Hi Folks!
Can anyone help-out on how to grid bias a 801A preamp using battery?
I've great success with cathode and filament bias, but, curious to tryout how it'll sound like with battery bias on grid?
Currently, I've 380vdc coming from a MJ Statistical Regulator feeding the left/right LL1668/25mA plate choke.
Input is just a 100K pot and output is 1uf>TVC (Dave Slagle's).
So, what battery should I use and how do I determine the current passing thru?
Thanks A Lot!
Zekk
Can anyone help-out on how to grid bias a 801A preamp using battery?
I've great success with cathode and filament bias, but, curious to tryout how it'll sound like with battery bias on grid?
Currently, I've 380vdc coming from a MJ Statistical Regulator feeding the left/right LL1668/25mA plate choke.
Input is just a 100K pot and output is 1uf>TVC (Dave Slagle's).
So, what battery should I use and how do I determine the current passing thru?
Thanks A Lot!
Zekk
For battery grid-bias, you would connect the neg of battery to grid-leak resistor and pos of battery to chassis/earth.
To determine the voltage battery needed you would need to decide on operating-point of tube by consulting its data-sheet and drawing load-line for the load resistance the tube has in its plate circuit, determine the required current through tube at whatever plate voltage you intend to use.
BTW--Whats a 'Dave Slagle'...?
To determine the voltage battery needed you would need to decide on operating-point of tube by consulting its data-sheet and drawing load-line for the load resistance the tube has in its plate circuit, determine the required current through tube at whatever plate voltage you intend to use.
BTW--Whats a 'Dave Slagle'...?
I use battery grid bias in my 26 preamp. I love the sound. After trying a few different variations I found that applying the bias through the secondary of an input transformer sounded far superior to using a DC blocking cap. I use a Lundahl LL1676 wired for a 2:1 stepdown since the mu of the 26 (8) is a little on the high side for the Slagleformers I use on the outputs. The transformer gives a bonus benefit of allowing you to flip the polarity of the input signal to compensate for the polarity inversion of the gain stage, and also provides galvanic isolation to prevent ground loops. Finally, the transformer acts like a grid choke rather than a grid resistor, helping to keep the bias stable in the face of grid current.
You would simply wire the negative of the battery to the ground side of the input transformer secondary and the positive of the battery to ground.
You would simply wire the negative of the battery to the ground side of the input transformer secondary and the positive of the battery to ground.
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Dave Slagle: intact audio
He's a long time friend of mine and his inexpensive TVC is a great alternative to a passive line stage if just a little gain is needed..
Guys, thanks for responding!
My amp building is based on the generous contributions from the kind folks of this Forum. I strongly beleives I'll be safe by following the schematics and alternatives operating parameters projected.
Followed the circuit for #26 preamp with a 9v battery wired to grid, and a current meter wired in series to the B+ supply to LL1668.
Current shoots up to 50mA passing thru per choke and I immediately pulled the plug to prevent damaging the componants downstream.
So, you see, It's all trial and error but with lots of common sense.
Finally, based on the limitation of my plate choke of only 25mA, what would be the best compromise, and the type of battery I should use?
Best Regards!
Zekk
My amp building is based on the generous contributions from the kind folks of this Forum. I strongly beleives I'll be safe by following the schematics and alternatives operating parameters projected.
Followed the circuit for #26 preamp with a 9v battery wired to grid, and a current meter wired in series to the B+ supply to LL1668.
Current shoots up to 50mA passing thru per choke and I immediately pulled the plug to prevent damaging the componants downstream.
So, you see, It's all trial and error but with lots of common sense.
Finally, based on the limitation of my plate choke of only 25mA, what would be the best compromise, and the type of battery I should use?
Best Regards!
Zekk
As shown below.
The resistor and cap across the secondary are to eliminate (snub) transformer ringing on square waves and the cap across the battery is to lower its impedance on very high frequencies. Neither are required, although I prefer the sound with the RC network in place.
Attachments
Are you still using the 801a in that circuit?
If so , and if it's with the B+ you mentioned in your first post and the battery bias of only -9V , 50mA isn't surprising.
If you are using the 801A and you have more batteries then try putting two more in series with the first for a total of 3 batteries and bias voltage of -27VDC. That should put you in the ballpark, somewhere just below 25mA.
