will a 45 DHT amp work with 96 dB speakers

[Mike Stehr]

Those 801A's are going to force me to learn DC heating if I'm going to use them for anything. I could only imagine what 7.5 volts of thoriated tungsten filamant sounds with alternating current.
A 45 would be easy, you can just heat those with AC. The filament voltage is minimal and easy to compensate for.

But them 801A's sure glow purdy. I want to eventually use them for a linestage, I have some 600 ohm line out OPT's that match up with the 801A's 5K plate resistance. But the linestage will be huge, and getting it with minimal noise will be a challenge.

 

[mhardy6647]

Those 801A's are going to force me to learn DC heating if I'm going to use them for anything.

Is that true? I am always a little leery of DC filaments on DH cathodes. Are the thoriated tungsten filaments more or less susceptible to cathode stripping than "regular" ones? Hmm... wonder if anyone's ever regenerated HF AC (pulsed DC) to heat filaments instead of DC? Maybe that's completely nuts, but at least I am thinking (even on my day off)...

 

[Mike Stehr]

"Is that true? I am always a little leery of DC filaments on DH cathodes. Are the thoriated tungsten filaments more or less susceptible to cathode stripping than "regular" ones? Hmm... wonder if anyone's ever regenerated HF AC (pulsed DC) to heat filaments instead of DC? Maybe that's completely nuts, but at least I am thinking (even on my day off)..."

Good point Mark.
I've never really pondered the fact of inrush current with a SS DC bridge for a filament circuit.
All the tube amps I have are AC heating, I have no experience with building a DC PS for a filament/cathode, I think I better start.....
I do know that a straight 7.5 volts AC is gonna be rather loud with respect to line noise. I've heard a 300B amp with 5 volts AC, it's like a having a interconnect unhooked flapping in the breeze.

From what I understand, thoriated tungsten filaments can deal with cathode stripping with a little less chance for failure. But this is with regards to stripping a cathode to get more emission out of a already wanked tube.

But no, I don't wanna do that every time I power up a linestage.....

As far as high frequency (pulsed DC heating) is concerned, I've read posts on Joenet on the issue, and I'm sure there is comments on AA over the subject.
But I haven't seen any meat and potatoes way of implementing this pulsed DC heating so the average gomer can try it. Maybe I haven't looked hard enough....

I dunno....a guy doesn't want to blast 7.5 volts of DC to the 801's right off on turn-on, I know that. I'm not sure if the 801A was designated for DC heating in the data like the lower voltage tubes such as 45, 2A3, etc....
I was thinking more towards noise than inrush current. Maybe some sort of timed circuit that gradually ramps up to 7.5 volts over the course of 20 seconds?

 

[tubino]

I dunno....a guy doesn't want to blast 7.5 volts of DC to the 801's right off on turn-on, I know that. I'm not sure if the 801A was designated for DC heating in the data like the lower voltage tubes such as 45, 2A3, etc....
I was thinking more towards noise than inrush current. Maybe some sort of timed circuit that gradually ramps up to 7.5 volts over the course of 20 seconds?

Mike, IMO that's overkill. If your DC supply is adequate in voltage (or a bit over), a simple resistor after the caps will do quite a lot to prevent a problem with inrush current. I built an overkill 845 fil supply that way, and see no problem on startup.

A step up from that, but still simple, is the kind of resistor specifically designed to get shorted out of the circuit on warm-up, by using a bimetal coil to make contact.

I would suggest putting the ramp-up (or delay) on the B+ supply instead.

 

[Jack G]

I dunno....a guy doesn't want to blast 7.5 volts of DC to the 801's right off on turn-on, I know that. I'm not sure if the 801A was designated for DC heating in the data like the lower voltage tubes such as 45, 2A3, etc....
I was thinking more towards noise than inrush current. Maybe some sort of timed circuit that gradually ramps up to 7.5 volts over the course of 20 seconds?

I have some SETs that will take 10/10Y/801As, and have to use DC(AC was too noisey). Its not a problem.
Jack

 

[mhardy6647]

I built an overkill 845 fil supply that way, and see no problem on startup

Gary Kaufman likes to build overkill P/S's The one from his 211 amp will give you a tan if you stand too close!

http://www.the-planet.org/nnetg.html

 

[Mike Stehr]

Mike, IMO that's overkill. If your DC supply is adequate in voltage (or a bit over), a simple resistor after the caps will do quite a lot to prevent a problem with inrush current. I built an overkill 845 fil supply that way, and see no problem on startup.

