Matt C

Akai M8 / Roberts 770x preamp mod questions
« on: November 21, 2012, 08:35:06 PM »
So I'm in the process of modifying an Akai M8 machine to be used as mic preamps, and I'm hoping the people here would be willing to offer some help so I can better understand this circuit.  I'm new to electronics in general and new to tube circuits, so bear with me.

I took everything out of the unit and rebuilt it from the ground up as a single channel mic preamp.  the circuit is mostly based on the "Tablebeast Brute Force" design which can be seen below (let me reiterate that this schematic is not my own design.  thanks to Jesse/xfmr for sharing it). I've got everything more or less working, but I'm trying to learn as much as possible from this project (and end up with nice preamps), so I'm taking some time to read about tube circuits and try to figure out if there are any changes I can/should make for my own units.  Keep in mind I'm still just learning this stuff, so while I'm happy to make changes to the circuit, changing the design completely would be biting off more than I can chew.  here's a rundown of the issues/questions that have come up.  any help and advice would be greatly appreciated!:

- B+ voltage is 207 Vdc
- right now in the EF86 stage, quiescent plate voltage is only 52V, while the screen grid is at 79V.  my understanding is that we usually want the screen to be lower than the plate.  I didn't make any changes to that part of the circuit, so if this an indication that something is wrong?  or am I just incorrect in what voltages I'm expecting to see there?

- one main difference in my unit compared to the posted schematic is the input - I left out the 243R and 500k resistors on the input transformer secondary, and replaced them with a 500k pot across the secondary with the wiper connected to the EF86 control grid.  this seems to work but I'm worried it's an inelegant way to add an input volume control.  more specifically I'm wondering if it is creating problems because the part of the pot below the wiper is acting as a grid leak resistor, but when the pot is turned down low, you end up with a grid leak resistor that's much smaller than what usually gets used.  is this actually an issue or am I just making things up at this point?

- when adding a polarity switch, is it preferable to put it at the input, or the output?  one person recommended switching at the output, because he thought it was generally bad practice to switch low level signals if you don't have to.  my other thought, which I'm not sure is really valid, is that since a tube can distort a wave asymmetrically, flipping the polarity at the input could have a noticeable effect on the tone when pushing the tube hard.  any truth to that?

- am I correct in thinking that the input impedance of the preamp is essentially set by whatever resistance is across the secondary, reflected back through to the primary by the square of the turns ratio? so if I want the input impedance to be around 1.5k, and I use a 1:7 input transformer, I'd want around 73.5k (1.5k * 49) across the secondary?  Basically I'm confused about how to make all the resistances on the input (transformer zobel network, grid leak and grid stopper resistors, input volume control) play nice together and still end up with the right input impedance. the input transformer I intend to use is the Cinemag CMMI-7C which is 50,200:9.8k, and the datasheet recommends a damping network on the secondary.

- a more vague question, and maybe you can just point me toward some recommended reading, but when trying to reduce noise in this circuit, what are some ways of sorting out which noises are coming from where?  I tried the unit out briefly today and didn't have any major noise problems, but with the volume cranked up there was definitely some hum that I'd like to at least try to reduce. 

- right now all my AC heater voltages are about 0.7V too high. is it worth adding a dropping resistor in series with the heaters to bring them back down to the recommended levels, or is that extra voltage not significant?

« Last Edit: November 22, 2012, 09:56:35 PM by Matt C »


Re: Akai M8 / Roberts 770x preamp mod questions
« Reply #1 on: August 27, 2019, 08:51:55 PM »
Hey Matt,

      Did you ever work this out? By the way, I'd like to know what you used for an output transformer if you did. Thanks!

Matt C

Re: Akai M8 / Roberts 770x preamp mod questions
« Reply #2 on: August 27, 2019, 09:36:51 PM »
I had sort of forgotten about this. Eventually I decided not to use the schematic I posted above, and I just built a Redd47 preamp instead. That one got set aside indefinitely, both because I was too cheap to buy a proper output transformer, and because there was some major noise issues (I now think the voltage regulator tubes are to blame.)

Is there anything specific you're wondering about? I spent a lot of time tinkering around inside this thing.


Re: Akai M8 / Roberts 770x preamp mod questions
« Reply #3 on: August 28, 2019, 09:21:17 AM »
Well it's funny that you bring the REDD.47 up because my first instinct was to build two, as well—but the more I read about the OA2s the more I decided it would be above my head to troubleshoot. So instead I decided to try and stick with what's already inside the 770x. I've almost completed the RodC mod on one of the amps, but decided to look into different ways of dealing with the output—and that's what brought me to your original post.

Matt C

Re: Akai M8 / Roberts 770x preamp mod questions
« Reply #4 on: August 28, 2019, 09:55:32 AM »
well you could try just slapping an output transformer in there and see what happens, 15k:600 is pretty standard for this type of thing. You could get a relatively cheap one from Edcor for experimenting. I honestly don't know whether the 12AX7 cathode follower will be able to properly drive a transformer. If not, you could always try a 12DW7, it's a dual triode that's basically half 12AX7 and half 12AU7. I believe 12AU7s have better drive capabilities so that might be a better choice for the output stage.


Re: Akai M8 / Roberts 770x preamp mod questions
« Reply #5 on: August 28, 2019, 11:45:28 AM »
I'll certainly give it a try. Thanks Matt!


Re: Akai M8 / Roberts 770x preamp mod questions
« Reply #6 on: September 02, 2019, 10:17:23 PM »
I built 2 channels of the schematic Matt C has above (from Tablebeast) I don't know what it is, but I use it all the time. There's no negative feedback. The way it distorts is very pleasing. I prefer it for vocals and acoustic guitars over my Yamaha PM1000 and PM2000 preamps. It's great. I wouldn't mind pointers if there were anything I could do to the circuit to give it just a tad more air or openess up top, but it's pretty great for what it does anyway.
Used the Jensen 115 input transformers and Carnhill (I think 10K:600?) output transformers. But the non transformer out sounds the best if you have a high impedance load on it. And the cable run isn't too long.  I'm gonna build another one with Shure mic transformers and edcor outputs for the cheap. Saw one where the Tablebeast dude built a pretty close Altec 436 compressor into one of these.

Re: Akai M8 / Roberts 770x preamp mod questions
« Reply #7 on: September 09, 2019, 03:22:49 AM »


Related Topics

  Subject / Started by Replies Last post
10 Replies
Last post February 14, 2013, 07:58:59 AM
by xfmr
7 Replies
Last post January 22, 2013, 06:33:18 PM
by ruffrecords
4 Replies
Last post July 21, 2018, 06:42:30 PM
by smilan
1 Replies
Last post December 29, 2018, 05:50:42 PM
by Icantthinkofaname