Valves, I think I'm starting to understand what the fuss is about.

[tubehunter]

I think nick means voltage from the 6.3 winding on the transformer.

[lurcher]

Yes I did, but also yes, that Tent Labs supply would work fine.

Can't say it would make any difference though, I haven't tried it in a kel84. TBH, I never though it suffered from the heater supplies anyway.

[Jonboy]

Forget Mullards unless you have money burning a hole in your pocket try these Russians which are very good, i used them in my Leak Stereo 20 to good effect

[HighFidelityGuy]

Ah, I see. The heater winding on the power transformer is 6A so there should be enough headroom. I think.

The reason I thought of using this is that upgrading the diodes and adding the pi filter is supposed to increase the increase the s/n ratio quite a bit. So I thought if I'm going to change all that I may as well just use this module instead and get regulation on top.

Apparently low loss diodes also help the HT supply but seen as this will increase the voltage would I need to compensate for this?

Cheers.

[lurcher]

No, it was voltage headroom I meant. 6.3v windings have a peak voltage of 8.9v, a normal silicon bridge will drop about 1.2v, so that leaves 7.7v into the regulator. If you want 6.3v out that only leaves 1.4v across the regulator. Most regulators need 2v across them to regulate. Its doable, and thats what the Tent Labs unit does, but you need to use either a discreet reg, or a LDO regulator and low voltage drop diodes.

[HighFidelityGuy]

No, it was voltage headroom I meant. 6.3v windings have a peak voltage of 8.9v, a normal silicon bridge will drop about 1.2v, so that leaves 7.7v into the regulator. If you want 6.3v out that only leaves 1.4v across the regulator. Most regulators need 2v across them to regulate. Its doable, and thats what the Tent Labs unit does, but you need to use either a discreet reg, or a LDO regulator and low voltage drop diodes.

I see what you mean. The Tent Labs website says the module will work off a 6.3V heater winding so I guess it should be fine. Although it does seem to suggest that it can only supply 1A like this, so that might not leave enough headroom.

I'll give this some thought.

[HighFidelityGuy]

It's been a few weeks since I had my first experience with the KEL84, so I thought I'd give a bit of an update on what I've been doing with it.

After doing a lot of research into all the mods available for the KEL84 and some general reading up on valve amp design, I decided that I would see how far I could push it's performance in the hope that I could improve the slightly weak areas when compared against my mono-blocks.

As my KEL84 is an early version it lacks several improvements that were added later and also has a couple of minor design flaws. The flaws are in the wattage ratings of a few resistors which need to be increased. Also on the subject of resistors; I need to swap a couple to convert it into a power amp, I want to swap a few for Mills wirewound's and a few others for Kiwame carbon films. So in the end all the resistors are getting swapped. Some of the grid resistors are also a bit too far away from the valve bases, so I'm going to improve that. I'm hoping these changes will lower the amps distortion and clean up the sound a bit. Other users seem to have found improvements using these resistors, so hopefully I will too.

After that I'm going to swap the orange drop coupling caps for Mundorf Silver/Oil, polystyrene caps for silver mica and Jensen electrolytic bypass caps for Elna Silmic II. I'm hoping these changes improve the tonal balance and detail retrieval. I did a lot of research into coupling caps and the Mundorf Silver/Oil's seemed to be highly regarded as very good but not stupidly expensive like some Teflon caps. I'm hoping their slight brightness will compliment the Kiwame resistor's slight warmth to provide a nice balance. That's the theory anyway.

I'm also going to swap the HT supply caps but I've not decided what to use there yet. Possibly some Panasonic high ripple, low ESR caps. I also can't decide what values to use as there are lots of conflicting opinions on which should be increased and which shouldn't. I'm going to mull this over while I work on the other parts.

I decided to not bother with the Tent regulated heater supply as I couldn't guarantee that it would work properly with the existing heater winding. So I'm going to stick with the upgraded design that WAD recommended that involves swapping the standard diodes with Schottkies and dropping the extra voltage they allow through in a simple pi filter. I'll also swap the HT supply diodes for Schottkies at the same time.

After running the amp in it's standard form for a while I noticed that the mains transformer ran quite hot and made an audible mechanical buzz. So I picked up some silicone tubing off eBay that I'm going to use to make some dampening bushes to reduce the buzz. I don't think the heat of the transformer is a major issue as it's not too bad but I'm going to keep an eye on it. I could probably attach a heatsink using thermal adhesive I guess.

I have most of the parts I need now and I've removed all the old parts from the PCB. So I'm hoping to get the board populated with everything I have over the weekend and finish it all off next week. I'll try to post some photos soon.

[Reid Malenfant]

Hi Dave, i think you'll have a very difficult if not impossible job finding Schottky diodes rated high enough (voltage wise) to replace the HT supply diodes

Some kind of Fred would be fine (fast recovery epitaxial diode).

[HighFidelityGuy]

Hi Dave, i think you'll have a very difficult if not impossible job finding Schottky diodes rated high enough (voltage wise) to replace the HT supply diodes

Really. I'm sure I ordered the ones that WAD recommended. Perhaps they're not Schottkies and just some other type of ultra fast diodes. I'll check my RS order and let you know what I've got......

[HighFidelityGuy]

Ok, here's what I ordered: LINK
And here's the amps schematic: LINK Page 6

Have I got the right thing? Cheers.