03-24-2016 Post does not mapped to Knowledge Tree noviygera Chicago, IL Posts 177 Joined on 06-12-2009 Post #: 26 Post ID: 22576 Reply to: 21411 Resuming fiogf49gjkf0d After planning out the exit strategy for my DSP crossover, I am resuming my first Single Ended Pentode 307a tube amp project, the one mentioned earlier. I'd like to approach this with a certain degree of understanding so I have re-read carefully all posts and comments and this time I feel I have less stupid questions to ask. As I understand, choosing the proper transformers for a dedicated SEP will give me ability to tune the amp-speaker relationship (and this is what I want). But I want to lay down, what I feel, may be constants in the context of this project so that I can wisely narrow down the variables.The schematic shows 5K primary impedance on OPT. My mid horn is used between 220hz and 1.9Khz and in this freq range it's impedance sweep curve is between 25 and 14 ohms.Does this info help me select a proper OPT? Is it safe to assume that secondary should have single tap 14-16ohm and primary may have, perhaps, three taps 4.5K, 5K, 5.5K that will leave me with the single variable necessary to tune the amp-speaker relationship? Am I missing additional variables that will be used to further adjust OPT-speaker relationship?Who makes re-mappable OPT?I found SILK transformers are reasonably priced and will make custom. But to order custom I need to have a good idea of how to specify "custom". Gera

03-24-2016 Post does not mapped to Knowledge Tree Romy the Cat Boston, MA Posts 10,160 Joined on 05-28-2004 Post #: 27 Post ID: 22577 Reply to: 22576 It takes time sometimes. fiogf49gjkf0d Gera, tuning the amp loading is a tricky part. If you deal tube that you do not know and the driver that you do not know then there is no way for you to know the entry point into the game. I think the best way in this situation is not use what people recommend out there or what transformer makers advise to use. Your schematics show 5K into 16R, so use it for now but make sure that it is against 16R. As you set the things up you begin to listen the thing and begin to feel where you need to go, make sure that at this stage your tube is locked in proper operation mode.. The increase load to the tube will give you richer, slower, more harmonic, less detailed and more anti-euphoric sound. To a degree you can test it by shunting the driver by a good resistor of 20-40R. Divide the response of your driver on 2-3 virtual sectors if you horn is between 220 and 2K then observe 220, 1000 and 2000 regions separately as they might be differently impacted by different loading. You might like lower knee with one loading but upper knee with another. If you less load tube then you get "faster" sound, with more transients, details and faster decay. There is no known to me ways to test it beside changing transformer ratio. Transformer taps might be good in here if they made properly (feeling the full bobbin) but I prefer the re-mapable sections. Lundahl makes very friendly transformers for such experiments... A few warnings. As you change load make sure that you change the tube operation point and keep the tube in symmetric clipping. Idling tube more reduce power and reduce gain of your channel, so make sure that you re-adjust volume on the channel that you are trying to compare. Do not forget that your channel is not alone and it work accompanied with other channels. In some instances, an particularly in your where there is no HJF in band bath, it is preferable to have a channel to be driven a bit too hard (more idle tube)  and the shortage of harmonics at lower end to compensate by some overlapping of the next channel at the bottom.    It feels as your channels all have to be load in the same way for own reference impedance but it not might be the case. You need to load each channel in order to get a complete presentation from all channels that you approve. That might take time. Sometime only after a year or two you “get” that one channel might be driven more aggressively…   Rgs, Romy "I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

03-24-2016 Post does not mapped to Knowledge Tree JJ Triode Posts 99 Joined on 09-12-2007 Post #: 28 Post ID: 22578 Reply to: 21406 SEP loading and feedback fiogf49gjkf0d Novygera, If you are really going to run a single-ended pentode as a pentode (not triode-strapping the screen grid) then you will almost certainly need to use some negative feedback, whether global or local.  It is partly needed to reduce distortion, at least that is "common wisdom" but even more needed to reduce the output impedance, which is very high for a pentode running as such; like roughly 10-20 times higher than a triode, or triode-strapped tube.  Feedback gives you another variable to play with, and the subjective effects are somewhat similar to what you get with what Romy calls reduced or lighter loading: more feedback = more transients, "tighter" sound but eventually more hardness.  At least this is true for voltage feedback, such as globally from the output positive tap to the input tube cathode.  There is also such a thing as current feedback, which may sound differently. SEPs are kind of strange animals.  There is a lot less information about them out there than for SETs.  You can find some discussion on local (like the RH amplifiers) versus global feedback, and screen power supplies (gas tube regulation at significantly lower voltage than the plate is advocated by some.) Good Luck, JJ

03-25-2016 Post does not mapped to Knowledge Tree Romy the Cat Boston, MA Posts 10,160 Joined on 05-28-2004 Post #: 29 Post ID: 22579 Reply to: 22578 No pentode intended fiogf49gjkf0d I would like to point out that whatever I said might or might not be applicable to amp with pentode in output stage. I have absolutely no experience with driving speakers with pentode and I never used any feedback in my tube amps. So, I just do not know if anything that I know (neither I care) has any relevancy to pentode amps. "I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

03-26-2016 Post does not mapped to Knowledge Tree noviygera Chicago, IL Posts 177 Joined on 06-12-2009 Post #: 30 Post ID: 22580 Reply to: 22579 Mappable transformer fiogf49gjkf0d Romy, I think I'll try your suggestion. When you say "mappable transformer" do you mean one with multiple primary taps, like 3.5K, 5.5K, 8K?Gera

03-27-2016 Post does not mapped to Knowledge Tree Romy the Cat Boston, MA Posts 10,160 Joined on 05-28-2004 Post #: 31 Post ID: 22581 Reply to: 22580 Many of them. fiogf49gjkf0d Many transformers have multiple sections for primaries and multiple sections for secondary. You can map them in any way you want, parallel or in series, or even if you want you can love a section unused which is controversial and I am not widely support. Any transformer maker know most of the tube and will give you a good starting point to experiment. For instance Lundahl is represented in US by K&K audio and Kevin Carter who runs the shop will be very instrumental for you to figure out what you need. "I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche