Advice on output transformers - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

cathoderay57 · 4th Jul 2020, 11:45 am

Welcome back, glad to hear that you are OK! Kind regards, Jerry

cathoderay57 · 17th Jul 2020, 9:54 pm

I was looking in my spare valve box and found an ELL80. It tested 100% on my valve tester. I also keep a small stock of the RS Components output transformers, same as the ones that Diniz used, because I quite often need them to repair radios. So I thought it might be fun to build a replica of the Grundig amplifier (just the ELL80 and ECC83 stages) and see how it performs. Photos attached. Visually it's a bit of a "rat's nest" but in fact it works well. I made the same mistake as Diniz and wired the output transformers the wrong way around (squealing feedback) but soon put that right. In post #16 Diniz says that one of his output transformers got hot while the other didn't. I ran the amp for about 20 minutes and there was no noticeable rise in temperature on either of them. I am running at slightly lower HT than the Grundig spec (incidentally when I quoted the voltages in earlier posts I misread the service data - the highest voltage readings were for pickup (PU), which is the configuration I am using):

ELL80
Pin 1 (Grid 2') --> 249 vdc (Data says: PU 286v)
Pin 3 (Anode') --> 237 vdc (Data says: PU 276v)
Pin 7 (cathode) --> 9.0 vdc (Data says: PU 9.8v)
Pin 8 (Anode) --> 237 vdc (Data says: PU 276v)
Pin 9 (Grid 2) --> 249 vdc (Data says: PU 286v)

ECC83
Pin 1 (anode 1) --> 139 vdc (Data says: PU 152v)
Pin 6 (anode 2) --> 135 vdc (Data says: PU 152v)

I made minor and insignificant changes to the circuit because I didn't always have exact replacement components:

1. Volume control R68/69 I didn't have one with fixed tappings at top and bottom so used a 2.2M stereo pot with single tapping at the bottom of each pot track.
2. R83/R84 I used 2M instead of 1.8M.
3. I didn't have a stereo 500R pot for the balance control R75 so used two 270R resistors between each cathode of the ECC83 and C65.

I am driving the amp from a home-made preamp with active filters, using a turntable with moving magnet pickup. As to performance, the sound is nice and clear up to about 75% of the preamp volume control (vol control of amp always set to max) and above that distortion does occur. Tomorrow I will experiment with higher HT and see what difference it makes. Interestingly the max anode and g2 voltage for the ELL80 in valve data is supposed to be 250 and so in the Grundig the valve seems to be having a hard life! Also in Post #55 Diniz mentioned that the feedback resistors R81/R82 shown as 12k on the service sheet measured 1.9k on his set so I will try out lower value resistors and see what happens.

Cheers, Jerry

cathoderay57 · 18th Jul 2020, 1:06 pm

Today I cranked up the HT by connecting directly to the smoothed cathode output of an EZ80 rectifier powered by a 250-0-250v transformer. Revised valve readings as follows:

ELL80
Pin 1 (Grid 2') --> 273 vdc (Data says: PU 286v)
Pin 3 (Anode') --> 262 vdc (Data says: PU 276v)
Pin 7 (cathode) --> 9.8 vdc (Data says: PU 9.8v)
Pin 8 (Anode) --> 262 vdc (Data says: PU 276v)
Pin 9 (Grid 2) --> 273 vdc (Data says: PU 286v)

ECC83
Pin 1 (anode 1) --> 152 vdc (Data says: PU 152v)
Pin 6 (anode 2) --> 149 vdc (Data says: PU 152v)

Performance improved marginally - about 80% full volume before distortion becomes noticeable. Haven't tried lower value feedback resistors yet.

cathoderay57 · 19th Jul 2020, 8:22 pm

Just for completeness I substituted 2k resistors in the negative feedback line R81/R82. Perhaps as to be expected, the gain of the amp reduced by approx 50%; I was surprised it was as much as that. Still some distortion right at the top end of volume. I think the circuit was intended to use 12k resistors so if you are tracking this thread, Diniz, I recommend you change the ones that you measured as 1.9K and fit 12k resistors. Also, if possible, get the ELL80 valves that you have used tested to make sure they are still balanced - unbalanced pentodes is probably the most likely reason for one output transformer getting warm while the other remains cold, and also most likely to be causing your distortion at mid-volume levels. With a perfectly balanced ELL80 I found the sound to be undistorted until I turned it up to very loud listening level.

Diniz Diniz · 2nd Aug 2020, 1:34 am

Hi jerry!
Today I re-cheked r81/r82 and they measured 1.9kohm when installed in the circuit. But when with one lead disconnected the measured the specified 12kohm. Don’t really know why...
Despite that, the set is playing quite good.
Cheers,
Diniz.

