Can I use this PT for something?

[Phil_S]

...With a big fat preamp that has too many stages....
Goin' for gain, if I haven't been clear on that one.

Why not build an Express preamp? IIRC the SE Express has already been done and you. Look around for it.

I feel awful taking peoples hard work they've done, and cashing in...

IMO, they wouldn't have published it if they didn't mean for it to be used. They put it in the public domain knowing others might use it. There is nothing to feel bad about.

I love you guys.

OK, let's not get carried away. Quit winking at me. It's creepy.

+1 on using a 6V6. You might have enough for a 6L6, but I think it will be close. You are probably pushing too far for EL34, KT88, etc.

As has already been said, a SE amp is not very efficient. Pushing that PT to the limit may not be particularly wise.

Your question about using it just because you have it...well, in the end that's up to you, but it is as good a reason as any to build an amp. There is everything right about doing it and nothing wrong that I can see.

I don't know what sort of tone you are really looking for. You may want to rethink your power amp section. The tube I always think of when you need low current draw but have high plate voltage is the 6SN7. This is a dual triode that is good for about 5W in push pull and a single tube (two triodes) will want to see an OT primary of about 22K (though they are very forgiving). If you use 3 of them, you get to about 15W with an 6K to 8K OT very nicely. Don't look for power amp distortion. You'll need to get all your dirt from the preamp. You will get to experiment some with the PI, as well. I built one amp using 2x 6SN7 and used a concertina PI. The LTP might be too pushy for these triodes which is why I said you've got to do all the tone shaping in the preamp. I might look here for the OT20PP http://www.musicalpowersupplies.com/products/ though I suppose there will be lots of choices in this power range and primary impedance. Whether you do something like this depends on how big your sense of adventure is and be prepared to end up with something that may be different from what you are going for.

[Meat&Beer]

Wow, so many choices and things to ponder!!

Here's where my head is at; The Raven CKD - (The amp I've been at for almost 3 years, it has its own thread) I need to make a bit of progress on that before I move forward. I think I'll update that thread and see if you guys can help me with a really baffling problem that myself and amp tech friend can't seem to thwart. It's time...

Hopefully that'll be semi quick... (Fingers crossed!)

This thing.. I'm really liking the idea of a SE ghetto UL. If using a KT88 means pushing this OT to it's limit, that's exactly what I do not want to do. I checked out some champ circuits with a 6V6, that looks promising to me.

The PP with 6SN7's also has its benefits, but I think I had my heart set on G-UL the moment Matt mentioned it. I am completely content with deriving all the dirt from the pre as opposed to the PA, as I tend to prefer this anyway. To my ears, most power tubes break up in a way that I'm not particularly fond of, so I tend not to really crank an amp to the point where it's heard too much.

The limitations are there for the power side with this OT, I wonder how limited it makes my preamp tube count? I'm hoping for 3x12AX7; 2x12AX7 + 1x12AU7; 2x12AX7 + 1x6AV6 + 1x6C4 etc... End result is 5-6 stages moderately pushed with a couple switchable cathode caps, otherwise capable of lots of grit without them.

I know the filament winding(s) can handle these, but can I pull all that B+ for them out of the power supply? Assuming I go with a SE 6V6, or similar, that is.

Hey, thanks again for all of this discussion and kind direction.

Cheers!

[Phil_S]

For preamp tubes, I'd stick with 12AX7's, which will require maybe 2.5mA each triode give or take.

A 12AU7 will need around 10-11 mA per triode, making them too "expensive" and the same is true of the 12AT7.

The 6AV6 is same as 12AX7, but you only get half the number of triodes while it sucks up the same 0.3A of filament supply so I'd avoid those.

The 6C4 is one triode equal to the 12AU7, so, again, too much plate current, but the filament supply is truly half!

12AY7 (less gain, lower Ra spec) use 3mA per triode, maybe making the list of possibilities. The 5751 is good for about 1mA per triode.

The only low current pentode I can think of is EL95. I seriously doubt you can operate those at 380V, but a pair will only sip less than 60mA. At higher plate voltage, probably less. A pair is probably good for 5W.

Lots to think about.

[gingertube]

6X5 or EZ80 or EZ81 rectifier.
6V6 Single ended output - rip of a Champ or use this -
http://www.ax84.com/static/corepoweramp ... ematic.pdf
Choose your own preamp from AX84 to suit your preferences
http://www.ax84.com/corepreamps.html
Cheers,
Ian

[matt h]

(deleted)

[Phil_S]

Matt H is right. We are not listening. That PT will make a nice PP 6V6 amp with FW and choke input. If that PT is primary rated for 115VAC, I'm thinking the secondary will be ~4% above that -- 390-ish. Limited filament supply on that PT.

Are there more possibilities if a separate filament transformer is added? With a tube rectifier, you can probably push B+ down to 350-ish, maybe lower?

This also solves a "problem" for me. I've got a bone pile with a couple of nice chokes (Stancor 4H 250mA, Vox 20H 100mA) that are up to the task and a 750V CT 100mA transformer looking for a place to go. This is nearly the same problem as the OP. That old PT must be meant for 110V as the unloaded secondary (primary input 119VAC) is 854 and I'm guessing under load around 800. I'm seeing a bright future for that PT all of a sudden, maybe a pair of 6L6 as it's got lots of filament on it.

