• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

#26 pre amp

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#763
Hi Andy,

I will test them in my 801A linestage which is currently cathode bias, not filament bias 😱

In my 'commercial' designs I only use filament bias if the customer explicitly requests it, due to the excessive heat dissipation (with tubes like 801A).

I will consult with Rod after I tested them before I write anything. No matter what will be the outcome, keep in mind everybody is biased, including me, so I encourage people to try and listen for themselves. Also what is better in one set up might sound worse in another.

If someone wants to do the comparison himself, I can offer the filament chokes on a loan basis. So no risk, just shipping costs.

Since filament bias seems to gain popularity, I will write up an article on my blog about it sometime soon. Speaking of filament bias, have you also tried the next step beyond? Filament bias with 'DirectPath'? A gain stage based on UX201A absolutely without capacitor in the signal path.

Here ist the schematic:

ImageShack® - Online Photo and Video Hosting

The PSU:

ImageShack® - Online Photo and Video Hosting

A foto:

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PSU:

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Maybe it's time to rebuild this in a new chassis ...

Best regards

Thomas
 
#765
Hi Andy,

andyjevans said:
Thomas - did you play with Meccano as a child?
Click to expand...

Many people ask me that ... 😉 This is my old style to build things. I still like it for quick prototype builds.


This is how my stuff currently looks like (boring, I know)...

Linestage with 801A and LCR Phono with EC8020:
ImageShack® - Online Photo and Video Hosting

A pic of some amps demoed at ETF09:
ImageShack® - Online Photo and Video Hosting

Closeup of 801A power amp:
ImageShack® - Online Photo and Video Hosting

300B with switchable filaments AC/DC:
ImageShack® - Online Photo and Video Hosting

Best regards

Thomas
 
#767
Starting 26 preamp

Hi,

Received today the tubes, I will use Salas Shunt High Voltage regulator (can I share for the 2 tubes or I need one reg for each tube?) for B+ wich current I need to adjust? & Rod Coleman for heater filament (I suppose I need two, one for each tube?), could you remind me the last schematic (I have page 36 post 353), it's the last? I mind that with SSHV don't need plate choke right?

Attached a photo with the 3 tubes Cunningham 26, someone could do a look to see if all three are good?

Cheers

Felipe
 

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#768
Merlin,

you could use one or two SSHV, but you need either a plate choke or a plate resistor, I use two SSHV and plate chokes. Also one of Rod' s fillament per channel, with a separate transformer, around 30VA each.
Regarding tubes, you never know if you don' t try. I did' t have good luck with RCA, and then I had 4 Sylvanias, and after a while one of them had strange noise and I replaced it. But so far I like the sound of Sylvanias better.
You could start with cathode bias and around 150V B+. I use battery bias now.
 
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#769
vgeorge said:
Merlin,

you could use one or two SSHV, but you need either a plate choke or a plate resistor, I use two SSHV and plate chokes. Also one of Rod' s fillament per channel, with a separate transformer, around 30VA each.
Regarding tubes, you never know if you don' t try. I did' t have good luck with RCA, and then I had 4 Sylvanias, and after a while one of them had strange noise and I replaced it. But so far I like the sound of Sylvanias better.
You could start with cathode bias and around 150V B+. I use battery bias now.
Click to expand...

vgeorge,

thanks for support, I have a lot of questions without answer:

Tx for B+, value in volts & power needed to adjust 150V, wich current to use only one SSHV?
Tx for Rod Coleman heater filament, value in volts & power needed or better shoul I ask to Rod?
Sorry my ignorance, cathode bias is a resistor or a resistor bypassed with a cap, values?
It's absolutely mandatory the Tx in a separate box?

Cheers,
Felipe
 
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#770
As you need to burn around 25mA on the shunt plus 6 mA per channel, total 42 mA for one shunt minimum.
Rod will tell you everything about his regulators in a very detailed manner.
You could start with a 1,8kohm to 2.2kohm bypassed with 470uF on the cathode.
If you don' t use a seperate box for transformers be sure to decouple the tubes from the chassis.
For me there was not enough space to put all the caps, chokes and transformers in a single chassis anyway. I am going to transformer coupled soon.
 
#772
200 H at 5-10mA for plate choke should be fine. I am using LL1667 25mA now, but I will replace it with a LL1660 5mA soon.
Transformer, it's up to you to calculate, depending your plan.
I am using choke input, so with my 310-0-310 150VA I get 225V before the regulators. It also depends on the rectifier tube you are going to use. If you are going with cap input you will need a smaller trafo.
Try to use PSUD2. It will help to simulate the various senarios.
 
#774
merlin el mago said:
vgeorge,

thanks for support, I have a lot of questions without answer:

Tx for B+, value in volts & power needed to adjust 150V, wich current to use only one SSHV?
Tx for Rod Coleman heater filament, value in volts & power needed or better shoul I ask to Rod?
Sorry my ignorance, cathode bias is a resistor or a resistor bypassed with a cap, values?
It's absolutely mandatory the Tx in a separate box?

Cheers,
Felipe
Click to expand...

Hi Felipe, Click on my name to send some PM. Send me an email address, and I can send some PDF manuals. These have all the infos on how to choose transformer, heatsink, and other parts to make good-sounding filament heater, with no hummm!

For 26 DHT, choose 6.0V or 6.3V transformer, 25 .. 30VA
 
#776
merlin el mago said:
LL1660 is an interstage / line output tx not a choke?

About power/voltage needed for tx I will use this psu
Click to expand...

You can use an output transformer, provided it has enough inductance for the tube you will use. It will help me to reduce gain and output impedance also. You can find more info in this thread if you search.
If you use a ss bridge, you can get away with a 0-150V and 100VA transformer. I prefer tube rectification.
 
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