18-04-2013, 09:14 PM
This is the first stage of making two of these.
First is the germanium unit for testing with a turn table.
I have a pair of language school printed circuits I salvaged from a skip in the 1980s that were kept for the transistors.
These were equalised for a tape head and few modifications are required.
A quick look in the Mullard book soon revealed that a single capacitor could be added to get a first shot at the correct curve by an intelligent guess.
As they had been stored for 30 years the caps needed to be reformed and this went well and a microphone proved that there was plenty of gain to play with.
I even retrieved a circuit diagram for them.
Now it was 15nf across R9 and R10.
On first sound there was a huge signal driven to clipping.
The gain was like a wild animal and needed taming badly.
I tried a coupling capacitor from TR3 collector and still needed a pot to stop it driving the output amplifier into heavy distortion before the signal got to the volume control.
The equalisation came in pretty well and can be tested with my sweep oscillator later to get it a bit closer to the graphs with a little bit of fine tuning.
Any better circuits that could be used to get better ideas for simple modifications to these neatly built boards would be welcome. They are just too well made to be ripped apart to sell the transistors. At the same time I would like to keep them looking good.
The next go at the gain was to take out one end of C5 and add a capacitor connected directly to the cartridge. This got the gain spot on so that the output can now come from C8 and go direct to the amplifiers.
So now I am looking for a case for the preamp.
Now the lineup for the complete gram is (GET536, GET536, EF37A, ECC33, EL37, EL37, GZ32) per channel. I will be fitting a tripler running from the spare isolated heater winding in the 110V transformer. The amplifiers are 110V only and have a matching auto-transformer.
Now you know what my avatar looks like with the covers on.
First is the germanium unit for testing with a turn table.
I have a pair of language school printed circuits I salvaged from a skip in the 1980s that were kept for the transistors.
These were equalised for a tape head and few modifications are required.
A quick look in the Mullard book soon revealed that a single capacitor could be added to get a first shot at the correct curve by an intelligent guess.
As they had been stored for 30 years the caps needed to be reformed and this went well and a microphone proved that there was plenty of gain to play with.
I even retrieved a circuit diagram for them.
Now it was 15nf across R9 and R10.
On first sound there was a huge signal driven to clipping.
The gain was like a wild animal and needed taming badly.
I tried a coupling capacitor from TR3 collector and still needed a pot to stop it driving the output amplifier into heavy distortion before the signal got to the volume control.
The equalisation came in pretty well and can be tested with my sweep oscillator later to get it a bit closer to the graphs with a little bit of fine tuning.
Any better circuits that could be used to get better ideas for simple modifications to these neatly built boards would be welcome. They are just too well made to be ripped apart to sell the transistors. At the same time I would like to keep them looking good.
The next go at the gain was to take out one end of C5 and add a capacitor connected directly to the cartridge. This got the gain spot on so that the output can now come from C8 and go direct to the amplifiers.
So now I am looking for a case for the preamp.
Now the lineup for the complete gram is (GET536, GET536, EF37A, ECC33, EL37, EL37, GZ32) per channel. I will be fitting a tripler running from the spare isolated heater winding in the 110V transformer. The amplifiers are 110V only and have a matching auto-transformer.
Now you know what my avatar looks like with the covers on.