If so , and if it's with the B+ you mentioned in your first post and the battery bias of only -9V , 50mA isn't surprising.
If you are using the 801A and you have more batteries then try putting two more in series with the first for a total of 3 batteries and bias voltage of -27VDC. That should put you in the ballpark, somewhere just below 25mA.
Now I got it!
What'll be the type of battery suitable, taking into consideration the space constrain?
Thanks!
I don't know what your space constraints are.
You'll first have to confirm the correct bias voltage. If the earlier guesstimate of -27VDC is right (just for example!) then I might try a couple of A23(12V) and a 3v button. Or 3 standard 9V batteries. Other specialty sizes are around (like Exell) but whatever is easy is where I'd start.
I'm not a battery bias aficionado but I have battery powered thingies like everyone else and find that sometimes the connections get resistive and you have to open things up and spin the batteries to get a better connection. If counting on a series of different batteries to keep the tube from running away, I'd want to make sure the connections were good and stay that way.
You'll first have to confirm the correct bias voltage. If the earlier guesstimate of -27VDC is right (just for example!) then I might try a couple of A23(12V) and a 3v button. Or 3 standard 9V batteries. Other specialty sizes are around (like Exell) but whatever is easy is where I'd start.
I'm not a battery bias aficionado but I have battery powered thingies like everyone else and find that sometimes the connections get resistive and you have to open things up and spin the batteries to get a better connection. If counting on a series of different batteries to keep the tube from running away, I'd want to make sure the connections were good and stay that way.
As someone mention somewhere, it's all a compromise!
Same sonics as using 1K cathode resistor bypass with a 33uf tantalum cap.
Filament bias rank supreme but hate the heat generated.
It's the best sounding preamp I've ever own comparing to my past CJ Premeir 14, PAS 3A, HF-85 and Citation pre.
I've learned dearly that in hi-fi, don't compare, or, it'll be a never ending journey.
Enjoy your Hi-Fi!
You're not complaining are you ?
Interesting that you find the battery bias about the same as cathode bias. What will you do now?
Not at all! Love it and sound as good as my friend's expensive Shindo.
Now that I've a winner in hand, might have it re-work and dress-up. I'm using hook-up wires unwound from Telefunken power trans for it's pure copper contents. Is it better? Can't differentiate but feeling comfortable listening.
Good day!
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Sweet ! Glad it worked out.
I was interested in your 100K pot in and Slagleformer out. Which do you usually use for volume adjust?
A while back I was experimenting with a phono stage circuit that didn't like the variable impedance of the input TVC in the line-stage so I moved it to the output and replaced it with a 100K resistor. That phono stage circuit isn't there anymore but somehow the line stage hasn't been switched back. I guess it works
(I should mention though that my circuit is biased at 3mA and likes the autoformer on the output to give it some line driving help. It works pretty well at lower volume setting but from about 3/4 volume up it's not so good.)
I was interested in your 100K pot in and Slagleformer out. Which do you usually use for volume adjust?
A while back I was experimenting with a phono stage circuit that didn't like the variable impedance of the input TVC in the line-stage so I moved it to the output and replaced it with a 100K resistor. That phono stage circuit isn't there anymore but somehow the line stage hasn't been switched back. I guess it works
(I should mention though that my circuit is biased at 3mA and likes the autoformer on the output to give it some line driving help. It works pretty well at lower volume setting but from about 3/4 volume up it's not so good.)Sweet ! Glad it worked out.
I was interested in your 100K pot in and Slagleformer out. Which do you usually use for volume adjust?
A while back I was experimenting with a phono stage circuit that didn't like the variable impedance of the input TVC in the line-stage so I moved it to the output and replaced it with a 100K resistor. That phono stage circuit isn't there anymore but somehow the line stage hasn't been switched back. I guess it works
As mentioned earlier, my built is all trial and error. Initially, the TVC was on the input side but it don't works for me. Re-positioned to output and permanently setted to the 33dB terminal (3rd position), then, use the 100K pot to adjust volumn. By doing so, I removed the selector switch and reduce the number of bridges the signal had to cross.
Also removed my 21st century regulator and Rod Coleman's filament regulator. The only IC used was a SSHV supply to 2x OD3.
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