Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on February 10, 2021, 06:33:39 pm
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Work done on amp: Clean and service amp, replace all filter caps(F&T`s), power supply and pwr tube screens resistors with 3w metal oxide, blackface power tube bias circuit and PI circuit to AA864 spec with all new resistors and caps, swapped leads on speaker jack because of relocation of NFB loop, replace all carbon comp plate load resistors with 1w carbon film, re-bias amp and check all operating voltages. All is fine amp sounds stellar. Played it for a couple of days before giving it back to the owner. The owner plays it a lot for about a month and calls saying, the amp starting making noise like rumbling thunder, crackling, hissing and popping. It would start and stop at random. I told him to swap the tubes out with known good tubes. He did but no change. I get the amp on the bench using my own known good tubes I traced the noise to the input of the PI. I lifted the down stream leg of the PI input cap, both channels are clean and quiet. I took voltage readings when QUIET PI pin 1 = 250vdc pin 6 = 248vdc pins 3&8 = 92vdc Plates & screens of pwr tubes = 445vdc. When making NOISE PI pin 1 = 181vdc pin 6 = 272vdc pins 3&8 = 110vdc Plates & screens of pwr tubes = 440vdc. Checked grounds, I tried chop sticking all over every cap and resistor and connections , reflowed all solder joints, clipped in a 22uf/500v across the PI plates cap node, clipped in a new choke and my test opt, and lifted the eyelet board, removed and inspected the insulator board, put a clear plastic insulator under eyelet board, fired it up. No change. I noticed both the eyelet and insulator boards have more wax on them that I have ever seen before. Any ideas what is causing the voltage changes in the PI and PA circuits? Any help is much Appreciated. Thanks in advance: Punky https://el34world.com/charts/Schematics/files/Fender/Fender_bassman50.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_bassman50.pdf) https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa864_schematic.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa864_schematic.pdf)
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You have checked a lot already. If it sounds like loud static or like an old radio dial changing stations.....I had that problem once and of all things, it ended up being bad filament wires coming out of the PT. This PT had been used a ton and the constant twisting of the wires eventually shredded out by the paper in the PT.
May not be what you are experiencing, but it's all I can think of.
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Wdelaney72. thanks for the reply. I had some time to work on the amp today. I replaced the 9 pin PI tube socket and wires from the socket to the board. While doing that I noticed the heater wire polarity was different for each tube so I rewired them the way I was taught. Didnt help, noise still there and weird voltages on PI. I am starting to think the board is somewhat conductive in that area. On the board near the PI circuit are two empty eyelets, where on some of the other Bassmans`s are the two 220k mixing resistors. I put my MM on them and chassis ground to check for voltage. One has 780mV and the other about 500mV when the standby is in play and only a few mV when in standby. I thinking of severing the board in that area and build a new board for the PI, PA and power supply connections in that area. Anyone have any other suggestion? :BangHead:
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Anyone have any other suggestion? :BangHead:
Show us good picture of the amp circuits.
PI may not receive B+ voltage or something like that
Check wiring and solders from power supply.
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The PI tube socket would have been my suspect, but you replaced that. Have you gone through the signal chain with a probe and listening amp to see if you can pinpoint a noise source? I have been surprised in the past as to what I find. And can you find where else in the circuit the voltage is fluctuating?
Your comments on the board - The amps built at that time frame do seem to have a lot more wax on them. One of my amps is a Bassman 50 that I have rewired 3 times into different configurations. I just finished making the normal channel a 6G6 and the old bass channel similar to the 864 musings amp that Steve and Dwinstonwood cooked up. Eyelets that I had resoldered a few times have become really cruddy and I've tried cleaning around them but I cannot get channel 1 as quiet as I think it should be. I dislike the eyelet boards anyway, so I am thinking about making up a turret board for it, as that 6G6 channel is a keeper.
