I have two ARC D76A stereo amps that died because the driver card G10 boards and electrolytic caps fried with radiated heat from the output tubes. I'm not restoring them, I'm reusing the XFMR's to make the best amp I can. My speakers dip to 2.7 ohms.
I'm going to parallel the output tubes of the two sections of each stereo amp, and run them with a single driver stage per chassis.
The first stage is a Kimmel Mu follower card marketed by Wellbourne in the '90's that sounds just wonderful in my Dyna Mk III. It uses a 5751 or 12ax7 for its triode, and has the pentode section of a 6U8Aa as a load/CF, feeding the triode section of the 6U8A as a concertina PI.
I intend to run a cap coupled, 6N1P LTP diff amp coming out of the concertina PI, and directly couple each output to a 6N1P CF and thence to its two output tubes (per phase), KT120's. This is ala the ARC VS110.
I'm not sure I need the extra gain of the LTP diff amp, I could just cap couple the Kimmel PI output to a 6N1P CF, but the ARC output xfmr's allow grounding the output tubes' cathodes through the OPT secondary, a local feedback. I'm not sure how many dB are used up, hence the need for gain, I think.
I would appreciate any comments on the above, but here's my question:
The ARC 76A goes to a lot of trouble to create a regulated screen supply (that also feeds parts of its driver card). The (latter design) VS110 just tags the screens to the B+ through a 100 ohm R. I haven't found the VS110 power supply schematic, but could easily imagine it to have a regulated B+. (I understand that my B+ voltage is too high (590V) to do that conservatively on my amps.)
What are the Pros and Cons of taging the screens to the B+? I thought it was considered better to reference the screens at a fixed value above the cathode and let the B+ sag as it may, hence the regulated screen on the D76A, or at least a separate screen node in the PS. The old Fender 5E8A twin and the Vox AC10 SRT were the previous use of that screen to B+ that I was aware of, neither hifi.
Thanks.
Dan
I'm going to parallel the output tubes of the two sections of each stereo amp, and run them with a single driver stage per chassis.
The first stage is a Kimmel Mu follower card marketed by Wellbourne in the '90's that sounds just wonderful in my Dyna Mk III. It uses a 5751 or 12ax7 for its triode, and has the pentode section of a 6U8Aa as a load/CF, feeding the triode section of the 6U8A as a concertina PI.
I intend to run a cap coupled, 6N1P LTP diff amp coming out of the concertina PI, and directly couple each output to a 6N1P CF and thence to its two output tubes (per phase), KT120's. This is ala the ARC VS110.
I'm not sure I need the extra gain of the LTP diff amp, I could just cap couple the Kimmel PI output to a 6N1P CF, but the ARC output xfmr's allow grounding the output tubes' cathodes through the OPT secondary, a local feedback. I'm not sure how many dB are used up, hence the need for gain, I think.
I would appreciate any comments on the above, but here's my question:
The ARC 76A goes to a lot of trouble to create a regulated screen supply (that also feeds parts of its driver card). The (latter design) VS110 just tags the screens to the B+ through a 100 ohm R. I haven't found the VS110 power supply schematic, but could easily imagine it to have a regulated B+. (I understand that my B+ voltage is too high (590V) to do that conservatively on my amps.)
What are the Pros and Cons of taging the screens to the B+? I thought it was considered better to reference the screens at a fixed value above the cathode and let the B+ sag as it may, hence the regulated screen on the D76A, or at least a separate screen node in the PS. The old Fender 5E8A twin and the Vox AC10 SRT were the previous use of that screen to B+ that I was aware of, neither hifi.
Thanks.
Dan
Both amps are dead? These should be very easy to repair.
You have twice as many transformers as you need for such a project, why not fix the amps and sell them, then buy something more suited for what you want?
You have twice as many transformers as you need for such a project, why not fix the amps and sell them, then buy something more suited for what you want?
I have two stereo amps without circuitry, and have no interest in making a restored D76.
I am paralleling sections. I have no extra xfmrs. I expect to produce near 140 watts on the 16 ohm taps, which with paralleled outputs will be the 8 ohm taps.
Because my speaker Z is low, I'll run the net 4 ohm tap.
I am paralleling sections. I have no extra xfmrs. I expect to produce near 140 watts on the 16 ohm taps, which with paralleled outputs will be the 8 ohm taps.
Because my speaker Z is low, I'll run the net 4 ohm tap.