View Single Post
Old 4th Feb 2023, 9:46 pm   #14
Valvepower
Octode
 
Valvepower's Avatar
 
Join Date: Aug 2008
Location: Rayleigh near Southend-On-Sea, Essex, UK.
Posts: 1,852
Default Re: My first single ended stereo amplifier build

Hello Gabriel,

That's a cracking build.

The PYE Super Black Box (SBB) has a 33% UL tap and a pair of ‘period’ Gilson SE EL34 transformers I have in the workshop, dating back to the late 50’s early 60’s, have a 40% UL tap.

As pointed out by kalee20, finding the UL ratio is a case of placing a known voltage across the HT and Anode winding and measuring the voltage on the UL tap to ascertain the ratio – this is the method I used to find the UL tap on the two SE o/p t/x’s.

From memory the SBB ran the EL34 with roughly 280V on the anode, 2700Ω anode load and a 150Ω cathode resistor, and this didn’t fall that far short of 10 Watts output – I’ll have to measure it again.

I like the use of the SRPP driver.

Personally, the SRPP is part of the same family as the Bootstrap amplifier and follower, and Jeffery High-Gain Phase splitter. Recently I built a 10-watt push-pull EL84 amplifier using the Jeffery High-Gain Phase splitter (this is the all AC coupled variant of the SRPP), which worked fine, however there were a few problems caused by stray capacitance in the anode circuit of the bottom bootstrapped valve. I suppose this due to action of the bootstrapping increasing the impedance and thus the gain – but as an old Physics teacher said “you don’t get owt for nowt”..

Jeffery, himself, described an EL84 amplifier implementing the ‘Jeffery’ driver using a 6BR8, however I used E180F and a triode connected EF184.

I must admit to having thoughts of using the Jeffery High-Gain Phase splitter (after Bailey and Short) with a singled ended output to drive a SE connected o/p valve. Maybe give the 5B/254M SE a go?

Just thinking out the box here (sorry for the David Brent speak!) regards the hum… Was it the same on both channels? As you said it’s OK now, and best leave it that way, however…, just a thought, could there be some ‘power supply filter noise’ finding its way into the driver ground(s), which was eased by swapping the choke and resistor? However, looking at the grounding, as best I could, all reservoir and smoothing capacitors all seem well anchored to a sound grounding point in the middle of the chassis… I suppose this kind of power supply noise would have been more of a buzz than a hum though.

Terry.
Valvepower is offline