I looked at the schematic and as I suspected, none of the selenium rectifiers are not involved in the 600 volt supply. CR2 A&B generate the 12.6 volt filament voltage and can be adjusted with R1. No problem there. The other selenium rectifier CR1 is to generate a negative bias supply.which is also adjustable and further clamped by Zener CR3. The high voltage is rectified by CR4 and CR5 which, I assume are silicon diodes that you have not touched. The diodes you used for the selenium replacement are an overkill but should not be related to your problem. Are you sure the -38 is really there and is in fact a minus voltage? If so, at this point I would check the CR4 and CR5 diodes and other parts of the 600 volt circuitry to see if you blew something else when working on this unit.
I wonder where exactly the 600v next goes ? I see that there is a further
1:1 transformer-isolated supply to the "regulator filament". As they've gone to this much trouble to isolate it, could it be a regulator for the 600v maybe ??
I'd disconnect the relay (S-1 and VS3) and comfortably check all other voltages (value, polarity, ripple). If ok, I'd power off and check the electrolytics in the 600 V circuitry (otoh, if they do not look exploded, they are probably ok). Then the fault is somewhere else in the device.
BTW, what is connectors 2 on CR4 and CR5? Does somebody switch from 8 to 2 at some point?
This is also a problem. The original 1N1239 rectifiers only have three pins on the bottom at locations 4, 6, and 8, and the 'key'. There is no pin on the device at #2, but the two wires from the relay switch pole that is NC (until the coil is powered) are tied to #2 the socket. (It's not connected to chassis ground either, which might be inferred by the schematic - they're just meaning that two wires are tied to #2 on the socket, even though there is no pin on the 1N1239 at that location.) I ordered newer replacements for these from americanmicrosemiconductor.com - they were the only place on the internet that had exact replacements for the '1N1239' Sarkes Tarzian center tapped rectifier. HOWEVER, when the new replacements - that look much more like a metal envelope vacuum tube than the originals - they have all pins and are shorter, there's no 600V on the output. Just a few millivolts of nothing. The newer units must be wired differently than the originals. The 1N1239 was a replacement for the
5R4 vacuum tube rectifier; I also located another online vendor that sells 'C-Cap' rectifier replacements and he states that his 5R4 replacement should work. I'm debating whether or not to order some to test though, since the originals seem to be oddballs of some type. And essentially what the relay does, is shunt resistor R5, as pins 2 and 8 seem to be looped no matter what position the relay is in. This one is really a pain - I've never encountered a PS like this one...
All the "relay" does is to complete a low resistance path from the 600 V voltage source, to the filter caps AFTER a time delay has (gently via 15k) pre-charged the filter caps.
Disconnect the 600V lead from the PS to the Amp and see which way the problem is. If the fuse still blows, trouble shoot the PS. You do have a voltmeter?
Yes, the weird part about this is that before doing any work on the power supplies (i.e. everything working) the output voltages with no load were around 17 on the 12.6 legs, -38 on the bias, 6.7 - 7.2 on the 6.3 volt legs, and nearly 930V on the "600" output. I used a Variac to bring things up slowly after doing the modifications to check the output and it was nearly the same on all outputs (again, no load). Hooked the cables up to the amp and used a Variac again, and the 17 was pulled down to around 9.7 once under load, the -38 remained about the same, the filament voltages were all within 6.3 - 7.1V, and the plate side of the output transformer read about 500V, but once the voltage got nearly up to full input voltage and the relay closed (110-120) the 5A fuse popped. If I disconnect the 600V connection at the amp - the fuse doesn't blow. The amps and components test fine and no changes have been made there since everything was working.
