Disconnected speaker etc - UK Vintage Radio Repair and Restoration Discussion Forum

hillmanie · 13th May 2020, 10:00 am

Scraps of info trickle back into memory randomly like the following. I stumble across them in the Conference with the usual helpful hints.
1. No load on speaker transformer:
I have to confess that I've powered up some old recorders with the speaker dis-connected.
How much damage will this do to the output valve? I seem to remember that the more powerful the amp the more harm done?
2. Valve damage:
Don't know if this type of valve is used in any tape recs. but I think it was that if the wrong voltage was applied to the grid (screen grid?) of a Tetrode valve it was wrecked. I forget how this might happen - short in the supply circuit I suppose.
TT

kalee20 · 13th May 2020, 1:08 pm

No load on a valve amplifier is generally considered bad practice.

But really, a triode output stage will be OK. And a pentode or beam-tetrode, with negative feedback.

Reason for it being bad, is that with no load and the volume wound up, really high voltages can be developed across the pure inductance of the output transformer being hit by current being rapidly slewed from something to nothing. Something could die - maybe the output transformer, maybe the valve or valveholder sparking across (I've had the latter).

Screen-grid damage? Well, disconnecting the anode but leaving screen-grid connected won't do it any favours, because the entire cathode current will head to the screen - which, being thin wires, can only dissipate a fraction of what the anode could. Again, I've seen it happen. Grid glowing bright orange is not a pretty sight. No valve was wrecked, but it's possible a bit of gas was evolved.

hillmanie · 13th May 2020, 1:37 pm

Thanks for very clear explanation of these questions. Subject to digging out the speakerless chassis concerned I think the valves were ECC83 and ECL82. (Latter the output?) Maybe I got away with it

garry71 · 13th May 2020, 3:23 pm

The Ferguson 441TR has an ECL82 power amp with a speaker off switch. The switch just puts an open circuit on the output transformer secondary just after where the negative feedback loop taps off.

The ECL82 is a triode and output pentode in one valve.

DMcMahon · 13th May 2020, 6:17 pm

On the Grundigs TK 30 & TK 35 I am presently working on, the loudspeaker switch just open circuits the connection between the speaker and the output transformer secondary winding. There is no -ve feedback connection and the only other connection is a wire to the outputs DIN. There is a second secondary winding that does have -ve feedback I think.

I have occasionally run without speaker load without incident, for any long duration running with no speaker load would remove the output pentode EL 84 in my case.

Some of the other Grundig models, switch in a dummy load resistor when the speaker is switched off.

Reelman · 13th May 2020, 6:28 pm

Quote:

Originally Posted by DMcMahon

On the Grundigs TK 30 & TK 35 I am presently working on, the loudspeaker switch just open circuits the connection between the speaker and the output transformer secondary winding. There is no -ve feedback connection and the only other connection is a wire to the outputs DIN. There is a second secondary winding that does have -ve feedback I think.

I have occasionally run without speaker load without incident, for any long duration running with no speaker load would remove the output pentode EL 84 in my case.

Some of the other Grundig models, switch in a dummy load resistor when the speaker is switched off.

I wondered about that when I was looking over the circuit myself. However, as has just been pointed out it is not a problem when negative feedback is applied. In this case the NF is applied by a separate winding on the output transformer. Effectively even when the ‘speaker is disconnected there is a load.

Peter

DMcMahon · 15th May 2020, 8:26 pm

Yes agreed.

ricard · 17th May 2020, 5:35 am

Valved Tandberg machines are unique in that the output stage is intended to be run unloaded. There is am integral and permanently connected 51 ohm resistor across the output transformer secondary. which together with the rest of the design I'm supposing is enough of a load to avoid damage. Or would the negative feedback that is also employed be the main reason for this?

At any rate, it makes these old Tandbergs ideally suited for operating with modern day equipment rather than trying to use a pre-amp output which is of too high impedence to deliver much of a signal when used with transistorized equipment. Also coupled with the fact that come the 1960s, there were phono jacks directly on the back of machines, including one for the speaker output - no adapter cables needed.

13th May 2020, 10:00 am	#1
hillmanie Pentode Join Date: May 2019 Location: Galway, Republic of Ireland. Posts: 208	Disconnected speaker etc Scraps of info trickle back into memory randomly like the following. I stumble across them in the Conference with the usual helpful hints. 1. No load on speaker transformer: I have to confess that I've powered up some old recorders with the speaker dis-connected. How much damage will this do to the output valve? I seem to remember that the more powerful the amp the more harm done? 2. Valve damage: Don't know if this type of valve is used in any tape recs. but I think it was that if the wrong voltage was applied to the grid (screen grid?) of a Tetrode valve it was wrecked. I forget how this might happen - short in the supply circuit I suppose. TT

13th May 2020, 1:08 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: Disconnected speaker etc No load on a valve amplifier is generally considered bad practice. But really, a triode output stage will be OK. And a pentode or beam-tetrode, with negative feedback. Reason for it being bad, is that with no load and the volume wound up, really high voltages can be developed across the pure inductance of the output transformer being hit by current being rapidly slewed from something to nothing. Something could die - maybe the output transformer, maybe the valve or valveholder sparking across (I've had the latter). Screen-grid damage? Well, disconnecting the anode but leaving screen-grid connected won't do it any favours, because the entire cathode current will head to the screen - which, being thin wires, can only dissipate a fraction of what the anode could. Again, I've seen it happen. Grid glowing bright orange is not a pretty sight. No valve was wrecked, but it's possible a bit of gas was evolved.

13th May 2020, 1:37 pm	#3
hillmanie Pentode Join Date: May 2019 Location: Galway, Republic of Ireland. Posts: 208	Re: Disconnected speaker etc Thanks for very clear explanation of these questions. Subject to digging out the speakerless chassis concerned I think the valves were ECC83 and ECL82. (Latter the output?) Maybe I got away with it

13th May 2020, 3:23 pm	#4
garry71 Tetrode Join Date: Apr 2020 Location: Birmingham, West Midlands, UK. Posts: 88	Re: Disconnected speaker etc The Ferguson 441TR has an ECL82 power amp with a speaker off switch. The switch just puts an open circuit on the output transformer secondary just after where the negative feedback loop taps off. The ECL82 is a triode and output pentode in one valve.

13th May 2020, 6:17 pm	#5
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Disconnected speaker etc On the Grundigs TK 30 & TK 35 I am presently working on, the loudspeaker switch just open circuits the connection between the speaker and the output transformer secondary winding. There is no -ve feedback connection and the only other connection is a wire to the outputs DIN. There is a second secondary winding that does have -ve feedback I think. I have occasionally run without speaker load without incident, for any long duration running with no speaker load would remove the output pentode EL 84 in my case. Some of the other Grundig models, switch in a dummy load resistor when the speaker is switched off.

15th May 2020, 8:26 pm	#7
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Disconnected speaker etc Yes agreed.

17th May 2020, 5:35 am	#8
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Disconnected speaker etc Valved Tandberg machines are unique in that the output stage is intended to be run unloaded. There is am integral and permanently connected 51 ohm resistor across the output transformer secondary. which together with the rest of the design I'm supposing is enough of a load to avoid damage. Or would the negative feedback that is also employed be the main reason for this? At any rate, it makes these old Tandbergs ideally suited for operating with modern day equipment rather than trying to use a pre-amp output which is of too high impedence to deliver much of a signal when used with transistorized equipment. Also coupled with the fact that come the 1960s, there were phono jacks directly on the back of machines, including one for the speaker output - no adapter cables needed.