Just finished a 6G5-A Pro copy -- inconsistency between layout and schematic --

[mort]

if V6 is the phase inverter, it's WAY off!
compare with schematic above

check your cathode and tail resistors (820 and 6800) in the PI

ah, you are correct!

I followed your observation and found where I had not connected the 6800 resistor to the presence pot. I've corrected that problem and now I have these voltages on v6

1-309.8
2-20.87
3-32.47
6-310.8
7-22.0
8-32.47

I now have more output but it is still badly distorted and not full.

I'll keep looking for forgotten connections. I'm very methodical about doing my wiring but usually I'm able to do it all in one sitting. This one was too big to do that and I did it in 3 sittings. Must have messed up my system....

[mort]

It looks like there's another inconsistency between the layout and the schematic. Take a look at the bass pots on the layout, with the right lug shown empty and the center lug going to the 70K tap of the treble pot. (this is how I wired mine)

Am I correct in thinking that the right lug should get the 70K tap of the treble pot and the center lug should connect to the left lug & 6800 resistor, according to the schematic??

[Jazz P Bass]

The layout is correct.

[mort]

ok thanks.

I've given up on this one for today.

Shouldn't there at least be a little bit of resistance across the secondary of the OT? I pealed back the shrink tube on the 16 ohm tap and there were a total of about 4 ohms across it and the black common. I checked a new 5e3 OT and it had 15 ohms across its secondary...

[Tom Phillips]

Shouldn't there at least be a little bit of resistance across the secondary of the OT? I pealed back the shrink tube on the 16 ohm tap and there were a total of about 4 ohms across it and the black common. I checked a new 5e3 OT and it had 15 ohms across its secondary...

Since you are measuring the DC resistance I would expect the reading to be 1 Ohm or less across both transformer secondaries. Most likely, your Ohm meter doesn't perform well for very low resistance readings. Otherwise, such high readings (4 Ohms and especial 15 Ohms) would indicate that there is something wrong with the transformers. If you short your test leads together what resistance reading does your meter display?

The issue here is the difference between "impedance rating" and DC "resistance." They are both assigned unis of Ohms but they are technically very different things.

[mort]

If you short your test leads together what resistance reading does your meter display?

about 3 ohms. To get my numbers I was subtracting 3 from the actual reading.

I would think there would HAVE to be some kind of resistance in the secondary though. More than an ohm anyway... If it's pretty much continuous then why would it matter which wire went to the speaker positive?

[Tom Phillips]

... If it's pretty much continuous then why would it matter which wire went to the speaker positive?

The tapped secondary is, as you say, a continuous piece of copper wire. However, although there are only slight differences in resistance between the taps, it is the number of turns used for each part of the secondary winding that determines the impedance. The wire is relatively short so the resistance is low but the number of turns is sufficient to produce the required impedance difference.

For example, I just measured the DC resistance of the secondary on a 50W output transformer I had available.
The measured resistances between the common secondary wire and the taps are as follows:
4 Ohm Tap: 0.286 Ohms
8 Ohm Tap: 0.446 Ohms
16 Ohm Tap: 0.687 Ohms

The absolute resistance reading is not critical but all similar output transforms will have relatively low DC resistance values. None should be as high as you reported. If the resistance was 15 Ohms then about half of the output power would be dissipated as heat in the transformer.

[mort]

hhmmm ok. So my meter isn't good for low resistance reading... thanks for the clarification. This is the first time I've even run across a situation where reading so low would be helpful.

[Jazz P Bass]

Time to purchase a relatively decent meter.

Your work is only as good as your tools.

[Tom Phillips]

Try a fresh battery in your meter. When the battery voltage drops even high end meters show wildly incorrect readings.

[gui_tarzan]

I like this thread, it's frighteningly similar to my 5F6-ish build. Not good that you're having trouble, Mort, but that someone is having a VERY similar problem to mine. I never did figure out why I have almost no output so I just set it aside for now.

[Jazz P Bass]

The trick to finding out Why you have low output is to figuring out Where the signal is being lost.

Tube amps are relatively stupid creations.

The preamp tubes are biased to the center of the signal swing.
Class A.
Send a signal in & see what comes out.

On a Push- Pull output section the tubes are biased 'barely' on.
Class AB.
Send a signal in & see what comes out.

If a low output condition is being observed, it is up to the tech to find out , first of all, Where the signal is deficient.

The task at hand then becomes Why.

