didit wrote: ↑Sat Jul 29, 2017 10:54 pm
10-15% unclear on the proposal. An updated schematic with the added triode pair please?
Best .. Ian
Ok... this was a lot harder to put into schematic form than I expected, but here it is. With no values for the components calculated yet, just an illustration of what I was thinking:
[...]
It seems like it should work as a way to switch between ac coupled cathode follower used to buffer between the EF 86 and the EL34s and adding a gain stage + DC coupled cathode follower for a presumably significantly higher gain in the preamp. I'm wondering if it would actually need to be a 3pdt switch to change anything else on the stage, like removing the connection from the grid leak resistor to the CF triode grid when it is set to DC coupling? I assume that if this is a workable idea, someone else has already done it, but I don't know where to look for an example.
No rigor & arithmetic behind me, but unnecessary to add any gain driving an EL34. However, what appears you've roughly sketched is an OD triode. That could be interesting. I'd split and relocate R11. Put a sensible grid stopper right on the pin of each triodes instead. Still head scratching about R12. Definitely put trimmers if not full faceplate control around the OD stage for inevitable gain/distortion tweaking. Three elements sharing B+ 300V, pentode & both triodes, needs parallel isolated sources. Look again at the Clubman for power design inspiration. Cannot imagine recommendation for "pop" suppression on switching. Perhaps someone with more experience in relays will pop in and advise.
Perhaps though, another simple case of "unused triode syndrome". If so, let it be.
Seeing 100 ohm resistor at the rectifier output and will again suggest a choke instead -- e.g., Hammond 157Q.
didit wrote: ↑Sat Jul 29, 2017 10:54 pm
10-15% unclear on the proposal. An updated schematic with the added triode pair please?
Best .. Ian
Ok... this was a lot harder to put into schematic form than I expected, but here it is. With no values for the components calculated yet, just an illustration of what I was thinking:
[...]
It seems like it should work as a way to switch between ac coupled cathode follower used to buffer between the EF 86 and the EL34s and adding a gain stage + DC coupled cathode follower for a presumably significantly higher gain in the preamp. I'm wondering if it would actually need to be a 3pdt switch to change anything else on the stage, like removing the connection from the grid leak resistor to the CF triode grid when it is set to DC coupling? I assume that if this is a workable idea, someone else has already done it, but I don't know where to look for an example.
No rigor & arithmetic behind me, but unnecessary to add any gain driving an EL34. However, what appears you've roughly sketched is an OD triode. That could be interesting. I'd split and relocate R11. Put a sensible grid stopper right on the pin of each triodes instead. Still head scratching about R12. Definitely put trimmers if not full faceplate control around the OD stage for inevitable gain/distortion tweaking. Three elements sharing B+ 300V, pentode & both triodes, needs parallel isolated sources. Look again at the Clubman for power design inspiration. Cannot imagine recommendation for "pop" suppression on switching. Perhaps someone with more experience in relays will pop in and advise.
Perhaps though, another simple case of "unused triode syndrome". If so, let it be.
Seeing 100 ohm resistor at the rectifier output and will again suggest a choke instead -- e.g., Hammond 157Q.
Best .. Ian
Theres definitely some unused triode syndrome involved, but still I want to see if it goes anywhere useful...
As for the choke, I have one between the plate node and the screen node, are you suggesting putting another one in before the first node or are you suggesting I move the existing to that location and use a resistor between the screen and plate notes instead?
nickfl wrote: ↑Mon Jul 31, 2017 4:53 am
As for the choke, I have one between the plate node and the screen node, are you suggesting putting another one in before the first node or are you suggesting I move the existing to that location and use a resistor between the screen and plate notes instead?
2nd choke, hum reducing & providing regulation. Resistor PI filter would too, just less so. Leave space, try the build without. Adds ~$20. Bothered by hum then add it.
In SE amps, an HT choke in a CLC filter between the rectifier and the first (Plate) supply node will do a lot to reduce ripple hum. Back in the days of small (~8uF) reservoir caps, it was common to see whole-of-supply HT chokes in many amps. But you need a choke with a DC-rating sufficient to handle the whole current. In an SE amp with an EL34 sitting at 300V on the plate and idling at 100%, I'd use a 120mA rated choke. This allows plenty of room for the standing plate current (about 80mA) plus an extra 20% for the peak current and a bit more besides. (FWIW, 120mA chokes are commonly available as screen supply chokes in Marshall 1959 Superlead 100W amps)
I think I may shelve this for a while, its really stretching my knowledge level to the limit. Which has been a good thing, I've learned a lot in the last couple of weeks messing with this, but I think I need to understand a little more about the way stages interact with each other before I am able to really understand if this will work well or not. I've got another project in mind I may try first, which fits in better with my current comfort level of splicing together pre and power sections from existing designs.
