Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: CJ_Audio on July 13, 2020, 10:37:53 am
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Hello, after reading a lot of useful information in the forum, I've decided to register and write a post about this specific problem.
I'm restoring a Guyatone GA-1100DB. After a full recap, putting new tubes and adjusting the bias, the amp sound really good except for this noise: https://soundcloud.com/project_01/noise/s-tPaYy6trfRX
Some info about the noise:
- It's controlled by the master volume (vol at 0 = no noise)
- Present in both channels even with both volumes at 0.
- I'm in Germany where the standard voltage is 230 V and the frequency is 50 Hz
- The chassis is grounded.
Keeping that in mind, makes me think that the problem is after the second half of both 12AX7 and before the master vol potentiometer.
I've attached the schematic. Please ignore the reverb and tremolo section.
Any help will be much appreciated!
Cheers,
Cris
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Could you please post a few pictures of the inside of the amp? In particular, enough to see the input jack, tube sockets, and grounding points?
Thanks! Cool amp!
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How loud is that noise? Pull V1. Still got noise? If so, leave V1 out and also pull V2. Still got noise?
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Welcome to the forum.
Looking a schematic;
Heaters don't have center tap at xfmr secondary, it is a must ; use two 100 ohms resistor from each side to ground .
Did amp have 3 prong power cord ?
Amp may need a shielded wire from center volume to tube grid.
V1 noisy tubes ?
Bad or no ground at inputs jacks, check normaly close contacts to ground.
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Noise test must be made with nothing plug at the input jack. Is that what you do ?
Did the amp make this noise before you work on ?
Please report.
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> Heaters don't have center tap at xfmr secondary, it is a must
Two heater windings. The one for the small tubes shows a hard center-tap. That's fine. The one for the big tubes is indeed floating. At the high signal level here, this should be fine. Unless a big tube has grown an internal short?
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Are the filter caps old, like originals?
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The signal-to-noise ratio is quite low, but playing a quiet song would be a problem. Pulling V1 and V2 didn't affect the noise (one at the time and both at the same time).
The amp does have a 3 prong power cord (ground connected to the chassis with one of the PT screws). I'm testing with nothing plugged in. The amp didn't work when I got it, so it's difficult to compare to a previous state.
Filter caps are all new.
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Why did you say ; "Please ignore the reverb and tremolo section " ?
There is no reverb or vibrato ?
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Why did you say ; "Please ignore the reverb and tremolo section " ?
There is no reverb or vibrato ?
No reverb or vibrato in the 'DB' model.
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Thank you.
Noise may come from power supply feeding last stage from PI. How is the filtering ? Can you read AC volts/ ripple at Power Supply ?
With 'scope read 12AT7 pin 2 and 7 ( input grids)
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I just measured voltages around the 12AT7.
In the schematic where it says:
+83V --> I'm getting +62V
+448 --> I'm getting +390
I don't have access to a 'scope right now, but I'll borrow one sometime this week.
Thanks!
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I just measured voltages around the 12AT7.
In the schematic where it says:
+83V --> I'm getting +62V
+448 --> I'm getting +390
I don't have access to a 'scope right now, but I'll borrow one sometime this week.
Thanks!
Only this, it is not the noise issue if voltage is DC. Unless filter caps are not working.
How is the AC voltages ( at all filter caps ) as I ask you ? I need to see ripple voltage.
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AC voltages at all filter caps are between 23V and 24V. That doesn't sound right, correct?
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That doesn't sound right
nope that isn't right. 4Vac at PA tap is about max normal, 20-40mVac at V1 tap about normal.
I'd check 4 blown rectifier
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That doesn't sound right
nope that isn't right. 4Vac at PA tap is about max normal, 20-40mVac at V1 tap about normal.
I'd check 4 blown rectifier
I agree and check for bad connections/ solder at filter caps and on all DC power rail
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4Vac at PA tap is about max normal, 20-40mVac at V1 tap about normal.
Actually this thing is cheap-capped and at full power may run 45Vpp or 15V avg AC at the first cap. Less at idle, true, but probably not 4V?
Even this is "acceptable", IF the screens and small tubes are much cleaner. They should not be all the same AC! That suggests only one is is grounded and the others are not helping.
