Tube amp build (s)

[DrNomis_44]

Oh, now it looks like my voltages have disappeared lol. I'll try again tomorrow, ha ha ha

Were you measuring the voltages with the black probe firmly connected to the chassis?, if so, could be that the mains fuse blew for some reason, I'd investigate that if that's the case.

Just had a quick look at the Rob Robinette eyelet board layout you posted, and it appears that both the power amp ground-bus and the preamp ground-bus don't have any ground wires going to the chassis, which might be why you're getting funny voltage readings, but then-again I might be mistaken.

[JohnH]

Were you measuring the voltages with the black probe firmly connected to the chassis?, if so, could be that the mains fuse blew for some reason, I'd investigate that if that's the case.

Just had a quick look at the Rob Robinette eyelet board layout you posted, and it appears that both the power amp ground-bus and the preamp ground-bus don't have any ground wires going to the chassis, which might be why you're getting funny voltage readings, but then-again I might be mistaken.

Oh, no sorry - I meant that I'd posted the voltages but the post has disappeared!

I'll post them again now

[JohnH]

Voltages, for those interested:

Wall voltage - 246V

Filiments - 6.1 V

B+1 - 368
B+2 - 332
B+3 - 241

Rectifier
Pin 2 - 8 - 4.7V across the pins
Pin 4 - 350
Pin 6 - 350

V1
1 -120
2 -
3 - 2
4 - fil
5 - fil
6 - 120
7 -
8 - 2
9 - fil

V2
1 - 155
2 -
3 - 3
4 - fil
5 - fil
6 - 193
7 - 3
8 - 45
9 - fil

V3
1 -
2 - fil
3 - 365
4 - 334
5 -
6 -
7 - fil
8 - 22


V4
1 -
2 - fil
3 - 365
4 - 334
5 -
6 -
7 - fil
8 - 22

[JohnH]

I'm using this voltage chart to check against:

[Simon Barden]

I can't edit that last post for some reason, but I should add heaters measured 6.1 across the pins, not referenced to ground.

I did have a bit of a weird thing measuring the heaters though. I measured the high voltage secondaries with black probe on the chassis and red probe on the pin (and got a stable reading of 350) but when I did the same for the heaters it just kept rising way past the 3v I expected (I took the probe off when it hit 45v because it freaked me out). Measuring across the pins gave me 6.1 which seems reasonable but I didn't think I'd get a runaway reading with the black probe to ground?

Is that just me missing something obvious?

Is the heater negative side connected to ground? If not the differential voltage can remain constant but the voltages relative to ground will float and could read up to almost any voltage present in the amp. If it's not grounded, then it either needs grounding or you need both sides of the feed tied to ground through their own same-value resistors or a more complex circuit that adds about 20v dc to the ac voltage, which can reduce heater-induced hum. You don't want the circuit floating as you can then exceed the maximum heater to cathode voltage and damage the valves.

[DrNomis_44]

At a glance, the voltages on the valve pins do look pretty normal, or pretty much what you'd expect, so, looking good so far, if the power transformer doesn't have a center-tap wire for the 6.3V AC heater supply for valves V1 to V4, you can make a virtual center-tap by using two resistors of same value, say 100 Ohms, connected as per this diagram:




Sometimes the heater-supply is deliberately made to float at a voltage above chassis ground in order to reduce the heater-cathode voltage so that no short-circuit or leakage occurs, from memory, the maximum heater-cathode voltage rating for a 12AX7 is about 100V or so, but I could be wrong, in any case, the maximum heater-cathode voltage should not be exceeded.

[JohnH]

Ah, that's good to know Doc.

I'm actually wondering if the red glow is even unusual. I've never looked closely at a power tube operating normally, and these pics from Rob Robinette's site make me wonder if all I'm seeing is cathode glow. For some reason I just assumed that the only glow would come from the little bits at the top (which I assumed were the heaters, but honestly don't know).

My tubes:




Pics from Rob's site:

[JohnH]

I tried getting a pic in the dark, but this was the best I could do...

[DrNomis_44]

I tried getting a pic in the dark, but this was the best I could do...

That red glow from the center of the tubes/valves is the filaments, or heaters glowing, which is normal, in order for a tube/valve to operate properly, the cathode needs to be heated so it glows a dull orange colour, when this happens electrons will boil-off the surface of the cathode much like water-vapour boils-off the surface of boiling water.

If you see a blue-coloured fluorescence on the inside of the glass envelope of the 6V6 power tubes while they are operating, that's normal and is caused by a phenomenon known as secondary emissions, basically the electrons from the cathode hit the anode with enough force to knock other electrons out of their orbits.


On the other hand, if you see the anode of one of the 6V6 power tubes glowing a cherry red colour, that's called "Red-Plating" and it indicates that the 6V6 tube is conducting too much current.

[JohnH]

Excellent! In that case the amp is a total success, and I was worrying for nothing, ha ha. Thanks doc.

Now to tolex the box and then start on the next one.

I thought I'd have the box done by yesterday, but somehow managed to order the wrong feet, handles and corners. I have no idea what I was doing, but clearly not concentrating.