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Old 29th July 2011, 02:34 AM   #1
rhone is offline rhone  Australia
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Default Vacuum State FVP5A voltages

I have just finished a build of the line stage part of Allen Wright's FVP5A circuit and would appreciate some opinion as to the voltages I have measured.
Schematic is below. My PS is CLCRCRC with tweaked resistor values to get the desired 250V B+.
I am getting 160-170V on the anodes of the lower tube. Now this may be OK but my feeling is it's a bit high. What do you reckon?
Measured currents are about 3mA through the gain tube and 10 mA down the cathode follower bit.

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Old 29th July 2011, 02:41 AM   #2
SY is offline SY  United States
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Yes, too high. It should be about 102-105V. Disconnect the CF and see what plate voltage you get on the voltage amp. Adjust the cathode resistor to get 100V on the plate (like it says on the schematic), then hook the CF back up.

Alternatively, if you have a pile of ECC88-oids around, you can change the input tube until you find one that gives you the right plate voltage for that design.
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Old 29th July 2011, 04:07 AM   #3
rhone is offline rhone  Australia
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Thanks for that SY.
I have been using my "least desirable" 6922s to test the circuit. I'll put in the good ones and then adjust that resistor as necessary.
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Old 1st September 2012, 03:43 AM   #4
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Default My creation of FVP5A

First off, the passing of Mr. Wright is sadly missed by everyone. I came across the discussions regarding the DIY of this excellent pre-amp. I like to share some pics of my humble creation. However, I have experienced some problem and will start trouble shooting tomorrow. I used it to drive my 2A3 SE power amp which has has a EM84 electronic eye as VU meter. Even if there is no signal to the phono, once in a while I find there is a sudden quick surge of signal and can be seen from the EM84 beam closing and at the same time I can see the cone of my 96db Fostex 206e moves. If LP is playing, it can be heard as a sudden "hiccup". It appears that the circuitry is not very stable and causes some sporadic oscillation. After reading the discussions, I noticed that the plate of the lower tube of the one I built also has about 170-180 v. if the B+ is 250v. Tomorrow I will attempt to adjust the 2K2 to get about 100-105v and see if this is the cause and report back.
Also I wonder If R1 should be 47K for mm cartridge instead of 100R as indicated in the schematic ? could it be a mis-print ?
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Old 11th September 2012, 09:32 PM   #5
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I don't understand SY's "disconnect the CF..." However, I have to decrease the 2K2 resistor to about 500-500R to get around 105-108V on the anode of the lower 6922. Because of this the B+ has dropped from 250V to only 215V. I hope it is still OK. For rectification I am using an isolated 110V/220V transformer - 4x FR205 diodes in bridge -> 40uf motor run cap-> 15H 70mA choke->100uf Mundorf electrolytic->100R 10W->100uf Mundorf. The sound is very good and the random "oscillation" or "signal surge" has disappeared. Any comments will be appreciated.
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Old 11th September 2012, 11:55 PM   #6
rhone is offline rhone  Australia
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Disconnect the CF means disconnect the 100R resistor from the gain stage anode. This allows the gain stage to temporarily operate independently of the final "totem-pole" stage for adjustment that cathode resistor. I ended up with 430R, so we're in the same ballpark. Whether your B+ is now too low, I don't know. 10% either way is usually fine but you are outside that range.
There is a typo in the text below the schematic. It should be R2, on the FET source pin, that is 100R for MM. R1 is 47k for MM.
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Old 12th September 2012, 03:08 PM   #7
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By decreasing the 2K2 cathode resistor of the first stage you have increased the current draw considerably. This is not desirable and accounts for the sagging of your +B. You need to increase the load resistor to about 33K and see how your voltages settle down.
Surprisingly the amp will settle down to operate nicely whatever voltage is on the plate of the first stage - but the tone of the amp will vary quite substantially over a range of voltages. It maybe an idea to temporarily use a pot to set the voltage to whatever sounds best to your ears.

Shoog
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Old 12th September 2012, 10:32 PM   #8
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Thank you for your input rhone and Shoog.
I am getting 221v. after the 15H choke and 215v after the 100R cap. I will try to eliminate the 100R and the second 100uf Mundorf electrolytic and get the 221v right after the choke and see what happens. I know by eliminating the 100R and the 100 uf cap. I will compromise a bit on the DC smoothing, but it worth the try in order the get closer to the desired 250v B+ (within 10% as advised by rhone)
Shoog: Do you mean to increase the plate resistor of 25K to 33K ?
I have to do some traveling for a couple of weeks and I will report back after trying different tweaks after my return.
Thank you both again.
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Old 12th September 2012, 11:07 PM   #9
rhone is offline rhone  Australia
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Quote:
Originally Posted by Shoog View Post
By decreasing the 2K2 cathode resistor of the first stage you have increased the current draw considerably. This is not desirable and accounts for the sagging of your +B. You need to increase the load resistor to about 33K and see how your voltages settle down.
Surprisingly the amp will settle down to operate nicely whatever voltage is on the plate of the first stage - but the tone of the amp will vary quite substantially over a range of voltages. It maybe an idea to temporarily use a pot to set the voltage to whatever sounds best to your ears.

Shoog
Thanks for your input, Shoog. I have recently been going back thru' the archives and enjoyed reading about your build of this excellent pre-amp. The way Allen has drawn the circuit we get 6 mA through that gain stage, do you think this is optimum?
I know you experimented with various alternatives to the "plain vanilla" cathode resistor on that stage. What do you think worked out best?
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Old 13th October 2012, 12:03 AM   #10
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Hello Shoog and Rhone,

As suggested, I have replaced the 25K plate resistor with 33K. The plate voltage (1st half of line stage 6922) has now dropped from 107Vdc to 90Vdc. I replaced the 2K2 with 750R (instead of 500R before) and getting 114-118Vdc. Obviously this has helped reducing the current draw from the B+. I am able to maintain the B+ @ 228Vdc (better than the 213Vdc before). The line stage seems more stable and no more sign of oscillation is observed. Any other suggestions would be appreciated.

Regards,
Ken
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