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|16th May 2012, 05:38 AM||#1|
Join Date: Feb 2009
Location: Minneapolis, MN 55404
SOLD! Vintage Fisher 500C Receiver with New Output Tubes $575 SOLD!
The Fisher 500C may be the best sounding vacuum tube receiver ever made.
This Fisher 500C in great condition with walnut case! All of the original brass knob trim is intact. You can leave the original patina if you prefer or you can polish them as you see fit. As you can see the faceplate is in excellent condition. All of the indicator lamps work.
I did not to anything to the cabinet. It could probably use oiling and a bit of touch up staining. I would have done it myself, but some people are very picky about how to care for the wood cabinet, so I thought it best just to leave it up to the buyer to do their way.
I can send you additional photos upon request.
All of the tubes will be removed for shipping. Each tube has blue 3M masking tape with the corresponding tube number written on it so you know where to install it.
I would recommend that the walnut cabinet be shipped separately from the chassis. I will not be responsible for damage to the case during shipping if the amplifier and case are shipped together.
It should be mentioned that I am an electronic technician and have been repairing electronic equipment for over 40 years.
Some restoration, improvements, and basic preventative maintenance has been performed.
What was done:
1a. An SDS Audio Labs electrolytic capacitor replacement board was assembled and added.
2a. The B+ rectifiers were replaced.
3a. The bias supply selenium rectifiers were replaced with silicon rectifiers.
4a. The coupling capacitors in the power amp section section were replaced with polypropylene capacitors.
5a. The electrolytic cathode resistor bypass capacitors in the power amp section were replaced.
6a. The output tube grid bias resistors were changed from 330Kohm to 267Kohm.
7a. The grid stopper resistors on the output tubes were replaced.
8a. Resistors were added to the output tube cathodes to act as fuses in the event of a tube short and also to facilitate the measurement of the output tube bias current.
9a. Test jacks for voltmeter leads and bias adjusting trimpots were added.
10a. Resistors were added in series with the filament windings to reduce the filament voltage back down to 6.3 VAC.
11a. The filament wiring to the output tubes was modified.
12a. The tubes were tested and replaced as needed.
13a. The output tubes were replaced with new matched JJ brand 7591's.
14a, The voltage amp and phase splitter tubes in the power amp section were replaced with new matched JJ brand 12AX7A tubes.
15a, The aging accessory AC receptacles in the rear were replaced with new polarized receptacles.
16a, The AC power cord was replaced with a polarized power cord.
17a. The switches and controls were cleaned.
Long winded explanation of why this was done:
1b. The electrolytic filter capacitors in vintage amplifiers are usually dried out and in need of replacement. Today's electrolytic capacitors are also vastly superior to the older capacitors and they improve the sound quality of the amplifier. The easiest way is to replace the filter caps is with the SDS Labs capacitor replacement board which I assembled and installed.
There was no need to leave the original output capacitors on the top of the chassis, except for cosmetic reasons, as some people prefer the stock look. Some people remove the guts of these capacitors and install modern capacitors inside. This is referred to as stuffing. I won't do it because it leaves the capacitors without any airflow and they could run too hot. It's better to install the replacement capacitors inside the chassis.
2b. The original B+ rectifiers were weak and prone to failure so they were replaced with new larger silicon diodes.
3b. When the stock selenium rectifiers get weak they don't produce enough negative bias voltage and too much bias current flows through the output tubes causing them to fail prematurely. The SDS Cap board replaces the troublesome selenium rectifiers with more reliable silicon diodes.
4b. The stock mylar coupling capacitors are known to fail with age and will cause an output tube to fail. Replacing them with modern polypropylene improves reliability and improves the sonic quality.
5b. The cathode bypass electrolytic capacitors are usually dried out and need of replacement.
6b. Fisher exceeded the maximum value for the grid bias resistors that was specified by the output tube manufacturers. This excessively high resistor value can cause the output tube control grids to become positive and allow too much plate current to flow and damage the output tubes. Why they often got by with these higher resistance values with hte tube os yesteryear, the 330Kohm resistors might not work well with today's output tubes. The output tube coupling capacitor values were also increased slightly to compensate for the lower grid bias resistors.
7b. The grid stopper resistors on the output tubes were replaced for reliability. A failed grid stopper resistor can cause an output tube to fail.
8b. In the case of a tube short, the added 10.0 ohm bias resistor will burn out like a fuse protecting the output transformers. Better a 10 cent resistor failing and an expensive output transformer.
A single 10.00 ohm resistor was added in series with the cathodes of each pair of output tube pair. By using a singles cathode resistor shared by each pair of tubes, additional differential action is added and distortion is reduced slightly.
The output tube cathode resistors allows the output tube bias current to be easily measured.
9b. The stock Fisher 500C did NOT have any provision to measure the output tube bias current, nor did it have any way to adjust the bias current except for a service technician physically changing resistor values, which is times consuming and expensive.
Test jacks for your voltmeter leads and bias adjusting trimpots were added to make output tube bias current adjustment easy. Only one bias adjustment trimpot is used per channel with one 10.0 ohm resistor shared by each pair of output tubes. An individual bias trimpot per output tube is a solution looking for a problem. In fact it can make matters worse. Even if you adjusted the bias current for each output tube individually, the output currents would not necessarily be matched at high current levels during loud passages. The output tubes need to be matched so that their idle bias currents are matched and also that the output currents are matched at high signal levels. So it's best to use a single cathode resistor per pair of output tubes and one bias pot per channel and use matched output tubes.
10b. The filament voltage runs high in vintage equipment because it was designed for lower powerline voltages than we use today. Some method needs to be used to lower the filament voltage to the correct value. Excessive filament voltage causes your tubes to fail prematurely.
11b. The stock filament wiring allowed unequal voltages to the filaments of the output tubes. If the filament voltage on each tube is not identical the cathodes can run at different temperatures and change the characteristics of the tube. Not matter how well matched your tubes are, if the filament voltages on them are not identical the tubes will no longer be matched. It's a simple matter to reconnect the filament winding of the power transformer to feed opposite end of the output tube section. This makes the filament voltage on each tube socket IDENTICAL and allows the output tubes to stay matched. Fisher goofed. This is a common problem with most vintage stereo equipment. WHAT were they thinking? This improved filament wiring technique is even mention in output tube data sheets.
12b. Replaced worn out tubes as necessary.
13b. The original output tubes were worn out and in need of replacement.
14b, The voltage amp and phase splitter tubes in the power amp section were worn out. While not required, using matched tubes here will help keep the open loop gain similar between channels.
15b. The accessory receptacles were cracked and in need of replacement. Polarized receptacles were added to allow the use of modern equipment with polarized power cords.
16b. It's required electrical code to to supply polarized receptacles with a polarized source of power.
17b. Switches and controls get dirty, especially as they get older. Spraying them with a cleaner/lube makes them work properly and the lube adds a protective film to lubricate contacts and reduce the formation of oxides.
Last edited by Scott Novak; 7th September 2013 at 07:53 PM. Reason: SOLD
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