Preamp

lpd · 31st December 2004, 06:09 AM

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).

audiousername · 31st December 2004, 11:02 AM

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.

anatech · 2nd January 2005, 11:09 PM

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

lpd · 3rd January 2005, 12:22 AM

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.

anatech · 3rd January 2005, 01:46 AM

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

lpd · 3rd January 2005, 02:11 AM

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.

anatech · 3rd January 2005, 03:07 AM

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

lpd · 3rd January 2005, 04:37 AM

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.

Original Burnedfingers · 3rd January 2005, 12:05 PM

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.

audiousername · 5th January 2005, 10:01 AM

Quote:

Originally posted by burnedfingers
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.

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.

31st December 2004, 06:09 AM	#1
lpd diyAudio Member Join Date: Nov 2003 Location: BC	Preamp 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).

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Tube Preamp Popping on Source Input Change (?) / 6DJ8 Preamp On Chip Amps	MartyM	Tubes / Valves	3	16th October 2008 05:52 AM
Guitar Preamp = MIC Preamp = other preamp ,,are they all same?	Paswa	Solid State	1	12th March 2008 06:16 PM
twisted pear Preamp/other suggestions for Preamp kit	LukeDuke	Chip Amps	1	1st February 2008 10:57 PM
Electret mic preamp - will phono preamp work?	darthmullet	Analogue Source	6	12th October 2007 08:32 AM
wiring preamp outs on a preamp	bonsai171	Solid State	4	22nd March 2004 11:13 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

31st December 2004, 11:02 AM	#2
audiousername diyAudio Member Join Date: Jul 2004 Location: Melbourne, Victoria	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.

2nd January 2005, 11:09 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

3rd January 2005, 12:22 AM	#4
lpd diyAudio Member Join Date: Nov 2003 Location: BC	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.

3rd January 2005, 01:46 AM	#5
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

3rd January 2005, 02:11 AM	#6
lpd diyAudio Member Join Date: Nov 2003 Location: BC	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.

3rd January 2005, 03:07 AM	#7
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

3rd January 2005, 04:37 AM	#8
lpd diyAudio Member Join Date: Nov 2003 Location: BC	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.

3rd January 2005, 12:05 PM	#9
Original Burnedfingers diyAudio Member Join Date: Nov 2002	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.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)