UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - OK need assistance or clarification of output transformer values

AdrianH · 30th Mar 2020, 12:09 am

I am new to actually making something with valves.

I am looking to make a simple audio amp from some 6021W valves and have found a few circuits to get me on my way. But I would like to work a few things out in my head better.

The valve is a sub miniature double triode (ECC70), now from the data ra is typically 6K5 Ohms per triode.
I intend to use two valves in push pull output, that is, the two triodes per valve in parallel acting as one valve. So I was initially thinking that the ra will be halved per valve to 3250 Ohms, but then thinking why, the anode voltage drop would be double if going through a single anode load, so perhaps the ra will stay at 6500Ohms, then I just get confused and head hurts. Probably over thinking it, just build it and see if it works!

Next question, I will be trying to get a Hammond 125B output transformer, the static anode current per triode will be around 6.5mA so 26mA typical in the transformer. and if I drive an 8 Ohm speaker think I would be using output pins 4&6 giving an input impedance of 12K8 Ohms end to end of the transformer.

Have I lost it completely or am I on the correct lines?

TIA

Adrian

I need to do something whilst on lock-down.

30th Mar 2020, 12:09 am	#1
AdrianH Octode Join Date: Mar 2020 Location: Blackburn with Darwen, Lancashire, UK. Posts: 1,573	OK need assistance or clarification of output transformer values I am new to actually making something with valves. I am looking to make a simple audio amp from some 6021W valves and have found a few circuits to get me on my way. But I would like to work a few things out in my head better. The valve is a sub miniature double triode (ECC70), now from the data ra is typically 6K5 Ohms per triode. I intend to use two valves in push pull output, that is, the two triodes per valve in parallel acting as one valve. So I was initially thinking that the ra will be halved per valve to 3250 Ohms, but then thinking why, the anode voltage drop would be double if going through a single anode load, so perhaps the ra will stay at 6500Ohms, then I just get confused and head hurts. Probably over thinking it, just build it and see if it works! Next question, I will be trying to get a Hammond 125B output transformer, the static anode current per triode will be around 6.5mA so 26mA typical in the transformer. and if I drive an 8 Ohm speaker think I would be using output pins 4&6 giving an input impedance of 12K8 Ohms end to end of the transformer. Have I lost it completely or am I on the correct lines? TIA Adrian I need to do something whilst on lock-down.