idea's for a valve preamp

[fisher x 100 a]

looking for a valve preamp.

I have a Rega with Elys 2, Output: 6.8 - 7.2mV

fisher valve amp -the phono input are - HI : 16mv - low : 3.5mv

any suggestions/ideas great-fully accepted

playing records are just too trebbley and bass is all muddled at moment,

thanks

 

[kevinkr]

Which model Fisher integrated, and has it been treated to fresh supply and coupling capacitors?

It may also be that you could rebuild the phono stage with a better circuit, something passively equalized with a ECC83 in the front end and a ECC88 in the second stage perhaps..

Or you could build SY's new phono stage design which should provide pretty good performance, although you will need to take steps to minimize interconnect capacitance with MM cartridge..

 

[fisher x 100 a]

its a fisher X 100 A, the amp has been worked over twice. Really want to keep this original as possible also. In saying that - buying five 12ax7 and 4 el84 nos tubes would be expensive if keeping original. running sovtek el84m and jj 12ax7.

wondering if the resistors etc on the phono stage are not matched to the rega of today era's.

 

[picowallspeaker]

Neither the x 100 t -transistorized- phono stage was capable of playing frequencies lower than 100 Hz .

 

[kevinkr]

Neither the x 100 t -transistorized- phono stage was capable of playing frequencies lower than 100 Hz .

I suspect given his moniker that he has a Fisher X-100-A which has all vacuum tube based circuitry - no transistors anywhere.

If you meant the output transformers - these are good for 30Hz or better.

I somehow doubt in the heyday of the LP that any reputable hifi manufacturer built an RIAA phono stage that could not reproduce below 100Hz...

The Fisher X-100-A

 

[fisher x 100 a]

Neither the x 100 t -transistorized- phono stage was capable of playing frequencies lower than 100 Hz .

hi there - could you please explain this - wont the fisher play a rega or.....

😕😕

 

[picowallspeaker]

I have (somewhere) the phono and tone/vol PCBs which I removed from the chassis-the old outputs were blown..probably somebody wise attached it to our mains (220)-it did have two interstage transformers!-and I tried the two circuits separately (12 volt operating)...the phono being a little constipated ...

 

[kevinkr]

I have (somewhere) the phono and tone/vol PCBs which I removed from the chassis-the old outputs were blown..probably somebody wise attached it to our mains (220)-it did have two interstage transformers!-and I tried the two circuits separately (12 volt operating)...the phono being a little constipated ...

I think you are talking about altogether another amplifier, please reread the original poster's comment about using a Fisher valve amp (amplificatore solemente a valvole, non e transistorizato) - the Fisher X-100-A has no pcbs and NO transistors, and was manufactured in NY around 1961 or so. . I have worked on a number of these amps, the one you refer to is probably the later solid state unit..

 

[picowallspeaker]

Yes it is tran-sis-tor-ised ! But as I see ,it may have the same flaws 😛
TX-100 Kevin you were right ,late sixties ,115-135V 50/60 Hz .The magic had just begun 😀

 

[Eli Duttman]

Fisher phono sections are not known for good bass extension, sorry. The simplest change is to replace the phono section tubes with Sovtek 12AX7LPSes. The Russian tube is clean and quiet, along with being well balanced.

IMO, you can't retain the OEM circuitry and improve the sound. If it were my unit, I'd change the phono section to RCA style. The attached schematic shows how.

 

[kevinkr]

Fisher phono sections are not known for good bass extension, sorry. The simplest change is to replace the phono section tubes with Sovtek 12AX7LPSes. The Russian tube is clean and quiet, along with being well balanced.

IMO, you can't retain the OEM circuitry and improve the sound. If it were my unit, I'd change the phono section to RCA style. The attached schematic shows how.

I think Eli's proposal to use this circuit would be a rather significant improvement over the stock circuit which in addition to other faults probably had insufficient loop gain to maintain proper bass response below 50Hz or so..

I used the 12AX7LPS extensively and like it a lot, sounds good, has a hum canceling filament that works pretty well, and seems to last a long time. (I'm using a bunch of them in the error amps of my system power supplies, and after 6yrs and thousands of hours of use they are still doing the job.)

 

[rotaspec]

To the original poster, I have posted this here as well as on the NZ forum:

I just looked at the diagrams posted on diyaudio. I can see potential for problems with an old amp like this particularly with the switches. So first off, be sure you have the rotary selector switched to the phono position AND the phono/tape slider switch in the phono position. If the slider is in the tape position, you will end up with sound as you describe.

If that is not the problem, then the switches themselves could be needing a clean (Deoxit). Having the phono signal go straight through a switch without amplification first is asking for trouble, but I see why they did it - they could use the same gain stage for pickup and tape heads. As no-one uses tape-head direct these days (now tape machines all have their equalisation built-in), you could afford to replace the entire stage with a design like Eli posted. However, that will take someone with experience to make it work within the Fisher chassis.

Gary