Hey all. i've been working on this amp for months and can't get it right yet.
Its an old teisco checkmate 60. Originally it came with EL34 although the original amp used 6CA7
The owner re-did the phase splitter so i have no idea how the original looks like. The lack of documentation
available on the internet about these Teisco amps makes me feel absolutely frustated.
I replaced the PI section with the one shown on the Checkmate 50 circuit, which is a classic long tail pair
phase splitter. Replaced the Grid Resistors on both tubes to 5.6K instead of 3.3k shown on the schematic
of the Checkmate 50 that looks very similar to the power stage of the Checkmate 60.
This unit has a tremolo with an optocoupler , it looks really like the one on the checkmate 50 and a lot like the silvertone 1484 in some ways.
Since this amp is totally third/fourth hand there is a lot of mods in it.
What i've done so far is replace the majority of caps including coupling caps and filters. Replaced every resistor (originals were carbon resistors) with 1% metal film and carbon film
for the plate resistors. Screen resistors were increased from 470ohms to 1K ohms
The pre-amp has a HUGE hiss and hum. I did the tricks of installing 100 ohm to ground resistors from the heater taps to ground to reduce hum , which worked nicely but still
got a HUGE amount of white noise, even when all the controls are off.
I wonder if installing grid resistors (33K to 68K?) on every pre-amp tube stage will reduce this symptom.
The amp originally used a tube rectifier. with 100ufx450V filter caps (5AR4 tube)..which is kinda weird.
They converted it to a full wave rectifier which i improved a bit.
There is a weird arrangement on the bias and bias splitter resistor section which does affect the overal frecuency response of the amp
I haven't been able to find the original schematic so i can restore the original pre-amp. I'll attach the schematic for the checkmate 50 . They are quite close
except for the "bright" switch arrangement.
Any info/tips would be appreciated in this build. I've lost a lot of hair doing some design, trying everything i have on hand to reduce hum and noise.
Its an old teisco checkmate 60. Originally it came with EL34 although the original amp used 6CA7
The owner re-did the phase splitter so i have no idea how the original looks like. The lack of documentation
available on the internet about these Teisco amps makes me feel absolutely frustated.
I replaced the PI section with the one shown on the Checkmate 50 circuit, which is a classic long tail pair
phase splitter. Replaced the Grid Resistors on both tubes to 5.6K instead of 3.3k shown on the schematic
of the Checkmate 50 that looks very similar to the power stage of the Checkmate 60.
This unit has a tremolo with an optocoupler , it looks really like the one on the checkmate 50 and a lot like the silvertone 1484 in some ways.
Since this amp is totally third/fourth hand there is a lot of mods in it.
What i've done so far is replace the majority of caps including coupling caps and filters. Replaced every resistor (originals were carbon resistors) with 1% metal film and carbon film
for the plate resistors. Screen resistors were increased from 470ohms to 1K ohms
The pre-amp has a HUGE hiss and hum. I did the tricks of installing 100 ohm to ground resistors from the heater taps to ground to reduce hum , which worked nicely but still
got a HUGE amount of white noise, even when all the controls are off.
I wonder if installing grid resistors (33K to 68K?) on every pre-amp tube stage will reduce this symptom.
The amp originally used a tube rectifier. with 100ufx450V filter caps (5AR4 tube)..which is kinda weird.
They converted it to a full wave rectifier which i improved a bit.
There is a weird arrangement on the bias and bias splitter resistor section which does affect the overal frecuency response of the amp
I haven't been able to find the original schematic so i can restore the original pre-amp. I'll attach the schematic for the checkmate 50 . They are quite close
except for the "bright" switch arrangement.
Any info/tips would be appreciated in this build. I've lost a lot of hair doing some design, trying everything i have on hand to reduce hum and noise.
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