AKAI M7 M8 / Roberts 770 Mod updates

[xfmr]

For small signal tubes you can run them at a lower voltage like 5.7v and get massive increases in life of the tube. When you do this, the tube gain goes down some from the nominal, but is more stable over the longer life of the tube.

As for your desire to add another EF86, yes of course you can do this. This does however get you into a much more involved mod as you will have to rewire the power tube socket at the very least. Luckily the heaters are going to be the same (pins 4 and 5) on both the EL84 you are removing and the EF86 that you want to replace it with. You can also move the original EF86 to the main heater supply if you like. The EL84, once removed, leaves a TON of extra heater headroom at the transformer tap. Also, keep in mind you are already gaining some headroom from removing the 6AR5 erase tube (or by not having it in the amp at all since there is only one erase circuit per two channel tape deck) so you may want to consider replacing the 6AR5 spot with another EF86 and keep the EL84. If you DO replace the 6AR5 or blank spot with something and want to keep the 7 pin socket you can use a 6AU6 or 6AK5 for a different flavor of pentode preamp with similar gain (depending on how you set it up of course).

The Akai Roberts power transformer RP-102 has 1800mA @6.3v available on its biggest heater tap. If you have the 6AR5 erase tube which eats 400mA and EL84 power tube which eats 760mA removed from the circuit you have freed up 1160mA from the tap and can use that headroom for anything you want. The EF86 draws 200mA @ 6.3v, the 12AD7 is 450mA @6.3V, and the rectifier tube has its own 600mA tap for its heater. Now a 12AX7 only draws 300mA @ 6.3v so you can grab an extra 150mA just by swapping the stock 12AD7 for a 12AX7 for a maximum of 1300mA available.

In my conversions I take everything apart and rebuild a new circuit in the chassis. One of my designs is a two channel microphone preamp that uses an EF86 feeding a proper line amp output that uses both triodes of a 12AU7. I fit two of these circuits in the one chassis. So, it has 2x EF86 @ 200mA and 2x 12AU7 @ 300mA for a total draw of 1000mA leaving tons of headroom.

You know what I do with that 5.7 volt tap? I move it over to feed the lights on the meter. Undervolting them will increase their life significantly which is great because they are a pain to replace as these 'toy' meters were not really designed to be serviced. This also has the bonus effect of keeping the lights off the heater tap going to the audio tubes.

[Matt C.]

great information, thanks. I intend to do a more complete tear down/build up modification of the circuit, but I'm just now learning how all this stuff works, so I'm taking it slow and using Rod's "Boris" schematic as my first goal (minus the power amp section). I have two full M8 units (four original channels) to play with, so once I wrap my mind around the circuit I'll experiment with different stuff. I like your idea of the two channels of EF86 feeding 12au7 outputs.

thanks again, I'm sure I'll be back with more questions.

[xfmr]

Here is a blog post with a full schematic if you want to mess around with my EF86/12AU7 design. I call it the 'Brute Force' as it heaps a ton of gain on with no feedback!

http://xfmr.blogspot.com/2012/06/brute- ... -hyde.html

[Matt C.]

okay, back with a few more questions about this project. I'm knee deep in it and it's passing audio but I have a long way to go before it's all finished.

- I removed basically everything except for the PSU section and the EF86 stage. right now the B+ voltage at the EF86 stage is about 300Vdc. is this too high? to lower it would I just put another resistor in series with the + side of the power supply to drop some of that voltage? if that's the case, is there any easy way to calculate what that resistor's power rating would need to be?

- is there any reason to tie the big center pin of the tube sockets to ground? does this actually do anything, or is that just a convenient spot that often gets used for local grounding?

- in that "Brute Force" schematic, if I only want to do the impedance balanced output, can I get rid of that 220k resistor?

thanks!

[xfmr]

- I removed basically everything except for the PSU section and the EF86 stage. right now the B+ voltage at the EF86 stage is about 300Vdc. is this too high? to lower it would I just put another resistor in series with the + side of the power supply to drop some of that voltage? if that's the case, is there any easy way to calculate what that resistor's power rating would need to be?

It is not pulling much current so any 1/2 watt resistor will work fine. Because it is not puling much current, you are getting nearly the full voltage coming off the rectifier. It is usually somewhere between 300 and 320 volts. I am guessing you kept the choke and the 5k resistor? I would say you need at least one more RC (resistor/capacitor) stage to further filter the PSU and to knock the voltage down. I like to see about 150-180 volts feeding the plate resistor of the ef86/6267. Try a 47k 1/2 watt resistor and an additional filter cap after it of 20uf. I like the Vishay/Dale CCF60 series, they are technically 1/2 watt, but are rated to 1 watt and are 1% metal film.

