Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Carlsoti on November 03, 2022, 04:51:02 pm
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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.
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Voltages and pics.
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Couple more pics. Any insights would be greatly appreciated.
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Not a perfect one but seems your amply is in the cathegory of SPAGHETTI AMP
If you are really but really in trouble ....
dismant all components (leave inside transformers and sockets) then plagiarize a Turret Board that fits with what is on the chassis
Other option, dismant also sockets, get an aluminium sheet and fix it on the top of the chassis, this way you have a blanck space and you can drill socket hole were you want
This is a near perfect Spaghetti Amp
(http://sluckeyamps.com/smoky/rr3.jpg)
Franco
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V3 pin 4 grid must have a resistive path to ground. Otherwise no bias can be established.
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@sluckey, It was buried down in the post script, but I had caught that before posting, tested it with a 1M resistor, and it seemed to make things worse. Should I drop that back in place, then start de-bugging the new problems? Could the 1M resistor I used be too high a value? I'm short of 500K resistors at the moment, but I suppose I could wire the pot in temporarily, if need be.
Looking a the schems again, I see I also pulled out a 1.5M resistor and a tiny cap in parallel that was between the V2 plates and V3 grid. I initially figured it was part of mixing the phono and pre-amp signals and wouldn't necessary, based on how it was drawn on the service schematic. Now I'm thinking it may have also functioned as a grid-stopper for the PI, but I'm not sure that's the case with that cap across it, and the fact that there are no other grid-stoppers in the amp. I think maybe I try both?
Edit: I found a 470K that's risen to 500K in the bag of parts I pulled. In addition to that, I grabbed a cap/resistor combo I thought was the 39pF/1.5M, but I it's a 100K and 200+pF from the input of the phono circuitry. The circuit is now more stable than I've ever heard it. All mildly overdriven, even with low output strat p/u. Output volume is low, but with this little bit of progress, it feels like a huge weight has been lifted. GF will be home soon, so new voltage checks and some oscilloscoping will be done tomorrow. Thanks Steve!