EZ81 with B+ on the filament ?

[Ronquest]

I just got a Blackstar Artisan 15, one of their early handwired amps.  The EZ80/81 rectifier has the filament jumped to the B+.  What would be the reason for this?  I couldn't find any info on what benefit this would have for this style amp.  Some amps have EZ80 and some EZ81.

Attached is a schematic of the amp (mostly accurate) and in this amp pins 3 and 4 are connected.

Revised Schematic is now below. "Blackstar-Artisan-15H 201.jpg"

[sluckey]

Attached is a schematic of the amp (mostly accurate) and in this amp pins 3 and 4 are connected.

   If that's true then there cannot be a grounded center tap on that filament winding!

[WimWalther]

Attached is a schematic of the amp (mostly accurate) and in this amp pins 3 and 4 are connected.

   If that's true then there cannot be a grounded center tap on that filament winding!

Nope, that wouldn't work at all, would it?

Guess? They found that tying the fils supply to B+ was either quieter or necessary to prevent arcing vs. grounding the fils CT?

[DummyLoad]

Attached is a schematic of the amp (mostly accurate) and in this amp pins 3 and 4 are connected.

I drew that schematic - What is(are) the error(s)? It was based on the unit I had. Yes, I had same query about the EZ80 - But  the unit I had DID have 6.3V CT grounded and ONLY pin 3 connected to B+, The unit you had/have on your possession is obviously a different revision/tube line-up? 

It would be safer to float the 6.3V winding through a virtual CT to ground without a grounded center tap connection, obviously the unit you have has that revision? 

--Pete

[Ronquest]

Attached is a schematic of the amp (mostly accurate) and in this amp pins 3 and 4 are connected.

   If that's true then there cannot be a grounded center tap on that filament winding!

I'm thinking there must be 2 windings for the filaments.  I didn't have time to sort through the amp before needing to get the amp back in the head cab.  Just noticed the connected filament pin and B+.

I drew that schematic - What is(are) the error(s)? It was based on the unit I had. Yes, I had same query about the EZ80 - But  the unit I had DID have 6.3V CT grounded and ONLY pin 3 connected to B+, The unit you had/have on your possession is obviously a different revision/tube line-up? 

It would be safer to float the 6.3V winding through a virtual CT to ground without a grounded center tap connection, obviously the unit you have has that revision? 

--Pete

Your schematic has been helpful, even prior to buying this amp.  It gave me a good idea of what was going on with this design.  Blackstar has the EZ81 in their advertising and owners manual, but mine also came with an EZ80 EZ81 (Tube DR), whether that was stock or not, who knows.  Maybe someone wanted to lower the B+ with the EZ80 vs 81.  Blackstar also advertises this amp as class A 15 watts and 5 watt in triode mode. Marketing!
They apparently changed up the filter cap size and order from:
  47uF - choke- 47uF-22uF-22uF  Pete's schematic
100uF - choke- 47uF-47uF-47uF
100uF - choke-100uF-47uF-47uF
  47uF - choke-100uF-47uF-47uF  this is my version

The 100uf cap is over spec on an EZ80/81 with max rating of 50uF. 

I did notice a few other value changes and will go over this amp vs your schematic and post those differences. I will say they run the EL84s way above spec.


Question still remains, why would they tie B+ to the filament?

Edited to clarify EZ81 vs EZ80, I mistook the TAD EZ81 for an 80 due to voltage change with new JJ EZ81. 

[pdf64]

… why would they tie B+ to the filament?

You seem to indicate that the heater circuit for every valve is elevated to the HT DC voltage; if so, are you sure that’s correct?

Normally EZ81 have their own 6.3V winding on the mains transformer, totally separate to the 6.3V heater circuit for the other valves.

[Ronquest]

… why would they tie B+ to the filament?

You seem to indicate that the heater circuit for every valve is elevated to the HT DC voltage; if so, are you sure that’s correct?

Normally EZ81 have their own 6.3V winding on the mains transformer, totally separate to the 6.3V heater circuit for the other valves.

I'll get to testing all the filaments as soon as I can, but I'm thinking there are 2 windings.  Why tie B+ to the filament of this rectifier valve?   

[sluckey]

Why tie B+ to the filament of this rectifier valve?

The EZ81 has a maximum heater to cathode voltage rating. V_h-k(max) = 500V

Connecting the filament to the cathode ensures this max rating will not be exceeded.

https://tdsl.duncanamps.com/pdf/ez81.pdf

[Ronquest]

Why tie B+ to the filament of this rectifier valve?

The EZ81 has a maximum heater to cathode voltage rating. V_h-k(max) = 500V

Connecting the filament to the cathode ensures this max rating will not be exceeded.

So, do you think this was just for extra insurance, even with less than 400V B+ ?  No improvement to the amp's filtering, hum, or sonics?  As I haven't seen this in any other schematics with the EZs.

[DummyLoad]

So, do you think this was just for extra insurance, even with less than 400V B+ ?