- is there any reason to tie the big center pin of the tube sockets to ground? does this actually do anything, or is that just a convenient spot that often gets used for local grounding?

Local grounding. But keep in mind the little floating panel that the ef86 and 12AD7 rest is ground isolated, so you need to make sure you have the middle lug of the tag strip attached to it grounded.

- in that "Brute Force" schematic, if I only want to do the impedance balanced output, can I get rid of that 220k resistor?

You still need the 220k resistor no matter what, without it the circuit would be unloaded with nothing plugged in. Tubes always like to see a load, even while idling.

[Matt C.]

thanks. one more question I forgot to ask: is there a good source for reasonably priced axial lead electrolytic capacitors? they seem hard to come by and overpriced

[xfmr]

tedweber.com has great prices and good reliability on their axial caps.

[Matt C.]

okay new question, this time about the heater supplies. originally the two hum balance pots had their wipers tied to the power tube's cathode (am I correct in assuming this is for an "elevated AC heater" scheme?), but now that i've done away with the power tube circuitry, that's no longer an option. It seems like some people just ground the wipers, other people get their elevated voltage from the screen grid of one of the tubes, and sometimes people put a voltage divider between B+ and ground and get their voltage there (which seems like a good option since it would also serve as a bleeder for the PSU caps). Any tips on which of these options would be preferred? since the heaters were elevated in the original circuit I'm inclined to keep them that way, but I don't know enough about it to know whether it's actually necessary.

thanks!

[xfmr]

Does your unit have the little pots mounted to the own panel and adjusted from the front of the unit or do you have the simpler type wired directly to the tag strip and adjusted from the rear? If it is the first type, try simply grounding the middle lug and trimming the pot until any hum is cancelled out. You may want to spray the adjustment pot with some deoxit and fader lube for optimum performance and hum rejection. If it is the second type of trimmer pot, just lose it and replace with two matched 100 ohm resistors and ground the new center tap created by the two resistors.

[Matt C.]

mine has the hum balance pots on the front. I'll just ground the wipers. thanks.

[xfmr]

If you want to mess around with raising the center tap above ground, try a voltage divider with a 1M and 10k resistor/ 25uf cap in parallel. You run the 1M resistor from B+ after the choke and connect to the 10k resistor / 25uf cap which are grounded. Place your center tap at the intersection of the 1M and 10k resistor / 25uf cap.

[Matt C.]

so everything in my M8 preamp is more or less working, but I'm trying to sort out a couple more issues.

first is that right now in the EF86 stage the screen voltage is higher than my plate voltage, which seems backwards. B+ is around 200V, plate is sitting at 50V and screen is at 70V. seems like the quiescent plate voltage should be higher, but maybe I'm wrong here. I haven't changed the values of the plate and screen resistors yet.

other thing is noise. any clues about common sources of noise would be great. here's what i'm getting:

when scoping the output with the volume cranked all the way up, i get some 60Hz hum (around 1.5 Vpk-pk) and some 40kHz noise (i think around 0.5Vpk-pk, if i remember correctly). I think there was also some noise at around 5kHz, which seemed weird to me. when I did listening tests this noise was not overbearing at all, but enough that I want to try to minimize it.

part of the problem may just be from not using shielded wire yet. when I do put in shielded wire for the signal lines, am I right in thinking I should just attach one end of the shield to ground and let the other end float?

I assume the 60Hz hum must be coming from the AC heaters. the hum balance pots don't seem to make much of a difference when rotated. The heater lines are twisted together and generally kept away from other wires, I don't know if they're inducing noise right at the tube sockets or if it's something else I'm not thinking of.

I don't know where the 40kHz noise would be coming from. it seems to be present at the EF86 output and then gets amplified by the 12AD7. I tried adding a grid stopper to the EF86 but that only made matters worse, although maybe I mis-calculated the size?

[johnblue]

first off, all you guys are genius'

i have been studying this thing for a long time, i am an absolute novice, but i think i can handle this, i am very close to making it happen, but i got lost with the dummy 8 ohm resistor.

its mentioned early in the directions, but i kinda just skipped it,

can anyone point it out in a pic, or explain it?

i am working a roberts 770, left channel for now,

also, if anyone has more pics of the childers 2x1 preamp inside guts, i could use all the help i can get, this is the first thing like this i have ever done.

i am a visual guy, so all that helps greatly.

also, for now i have mic input transformers from a tapco 6200b, it has 6 leads out, 3 and 3 on each side, what do you guys think of this? will they work? and... if they suck, where i might find something better for reasonable prices?

