audio-talk

So, would 3.5K (approx) be optimal ?

That statement is worth pulling apart a little, as it assumes there is but one "optimal", but there isn't. It depends on what you want, max power, minimum distortion, maximum swing, long valve life, they can all lead to a different choice.

Heavy rain here. Spud time!

Va @ 180V, bias 2V via Red LED, sorting through my handful of 6C45's to try and find a pair that match close at that operating point. 2K5 Hammond 1627SEA installed.

Thanks for the cf heads up too Paul, I've only just seen your reply, apologies for the delay. I've got a simple Chinese eBay 6J1 cf stage kit of parts to make up for just such occasions.

curious to know Dave what current you are running through the LED's.....years ago tried LED and disliked the sound....found out afterwards I wasn't running them hot enough.

A whopping 32.5mA at the moment Neal.

Its taken all morning to get a pair of 6C45 that are similar in current at the 180V OP.
Hooray! Got 2 that are stable at 32.5mA.


Anode dissipation is currently 5,85W at quiescent, Sound wise its got '"fuller", not squashed/compressed, just thicker.

As a 'whole thing' I'm wondering about what effect altering the PSU would have on the sound now? At the mo its running off 2 x Bench supplies for HT, in the past I've used these for many project mostly because I've been lazy and they're just so easy reach up and use.

Seeing as we're using such a low current I'm wondering about Shunt Reg which I've never done before. I know folk here have done a few low current projects that used them to great success like Phono stages.

Worth a punt? If so, Where do I start?

Lots of places to start

http://diyaudioprojects.com/mirror/memb ... /reg5.html

Loads of stuff on tubecad

There is the single triode with the load resistor of 1/gm to cancel power supply noise that audio note seem to have patented even though it was in wireless world decades before.

There is the more complex version Mark uses

Ta. More to it than I thought as always, erks. I was naively hoping it was just a case of strapping to Gas Tubes across an HT rail!

I'd better do some reading and learning.

For some odd reason the notification email for this topic was treated as spam ..

Mike H wrote: ↑Wed Aug 09, 2017 4:11 pm For some odd reason the notification email for this topic was treated as spam ..

^^ Such is the quality of my builds.... doomed to the trash box before they even get heard!

Ssshhh, I'm reading and concentrating here in the Library

In the Blencowe book on PSUs for Tube Amps theres a section that uses 6SN7 and ECC81's as active elements across the HT rail doing the shunt work. Lots of ways to do it it seems.

Dave the bass wrote: ↑Wed Aug 09, 2017 3:14 pm Ta. More to it than I thought as always, erks. I was naively hoping it was just a case of strapping to Gas Tubes across an HT rail!

Well you could try it like the phono amp supply , use a CCS then a pair of OD3's to ground ( 300V ? ) , but it's on the limit in terms of shunting , being a power amp - the OD3's could idle at 30ma, so go up & down 10mA. Might work - after all the big T-Rex supply I built for the 300B amp only shunts 50mA in the KT88 and sounds great. The T-rex supply has a much lower output impedance.

Yeah, there's a few ways around it it seems. After todays 'playing'/learning' session I now know I need a PSU that can supply 190V @ 32mA which accounts for the 8V drop across the OP TX Primary and the 2V for the Cathode Bias and the actual 180V Anode voltage on each channel. Reading various pages on the web and the Mr Blencowe PSU book revealed a stabiliser valve called a 85A2 which at 85V .... 2 of them in series would give me bang on the nose 190V. Genius I thought, except they're tiny in both size and shunt current capacity, doh. Genius badge confiscated by the design Prefect!

Then a penny dropped.

I've got a breadboarded 2 channel 6EM7 Series pass regulator PSU sat under the bench that I've used in the past to supply an ill-fated 6EW7 amp that was uber-pants. HT was 250V IIRC... I wonder if I could fettle that to a shunt regulator. 6EM7 has a small triode for the error amp/feedbck amp and meaty-ish power triode for shunt duties. Hmmm...

Why not try it as a series reg first, and then try as a shunt.

I could if I can get it working correctly, I powered it up today to work out what the HT was and noticed the Pots aren't controlling HT at the moment, theres also 2 TX's missing that I've used for another project but they were for 6EW7 heaters IIRC.

'Needs work' I'd say. I also need to draw out what I actually made here, I think it was PSU you helped me create a few years ago.