A 2 or 3-channel SE 'Horn-amp'

pre65 · Post by **pre65** » Wed Nov 06, 2013 8:59 pm

I used 6S4s and 6B4-g on one amp I had with IO (International octal) sockets with no problems.

See Mike H tip in previous post.

IslandPink · Post by **IslandPink** » Wed Nov 06, 2013 9:12 pm

OK, following-up on Mike's advice, I have 6.2V DC to the valve that is supplied by the 'Thorsten' supply . It is glowing very slightly when the lights are off. Pretty surprising really , would have expected more glow .
The other one ( supplied by a Lehane V-reg supply ) is showing 4.6V DC .
This is odd because this supply could happily light a 6N6P at 0.75A from a 7V AC input from a toroid . I guess it's dropped-out of regulation.

I have no more energy this evening to do anything .

Project is now touch & go for the weekend, it will be a monumental ball-ache to take this supply out and fit something else ...

Thanks for the help .

IslandPink · Post by **IslandPink** » Wed Nov 06, 2013 9:18 pm

Andrew -
There's still a healthy 7.2V AC going into the supply .
Is the voltage-out and the resistors ( 560 + 56 nominal ) used to set that voltage, dependant on the current-out ?
Is there some mileage in raising the resistance in that location ?

I wish I'd put a pot in there now

Mike H · Post by **Mike H** » Wed Nov 06, 2013 9:37 pm

This is the trouble with filament regs, getting them to work

6C4C is 1 Amp isn't it, so is 6B4G the same?

But that is the hot current of course, cold filament resistance is <= 1 tenth what it is hot.

IslandPink · Post by **IslandPink** » Wed Nov 06, 2013 9:48 pm

Yes, most frustrating, Mike.
But this supply does work , has been checked at 0.75A .
The 6N6P ( used before ) and the 6B4G are both oxide-coated filaments, so the cold resistance should not be too much different .

The 6B4G is also 1 amp, same as 6C4C , yes .

Andrew · Post by **Andrew** » Wed Nov 06, 2013 10:15 pm

So to recap, you have a Vreg with 7V AC RMS in trying to supply 6.3v DC?

If that is the case I think you need a 9VAC RMS in, just from memory 6.3v DC from 6.3v-ish AC (even 7v AC) is hard to achieve even with a simple 3 legged reg let alone a double reg like the Vreg as there's two regulation voltages dropped.

I can bring a bench supply down to Owston if it helps or a 9V TX.

Andrew

Mike H · Post by **Mike H** » Wed Nov 06, 2013 10:22 pm

0.75A is not the same aa 1A.

I agree with Andrew, what I found is you need enough 'overhead' to run the reg chip(s). LM317 needs 3V minimum across it I think. For example.

So that would be 9.3V minimum on the input side for 6.3V out. If it were a LM317.

IslandPink · Post by **IslandPink** » Wed Nov 06, 2013 10:30 pm

Thanks Andrew
7.2V AC-in actually , if that makes any difference

It would help if you put together a simple table for us to refer to ... ( as requested... )

Just had a blinding light of realisation-
AC heating

It is a 700Hz to 20k channel, after all !!

pre65 · Post by **pre65** » Wed Nov 06, 2013 10:31 pm

I agree with Andrew and Mike.

Andrew · Post by **Andrew** » Wed Nov 06, 2013 10:38 pm

IslandPink wrote:Thanks Andrew
7.2V AC-in actually , if that makes any difference
It would help if you put together a simple table for us to refer to ... ( as requested... )

Just had a blinding light of realisation-
AC heating
It is a 700Hz to 20k channel, after all !!

I'll bring a 9v TX if you want to provide somewhere to hook up to on the amp, you may get intermod even higher up teh spectrum with the 50Hz so DC will be better.

So far I have been asked for 5v, 6.3v and 12.6v, the first two both require a 9v TX.

Andrew

IslandPink · Post by **IslandPink** » Wed Nov 06, 2013 10:42 pm

Ok, thanks.
I will see how it sounds tomorrow .
Fingers crossed the rest of the amp works , he he

Solid-state eh ? - it would steal your face if it had the chance ...

Mike H · Post by **Mike H** » Thu Nov 07, 2013 2:04 pm

OK 7.2VAC in, you'd think you get 1.414 x = DC, but no, the current load is so large that it pulls it down to nearly the AC level. And the ripple is large.... guess how I know ...

UNLESS you have a transformer that is over-spec on the current (say double), then it will hold it up better. It's all about winding resistance I think.

IslandPink · Post by **IslandPink** » Thu Nov 07, 2013 6:37 pm

Yes, I'm always too optimistic about power supply voltages .

Meanwhile - there's quite a lot of 'stuff' in this amp, all of a sudden :

IslandPink · Post by **IslandPink** » Thu Nov 07, 2013 8:13 pm

Filament voltages all OK now. AC on the upper 6B4G.
HT voltages all near-enough correct .

166V on 4P1L anode perhaps a little low - could change cathode R from 110R to 120R tomorrow .

All hooked-up and playing now . Sounds better listening in the hall than in the front room

Ouch ! No shortage of gain, anyway !
Upper 6B4G fil AC is tied to cathode resistor on one side

, so that isn't very good ! - must do a virtual centre-tap tomorrow , as there's a lot of 100Hz etc coming out of the Azurahorn - probably making the upper mids etc. sound a bit aggressive .

Now for a bit of Ginger Baker to run things in ....

IslandPink · Post by **IslandPink** » Thu Nov 07, 2013 9:17 pm

The sound is getting better but I will have to get rid of the hum a.s.a.p. tomorrow morning !
See pic -

audio-talk

A 2 or 3-channel SE 'Horn-amp'

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