Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: CrocMule on November 14, 2022, 04:03:02 pm
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I pulled my Pathfinder out of storage a few days ago and tinkered with it for a while. It sounds absolutely fantastic, but I'm wondering if I'm shortening the life of V1. (as-built schematic attached)
I left the grid leak resistor for the "MIKE" input stock at 18M, instead of changing it to 1M (for guitar). I tried different values there, from 1M up and decided that the stock 18M sounds best. It's a huge increase in gain. I'm sure it's an impedance mismatch, and It sounds like I'm overloading the input (with tame single coils, no pedals), but it sounds so wonderful -- throbbing warmth, sustain for days, and at a tolerable SPL. It even cleans up well -- at 2.5 on the "MIKE" knob it's clean and almost full volume. From there it gradually gets warmer and dirtier until you're swimming in swampy overdrive.
Apart from the removal of the phono input and the input and output octal sockets and related wiring, the schematic is stock for the MIKE input, except for the addition of a single 1.5k cathode resistor where there was none on V1a.
I have tried bypass caps on V1b and V2 (6AU6), but removed them since they just seemed to make the bottom end muddy. I've also tried lower values for the V1a plate resistor, but they removed too much bottom end, so I kept that at the stock 560k.
I'd love to keep the input grid leak resistor at 18M, but I don't want to be frying 12AX7s either. Any thoughts?
Thanks!
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> I don't want to be frying 12AX7s
It has resistance in the plate path, right? So it can't runaway to infinity. The "worst" can happen is all 300V appears in the 600+k plate resistance, and that is 0.15 Watts. Worst for the tube is half that voltage, and that makes 0.037,5 Watts in the 12AX7 plate, which is good for a Watt or more.
It is hard to kill a 12AX7 because it does not want to conduct much.
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Thanks @PRR
Exactly what I was hoping to hear! I really don't want to change it -- I'd all but forgotten about the thing, now it's my new favorite (again).
It literally came from an old church lady who brought it to trade in on a new PA at the music store I was working at. I wasn't there, but as soon as I came in my next shift it caught my eye, and I asked the manager about it. He said he gave her $15 for it (it was the mid '90s). I pulled $15 out of my wallet and it was mine. I wish I had been there -- I would have loved to see the deal go down.
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Here's the original schematic, if anyone is interested.