Maybe run the screens from a second transformer, its CT at the lower voltage and the ends of the transformer capacitively coupled to the UL taps on the main tranny.
Well I've done mucking about with these 807's and OPT's and have tried various OP stages, namely AB1, AB2 and extended class A, and have been surprised at the results which are listed in brief.
Class AB1) after sorting out instability issues I tested the amp, THD below 1% at 10v RMS OP, flat frequency response from 100hz to 30khz. Music test left me cold, yes it was hifi, but it didn't get me Dad dancing round the living room.
Class AB2) After sorting the driver stage out paralleling the twin triodes in a 6H8C, one 6H8C per side of the PP stage & building a regulated 300v PSU for the screen grids - tested not so good, higher THD and prone to going gaga at clipping, F response still good, a lot of 3H. Music test, ahooo! Now were talking, silly gurt big smile on the old mush, music sounded alive, sparkly. You know when an amp sounds good, you reach for the volume control, crank it up, slip on some classic 70's roots reggae and your off skanking, making your kid/s laugh their ar*es off. However, although I loved this OP stage, the OPT pri wasn't optimal, other building issues too, that siad I am 100% definately going to build a AB2 amp.
Extended class A) For those not familiar with this configuration, all it is, is two 807's strapped as triodes, two as beam tets which are near cutoff at low volume but kick in when more power OP is needed. I got this one well wrong at first, bad driver/LTP AC balance and was actually applying PFB - THD 8%! 10% or higher when I cranked it up. Music test, sounded great, same result as AB2. Rectified my mistakes, THD now 0.05% at low OP power, F response flat as a pancake, music test, one word - sh*t.
Best qualify that last statement, with good THD readings the sound is different, hifi - bass, mids trebles all balanced, I'd describe it as controlled, sauve, a good metaphor would be, hifi type specs - chatting to a well dressed intelligent high society lady about art, culture and 15th Century Siamese birthing hats. AB2 & everything screwed up, every thing wrong, extended class A - talking to a vivacious beautifull young lass at a party, you feel alive, all sparkly, like being 16 again having your first proper drink.
Although this has taken weeks of mucking about and I've no real data to make a proper comparison it could be that designing and building a valve amp to be "wrong" is an option worth exploring.
Andy.
Class AB1) after sorting out instability issues I tested the amp, THD below 1% at 10v RMS OP, flat frequency response from 100hz to 30khz. Music test left me cold, yes it was hifi, but it didn't get me Dad dancing round the living room.
Class AB2) After sorting the driver stage out paralleling the twin triodes in a 6H8C, one 6H8C per side of the PP stage & building a regulated 300v PSU for the screen grids - tested not so good, higher THD and prone to going gaga at clipping, F response still good, a lot of 3H. Music test, ahooo! Now were talking, silly gurt big smile on the old mush, music sounded alive, sparkly. You know when an amp sounds good, you reach for the volume control, crank it up, slip on some classic 70's roots reggae and your off skanking, making your kid/s laugh their ar*es off. However, although I loved this OP stage, the OPT pri wasn't optimal, other building issues too, that siad I am 100% definately going to build a AB2 amp.
Extended class A) For those not familiar with this configuration, all it is, is two 807's strapped as triodes, two as beam tets which are near cutoff at low volume but kick in when more power OP is needed. I got this one well wrong at first, bad driver/LTP AC balance and was actually applying PFB - THD 8%! 10% or higher when I cranked it up. Music test, sounded great, same result as AB2. Rectified my mistakes, THD now 0.05% at low OP power, F response flat as a pancake, music test, one word - sh*t.
Best qualify that last statement, with good THD readings the sound is different, hifi - bass, mids trebles all balanced, I'd describe it as controlled, sauve, a good metaphor would be, hifi type specs - chatting to a well dressed intelligent high society lady about art, culture and 15th Century Siamese birthing hats. AB2 & everything screwed up, every thing wrong, extended class A - talking to a vivacious beautifull young lass at a party, you feel alive, all sparkly, like being 16 again having your first proper drink.
Although this has taken weeks of mucking about and I've no real data to make a proper comparison it could be that designing and building a valve amp to be "wrong" is an option worth exploring.
Andy.
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Loved the review! You're not from Zummerzet by any chance, with the 'gurt'? 🙂
Music and the amplifier are inextricably linked. I saw a review the other day where the reviewer was describing different pieces of music on an Acoustic Research amplifier. After one song he wrote ' ... by then the tears were flowing uncontrollably ...'. I hope I never go deaf!
Music and the amplifier are inextricably linked. I saw a review the other day where the reviewer was describing different pieces of music on an Acoustic Research amplifier. After one song he wrote ' ... by then the tears were flowing uncontrollably ...'. I hope I never go deaf!
Yep, gurt is a good word, I like old words like fettle and jiggit. No, not from Somerset, but have travelled a bit and read the odd book, picking up words and phrases from here and there... the only downside to using archiac language being folk don't understand what the flegnog I'm going on about half the time, still, I'm getting old so am allowed to be eccentric.
Have reconfigured the front end and PS to now add distortion, just have to optimise so I get the right distortion, but part of the trick apart from biasing is to unbalance the amplifier so that CMRR isn't good. Quite good fun this, though it may get purists and engineers hot under the collar.
