Rauland Borg 6L6 amp Need Info

[desperateaudio]

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.

[kevinkr]

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.

[Tom Bavis]

There's a Rauland 426A listed in Sams Photofacts.. no 4260.

[desperateaudio]

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.

[desperateaudio]

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!!!

[kevinkr]

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!

[PRNDL]

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.

[desperateaudio]

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.

[desperateaudio]

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.

[SY]

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?).