A step up from that, but still simple, is the kind of resistor specifically designed to get shorted out of the circuit on warm-up, by using a bimetal coil to make contact.

I would suggest putting the ramp-up (or delay) on the B+ supply instead.

I forgot that Dan Marshall had given me a pair of these little DC regulated supplies.

Xentc Inc. Model XE15-3

They put out like 3.5 volts DC at 3 amps, beefy little things, adjustable overvoltage protection, and voltage adjustment.
They say 4 volts, but I think they could only eek out 3.5 volts.

He even showed me how to strap the two together to come up with 7 volts, but it was so long ago I can't remember how!
I know that isn't quite 7.5 volts, but the current should be plenty enough.
I guess I can sit here and ponder about it, or hook the things up and measure the voltage and figure up the current.
Maybe figure out how to strap them together without frying them.....Dan does have a whole box of them....heh

RCA 801A data says AC or DC. AC must be with respect towards telephony or class B circuits. I dunno, I can't imagine 7.5 volts AC heating in a radio output circuit, even in 1934. I doubt PP would help either.
Who knows...

 

[tubino]

He even showed me how to strap the two together to come up with 7 volts, but it was so long ago I can't remember how! [snip] RCA 801A data says AC or DC. AC must be with respect towards telephony or class B circuits. I dunno, I can't imagine 7.5 volts AC heating in a radio output circuit, even in 1934. I doubt PP would help either.

PP would help a lot, as long as the phase for the filaments were properly wired. It hurts to figure it out, so I'd probably solder one, clip lead the other till I knew I had it right, then solder. I heard a PP 6A3 (or maybe it was 6B4G) that worked well on AC.

On the reg supplies, for 7v wouldn't you just stack them, using the pos from A to the neg for B, and then your 7vdc are the two remaining leads? And you have a "center tap" to balance the two supplies...

 

[Mike Stehr]

On the reg supplies, for 7v wouldn't you just stack them, using the pos from A to the neg for B, and then your 7vdc are the two remaining leads? And you have a "center tap" to balance the two supplies...

Ok, I've been messing with these little supplies with a Variac and DMM.
They can do 3.75 volts no problem each, they can do up to 4 volts at 3 amps.
One unit needed the overvoltage protection adjusted. You would get up to three volts and the overvoltage would drop it back down to a volt.
A little noodling cured that. Monitoring the DMM for voltage drift, one just sits there like it should.
The other has a ever so slight drift towards positive.
That might not be a issue, I'll have to watch it over a longer period to really see how far it will drift.

These supplies only have a positive and negative for output taps, no center taps.
I jumped the positive of one supply to the negative of the other, then tapped the DMM on the other taps. (I think that's what you had suggested.)
All I get voltage-wise is 3.75 volts.
I coulda swore I had them putting out a good 7 to 8 volts together the first time I measured, and then the overvoltage kicked-in on one unit and it dropped.
Maybe I had it positive to positive......

 

[tubino]

These supplies only have a positive and negative for output taps, no center taps.

I just meant you'd have a pseudo-centertap at the stacking point, where pos of one meets neg of the next... that would enable you to set the PS outputs so that they would balance... and also give you an access point for fixed bias, return to ground, etc. for the cathode.

I don't understand why you don't get 7v though when stacked...

 

[Mike Stehr]

Ok, I've been messing with these supplies, and did get them to double wired in series. The Variac had a bad connection at the recepticle.

I was heating two 6.3-volt octals the other night, they do work but seem to get a little warm over a course of 15 minutes.

I just tried one 801A, and the two supplies in series seemed to pull the filament up nice and easy.
If I could find another junk 4 pin socket, I'll try the other.

Sorry about the pic in the dark, that sucks!

Mike.

 

[Mike Stehr]

Here. More blurry photos....ugh.
Anyways, this the way a 801A is supposed to glow.

The little DC supplies wired in series don't seem to wanna heat both however....bummer.....