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Old 4th January 2013, 04:12 PM   #1
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Default Almarro A205A as experimentation platform

I got my hands on cute little Almarro A205A MkII amp and the first thing I did when I received it was to unbolt bottom cover and inspect the circuitry. It is EL84 SEP with feedback. Its point to point construction and ample space will lend itself well to tweaks and modifications that would make this amplifier more flexible in terms of configuration.

I would like to make the amp Pentode - Triode mode switchable. Other things to try would be adjustable NFB. The EL84 socket pins 3 and 6 are not connected together, tying them together or installing "Hazen mod" cap would enable the use of SV83 pentodes.

Has anybody tried any modifications other than obvious coupling cap changes and what were the results?

Link on "Hazen mod": Anyone try the "Hazen mod" to G3 supressor grid

6 moons review: 6moons audio reviews: Almarro A205A MkII
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Old 4th January 2013, 05:14 PM   #2
JimW is offline JimW  United States
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To run sv83 you will need to reduce the value of the cathode resistor, and pay attention to the g2 ratings. The ruggedized version (6p15p-eb) will stand higher g2 voltage, but you are still limited to 1.4w g2 dissipation. I haven't tried the hazen mod, but I run them with g3 tied to the cathode. They are good sounding tubes, in triode mode, I think they may sound better than el84's, which is a very good sounding tube. I run sv83 in a rh-partial feedback style amp that has been set up for sv83 (lower value cathode r than el84, and regulated g2 voltage of 195).
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Old 4th January 2013, 05:47 PM   #3
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Hi Jim,
Thank your for your suggestions. I will measure the voltages on the amp and calculate the proper value of Rk for SV83. I think SV83 is close enough to EL84 to be run without modification of Rk values in Almarro. The idea is to make tubes interchangeable without having to swap internal components.
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Old 4th January 2013, 06:57 PM   #4
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In writing SV83 I really mean 6p15p-EV which is closer to EL84
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Old 4th January 2013, 06:58 PM   #5
Cassiel is offline Cassiel  Libya
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OK, you basically have three different choices for three very different kind of sounds: as it is (pentode with GNFB) , triode with NO negative feedback (just break the feedback loop somewhere) and what is called Schade feedback. It is a question of personal taste. I think you should try all of them. In my opinion the one you have now is the worst sounding of the three. For the 'Schade mod' you need to change the input tube and its values. Just a few resistors, nothing complicated. To find out the values for those resistor check the RH84 schematic out. Easy enough to find. That one uses a 12AT7, you can also use the ECC88 but it requires other values and the pinout of the tube is different. More work.

The "Hazen mod" is a very minor mod compared to the ones I just listed you.
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Old 4th January 2013, 07:20 PM   #6
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Thank you for suggestions. Again, the goal is to leave the amp basic structure intact, allowing swaps and changes with one or two switches - no major surgery.
I put a 0.1uf cap between G3 and k and plugged 6p15p-ev tubes just to try - it sounded good at lower levels, distortion on higher levels, not good. I suppose I will stick to EL84 pentode/triode configuration for the sake of simplicity of the mod.

Triode/Pentode switch looks very easy to implement.

Last edited by ClefChef; 4th January 2013 at 07:33 PM.
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Old 5th January 2013, 12:01 AM   #7
Cassiel is offline Cassiel  Libya
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Before installing a pentode/triode switch I would just make sure triode mode is a valid option. You may find 2 Watts is not enough power for your speakers. Oh and disconnect the negative feedback when in triode mode. Otherwise it makes little sense.
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Old 5th January 2013, 12:06 AM   #8
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I have the amp connected to a pair of tekton ob4.5 that are 96db or so. Fostex126 will really benefit from better triode resolution. 1.5w for them is plenty.
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Old 5th January 2013, 01:47 AM   #9
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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.
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Old 5th January 2013, 02:05 AM   #10
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Here's the rendition of Almarro power supply. The HV secondary could be configured to 220v CT or 250v CT by placing two fuses in different slots, currently 250v is selected. Heaters are AC fed from 6.3v @2.5A secondary which is probably not enough - PS transformer gets real hot during operation.

B1 - EL84, B2 - EL84 G2, B3 - 12AX7

One way to improve power supply could be bypassing second and last electrolytic with small value quality film caps. Large film cap instead of lytic in the last position would be ideal, but fitting may be an issue.
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