Let's put together a modular pre

[skyboltone]

Greetings Lads:
I'm designing an amp around a completely unknown (to me) power tube. I've ordered an OT from Heyboer and will use a 325MA Hammond Torroid 236V PT and a modular DC power supply for all the filaments.

I would love to build up a pre-amp of known tonal quality. Either maybe an AB-763 or a Blackface Bassman, OR? I would like to make this thing modular, with an extra socket for a clipper stage, an extra socket for an effects loop, and a seperate PI. Each of these elements should be connectable via RCA or BNC jacks with low capacitance cable.

The idea here is to build up a power supply, bias supply and the power amp section, hook it up to something reasonably predictable by way of a preamp and then work backwards, adding to the new design and subtracting modules from the known pre-amp.

Have any of you done something like this? Does anybody think this is a good idea? Is it workable? Are we gonna get distracted by the connection issues?

Waddaya think?

Dan H
Stuck in Frisco for now.

[nickt]

Good idea, AX84 discussed this once or twice - haven't been there for a while so don't know if it went anywhere.

Like the idea of the RCA's - how would you do power and grounding?

[CaseyJones]

It's been done... sorta.

Remember the old Seymour Duncan modular? If they were really cool Seymour would be selling reissues by now, guess he's got his hands full makin' pickups.

Of course there's that little Randall although near as I can tell I can build complete amps for what the modules sell for. Not much point to point goodness there, either.

Starting with a power supply is a great idea. 325ma sounds like a lot. You'll save a lot of hassle if you leave all the filaments connected at all times, it sounds like that's what you're going to do. Use mini toggles to lift the plate supply from each unwanted tube stage, that way you don't have redundant circuitry hissing away. It seems to me like all you really need is a mini jackfield, if you made a mini patchbay using say 1/8" phone plugs you could route your signal where you wanted it.

Toroid=donut=round. If you arranged your preamp tubes in a circle around your patch bay you'd have half a chance of keeping your signal path short.

Hey, I just had another idea. Route signal only to an octal socket, that gives you a hub to route to / through. Use octal plugs with jumpers to re-configure. Hell, spray paint 'em pretty colors... say the red plug cascades as many gain stages as possible, the green plug inserts reverb and drops a gain stage or two, the blue plug adds an effects loop. Octal plugs are self indexing. Could be interesting...

[Andy Le Blanc]

an old fashioned approch was to build each tried and tru componant its own
box and lay them out.... may be a separate power supply..... good for hifi
science fair look...... nice boxes etc..... but not portable

You might want to try building the moduals on their own plate....just big enough
to the tube socket and a component board.....then line up the plates
useing a rail system for ground bus and power....each modual can still be
point to point

I've been playing with mounting tubes and a point to point affair horizontally on
verticall metal strips inside a chassis... heat hasn't been an issue with the 9 pin miniature tubes...
I havent found a mounting precaution yet either
this might be an ok compromise ....ive seen this approach in older tubed lab gear also

you will want to make your cable or wire interconnects.... all those little
capacitive values add up

[skyboltone]

Great input so far guys, thanks.

I kinda see this thing as a 17" by 4" Hammond chassis with it's own PS. Say a 350-0-350 at about 30-40ma choke input. A filament supply for 12XX7 tubes. That gives us about a 310V rail with a choke input filter. I'm not sure if it's necessary to have adjustable voltages on the rail. I think that part of the tweaking happens in the actual build. Anyway should have as smooth a supply as possible. There are only going to be four take off points. Pre PI and post PI. Switching the plates off with little switches makes lots of sense. Very close coupled switchs to separate signal elements. Let's say the design I'm working on has no effects loop or clipper stage, so I just jerk the tube on that socket and disconnect the modules from each other. Like this:

Input/tonestack/recovery----------clipper-------loop-----PI
| | | |
----------out----------------------------out----------out-----out

Sonafabiscuit @#!&*%@#@ I forgot you can't do text drawings on this thing. Add little switches between sections and imagine spaces between the vertical carots.

Casey, the 325MA doughnut is because Hammond makes a 210ma/236VAC and a 325ma/236VAC doughnut. Like the old Mercedes ad, Ve maak 5 cylinder engine because 6 iss to many and 4 iss too few! I'm going to run 4 7551s in parallel PP. I need about 260ma.

Anyway, the modular pre is strictly a piece of test equipment. I don't care if Seymour made any money on it or not. It's just a tool so that I can build tone backwards from the power amp in new designs. Its sequel tool would be a stand alone power amp with a disconnectable PI and some options regarding feedback and presence.

Dan

[benoit]

Not sure if this is what you were going for but the code tag works for what you want to do. You'll want to make sure to write it out in a program (something like notepad, if you're in windows) that uses a fixed-width font otherwise it won't come out right.

