cv wrote:

Btw, did you consider fixed bias to the output stage via the interstage secondary?

Today is a “special” day in the subject of this thread – today the last waited piece of the projects – the inner-stage transformer will be shipped to me and I will have no more impediments to render the project.  However, the intricacy of the project is not in the technicalities of new MF amp but in definition of the project’s success or in the visualization of what kind “new” MF midrange I would like to get out of Macondo’s MF channel.

I have written somewhere that since the problems with electricity are gone I was not pleased with the sound of my MF channel and that Macondo MF begins to remind me the way in which the field-coil drivers typically sound. I remember that I used even harder words then “do not like” – I was saying that I hated it. In the same post I was observing my continuing over the last 7 years increased of load over the plate of the tube that drives MF. Well, I think that I went overboard with loading of that anode and now the time to go slightly back, thanks the electricity clearly offers the opportunities not to fear.

After a lot of thinking at this point I have absolutely clear visualization what MF I would like to have as the result of my new MF channel. What I have now is not as bad as I was presenting it. My current MF are very elegant. They are inconspicuous with good ability to discriminate dynamic and tonal nuances, with a reasonable absolute tone, injected.  It can play hostile or peaceful but it kind of on a withholded, moderate side. It rather is overly-polite then neutral (in my definition of neutrality) and it does not step into the dangers water of brutality in the same way as my Vitavox S2 use to able to step in a few years back.

OK, what I would like to change? The key is not to hurt anything whatever I value and what’ve is good. In addition to that I would like to increase the ability of sound to accelerate without shortening the distance of the Sound need to stop after the acceleration. It is not that I need “faster” sound but I rather would like to get an ability to go faster but without actually going “faster”. Ironically the things that I would like to change in my sound are very same things that that I do not like in the sound of the today’s Boston Symphony.  The BSO does not “rape strings” when Sound is called upon it and they also stay overly save. This “string bite” is a tricky moment in audio, as it is very easy to produce, or more accurately to say overly-produce but incredibly difficult to control. It is not about an introduction of “edge” in upper MF – most of the HF speakers have it and I do NOT like it. The “edge” gives a Boolean-type “bite” and it impresses the Morons during the audio trade shows but it insult my audio objectives and listening preference. What I would like to have is Sound that gently and gradually enters into the well-controlled “string bite”, does there the necessary well-defied things and then gracefully lived the “edge”. I think in my new environment the type of the tube, the loading, the type of driver, the diffusion or sharpening the MF with tweeters and a new other flexibilities that I have with the new DHT channel  might give me a chance to play with it and to get the result that I would like to get. I use to have it but now I would like to get it at a new level of “cleanliness”.

The journey is promised to be interesting…

OK, after walking around the Melquiades and thinking where to local the new channel I decided to put it atop of the budge in own little Island. The Island is aluminum made with heavy conductive nickel coating to prevent any EMI/RFI

http://www.elexp.com/che_5944.htm

Of course it is not finished but it gives a good idea of what it will be.

I would be able to file the project this weekend but I am off to NY City to the rest of the week. Anyhow, I hope I will have the first results sometimes new week.

In my upcoming DH Milq MF DSET I will have an Innerstage transformer and output transformer. The loading of output tube has known effect to harmonics vs. transients, it is know, it is predicable and it is manageable. The Innerstage transformer might or might not have a resistor in secondary that impacts the loading of the driver stage. The loading of the driver stage also has the harmonics vs. transients consequences.

So, what I wonder: how different sonically the impact of Innerstage loading from the output transformer loading. My Innerstage transformer is reconfigurable 1:1, 2:1, 3:1, and 4:1. So, would the change in the Innerstage loading twice have the same sonic effect as the change of the output transformer twice (let discard the gain change or power change)? Does anybody have this experience to share?

