One-Bottle Preamp

[khstudio]

Thanks Dave,
This topic is long & shoots all over the place... got lost :?


skipwave,
Hey I didn't realize the MI-69 was an input TRZ... Thanks.
I try it & see.

 

[skipwave]

[quote author="khstudio"]Hey I didn't realize the MI-69 was an input TRZ... Thanks.
I try it & see.[/quote]

The Sescom website is vague, and states +18 db Max Power Level Primary 600ohms. The implication is that the number is +18dbm.

The output of the One-Bottle could be higher than that, but you might like the sound of the transformer saturation. OTOH, the spec is for the primary, which you would be using as a secondary. I don't know how, if at all, that impacts the spec.

 

[khstudio]

Another good reason for the output option switch.

So right after the cap & pulldown resistor I can install my switch?

Skipwave, did you increase the output cap & to what value?

 

[Kingston]

I just made a cool discovery. :grin: I'm sure most experienced people here know about this already.

I did some A/B testing with and without the 220k tiedown resistor. There's is a subtle but audible difference with the OEP 4:1 transformer saturation. With the resistor the general output of the amp was cleaner, but slightly more boring. Without the resistor the saturation sounds a whole lot better, less rough, and it somehow seems to "contour" the sound in a good way. When the transformer isn't working near its limits the difference is very small, but still there. Does this have something to do with the 2.2uF cap (C4)? What will happen to the sound if I put a bigger cap there?

I don't understand the design well enough to explain why this happens, but the tiedown resistor definitely stays out. :thumb:

I'm tempted to start to play with the 330ohm vs. 1k plate resistor now.

 

[khstudio]

12AV7 @ Antique Electronic Supplies

ON SALE FOR $1.75 EA :shock:

Kevin

 

[NewYorkDave]

330ohm vs. 1k plate

That's the cathode resistor.

Raising the value of the cathode resistor will reduce the open-loop gain (thus rendering the negative feedback less effective) and increase the plate resistance, making the triode less capable of driving the transformer at high levels. I actually started with 1K when I began the design, but ended up with the values shown after a lot of careful tweaking/optimization/whatever ya wanna call it. But hey, go ahead and try it. It won't hurt anything.

The tiedown resistor is in shunt with the load and reduces the total load impedance seen by the tube. This will result in slightly higher distortion and slightly lower maximum output level. The value of the resistor was chosen to be high enough for minimal shunting effect but low enough to be effective as a tiedown for the fairly large output coupling cap. The tiedown is necessary if you're not using an output transformer but if a transformer is permanently installed there, there's no reason for it.

By all means, try a bigger output coupling cap if you have the room. If I build another one of these I'm going to use a bigger enclosure so I'll have enough room to use a 4.7uF/400V Solen. (I even noted on the schematic: "C4 may be higher value if space permits"). Your low-low bass will be extended by an octave although I don't know if you're going to hear it or not since most of the "improvement" will be in the subsonic region.

$1.75 EA

That's average street price. I don't think I've paid more than two bucks for an AV7. It's a good, inexpensive tube. That's why I use it.

 

[khstudio]

Dave, I'm amost there...

I asked you about a Stancore OT & you said it would work but I made a mistake, it's a Sescom input rated for +18dbm. Will this work & what are the downsides of using an input iron for output? If you think it's a bad move I'll just order an Edcor or something designed more for an OT.

I posted the specs earlier on the Sescom.

 

[NewYorkDave]

I think the answer is right there in the specs.

+18dBM rating tells us it's definitely a line-level transformer.

"LI, LO" means "Line Input or Line Output" (according to Sescom's page for the MI series).

So yeah, use it as a 15K:600 output transformer. It should work OK. The One-Bottle has a max output level of about +18dBM so you'll be within the transformer's ratings.

I get the feeling this might be your first tube project. Whether or not that's the case, please be careful. 300V can bite ya.

 

[khstudio]

[quote author="NewYorkDave"]

"LI, LO" means "Line Input or Line Output" (according to Sescom's page for the MI series).[/quote]

Cool, I didn't know that.

I get the feeling this might be your first tube project. Whether or not that's the case, please be careful. 300V can bite ya.

