DHT circuit development for noob:)

[mltube]

I am a 4yr noob, with tubes and ideas but with indecision.
I have been reading posts around DHT designs but as a noob have not ventured yet. I have the following tubes in multiples...Triodes 30, 31, 1b5/25s, 1h4g, 1h5g, 1g6gt, And pentode varieties 6A4/LA, 1a5gt, 1c5gt, 1f5g, 1g5g, 1q5gt, 1t5gt, and all of the seven pin varients.
The list could go on forever due to many tube caddies full...
I have collected so many so that I could design whatever I could dream of.
I built from scratch many varieties of se and pp of most common output tubes, el84, 6v6, 6l6, el34, kt88, and many horiz output tubes using proven schematics. (all for personal use and to give to other family members)

The design I would like to begin with involves the 6A4/LA for output SE 8Kopt. B+ @170 just under max rating with PO @1.25w.

With so many possible tubes to use could someone suggest a good circuit to begin with? I will post Pic's of the progress as time goes by.

I have unfortunately been diagnosed with PAP terminal lung disease, stuck at home now on O2 , but spirits are high and I never stop working on amp ideas. Just hoping for a little assist with possible design.

Making amps for my kids now, so they have stuff to keep as a memory of what I do and have done. I have kept detailed shop notes and drawings to pass down for my son, he is so interested in the hobby.

The only test equip that I have at the moment is a simpson tube tester and ss multi-meters but all is well with judgement and common sense.
Thank you in advance for Idea help.
Michael (mltube)

[Richard Ellis]

Mike:
Sorry to hear about your condition..........Hey, no one said this ride was going to go on forever.


What is that 6A4/LA anyway? A directly heated Power Pentode with the suppressor grid tied to the center of the DH Cathode?, wow what a weird one.
I would look up why this little job is internally wired as it is. Sounds like one of those 'so-called' specialty tubes.....built for a very specific purpose.
I would go parallel on the output, SE of course, ultra-linear taps......But this wierd internals might throw you a curve.......maybe for the worse, or you might come up with a novel circuit because of it. You can't quite get 170 VDC from an isolation TX, so a slightly less than 220 secondary, say a 200 Vac secondary will get you 170. Do you have PSUD II? Research the tube to learn more.
Does anyone here know of this tube???The 6A4/LA........& what it's for??

__________________________________________________ ____Rick......

[Arnulf]

Originally Posted by Richard Ellis

What is that 6A4/LA anyway? A directly heated Power Pentode with the suppressor grid tied to the center of the DH Cathode?, wow what a weird one.

The filament is center-tapped because (unlike in indirectly heated tubes) cathode will always be at different potential at each end (regardless of whether the filament is AC or DC powered). This ensures potential difference between cathode and supressor is minimal and as uniform as possible.

@mltube: 1.5W maximum dissipation means less than half of that as useful output power. With B+ of 170V, the output will swing above 180V (which appears to be the maximum allowed anode voltage) on almost every cycle as transformer primary acts just like a choke load. Transconductance of 0.0025 means it'll take a serious voltage swing at grid to get ~20 mA peak swing at the output (to get you close to the 0.75 Po estimate) - an unrealistic prospect for line level input signal.

Can you describe your goals for this amplifier so we can better understand what you are after and help you accordingly ? What kind of load do you intend to drive (headphones ?), what is your input source (CD player, iPod, computer, turntable ...) ?

[andyjevans]

Hello!

I'll leave the output to others, but say a word about the input. The 30 and 31 are good tubes, and that would make a good start. Also they are UX4 sockets and that's also good, since you could upgrade to 01A into 71A if you wanted, or even 26 into 10Y with bigger filament requirements. But let's start with 30 into 31. Max plate voltage on both is 180v, so that's pointing in the direction of direct coupling the first two stages and interstage coupling to the outputs - no coupling caps. I'm just working on a PP amp right now doing exactly this - 01A direct coupled into 71A, interstage coupled to 300b. I haven't finalised the circuit yet, but looks like I'll have about 360v on the drivers, which means 180v on the cathode. With the 71A it'll be about 45v bias, but with the 31 it would be 30v bias for 12mA. So that would put the grid at 150v. So a cathode resistor of 15k, which should be at least 7W or better 12W to keep it cool - the dark green wirewounds are nice - Vishay do a 15k at 11W.

So take 150v back onto the plate of the 30, and it looks like 2.8mA and 10.5v bias. So say 340v HT here and 68k plate resistor, and cathode resistor of 3.8K. I hope these figures are correct! They are just a quick calculation. I have a SE 300b amp which is 30sp (VT-67) into 31 and it's cap coupled right now, so I could convert that and try it! Interstage could be a Hammond series - the 126C should be good. I even have a pair of those to try myself. should be a really good sound.

My existing 300b SET uses 30 into 31 as stated - all cap coupled. I use two 150v octal glow tubes for the input section giving 300v on the 30 and 31. The 30 has 47k plate resistor, 150v on plate, 3.9k cathode resistor, 11.2v on cathode and 3mA. The 31 has 15k plate resistor, 178v on plate, 2.4k cathode resistor, 19.5v on cathode and 8.1mA. that should work for starters, since the amp works! Note I use a 30sp (VT-67) which sounds a bit better and is less microphonic. Has 120mA filament draw.

