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Akai Guitar Amp Project - Round 2

sloober

sloober

Super Member
Some of you may have seen the guitar amp I made from an Akai/Roberts RTR. Came out great, and everyone loved how it looked, but it lacked a real guitar amp sound.
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I tried this and that and then decided to start from scratch. I have about 6 of these monoblocks now, so that part was easy. I took the nastiest one and stripped it to a bare chassis. I painted the chassis with some silver paint, and reinstalled the blank tag board and sockets, 3 of them anyway. I then began searching for a suitable schematic. Ended up going with the Fender Champ Amp AA764. It is single ended and uses a 6V6 for output. I subbed a 6AQ5A for the 6V6 and a 6X4 for the 5Y3. I also added the choke that was there to the first stage of the PS. I have it just about ready to go, but want to check with my buddy with his scope and equipment first. If this sounds good, then I will paint a faceplate and make it look nice. Then make 3 more and see how they do at the local guitar shop. I may do one in paisley just for the heck of it!
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Very cool! :thmbsp:

Do you happen to have a schematic of this amp? I have a few of these myslef and would like to build at least one into a guitar amp.

Or even better would be a parts layout :smoke:
 
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Stock transformers, and choke. Yes that is the schematic I used. Here is mine slightly modified. Again I am not done, so you may want to wait. So far I am not getting input from the guitar cord, so I think I have a wiring issue. Also have to be sure the feedback is the right phase or it will be all wacked out.

Akai-Champ
 
Will have to see. Will have it on a scope tomorrow. I am getting some sound out of it, but not from the first half of the 12AX7. The input or tone wiring may be messed up. For such a simple amp, there sure are lots of wires running about. If I could use it with or without feedback, I could easily mount a switch!
 
Still no time to test it, but soon. I think I just found a possible problem. The 6AQ5A is listed as being equivalent to the 6V6 and 6V6GTA WITHIN ITS MAXIMUM RATINGS!

Max plate voltage for the 6AQ5A is 250v, the 6V6 is 350. My transformers are putting out at least 300, so time for another output tube. I'm thinking I'll just go back to the 6BQ5 or 7189. Damn...
 
I don't care how it sounds, I play guitar and that is the coolest looking amp I've ever seen.... and I've seen a lot. I WANT!!!! Seriously, amazing.
 
A vintage 6AQ5 may well be capable of running at 300+ volts even if the max rating was 250. Just look at any vintage Fender schematic and see the voltages they were running tubes at in the late 50s and early 60s... way over the maximum ratings. Those same amps are running beautifully today with 5% higher voltages because of the increase in line voltage. If you're getting "some" sound out of it and the 6AQ5 isn't redplating or arcing, the problem is likely elsewhere IMO.
 
I will need to investigate with the help of my friend. I found a few errors in my input wiring, but now all I get is a squawk....
Thanks Floridasam, my first attempt and I don't even play very well at all.
 
Success! Found a resistor I used the wrong value for...several times larger! Anyway, got sound but not all that loud. So I removed the feedback, and WOW, twice as loud and sounding real good. Gonna check it on the scope tomorrow.:banana:
 
So are you going to need another amplification stage with the feedback? You know we'll want audio samples, both with and without the feedback.

I have an old Akai tube R2R, it works (well, last time I powered it up, a few years ago) so I don't know if I want to tear it apart. It would be nice to have a junker to do something like that with.

I remember your original thread - have a link to it offhand (might be good to add that to your first post here)? Where did you get the speaker and what model is it (I'm guessing either Antique Electronic Supply or Parts Express)?
 
Saw that you mark your orange drops with a Sharpie the way I do....chuckled when I saw the pic.

Why they have to go six decimal places then subtract 3 or 4 is beyond me....just put the value on the cap(?).

By the way....good luck with the continued upgrades to that (those?) amps. Great idea!
 
Sounds like you are close.

When you have it all done please p[ost the revised/final schematic and pics as I will be following along soon.

Thanks

:thmbsp:
 
A friend of mine just picked up an M-8 for me. I haven't seen it yet but apparently it is in working condition. Haven't decided what to do with it but I've always wanted to build a quitar amp and it sounds like a good candidate. I've rebuilt several R2Rs and when you get done that's what you have. Another R2R. I have several nice ones and don't use them and it's hard to get back what was put into them. I don't have a lot of experience with tube stuff and there is a lot of info on modding the M-8 so I will probably convert it. Now I need to decide. Hi-fi or guitar.

sloober, can you attach a link to your earlier thread?
 
If you don't have quite enough gain or drive in the amp you can try dropping the 1500 ohm cathode resistor in the first 12AX7 stage down to 1000 ohms, or even 820 ohms. You'll lose some of the clean sparkle and headroom, but it's an easy thing to try. Also, you can increase the value of the 250pF cap in the tone stack. If you increase it a lot, your treble control will become like an overdrive control. I've got a .15uF film cap in place of the 250pF on one channel of my vibrolux, and when you put the treble control up that channel become a screaming overdrive monster. Sounds much better to my ears than amps with additional preamp tubes. I usually parallel the two channels in my amp... one with that mod, the other stock and nice and clean. That way I can keep the warmth, chime, and clarity of the stock Fender circuit while adding some drive and harmonic content from the other channel. The amp responds well to the guitar's volume control.

I'm assuming your circuit is what you've shown in the Akai-Champ marked-up schematic.
 
i should try those tricks on some of my amps...thanks, JonL.

when converting a low powered amps such as these reel deck amp or other console amps maybe, and if you're keeping the stock circuit, what are the things that you would do other than basic restoration? do you take out certain part of the circuit?
 
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