Andy.
Have reconfigured the front end and PS to now add distortion, just have to optimise so I get the right distortion, but part of the trick apart from biasing is to unbalance the amplifier so that CMRR isn't good. Quite good fun this, though it may get purists and engineers hot under the collar.
Andy.
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Reminds me a bit of Rambling Syd Rumpo singing Green Grow My Nadgers, O.
I watched your YouTube ravings and found them most 'taining indeed. Observing closely as I have a few 807's in the mail and speeding toward me, and a boring Williamson isn't a good use for them.
I watched your YouTube ravings and found them most 'taining indeed. Observing closely as I have a few 807's in the mail and speeding toward me, and a boring Williamson isn't a good use for them.
That dates you, Kenneth Williams want it?
"I watched your YouTube ravings" Ravings perhaps is a bit strong, makes me sound like The Donald, who is verifiably insane, but take your point and hope they were helpful, or hey, 'taining.
Had a bit more of a muck about with this yesterday and re-fettled the front end and PS aiming to optimise 2H; LTP 6H8C, Ra 33k's & 10k pot, so Ra = 38k CCS set to 8-10mA. So, powered it up and fed OP into SC and used Soundcard Scope's FFT graph. The OP stage has two DC balance pots to balance DC to each side of the PP OP stage as well as adjustable bias. I fiddled with these to get max 2H, this was adjustable over 10dB, 2H was about 10 - 15dB higher than 3H, this at about 5v RMS OP, @ 1khz.
Adjusting the LTP "un-balance" pot also effects harmonic component, but to a lesser degree. Turning the pot clockwise decreases the triode that has g1 strapped to gnd, anode load, this increases 3H & lowers 2H, but over only a few dB. Turning the post CCW does the opposite.
I also made NFB adjustable and also tried applying PFB, this obviously affects the harmonic component a lot. Applying PFB in greater amounts increases 3H to the point where 3rd, 4th, 5th H rise in equal amounts. Doing the same with NFB equalizers harmonic content, but is less dramatic.
OFF with the dummy load and connected the speaker, then tried all of these whilst listening to music. Applying PFB "thinned" the sound, to the point of harshness. Unbalancing the OP stage gives the most interesting but subtle change, making the bass more "creamy". Adjusting the LTP does change the sound, but it's very subtle, I'd need to increase the pot to 20-25k.
All this is bleedin obvious to most of us, I was just after a quick & dirty test to get some ideas for possible further, future experimentation.
Started work on the chassis too, I now have the layout sorted and am pretty sure I'm going to build the extended Class A amp, with current PSU etc this will give me 28w though it's capable of nearer 50w. Am going to try a SS tone control as well, before the valve IP stage or as part of the NFB, something like a Baxandall.
Andy.
"I watched your YouTube ravings" Ravings perhaps is a bit strong, makes me sound like The Donald, who is verifiably insane, but take your point and hope they were helpful, or hey, 'taining.
Had a bit more of a muck about with this yesterday and re-fettled the front end and PS aiming to optimise 2H; LTP 6H8C, Ra 33k's & 10k pot, so Ra = 38k CCS set to 8-10mA. So, powered it up and fed OP into SC and used Soundcard Scope's FFT graph. The OP stage has two DC balance pots to balance DC to each side of the PP OP stage as well as adjustable bias. I fiddled with these to get max 2H, this was adjustable over 10dB, 2H was about 10 - 15dB higher than 3H, this at about 5v RMS OP, @ 1khz.
Adjusting the LTP "un-balance" pot also effects harmonic component, but to a lesser degree. Turning the pot clockwise decreases the triode that has g1 strapped to gnd, anode load, this increases 3H & lowers 2H, but over only a few dB. Turning the post CCW does the opposite.
I also made NFB adjustable and also tried applying PFB, this obviously affects the harmonic component a lot. Applying PFB in greater amounts increases 3H to the point where 3rd, 4th, 5th H rise in equal amounts. Doing the same with NFB equalizers harmonic content, but is less dramatic.
OFF with the dummy load and connected the speaker, then tried all of these whilst listening to music. Applying PFB "thinned" the sound, to the point of harshness. Unbalancing the OP stage gives the most interesting but subtle change, making the bass more "creamy". Adjusting the LTP does change the sound, but it's very subtle, I'd need to increase the pot to 20-25k.
All this is bleedin obvious to most of us, I was just after a quick & dirty test to get some ideas for possible further, future experimentation.
Started work on the chassis too, I now have the layout sorted and am pretty sure I'm going to build the extended Class A amp, with current PSU etc this will give me 28w though it's capable of nearer 50w. Am going to try a SS tone control as well, before the valve IP stage or as part of the NFB, something like a Baxandall.
Andy.
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'twas indeed.
Well, I could have used wafflings, or blatherings, or any number of other suitable words, but the only one that fits The Donald is "rantings." He is indeed certifiable.
Carry on.
Yep if you want UL. Divide the 40% UL taps to ground with a potential divider say by 2/3. This will reduce the voltage. Feed this into a dual cathode follower which will buffer and feed the cathode outputs into the screens of your 807's. Or you could use 3 resistors one to HT one to 807 plate and one to ground. By adjusting these you can control both the voltage and the amount of UL.
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