I've run out of talent and patience for this PITA. Any insights would be greatly appreciated.
TL;DR: HF oscillations and bad decisions are driving me mad. Please help.
A buddy got this head in lieu of payment for other gear and asked if I could breathe some life into it as it sounded pretty stale, like a PA does. Additionally, he asked that the interactivity between the channels be reduced. It had been worked on before, but it doesn't seem as if the circuit had been modified much aside from a couple replaced coupling caps, an added barrier strip for speaker connection, and some odd wire choices. Volume pots were wired with big solid wire more suited to heater wiring. I do see evidence of severe problems in the past; inner chassis bits are bent, as if it was dropped or thrown, and a big burn mark from a wire giving up the ghost. Probably the mains line into the unit. It did have a hass-alfed 3-wire mod done before I ever saw it. The death cap had been left in; I've since took it out.
The amp worked but had some issues when it arrived. Played ok at low volume, but would feedback/oscillate at higher settings, exacerbated by crosstalk between the channels. Tube swaps did not improve the situation. I bypassed/subbed each section of the power supply caps with no improvements. I replaced the last cap in the power supply (8uF) with a new part that is MUCH smaller for the sake of having more room to work around V1 and V2. I did the same with the cathode bypass cap on the power tubes.
Against my better judgement, I decided to proceed with modifications. Phono circuitry was gutted. V1 and V2 sockets were re-wired for use with 6SL7GT/6SN7GTs. Coupling caps were replaced enmasse as DC was showing downstream of a few. Coax was run from V1 coupling caps to the pots and back to V2. V3 was left alone as the shared cathode in that stage shouldn't contribute to interaction between the channels. I later stumbled across a circuit to switch between grid-leak bias and
cathode bias input. Since my guy is a dyed-in-the-wool fender guy, I thought having something closer to his "normal" would be wise. I installed this on only the second channel utilizing the original mag/mic switch. Additionally, I replaced some plate resistors and I've isolated the input jacks and ran grounds to the bus wire.
When I fired it up, it was still wonky. about a 3rd of the way up, the tone control sets up a high frequency oscillation that is pervasive throughout the unit. I think it was up in the region of 21k. Output is low/distorted, not terribly different that it had been. I decided it was time to map out an accurate schematic with voltages. Attached are the factory schematic from inside the amp, a service manual version, and the modded version that I worked up. Also, in the next post some pics and voltages, so you have an idea of the mess I'm working with.
I'm at the "F' IT!" stage, just before gutting and completely re-wiring the thing. Please suggest some path forward.
P.S. While prepping all of this, I noticed a possible problem that stems from when I gutted the phono circuitry. It seems the PI grid WAS ground referenced through the phono volume control, until I pulled it. I dropped a 1M resistor in from p4 to ground, but that was NOT the right move. Lots of gnarly sounds came out of the amp with no input and the volumes down. Power tubes' glow was dancing in rythym to the motorboating/oscillations, so I quickly shut it down and have since pulled that resistor, so the modded schematic is an accurate representation of what I've got in front of me.
Topic: 50's Allied/Knight 93-SZ-682 PA to guitar conversion gone to hell. (Read 2755 times)