Hey everyone. I have something of interest that I can not find any info on no matter where I look on the web. I have checked evey forum and even the guy that sold them to me has no idea. I have a pair of Rauland-Borg push pull tube amps. They are identical 6L6's on the outputs and what appears to be 6SL7 as the driver and 6SN& as the phase splitter with an interstage transformer between that nad the 6L6's. Problem is when I try and plug up to the input, which by the way was originally via a male octal plug into the normal female octal socket she will not play. All voltages check out perfectly at every location you expect them to be. All the tubes are new. The model number on these amps is RA4260A. I am drawing up a schematic and I will post it when finish. The input arrangement on the 6SL7 is very odd. The input is to one of the grids (Pin 4), both cathodes (Pins 3 & 5) are tied together and dropped to ground via a 1K and 33K resistors in series, both are by passed via a 40uf 150vdc cap. Then there is a 470K resistor from the 245vdc supply from the load resistors of the 6SL7 going between the 1K and 33K cathode resistors. The plates (Pins 2 & 5) are loaded via 100K resistors from that same 245vdc supply. From each plate of the 6SL7 you have a .1uf cap going to each grid of the 6SN7. Then from the pin 5 plate (Input Section of the 6SL7) after the .1uf cap at the Pin 4 grid of the 6SN7 you have a 220K resistor going back to the Pin 1 Grid of the 6SL7 which also has a 56K resistor going to ground but from the Pin 2 plate of the 6SL7 after the .1uf cao you just have a 220K resistor going to ground. In all my research of my catalog of schematics as well as my RCA Radiotron Designer Handbook and can no input circcuit that looks like this. Please help. Thanks in advance.
Sounds like it might be a modified paraphase splitter, but definitely do post your schematic. (It could be something entirely different and your schematic should make it clear as to what it actually is.)
Do these amplifiers use fixed bias in the output stage and if so does the bias supply possibly feed anything else in the front end?
What are the dc voltages on 6SL7 pin 1 and 4? They may not be at ground potential, and if that is the case a coupling cap from the octal socket to pin 4 of the 6SL7 could be required.
Do these amplifiers use fixed bias in the output stage and if so does the bias supply possibly feed anything else in the front end?
What are the dc voltages on 6SL7 pin 1 and 4? They may not be at ground potential, and if that is the case a coupling cap from the octal socket to pin 4 of the 6SL7 could be required.
I will get the schematic up by Monday at the latest. I am way this weekend. As for fixed bias the answer is yes. It uses two 5U4GB's for the 6L6/OPT voltage and two 5Y3GT's. One for the negative bias voltage and one for the input and driver voltage.
As for the vdc on the 1 & 4 of the 6SL7 I will check when I get home Sunday afternoon and post.
There is a very strange arrangement at the input. There are two wires from the input socket to a dual 500K volume control. Then one of the two wires from the volume control goes to pin 4 of the 6SL7 and one goes to a solder post attached to nothing. All these input wires have a grounded shield casing and neither wire crosses or touches the other. I need to test the 500K pot to make sure each channel is separate.
Also as a test I placed a test clip from the input wire on pin 4 of the 6SL7 to pin 1 and I get the most clear beautiful sound. Having owned many tube amps I was pleasantly surprised. Problem is the max volume is very low. As if people were sleeping and you wanted to here music but not upset. It started me thinking that may be these amps mush use a preamp?
I'll post schematic by Monday. Love to here your thoughts and thanks for the points of view.
As for the vdc on the 1 & 4 of the 6SL7 I will check when I get home Sunday afternoon and post.
There is a very strange arrangement at the input. There are two wires from the input socket to a dual 500K volume control. Then one of the two wires from the volume control goes to pin 4 of the 6SL7 and one goes to a solder post attached to nothing. All these input wires have a grounded shield casing and neither wire crosses or touches the other. I need to test the 500K pot to make sure each channel is separate.
Also as a test I placed a test clip from the input wire on pin 4 of the 6SL7 to pin 1 and I get the most clear beautiful sound. Having owned many tube amps I was pleasantly surprised. Problem is the max volume is very low. As if people were sleeping and you wanted to here music but not upset. It started me thinking that may be these amps mush use a preamp?
I'll post schematic by Monday. Love to here your thoughts and thanks for the points of view.
Some other quick notes. I have tried different tubes as well as tested them all and problem still remains. I have replaced the bypass cap on the 6SL7 cathode and that did not fix it. I also replaced the 2 coupling caps from each stage of the 6SL7 to each cooresponding stage of the 6SN7 and the problem still remains. Schematic will be up loaded Monday as a PDF. Thanks again!!!
Seeing the schematic should answer a lot of questions, once I see it I will know the answer to some of your current questions. Have a good week-end! 😀
It sounds like the previous owner may have unsuccessfully tried to modify the amp for guitar.
The most obvious error would be using a 9-pin instead of octal pin outs for the 6SN7, which may explain the weird wiring.
The most obvious error would be using a 9-pin instead of octal pin outs for the 6SN7, which may explain the weird wiring.
Nope these were untouched. It was painfully clears that was the case when I first exammed them. Thanks for the suggestion anyway. I will return home tomrrow and place schematic up ASAP. Thanks for all the good ideas.