Likely was done to overcome poor quality current production rectifier tubes.


--Pete

[sluckey]

So, do you think this was just for extra insurance, even with less than 400V B+ ?  No improvement to the amp's filtering, hum, or sonics?  As I haven't seen this in any other schematics with the EZs.

400+ B+ is not the issue. The peak AC pulses on the cathode are much higher. For example a 350-0-350 PT will put 495V peak pulses on the cathode.

[WimWalther]

Guess? They found that tying the fils supply to B+ was either quieter or necessary to prevent arcing vs. grounding the fils CT?

Ahem..

And IME the fills don't need to be directly (DC) coupled to prevent noise  or other issues. You can try using a 0.05uF 1KV cap to ground or B+, or possibly a high-value resistor - or both.

I was able to shut down a huge filament hum in the original ASMS M-60 Mk.II by tying driver AC fils to chassis with a single 0.05uF ceramic.

The original M-50/60 used 12AT7 / 12AU7 drivers, and had no filament hum. But when we switched to the Mk.II circuit with all 6SN7 drivers, the hum popped out of nowhere. Tube type didn't matter, same hum with new Russian & Chinese, NOS GE / RCA, JAN-PHILIPS 6SN7GTA..

[pdf64]

The 2 points Im aware of are to avoid allowing more than minimal V AC between cathode and heater, eg not allowing it to float with a high impedance to circuit common.

And to avoid excessive V DC between cathode and heater, well under the limiting value, to avoid stressing the insulation.

[scstill]

Normally EZ81 have their own 6.3V winding on the mains transformer, totally separate to the 6.3V heater circuit for the other valves.

Why would sharing the filament winding be a problem since the EZ81 is an indirect heater with HVDC connected to cathode not filament?

I have a low power SE build where the EZ81 (6CA4) and tubes share the filament winding.
The 6.3v winding center tap is connected to power tube cathode about 14v. The amp is fairly quiet.
https://stillampd.com/silvertone-deluxe-micro-04

[pdf64]

Normally EZ81 have their own 6.3V winding on the mains transformer, totally separate to the 6.3V heater circuit for the other valves.

Why would sharing the filament winding be a problem since the EZ81 is an indirect heater with HVDC connected to cathode not filament?
 …

The problem is that it puts significant voltage stress, several hundred volts, on the insulation between the cathode and its heater.
Heater elevation is great, but I suspect 14V won’t make a material difference to that.

[sluckey]

This idea of having a separate filament winding for the EZ80 and/or EZ81 seems to be a relatively modern idea. I guess the idea has merit. However, the original amps we love to clone used only one filament winding for all the 6.3V tubes. Browse through the schematic library and see for yourself. Marshall 1974 (18 watt), all the little Vox amps, Hammond organ amps are examples of amps that have the EZ81 heated from the same filament secondary as all other tubes.

I built the Marshall 1974 back in 2006 when I was just beginning to build amps after a 30 year vacation. I used the popular Heyboer PT because it was available everywhere. It had a separate winding for the rectifier. This 6.3V winding also had a 5V tap so you could use EZ81 or 5Y3, your choice. That is the only amp I've built that uses a separate filament winding for the EZ81.

However, none of the PTs I've used exceeded that 500V_H-K(max) parameter. As long as that parameter is respected I will have no concerns about sharing a single filament winding for all tubes.

[Williamblake]

Separate windings for EZ80 where the rule, I have seen lots of lab equipment from the 50ties and the EZ80 always had it's own winding.

[sluckey]

Separate windings for EZ80 where the rule, I have seen lots of lab equipment from the 50ties and the EZ80 always had it's own winding.

I have seen lots of 50s/60s guitar amps and organs that did not have a separate winding.

[PRR]

Lots of hi-fi audio didn't bother with separate windings.

Even Philips radios.

That WAS one point of these insulated cathode tubes: simpler builds.

[pdf64]

I suspect that stuff built to a price didn’t bother with a separate winding, stuff aspiring to a higher spec did.
eg Charlie Watkins’ Dominator, which strongly inspired the Marshall 18W https://el34world.com/charts/Schematics/files/Watkins/Watkins_dominator.pdf

I think it was Tomer (!) who was a bit cynical of the 500V h-k insulation limiting value, as those rectifiers use the same insulation material as regular receiving valves with a 100 or 180V limit.

[sluckey]

Original Marshall 1974...

[Ronquest]

Got the amp back on the bench.  There are two 6.3V windings.  The rectifier winding (no center tap), the preamp and power tubes winding has a center tap that's grounded at the power supply filter caps.

Also noted a few more differences from Pete's schematic, caps, resistors, HT layout.  Noting so far that looks major.   I'll spend some more time soon and post a revision for this 2011 amp.

[DummyLoad]

Got the amp back on the bench.  There are two 6.3V windings.  The rectifier winding (no center tap), the preamp and power tubes winding has a center tap that's grounded at the power supply filter caps.

