I sniped a custom 6SL7 preamp on E-Bay for a whopping $16.00. Uses a 5AR4 outboard power supply (choked b+), and I am wondering if my voltages check out ok. Heaters are 6.8 (which is a little high), plates are 225VDC and grid is -2V. Does this seem ok for a linestage? It uses two 6SL7's one for left channel and one for right. Input caps are Hovland .22@400VDC and outputs are Auricap .47@600VDC. There is a .047 cap strapped from the heater input to ground, as well. I'm fairly new to tubes, but am a fairly quick learner any help is appreciated. Just wondering if I should zap down the heaters, or just plug into a variac and ramp to 6.3?
I know the power supply is overkill but its what it came with and I'm not complaining. Tranny is a Hammond 278X (400/0/400/5/5/6.3).
I know the power supply is overkill but its what it came with and I'm not complaining. Tranny is a Hammond 278X (400/0/400/5/5/6.3).
Nice buy! The power transformer alone is around US$80 
You're supposed to keep the heater voltage to within 5% I think. In this case it's about 8% over. Heater power in this case is increased by about 17%.
It would be nice to reduce the voltage to the rating if possible - it may extend the life of your valves. It doesn't matter how it's done, really. Series resistors are wasteful of power, but are convenient, and anyway valves aren't terribly efficient in any case.
You're supposed to keep the heater voltage to within 5% I think. In this case it's about 8% over. Heater power in this case is increased by about 17%.
It would be nice to reduce the voltage to the rating if possible - it may extend the life of your valves. It doesn't matter how it's done, really. Series resistors are wasteful of power, but are convenient, and anyway valves aren't terribly efficient in any case.
Hi lpd,
What a great buy! Even if it has problems you are off to a great start. Heck, the shipping to Ontario is more than what you paid.
So, how does it sound?
-Chris
What a great buy! Even if it has problems you are off to a great start. Heck, the shipping to Ontario is more than what you paid.
So, how does it sound?
-Chris
This thing stomps all over my modded Pas. The Pas has a purist linestage with oil caps, Allen Bradley resistors and an outboard power supply using a seperate heater supply. The b+ is choked as is the filaments. The 6SL7 appears to have space between instruments (I've never really noticed this until now), and a great soundstage. The only identified problem is the heater voltage is somewhat high. It runs at 6.8 and I might tweak it down to save tube life. Plates are within 3volts of one another and operate at 225v and grid is -2 (I believe this is class a). Its a bit microphonic and the tubes tested weak, but for $16.00 I'm not complaining. I'm going to play with some tubes and see what I come up with.
For my first schematic project, I'm going to draw it out and then hopefully I'll post the schematic here so someone might be able to explain it to me.
For my first schematic project, I'm going to draw it out and then hopefully I'll post the schematic here so someone might be able to explain it to me.
Hi lpd,
A schematic would be interesting. Some of us might build this preamp. I would use a resistor to drop the filiment supply and be done with it. New tubes might really wake this unit up.
-Chris
A schematic would be interesting. Some of us might build this preamp. I would use a resistor to drop the filiment supply and be done with it. New tubes might really wake this unit up.
-Chris
Well not to stir up flames here, but right now its feeding into a Threshold S/150 amp, as my Bogen tube amp hasn't arrived yet. The Threshold is the closest I've come to tubes on a sand amp and is very enjoyable.
Hi lpd,
No flames. I have a Nakamichi TA-2 rcvr in the bedroom - stassis design. Sounds great. Very smooth unlike most SS. My living room has a Marantz 300DC driven by a Luxman C05. Even better. I bring out an Eico HF-87a at times. Really nice and a little different.
My point is that there are many great sand amps, as well as tube amps. I enjoy anything that sounds good. I like my fire suit.
-Chris
No flames. I have a Nakamichi TA-2 rcvr in the bedroom - stassis design. Sounds great. Very smooth unlike most SS. My living room has a Marantz 300DC driven by a Luxman C05. Even better. I bring out an Eico HF-87a at times. Really nice and a little different.
My point is that there are many great sand amps, as well as tube amps. I enjoy anything that sounds good. I like my fire suit.
-Chris
I'll draw up the schematic this week. The volume has a little too much gain for my tastes. At 7:00 its the highest I'd ever listen. Might have to modify a bit more. I like the volume to sit at 12:00 when at full tilt.
When using sand amps you might want to throw a couply of zenier diodes across the input of them just in case there is a malfunction. Its a lot easier than rebuilding them if they blow.
burnedfingers said:
You should be fairly safe against almost any fault involving DC on the outputs as long as the output coupling cap doesn't go leaky though.
Quote:
You should be fairly safe against almost any fault involving DC on the outputs as long as the output coupling cap doesn't go leaky though.
The key word here is fairly safe. Why chance being fairly safe? Add $1 worth of 3.3v zenier diodes and protect your investment.
Two zeniers on each channel will NOT change the sound and WILL
protect your sand amp.
Note***
The last line stage I built(1 month ago) had 600v $30 ea coupling caps but still managed to do $50 in parts(damage) to my SAE amp. I repaired the SAE and added the zeniers to the input jacks......no more worries.
You should be fairly safe against almost any fault involving DC on the outputs as long as the output coupling cap doesn't go leaky though.
The key word here is fairly safe. Why chance being fairly safe? Add $1 worth of 3.3v zenier diodes and protect your investment.
Two zeniers on each channel will NOT change the sound and WILL
protect your sand amp.
Note***
The last line stage I built(1 month ago) had 600v $30 ea coupling caps but still managed to do $50 in parts(damage) to my SAE amp. I repaired the SAE and added the zeniers to the input jacks......no more worries.
burnedfingers said:
Eeek
Did you ever figure out what caused the fault? The zeners will protect the input of the amp, but the preamp shouldn't be out to destroy the inputs anyway. Maybe a little bit of valve-sand rivalry 😀I probably wouldn't have posted that if I ever had an input stage destroyed.
Dont' forget to load the input with low value resistor to avoid parasitic feedback that can be created with the addition of the Zener Diodes on the input. Say 220 to 250 ohms.
Regards
Anthony
Regards
Anthony
When a tube preamp is turned on or off the output will swing through a large range. If the coupling cap is a high enough value, the charge / discharge curent can damage the input transistors. Really bad if the amplifier power is on.
Zeners may affect the sound and you have to suze them for peaks in the music. 1N4148 in series with the zeners may help in that respect. The best solution is a relay that shorts as soon as power is removed in the preamp and stays shorted for a while after power on. This will not affect the sound as much as diodes.
-Chris
Zeners may affect the sound and you have to suze them for peaks in the music. 1N4148 in series with the zeners may help in that respect. The best solution is a relay that shorts as soon as power is removed in the preamp and stays shorted for a while after power on. This will not affect the sound as much as diodes.
-Chris
How would the zener diodes be connected? Sounds interesting. I had a bad experience with a tube pre-amp connected to a SS amp and would like to avoid a potential problem in the future.
Nelson Pass used a Zener network in his SOBOZ design. If you go to passdiy.com you can download the article. He describes in some detail how the circuit works, just be sure to follow his advise about the 220R resistor on the input.
Regards
Anthony
Regards
Anthony
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