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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Tue Apr 12, 2016 9:52 pm
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Rock Star
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Abe, depending on the current that PSU can supply --- a 6V6GT push-pull, Class "A," cathode-biased amp would be perfect.

If the current output is low... still can do a 6V6GT single-ended amp.

;)


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Thu Jun 30, 2016 12:43 pm
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Aspiring Musician
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Hello all,

Just an update, I ended up leaving it mostly as-is. I posted the schematic I followed and it turned out pretty ok. I haven't tried substituting the 220k plate resistor for a 330k to see how much more gain it adds but I think I'd like to try it. It's a pretty clean 12'is watts or so. Also, I'm interested in what some of you may have to say regarding the phase inverter. It's been discussed that it's not really set up for guitar use.... so what would make it suitable for guitar use?

Thanks again for everyone's contributions to this thread.

:D

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Thu Jun 30, 2016 6:42 pm
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Abe,

A 330k-ohm resistor would lower the plate voltage onto the input ECL86 (versus OEM 220k-ohm). I would not do this mod. What I would do is look for ECL86 (6GW8) tubes to stock up. Philips or Ei are good ECL86. As are GE or RCA 6GW8.

If the amp is unstable, go to a 220k-ohm grid-to-ground on the output section grids. 470k-ohm sets a pretty high bias. 1500 ohm grid stoppers may also be nice for guitar work.

The 20nF coupling caps between the PI and output is prolly to reduce bass response (and instability). But, you may have very weak bass for guitar use. You may need to experiment with cap mfd here. Grid stoppers and screen resistors may need to be added as you increase the mfd of the coupling caps. As extra bass (and thus voltage) could induce instabilities.

Thanks, for updating the thread. Good luck! Tell us how it sounds.


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Thu Jun 30, 2016 11:12 pm
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Aspiring Musician
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Thanks for the suggestions. What I'll do is jumper in some 470k resistors in parallel with the original 470k's to bring it to 230k for testing purposes. If I like the way it sounds, I'll replace the 470's with 220's. I think I can also apply that principle to the coupling caps to up the value and see if that opens up the bass. I'll update when done. Also, i'm testing the amp by running it into a 8 ohm 212 Bogner cab loaded with a celestion greenback and a 70th anniversary. Just a little fyi.

One more thing, on the original hammond schematic, it runs a 82k resistor from the output of the o.t. transformer back to the cathode of v1. I take it that they were incorporating negative feedback. Why would it be removed from this current schematic? I'm thinking that it might help bring some aggressiveness back into the amp....just a thought.

Thanks Steve.

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Fri Jul 01, 2016 7:07 am
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Dunno why the global negative feedback was removed. The ECL86 tube is not the most linear. It needs GNFB. At least to get some volume without THD to the max. :lol:

Try putting the GNFB back in and play through the amp. You can play with the GFNB values or even sub in a pot to adjust it (ala: Fender's brown and blonde era "presence" controls).

IMHO, this amp is going to need a very efficient speaker(s) to project sound into a medium or larger setting.


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Mon May 29, 2017 12:41 pm
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Hello all, I'm revisiting this conversion. I wasn't completely satisfied with the amp so I decided to do a scratch build. I'm going to take this sTage by stage, the way we did our heath kit radios back in the days. I've gutted the chasis and wired in the power supply section. Next is the filter stage. I'm going to go with the 5f2a filtering scheme, one 33uf and two 8uf, as the ez81 has a maximum capacitance of 50uf listed in the datasheet. I'm open to all suggestions, recommendations and constructive criticism during this build.
Thanks in advance.

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Mon May 29, 2017 12:46 pm
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BMW2002Ti wrote:
Abe, it needs a PT? I found a re-issued PT site. Apparently the 1k-ohm resistor is important for use with the 6.3VAC EZ81 rectifier.

http://www.tompetro.com/TREKII/MRT1.html


I'm assuming this needs to be inserted in series with one of the filament wires to drop the voltage to 6.3v?

Thanks

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Tue May 30, 2017 12:01 am
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"I'm assuming this needs to be inserted in series with one of the filament wires to drop the voltage to 6.3v?"

No, my interpretation is a bit more complicated. The EZ81 is susceptible to reverse voltage from the non-conducting anode (during on-off phase of the rectifier). The one-ohm resistor is there to lower the reverse anode on the non-conducting plate. It's a rather delicate rectifier. Most 5VAC ones have no problems with this issue.

If your PT can handle the extra current needed, a WE 412A or the Bendix 6754 rectifier maybe a better choice. Esp in a guitar amp. Much, much more hardy. The 6X4 (EZ81) needs 0.6-amps of current. The WE412A and the 6754 need 1-amp.

http://www.hifido.co.jp/photo/15/277/27724/a.jpg

https://thumbs.worthpoint.com/zoom/imag ... 619082.jpg

Run 8-10mfd on the first filter stage off the rectifier. Or run a choke, for better filtering. 16mfd-20mfd in the other stages should be fine.


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Tue May 30, 2017 12:05 am
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Bendix 6754 datasheet:

https://frank.pocnet.net/sheets/131/6/6754.pdf

Western Electric 412A:

www.westernelectric.com/spec_sheets/412A.pdf


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