Getting wacky with top caps

[deafen]

I've been idly playing with the idea of building a guitar amp with all top-cap tubes (with two exceptions), the weirder the better,. To that end, I've bought (cheap) a Hammond AO-14 chassis, which has seven octal sockets and one mini 7-pin, a power transformer that puts out 365/0/365 and at least 2.5A@6.3V, and an OT for two 6V6s. I also picked up a 10A@2.5V filament transformer for a song.

I'm open to suggestions, and I'll have a couple questions along the way. Designing this is a project to keep me busy when I'm not able to actually be soldering on my other stuff, so I'm in no real hurry. I'd like to end up with something that can go from a nice warm clean to reasonably raunchy, but not looking for Mesa-style high gain. Probably will voice it closer to early Marshall than Fender - I've got that well-covered with other amps - but I'm going to need to experiment. There will be a master volume, but I haven't settled on what kind or where.

Tube selection criteria:

• Must have top cap if at all possible. Preferably two!
• Must be octal (or use a socket I can easily retrofit, like medium 4-pin)
• Must not be ridiculously expensive.
• Must be as cool looking as possible.

Here's what I ended up with on my first pass:

• Rectifier: 2x3B28 in full-wave (thus the 2.5V traffo)
• Power: 1x815 in push-pull (8K output impedance matches the donor OT)
• PI: 6J6 in paraphase in the 7-pin slot (exception #1 b/c I can't find any mini 7-pin tubes with a cap)
• Preamp: Some combination of 3 of the following:
  • 2C22 (two caps! but only 20 mu)
  • 6F8G (dual 20 mu but only one cap)
  • 6F5G (single 100-mu but very expensive!)
  • 6S9D (single 100-mu, cheap, wacky looking)
  • 6B6G (100 mu, I would ignore the diodes)
  • 6S8GT (100 mu, I would ignore all three diodes)
  • 6T7G (62 mu, ignore the diodes, only 175mA heater current)
  • EF36 or EF37 (high gain pentodes, EF37 low microphonics and AWESOME red coating)
• OD3 in the last spot to regulate the screen supply (exception #2 because it looks badass)

Questions:

• Do any of these choices look like a terrible idea? I know the 3B28s are absurd overkill for the 500V I expect to run, but they look so cool!
• Can I get enough gain out of three 20-mu triodes to do a proper preamp *and* TS recovery?
• If not, I assume I'd want one of the 100-mu triodes at the input.
• Alternatively, should I consider doing something out of the box for gain like running a GMI-6 (dual 7.5W tetrode) triode-strapped in the preamp? Or an 807?
• Are there *any* mini 7-pin tubes with a top cap? I know it sounds ridiculous, but still!
• I obviously only want the OD3 for looks. Is there something equally as cool-looking that has a cap?

Now if only the GU-81M didn't need 140W just to light the filament ... ��

 

[Printer2]

6K5G, 6Q7G

 

[Tubelab_com]

I built a HiFi amp many years ago, that I called the multi headed alien. It used a pair of 7193's which is like a 2C22, half of a 6SN7 with two caps, and an 815 per channel. The 815 is a pair of 2E26's inside a fat bottle with two caps. I used 6600 ohm OPT's because I had lots of them. It looked neat and worked good, but I wound up giving it away.

I have collected most of the parts for MetallicAmp, all tube, no glass. This will be a guitar amp built with all metal vacuum tubes. The idea is to build something that could have been built 70+ years ago, using only parts that were available before I was born (1952). I'm still working out the channel switching details.

Relays are an option, and a likely choice for some circuits, but I can't find a verifiable date as to when CDS photoresistors became available.

 

[deafen]

6K5G, 6Q7G

Ooh, those look like good candidates. Thanks!

 

[deafen]

I built a HiFi amp many years ago, that I called the multi headed alien. It used a pair of 7193's which is like a 2C22, half of a 6SN7 with two caps, and an 815 per channel. The 815 is a pair of 2E26's inside a fat bottle with two caps. I used 6600 ohm OPT's because I had lots of them. It looked neat and worked good, but I wound up giving it away.

Sounds very cool! I love the 2C22/7193, although I find it a little annoying that the only octal triode with two caps is a single, while the only double triode I've found with a cap (6F8G) just has the one. There are plenty of dual output tubes with external electrodes (815, 829, etc.), but nothing for the preamp.

I have collected most of the parts for MetallicAmp, all tube, no glass. This will be a guitar amp built with all metal vacuum tubes.

The donor chassis will arrive with five metal tubes: two 6V6, two 6SJ7, and a 6SC7. That's a whole single-channel guitar amp right there. Hmm ...

The idea is to build something that could have been built 70+ years ago, using only parts that were available before I was born (1952). I'm still working out the channel switching details.

Relays are an option, and a likely choice for some circuits, but I can't find a verifiable date as to when CDS photoresistors became available.

Maybe an HVAC contactor? I like the idea of a big heavy mechanical *THUNK* when you switch channels.

 

[Printer2]

Maybe an HVAC contactor? I like the idea of a big heavy mechanical *THUNK* when you switch channels.