Here's my guess. Some of this may be way off, but maybe it frames the situation.
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I'm getting 23.7V AC where you wrote "15V avg"
Thanks!
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I'm getting 23.7V AC where you wrote "15V avg"
Thanks!
Something wrong, check rectifier diodes.
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I'm getting 23.7V AC where you wrote "15V avg"
Thanks!
Something wrong, check rectifier diodes.
I've checked the diodes with a multimeter and they seem fine. Is there any old diode's problem that I can't detect with a basic multimeter?
Also, I've noticed that I have a 10k resistor between the filter caps, when the schematic says 2.2k
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problem that I can't detect with a basic multimeter
High volts and big current come to mind, thermal breakdown.....
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Solid state diodes, old or new; reading is same with multimeter and you will know if they are good or not.
What are your reading on each one ?
Did you disconnect diodes from circuit ?
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I've checked the diodes with a multimeter and they seem fine. Is there any old diode's problem that I can't detect with a basic multimeter?
Also, I've noticed that I have a 10k resistor between the filter caps, when the schematic says 2.2k
The 10k resistor had been used on some versions of the amp....you just get lower voltage for the preamp, so no worry.
Why you don't change the diodes? It's cheap and you are sure, they are OK.
ATB KH
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I disconnected the diodes to measure them, all good. I'll change them, it's so cheap anyway. 1N4007 right?
I've noticed I few things: the PT is 370V and not 355V like in the schematic, the heater's CT is not connected straight to ground (still grounded, check picture) and the filter caps after turning the amp off discharge really fast.
Thanks!
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see attached - broken traces? or just difficult to see? if one is broken, you'd have a upset bridge.
--pete
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I disconnected the diodes to measure them, all good. I'll change them, it's so cheap anyway. 1N4007 right?
I've noticed I few things: the PT is 370V and not 355V like in the schematic, the heater's CT is not connected straight to ground (still grounded, check picture) and the filter caps after turning the amp off discharge really fast.
Thanks!
1N4007 are good, no need to change them if they are good. Look you don't know ......as I don't see you reading !!
Use heat sink when solder new diodes.
PT voltages ( and other voltages ) on schematic are with load, with amp working.
No load or half load, voltages always rise.
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this is a 100W amp - i wouldn't use 1N4007 diodes. use 1N5408. those 1A diodes are getting hot - see burned board under bridge diodes.
--pete
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this is a 100W amp - i wouldn't use 1N4007 diodes. use 1N5408. those 1A diodes are getting hot - see burned board under bridge diodes.
--pete
Oh ! 100 watts , my bad ...
if original diodes are good why put new ones ? Unless OP don't know what he is doing
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if original diodes are good why put new ones
solid state devices don't always play well with just a meter. they will breakdown with heat, voltage and current that a meter can't replicate, but it can measure "under load" or dynamically
I agree with DL, 1a in a 100W's is not a great replacement
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I've fixed it :guitar1
There was a 250pf capacitor in the master volume potentiometer between the preamp signal and the 0.001uf cap. This one is not in the schematic, I just removed it and the noise went away.
I'm still changing the diodes for the 1N5408. These are getting crazy hot.
Thanks everyone for the help!
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Bad solid state diodes I replaced over year, always read bead with meter.
ohmeter ; infinite one side or overscale
200 K to 300 K other side
With power ON ; voltage across; 0,7 volts ( 0.5 volts for Germanium)
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1N5408 is a 3 amp diodes ! Something might be wrong with amp .
Which number are original diodes ?
Congrat about the caps, you do great work
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I've fixed it :guitar1
There was a 250pf capacitor in the master volume potentiometer between the preamp signal and the 0.001uf cap. This one is not in the schematic, I just removed it and the noise went away.
There ;
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The diodes seen are possibly not original. Many of these amps used "pill" rectifiers. These were very good for their day, but that was a long time ago, also many techs replace rectifiers blindly.
Agree that 1A parts are just barely marginal most days, and you should *always* over-size at least 2X for the bad days. 1400V 3A does not sound wrong to me.
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Hello, this is the result after the restoration. The low hiss comes from the tape echo.
Thanks to everyone for the help to fix the amp. I kinda wanna buy another broken tube amp now :laugh:
Cheers,
Cris