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Old 5th January 2013, 04:01 AM   #11
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Quote:
Originally Posted by Eli Duttman View Post
An obvious improvement to pentode mode is employing regulated g2 B+. It looks like there is plenty of room for a Maida regulator.

If you are going to use the 6П15П in triode mode, follow the DECWARE SE84 example and tie g2 to the plate via a 1KOhm resistor. Protect that fragile screen against excessive dissipation.
6p15p may require changing operating points that are suitable for this tube only, which is not what I'm after. It is an interesting suggestion, however.
I am a big fan of regulation, - B+, heaters, bias, CCS, etc. Not on this amp...

I bypassed B1 and B3 electrolytics with 0.47uf auricaps i had in my bin - audible improvement.
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Old 5th January 2013, 04:13 AM   #12
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You have heater current to spare. If you want the power trafo to run cooler, use a 47 μF. 'lytic in the 1st filter position and the 470 μF. 'lytic in the reservoir (2nd) position. B+ rise will slow down too.

What is the # of the OEM SS diodes? UF4007s cost approx. 15 cents each and produce much less switching noise than other PN junction diodes do.
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Old 5th January 2013, 04:25 AM   #13
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Originally Posted by Eli Duttman View Post
You have heater current to spare. If you want the power trafo to run cooler, use a 47 μF. 'lytic in the 1st filter position and the 470 μF. 'lytic in the reservoir (2nd) position. B+ rise will slow down too.

What is the # of the OEM SS diodes? UF4007s cost approx. 15 cents each and produce much less switching noise than other PN junction diodes do.
I couldn't see the numbers on diodes - too small and soldered blank side up. Could be of MUR series...

Attaching schematics with component values.
I suppose there are variations in power supply modules (at least from the pictures that I have seen), also the schematics are inaccurate in terms of NFB wiring and headphone jack connection. The signal ground to chassis is connected at 12AX7 socket via 5 ohm 5W resistor. There is also a thermistor near the power switch (at least it looks like one, black disc of appropriate size), probably to save fuses from blowing on the initial monster 470uf cap charge.
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File Type: gif Almarro A205A schematics.gif (37.6 KB, 181 views)

Last edited by ClefChef; 5th January 2013 at 04:47 AM.
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Old 3rd March 2013, 03:27 AM   #14
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Just to follow up...I finally got bored (annoyed) with Almarro and "swapped" it for a pair of Wright WPA 3.5 monos - sonic bliss.

I acquired Almarro A205a following reviews, some of them high on superlatives, 6moons, stereophile, etc. Graphomaniac poetry aside, Almarro is quite mediocre in terms of SQ, especially in pentode mode. Nothing special in circuitry, power supply or parts quality - everything is plain vanilla, - silicon diode rectification, AC filaments, CLCRC B+, usual cathode bias, cheap electrolytics everywhere...Meh.
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Old 3rd March 2013, 03:43 AM   #15
Cassiel is offline Cassiel  Libya
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SEP with global feedback: meh.
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Old 3rd March 2013, 04:38 AM   #16
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SEP with global feedback: meh.
GNF is to be avoided.
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Old 28th September 2013, 11:30 PM   #17
maryc is offline maryc  United States
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Hi folks, it looks like this thread is fairly old, but I thought I'd post to see if anyone could help me. I picked up a used Almarro a205a a few weeks ago. Today, when I was changing capacitors, I noticed that one of the resistors had been snipped out of the circuit. I'm not an electrical engineer, and although I've been reading Morgan Jones' "Valve Amplifiers" book, I'm only on page 79. I'm assuming that this modification broke the negative feedback loop? (The amp plays.) But I don't know for sure.

I've already closed up the amp (plus I don't have a decent camera at home today--left it at work), but if you'd allow me to attach a very-bad visual aid, I've drawn a red line pointing towards the resistor that has been removed (it's on the right side of the photo, underneath the lead from the V-Cap.) If you are familiar w/ the amp, it's one of the two resistors that has had the leads bent through a couple of 90-degree elbows.)

1.) Should I try to get my hands on a circuit diagram and solder this resistor back in?
2.) If so, can anyone give me an idea which resistor this is, or what I should look for on the circuit diagram in order to find out the value? From looking at fuzzy images online, it doesn't seem to have the same value as the symmetrically-placed resistor on the other side of the socket.

Thanks, folks. --Mary

Click the image to open in full size.
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Old 29th September 2013, 04:56 AM   #18
maryc is offline maryc  United States
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Nevermind, I figured it out. It looks like a 15 kOhm and I found some diagrams that seem to indicate that's correct. Since I'm playing around, I'll probably solder one back in just to see what it sounds like w/ the negative feedback.
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