300b clone

[NealG]

RE DC heaters:

I'd forgotten about this. When Mel first built his amp. it suffered from severe hum. If you placed your hand near the amp. the level would change. It transpired he had omitted the centre tap earth wire for the 6072/5687 heaters....don't make the heaters float unless you like to hum along....

Using Neal's formula and my last set of measurements:

HT 580v
Anode 578v
Cathode R 1k5
Cathode 97v

Dissipation ((578-97)*97)/1500 = 31.7w

Lord, it doesnt look like you've drop the HT voltage, is that correct.

My mains is very similar to Neal's, I take it your having no problems with 1K32?

[Greg.]

Hi Stu,

I've heard from Mel. He states....

At the moment, my mains is bang-on 240V. Not sure about HT, but I seem to recall it was about 560V. My R12/13 are the original specs (1K2). So far, touch wood, no real problem with dissipation; only one 300B valve failure (EH gold grid) since it was first built and that was when I was messing about with it.

Also from Steve Grimshaw......

Mains voltage in Brighton measured at 242V and it does seem pretty standard at that, with just small variations of a volt or so although I did once record 247V!
The amplifiers HT is 556V. I did use dropper resistors on the HT, can't remember off hand what value, but do I remember that they were the same value as you used. The HT before the dropper resistors is 567V. Originaly I was using the 1K2 cathode resistors on the 300bs without any problems on the valve arts. I am now using TJ mesh plates and in line with advice from Jac music I have reduced plate dis to @23 Watts. Sounds lovely to me :-)

Hope that helps.

Best wishes,

Greg

Stu,
Correct.
My mains hovers between 241v and 247v AC, I think it was circa 245v when I took those readings. I also still have the original kit's 27k 9W resistors, though next time I place an order I might get some of the 27.4k fancy ones that the 'Clone Gang' ™ () use.

I'll start with 1k5 and see what that does, probably use white coffins 'till Im happy, then I'll join the Clone gang with the flashy Mills variety!

[Richard]

One of my 300Bs has a hot spot visible on the plate in a very dark room (same 1k3 cathode Rs as Neal's). It may just be a less-good valve but this amp does run them quite hard at a nominal 35W plate dis. I'll look at the other types I have and see if they run red.

I've bought 1k5 resistors and will measure voltages etc tonight to see what's going on. Will also scope and see what the power and waveform looks like at clipping if I alter bias.

Rich

[Richard]

OK, did this last night,

Mains 236V
HT 567V
voltage at anode 560V
voltage at cathode 102V
cathode Rs measured 1313R
so plate dis = (102/1313) x (560-102) = 35.5W

Changed cathode Rs to 1512R
Mains 236V
HT 570V
voltage at anode 564V
voltage at cathode 104V
so plate dis = (104/1512) x (564-104) = 31.6W

Power output and waveform at clipping appears identical.

Sound seems identical but no plates are glowing red now so this looks a sensible change. How much the plates glowing red matters I don't know but lowering plate disippation by 10% will probably prolong valve life by at least that much.

Rich

[Greg.]

Hi Richard and Stuart,

I concur with your findings. I expect that I can remove my HT dropper R's and get about the same result, based on what others here have found, therefore ensuring plate dissipation plus or minus around 30W which seems to keep the 300B's comfortable.

Best wishes,

Greg

Are you going to keep with the HT droppers Greg?