One thought though, and I'm not sure if it makes sense or not; I measured the resistance of the selenium I removed in the bias circuit
- which shares the "common" with the 600V supply - and it was about
1.4 Meg. The resistance across the silicon diode I replaced it with is only around 830K. Forward voltage drop aside - is it possible that since the new component doesn't have the same resistance characteristic as the new that adding a "dropping resistor" per se would increase the resistance and balance out the circuit? I've put a
1/2 watt 330K resistor in series with one of the 10A 600V diodes that I used, and the resistance then measures around 1.4 Meg, but I doubt a
1/2 watt unit would stand up to the circuit load. I'd probably have to have at least a 10W unit, and 330K in 10W is hard to locate. Does this make sense? I'm running out of ideas with this thing... :/
Measured in or out of the circuit? Forward or reverse? At what voltage was the resistance measured? It's been a while since I worked on anything using a selenium stack, but....... They age. The forward resistance goes up. I was not ever impressed with the reverse resistance, and 1.4 Meg might be ok. When changing them out with silicon, I always added a series resistor. As I recall, I commonly used about 300 Ohms for a 75 to 100 mA selenium change-out, but 50 to 100 Ohms might be more appropriate for this PS
That is too Low. The forward resistance should be only a few Ohms, and the reverse will be many MegOhms.
Putting that much resistance in series with the diode renders it useless. The Amp is going to draw a lot of current, so any resistance needs to be relatively low, and it needs to have a much higher power rating. Ohm's Law is your friend.
Recheck your silicon diode's front and back resistances. If they don't agree with what I posted above, either they are defective or your test voltage is not suitable.
It may be that one of the filter caps is breaking down. Do you have any spare 40 to 80 Mfd, 400V caps?
Assuming that the power supply runs without blowing the fuse, when it is disconnected from the power amp, then looking at the schematic, it seems to me that tracking down the problem will be relatively simple. But before going any further down that route, can we get a bit clearer on how exactly you have gotten to the point that you are at now? First, did the psu / amp combination ever work before you started work on it, or has it been blowing fuses all along ? The LT selenium reccies that you replaced just for reliability's sake can be discounted I think. What was the reasoning behind replacing the HT rectifier packs ? As you imply that these are differently wired from the originals, could this be anything to do with the current problems ? I'm sure that you have described some of this reasoning elsewhere in the thread in odd bits, but it would be helpful if you could just go through the whole of how you have reached the point you are now at, all in one go.
The forward resistance of a typical silicon diode will be about 700 ohms to
1k ohm, measured on a typical analogue multimeter. Reverse resistance will be, to all intents and purposes, infinity.
Reading again where you currently are, and how you got there, it seems that before you started 'modifying' the rig, it worked. The only implication of this is that the current problem must be down to something you have done yourself. Looking at the sorts of values of resistor that you are trying to work with, it is clear that your understanding of how this equipment should work, is a little sketchy. Whilst tube based equipment is actually very simple in design concept, and normally a breeze to troubleshoot, it is actually quite 'specialist', and if you are not used to working on it, regrettably, you may be getting yourself out of your depth on this one.
Unless you have a clear idea of exactly how to troubleshoot the problem, I think that it might be time to abandon your efforts, and call in some on-the-spot help. I'm not trying to deliberately put you off - or put you down even - just offering some good honest advice with the experience of many years at this.
Oh, that explains it. Then the 5R4 ist a directly heated rectifier tube and the 5 Volts AC are the heater power. This is, you may disconnect 12 and 13 from the mains transformer when using selenium or other semiconductor rectifiers.
Investments into faulty equipment mostly turn out wrong, if a sound reason for the purchase is not present :-).
To give the amp tubes time to heat up before anode etc power is applied. Very reasonable.
It looks quite ok once we understand everything :-).
Now, once the power supply seems to be ok, I would follow the 600 V wiring into the amp with an Ohm-Meter and try to locate a low resitance to ground. Next I would check all anode-to-grid caps like eg C19 at V8b. If some of those leak, the grid will not be negative enough and anode current will go through the roof. I'd disconnect T2 on the primary side and measure the resistance of the windings and the resistance to ground, to prove that the transformer is still ok.
Well, much everything like in a common radio repair :-).
Actually made some progress. Yes, a previous poster was correct that my understanding is "sketchy" :) That's why I'm posting here...
