Greeting all. 🙂 New to this forum.
I originally posted this over on SSGuitar.com but that site has been down for about a week now. 🙁
Posting this here in case anyone has need of the schematic for this amp. 🙂
Found this on ebay, paid very little for it. 😀
Edit; I know JMFahey was/is interested in this...
Edit 2; updated the pre-amp schematic.
This had some issues... tremolo not working.
No success finding a schematic so I traced the circuit board and created the schematic from that.
Probably the only one on the whole intergoogle. 😀
Some pics of the inside stuff...
Cheers!
I originally posted this over on SSGuitar.com but that site has been down for about a week now. 🙁
Posting this here in case anyone has need of the schematic for this amp. 🙂
Found this on ebay, paid very little for it. 😀
Edit; I know JMFahey was/is interested in this...
Edit 2; updated the pre-amp schematic.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
This had some issues... tremolo not working.
No success finding a schematic so I traced the circuit board and created the schematic from that.
Probably the only one on the whole intergoogle. 😀
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Some pics of the inside stuff...
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Cheers!
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Thanks, I have one of those National GA920P amp and cabinet en route to me as I type this.
I'm interested to see just how it compares circuit-wise to this Audition amp.
I also have one of these..
This Teisco Checkmate 21 is very similar looking to the Audition but quite different, circuit-wise.
The "21" has a much smaller reverb tank, smaller output transistors (3 watts at best), a very weird tremolo circuit, and other circuit oddities.
In the distant past, as a teenager I had a Silvertone amp that looked identical to the Checkmate 21 with of course, the exception of being branded Silvertone and with a silver faceplate.
Wish I still had it... it somehow got away from me years ago... 🙁
Despite extensive searches, I have never seen another one like it.
I suspect that particular Silvertone amp was only sold thru Sears Canada for a very short time in the early 70's.
I'm interested to see just how it compares circuit-wise to this Audition amp.
I also have one of these..
An externally hosted image should be here but it was not working when we last tested it.
This Teisco Checkmate 21 is very similar looking to the Audition but quite different, circuit-wise.
The "21" has a much smaller reverb tank, smaller output transistors (3 watts at best), a very weird tremolo circuit, and other circuit oddities.
In the distant past, as a teenager I had a Silvertone amp that looked identical to the Checkmate 21 with of course, the exception of being branded Silvertone and with a silver faceplate.
Wish I still had it... it somehow got away from me years ago... 🙁
Despite extensive searches, I have never seen another one like it.
I suspect that particular Silvertone amp was only sold thru Sears Canada for a very short time in the early 70's.
Thanks 🙂 🙂 🙂
Very interesting 🙂
As told you before, besides the Electronics side of the problem, I wanted to see the often ignored (yet SO important) "mechanical" aspect, meaning speakers, transformers, in this case the Reverb tank, actual layout inside the chassis, etc.
Hope you read my last post in SS Guitar, if not I repeat it here: those mystery "lentils" measuring 22 ohms are NTC thermistors, an important part of the thermal bias/stabilization network, put them back or amp will overbias, overheat and may actually burn.
As of SSGuitar don't worry, I bet it will be working soon, it's just that site owner is happily enjoying a new arrival to the Family 🙂 and has not had a free second for site maintenance.
Very interesting 🙂
As told you before, besides the Electronics side of the problem, I wanted to see the often ignored (yet SO important) "mechanical" aspect, meaning speakers, transformers, in this case the Reverb tank, actual layout inside the chassis, etc.
Hope you read my last post in SS Guitar, if not I repeat it here: those mystery "lentils" measuring 22 ohms are NTC thermistors, an important part of the thermal bias/stabilization network, put them back or amp will overbias, overheat and may actually burn.
As of SSGuitar don't worry, I bet it will be working soon, it's just that site owner is happily enjoying a new arrival to the Family 🙂 and has not had a free second for site maintenance.
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JM thanks very much.
I did indeed see your last post over at SSGuitar and those NTC thermistors are now reinstalled on the board. 🙂
If you look at the PA schematic I have labeled them as therm1 and therm2. 😉
Thanks for letting me know about joecool and the SSG forum.
Glad to know he is ok and awesome news about his new family member!
The forum can wait... His new arrival certainly takes precedence.
I did indeed see your last post over at SSGuitar and those NTC thermistors are now reinstalled on the board. 🙂
If you look at the PA schematic I have labeled them as therm1 and therm2. 😉
Thanks for letting me know about joecool and the SSG forum.
Glad to know he is ok and awesome news about his new family member!
The forum can wait... His new arrival certainly takes precedence.
I got the amp back together enough for testing and tweaking.
I'm just playing around here....
The signal thru the pre-amp is clean, no clipping, but when it gets to Q1 in the power amp schematic and my scope probe on Q1 collector, the signal clips the positive half on peaks. (Reverb and trem at "0")
The negative half is fine.
I tried playing about with bias resistors R10 & 11 to no avail.... put them back to original.
Then noticed the NFB at R12 200ohm and R9 20ohm.
Installed a 1K pot instead of R12 and tweaking that to ~ 33ohm got a balance of still good volume, and less distortion.
Summarize:
High Resistance at R12 = Louder amp but lots of distortion and very harsh "screechy" treble response.
(this seem to be how some "Presence" controls in tube amps work)
Lower or less resistance at R12 = quieter amp but tone gets darker, less "sparkle".
I understand that R12 and R9 form a voltage divider and are a direct path to ground for the output signal, so of course reducing R12 makes it quieter.
Next gonna play with R9 and see what happens...
Any knowledgeable folks got some insight for me on this area of the circuit?
Thanks!
I'm just playing around here....
The signal thru the pre-amp is clean, no clipping, but when it gets to Q1 in the power amp schematic and my scope probe on Q1 collector, the signal clips the positive half on peaks. (Reverb and trem at "0")
The negative half is fine.
I tried playing about with bias resistors R10 & 11 to no avail.... put them back to original.
Then noticed the NFB at R12 200ohm and R9 20ohm.
Installed a 1K pot instead of R12 and tweaking that to ~ 33ohm got a balance of still good volume, and less distortion.
Summarize:
High Resistance at R12 = Louder amp but lots of distortion and very harsh "screechy" treble response.
(this seem to be how some "Presence" controls in tube amps work)
Lower or less resistance at R12 = quieter amp but tone gets darker, less "sparkle".
I understand that R12 and R9 form a voltage divider and are a direct path to ground for the output signal, so of course reducing R12 makes it quieter.
Next gonna play with R9 and see what happens...
Any knowledgeable folks got some insight for me on this area of the circuit?
Thanks!
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