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Shoog 6th April 2008 10:19 AM

My Tabor Clone is finished.
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Hi all,
This is my version of Gary Pimms Tabor amp. I was fascinated by the concept of the Tabor because I had done some extensive experiments with the partial feedback design concept and found it to sound very impressive wherever applied. The Tabor might be described as the ultimate partial feedback amp. Most implementations I have ever seen (or built) use triodes in the front end which tend to be run very current starved, this reduces the feedback element as it is what generates the feedback over the feedback resistor. Gary uses the 6AU6 and runs it at a generous 5mA (most run at less than 1mA). He also designed the original to combine the feedback resistor and first stage anode load - this bring sonic benefits. He also went to some trouble to eliminate any coupling caps. The resultant amplifier concept is extremely simple but the only drawback to the original design is that in order to achieve all these wonderful things he also designed a very complex and expensive power supply.
I spent the best part of a year coming at this from various angles. In the end I decided that most of the problems I encountered with the original design can be overcome by having a large split supply. I ended up with a design where I use a +/-300V supply which can easily be derived from a single 330V CT transformer. The negative rail draws about 30mA which makes it an easy thing to design, mine fits on a 4inch x 4inch peg board. The plates of the 6AU6 settle at about 0V. The CCS is the simple ring of two bias block biased up with a 12R resistor. Fresh 807's produce about 40V over the CCS which dissapates about 2.5Watts each - this requires a big heatsink with good air flow (mine currently run at about 70 centigrade which is too high). Over time the voltage over the bias blocks will reduce so it is important to give them this amount of voltage at the start. The +B rail requires a time delay circuit otherwise the 807's will attempt to draw excessive current before the rail is up and probably kill the CCS, it will also produce flash overs in the 807's.
The output stage is self adjusting and functions as a differential output. I am using toroidal power transformers as outputs (110V+110V : 6V). The input transformers I am using are LL1517 which need a low impedance drive - my response is a little down at 100hz because of this. I have mine configured as 2:1+1 step down. I am contemplating changing them out for mains toroidals with a 5:1+1 ratio to improve the bottom end response and reducing the overall gain further.

How does it sound. Very clean and detailed with bandwidth past 50kHz. Bass is tight. So far I think it is the best amp I have built.


Shoog 6th April 2008 01:27 PM

2 Attachment(s)
For all those lovers of amp porn heres a picture.
Ugly ain't it - you should see the insides:D


Zen Mod 6th April 2008 02:02 PM


Originally posted by Shoog
For all those lovers of amp porn heres a picture.
Ugly ain't it - you should see the insides:D





woodturner-fran 6th April 2008 08:58 PM

I know you had the teething troubles, but how would you rate the difficulty of build?


flyinglemur 6th April 2008 09:39 PM

Go on ireland!

Shoog 6th April 2008 10:05 PM


I know you had the teething troubles, but how would you rate the difficulty of build?
I would rate it as beyond basic, but not really any more difficult than the RLD which you have already mastered.

I just added a 120K load to the LL1517 input transformer and this seems to have tamed the strident top end I was experiencing. I may try triode strapping the 807's to see if it bring the gain down a little.


woodturner-fran 6th April 2008 11:53 PM

yeah, you know the way I'm thinking!

get up ya good thing!

Gary P 7th April 2008 06:41 PM

Before you try triode mode on the outputs increase the value of the screen resistors on the input stage. Increasing the screen resistor value reduces the gain and linearizes the input stage. The Tabor rev. 3 schematic has ~10K on each screen. IIRC, the high screen value cut the gain and distortion of the input stage 50%.

Shoog 7th April 2008 07:39 PM

Gary is that the series grid stopper resistor you are talking about. I would have thought that if it was, then it would increase the distortion by making the screen voltage more variable and less linear. What am I missing.


jduffy 8th April 2008 12:34 AM

That baby is rugged looking!

I love it. Nice stuff man.

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