Mystery amplifier

[Kokotoni Will]

Cathode voltages:
V3: 12.5v
V4: 12.5v
V5: 7v
V6: 8.8v

[Kokotoni Will]

Originally Posted by Richardgr

Did we shed any light on why all the valves had metal screens? Wouldn't that be a bad thing for a 7 pin output valve with a 13W dissipation?

See Radio Wrangler's post #106.

I've gone ahead and ordered that matched pair - the tops of the two suspect valves have gone clear, which if I'm correct means the vacuum has broken (see pic).

[Richardgr]

At the risk of being called a heretic, isn't using A2134s with those output transformers like throwing pearls to pigs? (Or throwing pigs to the pearls in this case)

[PJL]

Looks like the screens have melted the valves!
Without a specification for the transformer it is difficult to determine how much more power they would tolerate. It should be possible to get 10W per channel which is more than enough.
The spec says optimal a-a.load changes significantly with anode voltage/current but that can wait.

[Kokotoni Will]

I've desoldered the secondary transformer connections, and measured resistances.

On the left transformer, A-B and C-D both measure 0 ohms.
On the right, E-H and F-G both measure 0 ohms.
Everything else measures open.

So to answer Richard's question, it seems that the two transformers are wired differently simply because their windings run differently.

To measure the turn ratios, which pins of the primaries do I connect the heaters to?

[Kokotoni Will]

I went ahead using pins 1 & 3 of the left primary, and 7 & 9 of the right. Here are the results:

Left transformer
6.51V AC on primary pins 1 & 3
Voltage across AB: 0.298V
CD: 0.319V
ABCD: 0.589V

Right transformer
6.49V AC on primary pins 7 & 9
Voltage across AD: 0.286V
BC: 0.3V
ADBC: 0.616V

[Kokotoni Will]

I've taken voltage readings for all possible combinations of connections for both transformers and made a spreadsheet.

[PJL]

I am away from my pc for a few days and my mobile makes this very difficult.

[Kokotoni Will]

I forgot to upload the photo of the amp, with the pins numbered. Here it is.

[Kokotoni Will]

Originally Posted by PJL

I am away from my pc for a few days and my mobile makes this very difficult.

No problem. I really appreciate your help with this.

[trobbins]

To me, those measured voltages (using 50Hz Vac ?) suggest a 6k6 PP primary, and 3.5-4 to 15 ohm secondary.

It's also worth measuring the Vac from B+ (terminal 1,6) to orange screen (terminals 2 or 5) primary side taps (with previous 6.5Vac applied from B+ to an anode terminal), to confirm the screen turns ratio. These are no-load type turns ratio measurements, which may relate to a slightly different manufacturer impedance ratio rating/spec, but are usually quite close.

The 5-7% slight measured difference in secondary winding section voltages is a little concerning, as I'd have expected them to have somewhat tighter tolerance.

[Kokotoni Will]

Originally Posted by trobbins

It's also worth measuring the Vac from B+ (terminal 1,6) to orange screen (terminals 2 or 5) primary side taps (with previous 6.5Vac applied from B+ to an anode terminal), to confirm the screen turns ratio.

In other words, just to clarify, measuring 1-2 with 6.3V AC on 1-3?

[trobbins]

Yes

[Kokotoni Will]

I've measured voltages between those terminals (see attached), which are all about 2.7V. I've also taken new measurements for the turn ratios, after leaving the PSU on for about 10 minutes, which seemed to help the voltages stabilise, and with multimeter probes connected in both orientations (+ to -, - to +), as I found this gave different readings. There now doesn't appear to be any discrepancy above 2.5% between the two transformers.

[trobbins]

Good stuff. These have a 43% turns ratio screen tap - which would typically include/exclude certain makes of output stage valve for better performance. More modern valve datasheets (ie. from mid 1950's onwards) likely include specs and data for a particular screen turns ratio setup.

[Kokotoni Will]

Yesterday, I remembered that the four big cans atop the amp are in fact electrolytic caps. I haven't replaced them, so I realised they probably needed reforming.

PJL, I've got everything set up as per your instructions for reforming, with the lamp limiter connected, and a 40k load in series with the PSU and amp. However, adding my meter in series with everything else seems to halt the high voltage to the amp, and when set to current monitoring, it shows 0.00 mA / microamps. So, I'm having to reform the caps without the meter.

Roughly how long do you think I should leave everything powered up with the 40k load before switching to a 20k load, and then to 10k?

[PJL]

First Just checking...the resistor should be wired in series with the incoming HT to the amp.

You can monitor the reform progress by measuring the DC voltage across the 40K resistor and it should start off at the full HT and fall off as the capacitors charge up.

I am not sure I have given the right instructions for the transformer turns ratio test and will come back to you.

[Kokotoni Will]

Originally Posted by PJL

First Just checking...the resistor should be wired in series with the incoming HT to the amp.

You can monitor the reform progress by measuring the DC voltage across the 40K resistor and it should start off at the full HT and fall off as the capacitors charge up.

I am not sure I have given the right instructions for the transformer turns ratio test and will come back to you.

Yes, the resistor is between the PSU and amp.

I went ahead and after a couple of hours switched to the 20k load, and after another hour to 10k. Voltage across the load is currently sitting at 25v.

[trobbins]

If the 10k resistor has 25Vdc across it, then the incoming current is 2.5mA. It would seem that current is reforming the first 8uF e-cap, as well as the following 64uF + 32uF + 32uF filter caps.

Are you able to measure the voltage across the first e-cap, and the voltage of the incoming HT supply?

This test setup can also measure leakage current through the coupling caps C10, C11 and C2 in marked up schematic in post #141, by measuring the Vdc across the resistor connecting each coupling cap to 0V.

[PJL]

When you first applied HT without the resistor the capacitors were forcibly reformed otherwise they would have overheated and leaked. You could look for evidence of swelling/leaking on the caps as they might have been damaged. Assuming all is well, what is left is likely residual leakage current and is unlikely to do any damage, 2.5mA @200V is just 0.5W.