Here is the amp side on the schematic. The only change I made is that I installed a real RCA input jack right to Pin 4 of the 6SL7 with a 470K resistor to ground as normal. Also I changed out the .1uf, .0068uf and the 40uf electrolytic cap with new just for the heck of it. I tested the amps before any of these changes and the problem I discussed existed then and after the changes. To me it seems like the Paraphase splitter is set up for a preamp. Also if you look at the 470K resistor feeding back into the cathode of the 6SL7 that seems odd as well. I have never seen that before nor could I find it on any schematic I have looked at. Any help would be appreciated. I will post the power supply side tomorrow. Thanks again.
What plugs into that input socket? The 470k thing looks like a way to get some low current DC for biasing something (mike? preamp? heater string?).
It is an octal socket which would have received a plug. I never got the plug. The 470K is dumping 17vdc to that socket and yes I agree it powered something. I am wondering what it is doing for the cathode voltage other then elevating it to a stable number? Thanks all comments are welcomed.
I don't think it's there to stabilize the cathode voltage, just to provide volts for something else. It runs a little more current through the bottom resistor, but that's about it. Whatever it powers, it's something that draws minuscule current, which is why I thought about a microphone.
There are some other issues with the 6SL7 circuit as well - at any reasonable cathode currents the cathodes would be about 60V above ground as drawn, either the grids are not referenced to ground or that 33K more likely should be going to the negative bias supply.
The unused wiper connection doesn't make much sense either.
Can you tell whether this amplifier has been modified?
The unused wiper connection doesn't make much sense either.
Can you tell whether this amplifier has been modified?
For all intended purposes they look unmodified. Everything is old and similar in nature, construction, etc. Each amp has the same parts same manufacturers wired exactly the same way. So I think it is what it is. The schematic I posted a drew and redrew twice and double checked it to make sure that it was correctly drawn before I put it up. If you look at the oddly laid out wiring on the octal input socket it seems there is an expectation that when you pluged in the intended input socket it may have been expected that it would close a connection. Unless I am misunderstanding your thoughts the grids of the 6SL7 seem to be referenced to ground via the 500K pot and the 56K resistor? I still can't get the 470K resistor to the 6SL7 cathode? I see SY's point of view but I have not experienced this before. It appears that the 470K is doing nothing in the circuit other then the potential of powering so sort of low voltage item? As I stated earlier I changed out the input setup to be a standard RCA with 470K reference resistor to ground in place of the pot as normal procedure. I am now thinking that I should eliminate the entire cathode feed resistor setup shown and replace it with something more standard. Then rework the 6SL7 Pin 1 grid as well. Any suggestions. I would really like to keep the flavor of the original amp as best I can. I felt that since this amp seems rare and the circuit at least to me seemed odd the group would enjoy looking it over. Weight till I post the power supply. That is a real joy to look at. Thanks for all future suggestions.
My guess is that they wanted the amp to mute when the mystery plug was not inserted. The 470k+33k resistors accomplished that by biasing the 6SL7's WAY into cutoff. Something in the connecting equipment (or the mystery plug itself) shorted pin 3 to ground (pin 1 and/or 7.) This would correctly bias the 6SL7 allowing it to function normally.
Try shorting pins 3 and 1 (or 7) on the octal socket as a test. If that works then you can defeat the 'feature' by disconnecting the 1k cathode resistor where it meets the 33k and reconnecting to ground.
-- Dave
Try shorting pins 3 and 1 (or 7) on the octal socket as a test. If that works then you can defeat the 'feature' by disconnecting the 1k cathode resistor where it meets the 33k and reconnecting to ground.
-- Dave
Dave your the man. That did the trick. Now the only issue left is the secondaries. It breaks down like this. There are 6 of them. one black (Ground), then one each - white, yellow, orange, brown and green. They range in resistance from - 3.8ohms, 4.7ohms, 5.5ohms, 6.6ohms and 9.5 respectively. It has always been my experience that the lowest resistance coil on an unknown secondary setup would be say the 4 ohm tap then the 8 ohm tap and so on assuming 4 ohms is the starting place. I know I can take the tranny out run voltage in the primary then measure the secondaries and calculate. I was just wondering if anyone else remebered this old quick test as I remember it or if they have something better. Thanks again guys.
Well in a few posts back Tom called it. I got the Sams schematic which model number closely matched the amps I have and it came in this morning. I have attached it. This is the amps I have been talking with you guys about. Also as can be seen Dave was right on with his point on the input plug. Also take note of the secondaries. I have been listening on all the taps as a test and as is shown the lowest tap is 80 ohms. It is my recollection that in all reality this does not matter. The load is a reflected item. Therefore if you use an 8 ohm speaker on an 80 ohm tap you will reflect back to the primary differently. So if the primary is say 6600 ohms it may now act like it was 3300 ohms or 8800 ohms depend in the tap used and the load actually attached. Any opinions on that? It appears the server will not let me upload the PDF because it is to large. If anyone is interested let me know and I will gladly email it to you. I will try to strink it and up load later. Thanks again.
Finaaly a size I can post. I think you guys might find this interesting. My real issues is now if you put an 8 ohm speaker on one of these taps what will happen to the speaker? I have tried it for short runs (Half hour) with no ill effects. Thoughts? Thanks again folks.
Don't have time to do the math right now, but you can connect a low impedance speaker between two taps, say 80 and 125, for a reasonable ratio. Since it's only a small part of the secondary, the coupling will be poor - which costs high frequencies, (as if there are any in a PA amp...)
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