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Problem with 12ax7 phono stage

Basill

Member
2007-04-14 4:21 pm
Hi,
After building a simple ecc88 phono preamp i've decided that i need something with more gain. i've decidet to build pre from this link http://www.drtube.com/schematics/ai/ai500-ph.gif (it's from audio innovations 500 integrated amplifier), the problem is that i have 20mV of 50Hz sine, it wouldn't be nothing strange if the tubes were heated from AC, but i've build nice crc filter 2x4700uf-2ohm-2x4700uf. On the scope i don't get any anode distortions (the voltage is nice DC with only nV of some distortions). So what can it be? Before this i've made a lot of tube amps but i've never used ecc83/12ax7, so i'm not familiar with it :). (i've made an otl on 6sn7+5670 for my headphones own design, el84 SE amp, some modifications to chinese amps and lots of pramps ;), so there is no ground loop or anything like this :) ). When the ecc83 is plugged as a normal preamp (without the riaa correction) i get about 2mV of distortion (and it's also strage for me :) ). The input is shortened to the ground for the measurments.
 

kevinkr

Administrator
Paid Member
Please post some pictures of your build, seems likely that this is a layout issue. Did you use grid-stopper resistors mounted right at the sockets?

I assume the 4700uF - 2 ohm - 4700uF is the filament supply pi filter and not the HV supply.

If it is 50Hz that means it is probably a grounding issue or proximity to the mains wiring that is causing the issue.

Have you shorted the pre-amp inputs and measured the hum at the output?

I wouldn't say either of these designs is stellar, some important finishing touches are missing in both designs, and the later one IMHO has an equalizer stage that presents a relatively difficult load to the preceding stage.
 
Last edited:

Basill

Member
2007-04-14 4:21 pm
I will add the photos tomorrow (i don't have a camera :( ), the 4700 2oh 4700 is for filament (in pi shape :) ), for the anode i have 220uf-33k-220uf-33k-220uf (in pi shape ). No hum from anode DC . I also think that its a layout problem, but 20mV is a bit too much for such an issue, but the pre i've made on ecc88 was identical in layout and there was no hum (very low far below 0,2mV). I've measured everything on shortened input.
 
Good on you. Biasing the heater to a positive voltage is mentioned in the old RCA books as well as many others, but for some reason people keep grounding the centertap if there is one, or create a centertap with two resistors and ground that node.
For indirectly heated tubes, biasing the heater positive in relation to cathode is the right thing to do. (Centertap to ground is for directly heated tubes).

Why do u use those large caps if the heater is AC? Or did I misunderstand and it's DC?
 
Good on you. Biasing the heater to a positive voltage is mentioned in the old RCA books as well as many others, but for some reason people keep grounding the centertap if there is one, or create a centertap with two resistors and ground that node.
For indirectly heated tubes, biasing the heater positive in relation to cathode is the right thing to do. (Centertap to ground is for directly heated tubes).

Why do u use those large caps if the heater is AC? Or did I misunderstand and it's DC?

The heaters are DC, however I am sure there is some residual ripple on the supply which given the low signal levels present in the input stage couple through the cathode/filament insulation and get amplified. I elevate the filaments even when I use regulated DC on my filament supplies.

I suspect what he was hearing was probably 100Hz buzz unless the OP was using a half-wave rectifier on the filament supply. Pure 50Hz hum is much less obtrusive than the harmonic rich 100Hz sawtooth waveform typical of fullwave ripple.