Mystery amplifier - Page 10 - UK Vintage Radio Repair and Restoration Discussion Forum

Kokotoni Will · 13th Nov 2025, 1:30 pm

Grid measured 30v, cathode: 28v. A fail then.

Kokotoni Will · 13th Nov 2025, 1:32 pm

Does this 12E1 look ok:

https://www.ebay.co.uk/itm/136712994989

Kokotoni Will · 13th Nov 2025, 1:39 pm

I've just powered up the amp without lamp limiter. Good news: I now get between 235-290V HT, adjustable with potentiometer.

PJL · 13th Nov 2025, 1:50 pm

OK, that's very good but please check the DC voltage on 12e1 grid, cathode and anode, and the AC voltage on 12e1 heater. They may be running too high as this would have been designed for 230V AC and when running the power amplifier.

Most likely the 12e1 had low heater volts on the lamp limiter. I would prefer you to find a 100W filament bulb.

Kokotoni Will · 13th Nov 2025, 2:25 pm

Grid: 135V
Cathode: 269V
Anode: 599V
Heater: 125V AC

PJL · 13th Nov 2025, 5:31 pm

When you measure the AC volts for the heater you need to switch to AC volts and apply one probe to pin 2 and the other to pin 7. It is supposed to be 6.3V but I am expecting it to be higher.

600V DC is a scary voltage so be very very careful. I am also more than a little concerned that the ECC83 cathode follower (V3a) is running well over specification with with 600V on the anode and 135 on the cathode, it may breakdown!

Another option would be to run 2 40W bulbs in parallel?

PJL · 13th Nov 2025, 5:44 pm

I assume you had the 10x resistors in as a 10K load?

Kokotoni Will · 13th Nov 2025, 5:48 pm

Quote:

Originally Posted by PJL

When you measure the AC volts for the heater you need to switch to AC volts and apply one probe to pin 2 and the other to pin 7. It is supposed to be 6.3V but I am expecting it to be higher.

600V DC is a scary voltage so be very very careful. I am also more than a little concerned that the ECC83 cathode follower (V3a) is running well over specification with with 600V on the anode and 135 on the cathode, it may breakdown!

Another option would be to run 2 40W bulbs in parallel?

Gotcha. Will have a go tomorrow. I may be able to get hold of some bulbs this weekend. If not, I'll get onto Andy.

Quote:

Originally Posted by PJL

I assume you had the 10x resistors in as a 10K load?

No, I did not.

It turns out I blew a 2A mains plug fuse. Have replaced it with a 5A.

PJL · 13th Nov 2025, 7:00 pm

Put the load back as it might increase the 12e1 grid voltage and reduce the stress on the ECC83.

Kokotoni Will · 13th Nov 2025, 7:29 pm

Have measured 12E1 heater, it's 6.34V.
With the 10k load, V3 (ECC83) anode voltage sits at 580V.

PJL · 13th Nov 2025, 8:17 pm

its the 12e1 grid I am interested in. Hopefully its quite a bit higher than 135V with no load. You might want to add a 10K 5W load in permanently to keep the PSU under control when the amp is disconnected. It might also be sensible to add a resistor between V3b anode and the unregulated rail but I need to work out a value.

Diabolical Artificer · 14th Nov 2025, 8:57 am

I've a few 12E1's & similar valves if you need any, looks like your making progress, just needs finnagling well done, Andy.

Richardgr · 14th Nov 2025, 12:34 pm

I'm sure this must be a bit like cooking in a kitchen full of chefs

I'm not sure it is a good idea to replace a 2A fuse with a 5A one.

It is a bit suspicious that a 2A fuse in the first place - where is the load coming from?

I have one of those cheap devices you plug into the wall, and you plug into the device, that shows the amount of power (watts) being consumed. It has proved quite useful to sanity check the consumption of something under test, and it is also useful when determining a good fuse option and looking at peak load on startup.

Techman · 14th Nov 2025, 1:08 pm

It's normal for a 2A fuse to blow on switch on with transformers of that size, even with little or no load. You might get away with it not blowing for a few on/offs and then 'pop'! It just depends on which part of the mains cycle you happen to catch it on when you throw the switch - 5A is better.

Richardgr · 14th Nov 2025, 2:44 pm

I've experienced that with a toroid, but not a 'normal'' EI transformer. The toroid was tamed with a CL-90 inrush limiter. And a slow-blow fuse should deal with that as well.

Kokotoni Will · 14th Nov 2025, 2:55 pm

Quote:

Originally Posted by Richardgr

I'm sure this must be a bit like cooking in a kitchen full of chefs.

Not at all, all advice is welcome.

The 2A fuse just happened to be in the mains plug I salvaged to make a cable with the 3-pin Bulgin plug. I should have swapped it out beforehand, but it clearly slipped my mind.

Kokotoni Will · 14th Nov 2025, 3:28 pm

Quote:

Originally Posted by PJL

its the 12e1 grid I am interested in.

My mistake, I'll have another look tomorrow. Twenty 200k resistors in parallel it is, then!

Techman · 14th Nov 2025, 3:35 pm

Quote:

Originally Posted by Richardgr

I've experienced that with a toroid, but not a 'normal'' EI transformer. The toroid was tamed with a CL-90 inrush limiter. And a slow-blow fuse should deal with that as well.

But you're forgetting that this is the plug top fuse we're talking about - perhaps you don't have that type of fused mains plug in Sweden?

