Heaters for Phono stages - questions.

the road not taken.
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andyn
Posts: 6
Joined: Mon Nov 26, 2007 2:22 pm

Heaters for Phono stages - questions.

Post by andyn »

Sorry this post is a bit long.
Past experience of Stephen (of Izzy-Wizzy fame) and myself (we have worked both together and separately on pre-amps for 12 years or more) indicates that:-
1. regulated heaters following a cap input filter don’t sound as good as RC type feed following a choke input stage
2. negative feedback in heater circuits is as poor sounding as anywhere else
3. AC heaters are easier and sound better where the signal levels permit
4. DC heaters are probably mandatory for low level stages
5. common mode chokes prior to the tube are beneficial for DC heaters

Perhaps others might validate these observations, add to them or otherwise?

The result of the circuit using choke input filter, then choke/capacitor, then common mode chokes is that the heater voltages are a PITA to set and vary considerably as a result of the input supply variations. Ideally I would like to make things easier to set up and with considerably less heater voltage variation with the supply variations. It will be realised that the series resistance and the variation of heater resistance with applied voltage make the heater voltage variation larger than would be expected i.e 10% variation in utility voltage results in more than a 10% variation in heater voltage. In England the utility supply can be 216v to 253v.

So my questions are
1. what do other do
2. is regulation preceded by choke input OK

I have a differential Izzy-Wizzy design (and great it is too) using three stages (as I need more gain than Stephen). So I have for each channel, 2*D3a, 5687 and 7044. They are arranged to employ 12v DC heaters with the D3a’s wired in series.


Thanks

Andy
reVintage
Posts: 365
Joined: Fri Sep 07, 2007 9:54 am
Location: Sweden

Post by reVintage »

What´s the problem using a CCS? But if this is about indirect heating the hole discussion seems tobe a bit over the top :wink: .
Brgds
Lars
andyn
Posts: 6
Joined: Mon Nov 26, 2007 2:22 pm

CCS feed

Post by andyn »

I have suspected its a bit over the top for indirectly heated tubes.

But to ask you some questions,
1 would you have a CCS for each tube or for each channel?

2 what circuit would you use for the CCS?

3 would you set the voltage "correctly" for each tube or just go with the rated heater current regardless of the resultant heater voltage?

4 would you use a choke input filter?

5 do you use common mode chokes for the heaters - close to each tube?

There are probably more questions but those are foremost in my mind.

I do believe that the topic of heaters is important and often neglected.

Andy
PET-240
Posts: 23
Joined: Sun Jan 29, 2012 1:30 pm
Location: BrisVegas, Austalia

Post by PET-240 »

Heya Andy,

I have no real exp here aside from being told that ac sounds better than dc, however the guy that told me that was using a filtered 317 voltage reg supply, I have since read elsewhere that heaters prefer a CCS, this makes sense to me as the current is what causes the heat, not the voltage, simplistic, but hear me out, if we are setting the current to what sounds best, and what gives best hum, according to the valve specs, if the heater resistance is within spec, it says to me the voltage will also be within spec. I have been wanting to try the Salas SSLV1.1 shunt CCS for heaters, just had no time at present.
That's my two cents, hope it helps!

Thanks,

Drew.
jarek
Posts: 62
Joined: Wed Feb 02, 2005 10:02 am

Post by jarek »

Andy, in my heavy Pussy EAR834 clone, I use a huge choke input (0.3H/1A) supply with SiC diodes, followed by another LC cell, then RC and finally LT317A-based CCS for each individual tube, decoupled with a tiny wima plastic cap at the socket. The supply is overblown perhaps for heaters, but the same voltage at some point is used for biasing. This is not something which came after years of experimentation, rather from an intellectual
considerations. The reason to put a CCS for each individual tube is IMHO clear if you think what it should do: cut the way for a fraction of the cathode signal current to penetrate the heater circuit.

Cheers,
jarek

PS I set the CCS for the current: it is the current flow which is resposible for the thermonic emission. With good tubes the resulting votage will be ok too.
jarek
Posts: 62
Joined: Wed Feb 02, 2005 10:02 am

Post by jarek »

I pre-het the tubes for 2mins before switching the HT. This is done with a time realy in HT PS transformer primary: the relay is driven by the heater voltage, taken at the firs LC cell.
What I have recently observed is that the heater voltage drops a bit after switching of the HT supply on: from 13.15V to 12.80V...any idea why? The sensing R is 8.2R
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