Convert Tube to FET Circuit Back to Tubes?

[Puguglybonehead]

Okay, the story is, I was wanting to build a battery-operated amp for busking. I already had the power amp section (a ready-made TDA2003-based module from Velleman) and a decent speaker sorted. I wanted this to sound like my favorite tube amp I have here at home. I figured one of the many tubes-to-FETs amp-based pedal circuits would work best for a preamp. Swear to god, I'm not bragging, but the pedal that sounds closest to my #1 amp is the Steel String Singer. Seriously. My amp is a Supro-based, 6973 powered amp that I had custom made for me and then later had my local amp tech re-voice it and customize it further. The Steel String Singer is the closest in sound that I've heard. I bought two versions of the SSS pedal in kit form. Both are pretty much the identical circuit. Now, maybe it's been lack of sleep, social deprivation, poor nutrition or something, but I managed to render both boards non-working. One simply made no sound. I can probably come back to it one day. The other, started glowing when I powered it up and then I thought, "Hey! Wait a minute. There's no LED on that board!" Yup. shoulda' had another coffee first.

So, nuff said, I'm starting from scratch now on that preamp. I'd rather not fry any more FETs. I do have a wealth of unused tubes sitting around. Many of them are direct-heated type, like 6418s, which get used in microphone preamps, and 5672s, which are the tubes used in the Pentaboost and Pentadriver. I'm wondering if they could be wired as triodes rather than pentodes? (not a big fan of the pentode preamp sound) I could run the plates at 12V, which is what I'd be running the power amp at anyways. I can just use a C cell for the filament/cathode sections.

I can't find a schematic for the real Steel String Singer. Honestly, those amps have so many controls and switches and doodads on them that I wouldn't find most of it of use. I can find the pedal schematics, no problem. The pedal kits that I bought had nice, simple options: Gain, Tone, Volume. Nice and simple. My #1 amp is that simple. Bright and Normal inputs, a bit of a treble bleed on one and what seems to be a Big Muff based tone control and a volume control. (I'll have to ask my amp tech what exactly he did. All I know is it sounds amazing)

Anyone have any idea how I could convert a FET based circuit to using direct-heated tubes? I would be using 4 of them in this circuit. (I guess?) I know that Gate = Grid, Drain = Anode, Source = Cathode. To run in triode mode, do I tie one grid to the anode with 5672s? Do I tie pin 1 and 2 together? I'm guessing that the constraints on the cathodes in the 5672s will be a problem. They have to remain fixed at 1.25V or they fry. Am I better off starting completely from scratch with these type of tubes?

[GibsonGM]

Just from a quick read, tho a noble wish to undertake such a pursuit...my take is that the designer of the SSS pedal took note of what was great about the tube circuit(s) they wanted to emulate.    They built the SSS pedal around such, so it was now one generation removed from the tube stuff.

Along we come, thinking we'll now use tubes to re-create the once removed tube emulation thing, LOL....I'm not suggesting it can't be done, but maybe it would be a simpler process to do what they did...what was great about the original thing they were trying to sound like?   Then design your tube pre around that ("...start from scratch").   If you really want to give it a go, first thing seems to be getting the SSS pedal schematic.   Maybe simulate the stages in LT Spice or what have you, and then build the tube pre around what each stage is doing gain-wise and with respect to sound shaping, impedances and so on.  It MAY come close...or may be a disappointment.   Since it's so easy to tweak tube gain stages, I do think you'll get something you'll like in the end!

Yes, you can wire up pentodes like triodes and can find articles on this on the net (tho some may think that's a 'waste', but if you have them and want to use them....).  They are certainly less likely to be noisy that way but you get much less gain (not always a bad thing).  That cathode 1.25V requirement does sound like it will constrain you in terms of biasing.   Look up some 'submini tube preamp' stuff, and dive in!   Make a project of it and post so we can follow!  I'd like to do a sub-mini myself.

[teemuk]

What Steel String Singer kit are we actually even talking about?

I'm currently looking at one that is simple two SRPP stages in cascade and a Big Muff -style tone control. Needless to say, it has nothing even remotedly common to a Dumble amp of same name.

The runoffgroove's Umble design is somewhat closer to real Dumble circuit, but it's difficult to build those consistently. It may also be something that actually does not match your amp's characteristics, at least according to your description about Supro circuitry and Big Muff tone controls.

If you want to emulate those SRPP stages with pentodes then you don't want to convert them to triode mode. Pentodes as is have features closer to SRPP circuit characteristics than triodes. As is, the SRPP stages do nothing fascinating except provide a circuitry that is less dependent of (highly varying) FET characteristics.

[ashcat_lt]

You HAVE the tube amp.  The preamp circuit is literally right there.  Why Xerox a Xerox?

[Puguglybonehead]

I can't exactly clone the circuit in my amp as it's a high voltage circuit, not battery tubes. Also, it's a point-to-point rat's nest inside. I'll leave poking around in there to the pros. Going to start from scratch mostly I guess. I've been looking at the Pentadriver and also at the Oatley preamp kit I built a few years ago. Big Muff tone control for sure. I have 6418s, 5672s, 5678s and even some 5971 triodes, (even some 6977s if I want a light show) so lots of submini battery tubes to choose from here. I'll start a new thread on this when I've got something actually working on the breadboard.

[GibsonGM]

I suggest you leave room for tone control experimentation (daughter board, or do it on the BB before assembly, etc) so you can 'audition' more than one. You may not like the BMP tone control that much...I don't, but of course some do think it's great, too, and you should try it.   Not 'useful' enough for me, I set them 1 place and never touch it again; sounds like @ss either side of where I set them.   Some of the mods to them are ok ('presence' control mod and so on).

A BMT type is, to me, far more versatile...just my 2 cents.

[Puguglybonehead]

I suggest you leave room for tone control experimentation (daughter board, or do it on the BB before assembly, etc) so you can 'audition' more than one. You may not like the BMP tone control that much...I don't, but of course some do think it's great, too, and you should try it.   Not 'useful' enough for me, I set them 1 place and never touch it again; sounds like @ss either side of where I set them.   Some of the mods to them are ok ('presence' control mod and so on).

A BMT type is, to me, far more versatile...just my 2 cents.

Thanks. That sounds like wise advice. I looked at the Beavis article with the improved Big Muff tone control. I will probably breadboard this through a few versions. I do have an assortment of tubes to audition, as well as a few ideas to try out. Might try different plate voltages. I hate having too many knobs to deal with though. I used to have to keep a template of all the 'right' settings on my old Traynor Mk 3. Coming from an old-school punk rock background, I'm accustomed to chaos in live situations and prefer to keep the equipment as simple as possible. My favorite amp currently has only 'Normal' and 'Bright' inputs and simple 'Tone' and 'Volume' controls. For many years my only pedal was a simple boost, (modified into sort of a high-mid boost) that and the switch for the amp tremolo. If I can make something that I like the sound of with as few parts as possible then that's a win.

[GibsonGM]

I totally understand that!  I usually go gtr > wah > booster > amp.   And don't change much on the amp after a good tone is dialed in    

I just put together a practice amp with...a vol, and that's it, lol.    For what you're doing, I am picturing a bass cut, treble cut, and volume.    Perhaps with a 'pre-gain' after the 1st stage, depending on the topology you come up with.  There's a ton of stuff to look at out there for ideas!