RCA console amp RS-199C
- Thread starter HBrown
- Start date
the yellow/green fuzz on the top of the chassis is cadmium which is oxidizing. its poisonous, be very careful. read up on cadmium poisoning, use rubber gloves and a face mask if you try to remove it. Please wash your hand after handling the amp and before eating.
Yes, that's exactly right.
With the amp on but no sound to the input, you should get about 1.2v DC across that 1K resistor, afterward, plus or minus a bit, if everything is OK.
Regards,
Gordon.
HBrown
Active Member
Thanks Gordon. I made the swap, and she's alive. I performed the voltage check you suggested across the 1k resistors: 1.19V one side, 1.10V the other. When I did the check, with no signal applied I notice I am getting a bit of a hiss through one channel. When I plug in the RCA jacks from the source, with no signal playing the hiss goes away.
The overall volume of the rig seems a bit louder after the swap, but it's not a great difference. The sound quality is well balanced and pretty nice to listen to. It may be that I'm getting about all the volume a tired set of 6BQ5's is good for. It's certainly sufficient for near field listening in a small space.
The overall volume of the rig seems a bit louder after the swap, but it's not a great difference. The sound quality is well balanced and pretty nice to listen to. It may be that I'm getting about all the volume a tired set of 6BQ5's is good for. It's certainly sufficient for near field listening in a small space.
HBrown
Active Member
I've been listening to it some this week. I'm digging the sound I'm getting from this little amp. Anything else anyone see that I could do to improve more? What would be a robust modern replacement for the power supply rectifiers in this design? I think I'm going to order replacements for the remainder of the resistors and caps and then rewire this thing a bit neater. I've just been throwing any old parts I had on hand at it.
HBrown
Active Member
Thanks Gordon. I am going to try it in the console this weekend. I will be sure the volume knob is all the way down.
I am wondering if I still have something wrong hobbling the output somewhat. I do feel the tube swap added some small amount of volume, but I'm not sensing the drive gain increased by a factor of three. I'm running it from a CD player that has no problems driving most amps to full volume so I'd say I'm close to, if not definitely at the 2V of signal.
I also tried it with the CD through a tube-driven buffer (has a volume control, but no significant gain addition) I have which drives most amps just fine.
I'll drag out a true Carver pre-amp I have and see if that may drive it a bit harder.
There are still some resistors and caps that I have not replaced that could be drifted in value, but I had not checked as I was getting good sound. I'll go thru again and check everything I can.
I am wondering if I still have something wrong hobbling the output somewhat. I do feel the tube swap added some small amount of volume, but I'm not sensing the drive gain increased by a factor of three. I'm running it from a CD player that has no problems driving most amps to full volume so I'd say I'm close to, if not definitely at the 2V of signal.
I also tried it with the CD through a tube-driven buffer (has a volume control, but no significant gain addition) I have which drives most amps just fine.
I'll drag out a true Carver pre-amp I have and see if that may drive it a bit harder.
There are still some resistors and caps that I have not replaced that could be drifted in value, but I had not checked as I was getting good sound. I'll go thru again and check everything I can.
HBrown
Active Member
Everyone, It's been a year since my original run at sorting this little amp out. I like the sounds it makes and the size of the chassis is convenient. But even with the 6DT8 it just doesn't have enough drive. It is marginally acceptable driven from a Carver pre-amp but I want to be able to use the amp with a portable music player. As-is, this little rig has less output than a smaller SE chassis I have with 50EH5 tubes. I'd like to explore what I could do to add an additional gain stage and maybe a volume pot to it.
I have been studying the schematic at the beginning of this thread for the 199B that we were using as a go by. My amp is the 199C, but I never did find the specific 'C' schematic. I'm not the world's best at deciphering point to point wiring, but I'm getting better. I have to stare at the amp and schematics for long periods to even find some of the circuit features. I need to document any slight differences in how my 'C' variant differs from that 'B' in the schematic.
I do have a stash of parts from several 6BQ5 amps that I can scrounge parts from.
So, what tube complement should I use and does anyone have a schematic for a proper input stage for this amp?
Also, would it be feasible at all to transplant a power transformer and rectifier from a zenith 6BQ5 SE amp into this amp and get rid of the voltage doubler power supply? What all would that entail?
I have been studying the schematic at the beginning of this thread for the 199B that we were using as a go by. My amp is the 199C, but I never did find the specific 'C' schematic. I'm not the world's best at deciphering point to point wiring, but I'm getting better. I have to stare at the amp and schematics for long periods to even find some of the circuit features. I need to document any slight differences in how my 'C' variant differs from that 'B' in the schematic.
I do have a stash of parts from several 6BQ5 amps that I can scrounge parts from.
So, what tube complement should I use and does anyone have a schematic for a proper input stage for this amp?
Also, would it be feasible at all to transplant a power transformer and rectifier from a zenith 6BQ5 SE amp into this amp and get rid of the voltage doubler power supply? What all would that entail?
