AKAI M7 M8 / Roberts 770 Mod updates

tubetapexfmr · Post by **tubetapexfmr** » Mon Oct 19, 2009 1:23 pm

Rod, have you tried a triode/pentode switch for the EF86? I will be doing this on my new new Akai design with a three way switch: triode, pentode 1, and pentode 2 modes specifically. The different pentode modes move the G3 grid from the cathode to ground, while the triode mode straps G3 to ground and G2 to G1 with a 100 ohm resistor as a buffer.

RodC · Post by **RodC** » Mon Oct 19, 2009 1:35 pm

tubetapexfmr wrote:Rod, have you tried a triode/pentode switch for the EF86? I will be doing this on my new new Akai design with a three way switch: triode, pentode 1, and pentode 2 modes specifically. The different pentode modes move the G3 grid from the cathode to ground, while the triode mode straps G3 to ground and G2 to G1 with a 100 ohm resistor as a buffer.

Nope, I have done that with power tubes before but not in a pre. Let me know how that turns out.

I saw that on one of the data sheets I ran across, I always assumed I would loose too much gain.

RodC · Post by **RodC** » Mon Oct 19, 2009 7:00 pm

Update on the Edcor transformers, weird shit.

I have put these in 4 different preamps and did a few tests. When I use my delta 1010 interface things look as flat as they use to (Using some white noise through them)

When I hook them up to my Delta 44s everything from 1k and down starts rolling off. I can check the signal before the output transformer disconnected or connected and it looks fine, but on the other side when its connected to the 44s you lose all kinds of lows.

They are both rated at 10k input impedance.

makes you wonder how much interfaces reacting to different preamp outputs affect the sound!

I'm going to go through a bunch of pres and maybe post some results.

tubetapexfmr · Post by **tubetapexfmr** » Tue Oct 20, 2009 5:28 pm

I always assumed I would lose too much gain.

Well, yes in triode mode the EF86 does have a lot less gain, but it is a different sound for sure and another way to pad volume on loud sources.

tubetapexfmr · Post by **tubetapexfmr** » Fri Oct 23, 2009 1:14 pm

Instead of hijacking this thread any further I started a new thread dealing specifically with my Pre+Amp mod:

http://messageboard.tapeop.com/viewtopi ... highlight=

RodC · Post by **RodC** » Sat Oct 24, 2009 8:11 am

RodC wrote:Update on the Edcor transformers, weird shit.

I have put these in 4 different preamps and did a few tests. When I use my delta 1010 interface things look as flat as they use to (Using some white noise through them)

When I hook them up to my Delta 44s everything from 1k and down starts rolling off. I can check the signal before the output transformer disconnected or connected and it looks fine, but on the other side when its connected to the 44s you lose all kinds of lows.

They are both rated at 10k input impedance.

makes you wonder how much interfaces reacting to different preamp outputs affect the sound!

I'm going to go through a bunch of pres and maybe post some results.

Another update, even with a converter that works, sound decent, Im still loosing highs and lows. On all 4 preamps with these output transformers, what use to be a nice streight line up to about 16 - 18 K is now a frowney face with most freq response tests I have tried.

Im not blaming the transformers, maybe just not the right ones. Im going back to the direct outs whith the nice flat response.

aurelialuz · Post by **aurelialuz** » Sat Dec 12, 2009 12:03 pm

do you guys do anything with the shield connections on the EF86? or whatever you call them, 2 and 7. and why the hell is the EF86 not in the RCA RC-30?

tubetapexfmr · Post by **tubetapexfmr** » Mon Dec 14, 2009 8:27 pm

I ground pins 2,7, and 8. 2 and 7 being the shield, and pin 8 being the g3 pin.

lassoharp · Post by **lassoharp** » Sat Jan 02, 2010 6:05 pm

RodC wrote:
RodC wrote:Update on the Edcor transformers, weird shit.

I have put these in 4 different preamps and did a few tests. When I use my delta 1010 interface things look as flat as they use to (Using some white noise through them)

When I hook them up to my Delta 44s everything from 1k and down starts rolling off. I can check the signal before the output transformer disconnected or connected and it looks fine, but on the other side when its connected to the 44s you lose all kinds of lows.

They are both rated at 10k input impedance.

makes you wonder how much interfaces reacting to different preamp outputs affect the sound!

I'm going to go through a bunch of pres and maybe post some results.