Matt, thank you for the wake up call!

[matt h]

(deleted)

[Leo_Gnardo]

Well (harumph!) I WAS listening and although a choke filter guitar amp is rarer than rocking horse turds, I understood the implications and passed along my views on excellent filtering necessary for single end amps - hey the hi fi guys use it - it's all over the SET (single ended triode) column at Audio Asylum. You DO need a sturdy choke, because an insufficient one will heat up and buzz physically. So meat&beer rock on with your design and keep that avatar which whets my appetite. I swear I put on a pound every time I see it.
YUM! Like your amp's gonna be.

[Meat&Beer]

Well this makes me very happy, indeed! So the GULSEKT-88 can actually be a thing! I was wondering how we got from ~115mA back down to the uber limited 75mA. Thought I'd missed something for a second...

So, this now has the ability to be a Ghetto UL SE with a KT-88, and three preamp tubes. I have four 5751's, I put two of them in a phono pre that I just completed breadboarding this weekend, and it sounds fantastic with zero noise. I can use the other two here, along with a 12AX7 which would bring my current draw to somewhere in the neighborhood of 109mA?

I had no idea 5751's pull less than half the current a 12AX7 does. Also didn't know a 12AU7 was so hungry... Guess I should look at my tube manual before I go spitting off about things so casually.

I'm still unsure about the whole rectification thing, as it never got answered. It may seem like such a simple thing to some, but I am still just learning all these neat wizard tricks.

Tube or solid state, given the "go ahead" that this amp can in fact be a GULSEKT-88?

Limited filament supply on that PT.

Do you mean there are limited options for filaments, or limited on what it can run? I thought 2.75A, 1.5A, and the 7.3v tap which is 1A sounded pretty stout?

rarer than rocking horse turds

First of all, that made me laugh very heartily, thanks ya goof!

You DO need a sturdy choke, because an insufficient one will heat up and buzz physically.

May I ask what we would deem "sturdy" enough? With a capability of ~115mA would we be looking for something around 150mA? 200? Another item of curiosity is the Henry unit of measurement. Something I need to read up on, and some equation would no doubt give me the answer.

...and keep that avatar which whets my appetite. I swear I put on a pound every time I see it.

I make these burgers on occasion... Everyone I feed them to swears they're the best darn cheeseburger on the planet! If you're interested, I'd be more than happy to supply a how-to in the garage talk section. They are a special thing, for sure!

Once I get just a bit more info on this & that, the can and cant do's, I'll draft up a crude schematic undoubtedly wrought with errors for everyone to scoff at.

Thanks guys!!

[matt h]

(deleted)

[cbass]

I've looked into doing a choke input with some PT s I have that are high voltage.I found that for the price of a choke that's big enough I could pretty much buy a new PT with the correct voltage.

[Phil_S]

There are a number of guitar amps using choke input. Several Fender black/silverface models including the Twin, and some Vox amps (AC-30?).

IMO, that 7.3V tap is going to be problematic. With modern line voltage, expect more like 7.6V. You'll need to drop that by about 1v for it to be useful as 7.3 or 7.6 is too high. This can usually be done with the correct value sandblock wirewound resistor on one leg, placed between the PT and the first heater connection (math required.)

For a suitable choke, I'd look to buy one on eBay. I'm no expert, but I'd think something between 4H and 20H would be fine.
http://www.ebay.com/itm/CHOKE-Power-5-H ... 3a87420c10
http://www.ebay.com/itm/New-125C1A-tube ... 5af5c30408

Weber sells a 9H 120mA choke for $12:
https://taweber.powweb.com/store/022699sch.jpg

[matt h]

(deleted)

[John_P_WI]

I have not read the whole thread, but personally I think you are in for more work than expected. Don't most choke input filters use specially designed chokes to handle the swing voltage / current, hence the term swing chokes? Most are big $$, big core, air gapped etc. Use a regular choke = let the smoke out. IMO for the money you would spend on choke, you could buy the PT you want.

Personally, IF you must use the Xformer, I would suggest connecting the two primaries together in SERIES and connect the two 6.3v heater taps in series. The transformer is now effectively wired for 230v in. Yeah, yeah you have to mind the phases and determine the proper wiring connections etc. Should be able to knock the 840v secondary down to 420 @ 115v in.

That's my old school suggestion.

[cbass]

I have not read the whole thread, but personally I think you are in for more work than expected. Don't most choke input filters use specially designed chokes to handle the swing voltage / current, hence the term swing chokes? Most are big $$, big core, air gapped etc. Use a regular choke = let the smoke out. IMO for the money you would spend on choke, you could buy the PT you want.

Personally, IF you must use the Xformer, I would suggest connecting the two primaries together in SERIES and connect the two 6.3v heater taps in series. The transformer is now effectively wired for 230v in. Yeah, yeah you have to mind the phases and determine the proper wiring connections etc. Should be able to knock the 840v secondary down to 420 @ 115v in.

That's my old school suggestion.

That's what I was getting at the choke has to be designed to use in for a choke input.that equals big heavy and expensive