Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: proaudioguy on February 01, 2024, 11:46:25 am
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Comparing the Hoffman circuit to the original, there is a change from 150Ohms to 180, but there is also a diode in the circuit on the original. What is the difference? I have some IN4007 on hand I could add.
Also, how can I tell which way to orient the LED? I have no idea which side is the anode at this point.
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Comparing the Hoffman circuit to the original, there is a change from 150Ohms to 180, but there is also a diode in the circuit on the original. What is the difference? I have some IN4007 on hand I could add.
Also, how can I tell which way to orient the LED? I have no idea which side is the anode at this point.
A smaller resistor will allow more current and a brighter LED. Might be hard to tell difference in brightness between a 150Ω and 180Ω. The additional diode provides PIV protection. Hoffman's circuit relies solely on the PIV rating of the LED which is usually quite small. Adding the diode is an improvement.
A simple google search will show you a pin out diagram for a simple LED.
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I wired it the way it shows in the fender schematic. That being said I have some hum. The original PCB did not exibit this and so I am looking for advice on how to proceed.
Do you link the heater wires from the transformer are too long?
The heater wires for the tubes are tucked into the corner closest to the open end and they are twisted. Polarity is the same for both EL84s for maximum hum cancelling. Wondering if I should add a hum balance.
Bias is 25mA on one tube and 24mA on the other. Could the mismatch be the cause? When I first checked, it was 25 on one and 27 on the other. I swapped tube positions then it was 26 and 25. The same tube was higher in both cases. I turned the tube that was pulling more down to 25 and ended up with 25 and 24 just to be safe. I can’t get it much further. The bias pot is a CH from all the way down. I measured the bias voltage prior to installing the tubes and it ranges from -14 to -6.5VDC. I’m closest to the -14 side.
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I confirmed the humming tube is the phase invertor. I swapped V1 and V2 but the hum stays with the PI. Remvoe V1 and the SHHH goes away but the hum remains.
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I just noticed on the original schematic the heater wires are reverse for each EL84. H1 on pin 4 H2 Pin 5, then the swap. I would expect that to make the hum worse but fender appears to have done it on purpose. Why?
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Cranked it up this morning. Sounds good. I wish my BF amps broke up that way but still had the BF clean. I plugged it into a CTS 12” alnico. Very nice and huge. I could see it needing more tone controls to settle into a mix but all alone its really fat. The 10” in it is “ok” just not the best probably more to do with the size of the cabinet. I’d like to get rid of the noise. I am wondering if the huge bass response is the reason the hum is so apparent. Through the 12” in the bigger cabinet the low end is massive. I could use this as a bass amp at low volume.
The 820Ohm resister, is that responsible for the gain of the PI? Would a larger value lower the gain and a smaller value increase the gain? Would a bypass cap change the frequency response?
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> heater wires are reverse for each EL84.
Heater "phasing" makes NO difference on power tubes. Signal levels are 1000 times higher than at the preamp tubes.
Which knob(s) changes this hum? Try another bench, orientation. Try another circuit. What changes this hum??
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No knob changes the hum. Its there no matter what. I even moved the GRID WIRES on the PI. Pulling the PI tube makes it dead quiet. Pulling the preamp tube only gets rid of the HISS. I moved the excess heater wire away to the far side of the PT, no change. Nothing I do actually changes the hum level.
EDIT, turning the VOLUME control UP all the way changes the HUM. It nearly goes away. Odd cause when I pulled the V1 out it did NOT go away. That indicates to me there is a calcellation going on that is removing the HUM. Pulled V1 with the Volume all the way up and the hum comes right back.
Heater wiring is AC audio so in an AB amp it can be reduced on the power tubes (only the power tubes) if they are wired the same do to the phase cancelling of the hum, but that is precisely how they are wired, yet Fender’s schematic has them wired the other way around, but I don’t know how EL84s work so I figured I was missing something. Common Mode Rejection is a real thing though. Audio/ Sound I know well, in fact I have been asked to apply for 2 different professorships on the subject within the last 5 months. Tube amps, I’m just a hacker hobbyist. I don’t know where to go with this noise. The amp does have a ton of gain. I don’t want to lose the OD, but I wonder if lowering the gain or simply reducing the bass at the PI would help. Otherwise, maybe its a layout issue. I have no idea if this particular layout has been tested.
I found this post when searching on the topic....
