Nick wrote:
IMH, the biggest hurdle is understanding the load lines, and in realty, to understand those, you need to understand ohm's law.
One you have both of those, it makes a lot more sense.
Do you run evening classes in this Nick? I'm sure you'd have a class full of swots in no time and a heck of a lot of apples by the end of the first term?
So would I, there are papers from the age of valves, that I would love to understand properly, problem is my maths has atrophied with lack of use.
We could try and talk through some problems, or a simple use of load lines if thats of interest to anyone. But trust me, I only have a rough idea about this stuff, so if anyone thinks I am talking rubbish, please tell me.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Nick wrote:
We could try and talk through some problems, or a simple use of load lines if thats of interest to anyone. But trust me, I only have a rough idea about this stuff, so if anyone thinks I am talking rubbish, please tell me.
Bring a mortar board/pointy stick/bit o chalk/whiteboard/data-projector to the pub before WF3 and they'll be queuing round the block I reckon
I've done you a logo already
Wotcha fink? See the way I've spelt things wrong, it's a gift y'know...
Hi All
Yes I have problems with this load line stuff as well manly due to my lack of math’s causes of this is down to being dyslexic with numbers one good resin for wanting some software to do this at least the use of a software would get us in the right ballpark close enough for jazz perhaps some sort of step by step guide could be posted thou this may be a lot of work to explain in simple terms perhaps we should start a thread in the beginners section tube line for dummies well maybe this link will be a start http://diyparadise.com/tubeloadline/tubeloadlines.html
Hope this will help start this
Well the Sowter 600 ohm output transformers arrived today only ten days after I ordered them. Nice sevice and a really pleasant company to deal with.
The secondary is made up of eight windings in total, layered and doubled up to give four seperate coils.
This is a flexible arrangement as the configuration options are maximised. The secondaries can be arranged to give, with an 8 ohm speaker, reflected loads of 300, 600, 1200, and 2400 ohms anode to anode, and with 4 ohm speakers, 600, 1200, 2400 and 4800 ohms a-a. Rated at 12 watts, they are capable of handling 250mA of DC.
A little bit of maths and graph work, with the 6AS7 characteristics from Svetlana, gave a 1200R loadline for 9.8W continuous Class A power per channel; much better than the 4-5 watts I had been getting with Philip's Hammonds. To be fair to the Hammonds, this was because I could not get the primary impedance to go low enough, as the transformers were not designed for such low Ra valves. I'm running the output valves at about 3/4 of their maximum dissipation with 145 volts of HT and -60V of bias.
It is interesting that the predicted output of 9.8W should be achieved with an anode to anode load of 2400R; much much higher than the convention for triodes of 2x Ra. Other loadlines I tried all predicted less power with significantly higher levels of distortion. IIRC Morgan Jones found exactly the same thing when designing both his 6528 SE amp and the 13E1 Crystal Palace Push-Pull.
Because of the less than ideal linearity of the 6080/6AS7 family of twin triodes, one has to do a fair bit of work with the loadlines to find the sweet spot. Most other builders of 6AS7 pp amps have come out at around 1K per section (2K a-a) so I am in the same ballpark with 1k2 per triode section (2K4 a-a).
So, after all that, how does it sound. In two words, Bloody marvellous!
Thanks Philip for the loan of the Hammond transformers. They got the project up and running out of the ashes of the OTL.
I'll post more details on the sound quality after I have let the transformers bed in over the next few days.
Yes it did Nick and I still intend unshelving it sometime, but I liked the 6AS7 pp so much, I finished up rebuilding the whole of the OTL's twin PSU section to suit it.
It sounded so promising with Philip's Hammonds that I couldn't resist seeing what a pair of OPTXs suitable for the low Ra of the 6AS7s would sound like.
So was it going to be a new set of EL509s to possibly blow up again or the safe option? I chickened out I suppose calling the OTL project "ashes" was an unfortunate phrase and gave the impression that it was all over as far as I was concerned.