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bmccowan, the 1st thing I did was use my listening device to trace the signal. Both channels are clean and noise free up to the pin 2 input of the PI. When the noise starts the PI plate pin 1 drops 69vdc , plate pin 6 rises 24vdc the cathodes 3 & 8 rise 18vdc. The plates and screens of the pwr tubes dropped 5vdc. When the noise stops the voltages go back to normal. I check every cap and resistor with a meter before installing in an amp.
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PI pin 2 = 250vdc
Pin 2 is a grid and should not be anywhere near 200v.
Disconnect the NFB wire. Any better? I would leave it disconnected until you find the issue.
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I am not in the electronics league of Steve, PRR and others here, and I am sure they will jump in. But I think its possible that a resistor or a cap is in an early failure mode. That was shy I wondering if you had also checked voltages on various points on the board in addition to at the pins, and checked caps for leakage. You mention the noise starting - is it always after the amp warms up some?
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Posts crossed - I see Steve did jump in. His help will be better than mine. Good luck!
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Pin 2 was a typo, thanks for catching that Steve. I believe the other preamp plate voltages were unchanged when making noise, I will double check that. Yes the amp has to be warmed up about 30 to 45 minutes before noise starts.
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Steve, I did disconnect the the NFB wire but for got to mention it in my 1st post. Noise was still there.
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I re checked all the plate voltages in the amp. When the noise starts only the PI plates and cathode are affected. Pin 1 down 69v, pin 6 up 24v, pins 3 & 8 up 18v.
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I would check the voltage on either side of the PI plate resistors before the noise starts and compare the same after the amp warms and the noise has started. If that doesn't provide clues, I would check to see that no caps are leaking DC once the amp warms up.
But first I would see what else Steve suggests. :dontknow:
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Fixed!!! What threw me when I was checking signal noise with the listening device, I traced it to the input of the PI. I lifted the down stream leg of the 500pf silver mica cap and there was still noise on the PI circuit. At that time the faulty tube socket was making the noise. After replacing the tube socket and wiring everything up the noise came back in about 30 min but a little quieter. I took it for granted the silver mica cap was not leaking DCV when warmed up because it was in the amp for about 5 weeks. I lifted the down stream leg again clipped in another 500pf. Had the amp on for 2 hrs last night and 5 hrs today. Quiet as a mouse. Thank you for reminding me to double check the caps again. That makes two new silver mica caps go bad in a short time in the past 4 yrs. I always use heat sinks too.
Punky
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Yep, I've had a more than a few of those black silver mica caps go bad. I've changed to another brand of small pF caps - brown turdy looking ones - which seem to be more reliable
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tubeswell, do you mind sharing the brand name of those brown turdy looking caps?
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Rather than use a 500pF I would just use a .01µF of your choice. I never really understood why Fender used a 500pf on a couple of the bassman models.
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Glad you found it. It sucks when you are searching, eh? But feels great when you solve it. Now I need to take my own advice and check the mica caps in the noisy channel in my converted B-50. :icon_biggrin:
I bought some clear/silver 250 and 500pf caps a while back - those have been ok. I think they were polystyrene? I haven't seen them recently.
And I think I'll take Steve's advice on the .01uf caps.
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Steve, thanks for .01 tip. I`m going to have the amp for one more day. I am going to change it out to be safe. I would not want the owner coming back with the same problem.
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Fixed!!! ..... 500pf silver mica cap ........leaking DCV .....That makes two new silver mica caps go bad ...
What is the claimed voltage rating on the caps which have been disappointing you?
In the Old Days, Mica was always 500V, unless it was more. It was not worth shaving mica any thinner than that.
Today maybe it is worth it. To save space, or just to get more caps per rock.
That spot shows over 150V static, so 200V at the very least, and really 500V rating is not too conservative. This ain't no 3.6V cellphone.
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The cap is rated 500v, 272v killed it. Don`t know what the voltage on the other cap was that failed from 4 yrs ago. If I can remember what amp it was I could look it up. LOL