ARC SP-6A Tube Preamp

Well a little bit of progress, and a backwards step..

I've replaced the Zener stack and R66, the 150k resistor at the top of the stack. However, I'm not measuring 394V on the anode of V8, I'm measuring 354V, which indicates to my simple mind that there is too many volts being dropped across R66, hence the current through R66 is on the high side. Whilst the voltage from the PSU caps is at 667V.
Are you sure the Zeners are OK, all installed in the correct direction and all the right value?

If it were me I'd pull the 12AX7 and see if the Zener Voltage goes to the correct reading.

You may have to measure the Voltage across each Zener. There's not a lot of space for that- be careful!
 
Well a little bit of progress, and a backwards step..

I've replaced the Zener stack and R66, the 150k resistor at the top of the stack. However, I'm not measuring 394V on the anode of V8, I'm measuring 354V, which indicates to my simple mind that there is too many volts being dropped across R66, hence the current through R66 is on the high side. Whilst the voltage from the PSU caps is at 667V.

Coupled with two strange measurements, the 402v B2+ rail into the line stage is not reaching 402V, only to 384V and the voltage after the anode resistors of V4 and V5, the ECC83s in the line stage are oscillating between 174V to 182V at a period of about 10 secs. Cathode bias voltages for V5, V4 and V6 are all correct.

Also, the grid voltage on V6 6DJ8 in the line stage wanders around with the same period as the anode voltage swings.

On the audio output with nothing connected to the input sees the odd voltage blip and spike of 10mVs or so with no real pattern. Sound wise these blips sound like splats and coughs.

It does sound like the PSU on the 402V rail is messing about, certainly not coming up to voltage.

Meanwhile, the voltages to the MM stage are stable before and after the anode resistors.

I'm considering that the diodes and zeners in the PSU are suspect and quite old.

Any pointers would be greatly appreciated.
You say V6 in the linestage is a 6dj8. I thought the SP-6A used 12ax7 throughout including the cathode followers. A quick look at the SP-6a manual confirms this. If your unit has a 6dj8 perhaps it was modified in the field in which case there could be other changes as well.
 
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You say V6 in the linestage is a 6dj8. I thought the SP-6A used 12ax7 throughout including the cathode followers. A quick look at the SP-6a manual confirms this. If your unit has a 6dj8 perhaps it was modified in the field in which case there could be other changes as well.
Apologies, I hijacked this thread... mine's an SP-8 :)
 
That makes more sense. If my memory isn’t totally gone, I believe the SP-8 was the first model where ARC started using the 6dj8 and it was only as a cathode follower.
 
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Are you sure the Zeners are OK, all installed in the correct direction and all the right value?

If it were me I'd pull the 12AX7 and see if the Zener Voltage goes to the correct reading.

You may have to measure the Voltage across each Zener. There's not a lot of space for that- be careful!
Right, just measured the voltage across each of the zeners in the stack. All measure 47V bar the last one, on the gnd side, and that has 10V across it.
 
Right, just measured the voltage across each of the zeners in the stack. All measure 47V bar the last one, on the gnd side, and that has 10V across it.
So it could be a 10V device that got mixed in with the others- they all look the same so that's easy to do (speaking from experience...). I'd start by replacing it.
 
So it could be a 10V device that got mixed in with the others- they all look the same so that's easy to do (speaking from experience...). I'd start by replacing it.
Replaced the low voltage Zener and it was a 47v one, but I replaced it anyway. Measured across the replacement and that was still at 10v :(

Interestingly, I pulled the ECC83 and the voltage across the last Zener rose to 48v :)

Thank you for all the pointers, very very helpful.
 
Well a little bit of progress, and a backwards step..

I've replaced the Zener stack and R66, the 150k resistor at the top of the stack. However, I'm not measuring 394V on the anode of V8, I'm measuring 354V, which indicates to my simple mind that there is too many volts being dropped across R66, hence the current through R66 is on the high side. Whilst the voltage from the PSU caps is at 667V.

Coupled with two strange measurements, the 402v B2+ rail into the line stage is not reaching 402V, only to 384V and the voltage after the anode resistors of V4 and V5, the ECC83s in the line stage are oscillating between 174V to 182V at a period of about 10 secs. Cathode bias voltages for V5, V4 and V6 are all correct.

Also, the grid voltage on V6 6DJ8 in the line stage wanders around with the same period as the anode voltage swings.

On the audio output with nothing connected to the input sees the odd voltage blip and spike of 10mVs or so with no real pattern. Sound wise these blips sound like splats and coughs.

It does sound like the PSU on the 402V rail is messing about, certainly not coming up to voltage.

Meanwhile, the voltages to the MM stage are stable before and after the anode resistors.

I'm considering that the diodes and zeners in the PSU are suspect and quite old.

Any pointers would be greatly appreciated.
Slow cycling and "splats" is suspicious. If you have an oscilloscope, you could see if anything is oscillating or motorboating. The latter, slow cycling of the rails, is an issue I saw on several ARC preamps, usually due to a bad ground someplace -- they used to use a low-value resistor to decouple signal and chassis ground and those would fry. Be sure to use a divider and/or capacitor to keep the voltage within range of the 'scope probe; I usually used a 0.1~1 uF 600V cap in series with the probe and made sure to remove the probe first then cap to avoid voltage spikes taking out probe and 'scope.

