SF Twin Re-Cap

Stevem · Post by **Stevem** » Mon Dec 08, 2014 11:45 am

Good, you have plenty of bias voltage to keep the output tubes from running away and that 470 volts on pin 3 should also be on all of the number 4 pins on each socket , right?
If so then if the audio signal is getting out from the preamp and to the input of the PI section, then the amp should play for you!
If you do not have a O scope you can simply follow the audio signal with a voltmeter set for AC volts.
At the input to the PI you should read at least 15 volts of signal coming in even if the some of the preamp tubes are pretty weak.
In the vibrato channel V2 with a good tube should bring up 150mv of input signal to some 30 volts and as the signal passes on down stream and if all the tubes are ok then that signal level should hit the PI input!

LedZepp007 · Post by **LedZepp007** » Mon Dec 08, 2014 1:56 pm

Stevem wrote:Good, you have plenty of bias voltage to keep the output tubes from running away and that 470 volts on pin 3 should also be on all of the number 4 pins on each socket , right?
If so then if the audio signal is getting out from the preamp and to the input of the PI section, then the amp should play for you!
If you do not have a O scope you can simply follow the audio signal with a voltmeter set for AC volts.
At the input to the PI you should read at least 15 volts of signal coming in even if the some of the preamp tubes are pretty weak.
In the vibrato channel V2 with a good tube should bring up 150mv of input signal to some 30 volts and as the signal passes on down stream and if all the tubes are ok then that signal level should hit the PI input!

Thanks so much for your help man. Really appreciate it. I'll check some more voltages when I get home tonight.

LedZepp007 · Post by **LedZepp007** » Fri Dec 26, 2014 7:02 pm

OK, I finally checked the voltages again. My cheap RadioShack multimeter (which was only rated to 500VDC unlike the 750VDC that I believed) kicked the dust after one too many exposures to 500VDC +. Anyway, my fiancé was nice enough to get me a really nice DMM (Fluke 115) for Christmas. The continuity test has been extraordinarily helpful. I checked all of my ground connections. Somehow one of the 6L6's was still not to ground. I fixed that and then replaced my filter caps with new F&L's. I then measured my pin and bias voltages. Everything looked pretty good except for the voltages on the 12AX7 which were off by a lot. I didn't get a chance to test the amp for signal yet, but what effect would that 12AX7 being bad have? I'm guessing if the tube/wiring is bad that would partially explain why the vibrato wasn't working when I received the amp.

LedZepp007 · Post by **LedZepp007** » Tue Dec 30, 2014 11:50 pm

Quick question: I've done more continuity tests and found that both Of the speaker outputs are going to ground (both tip and sleeve). Is that normal?

martin manning · Post by **martin manning** » Wed Dec 31, 2014 12:12 am

The DC resistance of the secondary winding is very low, on the order of an ohm or two, so it hardly shows up.

ToneMerc · Post by **ToneMerc** » Wed Dec 31, 2014 12:13 am

LedZepp007 wrote:Quick question: I've done more continuity tests and found that both Of the speaker outputs are going to ground (both tip and sleeve). Is that normal?

Yes; the winding has continuity with one side of the winding connected to ground, while the other side of that winding is your tapped outputs.

TM

LedZepp007 · Post by **LedZepp007** » Wed Dec 31, 2014 5:49 pm

Thanks everyone!

I will continue to work on the amp. I think I must be very close to getting it back to 100%.

In the mean time, I think I'm also going to start a 5E3 build. I thought that working on this SF TR would be easier than building a simple amp from scratch, but it hasn't been.

LedZepp007 · Post by **LedZepp007** » Sat Jan 03, 2015 2:45 am

Things are looking decent now in terms of voltages. One continued odd reading. I changed the optocoupler out earlier (the old one was in bad shape) and now I'm getting negative voltage on the grid of one half of the 12AX7. Also, my plate voltages for that tube are still 80-100 VDC too high. Any ideas? Thanks!

Stevem · Post by **Stevem** » Sat Jan 03, 2015 1:25 pm

Turn the vibrato on and then check the voltage!

LedZepp007 · Post by **LedZepp007** » Sun Jan 04, 2015 5:46 am

Could only briefly test tonight (too late), but the normal channel appears to be functioning fine. Which is excellent.

LedZepp007 · Post by **LedZepp007** » Tue Jan 06, 2015 1:49 am

Rigged up a grounded RCA plug and checked the vibrato and it works as expected. Somehow I've ended up with a really good sounding, seemingly fully functional amp. It was a frustrating experience (bit off more than I could chew), but I learned a lot including very valuable lessons about electricity and circuit complexity. I now know that I want my dream amp to have as few components as possible -- how can it be good for tone to have the signal go every which way?

Thanks for everyone's help.

I think compared to the Silverface sound, the BF circuit sounds a little warmer and a bit more organic. The SF circuit seemed a bit more sparkly to my ears.

The vibrato and reverb are excellent.

I now realize just how useless that MV was. Jesus what a waste. Glad I cut it out of the circuit.

LedZepp007 · Post by **LedZepp007** » Fri Jan 30, 2015 5:13 pm

So, now that I've finished this off, I figured I'd do the Lar-mar PPIMV. I read into it a bit more to try and understand the theory/implementation. So in theory, a dual-ganged pot replaces the bias splitter resistors?

Would the attached layout work for the TR?

Would I need to add the 47pf cap?

Thanks!

LedZepp007 · Post by **LedZepp007** » Sat Jan 31, 2015 2:21 am

Any guidance on this would be greatly appreciated!

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