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Back to resto-mod planning

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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.
 

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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?
 
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 I´d google (in fact just did) "D shaft potentiometer" and browse the thousand options.

In the extreme, you don´t **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..
 
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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:cop: @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.
 
Rather than using the octal spot just for a CF buffer for the tone control section I thought it might be more useful to make it a gain stage to drive a simple balance control since that control was not yet included in the design. This allows me to keep the control layout for stock appearance and function. The AX7 should have no problem driving the SN7 grid and the SN7 should have enough mojo to drive the short cable run to the PA.

AmyPreWithBalance.png

For an idea of my intended layout I labelled the picture of the (now) preamp chassis. The spot for U4 is a connector for a MPX unit and will have to be drilled out for a 9pin socket. The spot for U6 is already an octal socket that was used for the rectifier. I will use Si diodes for rectification. Hopefully U6 is not so close to the transformer as to be a noise problem. Naturally the OPTs have been removed for use in another project.

SmAmyTopTrans.jpg
 
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Fleshing out the possibilities.

The original amplifier used completely passive controls leading into a VAS and following output tubes. One of the pots used in the tone controls was a 2.2M linear dual gang. So I decided to take a look at using it as a balance control at the input followed by the original 650K dual gang log volume pot.

In order to get more open loop gain and noise/microphonics performance I changed to first two tubes to 12AY7s. The better open loop linearity allowed me to reduce the feedback ratio for a bit more gain on the front end. so that a final gain stage was unnecessary.

With the possibility of thus reducing the bottle count by one I changed the output tube of the tone control section to a 12AU7 in hopes of supplying enough drive to handle the 1 or 2 feet of interconnect to the PA without using a follower (I could use the 6SN7 instead at the expense of more heater current load and a location closer to the PT). The output only dropped by 1dB with this substitution which seems like a reasonable trade-off.

In hopes of simulating a cable I added a cap across the output. even a load of 500pf (which I suspect would be a very poor cable) did not significantly affect the frequency response.

It seems like this might be a reasonable starting point to start building. If I find noise or loading problems I could then change to using a buffer for the TT and lowering values of the pots. If response is a problem I could also add the output buffering.
 

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The original amp used a tube rectifier. I plan to use sand there and was going to use that socket for another tube but it is very close to the PT so I think I will use it as a handy terminal strip for the diodes and early filter sections.

I got my head inside the chassis and noticed something interesting. There are two PT leads with the same color scheme (yellow and green stripes) but they are not wired together. One is one of the HV legs and the other is one of the 5V legs. Curious. The HV winding is 310-0-310 which is a good bit more than I need so I will probably use a pretty small 1st cap and several rc filter stages to get ripple down in the uV range.

I have played around in Duncan's PS simulator but I have yet to find a good write up on how to choose individual stage components for best filtering and stability. The HV secondary CT is already soldered to thht that I would install the primary star ground lug as close tp that as possible.

I figure 3 stars. 1 Input and gain section. 2 Tone control circuit. 3 Initial rectifying and filtering circuit.
 

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Given the low torque (no detent) I may play around with using 1/4" rubber fuel line for the union as a test of concept.

I didn't feel up to working on the preamp chassis layout today (tedious :D) so I did some loudspeaker experimenting. Originally I had intended to use some modern low efficiency 4" full range drivers (Mark Audio CHR-70) which sound very nice and my SE KT-88 amp to drive them. However that amp is Big and runs 🔥(I think I underspec'ed the PT a bit). Since the console is already cut for 8" woofers I thought to try some 32 ohm 8" woof/FR that I have used with excellent results in OBs.

I put one in the console and hooked up one channel of my Sony V series receiver up to it. It sounded pretty decent playing the local oldies station. At loud volume levels the power meter was running between 0.3 and 1 watt. Assuming that the meter is "calibrated" for 8 ohms we would have been running about 1/4 Watt. So if the meter is anywhere near correct they should be efficient enough for eve triode mode PP especially given that they will have subwoofer supplementation.

On their own they are pretty solid down to near 60Hz and up to about 7 or 8K. It should be fairly easy to integrate a tweeter for the top octave. So I guess for now I will hold off on cutting on the console. If I go this route all I will need to do is some bracing and damping.

With this level of efficiency it may be possible to go class AB with fairly cool bias. Lots of opportunity for diddling when I get to the PA part of the project.
 
No, just wondering. In The above I was really considering the CF as part of the second stage so planned to decouple there.

Tonight I was thinking about the almost non existent PSRR due to the follower. I wonder whether something similar to this would be a better idea. Seems to indicate about 1/3 as much PS noise as the other circuit. Not a huge improvement but perhaps there is another follower approach that has even better PSRR and still has the beef to drive the tone control circuit. Perhaps a Mu Follower for the gain stage...
 

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