Mystery amplifier

[Kokotoni Will]

I've ordered everything through Cricklewood - I ended up going for this replacement for PSU C4:

https://cricklewoodelectronics.com/Monacor-Audio-Bipolar-Pol...Super-High-Definition-1uF-250V.html

I've also ordered replacement resistors for all those that had drifted out of tolerance. It'll look neater if nothing else.

[PJL]

I have investigated your meter and from what I can determine it has an input impedance of 1M on DC and 0.5M on AC. This accounts for why we have odd voltage readings and probably gave rise to a misdiagnosis of C4.

We can continue to use it but need to be careful especially in the PSU where there are many high value resistors/low currents that will be impacted by the meter load.

The best option for the PSU is to check resistor values and replace as required. C4 & C5 still need to be replaced due to the risk from minor leakage.

[Kokotoni Will]

I've got another multimeter, a "Bside Smart multimeter" from aliexpress. I'll see if I get different results with that.

Correction to my post #154: All PSU resistors are well within tolerance, except R3, which measures 2.7M, and should be 2.2M.

[Diabolical Artificer]

See if you can get an old Fluke 25 DMM, there's a few on Ebay. Not cheap at £50 ish but well worth getting as they are perfect when working on valve amps etc. When I started out I bought several cheap meters, non lasted long.

Andy.

[PJL]

I would reconnect the old C4 and measure the HT output whilst adjusting your potentiometer. You are still on the lamp limiter but I would expect it to be able to reach 200+ volts.

You can use a series 10M resistor when measuring voltages and they should show 1/11th of the actual voltage loaded by 11M.

[CambridgeWorks]

In your posts 147 & 159 I note heater voltages for V3, V4 results are 0V & V2 1V
Was your meter set to dc or is this a typo?
Rob

[Kokotoni Will]

Originally Posted by Diabolical Artificer

See if you can get an old Fluke 25 DMM

Thanks Andy, I'll have a look.

Originally Posted by PJL

You are still on the lamp limiter but I would expect it to be able to reach 200+ volts.

I'm using a 40w bulb, might I be better off trying a 60w or 100w?

Originally Posted by CambridgeWorks

In your posts 147 & 159 I note heater voltages for V3, V4 results are 0V & V2 1V
Was your meter set to dc or is this a typo?
Rob

The meter was set to DC for all measurements. Even on AC though, I still got less than the expected 6v.

[Techman]

Originally Posted by Kokotoni Will

I'm using a 40w bulb, might I be better off trying a 60w or 100w?

The meter was set to DC for all measurements. Even on AC though, I still got less than the expected 6v.

You will do with only a 40 watt bulb. You won't get any sensible voltages until you're running on full mains. The voltages you're getting with the lamp limiter are only a rough indication.

[PJL]

You can do the functional test to see if the potentiometer is adjusting the HT voltage with the 300V DC you have. It has very little load at the moment so 40W is probably fine. If we put full mains on with no load (amplifier LT & HT) the voltages in the PSU might be excessive.

[Techman]

Originally Posted by Kokotoni Will

After posting some photos on a Coventry Facebook group, I think I've managed to make contact with the son of the man who made the amp.

You'd think that this chap would have some knowledge of when this amplifier was last used and some memory of whether it was all working properly when it was, did you ask him? Assuming that nothing has been messed about with or twiddled in between times then there's a good chance that all should be good, other than the need to replace a number of likely leaky coupling capacitors and any resistors that are more than would be expected out of tolerance.

The use of a lamp limiter is a good idea for initial tests and will stop anything bad happening, but the 40 watt bulb now needs to be changed for something a bit higher. Although a Variac is something nice to have, there's really no need for one unless you're going to do a lot of this sort of work going into the future.

[Techman]

Just to add, you're doing really well with this and learning a lot on the way. You've certainly showed your commitment with the reverse engineering of the circuit and I'm sure that there's many interested folk closely following this thread. VERY well done so far!

[Kokotoni Will]

Thanks for your comments, Techman. I'm very much enjoying the process.

The guy didn't say that he recognised the amp as belonging to his father - only that it looked like one that his dad had made.

Correct me if I'm wrong, but seeing that two grid stoppers are wired incorrectly, with no signs of anything having been altered by a third party, it may be that the amp has never worked.

I've looked around town for some higher wattage filament bulbs, but no joy. Bound to know someone who has some though.

[Kokotoni Will]

While I'm waiting for the replacement caps & resistors to arrive, I'd like to have a go at making up the interconnecting lead. I've got a couple of metres of 14AWG silicone wire, which if anything is a bit on the thick side.

I have a question about earthing. The PSU Jones plug has two separate earths: one from the mains, and one for HT out. However, the amp Jones plug has only one earth, which is soldered to the chassis. Should mains earth be joined to HT earth? If so, where?

[PJL]

From a safety perspective I would recommend the mains earth is directly connected to the PSU chassis within the PSU.

Have you tried the potentiometer test yet? We do need to tick that 'PSU regulator working' box.

Also, there are two LT windings run to the jones socket. How many are used in the amplifier? Each winding will be capable of different currents so we need to make sure we use the right one.

[Kokotoni Will]

The mains earth is already connected directly to the PSU chassis. I suppose what I should have asked was whether it should also be run to the amp.

I've tried adjusting the potentiometer with the new C4 installed, but it makes very little difference.

There are two LT connections on the amp Jones plug, but they are shorted together - see attached photo.

[PJL]

Is 'HT GD' connected to the PSU chassis too? There appears to be only 4 connections to the amplifier 'HT 'GD' 'LT' 'LT' and 'GD' and one side of the LT are connected to the amplifier chassis?

Not so good that the regulator is not working. You were measuring the HT voltage output on the jones socket which you have previously measured at 134V? I am surprised it fails with 302V regulator supply voltage. Can you put the old C4 back in?

If it still does not work then I will try modelling the regulator in LT Spice.

[Kokotoni Will]

No, HT GD is not connected to the PSU chassis.

I was wrong about the two amp LT connections - they aren't shorted together. One side goes to the amp chassis, as you have observed. Yes, there are only 4 connections to the amp.

I've tested again with the old C4, but still only get around 130v HT. Adjusting the potentiometer doesn't make any difference.

The old and new capacitors gave different voltages on 12E1 heater pins 2 and 7. On pin 2, 1.4v with old C4, 26v steadily increasing with new C4. Similar results on pin 7.

I'm not too bothered if I have to buy another 12E1 - they aren't too expensive on 'that' site.

[PJL]

Do you have a high wattage resistor in the order of 10K 2W? We need to add a small load to the HT output to test the 12E1. A couple of 22K 1W in parallel would also do. Then measure the 12E1 grid and cathode voltage which should show the grid as 30-40V lower than the cathode.

[Kokotoni Will]

I've got a bunch of 1/4w 100k resistors, would 10 of those in parallel work?

[PJL]

Yes, will do fine. I am expecting the 12E1 to fail this test!