Roberts reel to reel conversion, some quick Q's

[truth_hz]

So, I'm converting an old roberts reel to reel deck. this one has a twist though. Instead of the normal ef86-12ax7-el84 layout, it has 2 6au6's as the preamp sections.

I have been unable to find any schematics or info on this type of amp. So, using the more well known akai m-7, m-8 schematics, and tracing the rest, I came up with the attached schematic.

note: this is the bare amp, I removed the filtering around the el84. I added a tweed deluxe style tone control at the 1st 6au6 stage's volume.

I haven't fired it up yet, as I just finished up wiring today. Since there is so little info on these amps, I was hoping someone more knowledgeable could look it over for me.

the only things I'm not sure about are: on the 2nd input, does it matter if I use a shorting jack to ground the tip when that input is not in use. Since it is a mixer stage I'm not too sure. Also, do you think having 2 pentode stages is going to be too much gain? the tone control should bring it down a bit, as well as those 500k mixer resistors around the 2nd 6au6.

thanks for any advice anyone has to offer me.

[truth_hz]

fired it up today. sounds really good. 2nd input works great. the 1st input has way too much gain though. to the point that it's pretty loud at 0, and unbearable by 3 on the volume control. I 've tried a few 6au6's, and I'm going to try a 6be6 heptode tonight. otherwise, I'm not quite sure what to do, maybe add a series resistor in line with the volume control...need to drop some gain.

[truth_hz]

a little update:

tried some tube swapping, the 6ba6 sounded best in v1 6be6 sounded really good wherever it was. but I wasn't really satisfied, the way the volume controls on the 2nd input were affected the sound of the other channel, and it was just a little blah...

I decided to change it around a bit, making a more traditional stage out of the 2nd pentode, giving me the ability to make it a MV. Also, since I liked the 6be6 heptode so much, I decided to try it out, using grid 3 as the 2nd input.

I haven't fired up this incarnation yet....seems like all my projects sit for days before I even have time to try 'em.

here's a hopefully more readable schematic

[Fred2202]

Wow....with those cascading pentodes you are probably going to have more voltage gain than you could ever need. More than enough to fully drive the EL84. Try putting some voltage dividers in the signal path if you cannot control the gain with the pots alone.

[truth_hz]

Wow....with those cascading pentodes you are probably going to have more voltage gain than you could ever need. More than enough to fully drive the EL84. Try putting some voltage dividers in the signal path if you cannot control the gain with the pots alone.

the 6au6 was waaaaay too much. I'm sticking with the 6ba6 front-end. the 6be6 heptode, is actually pretty low gain. the 2nd input to grid 3 of the 6be6 offers a really nice clean, low volume tone for late night practice. I live in a townhouse, so it's important to me to have a low-volume option.

I still need to try out a few more oddball tubes, 6cg6, 6bd6, 6hr6, 6cs6 etc.


the normal input is great, goes from a nice clean up to about 11 o'clock, to full out grind by 3 o'clock on the gain. I'm calling this project done. It's got a great tone.

There's even an extra 7 pin socket I was thinking of using for a 6aq5 output tube option, but I think the plate voltage is just a tad too high for my comfort.

[busbyelec]

6au6 in Roberts

I have one of these preamps as well, out of a broadcast version of the Roberts tape deck. Mine has high and low impedance available on the mic input, as well as 600 ohm preamp output and 8 ohm speaker output. I found this one after picking up about 30 of the ef86/12ad7 variety... didn't even realize it had the different tube complement until I went to add an input transformer to it.