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Messages - lovatomi

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Amplifier Discussion / Re: Hohner 1250 Question
« on: February 23, 2021, 12:40:57 PM »
Thanks Jazz, other issues are entirely possible. Pardon my ignorance, transistors are quite new to me, but how does the diode bundle work to make the signal distorted?

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Amplifier Discussion / Re: Hohner 1250 Question
« on: February 23, 2021, 08:47:38 AM »
Apologies. I think perhaps the schematic hadn't been visible. Should be fixed now

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Amplifier Discussion / Re: 1967 Mosrite BG-500 Output Transistor Bias Question
« on: February 22, 2021, 11:41:28 PM »
Ha... never mind... crazy oscillations when trem engaged was caused by the reverb input jack touching the chassis. Who knew?! Everything sounding quite good now. Fingers crossed.

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Amplifier Discussion / Re: 1967 Mosrite BG-500 Output Transistor Bias Question
« on: February 22, 2021, 07:40:46 PM »
OK, using the guidance above it seems like the amp wants about 4.3 mV across those .1 resistors. With it set like that, frankly the amp sounds fantastic. Better than I could have hoped! Really sparkly clean. Then of course I go to switch on the tremolo/vibrato circuit and it starts screaming/oscillating like a madman. This was working pretty good before. Any chance my fiddling with the output transistors caused this? Is there a way to unfiddle it?

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Amplifier Discussion / Re: 1967 Mosrite BG-500 Output Transistor Bias Question
« on: February 22, 2021, 05:09:32 PM »
Thanks all! I discerned after a a little research that 50mA might be a safer current draw, so I reset the voltage across the emitter resistors to about 5mV. I have a pretty good meter (Fluke 115), but even that can't tell my anything about how close 0.1 ohms I actually am!

I will give the "Enzo" method a try. Stupid question, but when you say measure the current draw from the mains would replacing one of the fuses on the ac side of the PT with my meter reading amps be an acceptable way to do that? I assume when you get to the part when you "turn it back down a tad" you are trying to hit the point right before it begins drawing more current? Thanks again!


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Amplifier Discussion / Re: 1967 Mosrite BG-500 Output Transistor Bias Question
« on: February 21, 2021, 09:55:39 PM »
Thanks G1. Yes, they are 0.1 ohm resistors. When I checked one was at about 25 mV and the other was at 0. I adjusted them both to be about 25 mV. I suppose that would mean they are seeing 250 mA of current. Is there a current number I should be shooting for? The bias pots allowed for a sweep of about 0-50mV. Thanks!

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Amplifier Discussion / 1967 Mosrite BG-500 Output Transistor Bias Question
« on: February 21, 2021, 09:45:19 AM »
Hi all. I have an old Mosrite SS amp that I am working on getting back into good working condition. It had a few blow transistors in the output stage (as well as a couple light bulbs?!) and I have replaced the suspect parts. When i fired it up and scoped it i noticed I was getting a fair amount crossover distortion. I adjusted the bias pots to eliminate it visually and it sounds pretty decent as is. I know there is a more scientific way to do this, involving math and measuring voltage across emitter resistors. Just wondering if anyone could assist me in figuring out where to measure this and what I should be looking for on this particular amp. Output transistors are 2n3055 and are q30 and q31 on the schematic. Thanks!


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Amplifier Discussion / Hohner 1250 Question
« on: February 15, 2021, 02:45:05 PM »
Hello All, first post here, but I have been lurking for a while. Was able to sort out a Lab Series L5 with some good information I found on this forum. I know my way around tube amps pretty well, but am still a bit of a novice at solid state stuff. I was recently trying to sort an old hohner 1250 I ended up with. It seems like it's supposed to have a clean and dirty mode which is actuated by a pull switch on the preamp volume pot. When i got the amp it seemed to be in dirty mode permanently with the pull switch doing nothing. I believe the circuitry for this boost is all on a little circuit board attached to the volume pot. I found a schematic for a 1040 and I believe that is what my 1250 should look like as well. I attached that schematic. The little board attached to the pot is outlined in red. The weird thing is that my amp had all the same parts as the 1040 in the red outline, but they were wired up completely differently. My question is, does this schematic make sense? Is that bundle of 4 diodes coming off T101 supposed to go that direction? it was reversed in real life, which makes a little more sense to me. Either the original wiring was wrong, or this schematic is wrong. Or it could be a little of both. Thoughts on the schematic? Should that work? When i wired mine to match the schematic it was pretty quiet and had no switching capability. Thanks!

image_21488

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