Also noted a few more differences from Pete's schematic, caps, resistors, HT layout.  Noting so far that looks major.   I'll spend some more time soon and post a revision for this 2011 amp.

If I may, what's the S/N of the unit you're working on? If you care to share... I'd like to update the schema I drew to reflect changes that are in yours.


--Pete

[pdf64]

Original Marshall 1974...

Marshall’s ‘improvements’ to Watkins’ design were a single 6.3V winding and a hot switching standby; sheer genius 

Of course the Marshall cab was somewhat cooler than Watkins 3D sound cab 

[sluckey]

Of course the Marshall cab was somewhat cooler than Watkins 3D sound cab 

I don't know. That Watkins amp looks way cool to me. The Marshall 1974 looks like.... any other amp of the period. If I could have my choice, I'd pick the Watkins based on nothing but looks.

[JB]

Yeah but Marshall managed to squeeze one more watt out of it!

[sluckey]

Yeah but Marshall managed to squeeze one more watt out of it!

Did they? Or is that just a pencil stroke? 18 watts sounds better. 17 watts just sounds weird.   

[Williamblake]

Separate windings for EZ80 where the rule, I have seen lots of lab equipment from the 50ties and the EZ80 always had it's own winding.

I have seen lots of 50s/60s guitar amps and organs that did not have a separate winding.

There seems to be have been a different best practice back than concerning guitar amps and concerning lab gear. I remember you giving the explanation why that was the case but i forgot, i only remember it was not what i thought. Anyway i have never seen a circuit with the voltage amplifiers cathodes elevated to full B+ and i do have a couple of salvaged powertransformers with dedicated windings for the rectifier. I am happily using amps with with EZ80s on the same heater current elevated to somewhere around 40 VDC and that never was a problem. In oscilloscopes with two EZ80s philips elevated the rectifier for the very high voltage to that very high voltage so you had three heater windings elevated to three different voltages. My point being, it might not be worth the effort in a guitar amp, but having EZ80s heaters elevated to something different than the amplifiers heaters might just be ok as long as you can tell your heater circuits apart. You would notice some kind of fault being present immediatly if you mixed them up.

[sluckey]

Anyway i have never seen a circuit with the voltage amplifiers cathodes elevated to full B+

And we still haven't. In reply #21 Ronquest fessed up that the amp has two separate filament windings and the EZ81 is right by itself. So, it's quite alright to jump the filament to the cathode. That's exactly what is going on with the GZ34, except the jumper is inside the glass. I actually think this is a good idea. If I ever open up my Marshall 1974 clone again, I'll put that jumper on mine. That is, if I remember this conversation.   

[JB]

Yeah but Marshall managed to squeeze one more watt out of it!

Did they? Or is that just a pencil stroke? 18 watts sounds better. 17 watts just sounds weird.   

I was joking! I doubt neither of them got more than about 15W clean, depending on how you want to measure your distortion. But then distortion is something these amps do rather well. I’ve had the pleasure of playing an original 60’s example at gig volume with a selection of lovely vintage guitars. I was grinning for a long time after, despite the ringing in my ears. 

[sluckey]

I was joking!

Me too!   

[pdf64]

The ‘17 watts’ thing probably originates from the Mullard / Philips info, Watkins used the typical conditions published on the top right of p2 for the Dominator https://frank.pocnet.net/sheets/129/e/EL84.pdf

[Ronquest]

If I may, what's the S/N of the unit you're working on? If you care to share... I'd like to update the schema I drew to reflect changes that are in yours.

Here's the updates I could find.  Serial number is 201103UB2266
Also want to add, origionally thought the rectifier was an EZ80, I mistook the TAD EZ81 for an 80 due to voltage change with new JJ EZ81 and looking at the schematic that had an EZ80.  The TAD EZ81 was 340Vdc and the new JJ EZ81 is 366Vdc.

[PRR]

I think you will eventually want the 470k on V2.

[Ronquest]

I think you will eventually want the 470k on V2.

Thank you, your right, it's there.  Must have missed it after mudding drywall all day.  I looked for that a good while before eliminating it from the schematic.  Schematic will reflect change now.

For discussion sake, in this circuit the EF86 has negative feedback and is blended back in with the first 12ax7 gain stage right?  Thought being, they used an ef86 (marketing) to negate the problems with an ef86.  If the 470K were to be removed, isn't it just in parallel with the 15K/volume/tone?  What benefit is it and could it be eliminated?  Mind you I've never worked with ef86s and this is my first sighting in the wild.

[PRR]

It's not "blended back". It could be drawn clearer. V2 naked is high gain but the resistors constrain it to gain of ~~4.

[Ronquest]

It's not "blended back". It could be drawn clearer. V2 naked is high gain but the resistors constrain it to gain of ~~4.
https://el34world.com/Forum/index.php?action=dlattach;topic=31259.0;attach=111496

Well, that changes nothing... but my perspective.