Spitfire hum problem

[matt h]

(deleted)

[EtherealWidow]

Alright. Sounds good. Man... I'm going to be really frustrated if this build ends up sounding average.

[rp]

Alright. Sounds good. Man... I'm going to be really frustrated if this build ends up sounding average.

It could, Matchless cloning (all cloning actually) is more than just putting roughly equivalent parts together. Voltages and transformers really matter. They run the OT at 4K not 8K, and the TDS OT here matters IMO. In the end you'll get a nice little 18W screamer but if you want a Matchless diligence is in order. I use cc resistors and I found the 6PS caps work well here. They like USA 6BQ5s.

[EtherealWidow]

Got some good news and a question. Not a whole ton of hum now. Got the proper shorts sorted out. Amp is actually passing signal and playable! Here's my question, though: The PI grids (both of them) appear to have about 35VDC on them when referenced to ground (although it will be dragged down because of the DMM load). However, when I measure the voltage between the junction of Rt/Rk and the grids, there appears to be nothing. I'm assuming the 35VDC is being dragged down from 50V (what Rt is dropping)?

[matt h]

(deleted)

[EtherealWidow]

Alright... I hope this isn't too confusing. Rt is dropping 50 ish volts. If I measure from the cathode to grids, I get 1.2 volts.

Here's where it's unfamiliar territory for me, though. Measuring from the "top" of Rt to the grid, I get 35 volts. In a normal gain stage (with normal z in) this would be the equivalent of measuring from the bottom of the cathode (ground) to the grid (which would also be at ground potential under no signal conditions) . Since it's an LTPPI though, I'm guessing the 50 volts from the "bottom" of the cathode is being dragged down to 35?

I removed the tubes and checked all voltages and there's nothing there so it's not like the previous issue I had. I only get 35 volts at the grids with the tube plugged in, and it's just about the same voltage for both triode grids.

Another question: Right now I'm dropping the PT primary voltage by about 20 volts to get the secondary voltages more where they should be. I'm using a 50R, 10W resistor (a little too small according to my math). Needless to say, this thing gets HOT and will inevitably fail in the near future. Rather than being a brute and using a bigger wattage resistor, can I use 2 zeners in parallel and opposite polarity (since it's AC) to drop the 20V? I've never used zeners before and am still somewhat foggy on their operation and implementation.

[matt h]

(deleted)

[EtherealWidow]

Alright. Still working out some bugs. Got a nasty oscillation as I turn up the master. It's more prone to it as the gain and tone knobs go more clockwise. I set it to where it's oscillating a small amount and then chopsticked around to see if I could get rid of it. When I fiddle with the input jacks (Weber Cliff shorting types) I can get it to go away. Are the jacks the culprit? I know Weber stuff isn't the greatest quality.

Also, I'm looking for a way to reduce all the secondary voltages by reducing the primary voltage. As per my previous comment, I'm reducing them using a high wattage resistor at the moment but am looking for something much more efficient. I don't know how they used EL84's in this original circuit with the given PT. It puts out over 400V B+

[matt h]

(deleted)

[EtherealWidow]

Filament voltages are about 5.5 or something. It's within tolerance though. I measured the unloaded voltages on the PT before starting the build and did the math to determine resistor size. I know I could regulate B+ using mosfets, but like I said earlier, not to familiar with SS. Zeners would be pretty self explanatory, but I don't think I could use them on the primary side. I guess I could get a 100V one from mouser but would like to be able to shop locally for convenience sake.

[matt h]

(deleted)

[EtherealWidow]

No no. I was just letting you know that before I built anything, I did the math and found that a 50R resistor in series with the primary would drop the B+ to something manageable while still leaving the filament voltage at an acceptable level. There's nothing really inherently wrong with the amp. Most of the problems have been my own doing. The amp has (I suspect) only 2 problems left.

1: I'd like to use something more elegant than a 50R resistor to drop about 20V off the primary side, or about 100V off the B+ rail. I guess I could use a high voltage zener if it came down to it.

2: I think the input jacks are possibly causing the amp to oscillate somehow. Depending on amp settings, I can cause it to oscillate by wiggling the input jacks.

[matt h]

(deleted)

[EtherealWidow]

Alrighty. I'll look that up. Can faulty input jacks cause oscillating?

[matt h]

(deleted)