[Enzo]

In the overall view, transformers are the last thing on the likely failure list. And even then, a "shorted" OT secondary is about the last thing there. The secondary of an OT is a relatively few windings of heavy wire. It will have VERY low resistance. That doesn;t matter, because transformers do not work on resistance. They work on turns ratios, which is why the tap you use matters. It matters which wire goes to speaker positive so it doesn;t become a giant boat horn. Reversing the secondary is exactly the same as reversing the primary.


Bad bias is not the cause of very low volume. It can maker an amp sound sterile, or over the top, but it won;t erase most of your signal.


You built this from scratch, so as far as I know it has never worked. I say that because we approach this different from an amp that worked and then quit. In that case, we at least know all the parts used to work as we see them.

One of THE biggest mistakes builders make is wrong value parts. Like putting a 470 ohm where a 470k belongs or vice versa. Just because my drawer says it should be a 220k, doesn't mean it really is. I always check.

So power down and discharge.

At the phase inverter, from the cathodes to ground, I see 820, 6800, and 4700 ohms in the path to ground. 12,320 ohms. So measure pins 3,8 to ground. DO you get about 12k? I don;t care about a gew hundred ohms, but I don;t want to see 120k, for instance.

And try the power tubes. Got an ohm or less from pin 8 to ground? Pins 5 are the grids, with 220k to the bias supply, which itself is a 27k to ground, so each should read about 247k to ground. So do you get that roughly 250k?


There is a small 100pf cap plate to plate on the phase inverter. Disconnect one end of it, any help?


And what the heck, There are two speaker jacks, the main one has a shorting contact the extension jack does not. Did you use the different type jacks, and if so, is the shorting jack used as the main one, and is that the jack your speakers are plugged into?

[Enzo]

In the overall view, transformers are the last thing on the likely failure list. And even then, a "shorted" OT secondary is about the last thing there. The secondary of an OT is a relatively few windings of heavy wire. It will have VERY low resistance. That doesn;t matter, because transformers do not work on resistance. They work on turns ratios, which is why the tap you use matters. It matters which wire goes to speaker positive so it doesn;t become a giant boat horn. Reversing the secondary is exactly the same as reversing the primary.


Bad bias is not the cause of very low volume. It can maker an amp sound sterile, or over the top, but it won;t erase most of your signal.


You built this from scratch, so as far as I know it has never worked. I say that because we approach this different from an amp that worked and then quit. In that case, we at least know all the parts used to work as we see them.

One of THE biggest mistakes builders make is wrong value parts. Like putting a 470 ohm where a 470k belongs or vice versa. Just because my drawer says it should be a 220k, doesn't mean it really is. I always check.

So power down and discharge.

At the phase inverter, from the cathodes to ground, I see 820, 6800, and 4700 ohms in the path to ground. 12,320 ohms. So measure pins 3,8 to ground. DO you get about 12k? I don;t care about a gew hundred ohms, but I don;t want to see 120k, for instance.

And try the power tubes. Got an ohm or less from pin 8 to ground? Pins 5 are the grids, with 220k to the bias supply, which itself is a 27k to ground, so each should read about 247k to ground. So do you get that roughly 250k?


There is a small 100pf cap plate to plate on the phase inverter. Disconnect one end of it, any help?


And what the heck, There are two speaker jacks, the main one has a shorting contact the extension jack does not. Did you use the different type jacks, and if so, is the shorting jack used as the main one, and is that the jack your speakers are plugged into?

[mort]

So power down and discharge.

At the phase inverter, from the cathodes to ground, I see 820, 6800, and 4700 ohms in the path to ground. 12,320 ohms. So measure pins 3,8 to ground. DO you get about 12k? I don;t care about a gew hundred ohms, but I don;t want to see 120k, for instance.

And try the power tubes. Got an ohm or less from pin 8 to ground? Pins 5 are the grids, with 220k to the bias supply, which itself is a 27k to ground, so each should read about 247k to ground. So do you get that roughly 250k?


There is a small 100pf cap plate to plate on the phase inverter. Disconnect one end of it, any help?


And what the heck, There are two speaker jacks, the main one has a shorting contact the extension jack does not. Did you use the different type jacks, and if so, is the shorting jack used as the main one, and is that the jack your speakers are plugged into?

I have checked resistors that you have suggested and more, and all of them come up within normal range. I lifted the 100pF cap and the symptom is unchanged. And yeah my speaker jacks are wired exactly as you describe and I plug into the shorting jack. I've checked its wiring 10 times or more. I've double checked the inputs and they are wired exactly like the layout shows. I've followed all my pot wiring a few times and it looks complete and clean at this point.