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Old 26th December 2007, 11:23 PM   #1
pchw is offline pchw  United States
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Default Need help to check out this preamp schematic.

My friend struggled to get a pair of tube preamp and tube amp to work for a year. After one year of shipping to Canada to get it fixed, he completely lose his enthusiasm and decided to write it off. So, I purchased both gears from him and will attempt to bring them alive. Since the amp has some well known problem, I will tackle it later.

If I read it right, this preamp uses a 12 pin compactron (the number is almost all gone, I could tell what that is, it starts with a '6" followed by something looks like "0" or "C", after that everything is wiped clean) and the pentode section of the 6BM8 for the voltage regulation and constant source. Each channel uses 2 x 6SN7 with each tube paralleling 2 sections. The power supply wiring is quite a mess. So, I only traced the preamp's amplification sections and draw the schematic as in the attachment.
I have a few questions:
1. Is this SRPP? Not sure why it was quote as special mu-follower.
2. The cathode resistors of the 2 lower sections are 100R and 180R!!! One of them is obviously wrong. Does 100R look like the correct one?
3. Every one of the RCA jacks is grounded at the chassis? So, if I run into ground loop problem and want to isolate the safety ground, is it OK to use thermistor such as CL60 connect the safety ground to the chassis?
4. Since the compactron is totally unknown, the preamp will be useless if it goes south. I want to prepare to replace the whole regulated PS. Once the preamp is up and running, I will measure the B+ and loaded current. Is there any more that I need to collect?
5. The amp uses 4 6LB6 output tubes. I never worked with these RF tubes. Is there anything I need to pay more attention?

That's it for now, more to come, including photos if anyone is interested in :-)

Edit: corrected a mistake in the schematic.

TIA,
Attached Images
File Type: gif ma-preamp.gif (32.1 KB, 402 views)
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Old 27th December 2007, 12:05 AM   #2
Gordy is offline Gordy  United Kingdom
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Have a read of some of the Tubecad.com pages...

www.tubecad.com

http://www.tubecad.com/articles_2002...ted/index.html (notice the 'NEXT' link at the bottom of the page)

With the 6SN7 you could rewire it to be the first stage of a tubecad 'Aikido' type preamp...

http://www.tubecad.com/2004/blog0011.htm


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Old 27th December 2007, 01:35 AM   #3
pchw is offline pchw  United States
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Yeah, I looked at JB's SRPP articles. The preamp design seems to be some tweaked form of SRPP. Regarding to the Aikido, that will be another project. I have JB's PCB's around for quite some time, just need to finish up projects that have been started before firing up another :-)
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Old 27th December 2007, 04:44 PM   #4
kevinkr is offline kevinkr  United States
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It's just a mu follower, not an SRPP at all.

The 2uF shown doesn't need to be bigger than about a tenth of that value and could be replaced with a suitable smaller cap and reused as the output coupling cap instead. (Check the voltage rating)

The cathode resistors should be something like 820 - 1K..

The 6SN7 performs best with plate currents of 8mA and above, assuming that you will need about a 400V supply to account for the tube drops and the 80V or so lost across that 10K which incidentally can be reduced to as low as 3K or so with some small loss in gain if you need more headroom due to a lower supply voltage.

The grid of the lower triode needs a dc path for grid current so remove that 1.5uF cap at the input - add a 1M resistor at the volume pot wiper to prevent popping should it become intermittent.

Make sure that you bias up the filament supply such that the cathode to filament insulation voltage ratings aren't significantly exceeded. (I usually bias at ~100V in similar circuits.)
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Old 27th December 2007, 04:55 PM   #5
Jeb-D. is offline Jeb-D.  United States
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From a component count stand point you can identify that it's a mu-follower because of that 10K resistor and that 2uF cap. An SRPP won't have either of those. Most SRPP's won't have the grid pull-down resistor on the top tube either, the grid of the top tube will be directly tied to the plate of the bottom.

Kevin is Right that 2uF cap is overkill. For a 1M grid resistor a .1uF poly cap should do the job and sound better.
The rest of his advice is right on, as well.

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Old 27th December 2007, 06:40 PM   #6
pchw is offline pchw  United States
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I thought it was a SRPP because John Broskie's Tube CAD circuit guide had this circuit in the SRPP/Totem Pole amplifier section and called it Self Biased Totem Pole. I read MJ's book on the mu-follower again last night. These 2 circuits look so much alike, the nob was confused I think mu-follower is the right name :-)

There are separate heater DC supplies, one for the lower tiodes and one for the upper. The one for the upper is actually floated, i.e not connected to ground or a divider off the B+!! I will do the recommended enhancements. In this stock form, it is very good sounding, very natural, and very quiet. Kind of yet another stupid question, what is the benefit to parallel the 2 sections vs 1 section of the 6SN7?

Thanks!!
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Old 27th December 2007, 07:36 PM   #7
gianis is offline gianis  Greece
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A mu follower topology..
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Old 27th December 2007, 07:47 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by pchw
I thought it was a SRPP because John Broskie's Tube CAD circuit guide had this circuit in the SRPP/Totem Pole amplifier section and called it Self Biased Totem Pole. I read MJ's book on the mu-follower again last night. These 2 circuits look so much alike, the nob was confused I think mu-follower is the right name :-)

There are separate heater DC supplies, one for the lower tiodes and one for the upper. The one for the upper is actually floated, i.e not connected to ground or a divider off the B+!! I will do the recommended enhancements. In this stock form, it is very good sounding, very natural, and very quiet. Kind of yet another stupid question, what is the benefit to parallel the 2 sections vs 1 section of the 6SN7?

Thanks!!
Paralleling sections doubles transconductance reducing noise (lower internal impedance results in less johnson noise from the tube's equivalent internal resistance), lowers output impedance and increases available output current provided each section is operated at the same current as an individual section would be. Note that for two sections in parallel the ideal cathode resistances would be about half the values I stated in my previous post. (I actually prefer individual cathode resistors for each section which would be the values I mentioned previously - this allows each section to bias individually and obviates the need for close section to section matching for low distortion.)

The upper tube filament supply should probably not be left floating, but should be biased to a voltage somewhat higher than is present on the cathode when operating. (Lowest noise)

The lower tube filament supply should be biased about 20V above the lower tube's cathode voltage again to avoid odd little noises...
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