Comments on: Bespoke Mic Preamp based on Akai/Roberts circuit

By: Lance kimmons

Lance kimmons — Fri, 13 Aug 2021 18:33:06 +0000

In reply to Stephen Minnich. I’d like to ask you about this

By: Stephen Minnich

Stephen Minnich — Sat, 04 Aug 2018 03:24:44 +0000

I also have an old shure splitter. It was 5 bucks because the cannon plugs were sexually confused like most church/school gear. I had to fix that, not too easy either.
It needs no EQ or anything. I can plug in four #58 mics and send it into my ancient peavey PA head and they sound GREAT. SO CLEAN, SO FULL.
The best way EVER to add mic channels to a simple road PA.
Small and weighs almost nothing. Quality unsurpassed.

By: Stephen Minnich

Stephen Minnich — Sat, 04 Aug 2018 03:15:38 +0000

I have 3 of these old units now. 2 still with transport.
They sound fantastic.
Not only are they golden to playback and master recordings, and record microphones. They are utterly outrageous guitar amps.
I have driven these to the outermost gain imaginable and what they can do for a guitar is positively SICK.
I mean utter squalling mayhem without any breakup at all. AMAZING. TRULY BEAUTIFUL.
I made one into a portable head.

By: Peter

Peter — Thu, 31 Aug 2017 12:43:18 +0000

Hi Chris.

Sorry for this question comming years after date. Really love this website you build over the years.
I Wanted to build this preamp for a long time now and finally got around to it. Found al the parts i need!

Just for having my facts straight: You never did use the 6BQ5 right in M7 design right? Just ef86–>12au7 and then to OT?

Thanks for so much reading pleasure all this time.

By: Lacinato

Lacinato — Thu, 21 Jan 2016 23:49:35 +0000

Hi — just wanted anyone to know that the schematic you posted and lots more info on the Akai/Roberts stuff is now hosted at my site here: http://lacinato.com/cm/audio/robertsmod

Haven’t heard from Rod in a while, and the beyondsanityproductions.com site seems to be gone (for now, anyway).

By: chris

chris — Thu, 09 Apr 2015 15:27:42 +0000

In reply to Mike.

hi Mike. I can’t recall, but it must be a .5.

a 5uf foil cap would be HUGE, you would easily spot it in the image.

OT is visible beneath the green PP supply cap. it’s an edcor 15k:600. I have used these dozens of time. very economical and great performance. be sure to watch the orientation relative to yr PT cos they are unshielded.

I take it you are using the 6SJ7 as the input pentode; are u using the 6SN7 as the output driver? If so, the component values will vary quite a bit; i am not sure that I have ever seen a 6SN7 implemented quite the same way that the 12Au7 is used here (frankly, you MIGHT just be able to parallel the 2 6SN7 sections and use it as a straight-up R/C amp with 50k plate load and a DC blocking cap b/w the plate supply and the O/T – this might give you a low enough source impedance to drive a 15K primary output trans. MIGHT.).

good luck

By: Mike

Mike — Thu, 09 Apr 2015 13:51:44 +0000

Beautiful work, Chris. The value for C4 of that RCA two-stage input amp schematic varies between RCA tube manual editions. RC-17 shows C4 as 5 uf, while RC-20 shows C4 as .5 uf. Do you know which is correct? Also, does your Bespoke design use an OT? I’m following your lead here, using a 6SN7 and a 6SJ7.

By: chris

chris — Wed, 03 Sep 2014 23:08:43 +0000

Hi John. that is a pretty awesome circuit! I had never seen that either! I highly suggest that y’all check out the link on John’s comment – this (with an add’l 6J5 or 1/2 a 6SL7 at the front end) would make an awesome and unique lil gtr amp.

By: John

John — Wed, 03 Sep 2014 20:56:06 +0000

Nice work Chris,

There’s a lot of features crammed into this one!

b.t.w. I was looking at the source website for that cathode follower circuit you posted in the article and this caught my eye:

http://diyaudioprojects.com/Technical/Papers/Direct-Coupled-6V6-Cathode-Follower-Amplifier/

The way the o/t is also the cathode resistor is interesting to me, I havent seen that configuration before.