For the filaments, they are both 2v. The 30 is 60mA and the 31 is 130mA. Easiest DC filament supply for each tube is a LM1086 as current source. Adj to filament positive and resistor to set current between adj and output (pins 1 and 2) for 30 this is 21 ohms and for 31 this is 10 ohms. Use 2W resistors. Filament voltage should be just under 2v in this case, and as a general rule never over the stated figure in the data. Feed the LM1086 with a 6v secondary, Schottky diodes and at least 15,000uf cap - the negative of that cap goes to the negative of the filaments.

If you are interested in this kind of amp, by all means email me - I'm at home over Easter.

Andy

[andyjevans]

Forgot to mention layout. Your enemy here is hum, and putting all this in one chassis is going to make it big and heavy. So I'd consider one chassis for the signal and one for the power supply. Maybe you could keep the filament transformers in the signal chassis, and just use an outboard supply for the HT. I use a Speakon 4 pin connector for the HT, and for the filaments I use 4 pin XLR connectors for the inputs and 8 pin Speakons for the driver and outputs. If you have the filament transformers in the chassis you only need a connector for the HT, but keep the transformers away from the inputs and use a toroidal. Heatsinks on the LM1086 should be about an inch square. The main thing is to keep magnetic stuff like PSU chokes away from the signal chassis. Layout is a bit of a black art and one which I'm still learning, so I try and play safe.

Andy

[mltube]

[QUOTE=Arnulf;2138872]The filament is center-tapped because (unlike in indirectly heated tubes) cathode will always be at different potential at each end (regardless of whether the filament is AC or DC powered). This ensures potential difference between cathode and supressor is minimal and as uniform as possible.

@mltube: 1.5W maximum dissipation means less than half of that as useful output power. With B+ of 170V, the output will swing above 180V (which appears to be the maximum allowed anode voltage) on almost every cycle as transformer primary acts just like a choke load. Transconductance of 0.0025 means it'll take a serious voltage swing at grid to get ~20 mA peak swing at the output (to get you close to the 0.75 Po estimate) - an unrealistic prospect for line level input signal.

Can you describe your goals for this amplifier so we can better understand what you are after and help you accordingly ? What kind of load do you intend to drive (headphones ?), what is your input source (CD player, iPod, computer, turntable ...) ?[Used for sensitive 12" RCA vintage open baffle speakers with turntable]

[mltube]

The 6A4/LA is an old radio output pentode of DH design with PO of 1.4w @ 180vdc max. RL of 8k for se operation.
Michael(mltube)

[mltube]

Originally Posted by andyjevans

Hello!

I'll leave the output to others, but say a word about the input. The 30 and 31 are good tubes, and that would make a good start. Also they are UX4 sockets and that's also good, since you could upgrade to 01A into 71A if you wanted, or even 26 into 10Y with bigger filament requirements. But let's start with 30 into 31. Max plate voltage on both is 180v, so that's pointing in the direction of direct coupling the first two stages and interstage coupling to the outputs - no coupling caps. I'm just working on a PP amp right now doing exactly this - 01A direct coupled into 71A, interstage coupled to 300b. I haven't finalised the circuit yet, but looks like I'll have about 360v on the drivers, which means 180v on the cathode. With the 71A it'll be about 45v bias, but with the 31 it would be 30v bias for 12mA. So that would put the grid at 150v. So a cathode resistor of 15k, which should be at least 7W or better 12W to keep it cool - the dark green wirewounds are nice - Vishay do a 15k at 11W.

So take 150v back onto the plate of the 30, and it looks like 2.8mA and 10.5v bias. So say 340v HT here and 68k plate resistor, and cathode resistor of 3.8K. I hope these figures are correct! They are just a quick calculation. I have a SE 300b amp which is 30sp (VT-67) into 31 and it's cap coupled right now, so I could convert that and try it! Interstage could be a Hammond series - the 126C should be good. I even have a pair of those to try myself. should be a really good sound.

My existing 300b SET uses 30 into 31 as stated - all cap coupled. I use two 150v octal glow tubes for the input section giving 300v on the 30 and 31. The 30 has 47k plate resistor, 150v on plate, 3.9k cathode resistor, 11.2v on cathode and 3mA. The 31 has 15k plate resistor, 178v on plate, 2.4k cathode resistor, 19.5v on cathode and 8.1mA. that should work for starters, since the amp works! Note I use a 30sp (VT-67) which sounds a bit better and is less microphonic. Has 120mA filament draw.

For the filaments, they are both 2v. The 30 is 60mA and the 31 is 130mA. Easiest DC filament supply for each tube is a LM1086 as current source. Adj to filament positive and resistor to set current between adj and output (pins 1 and 2) for 30 this is 21 ohms and for 31 this is 10 ohms. Use 2W resistors. Filament voltage should be just under 2v in this case, and as a general rule never over the stated figure in the data. Feed the LM1086 with a 6v secondary, Schottky diodes and at least 15,000uf cap - the negative of that cap goes to the negative of the filaments.

If you are interested in this kind of amp, by all means email me - I'm at home over Easter.

Andy

Hi Andy,
Thanks for your design suggestion... I think i'll give it a go and work with your design. Thanks, I will send E-mail in a short while, picking up some audio gear that someone local wants to get rid of.
Cheers
Michael(mltube)