My 1961 build ST70 was totally stock. It has fabric insulated transformer wire, as does the PAS2 preamp. The first time I put tubes in they were GE 6CA7's and a 5AR4. The second time the 6CA7's were Mullards. I would set the bias current to exactly 1 new flashlight battery volts on my $10 R*****S****** meter and forget it. My B+ cap in 1970, I couldn't get (no internet) so I put in 5 axial Sprague atomlytics rated 450 VDC, the highest I could get. (original is 525 rated) Put a new selenium rectifier in 1970, probably the last one made. The MacIntosh salesman sneared when he saw my repaired unit, but the wattage and distortion were correct, and the caps never blew up. It is up for a recap, haven't been using it since 6CA7's went over $30 each. It was used 3-4 hours a day 1970-2000. I read in college that electronics were aged by on-off cycles, so I would turn the amps on and leave them on until I slept. Don't have an opinion on modern tubes, but I do know which country had quality control documents filled out for the brand owner, on their milk that made babies sick. One country still selling tubes was using tubes as late as the cold war MIG27, a package that impressed our experts mightily. I bought a Sovtek 5AR4 for the organ; it worked a month and was put back in the box. May install a cathode resistor, I've got 50 uf of film cap on the organ in place of the electrolytic Hammond can cap,
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i received the resistor you recommended but i need some help identifying the center tap on this triode electronics st70. it is a pretty hefty resistor but i think there is room for it
The H.V. taps are connected to pin 4 and 6 of the rectifier. They will both be the same color.
The H.V. center-tap will be that same color but with a stripe. It will connect to ground somewhere, probably on the negative side of the PSU capacitors.
Without having the Triode Elec schematic in front of me I have to make a guess.
On a stock Dynaco transformer the H.V. center-tap is red with a yellow stripe. If the transformer you have in yours follows that convention, you might look for it.
The H.V. center-tap will be that same color but with a stripe. It will connect to ground somewhere, probably on the negative side of the PSU capacitors.
Without having the Triode Elec schematic in front of me I have to make a guess.
On a stock Dynaco transformer the H.V. center-tap is red with a yellow stripe. If the transformer you have in yours follows that convention, you might look for it.
Looking at the photo of your amp that you posted in your origional post, I would guess that the H.V. center-tap is the red with yellow stripe.
However, I cannot be sure because the point that it attaches is obscured by some wires...
However, I cannot be sure because the point that it attaches is obscured by some wires...
The Triode transformer diagram shows it as the red/yellow line. It probably goes to a ground point with other wires there.
You'll get additional B+ voltage drop, added to what your 5R4 added. What value did you choose for the resistor?
im just experimenting a bit. i put in some 1n4007 diodes and am using a 5ar4 so its plates dont see any reverse voltage IIUC. one problem was that rectifier tubes kept arcing out so i went with SS rect, but this brought the voltage up even higher. i thought an SS module would be a drop in replacement but it isnt because the voltages are higher. someone said that these diodes will solve the arcing problem but the voltages are still a bit high so i was advised to install a 20R 20watt resistor on the CT.
the amp runs fine as it is at about 75% dissipation which is about 42 mA per tube. 20t020 said that the rectifier needs more series resistance so im following his advice and experimenting a bit.
i installed it but now i find that i cant bias the EH tubes up to 1 volt because the pots run out. any advice? the SED tubes are fine but still the pots are turned far clockwise
the plate voltage is down to about 430v which is nice
thanks a lot.
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Too much negative voltage, you can either increase the resistor (10K) between the two bias supply filter capacitors or decrease the resistor (10K) that goes to ground. If the tubes were running too hot and you couldn't adjust it low enough you would do the opposite.
Craig
Craig
Is that with the SS rectifier? With the 5AR4 I'd say a normal B+ with 122-124v line would be about 445ish +/- a little. Can you go back to the 5AR4 and check? I've heard other folks have had biasing range problems with different tubes. I don't know how the EH compare.
The 20R resistor may be a little more than necessary but it was a good starting point. And if the heat is tollerable you can reduce the wattage, of course. The ideal would be to use as little as necessary there. It might only need 15R or 10R. It just depends on how pissy any one individual new 5AR4 wants to be.
R10 and R11 are bias supply resistors i guess, they sit on the cap board right by the bias potentiometer eyelets. they are 10k ohms. what values do you suggest.
sorry, this is with 5AR4 rectifier. i think the heat is tolerable. i live at 51 degrees north. should i increase the value of the bias supply resistors? and if so what value do you recommend.
thanks
am i correct in thinking that if i set the bias at .85v instead of 1 volt that the tube will run less hot and therefore last longer?
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OK. How far off of 1v are you for the bias adjustment? What is probably happening is that with the 20R resistor the load on the secondary is lower now. That could make the (-) 55v bias supply voltage just a volt higher (-) which is why you are at the end of your adjustment pots. If you are close to 1v for your bias adjustment, I'd take a big sigh of relief, and run it that way to check the success of stopping the arcing of the 5AR4. If after a week of cycling and no arcing, I'd plan on pulling that 20R out and trying 15R. That may give you back the normal adjustment range, and closer to expected B+. I'd leave the stock bias resistors as is for now. Pull the diodes and see if you get a little more B+ and more adjustment range.
Also search around for others who have had biasing difficulty with any brands of tubes with a ST70. Changing the biasing resistors may be the last solution to get near a center position.
What do have in your junk box?, try 12K or so for the one between the capacitors OR NOT AND 8.2K for the one that goes to ground. Choose the option that fits the resistors you have, one or the other but NOT both. If you knew the voltage you were shooting for we could figure out the exact resistors to use, ideally you would center the pots and use whatever resistors to get as close as you can to the correct bias voltage. By doing it that way you'd have adjustment range up or down for aging tubes or different tubes.
Craig
Craig
Yepper...
.85/10/2=42.5ma; as opposed to 50ma suggested. Is that your maxxed out range? No problem.
yeah. that is max. what happens as tubes age will i run out of adjustment room? maybe ill pull the resistor for now and get a 10R one instead. it sounds great just without it at 42.5mA just keeping the dissipation down to under 80%. thanks for your help
yeah. that is max. what happens as tubes age will i run out of adjustment room? maybe ill pull the resistor for now and get a 10R one instead. it sounds great just without it at 42.5mA just keeping the dissipation down to under 80%. thanks for your help
Keep in mind as you adjust that power resistor, it's not just about having the right B+. It's purpose is to supplement the series DCR that a transformer lacks when using the 5AR4 and too little choke or too much input filter capacitance. You'll never know for sure just how low you can go with that until a tube arcs again. You could temporarily clip on another 20R across the current one to get 10R there for testing for B+.
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