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Post subject: Upcoming hammond ao 44 conversion
Posted: Tue Jan 19, 2016 8:29 pm
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Hello all,

I received a hammond ao-44 reverb amp recently and was thinking about converting it to a usable guitar amp. I've done research and found everything from simple conversions to full blown organ transplants. The one schematic that "tickled my fancy" was this one. You'll find the pdf way down at the end of the page. I like the simplicity of the circuit and everything looks to be in order, any suggestions or thoughts before I begin?

http://sluckeyamps.com/PeeWee/peewee.htm


The other thought I had was to convert it to a PTP Fender Pro Junior driving a 212 cab utilizing the PT, OT, and EZ81 rectifier. Anyhow, any comments or suggestions would be greatly appreciated.

Thanks.

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Tue Jan 19, 2016 9:59 pm
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Can you convert this? Yes.
Should you convert this? No.
You can install a lot of money and time and still not have much amp.
The first consideration is finding replacements for those rare power tubes.
The 200 volts on the plates is rather anemic. So the PT can't deliver much power and the OT probably can't handle more voltage
I would just use it as designed. The separate reverb amp and speaker does wonders for your reverb or time-based effects tone.


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Wed Jan 20, 2016 3:23 pm
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Tim,

I wanted to get some preliminary readings from the transformer. Based on 120v input, 270v ac from each secondary lead to ground with a combined 540 across both leads. On the heater lines, it's up at 7.1v across both leads and 3.5 from lead to ground. Quite surprising, wouldn't you agree? @ these voltages, would it not make a decent candidate? If not, then I will surely consider a stand alone reverb tank.

Thanks

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Wed Jan 20, 2016 3:51 pm
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I'm in agreement with Tim.

While with those available voltages you could conceivably drive a pair of 6V6GT's (assuming a primary winding impedance of +/- 8KΩ at the output tranny, which would solve the issue of those orphan power tubes), you could spin your wheels for quite awhile to work out the dynamics for a usable tone stack and matching the input level. I'm not familiar with the operating characteristics of the EZ81 rectifier but based on its availability (or lack thereof), it might have to go as well.

Arjay

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Wed Jan 20, 2016 4:24 pm
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Abe, I also agree with TimsAudio. Yes, there are these triode-pentode ECL86 (6GW8) are on Ebay. HOWEVER, good ones is another question.

http://www.ebay.com/sch/i.html?_odkw=ec ... 8&_sacat=0

If you already have a couple sets of these tubes, the resto would be relatively straight forward. However, if you are going to mod this amp to take a 12AX7 (for the triode half) and an EL84 for the pentode side --- you are looking at big mods on the chassis (like punching another hole for another socket). And re-doing the PSU to drive the new tube setup.

If you want to do this amp --- first look for good ECL86 tubes. My usual tube sources for these Euro tubes (Tube World and Upscale) do NOT have any NOS in stock.

BTW... there are NO triode-pentodes being made right now. And good NOS are getting rare. I have stocked up on a few 7199 NOS RCA blackplates, used in many old Gibson and Ampeg amps. The 7199 is NOT compatible with the ECL86.

Good NOS EZ81 (6CA4) can be found. Any US made 6CA4. Tesla and Philips EZ81. EH manufactures a new 6CA4. I have no experience with it.


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Wed Jan 20, 2016 5:28 pm
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Thanks guys for your comments and suggestions.

What I'm thinking now is to use this amp as a learning tool to familiarize myself and have a better understanding of tubes and circuits. I don't mind having to gut, drill, rewire,or even spend a few bucks in parts to get somethin or anything that can be of use at this point. What I think I need at the moment are the specs for the transformers. I can't seem to find the current rating on the PT, but it's gotta be at least 3 amps if the ecl86 Ih is .66a and the ez81 Ih is 1a. I'll keep looking and come to some kind of conclusion, but any suggestions would be appreciated.

Thanks again! :D

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Wed Jan 20, 2016 6:45 pm
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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


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Wed Jan 20, 2016 10:47 pm
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BMW,

The pt is fine. I'm just trying to find out what the max current rating is for the transformer so that I could find a practical use for it. Maybe I'll send those folks in the link you provided an email, they might know something. :D

Thanks!

