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Old 28th February 2013, 03:58 AM   #1
osci is offline osci  Australia
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Join Date: Jan 2013
Default Gibson amp project

Hi all,

My very first post on the forum, its sort an intro thingy and hopefully seeking input/advice about my next build I'm attempting.

I sincerely apologize for this being long winded, I'll say up front I'm not a qualified electronics tech or an electrician, as a matter of fact I have virtually no electrical experience just a healthy interest in field, I've always liked valve technology especially the warm tones the valve amps produce.

My wife is learning to play guitar and I decided to build a valve amp for her, she so excited by the idea, So after nearly 3 years of research and picking up bits & pieces of old valve equipment along the way and still a hell of a lot to learn, I've almost finished my second amp, which is from this link directly below.

http://www.valveheaven.com/The%20Lamington.html, I've built two in fact, Both amps are the same circuit with a slight difference for the second build.

The first I used the voltage multiplier as contained in the info from that link but the second I decided to use a 6V4 rectifier as I was recently very lucky to be given a box containing approximately 300 valves of which there are a stack of 12AU, AY, AT & AX7's, a couple of 6BQ5/EL84's, some 6BM8's, 6CM5's a couple of rectifiers and a heap of other types which I still have to sort through.

Anyway, the second amp, I decided to use the 6V4 rectifier, simply because I had it available and to see if it could/would work, I also had a 290 ct 290v power transformer from an old radiogram I won at auction for $6, to cut a long story short, I'm absolutely stoked with the results, My B+ target voltage at the anodes of the 6BQ5's was 320v when I first fired it up I got 315v at the anodes of 6BQ5's, the screens of the output valves maybe considered a bit low at 270v, However, to my untrained ears and very limited knowledge there doesn't appear to be any ill affects, in fact I think it sounds awesome (only my opinion).

I'll try to post some pics later tonight.

Anyway, for my next build the wife asked if its possible to build her an amp head, So I chose a Gibson GA-1RT-1 circuit to build http://www.drtube.com/schematics/gibson/ga-1rt.gif because it appears to be reasonably straight forward?

What would be peoples thoughts on building this circuit as an amp head?

I already have the hardware, most of it free or for very small prices at second-hand shops, the PT I'm hoping to use is a 260v ct 260v with 6v & 5v taps, this particular PT came out of an old test equipment that was running a 5Y3GT so I'm hopeful it can do the job, I already have the 5Y3GT 12AX7 & 6BM8,

The speakers I "hope" to use are 2 x 16 ohm 15w 12" speakers, I believe that if I connect the 2 speakers in parallel I will be able to run them at 8 ohm's? it may not be possible for a smallish amp like this to drive two 12 inch speakers, but if this is possible could anyone advise what type/size or value output transformer would I need to use?

For the reverb tank, I have the option of buying a unit off the shelf or building a unit, at this stage I will probably buy a unit although I do have a schematic for a single spring unit which uses 2 x Ronette type stylus's out of old record players and utilising the crystals in them as a drive & pickup. So all in all if I can build this its going cost only my time and well under a hundred dollars, simply because I have nearly all the hardware at my fingertips so to speak

I appreciate the time taken in reading this and really appreciate any input that may come.

cheers all.
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Old 28th February 2013, 07:53 PM   #2
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Join Date: May 2010
Location: Hawkes Bay
Hi osci.
1st impressions are:
- nice simple circuit
- but reverb circuitry appears incomplete
- easily drive 12" speakers depending on their sensitivity ( dB/w/m )
- clean amp only, will need boost in front for lead playing
Cheers
JimG
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Old 3rd March 2013, 11:27 AM   #3
osci is offline osci  Australia
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Join Date: Jan 2013
Thanks for that Jim much appreciated, The speakers I'll be using are Yamaha JA 3051's I was told they are out of an old organ, the look reasonably new, doesn't appear to be hardly any wear on them. I hope the reverb circuit is all there I fuess I'll find that out when I get to the build.

cheers
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Old 5th March 2013, 11:27 PM   #4
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Location: Adelaide South Oz
The reverb circuitry is complete - you need to know one additional thing, the reverb tank is driven from the speaker output, that is where those wires labelled red and black go.

The Yamaha speaker will work well but it would probably sound quite good with an old Rola 12" speaker from a Radiogram.
Here is Rola speaker and output transformer info for future reference:
Retrovox Data page

Cheers,
Ian
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Old 6th March 2013, 09:25 PM   #5
osci is offline osci  Australia
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ah ha Cheers Ian

Thats jogged my memory, I remembber reading something, I think its in the archives of the AX84 forum of this particular circuit about the reverb tank being connected/driven by the speaker output, from what I read it was described as a feedback type circuit.

Ian, when you say the "reverb tank is driven from the speaker output" by that, I'm presuming you mean the Red & Black terminals of the tank in the circuit are directly connected to the "output side" of the output transformer?

Unfortunately I used the last of the two 12" Rola speakers I had on my current project but I still have the output trasnformer that one of those Rola's was matched too, they actually came out of an old Astor radiogram of which I'll have a look for the spec's of the particular model to see its possible to use with this circuit.

and many thnaks for the Retrovox data page

cheers Ron
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Old 6th March 2013, 09:57 PM   #6
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Ron,
Yes wire directly across the speaker (the output side of the output transformer).
I would assume that red goes to speaker +ve and black goes to speaker -ve but if there is any hint of it being unstable then be prepared to swap these wires.
Cheers,
Ian
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Old 6th March 2013, 10:03 PM   #7
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Quote:
Originally Posted by gingertube View Post
Ron,
Yes wire directly across the speaker (the output side of the output transformer).
I would assume that red goes to speaker +ve and black goes to speaker -ve but if there is any hint of it being unstable then be prepared to swap these wires.
Cheers,
Ian
You are correct with the wiring. I have one.. No need to swap the red and black, there will be no noticeable difference, by the time the Hammond Springs have confused themeselves by reverberating!
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Old 6th March 2013, 10:11 PM   #8
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Location: Adelaide South Oz
Thanks Harleyjon - I offer my best guess / educated opinion but its always great to have that confirmed so as to not lead anyone astray.
Cheers,
Ian
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Old 6th March 2013, 10:13 PM   #9
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I wish you luck with your new project. Don't forget there is always help, by way of a pointer or two, out here.
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Old 6th March 2013, 10:16 PM   #10
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I recently repaired a Music Man (Fender) 130A.
It had an open circuit load resistor on the last ECC83 before the phase splitter. The resistor was OK ... it had never been soldered in!
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