Lindz wrote:
I have the unused 5v tap on my pt.. will that generate enough voltage using this board to switch one relay? Would be simple solution but underpowered a bit vs spec. Using bridge rectifier in board, 7805 regulator and the relay I have is Omron g6a-234p 5v also sourced through Hoffman. Not sure enough about the math to know if it will switch reliably powered this way.
I suppose it would also be easy enough to add a small 6.3v transformer instead... thoughts?
I'm not sure about the math either. But I can report that the line below which a relay does not trip seems pretty tight.
On my setup (using the Colossal regulator and relays) I put an LED in series with the coil and foot switch. An LED with a forward voltage drop of 1.8 volts caused the relays to never close. But LEDs with a 1.6 volt forward voltage caused to relays to work every time. I figure I'm ok since I'm using a 12v PT and a regulated supply. So, I think it is a good idea to use an extra PT in your case.
Thinking some more about 100K power tube grid resistors vs. 220K, anyone considered switching them? Anyone think this might be worthwhile?
If you put a 180K in parallel with each 220K, connected only at the top end ie to the grids, and to a SPST switch at the other end ie going to ground thru the bias supply, you could switch the 180K in and out making 99K when switched in. The 220Ks are always connected thru to the bias supply so no interruption of bias supply and should be no switching pop because it's switching to ground.
I would put it on a manual switch on the back of the amp, this is not something I'd think of as needing to be footswitched, although that could easily be done too.
I'm bored at work babysitting something on the CNC today thinking about how I am going to rout the supply for the switching on this amp..
I think I may have a Skipz circuits chsw board that might be an easier solution to this problem
Since I am on my phone right now, its hard for me to look at a schem to double check but am I correct that a PPIMV pot only sees bias voltages and not significant DC? From memory there are caps in front of the leads that go to the pots, right?
I ask because the Skipz boards have a 200v dc limit on the j-fets as I recall but assuming less than 200v runs to the CHSW board that would work like a charm and its small too..
I once put 220K Resistors in parallel with the 220K Grid Resistors in my old Marshall 1987XL. I can't recall much difference in tone if I'm honest, if at all!
I have my amp up and running (without the switchable masters, I do not have the resistors I need for the PPIMV's) and it sounds pretty good so far. Hope to get the switching finished on the weekend and will post a few clips shortly.
Seems to compress pretty nicely vs some of my other amps. Not as much as a bluesbreaker but somewhat similar without as much muddy low end
The Magnetic Components PT actual b+ is about 425v vs the 412v they publish on the spec sheet using the lower voltage taps so I guess I am a bit on the high side for a "low b+" plexi. Thought it would be a bit lower but I suppose I could add a zener dropping string to compare even lower b+ later.
I do have a small issue with the depth control (.0047uf/1 meg pot, 100k feedback resistor, wired as below)
It functions as expected from about 1/8 the way up to wide open on the pot but below 1/8 on the pot when the control is down to near off or is off I get a loud continuous "thump thump thump thump...." about 4 x per second. A bit of scratchiness on the pot down low as well
Does not sound like motorboating as I know it but does seem like some sort of low frequency oscillation
My amp has pretty low filtering like early Plexis - 16uf PI and Screens, 32uf mains - ala the Rockstah Mod 5 specs so I suppose it would be somewhat more prone to oscillation but I am unsure if that is what I am hearing. It does seem usable and effective as long as I do not turn it down too much
2nd pic shows the Resonance control on the left side with a purple wire running across to the speaker +
Would shielding or other placement be helpful?
Any other suggestions...?
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instead of the usual .0047/1M, I recommend .0022/250K Pot and keep the the 100K NFR. This is what Larry (Of Larry Amps) recommends. Not as woofy and much more stable.
martin manning wrote:Nice voltage chart David, thanks for posting. I have a 50W built to Jana's schematic, and it's the go-to amp around my house. I've been curious about the EH 6CA7, so I just ordered a pair to try them out. It'll take a week or so, but I'll report back.
Hey Martin,
Just curious if you ever got your EH 6CA7's, and if so, what you think of them?
Hi Lou, got them, checked them out with the uTracer, but then set them aside and haven't put them in an amp yet. I'll see if I can find the time in the next few days.
martin manning wrote:I have a 50W built to Jana's schematic, and it's the go-to amp around my house.
Ok, you've got my attention. I tried to find info on this spec but came up dry. Any chance you would post the schem or explain the important differences? I'm assuming this is a plexi, but maybe it's a JCM800? Apologies for the hijack. I couldn't help myself.
This is a great thread. I built an amp like Jana's but with 4X6V6 power section and I cant get it back from the guy I let try it. He decided he would use it on tour.
I have a question for the low B+ gurus. What choke do you recommend with what filtering values? (assuming standard 2XEL34 power section). I have the Mag Components 3 tap PT so I can use 390 or 412 B+ if that makes a difference for the choke value.
Thanks Martin! Looks like both links point to Jana's original thread. Can you update with the link to your build? I'm really curious about this (probably because I have a couple project amps I would try this on).
