Since a DHT may have measurable grid current, any reduction of drive source impedance will reduce the distortion of that stage.
Joel: "Difficult to drive" does seem to cover a wealth of sins
Variable input impedance Eg DHT
High miller capaciance Eg High mu triode
High drive level Eg Low mu triode
They all have difficulties. But that's what makes it fun.
Joel: "Difficult to drive" does seem to cover a wealth of sins
Variable input impedance Eg DHT
High miller capaciance Eg High mu triode
High drive level Eg Low mu triode
They all have difficulties. But that's what makes it fun.
Sy said
Well I've used both lots of times and wanted to try something different - and as Brett observed, it's simple to change to self bias if I don't like the diode. I might change it for a VR tube of appropriate rating too.... The point of this was to have a base to compare differences with the amp. my son is building a 2A3 PSE - so it's supposed to be a bit different but easy to adapt and experiment with...
and SY said
Actually I like interstage iron - simple as that. When I was designing studio monitoring amps (solid state at that time) I always ended up with some iron in the design because it measured and sounds better that way... where the iron was put in the design depended on the particular design. Some even had OPT
Of course having iron in the design meant these amps had zero or very low amounts of global feedback - so for sand amps they were good
What brought me back to valves was micro dynamics and tone... particularly on the reproduction of human voice
Well I've used both lots of times and wanted to try something different - and as Brett observed, it's simple to change to self bias if I don't like the diode. I might change it for a VR tube of appropriate rating too.... The point of this was to have a base to compare differences with the amp. my son is building a 2A3 PSE - so it's supposed to be a bit different but easy to adapt and experiment with...
and SY said
Actually I like interstage iron - simple as that. When I was designing studio monitoring amps (solid state at that time) I always ended up with some iron in the design because it measured and sounds better that way... where the iron was put in the design depended on the particular design. Some even had OPT
Of course having iron in the design meant these amps had zero or very low amounts of global feedback - so for sand amps they were good What brought me back to valves was micro dynamics and tone... particularly on the reproduction of human voice
James D
The more i look at your amp the more i like it. You obviously have a pretty good idea of what you're doing anyway. If i had to make it, i'd probably use separate power suppllies for the two stages. Please keep us updated on your progress. It will be interesting to know how diode bias compares to conventional fixed bias at the grids. Will only take you a minute to compare.
cheers
peter
The more i look at your amp the more i like it. You obviously have a pretty good idea of what you're doing anyway. If i had to make it, i'd probably use separate power suppllies for the two stages. Please keep us updated on your progress. It will be interesting to know how diode bias compares to conventional fixed bias at the grids. Will only take you a minute to compare.
cheers
peter
Re: Brett, many thanks - MS no thanks
http://klausmobile.narod.ru/testerfiles/6s45pe.htm
The ECC99 is a great tube, not a bad fall back at all, and it will drive a 300B no worries.
As you are running the IT in PP, the flux generated by the two quiescent currents will cancel, so you only need to spec the gap for the small <i>differences</i> that you'll find even with matched tubes. Recently Gary Dahl emailled me in response to a question about using the Lundahl PP IT's (no gap), and his tests showed more than satisfactory bass performance even with unmatched tubes. Adding the gap lowers the inductance, and hence the bass, so use the smallest gap you can get away with, maybe set for 5mA as I mentioned in my earlier post. GD is an engineer an Tektronix so I trust his measurements, and I grateful for the effort he took to help me with this.
Same logic for the output transformer, no gap. Because I'm familiar with the Lundahl iron, if it were me building this, I'd use an LL1635/5mA and an LL1663PP (5k:8 cathode bias) or LL1664PP (3k:8 fixed). Chech what's specced for the Amity/Aurora's.
I still prefer an active shunt reg, but the glows are simple and work, and look so <i>purdy</i>
I really like the look of the Aurora2, but with 813's: 50W and horns, hmmmm, can you say but Officer, I thought the old lady next door really liked SRV.
Good luck with the project
There are plenty of PP and even GE PP in oils that are reasonably priced. The BG super E (name?) might also workJames D. said:
I had some trouble taming the oscillation with the 6C45 in some circuits, wondering why it sounded so bad. When I put it on the spec an, a huge RF spike gave the answer. Grid stoppers tight at each grid pin, and individual cathode resistors (4) killed the oscillation dead.
http://klausmobile.narod.ru/testerfiles/6s45pe.htm
The ECC99 is a great tube, not a bad fall back at all, and it will drive a 300B no worries.