Sweet! Found it! Finally realized I should look at the old PCB and check the component values against the schematic, butterylicious, you get points for mentioning C1 could be .22uF, it was on the old board, and R6 was the other one, it's 56K on the schematic but 22K on the old board, changed that and it's sounding like it should now. I'm just happy because I can call this done and move on to a new project. Thanks for the help.
I tagged on a second 56K on MY board because the Tone stack does NOT sound the same as it did before, to me. I think the tone works much better. In an effort to LEARN. What are the affects of this resistor on the circuit? It appears to just be a way to throw away some gain before the Volume control. ???
BTW, I checked the photos on my original board and r6 is indeed 56K like the Hoffman board and the Fender schematic.
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FYI it pops when I shut it off. ANy ideas about that? Its wired Hot to fuse, fuse to switch, switch to transformer. Nuetral to switch to transformer.
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No progress on the hum but the tone is definitely better with C2, C10, C11, and R26. My value for R26 is a little low as I only had 470Ohm, so I parralleled 2 of them. It now has the fender sparkle without being harsh. The overdrive suffered so I removed the R26/ C11 and that got it there.
The mids when volume is cranked are great for OD. The mids when the volume is low (whether its the amp or the guitar volume) sound way too hot. Is it possible to use the volume control to dump mids when volume is low and progressively add mids as the volume is turned up?
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> heater wires are reverse for each EL84.
Heater "phasing" makes NO difference on power tubes. Signal levels are 1000 times higher than at the preamp tubes.
Which knob(s) changes this hum? Try another bench, orientation. Try another circuit. What changes this hum??
The PI buzzez regardless if V1 is there or not but the buzz is MUCH worse with V1 there. The If I touch a grounded wire to V1B GRID (where the volume wiper is soldered to it), the buzz gets louder. That ground is the one on the board that goes to the input jack. Its attached to the chassis through a star washer. It may need to have the metal under it sanded a bit. I added a 14G stranded wire to the other end of the ground buss that is soldered to the back of the pots and put a star ring terminal on the other end and tied it to the safety ground. Turned the amp up and it kind of went a bit nuts and started to squeel so I shut it off and pulled out V1. Now its behaving and the quieter buzz is there. I put a different tube in V1 and turned it on again. Its back to just buzzing loudly. I started moving the shileded mini cable that has the 100Ohm on the end of it soldered to V1B Grid Pin 7. As I moved the shielded wire around the buzz changed. It never got good, just changed. I would think if the volume was DOWN all the way, V1B would be quiet. I tried removing the 100Ohm on V2B Grid pin 7 and that did not help. I changed the entire mini shielded cable to a different piece. No difference. When the volume is all the way up you get the SHHH and the rizz goes away. If you then turn the tone down all the way you get some of the rizz back.Volume anywhere half way or below and the rizz is out of hand. I tapped on the mini shielded cable going to pin 7 V1B grid and its microphonic. Move it and the tone of the rizz changes. Tap on the tube itself or any of the pins or even V1A mini shielded with volume up all the way and its not microphonic. Only tapping right on the mini shielded cable is audible. I have the shield attached at the ground buss wire that is behind the pots. I removed the buss wire from the back of the pots and that made no difference. I’ve been in teh amp so much now its starting to get messy in there. Any links to that spray that cleans teh flux and dirt and bits of solder splats and flushes them down to one end? I’ve heard of it but never seen it and have no idea what its called. I have some contact cleaner here and that doesn’t do it. I tried to use that to clean the board before I installed it and it didn’t touch the flux left from doing the buss wires.
https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf (https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf)
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Why did you put those two 47Ω resistors on the V1 socket? They are redundant and should not be connected to the preamp ground.
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Why did you put those two 47Ω resistors on the V1 socket? They are redundant and should not be connected to the preamp ground.
How are they redundant? They are on the schematic and came with the parts BOM. I just moved them. A lot of people put the balance resisters on V1 to get the lowest hum. I’ve read this for years. The amps ith the hum balance built in have it right next to V1 as well. I have 3 of them, so I put it down there. I tried them on the other end on the terminal strip after I got that stuff off the turret board.
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How are they redundant?
They are redundant because they are also on the board just below the bridge rectifier...
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No they are not. As I stated in the post you replied to, I moved them off the board to get them away from teh rectifier. That photo is about 5 days old.