[tubetapexfmr]

The dummy load resistor looks like a little white inchworm with a metal nub on each end. You'll find it wired from the output transformer to the switchblock next to it. You probably didn't even know it WAS a resistor when you saw it as it is covered in a tightly woven fabric which helps to dissipate heat.

I don't think those Tapco transformers are going to have the right transformers with enough stepup to work with a tube circuit. Why not go with Rod's original suggestion and get a hold of an old Shure M68 or M67 mixers? Be careful to get the original M67 or M68, NOT the M67A or M68A versions which have PCB mounted transformers! The easy way to tell the difference from a distance is that the Shure you want generally have the large, round XLR jacks. The original version has all Male XLRs and usually has the older, larger, heavier, better-shielded transformers. The units with 1 XLR male and the rest the proper XLR female type is usually called a M68FC or M67FC (FC stands for female connector). Later models dropped the FC moniker as Shure switched to only making them with the female inputs as that became the standard.

Also, no offense to Rod, but his re-design of these monoblocks is far from optimal. For instance, using an EF86 by itself as a preamp without feeding it to a proper output line amp will lead to problems with loading because of the nature of its impedance. It works as a tape amp in the original design because it had a fixed output and the circuit was specifically designed to always have the same load. The volume pot only adjusted the playback volume in the 12AD7 circuit. Also, by placing a trim pot in between the input transformer and the first tube stage will introduce noise and can dull the tone considerably (think of how a Fender Strat gets dull when you turn the volume pot down). Plus, reusing all that old wiring, old caps, and excess circuitry will have you chasing your tail looking for circuit gremlins before long. Believe me, I started with Rod's designs when I was new to tubes, but I was lucky enough to have a tube guru look at what I was doing when I was trying to troubleshoot an early mod and he set me straight. It is better to build it right to begin with even if it takes longer and is more difficult than to spend the lifetime of the device tweaking it to get it to work instead of recording with it! I mean if you WANT to use a circuit that wastes the potential of the Akai / Roberts platform, then by all means go ahead. All that said, Rod has some good ideas in there, they just need to be evolved a bit, which is what I did with my Pre+Amp design. So my suggestion is to tear the whole thing down carefully and rebuild your own circuits in the chassis from the ground up. Use ALL new wiring, ALL new capacitors, and reuse most of the tubes and some of the resistors. Remove all the old solder, dirt, gunk, wax, dust, switching, mechanical tape junk, deoxit the sockets, strip the parts and wiring completely leaving the are tag strip and chassis bare. Now you have a platform that you can create anything with. The power transformer and choke are amazing. They have plenty of voltage and current handling capacity to do all kinds of stuff. That output transformer is no slouch either, and will handle a lot of different power tubes in the 5K impedance range from the stock 6BQ5 to smaller 7 pin guys like the 6AQ5 or 6AR5 (which just so happens to be the stock erase tube). Or you can take out the power transformer and fit two proper preamps in the chassis if you don't want to have a power amp on board.

Also, I know you're probably as tired of hearing this as I am, but with high voltage be freaking careful. Seriously. Even more than being watchful of the voltage, PUT ON SOME EYE PROTECTION when dismantling. It is so easy for hot solder to splatter when pulling these things apart. You DON'T want hot solder to splatter in your eye. Ask me how I know.

[johnblue]

hah! i found it!

it actually has the 8ohm marked on the white cloth very small.

i do not want to have a power amp, but i have and am learning alot by building it out this way, because there are the most detailed instructions and pictures.

i think that what you are talking about is relevant, and its a matter of taste, building from scratch is different from modifying a curcuit, some guys might want a different sound, i am sure that this Childers mod has a disticnt sound compared to a full redisign like you are describing, it seems to me that most of the people who might want this mod just want another sound option to throw in the mix, like... we got this old thing we can run our mics through... because they probably have really nice avalons also... but, me, i am trying to get something that i can count on for my mic inputs to my studio.

i am willing to do the work to get the best out of these puppies in the end.

what i really want is the most optimal mic preamp i can get, for my home studio.

after the full tear down of the left side as instructed, i can see the benefit of tearing it down further and cleaning and desoldering everything.

aethetically, the vu meter is pretty appealing, but i don't need it there if i could get a better operating machine without it.

i am right around the corner from firing this thing up, but i am surely going to do the full teardown you suggest for the easier right side, and start from scratch.

if you know anywhere or would be willing to snap some detailed pics and help with a different mod, i would be willing try it as well.