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Lafayette 250a

GordonW said:
Is the 7199 directly coupled between the pentode plate and triode grid, as opposed to having a coupling cap between the two sections?

If so, it could be a biasing issue... causing the 6U8 to run the triode section way too far down or too far up in voltage...

Regards,
Gordon.
Click to expand...

It is indeed Gordon. Could you explain how that messes up the bias (in simple language for the slow kid).:D Do you think putting a cap in there would help?
 
The cap isolates the DC on the plate from the grid but couples the AC (signal). I have not looked at the schematic so I am only shooting from the hip. If I am way off, I apologize, it is late tonight!
 
Pretty similar

Ok,

I checked the two different Scott 222C schematics and sure enough, they rotate the pentode section just like you say.

All the values stay the same on the Scott.

What is different are some of the resister values and the layout is a little different from the way yours is.

But only on the pentode section!

I kinda run out of steam there and I don't know if that means it is a different type of circuit or just set up a lil different.

http://hhscott.com/pdf/222C_LK48B.pdf <-- here is is set up for the 6GH8/6U8 tubes

I am simple, I would just rewire it to look like that and never know why it worked :)
 
I agree, check bias. But you may already have the components in place. Four separate diodes will work as well as a bridge. Let's face it, years ago, they didn't have a bridge and they used the individual diodes. Measure your voltages and see if they are OK and go from there, sound OK?
 
jaymanaa said:
It is indeed Gordon. Could you explain how that messes up the bias (in simple language for the slow kid).:D Do you think putting a cap in there would help?
Click to expand...

When they're directly coupled, the plate voltage of the pentode side is directly put into the grid of the triode. This will act to bias the triode (the phase inverter).

If you do it RIGHT, the triode will be "in the middle"... with about an equal voltage split in the two halves (plate and cathode circuits)- about one-quarter of the B+ value, dropped through the cathode and anode resistors, at idle. This allows symmetric voltage swing, through the phase inverter.

However, if the pentode is way off in plate voltage (due to its OWN bias), then the triode may be "jammed all the way to one side"... either almost all the way on or off... if it's way HIGH, this might be saturating the triode.

And, here's where it gets tricky- the pentode sets up its own bias, due to its transconductance and plate resistance, and the cathode and anode resistors. If you change the tube to one with DIFFERENT transconductance and plate resistance (and IIRC, the 7199 has a transconductance of around 7500 and a plate resistance of around .4 megaohm, while the 6U8 has a transconductance of around 5000 and plate resistance of around .4 megaohm- quite a difference), then it'll bias up differently on the same plate and cathode resistors, at the same B+ voltage... the 6U8 probably won't "turn on" to as much current, and therefore the plate voltage will be higher... possibly, throwing off the bias of the triode following...

Now, this can be dealt with... but, it'll require juggling the plate and cathode resistors of the pentode side, to make it bias up where the 7199 biases up. Trick is, to keep about the same gain... IIRC, there's a tutorial on pentode gain over on TubeCad... that should get you through how to calculate that... you should be able to get a combo of plate and cathode resistors to satisfy both conditions...

Otherwise, you need to just set up the pentode/triode with a coupling cap... which means you'll need to generate a reference bias voltage for the 6U8. If you want to do this, you can probably just use the values for the TRIODE side of the 6U8 from the Stromberg-Carlson ASR-333 or ASR-433... starting at the coupling cap between the pentode and triode sections, through to the plate and cathode resistor setup on the triode section. If you do this, it won't matter much where the pentode biases up... as long as it's at a reasonable plate voltage (somewhere between 55-100v on the plate)...

Regards,
Gordon.
 
kvflyer said:
The cap isolates the DC on the plate from the grid but couples the AC (signal). I have not looked at the schematic so I am only shooting from the hip. If I am way off, I apologize, it is late tonight!
Click to expand...

Yep, he may want to ADD a cap (and a method to re-bias the triode), to do just this. I think he's saying his amp does NOT have a cap...

Regards,
Gordon.
 
Hey, I just thought of something else... Jay, before going through all that's required to get a coupling cap in there, you might want to try changing the screen resistor on the pentode... to get the idle current (and therefore, the plate voltage) back where you need it... that might solve the bias problem?

Nice thing about pentodes... there's more than one way into that sucker... :D

Regards,
Gordon.
 
Well folks its official this thing sounds great!:music::D:music:
I picked it up from Jay last night and fired her up today.
It is only hooked up to two small bookshelves right now but DAMN.
I am born again tube!!:yes:
Well here she is in all her glory rescued from my buddys basement store room.
 

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hide the speakers

Hide the lil speakers and everyone will think it is coming out of the cabinet below it!

Seriously though, that amp looks Great!

Man, I hope there are some AK tube heads in Albuquerque.

Dan
 
I was mighty impressed by the sound of that one. Every now and then, the commercial guys hit great combinations, this is one for sure.:yes: You did a nice job on it too Brian.:thmbsp:
 
:banana: Glad to hear you guys fixed her up! :banana:

Have some fun with it and let us all know how you like it once you break it in! :thmbsp:
 
Thanks guys NOW its time for a new project maybe the small console its sitting on its tube, phono only Decca The Hudson IM HOOKED:yes:
 
:DI took some more pics with her top off
 

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Man, that looks awesome! What (brand) are the caps you guys used? Did you need to do anything with the PS besides caps?
 
Fran604g said:
Man, that looks awesome! What (brand) are the caps you guys used? Did you need to do anything with the PS besides caps?
Click to expand...

We used orange drops everywhere except the phono section where we used K42Y PIOs. Also, solen fast caps on the signal "in" to the pentode section of the 7199s. Power supply is stock, but strange as heck, with a 5k 20watt resistor going from B+ straight to ground.:no: Lots of room for mods there, in the future maybe.:yes:
 
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