Romy the Cat wrote:

In my upcoming DH Milq MF DSET I will have an Innerstage transformer and output transformer. The loading of output tube has known effect to harmonics vs. transients, it is know, it is predicable and it is manageable. The Innerstage transformer might or might not have a resistor in secondary that impacts the loading of the driver stage. The loading of the driver stage also has the harmonics vs. transients consequences. So, what I wonder: how different sonically the impact of Innerstage loading from the output transformer loading. My Innerstage transformer is reconfigurable 1:1, 2:1, 3:1, and 4:1. So, would the change in the Innerstage loading twice have the same sonic effect as the change of the output transformer twice (let discard the gain change or power change)? Does anybody have this experience to share?

A site visitor commented to my question above and I think the information and the view are valuable to share it:

"I guess I'm suggesting that applying a load to the secondary of a transformer never helps the sound of a properly designed transformer. There are four cases that I can think of where people purposely load a transformer and I'll brush on them briefly.

The output transformer:  The nature of this beast requires loading in order to deliver current, so this is the one case where loading is a necessary evil.  The whole purpose of this transformer is to deliver power to the load.  Every other audio coupling transformer is essentially there to couple voltage.

Secondary Loading to damp ringing:  I personally believe the lower the load (higher ohmic value) the better.   A 1:1 interstage feeding the grid of a tube essentially works 100% in the voltage realm since no current flows through the transformer.  It is only when you apply a load that current starts to flow and then losses become an increasing problem.  Often times people will look at the frequency response of a mis-applied transformer and think they are getting clever when they add a resistor or R/C network across the secondary to "fix" issues with frequency response (typically ringing).  The problem is these networks absolutely suck the life out of the music.  Here is the thing most people do not understand.  For a given source and load impedance a transformer can be designed to match those impedances without ringing.  Since a transformer manufacturer cannot offer devices for every possible combination they typically design a one size fits all approach and rely on secondary networks to explain  their choice of compromise.  One other thing secondary loading does to a transformer is impact the phase response.

Secondary loading to provide a proper load to the source:  This can often be a "kill two birds with one stone" approach where the added load both provides the proper load for the source and damps any possible ringning.  In the voltage realm the best example of this is the MC SUT.  I'll link you to a post I made about this that should cover the basics, sadly it seems to be falling on deaf ears.

Loading a transformer by a filter:  Many people find that lower impedance filters sound better than higher impedance ones and if transformers are involved, the low impedances provide a larger load. This like the output transformer becomes a necessary evil, yet it is still important to remember that a 4:1 to drive a 600R LCR should be designed quite differently than a 4:1 used as a preamp output which will be terminated by the cables and a 100K input resistor of the following amp.

My entire approach to magnetic design comes from collecting the needed info to make the proper choice of compromise for each specific situation.  I just did a pair of 8K nickel outputs that someone plans on using for a pentode.  This design had to be drastically different than the same 8K that would be driven by a triode.  Why you may ask?  

Well the 8K driven from the pentode has 80K source impedance and the 8K driven by a triode a fraction of that.  Sure both will "work" and to this day I have yet to see anybody make this distinction in transformer design.  I'm not sure whether the transformer makers are scared that if this info gets out they will need to double their product line of if they fundamentally do not get the difference in operation of the two since that is a circuit thing and they are magnetics guys."

*** Are you implying that in inner-stage transformer the sound  differences between the various ratios (discarding gain) will NOT be  ONLY because the change of loading?

I have today a local audio guy stopped by in my room and listened my playback. I have almost painful reaction from this listening session and could not wait when it will be over. The playback sounded very unpleasant, I mean so unpleasant that I was forced myself to stopped play in the middle of the tracks as I feel it was too insulting to listen. What annoyed me was incredible compression and distortions of MF, not the compression as it was during the electricity saga but it was different. I feel that the Focal tweeter in MiniMe nowadays is more dynamic and more articulate then  Macondo MF with is ridicules as Vitavox shall eat Focal MF on breakfast without creams cheese.