4th Jul 2020, 11:45 am	#61
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Advice on output transformers Welcome back, glad to hear that you are OK! Kind regards, Jerry

17th Jul 2020, 9:54 pm	#62
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Advice on output transformers I was looking in my spare valve box and found an ELL80. It tested 100% on my valve tester. I also keep a small stock of the RS Components output transformers, same as the ones that Diniz used, because I quite often need them to repair radios. So I thought it might be fun to build a replica of the Grundig amplifier (just the ELL80 and ECC83 stages) and see how it performs. Photos attached. Visually it's a bit of a "rat's nest" but in fact it works well. I made the same mistake as Diniz and wired the output transformers the wrong way around (squealing feedback) but soon put that right. In post #16 Diniz says that one of his output transformers got hot while the other didn't. I ran the amp for about 20 minutes and there was no noticeable rise in temperature on either of them. I am running at slightly lower HT than the Grundig spec (incidentally when I quoted the voltages in earlier posts I misread the service data - the highest voltage readings were for pickup (PU), which is the configuration I am using): ELL80 Pin 1 (Grid 2') --> 249 vdc (Data says: PU 286v) Pin 3 (Anode') --> 237 vdc (Data says: PU 276v) Pin 7 (cathode) --> 9.0 vdc (Data says: PU 9.8v) Pin 8 (Anode) --> 237 vdc (Data says: PU 276v) Pin 9 (Grid 2) --> 249 vdc (Data says: PU 286v) ECC83 Pin 1 (anode 1) --> 139 vdc (Data says: PU 152v) Pin 6 (anode 2) --> 135 vdc (Data says: PU 152v) I made minor and insignificant changes to the circuit because I didn't always have exact replacement components: 1. Volume control R68/69 I didn't have one with fixed tappings at top and bottom so used a 2.2M stereo pot with single tapping at the bottom of each pot track. 2. R83/R84 I used 2M instead of 1.8M. 3. I didn't have a stereo 500R pot for the balance control R75 so used two 270R resistors between each cathode of the ECC83 and C65. I am driving the amp from a home-made preamp with active filters, using a turntable with moving magnet pickup. As to performance, the sound is nice and clear up to about 75% of the preamp volume control (vol control of amp always set to max) and above that distortion does occur. Tomorrow I will experiment with higher HT and see what difference it makes. Interestingly the max anode and g2 voltage for the ELL80 in valve data is supposed to be 250 and so in the Grundig the valve seems to be having a hard life! Also in Post #55 Diniz mentioned that the feedback resistors R81/R82 shown as 12k on the service sheet measured 1.9k on his set so I will try out lower value resistors and see what happens. Cheers, Jerry Attached Thumbnails

18th Jul 2020, 1:06 pm	#63
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Advice on output transformers Today I cranked up the HT by connecting directly to the smoothed cathode output of an EZ80 rectifier powered by a 250-0-250v transformer. Revised valve readings as follows: ELL80 Pin 1 (Grid 2') --> 273 vdc (Data says: PU 286v) Pin 3 (Anode') --> 262 vdc (Data says: PU 276v) Pin 7 (cathode) --> 9.8 vdc (Data says: PU 9.8v) Pin 8 (Anode) --> 262 vdc (Data says: PU 276v) Pin 9 (Grid 2) --> 273 vdc (Data says: PU 286v) ECC83 Pin 1 (anode 1) --> 152 vdc (Data says: PU 152v) Pin 6 (anode 2) --> 149 vdc (Data says: PU 152v) Performance improved marginally - about 80% full volume before distortion becomes noticeable. Haven't tried lower value feedback resistors yet.

19th Jul 2020, 8:22 pm	#64
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Advice on output transformers Just for completeness I substituted 2k resistors in the negative feedback line R81/R82. Perhaps as to be expected, the gain of the amp reduced by approx 50%; I was surprised it was as much as that. Still some distortion right at the top end of volume. I think the circuit was intended to use 12k resistors so if you are tracking this thread, Diniz, I recommend you change the ones that you measured as 1.9K and fit 12k resistors. Also, if possible, get the ELL80 valves that you have used tested to make sure they are still balanced - unbalanced pentodes is probably the most likely reason for one output transformer getting warm while the other remains cold, and also most likely to be causing your distortion at mid-volume levels. With a perfectly balanced ELL80 I found the sound to be undistorted until I turned it up to very loud listening level.

2nd Aug 2020, 1:34 am	#65
Diniz Diniz Pentode Join Date: Nov 2016 Location: Lisbon, Portugal Posts: 187	Re: Advice on output transformers Hi jerry! Today I re-cheked r81/r82 and they measured 1.9kohm when installed in the circuit. But when with one lead disconnected the measured the specified 12kohm. Don’t really know why... Despite that, the set is playing quite good. Cheers, Diniz.