1. I try to avoid MCI power transformers because they can be all over the place. This particular one is no exception, it's supposely wound at 350VAC at 117V, now with 124V in it's a whopping 403VAC unloaded, which is about 30VAC higher than what it should be for a 124V mains.
Haven't used them yet, thanks for the tip and the saved aggravation.
2. For this cathode biased circuit, bigger iron is a waste. This is an honest 25W circuit. I think the OT is perfect
Good, I'll stop thinking about upping the OT in my 5C8.
3. Rolled several pre amp tubes could not get any 6SC7 anywhere near 160V.When I rolled the good glass stuff, the voltages were in the 135V range. I changed 6SC7 plate resistor from 249.9K to 180K and that did the trick to bump the preamp up in the 160V range which made a great difference in volume and headroom.
I had the same experience with V2 (cathode bias modded) in the 5C8 though I tried varying the cathode resitor instead of the plate's. Maybe when Fender measured they had a low measuring plate resistor. I will go try 180K too now that you say it makes a noticeable difference.
Tone: I went into this debating about whether to use the stock value caps or change the front end to .05 or .02 and dropping the cathode cap value as well. However, I wanted a stock circuit baseline. All would have been a good, IMHO it is far to bass heavy for guitar.
Good, if I it build I will go straight with .05 and 25mf. If you make these changes please follow up.
At this point it looks like TAG has stolen The Pedal Steel Forum's and The Hoffman Forum's thunder concerning 5B6 builds
Should have been Post Mortem......hey not depressed just yet....lol. I would use this PT again, I just would put a 550/600V eqivalent cap at the first node. I do need chase down that distortion issue though.
I don't think I'm going to refine this circuit for guitar as I think it will make a decent harp amp.
ToneMerc wrote:I try to avoid MCI power transformers because they can be all over the place. This particular one is no exception, it's supposely wound at 350VAC at 117V, now with 124V in it's a whopping 403VAC unloaded, which is about 30VAC higher than what it should be for a 124V mains.
Do you find them consistently high? AFAIK they quote voltages with all windings fully loaded.
ToneMerc wrote:For the first few secs until the 5U4 is warmed up it's hitting the first cap with 506 VDC...
I'm trying to make sense of this... I'd have thought the vacuum rectifier would make a soft-start as it warms up?
Martin, my experience has been that they are usually high. IC had the same deal with with Manzamp PT as well, 325V PT with over 500V loaded B+ using a GZ34. This Valco PT is only 125mA on the HT so the voltage gets pulled down a bit. I would use this PT again, if there was a 550/600V equivalent cap network.
Odd yes, I was expecting the soft start and thus wasn't really concerned with the voltage at the first cap. I ran the experiment about 6 times, it only does it when the tubes are cold. I may sub a few other 5U4 for grins.
I don't see anything about that in the data sheets I have, but I can imagine that the cathode temperature would rise faster if it is directly heated. Either way I'm surprised at a 25% overshoot in voltage relative to steady-state. Even with silicon diodes I'd expect an exponential rise with no overshoot.