"The solo starts now!!"






I have enough parts for a metal tube amp, also a coke bottle one with 6F6G's for output. Maybe next year.

 

[Tubelab_com]

Maybe an HVAC contactor?.....

THWACK!

"The solo starts now!!"

I would prefer something more subtle.

If LDR's existed in the early 50's, I would prefer to use them. This way a foot pedal and some pilot light bulbs could be used to morph from one channel to the other. If not, I will play with some 6SK7's to make VCA's.

I was at a hamfest swap meet several years ago when an acquaintance tried to sell me several boxes of old, mostly used vacuum tubes. I said I wasn't interested, and that he should sell them individually for $1 each from his table. At the end of the show he gave me all the tubes that didn't sell. There was one large box full of metal tubes, including 8 X 6V6's 2 X 6L6's and many other old radio tubes. They have been calling my name for a few years, so I plugged the 6V6's into an amp, 2 at a time. All work great....The 6L6's....alive, but not so great.

 

[thoglette]

6BG6? top-capped 6L6 in an octal socket

Or 807 or 1625 ..a topcap 6L6 in a 7pin socket and (used to be) dirt cheap

 

[deafen]

Starting to zero in on a topology and tube complement. I'm thinking fairly Marshall-esque, but the gain available in the tubes that I like will result in a lower-gain result, as far as I can tell. I'm pretty well settled on using an EF37A at the input, because of the cool red look and easy configuration for high gain. Oh, and I'm pretty set on fixed bias for the 815.

Socket-wise, I'm limited to 5 octal (or equivalent-size) and one B7G.

I've got a couple options in mind. First, one that maximizes gain and tube variety:

EF37A (mu=~100) -> Vol1 -> 6F5M (mu=100, cold clipper) -> 7193 (CF) -> TS -> Vol2 -> 6AB4 (mu=60) -> 6C8G (mu=48, LTP) -> PPIMV -> 815

Second, one that uses three 2-cap tubes at the expense of some gain, with a pre-PI MV:

EF37A -> Vol1 -> 7193 (mu=20, cold clipper) -> ½ 2C34 (CF) -> TS -> 7193 -> ½ 2C34 (mu=13) -> MV -> 6J6 (mu=38, LTP) -> 815

The Marshall 2203 preamp has an additional gain stage before the CF, but it also attenuates between stages. There just aren't many octal dual-triodes with caps - the 6C8G is the highest gain at 48. But to be honest, I'm not confident enough in my design and layout skills to do any higher gain than these anyway.

So ... do you see anything glaringly dumb about these choices? I know I'll find problems as I design the actual circuit, but do I at least have a workable starting point?

 

[deafen]

6BG6? top-capped 6L6 in an octal socket

Or 807 or 1625 ..a topcap 6L6 in a 7pin socket and (used to be) dirt cheap

I like those tubes a lot. Is there a way to use them in a preamp that makes sense? (I'm pretty solid on using an 815 for power.)

 

[thoglette]

I like those tubes a lot. Is there a way to use them in a preamp that makes sense? (I'm pretty solid on using an 815 for power.)

Not really, they're also nominal 25W pentodes/tetrodes.

Have a look at old radio circuits for topcapped tubes: the downside is that they are (mostly) single devices (only one top cap in most cases)

 

[Tubelab_com]

If you need or want another gain stage you can always replace the CF with a hidden mosfet. A couple of my "tube" guitar amps use mosfets for active loads to increase gain, source followers, and even split load PI's.

Why, because the leave more room for tubes, and in these places, they work better than a tube without affecting the sonics. Obviously these are not options where "period correctness" is desired as with my MetallicAmp.

Both of these are quite rare. The skinny tube is actually a 9 pin with a top cap, the EL804. The other is a 6BD5GT, a black plate 6L6 type with its wings clipped and stuffed into a 6V6 sized piece of glass. The EL804 is a TV video output tube which can be used as a high gain amp, or a low powered output tube. I don't know if your 7 pin hole can be enlarged, or if this is worth investigating.

 

[deafen]

If you need or want another gain stage you can always replace the CF with a hidden mosfet.

That's definitely a thought. I'm not obsessed with purism on this one, but I'm don't want to get too complicated with it. For example, I was already considering a mosfet-buffered loop, sluckey's single-triode tremolo (More tremolo?), etc.

Both of these are quite rare. The skinny tube is actually a 9 pin with a top cap, the EL804. The other is a 6BD5GT, a black plate 6L6 type with its wings clipped and stuffed into a 6V6 sized piece of glass. The EL804 is a TV video output tube which can be used as a high gain amp, or a low powered output tube. I don't know if your 7 pin hole can be enlarged, or if this is worth investigating.

If I could find more than one or two of those EL804s, I'd consider it, but they seem to be super rare. Rarer even that the other oddity I've added to the build ... I was able to get ahold of a few HY615s inexpensively, and they're even weirder-looking than the 7193, with reasonably similar specs, so I'm planning on using them instead. So here's where I'm at now, if I use a mosfet source follower ...

