Hammond Organ Amp Conversion

[omthode]

Hi everyone

This is my first post here so sorry if I'm breaking any conventions or rules.

I've come into the possession of a Hammond ao-14-1e amplifier from an M2 organ off of ebay. I'm planning on using it as the foundation for my first guitar amp build. The usable tubes from amp are:
2x 6SJ7's (I'm only planning on using 1 of them)
1x 6SN7GTA
2x 6V6's
1x 5U4GB

I've begun drafting up a schematic. I'm very much a newbie in this field, so I'd appreciate some guidance. I still need to figure how the power supply section will work (the voltages are just guesses), but I figure that will be easier when I can take measurements. For now I'd like to focus on the general layout and biasing. Essentially, is this design feasible for a simple bedside amp?

View attachment 771897

Many of the resistor and cap values have been pieced together from various other amps (Gibson GA 20, Route 6V6 Amp, Princeton Reverb B1270, etc.) and a few forums and articles. For the tone stack I simply plugged in the moonlight tone control. Just looking for someone with more experience and expertise to let me know if I'm at least on the right track here.

Thanks,
Oli

 

[omthode]

Don't think the actual schematic showed up on the last one, so just making sure:

 

[PRR]

> how the power supply section will work (the voltages are just guesses)

I noted that Hammond took a large negative back-bias for speaker field; however I think you are in sight of OK voltages. (I doubt the downstream voltages really matter.)

 

[omthode]

Thanks for the reply.

I noticed that the some of the line went off to negative supplies. I wasn't entirely sure why they did that, or if I should keep those same lines in my new schematic. Frankly I was pretty confused by their whole power supply, so I decided to just follow other examples.

> (I doubt the downstream voltages really matter.)

Are you talking about the B+ lines and plate voltages? If so, I tried to bias them properly, but I'm pretty much shooting in the dark without the amp in front of me.

 

[Gnobuddy]

...Essentially, is this design feasible for a simple bedside amp?

Firstly, welcome to diyAudio! 🙂

My guess is that there will be two problems you will face: the first is that the amp will be far, far too loud for bedside use, and if you try to turn it all the way down to usable loudness, the sound quality will be very "meh". Your amp should be somewhere in the ballpark of 10 watts - 15 watts output, and that is very, very loud when you feed it into a guitar speaker.

The second problem I think you might face is a lack of sufficient gain - maybe not enough to overdrive the output fully with a guitar using low output single-coil pickups. I say this because you only have two gain stages - the input pentode, and the long-tailed-pair phase splitter. And a long-tailed pair only has half the voltage gain of a normal gain stage.

On the plus side you have a low-loss tone control, so maybe you'll have enough gain to get by.

(By the way, if you do have enough gain, the amp will be insanely too-loud for home use when overdriven. But perhaps if you want to use it on stage?)

I've formed the opinion that "Bedside tube guitar amp" is a very difficult nut to crack. There have been many, many attempts by many people, but few happy endings.

I made a few attempts, too. My 15W amp was far too loud, so I built a 2W amp; it was far too loud as well. Most recently I built a 200 milliwatt (0.2 W) amp, which was about the right loudness for clean tones, but too loud when overdriven. 😱

For the moment, I've set all attempts to build a "bedside tube amp" aside; instead I went out and bought a Boss Katana 50. It's solid-state, but it's also the first modelling amp I've ever heard which actually sounds more "tubey" than my Fender Princeton Reverb does, and works better as a bedside amp than any of my DIY attempts ever did. 😱

The same Katana 50 has also been on stage with me twice now, once in a large hall, once outdoors. It needed to be much louder in these situations, and it also coped perfectly well there. None of my little DIY bedside amp projects would come anywhere near being loud enough for stage use. So the Katana 50 is not only better at being quiet, it's also better at being loud. 😱

(This is largely because it has a built in power output switch; you can set it to nominally 0.5 watts, 25 watts, or 50 watts. I use the 0.5 W setting at home, 25 W onstage in the hall, and the full 50W for the outdoor event.)

Don't get me wrong, your build might well end with a very nice, very loud guitar amp, about as loud as an 18 Watt Marshall. But I am doubtful that it will also be a usable bedroom amp...


-Gnobuddy

 

[omthode]

Thanks for the response. I honestly hadn't really considered those problems. I'm less concerned about the issue of gain, because my guitar has humbuckers. I also have revised the circuit since I posted this (as seen here) with an increased plate voltage and resistor going into the 6SJ7.

As for the power output, you make a good point. Is this something an attenuator could fix? I might look into building/buying one if volume is problematic.

I would prefer to use as many of the original tubes as I can, but if it really comes down to it, I might also consider saving one of the 6V6's for later use and building an SE amp. That would remedy both the gain and volume issues, or at least move in the right direction, no?

Thanks again

 

[Gnobuddy]

Is this something an attenuator could fix? I might look into building/buying one if volume is problematic.

The two solutions everyone talks about are (1) an attenuator, and (2) A VVR or similar variable B+ voltage.

I have some (not entirely positive) experience with DIY attenuators (an Lpad and one made with discrete resistors), and none with a VVR.

If you decide to try a DIY attenuator, do look up John DH's work. There's a link in post #41 of this diyAudio thread: https://www.diyaudio.com/forums/ins...net-design-5-driver-guitar-5.html#post5785471

If you decide to try a VVR, Google will turn up more. The simple version of the VVR is basically just a big MOSFET source-follower on a heatsink, and a potentiometer to set the DC voltage on the gate. It lets you dial-a-B+ voltage.

My suggestion is to build it without the VVR or attenuator for initial testing, but plan ahead for the possibility of the VVR - leave a little room for heatsink and MOSFET and VVR pot somewhere, maybe place a few turrets or a lug-strip where the MOSFET and associated components can go if you decide to use it.

...one of the 6V6's...SE amp...remedy both the gain and volume issues, or at least move in the right direction, no?

SE will only drop the loudness by 3 decibels, which is quite a slight change. It will go from "insanely too loud for the bedroom" to "very slightly less insanely too loud for the bedroom". And it will cost you a new output transformer, and maybe design changes to the power supply too. IMO, this is not a worthwhile change to make, not for your purposes at any rate.

To give you an idea: a typical guitar speaker puts out 95 - 100 dB at 1 metre distance when fed 1 watt of power.

Ten watts power output is the same as +10 dBW; we can add this directly to a speaker sensitivity to get SPL. So a 10 watt guitar amp driving a 95 dB@1W@1m speaker will spit out an SPL of 105 dB at 1 metre. (It could be 110 dB, if you use a more sensitive 100 dB@1W speaker.)

105 dB is 20 dB louder - one hundred times more acoustic power - than the 85 dB threshold above which OSHA says hearing loss starts to occur. Food for thought there. 😱

How quiet should a bedroom amp be? Nobody seems to have a really solid idea. But, for reference, a household vacuum cleaner is about 70 dB SPL.

For a bedroom amp, I made the assumption that a vacuum cleaner would be the loudest sound my neighbours would tolerate. That means you have to drop the SPL of the guitar amp from 105 dB down to 70 dB.

Guess how much power that would require into our 95 dB speaker? Three thousandths of a watt. Ridiculously low!

In practice, I found that estimate too conservative, and I could use a bit more, probably because guitars don't make sound continuously like a vacuum cleaner, and clean tones at least have relatively low average power compared to the big initial transient (pick attack). But 200 mW, overdriven, was definitely too loud in my apartment. For clean tones, 200 mW was not quite loud enough, and I would have liked to have a bit more.

In any event, a typical single-ended 6V6 guitar amp is still going to put out some 5 watts, i.e. more than ten times as much power - and maybe fifty times as much power - as you are likely to tolerate in your home. I don't think it will solve the bedroom-amp problem. 🙁

As to the gain issue - maybe you won't have a problem, I don't know. If it were my build, I'd play it safe, and leave room for one more tube socket and a front panel switch / pot, so I could insert an additional gain stage if need be.

But IMO it's always best to start simple, and only add complexity if unavoidable. It's much easier to test, trouble-shoot, and fault-find your build this way. If you start by building and testing the power supply, then the output stage, then the phase splitter, and work your way back towards the input jack, you'll find out soon enough if there really is a problem with too much SPL and/or too little gain.

Good luck with the project!


-Gnobuddy

 

[dotneck335]

..... I've formed the opinion that "Bedside tube guitar amp" is a very difficult nut to crack.....For the moment, I've set all attempts to build a "bedside tube amp" aside; instead I went out and bought a Boss Katana 50. It's solid-state, but it's also the first modelling amp I've ever heard which actually sounds more "tubey" than my Fender Princeton Reverb does, and works better as a bedside amp than any of my DIY attempts ever did. 😱 The same Katana 50 has also been on stage with me twice now, once in a large hall, once outdoors. It needed to be much louder in these situations, and it also coped perfectly well there. None of my little DIY bedside amp projects would come anywhere near being loud enough for stage use. So the Katana 50 is not only better at being quiet, it's also better at being loud. 😱 (This is largely because it has a built in power output switch; you can set it to nominally 0.5 watts, 25 watts, or 50 watts. I use the 0.5 W setting at home, 25 W onstage in the hall, and the full 50W for the outdoor event.)-Gnobuddy

Hmmmm....that;'s very interesting that you settled on the Katana 50. You say it sounds BETTER than a Princeton Reverb? IMPRESSIVE!! So the 500 mW setting works good for the bedroom? And you can still play it in a hall or outdoors? Even more impressive! I guess, for ~$230, it's a real bargain, eh? Guess I'm gonna hafta try one! Were your experiences in live situations done with a full band, with drummer?
And lastly, have you figured out how to battery-power it?

 

[Gnobuddy]

Hmmmm....that's very interesting that you settled on the Katana 50.

Yeah. I surprised myself, too. I've been extremely unimpressed with every modelling guitar amp / pedal / multiFX I've ever used before. But the video reviews of the Katana 50 sounded pretty darn good, and I found lots of people on the Internet saying it was the best practice amp they'd ever had.

That was enough to get me into a local music store to try one, and that experience was good enough for me to plunk down my debit card and take one home, and that experience was good enough for me to try it at the jam, and....you get the picture. 🙂

You say it sounds BETTER than a Princeton Reverb? IMPRESSIVE!!

I should qualify that before I get tarred and feathered and ridden out of town on a rail. 😀

My '65 Princeton Reverb Reissue has a factory-specified 440 volts B+ 🙂eek🙂 on the 6V6 plates. I bought mine lightly-used, and the previous owner had installed JJ 6V6S output tubes, because, he told me, other current-production 6V6s tended to fry in this amp.

With those JJs in it, my PRRI sounds sterile-clean even at the loudest volume I've ever turned up to - another sit-in with a friend's band, held at a good-sized hall.

This has been incredibly frustrating, as Princeton Reverb's have such a reputation for beautiful clean tones. But in my experience, my little hybrid Superchamp XD sounds considerably more "tubey" on its clean channel, than the PRRI does.

There are EHX-badged 6V6s in the SCXD, not JJs, which might have something to do with it; from what I've seen, the JJ6V6S looks nothing like old NOS 6V6 tubes, and instead appears identical to the JJ 7591. And who knows what Soviet-era military tube the JJ 7591 originally was, before they were re-labelled as 7591s and sold to the West.

At any rate: my PRRI has always lacked in those lovely "tubey" clean tones it was supposed to produce.

So the 500 mW setting works good for the bedroom?

As far as I can tell, the "power attenuator" in this solid-state amp is really just a stepped master volume. Switching from the "50W" to the "25W" to the "0.5W" setting progressively lowers the volume at the speaker, without having to touch any of the other knobs (gain, volume). But all the distortion comes from the preamp anyway, so the guitar tone you dialed in doesn't change much when you move the "attenuator" - it just gets louder or softer. It sounds like what we expect from an old-school tube amp with a speaker attenuator, in other words.

So the "0.5W" setting doesn't require you to actually be putting out 0.5 W of power (which is almost certainly much too loud in my apartment.) You can set the amp to 0.5W, and dial up the distortion and loudness you want with the gain and volume knobs, even if that corresponds to much less than 0.5 W going to the speaker. And I find that, on the clean and crunch channels, I can get the Katana to sound more "tubey" than my PRRI ever did.

And you can still play it in a hall or outdoors?

After the positive home experience, I took it to one of my regular jams. Small room, half a dozen other players, no drummer. No problem for the Katana, on the 25 W setting.

Encouraged, I took it on stage when I was invited to do a couple of songs with a friend's band. The drummer had quit recently to look after his wife, whose dementia was worsening. 🙁 Once again, no problem for the Katana, again on the 25 W setting.

The outdoor event was a jam put on by a friend living on a local farm. I took the Katana because expected it to be a small, intimate, and not-loud event. When I got there, there were pole-mounted speakers, an old few-hundred-watt powered mixer driving them, and a bass player with a 200-watt bass amp sitting next to my spot. No drummer. And I had taken my acoustic guitar, because this was going to mostly be singing and accompanying. (The Katana was set to its Acoustic channel, which has been a boon for me.)

There was also a keyboard player with his own small P.A., and three other guitarists with their own guitar amps. Two Fender Acoustasonics, and a third something-or-the-other.

To my surprise, the Katana 50 was easy to balance against the bass player's volume, and the rhythm guitarist with the Acoustasonic. The Katana didn't even break a sweat.

As the day went on, that bass player left. Another bass player showed up and took his place, this time with an Ampeg B15 V2 (15" speaker, 150 watts.) That 15" speaker gets loud when you're sitting next to it, I can tell you. And, once again to my surprise, the Katana 50 held its own. I wasn't trying to drown anyone else out, of course, just to "sit in the mix", but the Katana, on the 50W setting, did that just fine.

I guess, for ~$230, it's a real bargain, eh?

It's all about what you want, and what amps you already have...but if you think you have a use for a small combo amp, yes, I do think it's worth your time to try one out. The worst that could happen is that you got to spend some time in a music store. 🙂

Incidentally, I would have been thrilled to have paid $230 for a Katana 50. With the weaker Canadian dollar, list price is three hundred bucks, and then there are taxes to pay on top of that. I shoulda bought it from Amazon.ca, but I wanted to

I should add that the Katana 50 isn't perfect (duh!). The relatively small cab sounds a little "boxy", though my ears soon adjusted to it (and the more expensive and bigger Katana 100 is less boxy, if your budget extends that far.) Also, if I turn up the gain knob far enough, the amp stops being "tubey" and I can hear some distinctly solid-state harshness.

But the important thing is that I found I can actually dial in some very nice clean and slightly overdriven sounds. Add in the ability to get surprisingly loud, to get surprisingly quiet, to handle both electric and acoustic guitars, and the onboard effects, and this has become a must-have amp for me. I would rather lose my PRRI - what a shocker, even to myself!

And lastly, have you figured out how to battery-power it?

No, that I have not! 😀 This is the amp I bought so I could stop tinkering with amps, and just play guitar whenever I wanted to practise or have fun. It lives by the living room couch, next to my beater Epiphone electric guitar.

I have a feeling the Katana power supply is going to be fairly complex, probably with several different supply rails at several different positive and negative voltages. If the output is push-pull (and not a bridge), the power amp is going to need at least +/- 30V rails. And a battery to supply 50+ watts for any length of time is going to be big, heavy, and expensive.


-Gnobuddy

 

[dotneck335]

Here's an interesting comment from a fellow musician about the Re-issue Fenders:
So, having declared my love for the Deluxe Reverb after using a '71 SF at a recording session a few weeks ago, I picked up a recent (2014) '65 reissue to start the journey. I was thinking I would save up and spring for an early '70s SF (and I might yet), but a knowledgeable friend of mine made an interesting observation. He's a pro guitarist and has owned and operated a first rate local music store for decades. He pointed out that they did an A-B comparison at the store between a stock, original BF Deluxe and a reissue. First run through, the original won hands down. But(!), when they plugged the drri amp into the original BF amp's speaker, all concerned noticed very little distinction between the original BF and the drri. In other words, one of the determining factors in the tone of the older original Fender tube amps is a speaker that's had half a century to break in (vs. a new speaker). Not the only factor (I inject, because I can already hear fingers typing feverishly among the diehard Never-Reissue-ers among us), but one, and possibly a critical one.
So.....so far I like the amp quite a lot. True to the preceding anecdote, it sounded fairly "meh" when I first plugged in. After a brief checking out period, I took out the stock Italo-Jensen C12n and replaced it with a well broken in Weber 12F125b. Instant (major) improvement. Amp became louder (more efficient speaker I suppose?) without breakup, and the tone was greatly improved: lost some boxy mid, hard highs of the Jensen.

 

[dotneck335]

I also noticed that the B+ voltage you quoted on your Princeton R.I. (440v) is quite a bit higher than the old Black Face versions; I believe they were on the order of 400 volts. One of these power scaling kits (Power Scaling Kits: About | London Power) might do the trick of lowering this, and maybe allow full tone at bedroom levels. Just a thought. But, if you're happy with the Katana 50, WTF!!