Code: Select all

Input/tonestack/recovery----------clipper-------loop-----PI
           |                         |            |      |
----------out-----------------------out----------out-----out 

[skyboltone]

Not sure if this is what you were going for but the code tag works for what you want to do. You'll want to make sure to write it out in a program (something like notepad, if you're in windows) that uses a fixed-width font otherwise it won't come out right.

Code: Select all

Input/tonestack/recovery----------clipper-------loop-----PI
           |                         |            |      |
----------out-----------------------out----------out-----out 

That's it more or less Ben, I had some switching to isolate sections but took it out when it collapsed on me, but the point is made. As we progress here I'll do a visio drawing if folks other than me are interested.

The thing is I know the AB763 tone for instance. I know that blackface bassman. That way I'm not doing the whole design process at once. I can build the power amp and make sure it's working right before tackleing the PI. I have an ace in the hole too. I bought a 0-1000VDC 200MA adjustable power supply out of some medical research equipment off Ebay for $1. So I can see what sort of voltage the power section likes before designing the power supply.

A 310VDC rail may not be enough to power a PI correctly.

Thanks
Dan

[nickt]

Hi Dan,

You're thinking of a full featured amp with a patch bay - correct?

Edit: was just Andy's idea restated

Is there an issue with the tube amp Lego approach?

cheers
Nick

[skyboltone]

Hi Dan,

You're thinking of a full featured amp with a patch bay - correct?

Edit: was just Andy's idea restated

Is there an issue with the tube amp Lego approach?

cheers
Nick

Pretty much, I like Andy's interchangeable plates idea with a chassis that can provide power and connectability. I would like to reiterated though that this is a piece of test equipment to make it easier to build up new untried ideas. One may well have modules that look like Marshall or Vox or Valco or whatever. I think pre-amp voltage is important too though a variac would provide at least initial adjustability.

I'm trying to avoid the necessity of designing building an entire amp without knowing in advance how it will all fit together tonally. And then gutting it when things don't work out as anticipated. Some sort of very easily adjustable prototyping chassis would help learn how tone travels through the amp too.

[nickt]

Pretty much, I like Andy's interchangeable plates idea with a chassis that can provide power and connectability. I would like to reiterated though that this is a piece of test equipment to make it easier to build up new untried ideas. One may well have modules that look like Marshall or Vox or Valco or whatever. I think pre-amp voltage is important too though a variac would provide at least initial adjustability.

I'm trying to avoid the necessity of designing building an entire amp without knowing in advance how it will all fit together tonally. And then gutting it when things don't work out as anticipated. Some sort of very easily adjustable prototyping chassis would help learn how tone travels through the amp too.

Ok understood - I might try the "amp lego" approach but (given I said this two years ago on AX84 as well) don't hold your breath mate!

[CaseyJones]

Casey, the 325MA doughnut is because Hammond makes a 210ma/236VAC and a 325ma/236VAC doughnut. Like the old Mercedes ad, Ve maak 5 cylinder engine because 6 iss to many and 4 iss too few! I'm going to run 4 7551s in parallel PP. I need about 260ma.

Oh good... a motorhead tagent I can hijack!

Did Mercedes build a 5? I know Volvo, VW and Audi did. A 5 is kinda dumb, it's an oddball firing order and difficult to balance. To long to fit in a hole where a 4 is supposed to be and weaker than a 6.

And... congratulations, you pulled one out I'm not familiar with, 7551s?

Ok understood - I might try the "amp lego" approach but (given I said this two years ago on AX84 as well) don't hold your breath mate!

One of these days I'll have to winter over in Oz, our Winter is your Summer. I'd really like to know what kind of electronics surplus can be had down there, apparently a lot of the old military transmitters used KT66s. I like KT66s...

There are a TON of modular concepts goin' from way back to right now. Tektronics used some cool plug in concepts in their 'scopes, if you get a chance to rummage through an old Tek by all means check it out. All sorts of very tidy wiring and super duty parts that can be swiped and used in amps.

More recently rack mounted card frames have been used for everything from telephone switching to computer servers. The telephone company is constantly ripping out gear and upgrading, if you're in the right scrapyard at the right time you can get some very expensive connectors, switches and perhaps a power supply or two for very cheap.

I wish I had a source of vintage military avionics for cheap.

[skyboltone]

Oh good... a motorhead tagent I can hijack!

Did Mercedes build a 5? I know Volvo, VW and Audi did. A 5 is kinda dumb, it's an oddball firing order and difficult to balance. To long to fit in a hole where a 4 is supposed to be and weaker than a 6.

And... congratulations, you pulled one out I'm not familiar with, 7551s?

More recently rack mounted card frames have been used for everything from telephone switching to computer servers. The telephone company is constantly ripping out gear and upgrading, if you're in the right scrapyard at the right time you can get some very expensive connectors, switches and perhaps a power supply or two for very cheap.

I wish I had a source of vintage military avionics for cheap.

