Slartibartfast wrote:
As for the batteries, they would be charged while the amp is powered up.You would also start with a fresh charged set of batteries.
Oh! I didn't know that it would charge while in circuit. So then the battery would never 'die' and need to be recharged. That was my main concern.
EWBrown says I could also use a LED too. In fact I think he responded to my last post...
EWBrown wrote:The LED connects between the cathode and ground, just like a resistor (or battery). The cathode goes to ground, the anode goes to the tube's cathode, and it does not need to be bypassed, as the LED has a low effective impedance.
So basically any pre-amp tube that has about 1k for a cathode resistor (with cap) could have a LED instead? Does the LED light? If so that would be cool because the LED could double as an indicator that the stage is working.
any common 3mm low brightness red LED will work just fine.
Both halves of the 6SN7 run around 3.5 mA, the plate of the VA connects directly to the CF's grid. THe signal is than taken off the CF cathode, and run through a DC blocking cap, with a 100K to 470K resistor across the output to bleed off any DC on the output cap.
So because of the low current running through the circuit, you don't need to use any kind of resistors in series with the LED?
This will work with any common medium-mu twin triode, like 6SN7, 12AU7, 5963, 6189, ECC82, etc.
Thus it will not work with a stage that uses a 12AX7?
The schematic is similar to this one on Gary Kaufman's site:
http://www.the-planet.org/linestage2.html
Oh yeah! I seen this schematic before. As I said I have a bunch of XSN7 and XSL7 (x=insert voltage type) tubes. Most are 6volts, but I have also the 8 volt and 12 volt variants. So I must have come across this around the time I built the Rosenblit SRPP pre-amp.
Just replace the 1K VA cathode resistor with a red LED, and use a 6SN7 or 12AU7 instead of the 407A. B+ 150 to 160VDC, @ about 7 mA per channel. This may seem like a low voltage (the plate voltage is about 75V) but it operates well and sounds very good. I used a smaller output coupling cap, 1.0 uF, 400V (one of the green russian PIO caps)
Yeah, that voltage IS low. You sure that would be enough to push an amp? I typically see voltages around 225 to 300 volts on pre-amp tubes.
I used oneof the 100K stereo ladder step attenuators from Hong Kong (on e-bay) and dispensed with the 0.47 uF input cap. THe step attenuator makes a perfectly good grid resistor, and doesn't "pop" when rotated, as it is a make-before-break switch.
I am certainly going to use a 40% loudness tap control because I like the way it worked on the Rosenblit amp. (on that amp it was my ONLY tone control.
DC filaments are nice, but not truly necessary, and the filaments should be "biased" at around 37-40V above ground (or 1/4 of the B+ voltage), so that the reference voltage sits midway between the two cathode's voltages of 1.6V and 75V.
That helps to reduce / eliminate any hum or instability problems.
(This applies to SRPP drivers, as well).
Yes, I remember I had to do something like this with the Rosenblit pre-amp. It was a recommendation. However, the preamp does also have DC filaments. So it is dead quiet.
But I did know that if I do go the SRPP route with the PAS 'rebuild', I for sure would need a different power supply. SRPP's are usually picky about the type/size of power supply.
The B+ can be series or shunt regulated, if desired, but that isn't really necessary. Just icing on the cake ... Yellow_Light_Colorz_PDT_03 Yellow_Light_Colorz_PDT_02 :o
I would go that route on a custom pre-amp where I would have more room. Besides I usually like to make VR tubes visible from the outside of the pre-amp. Love the glow on those things!
Thanx for the info.
Geo