I was playing to my guest the different tracks and was thinking what the hell was going on with my playback and the saddest this that I have no idea. The leading blame is that my MF out tubes are gone but I can’t confirm it. The MF tubes still have an acceptable cathode emission but they work at full 8W for a year – a VERY heavy operation for this short-life tube. I can’t replace the tube as 30 other 6E6P that I brought from my storage were not acceptable by amplification or current – I need a full 35mA at 35 time of gain. So, I would say that one of 50 of my 6E6P can handle the operation – how long the tube might live in this mode? Not too long. The tube lost 1.5dB during the last month and MF sound a bit too soft. To compensate it in a way I was, perhaps stupidly decided to drive the tube even harder and now I have 200V and 38mA on the 6E6P’s anode.  However, the MF channel is loaded to Tribute amorphous core OPT with 40mA gap. I did measured how my Tribute entered over-powered clipping at 30mA a few years back when my former 6E6P was able to give 35 time of gain at 30mA. My current 6E6P can do only 32 times gain at 38mA-40mA, so I think I drove the Tribute core into saturation that comes as avalanche in amorphous case.  Driving the out output stage at lover current does not produce enough gain and killed the MF transients. I did not make any efforts to address it as my single-stage MF is living probably last week and will be replaced soon with two stages.

The guy who was listening my playback generally liked the Sound. What truly pissed me off was that I, as usually, asked if my guest can comment upon problems with Sound that he can recognize.  The guy made some semi-irrelevant critiques (in my view) but he did not stop the session with complain that it was absolutely un-listenable. I was not sitting at my sweat-sport and was way off the axis but at my location the sound of MF twisted my teeth. In his location he had to have the same effect only at 10-20db more focused and he did not complain!!!  Hm, interesting! If somebody played THIS type of Sound for me then I would spend a half hour of bitching about the heard instead of insulting myself by further listening.

Well, I do not know if my idea about the deal MF tube is right but I have no other explanations. Let hope that it is what it is as otherwise I have no idea what it is.

Paul S wrote:

Didn't you first become aware of MF "problems" when you got everything else working better via the PP2000, and the MF wound up dropping back to weak sister status with respect to system-wide PP2000 benefits?

Paul S wrote:

With luck, you will soon have again a midrange that is at least the equal of the rest of your system, and you can use most of the 6E6Ps instead of having to cull 1 of 50.

Romy the Cat wrote:

Do you know what I did for the last 4 hours? I was soldering one freaking cathode resistor for Milq DSET and was trying to figure out why the voltage does not drop on it. Do you know what the problem was in the end? The problem was that I just forgot that my filament transformers have center tap grounded. What a find!!!

Ok, it look like it is not a big deal and I might lift the ground on the filament secondary. However in this case the IDHT tunes heaters will not have reference to ground. I do not like completely floated filament. Well, they will not be completely floated but will have some reference via the DHT cathode but I do not know if it is kosher. At least need to re listen everything to see if it works.

I have two alternatives. First is to put a new DH filament transformer for 2.5V, 4V and 6.3V. The second solution is to put from my grounded 6.3V that I use for my IDH tube another transformers that will convert 6.3V into 2.5V, 4V and 6.3V only the secondary will be galvanic decoupled.

I am still considering both solutions…

Well, I do not know what DHT filament scenario I will choose. I consulted with different sources

http://www.intactaudio.com/forum/viewtopic.php?t=823

…but I still have no definitive opinion. There are few logical cons and pros for different implementation scenario and although I do have a bias to go for a dedicated DHT filament transformer but no one talks about the results in term of sound. So, I decide to try BOTH dedicated and decoupling transformers. Both of them have ordered, will see where it will lend me…

Meanwhile I took a temporary dedicated 6.3V 25A transformer that was laying around and put the DHT Milq together. It worked very fine. I did not listen the amp yet but rather just was observing how both 2A3 and YO186 work together in the SAME operation point.  They do OK, here are the numbers:

The 2A3, the Sovtech one for now: 315V on anode, 51.5V dropped on cathode resistor, which lives for the tube 263V.5. The plate current 43mA with anode dissipation 11.3W

The YO186: 317V on anode, 49.9V dropped on cathode resistor, which lives for the tube 267V.5. The plate current 49.9mA with anode dissipation 13.3W

It looks OK, but I probably will drive YO186 a bit softer, let say at 11W.  I can wait to start listen this thing…

