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Old 21st January 2018, 07:01 PM   #1
mashaffer is offline mashaffer  United States
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Default Back to resto-mod planning

After a long time out I am back to planning the resto-mod of my 1961 RCA console. The amp chassis was a very simple SE 6BQ5 with passive tone controls. It has three 9 pin and one octal socket. I plan to use a nice "little" SE-UL KT88 power amp that I have on hand so will be re-purposing the original chassis for preamp only so that I can make the tone controls active and make for a bit better sound quality in the process.

The PA has pretty low sensitivity so a reasonable amount of gain is necessary which is to be provided by the 6SN7 (unless I can find a less microphonic octal tube to use). The lines leading below the part of the circuit shown go to an optional low pass which would be on a separate chassis if I decide to bi-amp.

I began this a long time ago so I am not sure that I remember all of the details of why particular values were chosen so feel free to critique and suggest alterations.

If I end up using a ceramic cartridge for the turntable I plan to have a separately mounted chassis close to the table with a follower to provide the 1Meg input impedance required without compromising the noise performance. I may also go with a 250k volume pot instead of 500k to reduce noise and interference.

Any feedback appreciated as well as suggestions for low microphonic octal low/medium mu triode for final VAS.
Attached Images
File Type: png AmyPreamp.png (51.5 KB, 441 views)
File Type: png AmyPreampOutput.png (32.9 KB, 431 views)
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Last edited by mashaffer; 21st January 2018 at 07:03 PM.
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Old 21st January 2018, 07:05 PM   #2
mashaffer is offline mashaffer  United States
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Sorry pics are a little bit wide. will resize next time.
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Old 27th May 2018, 07:42 PM   #3
mashaffer is offline mashaffer  United States
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Some of the ideas on the linestage feedback strategies thread seemed useful for this project too. High input Z for the ceramic phono cartridge and a little gain added.
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File Type: png AmyPreWithGain.png (77.9 KB, 287 views)
File Type: png AmyPreOut.png (58.9 KB, 286 views)
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Old 27th May 2018, 09:50 PM   #4
Eli Duttman is offline Eli Duttman  United States
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JMO, put JFET source followers as close to the arm pillar as is possible. HF performance of piezoelectric carts. is poor enough, without cable capacitance exacerbating things.
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Old 27th May 2018, 10:42 PM   #5
mashaffer is offline mashaffer  United States
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Thanks Eli. Will keep that in mind.
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Old 28th May 2018, 12:50 PM   #6
mashaffer is offline mashaffer  United States
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Default Hard to find pots

I am having a very difficult time finding proper replacement pots for a resto-mod of a 60s vintage RCA console. I need 500K to 1M dual gang audio (with power switch if at all possible), dual gang 250K linear, and 500k single gang linear. Finding the values isn't too difficult but the format is 1/4" D shaft solder lug. The D is cut almost to the middle not just the small flattening of some. The shaft is about 3/4" but I can cut if needed.

I have tried the normal suspects but not having much luck. Any suggested sources?
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Old 28th May 2018, 01:22 PM   #7
JMFahey is offline JMFahey  Argentina
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You might try the "Guitar amp guys" such as Smallbear, EL34 World, Angela Instruments, etc. who often have parts to repair vintage Guitar amps.
I see that your main problem is the D shaft; personally Iīd google (in fact just did) "D shaft potentiometer" and browse the thousand options.

In the extreme, you donīt **really** care about 16 vs 25 mm body, PCB vs wire solder lugs, in a pinch not even 1/4" vs other diameter necks and so on, because all that can be solved one way or the other and remains "invisible" to user, I guess you want to keep original D shaft knobs for aesthetic reasons ... fine with me.

Absolute worst case, if you get a suitable pot but with, say, round or splined striated shaft, some handwork with a Dremel wheel can solve that too.

You can krazy glue a plastic shim inside a spline cut to make it "solid" again, and then grind it into a D shape.

Being in Argentina I have done all that and more, since for small parts Freight/Courier is often 5X the value of the part itself, plus 15/30 days delay, not worth it, so I have to make do with whatever I have available here..
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Old 28th May 2018, 02:59 PM   #8
mashaffer is offline mashaffer  United States
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Interestingly I thought I posted this in the parts forum. Not sure why it got here. Thanks for your response. I did search for D shaft and found some but mostly small single gangs or wrong values. The guitar guys do seem to have some single gang stuff.

I am not very skilled with grinders and hacksaws so was hoping for a plug and play. Cutting one side out of a slotted shaft and shimming might be an option. Hadn't really thought about that.
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Last edited by mashaffer; 28th May 2018 at 03:08 PM.
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Old 28th May 2018, 03:02 PM   #9
Moderation Team is offline Moderation Team
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@mashaffer - starting multiple threads on the same topic, in your case, restoring the RCA console, is not permitted by the Forum Rules, therefore, your threads have been merged.
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Old 28th May 2018, 03:05 PM   #10
mashaffer is offline mashaffer  United States
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Quote:
Originally Posted by Moderation Team View Post
@mashaffer - starting multiple threads on the same topic, in your case, restoring the RCA console, is not permitted by the Forum Rules, therefore, your threads have been merged.
Ah, OK. I was thinking it belonged there being a parts sourcing issue. No problem if this is the proper spot. Thanks for the explanation. I thought I had another senior moment.
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