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Thu Jan 21, 2016 12:11 am
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Abe, My guess is the PT and OPT are close to OEM blackface Princeton. Push-pull around 14-15 watts.

Schemo:

http://sluckeyamps.com/PeeWee/PeeWee.pdf


BF Princeton:

http://bmamps.com/Schematics/fender/pri ... _schem.pdf

FWIW: Most triode-pentode tubes were used in lower end table top radios & small consoles. European ones. And in US tube car radios. One triode-pentode could take the place of a duo-triode (12AX7) and output tube (EL84). With consequence of very high total harmonic distortion (usually ~ 10-20%) and not-so-good signal-to-noise issues. So, they were used in cheap amps.

One exception is the 7199, that RCA designed to higher standards than the ECL86. Internal shield to lower crosstalk. Better grid and support to lower microphonics. These 7199 were used in Dynaco and EICO home audio amps of the 1950-60s. Some Ampeg amps used them to drive higher power output tubes like the 7027A & 6550 in the V-series amps.


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Thu Jan 21, 2016 10:06 am
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Interesting idea.

Replace the tube EZ81 rectifier with a full-wave solid-state bridge (like 2 or 4 x UF4007). Now, use the empty 9-pin socket for a 12AX7. Replace the ECL86 tubes with EL84. Use BOTH halves of the 12AX7. One for input; the other for phase inversion (unlike the linked article). The phase inverter stage would remain a concertina design.

http://www.solorb.com/elect/musiccirc/AO44monoamp/


You'd need to eliminate the tone-control stage. I'd setup this part of the circuit similar to the 6G2 Princeton. Eliminate the Princeton's tone control. Utilize one-half of the 12AX7 for input/gain. Other for PI. No tremolo/vibrato section.

http://www.electronicstudio.net/schemat ... _schem.pdf

The concertina PI driving two EL84 in push-pull topology. But, with cathode-tied bias resistor. Class "A." Like the original AO-44.


Just a thought... :mrgreen:


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Thu Jan 21, 2016 2:57 pm
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Sounds interesting, let me draw it up on paper first and see how it looks.

Thanks for the input, Bmw.

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Thu Jan 21, 2016 8:09 pm
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My guess is bypassing the tone stack will give you some gain back. So, only one-half of the 12AX7 will be enough to drive the phase inverter section. Setup the second-half of the 12AX7, as a concertina PI to drive the EL84 tubes --- in a manner similar to how the BF Princeton PI drives the 6V6GT output section.


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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Sat Apr 09, 2016 5:10 pm
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Bmw,

I've been kinda busy at work but finally got around to doing some more research. The o.t. specs are 8k primary and 8 ohm secondary. Almost identical to the bfdr o.t. according to Richard Goodsell of Goodsell amps. If this is true, this might make for a very interesting project. I'm thinking princeton-ish type of circuit as you suggested.

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Sat Apr 09, 2016 6:15 pm
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socal323 wrote:
The o.t. specs are 8k primary and 8 ohm secondary.


Perfect for a pair of 6V6GTA's, assuming you can coax sufficient plate voltage from the power supply.

A single 15-inch speaker would yield plenty of volume without being overly loud. Think vintage Jensen C15S or equivalent.

Arjay

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Post subject: Re: Upcoming hammond ao 44 conversion
Posted: Tue Apr 12, 2016 5:04 pm
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Retroverbial wrote:
socal323 wrote:
The o.t. specs are 8k primary and 8 ohm secondary.


Perfect for a pair of 6V6GTA's, assuming you can coax sufficient plate voltage from the power supply.

A single 15-inch speaker would yield plenty of volume without being overly loud. Think vintage Jensen C15S or equivalent.

Arjay


Arjay,

I took some more readings at the power supply:

A 375v
B 370v
C 357v
D 337v

These were taken straight from filter caps.

What I did was to get the amp up to spec, as-is, to make sure it was working before digging in to it. I replaced the fliter caps and resistors in the power rail. Checked all other resistors and caps for drift and then proceeded to give it a test run. Initial results are good. I'm confident this will make for a fun/interesting experience.

As always, I appreciated everyone's input and expertise.

Abe

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