The 300 series diesels, 124 body were all 5 cyl. Some were turbo. I had one that I put 435K on in a little over 10 years. I sold it to aircraft mechanic and told him "This motor is wore out mister, you can't make it between fillups without adding a quart of oil" he said "I'll rebuild it next summer". He called me a week later and said; "yer right, I gotta rebuild NOW!"

The 7551 is a mobile communications tube intended for plate modulated AM transmitters. It's robust to say the least. With a 13V filament I doubt it ever went into military aircraft so I think it's for land based use. A lot of the NOS stock out there is branded Motorola. That tells you something. I want to try a quad because it's a much lower impedence tube than EL-84s and NOS RCA's are $6 apiece. Maybe it'll sound like a 6L6 type rather than like most 9 pin minis? I dunno. If it's crap I can convert over to EL-84s without much hassle. New OT though. The OT I had built will work with a pair of 6L6s into 8/16/32 ohms.

[CaseyJones]

Oh good... a motorhead tagent I can hijack!

Did Mercedes build a 5? I know Volvo, VW and Audi did. A 5 is kinda dumb, it's an oddball firing order and difficult to balance. To long to fit in a hole where a 4 is supposed to be and weaker than a 6.

And... congratulations, you pulled one out I'm not familiar with, 7551s?

I wish I had a source of vintage military avionics for cheap.

The 300 series diesels, 124 body were all 5 cyl. Some were turbo. I had one that I put 435K on in a little over 10 years. I sold it to aircraft mechanic and told him "This motor is wore out mister, you can't make it between fillups without adding a quart of oil" he said "I'll rebuild it next summer". He called me a week later and said; "yer right, I gotta rebuild NOW!"

Oh, those. Betcha wish you kept it... you could Barnum it off for $10k to some hippy from Mendocino for a biodiesel conversion!

The 7551 is a mobile communications tube intended for plate modulated AM transmitters. It's robust to say the least. With a 13V filament I doubt it ever went into military aircraft so I think it's for land based use. A lot of the NOS stock out there is branded Motorola. That tells you something. I want to try a quad because it's a much lower impedence tube than EL-84s and NOS RCA's are $6 apiece. Maybe it'll sound like a 6L6 type rather than like most 9 pin minis? I dunno. If it's crap I can convert over to EL-84s without much hassle. New OT though. The OT I had built will work with a pair of 6L6s into 8/16/32 ohms.

4x 7551 plus at least three or more preamp tubes? Hope that chassis is big enough.

[ted01]

Skyboltone,

I am currently trying this. I have just about completed the PS, which is built on it's own chassis. Hopefully this next week I'll be able to build one or two preamp modules. Then I intend to build a power amp section. All of these will be on seperate chassis. I'm using shrouded banana jacks/plugs to carry the B+, ground and filament voltages to each module. I will, at least initially, be using BNC connectors that are isolated from the chassis to carry the signal from one chassis to the next (and I'll lift the ground on one end to avoid a ground loop). When I'm done, I'm hoping that I'll be able to "plug and play" different modules together to see what the effect is. The first set of preamp modules will be based on the "1997 Humble Overdraft specimen, rev 1.2" schematic. I have already built a clone based on that circuit, so this will give me a sort of control group to compare with. I am uncertain about using the BNC connectors and RG58 cables to connect the chassis's together (what the heck is plural for chassis???). Hopefully the coaxial cable won't add a bunch of capacitance and such that will ruin the tone. At least this way, I can compare to the all-in-one-chassis amp I already have. I used those safety banana jacks and plugs as I am mightily paranoid about casually running a mess of wires around with such high voltages. I'm thinking I should use test probe wire rated at 1KV for the high voltage interconnects and still keeping hands and feet well away from the wires just to be on the safe side.

I'm also wondering if it might be a good idea to split the value of the coupling caps, putting one (half value) on the output side of one chassis and the other (half value) on the input side of another chassis. That way I'm blocking DC before it hits the BNC, but trying to adhere to the "best practice" of having the coupling cap as close to the grid.

Any thoughts?

Ted

[skyboltone]

Sounds good Ted:
I haven't started yet do to family commitments and general lack of motivation. I would shy away from RG-58. Most of that is pretty high capacitance stuff. There is alot of RG-174/178 around that is less than half as capacitive. Also, I think Brandon at bludotone.com has some ultra low cap stuff he's selling.

Yes to the test probe wire. Use red for the positive and black for neg/ground and those colors for NOTHING else. I'm not sure about the BNCs grounded at both ends is a problem unless it's a problem? If the signal goes from one chassis to the next through one wire only are ground currents going to circulate? I dunno. Cross that bridge one at a time I guess.

I see my project being in a long 24" strip chassis that I'll make myself. I'll only use shielded when I have to. It will have a pre amp only power supply. I'll always keep the power section separate. As I mentioned before I'm not trying to make a convertible; just a piece of test gear.

Keep us posted.
Dan