Thunderbolt circuit with KT120

[Hondo]

Hi all,

Anyone available to do a little schematic design work? I play bass, and I'm looking to buff the Supro Thunderbolt, using KT120 tubes, Fender Bassman 135W transformers, V1B used to allow for switchable single/parallel triode first gain stage input, switchable tube/silicon rectifier, and a few other little odds and ends. Most of the "little" stuff is pulled from RobRob's site. Thanks in advance.

/edit
None of the other little things include adding reverb or tremolo

[Phil_S]

So, are you building a Thunder Boltman? (Sorry, couldn't resist.) Carry on.

[Hondo]

Resist? Yes... At 1M... Turn about's fair play, my friend

[thetragichero]

do you really need a switchable rectifier? you'd need at least two tube rectifiers for the kind of current, and I'm not certain a "saggy" power supply would be all that desirable for a bass amp (if it is, you'd probably be better served with a big ole resistor before the first b+ node). bassman 135 power transformer also doesn't have a 5v winding so you'd need an additional 5v filament transformer

why kt120 vs the more commonly available (and likely cheaper) kt88/6550? right off the bat the bassman 135 power transformer only applies 7a on the 6.3v winding so you'd need an additional 6.3v filament transformer to run a quad of kt120 and even a single 12ax7
physically I'm not certain kt120 tubes would fit. I'd imagine they're bigger than kt88 and this is how close a quad of 6550s are too each other and the power transformer in a pa135 I'm converting (have a 6.3v filament transformer for the preamp tubes)



very close and that's without the cages (hadn't come in yet)

[Hondo]

I enjoy the sag of the tube rectifiers I already play through and I'd like to keep sag as an option. At the moment I'm looking at solid state rectifier with switchable sag resistors to emulate sag, and bypass it, as needed.

I'm not sure where you heard quad KT120? The Tbolt is a dual 6L6. I'd prefer to go dual KT120 (or even KT150) rather going quad 6L6, 6550, or KT88. But I would consider dual KT88. It's at the lower end of what I was looking for, but it is in range.

The Bassman 135 transformers are just what someone had recommended to me. From what I see, the KT120 is a drop in replacement for the KT88, so that BM135 transformers wouldn't work as is regardless. So it's either artificial tap, a more suitable transformer, or a custom wind.

I'm not talking about recycling an existing chassis and replacing the tubes, but building something new. From that stand pint I don't think tube spacing will be quite the same issue. But it is a good point and I'll need a suitable chassis.

Right now the biggest issue seems to be a suitable transformer.

[Stevem]

Just note that the up grade in tubes does not guarantee a gain / increase in clean head room if the power supply does not have a good amount of reserve current once peak wattage is reached in terms of what the output transformer can do!

In fact gargantuan wattage output tubes need to be idled at a higher current many times to equal the lower volume level punch of lesser tubes like a 6L6 / 7581 type , this increased idle then pulls back on the amps peak wattage.

Play around with Ohms law for a while in calculating wattage and you will see how different levels of I and E will produce the same wattage.

The power supply and output stage / tubes are most heavily loaded when the amp is pumping out only 60 to maybe 70 percent of its max wattage!

[sluckey]

You may want to consider a PT for the Sunn 200s...

https://el34world.com/charts/Schematics ... _200s2.pdf

Triode sells them...

http://www.triodeelectronics.com/mq782.html

[Hondo]

The Triodes look promising, thanks.

I've also been told to consider making a hybrid, with solid state power section and rectifier.

[sluckey]

I've also been told to consider making a hybrid, with solid state power section and rectifier.

Do you mean using a hybrid bridge rectifier? You'll still need a separate 5v PT if you plan to use that Fender Bassman 135W transformer.

[didit]

Hello -

Thunderbolt shares some similarity to Ampeg B15. If you search across various threads here on TAG you will find others have been at this before, looking to build with more power with familiar cathode-biased output stage driven by paraphase.

Suggestion to use big Dynaco transformers is good. Hammond Mfg has had listed transformers that were used for Traynor YBA-3 Custom Special (150W) and perhaps even the Super Custom Special (250W). Don't see them listed on the current website but perhaps can be ordered. Both were/are suitable high voltage+ current beasts.

Best .. Ian

[thetragichero]

antique electronic supply has the traynor custom special transformers, although they're quite pricy: https://www.tubesandmore.com/transforme ... cial-yba-3

[didit]

antique electronic supply has the traynor custom special transformers, although they're quite pricy: https://www.tubesandmore.com/transforme ... cial-yba-3

Yes. Those are listed in Hammond's online catalog. Far from expert on all amp variations & generations of Traynor YBA. That said, there was apparently 150W version of CS plus the awesome 250W SCS with a quad 6KG6/EL-509 output stage. Fairly familiar as SCS was popular regularly available backline rental from a music store in my home town back in the '70s. It was a fresh transformer set of that design from Hammond I'd had in mind.

Best .. Ian

[didit]

[...]

[Hondo]

@Sluckey -- Sorry, I meant keeping the tube pre-amp section and going solid state rectifier and power section.

I don't have a Bassman, so there's no concern at all about not using it, especially if it's not really appropriate here. It was just what someone had suggested to me. Originally we were thinking 4-6 6L6, and I think it would have been fine then.

@Didit -- thanks for the Traynor suggestion. That looks promising. I'll look for the B15 mods, too.

@thetragichero -- thanks for the Dynaco link (I'm call them that whether they actually are because it's a great name lol).

I don't think I get to complain about price if I'm trying to make a custom amp. So far looks like ~$350 for tramsformers and ~$250 for tubes (2x KT120, 2x 12AX7, 1x 12AU7 {paraphase inverter}, and 2x 5U4). Could be worse.

This might be a stupid question... Can I make this as a standalone tube pre-amp, and use an OTS solid state power amp?

[Stevem]

Lets go back to square one here!

Whatever you choose to build, will it be used manly for home usage as in practice, recording or band practice at home , or will it need to cover live gigs also with a hard hitting drummer and no sound reinforcement?