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Old 6th January 2011, 08:51 PM   #1
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Default Got unknown tube preamp board, need help!

Hi, long story short, I found a few empty boards in my stuff and wanted to make use of them. I was quite happy to discover what I believe to be tube preamp boards since I was planning on building one eventually. The trouble is, I have no clue how to populate the board. There is room for way too many components when I compare the board with popular tube preamp schematics.

I know these boards came from a company called caztech, and that the board was part of the spa-2 preamp, but little information is available on the web apart from that.

I could only guess some of the components, I am not an electronics expert but I know how to use a soldering iron. So, if anybody could help me find out what values I should use for the components , or for that matter which tubes fit that layout, it would prove to be very helpful to say the least.

By the way, hi, my name is John, I'm a math teacher who likes to modify and build audio equipment as a hobby.
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Old 6th January 2011, 09:18 PM   #2
kevinkr is offline kevinkr  United States
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This looks like a phono stage pcb to me, not a line stage from what I was able to trace out quickly.
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Old 6th January 2011, 10:19 PM   #3
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I draw a schematic by hand; it is definitely a phono stage.
R6, R7, R9, C6 and C7 are the components for the RIAA equalizing.
Two cascaded gain stages with a passive RIAA network in between.
There is a local feedback network around the input tube (C3 and R2).
V1 is the input tube for left and right; V2 the output tube for left and right.
Let me know if you need a well drawn schematic.
I will see what tubes might have been used (pin 4 is 6,3 volt DC I guess; pin 5 is ground).
Based on E88CC is should not be very difficult to calculate the values of the passive components.
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Old 6th January 2011, 10:43 PM   #4
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Caztech was a Canadian company that sold tube kits and finished products. They used to advertise in Glass Audio in the early '90's. Their stuff looked very nice for the price point.
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Old 7th January 2011, 12:19 AM   #5
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A phono stage! That would explain a lot.... I am quite pleased with that to be honest, but I have, once again, some questions.

Before building it I'd like to know if it would be a good idea to use it instead of the quad 33 from the turntable to a upgraded quad 405.

@ pieter: Yes! Please I would very much like it if you had any sort of schematic, even badly drawn on the corner of a napkin. It would save me quite some time searching around and I could go get the missing parts tommorow.

I'll search around for other boards to see if I can find the other parts of the preamp, but in the meantime, in case I don't have it, what power supply would best fit my needs for thr moment, assuming e88cc tubes?

Thanks again in advance.
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Old 7th January 2011, 01:15 PM   #6
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John,

Here is the schematic for one channel.
I would suggest a B+ of 200V; this must be a very good quality power supply.
When you take E88CC's, which are a good choice for a phono stage, good operating points are with 15k plate resistors (R5 and R11) and 390 ohm for cathode resistors (R2 and R8).
Maybe it's nice to invite some guys here to suggest the values for the other resistors and capacitors.
All coupling capacitors are parallelled pairs and I would suggest to use just one for each spot.

Pieter
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Old 7th January 2011, 01:17 PM   #7
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and (hopefully) the schematic....
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Old 7th January 2011, 02:02 PM   #8
SY is offline SY  United States
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I wonder why the input is cap coupled?
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Old 7th January 2011, 03:07 PM   #9
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Just for safety I guess; many designers seem to do this.
So far I see there is not that much gain in this preamp.
For grid stoppers I should not go over 100 ohms for noise reasons.
I would start omitting the local feedback around V1, and maybe bypassing the cathode resistors to "gain" some extra gain, but maybe there was a linestage further down.
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Old 7th January 2011, 03:22 PM   #10
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There was a linestage indeed. I will also build one later on, as well as a relay controlled input selector for the complete preamp. I still wonder if I'll find the linestage board or not... it should be in there somewhere...

I'll know more this afternoon anyway, I'll try to figure some of the other values as well.

Thanks, I'll keep posting my findings and build in the meantime.
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