• EF37-A
• Gain control
• 1/2 6C8G cold clipper
• 2x HY615 in cascode
• Attenuator
• Mosfet source follower
• Tone stack
• 1/2 6C8G
• Buffered loop
• 6J6 LTP
• Lar-Mar PPIMV
• 815

Now here's the fun part - there are several 11/16" holes in already in the chassis where there used to be some sort of physical pedal attachment ... so I could conceivably add as many as four 7-pin tubes without having to punch any new holes. Definitely considering an 0A2 or two to regulate the 815's screen supply(ies), and a 6AV6 for the aforementioned tremolo circuit.

Do I fear that this is turning in Homer Simpson's dream car? Yup. Between the big xenon rectifiers, all the top caps, the red-coated input tube, and a couple glowing regulators ... it's a lot. Maybe I should back off? Hmm.

Nah, screw it. Baroque is kind of my thing.

 

[Tubelab_com]

I was able to get ahold of a few HY615s inexpensively

I had a box full of those. I sold them to some ham radio guys quite some time ago. They are basically an acorn tube triode guts stuffed into a tiny octal bottle with two top caps.

Nah, screw it. Baroque is kind of my thing.

Baroque meets steampunk?

 

[deafen]

Baroque meets steampunk?

My future steampunk build will be a VT-127A (got two of 'em, but I can't afford enough transformer to light two 5V@10.5A heaters ) and a couple 8025s, maybe with a couple 717As and the weirdest rectifier(s) I can find.

It's kind of a shame that I won't let mercury tubes in my house, because I love the 866A. But between the big rambunctious dog and the 10- and 13-yo kids, that's asking for a hazmat emergency ...

 

[thoglette]

But between the big rambunctious dog and the 10- and 13-yo kids, that's asking for a hazmat emergency ...

Well, there's always the 833 (two top caps and a B+ over 2KV)

 

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First pass at a power amp design. Haven't done the math on much of anything, but this is the general topology I have in mind

• 6J6-based LTP. Probably not a TON of gain here, maybe 20 on each side? Shared cathode saves a jumper wire!
• Frondelli master volume (Amp Mods). Fewer components than the Lar-Mar. Datasheet for the 815 doesn't list a maximum grid leak, but existing circuits seem to like 220K, so I went with 250K pots.
• Fixed bias. Will sketch up the bias supply when I do the rest of the power supply.
• 0A2-based screen regulation. Because I can. I don't know if 150V is enough; if not, I might use series pairs of 0B2s to get 218V. Like I said, I've got lots of space for 7-pin tubes, and since these don't cost me any heater current, it's a win all around.
• I'm getting about 370 Vrms per leg on the PT. That should yield in the neighborhood of 500V after accounting for the 3B28s, and I'll see what it does under load and do what I need to get it down to 450V.

Between the stiff rectifiers and regulated screens, I would expect this to be a pretty tight, clean power amp. Which is fine, because I'm going to juice up the gain in the preamp.

Anything here seem wrong to anybody?

 

[mozz]

75 triodes have top caps and gain of 100. Might howl though, i have abunch for a futre project. I second the 6bg6 for a output tube.

 

[Tubelab_com]

Well, there's always the 833 (two top caps and a B+ over 2KV)

You know the Dumm Blonde One just had to "test" one of those. I set up this little test amp to characterize an OPT that I had custom made for the 833A. This only existed for a week back in 2005. Tube sockets for the 833A were not common, so I improvised....note the Vise Grips for the filament pins (10 volts at 10 amps) and hose clamps for the plate and grid.

That large metal object on the left is a power supply out of a Motorola ambulance transmitter, I tapped into its 1500 Volt at 1/2 Amp circuit with a suitable Radio Shack clip lead, the yellow one leading to a real "death cap" from a medical defibrillator (16 uF at 7.5 KV). The two chassis with wood bases in the center are my 845 SE amp. It provided drive for the big tube. The test OPT is on the right and the 500 watt load resistor is in front of it. The analog current meter behind the glowing tube shows 270 mA of idle current.

Sadly the OPT was not quite good enough for HiFi, and after a week of 'testing" the test setup was dismantled, never to see power again.

 

[Tubelab_com]

So, there is a powerful, but not quite HiFi test amp on my bench.....What do I do with it? After about 3 milliseconds of careful thought, I dug out my ADA MP-1, and a box with some speakers that could eat everything I could throw at them, plug in a Strat, dial up a preset on the MP-1 that I call "Jimi" and proceed to annoy the whole block. This could be heard two blocks away.

The guy running the pirate radio station in his garage down the street used to run a show called the "g-spot." Cute name for guitar lessons, where he would ask the listeners to play along. I took that invitation literally, put the speakers in the open window and set the controls for the heart of the sun.

This thing had 200 watts of single ended insanity. It was also deadly, one wrong move and you could be cooked. That's why there was a 1/4 inch thick sheet of LEXAN between me and it.

I included a few pictures of me abusing it. A week of my abuse, and the worse thing that happened was a tripped bench breaker twice when I got a howling feedback going.

The test amp was dismantled about 15 years ago, but I still the tubes, OPT and power transformer, but I never found the time and motivation to put it back together.