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Valvepower · 26th Nov 2022, 1:47 pm

Hello,

I recently built a Marshall JTM45/Fender 5F6 amplifier, using KT66’s and GZ34 the efforts of which, can be seen post #7 onwards in the post below...

https://www.vintage-radio.net/forum/...d.php?t=195449

This was made with a salvaged a mains transformer from a basket case Jason amplifier and a choke and output transformer from a Selmer Organ and it was these that decided where I took the HT feed to the output transformer. The reservoir and filter capacitors are 50uF.

The choke has a DC resistance of 193Ω and inductance of 10H and is meaty enough to carry the HT current to the output transformer, however, seeing the output of the amplifier was in the mid 20 watts the current being drawn would cause a voltage drop over the choke, so in order to preserve output power, I took the feed before the choke. The Jason mains transformer is 320-0-320V and is possibly 10-20V below what I’d have desired. But when salvaging you don’t have the choice and you incorporate what you have in the toy box.

If I were using ‘say’ a 50Ω choke I may well have taken it after the choke, but with 193Ω I decided to take it before the choke. So sometimes, the available parts can drive decisions as regards the circuit design. I suppose the ideal transformer would have been the Radiospares Heavy Duty 350-0-350V half shroud drop-through or similar.

I still have the amplifier on the bench, and I decided to do a couple of experiments/tests with the HT feed to the output transformer before or after the choke.

With the feed taken before the choke I got HT before = 363V, HT after = 355V and power output = 29.4 Watts

With the feed taken before the choke I got HT before = 369V, HT after = 341V and power output = 25 Watts

There is a loss of 4.4 Watts with the feed taken after the choke.

The other thing with the feed taken before the choke, is ‘ripple’ present on the clipping, whether this is a problem or not under ‘normal just under clip’ use, I’m not entirely sure, and will probably depend on the user/musician, however I remember when I was working with the guitarist Robin Trower, he noticed a difference between amplifiers at full power clipping (overload) in the USA and in UK and Europe. I put this down to the HT ripple in the amplifier being at 120Hz in the USA and 100Hz in the UK and Europe, this ripple ‘beat’ with the played note of the guitar to give a slightly different sound. These were the Marshall 100Watt Super Lead/Bass 'Plexi' amplifiers, where the HT to output transformer came before the choke.

I've attached a couple of pictures of clipping of the output signal with the HT feed to the output transformer taken before and after the choke. The ripple can be seen on the clip before, and clean after the choke.

Lastly, I agree with Aub as it depends on whether it’s a guitar amplifier or hi-fi amplifier, and generally with guitar amplifier it is taken form the rectifier/reservoir capacitor before the choke, however there is the odd amplifier like the Selmer amplifiers where it’s taken after the choke. Mind you, adding some resistance (using a choke) in the HT feed to the output transformer will cause more sagging of the HT thus giving a different sound, more so at overload when the current demand is at maximum.

Crikey, waffling on now!

Terry.

26th Nov 2022, 1:47 pm	#15
Valvepower Octode Join Date: Aug 2008 Location: Rayleigh near Southend-On-Sea, Essex, UK. Posts: 1,883	Re: Output transformer HT connection. Hello, I recently built a Marshall JTM45/Fender 5F6 amplifier, using KT66’s and GZ34 the efforts of which, can be seen post #7 onwards in the post below... https://www.vintage-radio.net/forum/...d.php?t=195449 This was made with a salvaged a mains transformer from a basket case Jason amplifier and a choke and output transformer from a Selmer Organ and it was these that decided where I took the HT feed to the output transformer. The reservoir and filter capacitors are 50uF. The choke has a DC resistance of 193Ω and inductance of 10H and is meaty enough to carry the HT current to the output transformer, however, seeing the output of the amplifier was in the mid 20 watts the current being drawn would cause a voltage drop over the choke, so in order to preserve output power, I took the feed before the choke. The Jason mains transformer is 320-0-320V and is possibly 10-20V below what I’d have desired. But when salvaging you don’t have the choice and you incorporate what you have in the toy box. If I were using ‘say’ a 50Ω choke I may well have taken it after the choke, but with 193Ω I decided to take it before the choke. So sometimes, the available parts can drive decisions as regards the circuit design. I suppose the ideal transformer would have been the Radiospares Heavy Duty 350-0-350V half shroud drop-through or similar. I still have the amplifier on the bench, and I decided to do a couple of experiments/tests with the HT feed to the output transformer before or after the choke. With the feed taken before the choke I got HT before = 363V, HT after = 355V and power output = 29.4 Watts With the feed taken before the choke I got HT before = 369V, HT after = 341V and power output = 25 Watts There is a loss of 4.4 Watts with the feed taken after the choke. The other thing with the feed taken before the choke, is ‘ripple’ present on the clipping, whether this is a problem or not under ‘normal just under clip’ use, I’m not entirely sure, and will probably depend on the user/musician, however I remember when I was working with the guitarist Robin Trower, he noticed a difference between amplifiers at full power clipping (overload) in the USA and in UK and Europe. I put this down to the HT ripple in the amplifier being at 120Hz in the USA and 100Hz in the UK and Europe, this ripple ‘beat’ with the played note of the guitar to give a slightly different sound. These were the Marshall 100Watt Super Lead/Bass 'Plexi' amplifiers, where the HT to output transformer came before the choke. I've attached a couple of pictures of clipping of the output signal with the HT feed to the output transformer taken before and after the choke. The ripple can be seen on the clip before, and clean after the choke. Lastly, I agree with Aub as it depends on whether it’s a guitar amplifier or hi-fi amplifier, and generally with guitar amplifier it is taken form the rectifier/reservoir capacitor before the choke, however there is the odd amplifier like the Selmer amplifiers where it’s taken after the choke. Mind you, adding some resistance (using a choke) in the HT feed to the output transformer will cause more sagging of the HT thus giving a different sound, more so at overload when the current demand is at maximum. Crikey, waffling on now! Terry. Attached Thumbnails Last edited by Valvepower; 26th Nov 2022 at 2:15 pm.