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I have an old Tape-O-Matic that I am thinking of converting to a Champ type amp.It has a 5Y3,6SJ7,2-12AX7s and a 6V6.I want to use 1-12AX7 ,6V6 and the 5Y3.A couple of questions-The Power trans is 350-0-350 at the first filter cap I am getting 500v,should I replace this or maybe use a 6L6.The Output trans has 3 wires on the secondary one is a black wire connected to pin 8 of the 6V6,what is this for? can I eliminate this so that I can cathode bias?
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T'man,
Are you measuring the B+ "at the first filter cap" with the rest of the tubes, in particular the 6V6, out of circuit? If so the unloaded PS will rise by about 40% or so. I'm not sure what a "Tape-O-Matic" is - is this a recorder or just a playback unit? The reason I ask is that if it's was a recorder there was probably a bias oscillator that also used a 6V6 or other small power output tube (although it is possible to "double up" on the 6V6's use but this would be odd). But if it was only playback then the 6SJ7 might have been the oscillator. I ask cuz having the bias oscillator - if it's present - out of circuit would also lessen the PS load and let the voltages increase and sometimes the bias oscillators were stuck into odd little corners of the chassis where they could be overlooked (especially if it is a smaller tube such as a 6AQ5 covered by a shield.
As to the 6V6 pin 8/tranny lead I suspect that this is a form of current negative feedback as part of the 6V6's plate signal is coupled back to the cathode through this winding. The purpose is to get a more linear frequency response from a cheap OT so for guitar amp use you can probably remove it with no problem (but you might have to add in a more conventional NFB circuit if yours proves to be unstable.).
Hope this helps.
Rob -
Rob,this was a record-playback unit.I have a correction it was a 6SN7 tube.I gutted everything except trannies, the 5Y3,1-12AX7 and the 6V6.I will check the voltages when wired up.The voltages I mentioned were with the 5Y3 only.By the way where the PT secondaries connect to the rectifier I measured 134v.Comment
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Well I got the amp fired up and it sounded good so I took some measurements.With no Output tube I got 500v at pin 3,and 495v at pin 4.With the tube -385v pin3,395v pin4.I also noted a pronounced hum until the amp warmed up.I tried a few changes to bring down the voltage,and connect the OT B+ in a couple different spots but the screen voltage stays 5 to 10 volts higher than the plate,other than the startup hum the amp sounds sweet.Could it be a bad filter cap or a problem with the OT?Comment
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T'man,
Your voltages seem fairly normal with the higher line voltages but I still wonder if there was another stage, now gone, that was drawing current which pulled the B+ down a bit. Like I mentioned you should have had an oscillator stage which could have, I guess, been the 6SN7, both plates together can dissipation 7.5W and draw almost 20 mA which would probably do it. If you don't mind a little more "sag" you can always add some resistance to the B+ circuit - or test a "s" load of 5Y3s to find one with higher voltage drop (hmm, the current production Sovwrek 5Y3 is supposed to have "unnaturally" low voltage drop - if you are using one of these replace it with a NOS types). If you're using cathode bias what's your output tube current/dissipation? Running the 6V6 "cold" - not drawing sufficient current - can cause the B+ rail voltages to rise. And, yeah, you probably need filters but new filters could make your voltages rise a little more so I'd concentrate on whether you've got a high B+ problem or not first.
RobComment
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Rob; I found that I had made a mistake in the supply connections and now have 368v on the plate and 362v on the screen.I see that that all the champ schematics have a feedback loop and that is the last item to install.So far it sounds great,its basically a 5E1 circuit.I have been looking at TUT vol3 and may use some of Kevins ideas.Thanks for your help.I'll let you know about the end results.Comment
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Well the initially the was good but dark,too bassy.I added the 22K feedback resistor-not much better.Then I used the TUT3 feedback circuit and what an improvement,not muddy and lots of bite.The cabinet has the original speakers (1-6X9,1-3" tweeter) and they sound like crap.I installed an extention speaker jack,plugged in my 1X12 JBL cab and it rocks.My only problem is that when turned up to about 3/4 volume it starts to squeal.I tried moving leads around,and I used shielded wire on the input.The only thing that made a difference was wiggling the Volume pot connections.But really there is no need to turn up that far because below that point it has everything from nice twang to sweet overdrive.Comment
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