I think the OTL really needs to be a breadboard thing that can be played around with, tweaked up and made sure of in terms of safety before being committed to a cased amp.
I have been doing a bit of further reading and think I can do a combination of Bruce Rozenblit drivers and John Broskie's output ideas using the 6AS7/6080s I already have rather than forking out for 8 EL509s at £25 apiece.
Some form of regulation and a slow start circuit should prevent the achilles heel of the cathode "fuses" on the 6080s blowing the tubes. I also intend reducing the output power to around 10WPC whilst at the same time making the PSU a much heftier affair than the Rozenblit one. Having been a commercial design, the T8 PSU was probably designed with cost in mind and was what he could get away with and still keep a semblance of reliability. TBH as a DIY builder I was not impressed with the PSU at all, but it could have been just me.
I'm hoping you guys can help me out with the regulated PSU as it will be a first outing with one of those.
In the meantime here are a couple of photos of the finished "Hilversum" amp. You don't want to know what it looks like inside The hypocrisy and ensuing shame of exposing the innards to public view would be too much for me to bear
Steve
Last edited by Cressy Snr on Sat Sep 01, 2007 11:05 am, edited 2 times in total.
I'll come back in a few days on that one Nick when I've specced it out.
Meanwhile, here's a rough estimate.
I'll be looking for three power supplies.
one to the pre at 250V@100mA
one to the driver stage at 550V@100mA
one to the output stage at +/- 100V@3A
Haven't decided on the bias scheme yet.
Then there is the question of heating the valves.
I'll probably go for AC in the first instance as the push pull amp is hum free.
We are probably looking at 6.3V 8A for the outputs and 6.3V 4A for the small signal valves.
Outstanding Steve, the amp looks so very very good. In a sense I wouldn't worry that much if the amp didn't sound excellent when it looks as attractive as this.
My best efforts look, as Philip Ramsey would say, 'like a sunday afternoon maplin amplifier' and they still hum here, get too hot there and may sound interesting but ... I am in the doldrums, can't seem to get anything right, don't listen to music any longer, even felt pretty good about a pair of speakers featuring some ancient alnicos I knocked up yesterday (cabinets pre built by Rega, adjusted and filled... and then dropped one of them as I was ferrying it back to the shed. Dent in the top and scratch in the side of the cabinet - bollocks! I've knocked the tops of two 6B4Gs in as many days and even though, using a voltage doubler, I've got the HT right, the gain is not so good. I suspect the american interstages I am now using where, in a previous build I was using $10 Hongkers interstages and was getting reasonably marvellous sound and volume .... gggrrrr and the dam' heaters are humming, just enough to make me crosssssss. I hate having to use DC.
Im not getting you down am I? I think I'll go and wash my head in a bucket of water. I've got one here, somewhere.
The power supply has now been substantially upgraded with the addition of a totally OTT 625VA 230-110V mains transformer and its associated solid state rectifier and smoothing caps. It supplies 145V @ 300mA to the two 6AS7 output valves.
The Sowter mains transformer now supplies 530V to the high voltage driver stage and the 200V to the two 12B4A preamp valves from its 2 HT windings. It also does the heating for the three stages one 6.3V winding per stage
Handing off the high current output stage duties to that huge toroid has paid large dividends, both in terms of much less transformer heating and the sound quality of the amp itself.
The power supply is not very sophisticated, relying on sheer brute force, capacitance and bludgeon power rather than regulators but it does a grand job and is virtually silent.
Just have to remake the cover as the old one won't fit over the transformer.
Bring a mortar board/pointy stick/bit o chalk/whiteboard/data-projector to the pub before WF3 and they'll be queuing round the block I reckon 
with numbers one good resin for wanting some software to do this at least the use of a software would get us in the right ballpark close enough for jazz perhaps some sort of step by step guide could be posted 
I suppose calling the OTL project "ashes" was an unfortunate phrase and gave the impression that it was all over as far as I was concerned.
The hypocrisy and ensuing shame of exposing the innards to public view would be too much for me to bear 