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I'm not an expert but you maybe able to copy the front end off of a similar amp that runs 12ax7 tubes. I also have one if these amps so I'm watching this thread closely lol.
HBrown
Active Member
That is my thought as well. I'm looking at a bunch of schematics trying to find something close enough. I need to buy some tube amp books and do some serious reading. There are a number of threads on this site and others on this chassis, but none where the low gain was ever sorted in a manner that I can follow to a working conclusion.
BinaryMike
Pelagic EE
Refer to the Resistance-Coupled Amplifiers chapter in an RCA Receiving Tube Manual. Like this one: http://www.tubebooks.org/tubedata/RC30.pdf
Greetings from an RS-199 fan ! I found this thread a bit late, but these amps can easily have very fine sound quality. Maybe, I can help...Before more modifying, perhaps a little rearranging of some parts will bring HBrown some satisfying sonics...
In stock form, these can sound lifeless, mostly due to the feedback arrangement. With the voice coil directly fed back to the 6CG7/6FQ7 cathode resistors, nice sounding gain will be tough to obtain. Whether an A, B or C version, the opt trannies are usually the same or very similar. Regardless of schematics available, we have to deal with the actual circuit on hand. So, as I have seen more than a few of these amps, some simple rearranging of the input tube's cathode resistor and capacitor arrangement is the first obstacle to overcome. RCA chose 3300 Ohms in parallel with 100 pf (tone cap and RF suppressor) as each cathode's load. Unfortunately, they "loaded down" this RC arrangement with a large, 2W 47 Ohm resistor and the voice coil secondary as the feedback and ground connection. More importantly, the input tube's 3300 Ohm cathode resistors are "floated" above the voice coil with the 47 Ohm in parallel to ground. Thus, the 47 Ohm is across the voice coil winding. It's no wonder we need more gain...Perhaps, within the console this sounded acceptable using RCA's chosen speakers. However, this cathode R-C and feedback arrangement literally restricts any gain possible when we want to use the amp as a stand alone.
If you can imagine the stock wiring, by lifting the 3300 Ohm from the small 100pf ceramic cap at the feedback connection (white wire), this could allow the 3300 Ohm as the cathode load, if we rearranged that lifted end and connected each 3300 Ohm from cathode to the ground buss. RCA always used a cool ground buss. Also, for simplicity at first, disconnect each 2W 47 Ohm from the connection point of the yellow speaker leads, which is also the white wire feedback connection. Simply bend back the 47 Ohm leads and safe them off. Then, we can now imagine that each 100pf appears as the feedback connection. Install a 33K or 22K resistor across each 100 pf. Therefore, each 6CG7/6FQ7 cathode will now have a 3300 Ohm cathode R. The white wires for feedback will have a 100pf in parallel with 22K or 33K (33K for less feedback and very open sounding IMHO, 22K for a bit mellower sounding...Try both feedback R values and you decide depending on your chosen speakers...). This rewiring will undoubtedly put a very large smile on your face. Then, we can concentrate on beefing up the power supply...
RCA was cost conscious, way too cost conscious. Their power supplies were usually wimpy, with small values of power supply reservoir capacitance. Two 100 uf voltage doubler caps "in series" means only 50 uf after the doubler diodes. The diodes should be replaced, upgrading to at least UF4007 diodes for safety and long life. Those ceramic caps across the original diodes can now be removed permanently as genuine UF4007 diodes produce little to no RF hash. Also, replace those old, dried out 100 uf doubler caps and increase their value to at least 200 uf each. Two 470 uf/200+V caps are even better. Bass impact and control will become dramatic sounding, very dynamic. Continuing this beefing up, the two 80 uf caps in that large multi-electro are now very old. You can either parallel newer electro caps to increase their cap value and ESR strength, or lose the multicap and install new, discrete cap parts. There really is plenty of room in these chassis for very beefy, high capacitance supply caps.
The shared 6BQ5 cathode R is 68 Ohms which is bypassed with a (now very old) 100 uf/25V cap, also within the multi-electro cap. While many designers prefer separate cathode bias resistors with separate bypass caps, if you choose this route, you will need two 136 to 140 Ohm 2W or larger cathode resistors and two 100uf/25V bypass caps. Try the green Nichicon Muse BP or Elna Silmic II types. Find the right supplier, as there are too many counterfeit caps out there these days. It's worth paying a bit more at Digi-Key for genuine caps. These specific types will undoubtedly sweeten the sound. Besides RCA saving $ with shared channel cathode bias, they probably did not foresee the possibly sweeter sound of this shared channel topology. By separating the cathode bias resistors and bypass caps, the sound will become a bit leaner or colder to some ears, yet possibly more accurate sounding.
