Guys , I built this amp .. it has been on here before .. built by https://www.diyaudio.com/forums/members/ddroukas.html
Anyways ..I have the amp working in the workshop , putting a signal though it .. however when I plug it into my system in the house there is very little volume ..
If I use my MC cartridge through a head amp then through my pre it has a bit of volume , but not as much as I would like .. I’m lost 🤔
Can anyone help I don’t know what to do next .. and how to solve my problem
The scope shot shows the amplification .. on one channel 6.4V PK to PK on the C3M and 50.8V PK to PK on the 300B ..
Anyways ..I have the amp working in the workshop , putting a signal though it .. however when I plug it into my system in the house there is very little volume ..
If I use my MC cartridge through a head amp then through my pre it has a bit of volume , but not as much as I would like .. I’m lost 🤔
Can anyone help I don’t know what to do next .. and how to solve my problem
The scope shot shows the amplification .. on one channel 6.4V PK to PK on the C3M and 50.8V PK to PK on the 300B ..
Attachments
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Wow, not a lot to go on here. How about starting with DC bias points. For AC how is the output loaded? Then, AC input, 3CM plate , 300B plate, and load voltages. No clipping on the scope measurements.
Normally the C3m have a gain of 78 in this configuration.What is the input voltage on g1 of the C3m?
Guys .. be gentle ..I’m a mechanical engineer ..🤷 ..and I can build the amp from the drawings but fault finding is the real art ..
I have attached the schematic with some Ac voltages and DC .. please let me know what else you need .. can you make it simple as well 😕
Cheers Brian
I have attached the schematic with some Ac voltages and DC .. please let me know what else you need .. can you make it simple as well 😕
Cheers Brian
Attachments
C3M
Anode Voltage 212v dc
Grid #2 130 v dc
Cathode resistor 240ohms
Heater voltage 20 vac
300d
Anode voltage 405V
Pin 3 grid. 50V.
Anode Voltage 212v dc
Grid #2 130 v dc
Cathode resistor 240ohms
Heater voltage 20 vac
300d
Anode voltage 405V
Pin 3 grid. 50V.
Based on the measurements thus far, it looks to me like there is a short in the output xfmr. Bias points look ok, and drive to 300B is reasonable. 300B output unusually low. Remove the power, 300B, and the load. Try to measure the inductance of the xfmr primary. It should be many Heneries, lookup it’s data sheet to get the expected value.
So,the amp works on the bench, what are you using as a load? What AC voltage do you get across a dummy resistor on the secondary of the OPT?
Then you plug it into a turn table and speakers but get little or no volume. So, first try a different input source like your sig gen or a PC audio out or more simply use your thumb/finger touched to the RCA IP lead hot/+ terminal, you should get a loud buzz. Also you could try using an old speaker instead of a dummy load for a bench test.
It's more likely to be your input source than a dodgy OPT, elliminate that first.
Andy.
Then you plug it into a turn table and speakers but get little or no volume. So, first try a different input source like your sig gen or a PC audio out or more simply use your thumb/finger touched to the RCA IP lead hot/+ terminal, you should get a loud buzz. Also you could try using an old speaker instead of a dummy load for a bench test.
It's more likely to be your input source than a dodgy OPT, elliminate that first.
Andy.
When I say it works on the bench I mean …. Put a signal in and get a signal out with some increase in volume when I wind up the Function Generator .. ..
I have a dummy load , and I used a 8ohm speaker ..
I tried a CD player with exactly the same result . very low volume ..
the pic shows CH1 - grid pin 3 of the 300b & CH2 - monitor output .. this is with a 2volt ac signal input ..
The input works fine on my other equipment ..
Attachments
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Not sure why the scope states those readings actually ..
Readings taken with a meter are
300b
Plate. 405v dc 10.4 Vac
Grid. -85v Dc. 114v ac
Filament Pin 3 50v dc
C3M
Pin 2 plate. 207 Vdc. 114Vac
Pin 6 grid. 2V ac
Hope this makes more sense ..
Readings taken with a meter are
300b
Plate. 405v dc 10.4 Vac
Grid. -85v Dc. 114v ac
Filament Pin 3 50v dc
C3M
Pin 2 plate. 207 Vdc. 114Vac
Pin 6 grid. 2V ac
Hope this makes more sense ..
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It looks like the C3Mis amplifying properly.
You have 114VAC at the grid of the 300B.
However you only have 10.4VAC on the plate of the 300B.
You should be approaching clipping with at least 200VAC on the plate.
Something appears to be wrong with the load on the 300B.
You have 114VAC at the grid of the 300B.
However you only have 10.4VAC on the plate of the 300B.
You should be approaching clipping with at least 200VAC on the plate.
Something appears to be wrong with the load on the 300B.