Richardgr · 14th Nov 2025, 4:10 pm

No we do not, fair point. It looks like there are two or 3 fuses on the power supply, so presumably they are specced for normal loads.

Kokotoni Will · 15th Nov 2025, 2:57 pm

12E1 grid measures 185V with the 10k load.

The new caps etc. have arrived. I've changed C4 for the new one, and HT with lamp limiter is now 145V, so a slight improvement. It will take me a while to solder all the new parts, as I'm trying to be as neat as possible, so please bear with me.

13th Nov 2025, 1:30 pm	#181
Kokotoni Will Pentode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 103	Re: Mystery amplifier Grid measured 30v, cathode: 28v. A fail then.

13th Nov 2025, 1:32 pm	#182
Kokotoni Will Pentode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 103	Re: Mystery amplifier Does this 12E1 look ok: https://www.ebay.co.uk/itm/136712994989

13th Nov 2025, 1:39 pm	#183
Kokotoni Will Pentode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 103	Re: Mystery amplifier I've just powered up the amp without lamp limiter. Good news: I now get between 235-290V HT, adjustable with potentiometer.

13th Nov 2025, 1:50 pm	#184
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 6,170	Re: Mystery amplifier OK, that's very good but please check the DC voltage on 12e1 grid, cathode and anode, and the AC voltage on 12e1 heater. They may be running too high as this would have been designed for 230V AC and when running the power amplifier. Most likely the 12e1 had low heater volts on the lamp limiter. I would prefer you to find a 100W filament bulb.

13th Nov 2025, 2:25 pm	#185
Kokotoni Will Pentode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 103	Re: Mystery amplifier Grid: 135V Cathode: 269V Anode: 599V Heater: 125V AC

13th Nov 2025, 5:31 pm	#186
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 6,170	Re: Mystery amplifier When you measure the AC volts for the heater you need to switch to AC volts and apply one probe to pin 2 and the other to pin 7. It is supposed to be 6.3V but I am expecting it to be higher. 600V DC is a scary voltage so be very very careful. I am also more than a little concerned that the ECC83 cathode follower (V3a) is running well over specification with with 600V on the anode and 135 on the cathode, it may breakdown! Another option would be to run 2 40W bulbs in parallel? Last edited by PJL; 13th Nov 2025 at 5:38 pm.

13th Nov 2025, 5:44 pm	#187
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 6,170	Re: Mystery amplifier I assume you had the 10x resistors in as a 10K load?

13th Nov 2025, 7:00 pm	#189
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 6,170	Re: Mystery amplifier Put the load back as it might increase the 12e1 grid voltage and reduce the stress on the ECC83.

13th Nov 2025, 7:29 pm	#190
Kokotoni Will Pentode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 103	Re: Mystery amplifier Have measured 12E1 heater, it's 6.34V. With the 10k load, V3 (ECC83) anode voltage sits at 580V.

13th Nov 2025, 8:17 pm	#191
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 6,170	Re: Mystery amplifier its the 12e1 grid I am interested in. Hopefully its quite a bit higher than 135V with no load. You might want to add a 10K 5W load in permanently to keep the PSU under control when the amp is disconnected. It might also be sensible to add a resistor between V3b anode and the unregulated rail but I need to work out a value.

14th Nov 2025, 8:57 am	#192
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 8,409	Re: Mystery amplifier I've a few 12E1's & similar valves if you need any, looks like your making progress, just needs finnagling well done, Andy. __________________ Curiosity hasn't killed this cat...so far.

14th Nov 2025, 12:34 pm	#193
Richardgr Octode Join Date: Oct 2019 Location: Täby, Sweden Posts: 1,110	Re: Mystery amplifier I'm sure this must be a bit like cooking in a kitchen full of chefs I'm not sure it is a good idea to replace a 2A fuse with a 5A one. It is a bit suspicious that a 2A fuse in the first place - where is the load coming from? I have one of those cheap devices you plug into the wall, and you plug into the device, that shows the amount of power (watts) being consumed. It has proved quite useful to sanity check the consumption of something under test, and it is also useful when determining a good fuse option and looking at peak load on startup. Last edited by Richardgr; 14th Nov 2025 at 12:41 pm.

14th Nov 2025, 1:08 pm	#194
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,589	Re: Mystery amplifier It's normal for a 2A fuse to blow on switch on with transformers of that size, even with little or no load. You might get away with it not blowing for a few on/offs and then 'pop'! It just depends on which part of the mains cycle you happen to catch it on when you throw the switch - 5A is better.

14th Nov 2025, 2:44 pm	#195
Richardgr Octode Join Date: Oct 2019 Location: Täby, Sweden Posts: 1,110	Re: Mystery amplifier I've experienced that with a toroid, but not a 'normal'' EI transformer. The toroid was tamed with a CL-90 inrush limiter. And a slow-blow fuse should deal with that as well.

14th Nov 2025, 4:10 pm	#199
Richardgr Octode Join Date: Oct 2019 Location: Täby, Sweden Posts: 1,110	Re: Mystery amplifier No we do not, fair point. It looks like there are two or 3 fuses on the power supply, so presumably they are specced for normal loads.

15th Nov 2025, 2:57 pm	#200
Kokotoni Will Pentode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 103	Re: Mystery amplifier 12E1 grid measures 185V with the 10k load. The new caps etc. have arrived. I've changed C4 for the new one, and HT with lamp limiter is now 145V, so a slight improvement. It will take me a while to solder all the new parts, as I'm trying to be as neat as possible, so please bear with me.