender did, putting the cap under a doghouse on top of the chassis. Replacing these caps is so easy.
Doghouse power caps.
Doghouse cover.
Here's a bit of Fender trivia: All the caps in that photo have the same polarity — therefore, it's probably a Deluxe Reverb or Vibrolux Reverb amp. In this case, it's a DR. Most 6V6GT Fender amps used cap cans.
Nearly all of the big boy 6L6GC amps had the first two caps reversed. MANY blown recap jobs were due to inattentive technicians.
Bassman caps.
Super Reverb.
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I knew that. I thought DR after I typed….reason I figured possibly Princeton was smallish output transformer.Here's a bit of Fender trivia: All the caps in that photo have the same polarity — therefore, it's probably a Deluxe Reverb or Vibrolux Reverb amp. In this case, it's a DR. Most 6V6GT Fender amps used cap cans.
Nearly all of the big boy 6L6GC amps had the first two caps reversed. MANY blown recap jobs were due to inattentive technicians.
Bassman caps.
Super Reverb.
I learned something new today, thank you! With the split load phase inverter, and multi section can cap, you would think it was a mono hifi amp.
Those Blackface and Silverface era Princeton Reverb amps used the other half of the 12AX7 as the Tremolo (Vibrato) driver. Click on the image to enlarge it.
The “bias wiggle” tremolo; I tried that circuit on a Vibroverb clone. I didn’t see any benefit, but the guy that bought it seemed to like it. It did seem to overdrive different, probably because no 50K load of the optocoupler type. I think that I went considerably higher value, don’t recall.Those Blackface and Silverface era Princeton Reverb amps used the other half of the 12AX7 as the Tremolo (Vibrato) driver. Click on the image to enlarge it.
I like Princeton's bias waggle tremolo. It's a bit smoother than the amps with the optocoupler bugs.
The best tremolos are the tube-driven oscillators. Those amps can require two or even three extra 12AX7 tubes, so they fell out of favor from manufacturers due to the extra chassis space taken and the cost. However, the tone of these amps is so tubular!
My 1992 Fender Pro amp and Reverb units.
Click on the image to enlarge.
I have replaced the master volume with that variable feedback “presence “ circuit on a couple of late silver face Twins….I like Princeton's bias waggle tremolo. It's a bit smoother than the amps with the optocoupler bugs.
The best tremolos are the tube-driven oscillators. Those amps can require two or even three extra 12AX7 tubes, so they fell out of favor from manufacturers due to the extra chassis space taken and the cost. However, the tone of these amps is so tubular!
My 1992 Fender Pro amp and Reverb units.
Click on the image to enlarge.