6AM5/EL91 vs A2134 - UK Vintage Radio Repair and Restoration Discussion Forum

Restoration73 · 3rd Apr 2021, 5:30 pm

I am currently renewing the audio side of my Eddystone 940, which uses
2 x 6AM5 in push pull driven by ECC82. I also have some A2134 which are
very similar. The situation is like EL84 vs EL86.
Anybody used A2134 for audio ? I don't need to rewire the valve sockets.
The main problem is the old composition resistors have gone high. TIA

Robert Gribnau · 3rd Apr 2021, 6:27 pm

I would not think that the 6AM5 compares to the A2134 like the EL84 compares to the EL86. The 6AM5 and the A2134 share the same pin-out like the EL84 and EL86 do, but that's about it. The EL84 and EL86 at least share maximum anode dissipation and mutual conductance, but still there are significant differences between the two (current, needed impedance of the primary of the output transformer, bias voltage, etc.). The 6AM5 and A2134 don't even share those.

Most important difference between the 6AM5 and the A2134 is their filament current. One 6AM5 takes 0,2 A, one A2134 takes 0,635 A.

The other important difference is that under normal operating conditions a pair of A2134 in push-pull will draw more than double the anode current and screen grid current than a pair of 6AM5's in push-pull would.

Probably the output transformer (and power supply) of your Eddystone 940 will not be able to handle those extra amounts of anode, screen grid and filament currents. Maybe it is possible to choose a working point for a pair of B2134's that would consume the same amount of anode and screen grid current as a pair of 6AM5's, but that does not cure the filament current problem.

The A2134 can surely be used for audio, but not as a practical replacement of the 6AM5.

Leon Crampin · 3rd Apr 2021, 7:26 pm

The A2134 is an N37 with a 6.3V heater. It is a device suited either to series regulator applications (often in paralleled pairs) or as an AF output valve for low supply voltages - as the N37 was used in TVs as an audio or field output device.

Although I'm not familiar with the Eddystone, I think it's highly unlikely that the power supply or the output transformer would suit these valves.

The EL95 is a possibility - often more cheaply bought as a PL95 which just needs a small heater ballast resistor for 6.3V operation.

Leon.

G6Tanuki · 3rd Apr 2021, 7:48 pm

Given that EL91s are really-rather-ubiquitous, having been used as doublers/triplers/drivers in loads of 50s/60s VHF mobile-radio stuff, it shouldn't be difficult to source them.

If you have dificulties - look to the CV416 (6F17) - which worked as the 121.5MHz PA stage and the associated two-valves-in-push-pull AM modulator-stage in the 1950s/60s "TR2002" VHF panic-radios.

http://www.radiomuseum.co.uk/TR2002.html

Restoration73 · 3rd Apr 2021, 7:55 pm

Thanks for the info - the 940 has a generous transformer but I might try it with the heaters powered separately.
EL95s require a socket rewire but they do well in p-p applications, I have seen them in
quite a few Grundig etc table radios. i also have N78 6AQ5 and others here. I'm lucky
this 1965 set works OK up to the volume control, so as usual it's back to replacing the
dud resistors. I'll report back on this.

Restoration73 · 7th Apr 2021, 2:58 pm

As expected the following resistors had significantly increased in value and were all
replaced with modern film types;R27,30,33,45,46,57,58,59,63,66 also C102 had dried up
Operation restored, I tried NOS EL91s and ECC82 Mullards in place of the original Brimars
but no change was noted. It was evident that this set had never received any repair
since leaving the factory so I think we can excuse Eddystone the original choice of
resistors. Excellent performer for a single conversion receiver. Job done.

Restoration73 · 15th Apr 2021, 10:52 pm

Having established issues caused by resistors, poor signal meter operation was found to
be due to the following 1W carbons changing value ; R35, 36, 37, 38. The first two ran
so warm I replaced them with 2W types (would have preferred wirewounds) Now the
meter can be zeroed and FSD achieved - although this is only a relative indicator.
A squeak when tuning was corrected when the dial pointer and brass rods were lubricated.
I do find the agc time constant is a bit slow.