I didn't replace the large Sarkes Tarzians, but did replace the selenium stacks. I do intend to put 5R4's back in the PS for rectifiers though, as due to their age they may be contributing to the overvoltage in the 600V circuit (i.e. reading 900+ when all's supposed to be present is around 600). Have since found out from another tech that the silicon's I used elsewhere in the PS circuit are no problem since they're not part of the HV circuit, and their output voltages under load measure normal. Will add a small dropping resistor though to prevent damage to the tubes. One problem was that there was a cap in the 600V circuit on the amp side that was conducting - basically a
600V to ground. Not good, and was causing the fuse to blow. Replaced it and got the amp functioning and not blowing fuses, but after applying a test signal from a tone generator, something 'opened' and I am getting some 60 hz buzz in the output, which I think might be one or both of the two 80/450 caps in the power supply that were damaged by the fuse blowing several times during troubleshooting. In addition, I did find that two other caps in the amp were getting bad (old can electrolytics) - made "new" ones and things got better, but still 'buzzing'. I also suspect some of the old can caps (lower voltages) in the power supply in addition to the large 450V units, as at this point there isn't anything else in the amp that is old and could be shorting or leaking. All of the signal caps have been replaced with Auricaps. Been a lot of work, but I think I'm close to completing the project.
Forgot to mention that the 'buzzing' only starts after the relay closes - so more evidence that it's either the 80/450's in the PS, or possibly a problem somewhere around the voltage divider/0A2/6DR7 portion of the circuit that is fed from the 600V terminal in the amp.
The relay itself isn't making any noise; it did 'buzz' a little before the fuse blew prior to fixing that problem, but it's a 60hz noise in the speaker now. Suspect the PS since most everything in the amp that could leak 60hz has been replaced.
There is no volume control as this is a power amp only, however increasing or decreasing the input signal doesn't affect the buzzing. Just a 60Hz drone...
It helps if you can check with a CRO to find if it is 60Hz or 120 Hz (120Hz will come from inadequate filtering in the power supply).
Work back wards logically to find the source. Initially leave only the output circuit feeding the speaker, and see if the Hum is still there. Then keep adding stages back to the input till the hum returns. You are then better able to analyse the situation.
Well of course it does, because before that the amp isn't working at all. A tube amp without anode voltage is just a refined heater.
Do not forget that this is a tube device. Some of the tube cathodes are heated with 6.3 Volts alternating current of 60 Hz. If there is a cathode-heater-leak (due to tube age) you get AC into the signal path.
Note that the preamp stages are operated with 400 Volts off the stabilizer circuit around V9 - V10. So have a look at C20 / C2 - they should neutralize AC ripple there.
I'm scratching my head about connectors 3 and 4 on TS2 named regulator filament. Somehow TS2 in the ps-schematic doesn't seem to be TS2 in the amp schematic.
It's a separate 6.3VAC supply for just the 6DR7 tube which is in the same circuit at the 0A2 voltage regulator, attached to 13 & 14 on the TS in the amp. I've isolated the problem to the power amp circuit - preamp section tests fine - and most likely something around the voltage divider/6DR7/0A2 portion of the circuit. When the biasing pot (25K unit in that circuit) is adjusted, the pitch of the buzz varies, which leads me to believe that the problem is somewhere in that circuit. I had replaced all the signal caps with Auricaps, following the black lead in/red lead out scheme recommended by the manufacturer, but C20 (I think) is a power supply decoupling cap and not just a signal coupling cap. The manufacturer recommends that in this application, the red should be the input and the black to ground. It was reversed, I put it in the right direction, but it didn't stop motorboating in the speaker. Auricaps aren't supposed to have polarity, and originally the old caps that had been replaced by another person were nonpolarized poly's and everything seemed to work fine. Guess that's what you get when you try to "upgrade"...
This is what's happened so far - the replacement of the seleniums (how this all got started in the first place) really wasn't a problem; the filament voltages which is all they affect are fine. When the PS/AMP were originall fired after replacing all signal caps with Auricaps and the large electrolytics with new units either separately or ganaged per a multi-section unit being replaced, the fuse in the PS blew after starting the amp, waiting for the relay/cap charge delay, and the b+ circuit engaging per the relay. Found that C21 was conducting, replaced it, and everything worked fine with no signal. Applied a
1khz signal to 4-5-6 on the feedback input terminal strip of the amp (goes directly to the power amp), something crackled briefly and the amp started "motorboating". When started now, the output is clear other than a bit of presence once the heaters heat up, but when the b+ kicks in the noise starts. I'm suspecting it might be the 80/450 caps and possibly the 100K resistors in the power supply that might have been damaged by some sort of issue with the b+ during the fuse blowing fiasco. I'm about to measure the frequency, but I think it's 120 hz and not 60 which as stated might indicate the power supply.
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