I made experiments running HF filaments from dedicated Hammond 6.3V transformer vs. lifting ground on my 6.3V IDH filaments. I did not listen Sound but rather measure the noise – the result was identical. In the best cases I was able to get 0.63mV at output – way bayonet of what I need to worry even with 109dB sensitivity. In practical term I prefer the lifting ground on my large 6.3V IDH filaments secondary. What I drive my HT filaments from a separate desiccated transformer then I need to be very accurate with grounds as I have 1.2K resistor between amps ground and the HD filaments. Any problem as the cathode of DH tube accept ground potential and plate current of the tube run away to 100s mA. I have a couple of accidents like this running accidently the Sovtech 2A3 at 100s mA. The good part that the large 20W 1.8K PS resistor act as a fuse … unsoldering itself from the circuit. To my surprised the immune to sound Sovtech 2A3 is still operation after of few stresses like that.

With the floating the secondary of my 6.3V IDH filaments I can do whatever I want and there is no damage to DH bias, or a need to isolate anything. Thinking further I realized that if I use a dedicated DH transformer of floating the secondary of my IDHT then in both cases I use my cathode resistor on DHT as biasing resistor for my heaters. It might not sound kosher but it is what it is in all cases. I have a dedicated better made filament transformer (bile like audio transformer) coming next week as well as filaments isolated (6.3V to 4V) transformer coming. I will see what they do in terms of Sound but I think I do not have a fear anymore to use my cathode resistor as bias for IDHT filaments.

There are however more complication then that. I made some fine adjustments in values and now I have both tubes 2A3 and YO186 runs 41mA . Here is the new draft of the circuit:

The operation is very stable and there is no noise. The amp produce OK sound (to my little control monitor, not to my main playback) and I have to tell you that the idea to have an amp with both tube sockets was brilliant idea. It is so cool that I might just name it and I have no idea why others do not do it.  The plate impedances of those tubes are very close and the amp might have as well the sockets for 300B/45 or AD1. Just shut down the amp, plug a new tube, adjust lowest amount of AC mV on the OPT secondary via the filaments attenuator and here we go…

Now is the problem. My DH DSET measures like crap. I did not go deep into it yet, will do obviously. I was running the sine wave trying to determine the LF cut off and making sure that the amp’s filter kicks in before both of transformers clipping will come to the picture. It was fine. I am running the Innerstage transformer for now as 2:1 and I was measuring with my scope with AC voltage that driver stage delegate to DHT’s grid. It was fine as well – it sits deep in A1. Something made me then to switch the generator into square wave and to see what amp outputs. Holy cow it is something absolutely not identifiable – at all frequencies with all amplitude. At 10K the amp shell do very good square wave but in my case it was a nightmare.  I do not know at this point what is ringing in there: the input filter (highly unlikely), the Innerstage transformer, the output transformer, the secondary of the Innerstage need additional damping, or something else. I know that until I have the acceptable square wave from my DSET then I would not even make any attempt to listen the thing. This is over 3.2kHz DSET it shell be handling square wave very fine throughout it’s whole range.  I need to blow each stage and each transformer with squares individually to find out what is the problem with this amp so far…

The voltages on the schematics are the actual live voltages driving a full power 5000kHz. The next 10mV at input clip the amp. I do not know yet what clip first’ driver stage, output stage of any of transformers. Anyhow, the full power set as a full 10 squares on the scope

Here I drive the same voltage all the way up until it hit -3dB. It is 23kHz. 

Then I drive the same voltage all the way down. The -3dB is at 2.4K

What is interesting is to drive the channel with something where transformers have no inductance. I took 500Hz. It is many dBs down and I added a lot of v/div. Still if spread it over the full screen then the distortions are visible but the tops are not clips – that is indication that the filter roles fast then minimum inductance – a good sign

What I do not like is below. This is a square wave into 5K that is perfectly in band-path. I did measured the same after the driver stage and blocking cap with 15K resistor on plate driven from 400V and the squares were MUCH better. So, it is not the filter and not driver state.

I was loading the inner stage secondary and it has no impact…