3rd Apr 2021, 5:30 pm	#1
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	6AM5/EL91 vs A2134 I am currently renewing the audio side of my Eddystone 940, which uses 2 x 6AM5 in push pull driven by ECC82. I also have some A2134 which are very similar. The situation is like EL84 vs EL86. Anybody used A2134 for audio ? I don't need to rewire the valve sockets. The main problem is the old composition resistors have gone high. TIA

3rd Apr 2021, 6:27 pm	#2
Robert Gribnau Heptode Join Date: May 2017 Location: Konongo, Ghana Posts: 513	Re: 6AM5/EL91 vs A2134 I would not think that the 6AM5 compares to the A2134 like the EL84 compares to the EL86. The 6AM5 and the A2134 share the same pin-out like the EL84 and EL86 do, but that's about it. The EL84 and EL86 at least share maximum anode dissipation and mutual conductance, but still there are significant differences between the two (current, needed impedance of the primary of the output transformer, bias voltage, etc.). The 6AM5 and A2134 don't even share those. Most important difference between the 6AM5 and the A2134 is their filament current. One 6AM5 takes 0,2 A, one A2134 takes 0,635 A. The other important difference is that under normal operating conditions a pair of A2134 in push-pull will draw more than double the anode current and screen grid current than a pair of 6AM5's in push-pull would. Probably the output transformer (and power supply) of your Eddystone 940 will not be able to handle those extra amounts of anode, screen grid and filament currents. Maybe it is possible to choose a working point for a pair of B2134's that would consume the same amount of anode and screen grid current as a pair of 6AM5's, but that does not cure the filament current problem. The A2134 can surely be used for audio, but not as a practical replacement of the 6AM5. __________________ Robert Last edited by Robert Gribnau; 3rd Apr 2021 at 6:55 pm. Reason: Typo

3rd Apr 2021, 7:26 pm	#3
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: 6AM5/EL91 vs A2134 The A2134 is an N37 with a 6.3V heater. It is a device suited either to series regulator applications (often in paralleled pairs) or as an AF output valve for low supply voltages - as the N37 was used in TVs as an audio or field output device. Although I'm not familiar with the Eddystone, I think it's highly unlikely that the power supply or the output transformer would suit these valves. The EL95 is a possibility - often more cheaply bought as a PL95 which just needs a small heater ballast resistor for 6.3V operation. Leon.

3rd Apr 2021, 7:48 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: 6AM5/EL91 vs A2134 Given that EL91s are really-rather-ubiquitous, having been used as doublers/triplers/drivers in loads of 50s/60s VHF mobile-radio stuff, it shouldn't be difficult to source them. If you have dificulties - look to the CV416 (6F17) - which worked as the 121.5MHz PA stage and the associated two-valves-in-push-pull AM modulator-stage in the 1950s/60s "TR2002" VHF panic-radios. http://www.radiomuseum.co.uk/TR2002.html

3rd Apr 2021, 7:55 pm	#5
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: 6AM5/EL91 vs A2134 Thanks for the info - the 940 has a generous transformer but I might try it with the heaters powered separately. EL95s require a socket rewire but they do well in p-p applications, I have seen them in quite a few Grundig etc table radios. i also have N78 6AQ5 and others here. I'm lucky this 1965 set works OK up to the volume control, so as usual it's back to replacing the dud resistors. I'll report back on this.

7th Apr 2021, 2:58 pm	#6
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: 6AM5/EL91 vs A2134 As expected the following resistors had significantly increased in value and were all replaced with modern film types;R27,30,33,45,46,57,58,59,63,66 also C102 had dried up Operation restored, I tried NOS EL91s and ECC82 Mullards in place of the original Brimars but no change was noted. It was evident that this set had never received any repair since leaving the factory so I think we can excuse Eddystone the original choice of resistors. Excellent performer for a single conversion receiver. Job done.

15th Apr 2021, 10:52 pm	#7
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: 6AM5/EL91 vs A2134 Having established issues caused by resistors, poor signal meter operation was found to be due to the following 1W carbons changing value ; R35, 36, 37, 38. The first two ran so warm I replaced them with 2W types (would have preferred wirewounds) Now the meter can be zeroed and FSD achieved - although this is only a relative indicator. A squeak when tuning was corrected when the dial pointer and brass rods were lubricated. I do find the agc time constant is a bit slow.