Voltage doublers allow very high capacitance, power supply caps. This beefy, power supply "reservoir" can yield awesome sounding bass impact combined with lovely bass detail. Once installed and tried, you will easily forget about converting to a tube rectifier for these amps. Remember that these amps were built when the wall voltage was only up to 115VAC. Plugging into a so-called 120 VAC outlet, which probably reads closer to 125VAC these days, will have that power tranny run very hot, very quickly. Vintage amps need an isolation tranny, a bucking winding for the primary, or more simply a Variac. 113 to 114 VAC in is safe and provides many hours of sweet sounding listening....
In stock form, these can sound lifeless, mostly due to the feedback arrangement. With the voice coil directly fed back to the 6CG7/6FQ7 cathode resistors, nice sounding gain will be tough to obtain. Whether an A, B or C version, the opt trannies are usually the same or very similar. Regardless of schematics available, we have to deal with the actual circuit on hand. So, as I have seen more than a few of these amps, some simple rearranging of the input tube's cathode resistor and capacitor arrangement is the first obstacle to overcome. RCA chose 3300 Ohms in parallel with 100 pf (tone cap and RF suppressor) as each cathode's load. Unfortunately, they "loaded down" this RC arrangement with a large, 2W 47 Ohm resistor and the voice coil secondary as the feedback and ground connection. More importantly, the input tube's 3300 Ohm cathode resistors are "floated" above the voice coil with the 47 Ohm in parallel to ground. Thus, the 47 Ohm is across the voice coil winding. It's no wonder we need more gain...Perhaps, within the console this sounded acceptable using RCA's chosen speakers. However, this cathode R-C and feedback arrangement literally restricts any gain possible when we want to use the amp as a stand alone.
If you can imagine the stock wiring, by lifting the 3300 Ohm from the small 100pf ceramic cap at the feedback connection (white wire), this could allow the 3300 Ohm as the cathode load, if we rearranged that lifted end and connected each 3300 Ohm from cathode to the ground buss. RCA always used a cool ground buss. Also, for simplicity at first, disconnect each 2W 47 Ohm from the connection point of the yellow speaker leads, which is also the white wire feedback connection. Simply bend back the 47 Ohm leads and safe them off. Then, we can now imagine that each 100pf appears as the feedback connection. Install a 33K or 22K resistor across each 100 pf. Therefore, each 6CG7/6FQ7 cathode will now have a 3300 Ohm cathode R. The white wires for feedback will have a 100pf in parallel with 22K or 33K (33K for less feedback and very open sounding IMHO, 22K for a bit mellower sounding...Try both feedback R values and you decide depending on your chosen speakers...). This rewiring will undoubtedly put a very large smile on your face. Then, we can concentrate on beefing up the power supply...
RCA was cost conscious, way too cost conscious. Their power supplies were usually wimpy, with small values of power supply reservoir capacitance. Two 100 uf voltage doubler caps "in series" means only 50 uf after the doubler diodes. The diodes should be replaced, upgrading to at least UF4007 diodes for safety and long life. Those ceramic caps across the original diodes can now be removed permanently as genuine UF4007 diodes produce little to no RF hash. Also, replace those old, dried out 100 uf doubler caps and increase their value to at least 200 uf each. Two 470 uf/200+V caps are even better. Bass impact and control will become dramatic sounding, very dynamic. Continuing this beefing up, the two 80 uf caps in that large multi-electro are now very old. You can either parallel newer electro caps to increase their cap value and ESR strength, or lose the multicap and install new, discrete cap parts. There really is plenty of room in these chassis for very beefy, high capacitance supply caps.
The shared 6BQ5 cathode R is 68 Ohms which is bypassed with a (now very old) 100 uf/25V cap, also within the multi-electro cap. While many designers prefer separate cathode bias resistors with separate bypass caps, if you choose this route, you will need two 136 to 140 Ohm 2W or larger cathode resistors and two 100uf/25V bypass caps. Try the green Nichicon Muse BP or Elna Silmic II types. Find the right supplier, as there are too many counterfeit caps out there these days. It's worth paying a bit more at Digi-Key for genuine caps. These specific types will undoubtedly sweeten the sound. Besides RCA saving $ with shared channel cathode bias, they probably did not foresee the possibly sweeter sound of this shared channel topology. By separating the cathode bias resistors and bypass caps, the sound will become a bit leaner or colder to some ears, yet possibly more accurate sounding.
Voltage doublers allow very high capacitance, power supply caps. This beefy, power supply "reservoir" can yield awesome sounding bass impact combined with lovely bass detail. Once installed and tried, you will easily forget about converting to a tube rectifier for these amps. Remember that these amps were built when the wall voltage was only up to 115VAC. Plugging into a so-called 120 VAC outlet, which probably reads closer to 125VAC these days, will have that power tranny run very hot, very quickly. Vintage amps need an isolation tranny, a bucking winding for the primary, or more simply a Variac. 113 to 114 VAC in is safe and provides many hours of sweet sounding listening....
