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Old 31st May 2018, 02:57 PM   #11
mashaffer is offline mashaffer  United States
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Join Date: Jun 2004
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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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Last edited by mashaffer; 31st May 2018 at 03:00 PM.
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Old 1st June 2018, 02:30 PM   #12
mashaffer is offline mashaffer  United States
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Default 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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File Type: png AmyPreWithFrontBalanceNoBuffer.png (95.5 KB, 119 views)
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Old 3rd June 2018, 09:16 PM   #13
mashaffer is offline mashaffer  United States
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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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Old 8th June 2018, 02:35 AM   #14
mashaffer is offline mashaffer  United States
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It turns out that I have the right value pots for the tone control on hand but with the wrong shaft. The diameter is close (24mm v.s. 1/4") but is about 1/2" too short and no flat. So I just need a way to extend it and add the flat.
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Old 9th June 2018, 04:19 AM   #15
Eli Duttman is offline Eli Duttman  United States
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I think you mean 6 mM., which is just shy of 1/4 inch. 6.35 mM. = 1/4 inch.

Use a rigid shaft coupling made of aluminum. Shorten the existing shaft on the control to the min. that will work with the coupling. File the necessary flat into the piece of stock that sticks out of the coupling, so the knob fits correctly.
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Old 9th June 2018, 08:04 AM   #16
mashaffer is offline mashaffer  United States
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Yes. 24 is the housing.
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Old 10th June 2018, 07:30 PM   #17
mashaffer is offline mashaffer  United States
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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 ) 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.
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Old 12th June 2018, 03:43 PM   #18
mashaffer is offline mashaffer  United States
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Advice please. Initially my plan was to decouple (separate PS cap and resistor) the plate supply for the first two.tubes from the last two but I am questioning how necessary it is. Would you split off the first 2 or 3 tubes or use a single B+ connection for all?
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Old 13th June 2018, 03:45 AM   #19
PRR is offline PRR  United States
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Without thinking:

You can run two sequential stages on the same filter node. Three "depends", and four "depends more".

Are you very adverse to just using a decoupler? It seems cheaper than thinking.
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Old 13th June 2018, 04:31 AM   #20
mashaffer is offline mashaffer  United States
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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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Last edited by mashaffer; 13th June 2018 at 04:48 AM.
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