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Discussion in 'Shock Brother's DIY Amps' started by robrob, Jan 2, 2021.
Some tube datasheets have charts to calculate THD (even and odd) using voltage and plate load.
I thought so at first, but the "screen tap" is on the other side of the transformer, resulting in negative feedback through the screen grid. I wonder about the math on that. This certainly is not normal practice. I would like to see the rest of that schematic. Not much signal should pass that 50 uF cap, though....
On second thought, that is just a filtered supply with the OT acting as a choke - the 1500 ohms and 50uF cutoff frequency is .47 HZ - no signal getting past that.
You're absolutely right, I was looking at one thing and imagining another. Still very interesting, I have not seen that arrangement before.
This was my first thought, and now I’m convinced. It’s a cost savings way to get a free choke out of an OT. I converted 2 of these to guitar amps. The first one I left mostly alone except for adding a preamp and it sounded awesome. The second I could not get to sound decent until I ditched the OT in favor of a Champ style one.
It looks like @tubelectron could help you: https://www.diyaudio.com/forums/tubes-valves/364867-6v6-single-stereo-hi-fi-advices-post6473468.html
Well @robrob , do not expect too much Hi-fi performance with the Hammond "Guitar Series" transformers, but OK : you may not need to go further, and nonetheless this will not prevent your amp from having a satisfactory tone, right ?
Personally, in the Hammond range, I would consider the 1628SEA (SE 5K 40%ULtap 30W) or eventually the 1629SEA (SE 6K5 40%ULtap 30W) which will be more able to respect bass extension due to their higher primary inductance. But OK again : it's definetly not the cost of a 125CSE !
Below is another experiment I made with a 6550 (KT120 projected) mostly with re-use parts :
As you can see, the output transformer (at right) is by far the bigger (3.5K 1K2ULtap 50W) :
Here are the square waves obtained with this circuit and that big OPT at 4.5W True RMS output : well, it's not bad for a SE amp, but not so amazing...
Here in comparison are the waveforms from my OTL (Output Transformer Less) 6080 amp project at 6.25W True RMS output : much better obviously, thanks to the absence of any OPT in the amp...
The big black casings on the pics below are the housing of the HV filtering of the OTL power stage - not an OPT !
All this just to show you the importance of the output transformer particularly in an Audio / Hi-Fi SE tube amp, even more than in a PP...
But again : it depends of course on how far you plan to go with your little 6V6 SE project !
Thanks @Tom Kamphuys ! I wonder how you could retrieve my quick post...
I do not post too much on www.diyaudio.com because I'm fed up to be criticized by some ugly and dogmatic so-called "experts" who are simply jealous of my builts, because my rule is : "no pics ? just blah-blah ? = no proofs, it never existed."
But it's me, OK ?
What about these for frequency response?
For this little amp I'm not looking for antiseptic tone so a true audio OT isn't needed to meet my design goals. I did look at the Hammond 30 watt audio output transformers but they are too large and expensive.
Dude, I was all set with my output transformers and you post that link. I think I'm going to have to use them. They look great, cost less than the Hammond and have ultralinear taps that I can put on a switch (or just ignore )
Working on the layout:
Thanks for the link @dankilling !
Yes, this is a cheaper intermediate alternative...
AFAIK I don't think that this brand is distributed here in Europe, conversely to Hammond - but I should re-check. If not, the shipping cost to France with customs will be prohibitive , compared to Hi-Fi Hammond models that I can buy from Germany !
Decided to go with a small headphone jack for input and move the volume control to the right side since most of us are right handed. I moved the power switch to the rear of the unit for improved 60Hz hum isolation.
Rob - am curious about the external humdinger/hum balance pot. Isn't this typically a set once and leave thing ?
Is it because this is an end user amp (or at least more than most DIY amps would be) ?
That's a really good point pmacaula. I'll change it to an internal or screwdriver type pot. This is why I post my layouts publicly during the design phase
@tubelectron Don Audio (Europe) has Edcor transformers.
I think I miss grid stoppers. Is that intentional?
The power tube does have a grid stopper. I left off the V1 grid stoppers because volume pot resistance acts as a grid stopper unless the volume is maxed.
Near final design: