Name a well-regarded classic tube amp that..

[WimWalther]

.. that has the following specs:

2x 6BQ5 / EL84 or 6V6
2x or 3x 12A?7 types
Basic tone / presence controls
Possibly a vibrato circuit

I have an old Hammond organ amp chassis** that I'd like to build up for use with guitar. Rather than reinvent the wheel, I'd like to use a decent classic-era circuit.

(** actually, two identical units)

Suggestions? Thanks!

[ThatRhythmMan]

The Fender 5F11 Vibrolux, one of my favorite amp circuits of all time. Also uses a 5Y3 rectifier.


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[andrew]

For single channel, the 5F11 Vibrolux (as mentioned above) is definitely a good choice. The 6G2 brown (and tuxedo) Princeton circuit is very similar.

If you want to go multi channel, the options open up more.

[WimWalther]

The Fender 5F11 Vibrolux, one of my favorite amp circuits of all time. Also uses a 5Y3 rectifier.

That pretty well covers the bases, I'd say. Nice suggestion.

About the only potential issue is the 5Y3. I don't recall my chassis running a 5V rectifier - or any tube rect for that matter. So that may need to be worked around.

[WimWalther]

If you want to go multi channel, the options open up more.

Nah, single is fine.

[sgosnell]

It's easy to put a solid-state rectifier in the socket. Lots of people use them to get a little more headroom, and they're readily available. You do need a rectifier of some sort.

[WimWalther]

It's easy to put a solid-state rectifier in the socket. Lots of people use them to get a little more headroom, and they're readily available. You do need a rectifier of some sort.

I took another look at those chassis, and sure enough they have a 5V rectifier socket. More than likely it took a 5Y3, but it could also have used a 5AR4. I'll have to see where the voltages fall.. or figure out which model these chassis came from.

ETA: Turns out that this is a Hammond L-100 chassis and the original rectifier was a 5U4.. which is a good thing IMHO. So it can use just about any of the 5V rectifier family - 5U4, 5AR4, 5R4, 5T4, 5Y3, etc.

But I'll most likely use the 5Y3 as that's what the Fender circuit uses.

[WimWalther]

So I'm looking over the docs for the aforementioned Princeton & Vibrolux amps, and I can't really see any differences beyond a couple parts values. The former uses 50uF filters vs. 16uF in the latter. There may be others but for some reason the schematics are only partially legible.. very blurry.

They're also drawn differently, though it seems they may have the same topologies.

But a question presents: What to do for the 3M-RA speed pot? I came across an ebay offer, but the cost is silly.. $8.50 + $15 ship for one piece.

Can't this be approximated with a 5M or 10M part with a law-faking resistor? It's just a speed control.

[sgosnell]

I tend to use Mouser for electronic parts, but even though they're not all that far from me, shipping often costs more than the parts if it's a small order. I think one is much better off ordering everything at once from a single supplier, to minimize shipping costs. If Mouser doesn't have it, you don't need it. Except for tubes. For those, you're on your own. I haven't bought one in 30 years or so.

[andrew]

You can find a CTS brand 3M Reverse Audio Taper pot for $3.95 from Antique Electronics Supplies:

Just a moment...


As you've discovered, the 5F11 tweed Vibrolux and the 6G2 brown Princeton are very similar circuits. Just looking at the two schematics, here are the only differences I could find:

-Vibrolux has 16uF filter caps (on the eyelet board). Princeton has 30uF filter caps (in a can cap). The voltage dropping resistors are also different values.
-Vibrolux has 3 inputs. Princeton has 2 inputs in the "normal" Fender configuration
-Vibrolux used a 2 Meg linear pot for the Speed control. Princeton uses a 3 Meg reverse audio taper
-The tremolo oscillator loop is slightly different. 1500 ohm cathode resistor on the Vibrolux versus 3300 ohm on the Princeton. The second 1 Meg resistor on the Vibrolux connects directly to ground, but on the Princeton it goes to the cathode resistor first.
-Not necessarily a schematic difference, but in terms of the layout, the Princeton is part of the first generation of Fender amps to have a separate daughter board for the negative DC bias supply.

[WimWalther]

As you've discovered, the 5F11 tweed Vibrolux and the 6G2 brown Princeton are very similar circuits.

So the Princeton is the later model, correct? Regards the differences in the two, should I go with the Princeton circuit as it's the more updated of the two? Or were they just trying to be done with goofy reverse-taper pots?

-Vibrolux has 16uF filter caps (on the eyelet board). Princeton has 30uF filter caps (in a can cap).

My chassis has 2x 50/50uF @ 450V FP style cans that seem to be perfectly serviceable. I formed them up to 425V without issue and would like to use them, but..

Would too much capacity negatively affect the character of the circuit? I mean, by increased resistance to sag (break-up) as expected?

The voltage dropping resistors are also different values.

Same issue / question as previous.

-Vibrolux used a 2 Meg linear pot for the Speed control. Princeton uses a 3 Meg reverse audio taper
-The tremolo oscillator loop is slightly different. 1500 ohm cathode resistor on the Vibrolux versus 3300 ohm on the Princeton. The second 1 Meg resistor on the Vibrolux connects directly to ground, but on the Princeton it goes to the cathode resistor first.

Do we assume the different osc circuit values are tailored to work with the two different control pots? Clearly, a 2ML pot is much more common than a 3MRA. I can fully understand wanting to do away with the latter.

[andrew]

The Princeton is the later of the two models and hence, the more "updated" one. That particular Vibrolux circuit is from the late 50s, whereas the brown Princeton ran from 61-64. I own a '64 Princeton (a 6G2 circuit with the tuxedo cosmetics) and can attest to its goodness firsthand. I've never had the pleasure of a playing through a 5F11 tweed Vibrolux in person, but have heard good things from plenty of folks. In short, I don't think you can go wrong either way. The differences are far from night and day.

In addition to less hum, larger filter caps will also result in tighter, firmer bass and less overall sag. For jazz playing, I generally consider a little extra filtering a good thing. However, tube rectifiers have a limit of how large the first filter cap can be and your 50uF cap is over the limit for a 5Y3. If you wanted to stick with the 50uF caps, you'd want to use a GZ34 rectifier, which will result in some higher plate voltages and less sag (Not necessarily a bad thing, it's all about taste and whether or not you want the extra headroom).

When it comes to the tremolo circuit, the tweed Vibrolux and the tweed Tremolux were the very first times Fender had put tremolo into one of their amplifiers. Perhaps the subsequent revisions later on were to work the bugs out. Fender used the same oscillation loop from the brown Princeton in the black Princeton Reverb, so it seems they were happy with it. (Actually, the one other tweak that's generally recommended is to change the 25uF cathode bypass cap on the tremolo oscillator to 4.7uF. This will eliminate the split second it takes for the tremolo to "catch up" after you turn the speed knob. Apparently Fender started making this change around '66 but never updated their schematics).

I don't mean to sound like a Princeton shill, but I think the other factor that's worth highlighting again is the input circuit. If you're familiar with and are a fan of how inputs 1 and 2 sound different on many of the other classic Fenders, the brown Princeton works the exact same way, whereas the tweed Vibrolux doesn't. The impedances are different. I know that I really like these two different inputs and vary them based on the pickups in the guitar.

Replace the tone control on the brown Princeton with a black panel tone stack and you'll essentially have a black panel Princeton non-reverb. Or do both and put it on a switch!

[WimWalther]

The Hammond circuit used a 5U4 rect. It also has a buffering resistor between the rect cathode (pin 2) and the first 50uF cap. So between the two, it's not a problem.

But if I want to run the Fender circuit, I will need to change things up.. one way or another.

[stevo58]

Hoffman (el34world.com) has CTS 3m RA pots for $4.49. Doug is my preferred source when I buy in the US, which admittedly isn’t often any more.

You might also consider a 5R4 rectifier.

The 6G2 is pretty much a 5F10 Harvard with a different input scheme and added tremolo. (The 6G10 Harvard is basically a tweed Princeton. Leo was frugal. Interesting that he basically swapped those two models.)

You can do a lot with a Hammond amp. If you check Doug’s forum, you’ll find endless Hammond conversion ideas.

[WimWalther]

You might also consider a 5R4 rectifier.

Sure, whatever works the best.. as I have 2A @ 5V to work with.

You can do a lot with a Hammond amp. If you check Doug’s forum, you’ll find endless Hammond conversion ideas.

Thanks, I'll check that out. It would be nice to find a component layout that works with the Hammond board, as I'd rather not reinvent the wheel. I'm sure this exact job has been done several times already.

[WimWalther]

Quick update here.

As suggested, I went to the Hoffman Guitars forum at Tube amp Information, Guitar amp Schematics, Tube amp Information and started a discussion on converting the Hammond A0-43 into a Princeton 6G2 circuit.

The wealth of assistance I've received is truly amazing, I can't thank folks enough. One member "sluckey" even went so far as to produce & publish a layout in Visio that implements the Fender circuit on the stock Hamond board. How cool.

It also turns out that the 10K gain adj. pot on the chassis is a perfect bias control for the new circuit.. and it's screwdriver adjustable. There are also 300R & 750R 3W parts that can be rewired as a ~1K 6W screen resistor for the new circuit. Add a 10K 1-3W and that's your B+ supply.

The FP cans (2x 50/50 @ 450V) are still perfectly good** on my chassis, so I have one 50uF section left over for what-may-come. You'll also want to sub a 5Y3 for the stock 5U4 to keep the B+ down to ~340V or so.

So two chassis are now in process. I'll keep one and probably sell the other, once I'm happy with them.

(** the A0-43 has a 64R 10W between the rect. and 1st cap. This really improves conditions for the cap section, which normally takes a real beating from ripple current.)