Got this little console amp a couple weeks ago, for the OPTs, and just didn't have the heart to part it out!!! 12AX7, 6V6 SE,,, First replaced the PS caps, and tried it out, it sounded pretty good so far, so I kept going...
Schematic shows the modifications in red... Pics of original and changes, so far...
It has a lot of gain, even more, since upping the coupling caps, and adding teh bypass caps on the output tubes, which improved the bass response, noticeably...
I have two thoughts on reducing gain a little, hopefully without sacrificing the bass... 1st, increasing the 6V6 cathode resistors,,, and/or, installing a feedback loop....
Any input or ideas are welcome,,, I'm still learning, this amp sounds pretty good as is, but its a good format for furthering education!!!
Thanks in advance...
Regards,
John
Schematic shows the modifications in red... Pics of original and changes, so far...
It has a lot of gain, even more, since upping the coupling caps, and adding teh bypass caps on the output tubes, which improved the bass response, noticeably...
I have two thoughts on reducing gain a little, hopefully without sacrificing the bass... 1st, increasing the 6V6 cathode resistors,,, and/or, installing a feedback loop....
Any input or ideas are welcome,,, I'm still learning, this amp sounds pretty good as is, but its a good format for furthering education!!!
Thanks in advance...
Regards,
John
Attachments
Thanks for replying,,, I've tried that in the past, and found on certain chassis it introduced hum (consider the ground lift switch)... I've read a lot about grounding and hum problems,,,seems there's two schools of thought on 2 wire polarized, and 3 prong grounding,,, as an electrician, I believe if these older amps were designed in times where knob and tube, and ungrounded Romex was the found in most homes, its OK to leave them 2wire...
Appreciate your thoughts...
Regards,
John
Appreciate your thoughts...
Regards,
John
I tried larger cathode resistors, which didn't seem to affect the gain or bass,,, just raised the B+.... Any ideas on adding the feed back loop on this amp? I remember it being suggested on a 50C5 SE amp build a while ago, and it seemed to get all the bass available from the small OPTs... I believe it would reduce gain a bit also?
Regards,
John
Regards,
John
Like your thinking,,,
Amplification factors ...AX7=100 (original),, AT7=60,, AY7=44,, AV7=37,, AU7+19.5/20...
I actually considered rewiring the driver to use 6SL7,, amp factor 70,, and one of my favorite input tubes,,, however, there's not enough room on the little chassis for an octal socket!!!
I asked about replacing driver tubes, (6SL7 and 6SN7), with out making any changes to to an amp before, and it got into a heated argument... I realize SN7 has twice the heater draw as SL7,,, So,, is there any reason I can't sub one of the mentioned input tubes for teh AX7, without changing components??
How about FB loop/
Regards,
John
Amplification factors ...AX7=100 (original),, AT7=60,, AY7=44,, AV7=37,, AU7+19.5/20...
I actually considered rewiring the driver to use 6SL7,, amp factor 70,, and one of my favorite input tubes,,, however, there's not enough room on the little chassis for an octal socket!!!
I asked about replacing driver tubes, (6SL7 and 6SN7), with out making any changes to to an amp before, and it got into a heated argument... I realize SN7 has twice the heater draw as SL7,,, So,, is there any reason I can't sub one of the mentioned input tubes for teh AX7, without changing components??
How about FB loop/
Regards,
John
OK,,, but, I've been playing this amp all afternoon,, its one of the cleanest sounding, little SE amps I've heard... Using a CDP, thru a pair of 15" Maggy speaker console/cabs...
What do you suggest gets changed? I am considering swapping some drivers, with lower amp factor, only to lower the gain...
Regards,
John
What do you suggest gets changed? I am considering swapping some drivers, with lower amp factor, only to lower the gain...
Regards,
John
RCA Resistor Coupled Amplifier section, See page 395:
http://www.tubebooks.org/tubedata/RC20.pdf
12AY7 RK=1.3K, RP=100K, AV=27
12AT7 RK=1.2K, RP=100K, AV=29
http://www.tubebooks.org/tubedata/RC20.pdf
12AY7 RK=1.3K, RP=100K, AV=27
12AT7 RK=1.2K, RP=100K, AV=29
I swapped in AU7 and AT7's.... the AU7's lower the gain to where I think is reasonable... Don't hear distortion with either of them,, however, I don't have a scope....
I appreciate the time you put in here,,, It looks like it should be changed to 1.2K cathode, and 100K plate resistor for AT7,,, but, I can't see where you got the figures... Also, AV=29, what is that?
What values for 12AU7?
I'm getting closer, but still learning,,, understanding teh tables in teh RCA manual would be a big step fwd!!!! I have RC-21, the one after yours....
Thanks again for chiming in...
Regards,
John
I appreciate the time you put in here,,, It looks like it should be changed to 1.2K cathode, and 100K plate resistor for AT7,,, but, I can't see where you got the figures... Also, AV=29, what is that?
What values for 12AU7?
I'm getting closer, but still learning,,, understanding teh tables in teh RCA manual would be a big step fwd!!!! I have RC-21, the one after yours....
Thanks again for chiming in...
Regards,
John
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