Make sure you are really on the 47 V, and not 10 V zener for V7. And that there is not some other (potentially stray) connection to that last zener.

You may have a bad tube (or two). You could try pulling some of the signal tubes to see what happens, though it could well be the regulator circuit (pass tube or error amp tube). I have not seen the entire schematic so you'd need to walk the circuit; be careful to not completely unload the regulator (there should be a bleeder resistor to keep it stable even with all tubes pulled but I do not know).

Edit: Didn't see Ralph's replies until after I posted, duh.
 
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Replaced the low voltage Zener and it was a 47v one, but I replaced it anyway. Measured across the replacement and that was still at 10v :(

Interestingly, I pulled the ECC83 and the voltage across the last Zener rose to 48v :)

Thank you for all the pointers, very very helpful.
So the Zeners are good. Sounds like there's not enough current to run the tube in addition to the Zener stack. Are the filter capacitors new?
 
So the Zeners are good. Sounds like there's not enough current to run the tube in addition to the Zener stack. Are the filter capacitors new?
The filter caps are the original ones they measure okay though with my Peak LCR meter. I have new ones on order...incoming from Mouser.
 
The filter caps are the original ones they measure okay though with my Peak LCR meter. I have new ones on order...incoming from Mouser.
As a general rule of thumb, any electronics this old should have new filter caps before powering it up or doing any troubleshooting. At this point anything made in the 1980s or before has suspect filter capacitors and most tube equipment up to the early/mid 90s is suspect. Filter caps in preamps tend to last longer as there is less heat, but they can load the power transformer (which they are not supposed to do) so you get all sorts of wonky stuff going on if this isn't dealt with first.

FWIW an LCR meter isn't than helpful when dealing with electrolytics. That is because they might have the capacitance, but can draw current when exposed to a Voltage. So the only real way to test them is to see if they draw current when powered up. That's really hard to do in-circuit.
 
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So the Zeners are good. Sounds like there's not enough current to run the tube in addition to the Zener stack. Are the filter capacitors new?
Mouser delivered today, so I've removed the original PSU caps for the HT and replaced them.

Unfortunately it hasn't made any difference to the line stage B+, wanders around. So the next step is to add a cap and series resistor to the B2+ line stage supply to reduce the PSU impedance. And I might replace the diodes in the regulator too, as I read that their performance is critical to the PSU.
 
Alright. Glad the caps aren't a variable.
What is the condition of the 12BH7?
Are you getting 10V on the cathodes of the 12AX7?
How does the power supply behave if there are no tubes in the audio sockets- does it hold Voltage then?
The 12BH7 is a brand new EH. Yes, 10.2V on the cathodes of the 12AX7.

I've not tried without the valves installed. I shall try that a little later tonight.

Output of U2, pin 6 is 6.4V.

I'm not sure what was going on with the original PSU caps, it looked like someone had removed them and reinstalled them previously, and left a bit of a mess...took some tidying up.
 
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Are the protection diodes D19 and DZ15 OK?
What is the Voltage on the cathodes of the 12BH7?
BTW just because a tube is new is not saying its a good tube.
It is very interesting that you say that. I'm not seeing 75v across ZD15, but both diodes test as diodes. I've run out of time today to rebuild that diode pair, but there's is tomorrow :)
 
Check the voltage on the heating pins of the 12bh7, it must be under 200 volts. UFK maximum voltage of the tube. If this is not the case, the tube will break very quickly and will probably quickly draw more current than the entire circuit can handle. There should be 400 volts on the cathodes for this to work, the heating must be raised to at least 220 volts. This is done with the resistor pair here.

20250118_002238.jpg

P.S often also a problem with srrp circuit, then tubes start to whistle at high frequencies.
 
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Check the voltage on the heating pins of the 12bh7, it must be under 200 volts. UFK maximum voltage of the tube. If this is not the case, the tube will break very quickly and will probably quickly draw more current than the entire circuit can handle. There should be 400 volts on the cathodes for this to work, the heating must be raised to at least 220 volts. This is done with the resistor pair here.

View attachment 143828

P.S often also a problem with srrp circuit, then tubes start to whistle at high frequencies.
Apologies for being stupid, but wouldn't I expect to have the heater pins on the 12BH7 "floating" at the B2+ HT voltage? I am measuring 398V wrt gnd on each of the heater pins, 398V is what the 402V B2+ is struggling to get to.

I tried today adding a local RC network to the HT of the first two valves in the line stage, to lower the impedance of the PSU. The 6DJ8 in the line stage is powered by a separate PCB track for its HT and I didn't want to start cutting tracks.

PXL_20250118_143322928.jpg

Anyway, with the RC in place and the 6DJ8 out of socket the voltage after the anode resistors for the Line stage ECC83's stabilised and stopped wandering, but the HT voltage stayed at 398V. Putting the 6DJ8 back into the line stage, remember that its HT bypasses the local RC network, made the ECC83's anode voltage begin to oscillate as before. And again the HT stayed at 398V.

Previously I had replaced ZD15 and D19, and D13 D15 with new parts.

Just for completeness this is the PSU schematic for the one I'm working on.

Mark-I-All-changes-200uf.png
My order from Mouser included every component of the PSU apart from the 2N6316 and the MC7824CK.

I'm really grateful for your help, but the PSU for the SP-8 I find particularly baffling :)
 

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