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Old 9th February 2014, 04:54 PM   #1
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Default Old Intercom guitar amp project

Well after finishing turning an old Webcore phono amp into a nice-sounding Princeton-ish clone, running 6Y6s with a 5Y3 (16W) or 6V6s with a diode full-wave rectifier (20W), I now have a little 12AU6/50C5/35W4 intercom to play with. It peaks out at 3.4W on the 50C5 plate, but want more gain.... will remove the xfmr that let the speaker work as a mic, and add a 12AU7 for a 2nd gain stage and tremelo. Using the Gregory Mark V as a guide. And add a 3-prong AC cord to keep the line AC off the chassis!
Attached Images
File Type: jpg 6Y6 CHASS 1s.jpg (214.6 KB, 152 views)
File Type: jpg CHASSIS REAR.jpg (31.2 KB, 144 views)
File Type: jpg BOTH CABS.jpg (23.2 KB, 145 views)
File Type: jpg GREGORY SCHEMATIC.jpg (9.3 KB, 151 views)
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Old 9th February 2014, 08:37 PM   #2
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Default What a difference a gain stage makes!

Added 1/2 a 12AU7 to my "intercom", and am now using all the headroom this little guy has to offer. Powered the heater from a Variac, but will add the tube to the heater "string", which will allow me to remove a fat 300R dropping resistor. Breaks up now above 7 on volume. Makes an amazing amount of sound thru a 12" cab I have hooked up to it! Amp rolls off high freqs too much, need to look for the cap causing it and change its value. Then add a 9-pin socket where the mic xfmr used to be, and start working on the tremelo....
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File Type: jpg XTRA STAGEs.jpg (241.4 KB, 140 views)
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Old 11th February 2014, 01:17 PM   #3
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I love how your test beds use the longest wires you can possibly find. haha...

Cool project, nice work. Gives me ideas to look for other old equipment to cannibalise instead of looking for broken guitar amps.

Could your treble roll-off be caused by Miller Effect? I can't really see from the schematic you posted, but you already know about playing with grid stopper values to adjust roll-off from your "Champrinceton" project.
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Last edited by doozerdave; 11th February 2014 at 01:19 PM.
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Old 11th February 2014, 03:20 PM   #4
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There is a method to my madness....long unshielded leads make for easy hookup, and if it hums or oscillates I know where to start looking. I found a 0.02uF cap across the OT primary, was killing the treble! Ready to mount a 9-pin socket and wire it up! Bought it for $40 (too much), but got that killer little P-P 6V6 phono for free at a garage sale where I was picking up a Raven RG-20 for $15. Just started talking tube to the guy, and the next thing I knew he was giving it to me! All evens out, all good. Hate to toss the cabinet, thinking of something to do with it....
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File Type: jpg IMAG1593ss.jpg (84.0 KB, 34 views)
File Type: jpg CHASSIS PRE TUBEs.jpg (228.2 KB, 32 views)

Last edited by LATUBEGEEK; 11th February 2014 at 03:36 PM.
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Old 11th February 2014, 06:54 PM   #5
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Put a speaker baffle on the front with a 12" right in the middle, seal it up and stand it on its side.
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Old 12th February 2014, 12:07 AM   #6
jjman is offline jjman  United States
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Same tube compliment in my 1959 Silvertone 1451. I use a Boss EQ pedal set for flat boost to increase overdrive potential when wanted. I measured the actual gain from the 12au6 and it's 100 ! I didn't believe it until I checked the specs for the 12/6au6 and saw the potential. It's just enough to overdrive the 50c4 a little with loud pickups. New 6" Weber speaker sounds good.
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Old 12th February 2014, 11:41 PM   #7
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You are using an isolation transformer - right?
We'd hate to loose you!
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Ian
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Old 13th February 2014, 04:23 AM   #8
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Thank you for your concern! I do want to stay "above ground". Have a question.... How does an isolation xfmr prevent the risk of electrocution? It only takes 15 mA going through your arms to stop your heart.... Where I work, we lift live 110AC all the time, tape off the leads with our bare fingers, and exercize "control" over the hazard with our technique. We have to work on most equipment "hot", and assume it is hot even when told it is "de-energized". On an amp, even with a PT you can have up to 400VDC when you hook your meter across a plate resistor. I do have a 3-prong AC cord, and my bench power is wired correctly. Another question - does a Variac afford the same protection as an iso xfmr? Thanks again!
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Old 13th February 2014, 04:56 AM   #9
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It doesn't prevent the risk of electrocution it just reduces the likehood considerably.

With the isolation tranny the circuit is isolated from active and neutral, you then need to spread yourself across the 2 secondary wires to get a shock. Without it you only need to be between one wire and ground and since your guitar is at ground anyway your'e half way there.

I'm talking about AC shock here as they are the really dangerous ones.
DC will tend to give you a sharp hit but after a sceond or so the muscles starts to relax and you can let go. This has to do with movement of particular chemical elements through muscle cell walls and they eventually become exhausted (after a few seconds). With AC the electrical voltage reversal allows muscle cells to re-polarize and then clamp just as hard all over again (at 50 or 60 times per second depending on where you live in the world). Muscle Cell repolarization time is typically 5ms. Another reason a lot of aircraft stuff is 400Hz. Its not just coz you can use much smaller lighter core trannies, it is actually much safer as 400Hz is too fast for muscle cells to properly repolarize so muscles relax and you let go rather than get "stuck".

P.S. I did my electronics training in a major teaching hospital's Biomedical Engineering Dept., and actually designed some muscle stimulators for their Physiotherapy Dept., Allowing a voltage reversal, after the stimulating pulse, such that the muscle cell could repolarize, was the key to strong non-painfull stimulation.

If the stimulating voltage is mains its also the key to getting you dead.


A Variac or any "auto-transformer" is a single tapped winding and offers no isolation at all. It will not reduce shock hazzard.

If you don't have an isolation tranny at least use a wall socket protected by a residual current (earth leakage) breaker.

Had you tried to post this over at the HiFi section There is a high chance your post would have been deleted by the mods. They seriously don't like amps with heaters spread across the mains and B+ generated by rectifying the mains. I've noted them deleting posts about these sorts of amps before.

Cheers,
Ian
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Old 19th February 2014, 01:03 AM   #10
jjman is offline jjman  United States
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I put the iso tranny on my Silvertone. Before I installed it, I strategically placed an adapter on the plug that allows it to pug in to the wall in only one direction, aka polarized, ensuring the neutral was the one on the chassis. The old amps came with non-polarized plugs. W/O that polarizing adapter, it's a 50-50 chance you will have the hot going to the chassis. It's a temporary way to increase the safety somewhat.

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