Another update, even with a converter that works, sound decent, Im still loosing highs and lows. On all 4 preamps with these output transformers, what use to be a nice streight line up to about 16 - 18 K is now a frowney face with most freq response tests I have tried.

Im not blaming the transformers, maybe just not the right ones. Im going back to the direct outs whith the nice flat response.

It may very well be the Edcors. There's been at least 2 or 3 cases I can think of where Edcor outputs had to be returned due to various problems - including having too low inductance on the primary. They do sometimes make mistakes.

Try sweeping the transformer by itself and compare it with the good one. If there is a problem the folks at Edcor are very easy to work with and I doubt you'll have any problem getting a bad one exchanged. Just explain what's going on.

Also may want to double check termination if needed.

StephenBOmaha · Post by **StephenBOmaha** » Mon Apr 19, 2010 8:51 pm

Hello,

I completed the "two in one" mods a few months ago and have been using both the ef86 pre and the 12ax7 mic pre with wonderful results. (Thanks for all the helpful articles and schematics, Rod!)

I am interested in using these amps as a stereo hi-fi too. Does anyone have any suggestions on mods that would allow me to use the amps both ways?

Currently, the input to the ef86 stage is 1/4". (I use an external mic transformer box when using this as a mic pre.) The 12ax7 stage is basically stock.

I put the 6bq5 tube back in and connected my b&w 601s. In order to get a clean and balanced playback I have to adjust the level from my computer interface to a very low output level and the 12ax7 volume pots are barely at 7 o'clock. (Here it sounds very detailed and three dimensional.) Any more signal from the computer or volume at the pot seems to make the high end a harsh and introduces noticeable distortion on the vocals. There seems to be this one very narrow sweet spot as far as these gain stages....

I also tried my phonograph player. Through the 12ax7 sounds harsh and lacks low end. I also tried the phono through the ef-86 into the 12ax7 input. Again a very narrow sweet spot, more distorted and missing low end.

I guess I am hoping someone can help me understand the input impedance on both pres...and what adjustments satisfy both recording and hi-fi requirements.

also, I found this suggestion on another board ,"replace the cathode bypass cap on the 12ad7 (25uf 25v)(c12 on schematic) with a larger value of 100uf or 330uf. "
good idea? What about the coupling caps? Any changes there?

As you can guess this is my first tube pre project and the vehicle through which I am learning the basics. Any help or resources is appreciated.

infiniteposse · Post by **infiniteposse** » Tue Apr 20, 2010 3:20 pm

I thought this might be of interest in this thread. I had a head cab made for my little akai. At the moment I'm just using it as an amp, but I love it.

Snarl 12/8 · Post by **Snarl 12/8** » Tue Apr 20, 2010 4:19 pm

infiniteposse wrote:I thought this might be of interest in this thread. I had a head cab made for my little akai. At the moment I'm just using it as an amp, but I love it.

That is freakin' beeeeooootiful.

llmonty · Post by **llmonty** » Mon May 17, 2010 1:54 pm

Yeah, really beautiful! Are you using it just for recording? Loud enough for you? I have 2 of these and need to put them through their paces soon as guitar amps and mic pres.

I had tested them quickly a while ago through what I had around and they sounded neat -- a bit dull and at their best when in big muff overdrive land.

They were modded by one of the guys that sells his stuff here on tapeop.

syzygy · Post by **syzygy** » Sun Jun 20, 2010 7:33 pm

Rod, have you tried a triode/pentode switch for the EF86? I will be doing this on my new new Akai design with a three way switch: triode, pentode 1, and pentode 2 modes specifically. The different pentode modes move the G3 grid from the cathode to ground, while the triode mode straps G3 to ground and G2 to G1 with a 100 ohm resistor as a buffer.

I would like to try a pentode/triode switch on the EF86 in my M8 monoblock. Can anybody suggest which pins should be wired differently & how? Currently the amplifer has been modified as per the 'Beyond Sanity' 2-in-1 preamp mod, retaining the output valve for use as a small guitar amp. Basically I'd like to be able to cascade V1 and V2, but at the moment the EF86 (V1) seems to have way too much gain for that purpose. I figured switchable resistors and/or caps in between V1 and V2 would do the trick, but the pentode/triode switch could be the best option. Any thoughts out there?

Dr Rubberfunk · Post by **Dr Rubberfunk** » Mon Jun 21, 2010 7:14 am

What a glorious looking cab! Great stuff.

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