NO, over 10 guitar amps & 2 LA-2A's but never a mic pre, I'm getting better but I don't FULLY understand everything including transformers. I ask a lot of questions to learn & keep me out of trouble. I realize that tubes are forgiving but I also feel there is an optimum balance in a circuit & like to stick to the book unless I FULLY understand what I'm doing.

Thanks Dave :thumb:

 

[skipwave]

[quote author="khstudio"][quote author="NewYorkDave"]

"LI, LO" means "Line Input or Line Output" (according to Sescom's page for the MI series).[/quote]

Cool, I didn't know that.

[/quote]

Me neither. How much more deceptively named could a product be? LI means Line Input, but MI means Mic Input only when its shown in addition to the MI that is just the arbitrary prefix of the product line. Ugh. :roll:

 

[khstudio]

A few more questions:

1 - What's the best way to wire a switch for the input impedance selector?

1=common
2=150
3=600
- a friend of mine who is a good tech said I should just short pins 2-3 to get the 150 instead of running all them seperatly to a switch... he said it could cause problems or ringing. True or False?

2 - How do I make my filaments DC off the 6.3v?

 

[NewYorkDave]

1. Your friend is wrong. Google the phrase "shorted turns."

2. You won't get well-filtered 6VDC reliably from a 6.3VAC winding. Just go with AC heaters.

 

[khstudio]

Yea, I think I misunderstood what he was saying & wanted to second check. good thing... Thanks.

I'll stick with the 6.3v ac.

 

[NewYorkDave]

By the way, thanks for the tip about the sale at Antique. They're actually $1.70 each, which is a little better than usual. I just ordered twenty for $43.55 total--that works out to only $2.17 each, shipped, and I don't have to go through the eBone bidding nonsense.

 

[khstudio]

Dave, no problem.
I may need a little more help around here in the future... my friend, the tech guy that was helping/teaching me past away yesterday. I was going to finish my 1 bottle last night but couldn't. He was a one of a kind old head, loaded with knowledge... his name was Dave too.

I just started DIY 2 years ago and wasn't even close to being finished asking him questions. He would fix what I fucked up & show me how to make it better.

Thanks for the cool projects & being patient.

kevin

 

[Brown Note]

12AV7 @ Antique Electronic Supplies

ON SALE FOR $1.75 EA

FYI they're NOS GE's.


I'm still trying to work the bugs (layout issues) out of my first one bottle before I build a second one, and I'm wondering...

Which wires should be shielded?

and...

Is 15k:600 ok for the output?

Thanks for the cool projects & being patient.

Amen to that.

 

[skipwave]

[quote author="Brown Note"]Is 15k:600 ok for the output?[/quote]

Yep. 10k is minimum, higher is fine. I used 25k:500 for my output transformers and it sounds great. We talked about it somewhere in this labrynthian thread.

[quote author="Brown Note"]

Thanks for the cool projects & being patient.

Amen to that.[/quote]

Somebody shout "AMEN!"

I'm just trying to help out with fielding the questions so Dave can spend more time cooking up cool designs.

 

[NewYorkDave]

hey're NOS GE's

Please tell me they're not 5965s or some other 4-digit type being sold as an "equivalent." That thought occurred to me--after I'd placed the order, of course--because of the fact that NOS bulk packs of 5965s and similar computer tubes have been turning up on the market at low prices. Oh well... If 20 5965s show up here, I'm just gonna call 'em and say "that's not what I ordered..."

 

[khstudio]

I hope they're NOT too.

 

[CJ]

Usually Antique has more than one brand for each tube number.
Usually they ask you which one would you like.
If they do not ask, you ask them!

Cool place. I was rapping with one of the sales guys for a while, he says all kinds of famous freaks order there.
"Yeah, Neil Young just bought some 12AY7's for his deluxe the other day".

If you nebver buy from them, at least get their catalog.
It's like the Blue Book for tube prices.
Extremely handy. I look at it every day it seems.

And they update it as trends get crazy.

Worst tubes to get spiked in the last 5?

I would say

KT-66
6386
417-A
EL-34 Mullard
7189-A