Played thru an Ulbrick Arena recently and had a look inside it too. It was SS rec, but had a 50R after the rec, switched for an 'old' feel. Basically a bassman circuit with split cathode front end, excellent sounding amp btw.
i preferred it in the 'new' pos tho.
Been using my AC30 build sans 2 tubes a lot lately, just love it. Much cleaner than what im hearing with this xits, but very usable.
azatplayer wrote:What still puzzles me with the PPIMV is i can see 2.2M R's on the dual gang pot, so one would have to assume its a 250k dg pot, and theres a black wire to ground coming from the pot so there can not be a ground reference at the end of the 220k pair. So, what are they there for? I gonna have to sub some in to an amp and find out!
Looks like they're using something like Metroamp's LarMar PPIMV (a variation of the Ken Fischer PPIMV).
Colossal wrote:I've enclosed a PDF of what I can guess from the gutshots. I *think* that DG 250kA/2M2 Master Volume pot is grounded to the cathode bias R/C pair. You can just see a black wire running the DG pot, it disappears under the power on/standby/off switch and then you see it run just under the turret that the black Sprague cap and R are connected too.
I can't quite make out what the grid resistors are on the power tube grids. The bands look brown-brown-something or red-red-brown but those don't add up to be grid resistor values that seem "normal".
I am pretty certain the PI plate loads are 100k/100k to give some imbalance and a lot of 2nd order harmonics to the power amp. This is done in the 18W variants.
Hi,
Thanks a lot for this. I agree with the 100k/100k on the PI plates. I also agree on the PPIMV - that ground connection makes sense.
PI input cap is 0.01uf and goes directly from V2 to V3 (it's not on the board).
If the grid stoppers to the power tubes have two red bands then they likely are 2.2k which would be a bit higher than the ubiquituous Vox values. But they could be 22k as well which would limit the drive to the power tubes a bit.
Thanks for clarifying the values (you guys have good eyes!). I'll post a schematic later (today probably) based on your observations and the discussion here.
Colossal, in your schematic the 220k's and 100p cut cap should be between the 0.047u PI coupling caps and the MV pots, and I think the junction of the 220k's should be grounded.
martin manning wrote:Colossal, in your schematic the 220k's and 100p cut cap should be between the 0.047u PI coupling caps and the MV pots, and I think the junction of the 220k's should be grounded.
Excellent catch Martin, I really appreciate you going over it. Will post full schematic with as much information as we have and revisions soon.
martin manning wrote: and I think the junction of the 220k's should be grounded.
We are still unsure about that. But even if they were grounded the 2.2M safety resistors on the PPIMV would still be needed in case the pot wiper lifts.
A draft schematic is completed (will post later). One thing though...to my eye, it looks like all PS caps are grounded through the standby/power switch. Does that look right? (notice on the photo that a bunch of green wires terminate on the upper left hand lug of the standby/power switch).
Some people connect leads to an unused lug of a switch as a solder point which is something I wouldn't do. It is not part of the switching just a place to connect wires.
So im sorry for keeping on about the MV, Martin i got your reply when i posted the question on metros site. Cheers dude.
With the 220K's grounded, we end up with them in parallel to the pots and saftey R's in the MV.
Now if a suggested value of 250k with 2M2 were used, we end up with about 110K total resistance. Sooo, is that value ok as a grid leak? 220k or 470k is what we see on some other 18 watt builds.
What happens with the lower resistance here?
Even using higher value pots a saftey R's, we will never get close to the 220k values, with 1M pots and 10M R's would still only get to 175k or so.
Just need to know what effect that has on operation.
Cheers doods!
You guys have to see this. This dude is a master craftman for sure.
You have to scroll down thru the image panel on the left, about 3/4 way down, theres a gutshot of one of his big amps. Makes matchless look less than.
