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OT impedance

6/29/2004 1:43 AM
Steve Rathmann OT impedance
I'm working on a project using some salvaged parts including the OT from a SF twin reverb and instead of 4 6L6s I'd like to use 6V6s.  
I just wanted to verify that my thinking is not backwards. If the original setup was 4 6L6s which saw a 2K primary into 4 ohms, then for 4x6V6s I would want to double the load so the tubes see 4K.  
Does 4K sound like an appropriate load for a quad of 6V6s? What would the ideal load impedance be?  
Steve Rathmann
6/29/2004 1:52 AM
4k sounds fine; they'd probably work fine into the stock Twin OT as well (at 1.7k-ish). Ideal is a complex question. Ideal in what sense? THD in clean operation? Best overdrive harmonics? Highest power output? JM2C - it's going to vary depending on what your specs dictate. Personally, I'd try the twin OT as is, and then try doubling up the load to see if I could hear or measure a difference.
6/29/2004 4:04 AM
Steve Rathmann
Thanks Speedracer. I guess by "ideal" I meant the load which will "provide maximum power transfer" since this is the expression that often comes up when people discuss the negative consequences of applying a non-optimal speaker load to an amp.  
The amp will run two power modes: one with high B+, negative grid bias, and NFB; the other with lower B+, cathode bias, and less or no NFB. The idea being to try to have both the clean-and-loud thing (well, relatively speaking with 6v6s anyway) and a tweedesque power amp clip/compress thing.  
I'll take your advice and make the final judgement empirically by trying out both 4 and 8 ohms loads. At least I know I'm not likely to damage anything in either case.  
6/29/2004 5:50 PM
where the idea of 'ideal' goes to heck with pentodes is that the max power transfer point that may work nicely with triodes is a bad place to go with pentodes (lots of THD) so the actual operating point is series of compromises - basically does it sound good and not melt? ;) Was not trying to break balls.. there just is not a 'hard fast' way to arrive at the 'ideal' OT imp for a given pentode and B+ level. In the design books they go for min distortion generally, but if you follow the dist graphs, you get different mixes of even and odd and you may prefer to have more of one than the other based on taste and testing which will alter where you want the imp. Trainwreck wound up in the 5k range IIRC for their EL34 amp even though the world was weaned on 3k4 primaries.. bc he liked what he heard I gather. Is one better than the other? No.. just different. :)  
Have fun!
6/29/2004 7:40 PM
Does the distortion actually occur in the OT or does it occur in the output tubes because they see a mismatched output? I'm having a hard time seeing why distortion would occur in an OT tranny unless you oversaturate it which would be a dangerous approach prone to overheating.  
Also, do the tubes run hotter when they use a mismatched tranny?
6/29/2004 9:01 PM
distortion is in tubes - not talking clipping, just harmonic distortion (wave is not the same shape as the input). The slope of the load line will indicate the peak current, THD, etc - and the slope is also the load imp. And you thought you'd never use geometry. ;) They may or may not run hotter depending. My basic point was that if you use the metric of best power transfer for a pentode load, every commercial OT is mismatched.. Distortion occurs becuase the the up and down swing from the quiesent point (idle) do not share the same voltage/current swing characteristics - it's some variation on a funhouse mirror. Input signal is one way, output is another - eg: distorted. Again, not talking about clipping or overdrive here.. just basic THD.  
shameless plug perhaps?(but a decent pic of what is going on) here:
6/29/2004 11:38 PM
Steve Rathmann
You sure aren't making things easy here. :) (BTW no worries about "ball busting". Words like "best", "ideal", "optimal" must always be qualified for context. Part of the learning process though is understanding what the range of contexts is.) Loadlines and impedance are by far the most daunting aspect of amp building for me to wrap my brain around in a specific mathematical sense. There's a lot I'm able to do intuitively with topologies and "grafting" bits and circuits that I encounter together to ultimately come up with an amp design that will accomplish what I want it to.  
But I don't really have a sense of the affects of impedance. I've always heard of this in the context of someone wanting to use a load other than specified on the amp or pulling tubes - the rule of thumb that you can double or halve the "expected" load typically without dire consequence. Now I see why that is so.  
So the plot thickens somewhat...  
I'm now toying with the idea of switching to a pair of 6L6s for the high power mode, and to 2-6V6s for the lower power. Based on what we've discussed I think I would want to use a 16 ohm load to reflect a 6k8 primary which would be the best compromise between the cathode biased 6v6s (approx. 375volts) and the fixed bias 6L6s (470v).  
But the bottom line to take away I suppose is that there is room for experimenting and making decisions based on empirical observations.  
I'll have to spend a bit more time digesting your bit on load lines, and I know there's some good info a Steve Bench's site. Actually what I hadn't quite realized before reading your article was that the b+ at the plate changes A LOT. Now I understand how B+ is directly related to clean headroom. I wished I could just snap my fingers and suddenly "grok" all this information though.  
Anyway, thanks for your comments.  
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