As you are running the IT in PP, the flux generated by the two quiescent currents will cancel, so you only need to spec the gap for the small <i>differences</i> that you'll find even with matched tubes. Recently Gary Dahl emailled me in response to a question about using the Lundahl PP IT's (no gap), and his tests showed more than satisfactory bass performance even with unmatched tubes. Adding the gap lowers the inductance, and hence the bass, so use the smallest gap you can get away with, maybe set for 5mA as I mentioned in my earlier post. GD is an engineer an Tektronix so I trust his measurements, and I grateful for the effort he took to help me with this.
Same logic for the output transformer, no gap. Because I'm familiar with the Lundahl iron, if it were me building this, I'd use an LL1635/5mA and an LL1663PP (5k:8 cathode bias) or LL1664PP (3k:8 fixed). Chech what's specced for the Amity/Aurora's.
I was really tired when I first saw he post, and thought it was an XLR connector. D'oh.
I still prefer an active shunt reg, but the glows are simple and work, and look so <i>purdy</i>
I've been watching the project develop, and it's great. Take lots of note about Lunn's comments about the PSU. I was hugely sceptical, until I tried it all. It makes a big difference on my IHT amps too.
I really like the look of the Aurora2, but with 813's: 50W and horns, hmmmm, can you say but Officer, I thought the old lady next door really liked SRV.
Good luck with the project
I will post my progress with the project, it will take me a while as I have several projects on the go and not a lot of spare time. The first one is make sure my son doesn't hurt himself building and testing his 2A3 PSE...he's 14 and it's his first amplifier build and whilst he is used to electronic circuits he is not used to valves and B+ level voltages...
Next one is to build my speakers - I'm building Bd-design Quasars. Open panel design and about 98dB sensitivity...
Then I get to build this amp... mind you I always find the design is what takes the real time unless it's a rehash of a previous one. I haven't seriously analysed this one yet but I will before I build it.
Brett
many thanks for the link. I have been working from this one...
http://digilander.libero.it/paeng/high_sigma_vacuum_tubes_part2.htm
it's a useful site with some interesting work on it.
From the bit of reading I've done I was expecting quite a large static mis-match between the 6C45s but I take your point about IT transformer. I intend to try the Lundhals and also some Sowter trannies built to my spec.
I've built a few pp amps in my time but none with DHTs and none for 20 years! So most of my experience is with El34 and KT88s in pentode or triode mode. I always found that there was a fair degree of dynamic mismatching between the pairs even between tubes with a good static match. This lead me to specifying pp transformers with a dc current capability at around 20% of Ia. When properly spec'd and built they measured and sounded better than a standard pp. Of course they are bigger, heavier and more (much more) expensive as well....
Your amps. sound really interesting and tearaway's into horns - they're gonna rock - was my sons response!
ciao
James
Next one is to build my speakers - I'm building Bd-design Quasars. Open panel design and about 98dB sensitivity...
Then I get to build this amp... mind you I always find the design is what takes the real time unless it's a rehash of a previous one. I haven't seriously analysed this one yet but I will before I build it.
Brett
many thanks for the link. I have been working from this one...
http://digilander.libero.it/paeng/high_sigma_vacuum_tubes_part2.htm
it's a useful site with some interesting work on it.
From the bit of reading I've done I was expecting quite a large static mis-match between the 6C45s but I take your point about IT transformer. I intend to try the Lundhals and also some Sowter trannies built to my spec.
I've built a few pp amps in my time but none with DHTs and none for 20 years! So most of my experience is with El34 and KT88s in pentode or triode mode. I always found that there was a fair degree of dynamic mismatching between the pairs even between tubes with a good static match. This lead me to specifying pp transformers with a dc current capability at around 20% of Ia. When properly spec'd and built they measured and sounded better than a standard pp. Of course they are bigger, heavier and more (much more) expensive as well....
Your amps. sound really interesting and tearaway's into horns - they're gonna rock - was my sons response!
ciao
James
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