Teeco conversion

[angelodp]

Ok let me try again. I did have some errors that were pointed out and have corrected them. I could tear this out and build a 5C3, but I kinda like what I am hearing at this stage. I am simply asking if you can see
a way to get the tone-stack to a more responsive place. I have tried a variety of stacks and come back to this as it is the only one that gets the tone I like. Its a bit like a raw amp with only volume. And NFB which is cool.


I hope you can work with me as I do not have the engineering background of many here.

[Latole]

I don't know about the tone stack, I did not look at it.

What I see is there is no heater center tap, build one with two 100 ohms resistors

[angelodp]

Hi Heaters are elevated to cathode. Thanks

[sluckey]

Bottom half of 6SC7 grid needs a resistor. I don't think your PI schematic is correct. I'm betting it's a paraphase inverter but it looks a bit tangled up. I doubt the amp is actually wired that way.

[angelodp]

Sluckey, thanks for your eyes on this. I omitted to show the 1M on that bottom 6SC7. I have looked at the PI and it is wired this way. Would posting pics help clarify. Appreciate your help.


Ange

[angelodp]

Here are some pics of the PI area

[angelodp]

Chassis shot

[angelodp]

Is it a "floating paraphase" per Valve Wizard

[bmccowan]

On your tone stack question - you state that you have tried a variety - please share what you have tried as there is little point in suggesting things that you have already tried.

[angelodp]

I tried a standard TBM Fender stack, a TB stack also Fender.

[angelodp]

Layout - BTW this amp plays and it has a great blues breakup tone even at 3 on the volume. Great is subjective, but I like it.

[bmccowan]

I could easily be wrong - happens all the time - but I'm not sure that your bass control is doing much? Anyways, lots of documented tone stacks to try - Brownface Fender stacks are good TB stacks and there is Baxandall/Jones. I have liked a standard Fender style tone control combined with a Framus mid control for some of my PA conversions. All are documented on the Forum, Rob Robinette's site and multiple other places.

[yorgle]

Hi Heaters are elevated to cathode. Thanks

I'm curious: is this method better, worse, or just different than connecting an artificial center tap to the cathode, e.g., x and y each connect through a 100R resistor to the cathode pin? 

[bmccowan]

I'll throw in my two cents on the elevated heater question, which is all experience - no theory. All three methods work: CT, elevated CT, and artificial CT. None perform hugely differently, and the small differences seem to depend on the circuit. I have only used an artificial CT if there is no CT lead present. Sometimes if I move the CT from chassis ground to power tube cathode there is a slight decrease, or increase in hum. But usually I hear no difference. I have worked on a lot of old Gibson amps and almost all have elevated heaters. Sometimes I forget and ground the CT - no difference to my ears.
Valve Wizard website has a piece on this.

[PRR]

...small differences seem to depend on the circuit. ...

And the specific tubes. Some leak one way, some another way, always more or less every time you dip into the carton. On our weekends we can swap-away troubles. Build a million amps and you like to know that even the less-good tubes will play OK for the buyer.

It should be clean DC. It should be several volts. Positive or negative may work different but rarely enough to be worth the considerable trouble. If most cathodes are near-ground there's no reason to elevate more than 50V-70V, and 20V (power cathode) is often as good as any other (and free). If you have long-tales or cathodynes or cathode followers or wacky-stages you may need to elevate 100V or more.

[yorgle]

More specifically, I’m curious about the one leg of the heater circuit being connected to the cathode as compared to two resistors converging at the cathode.  It’s not really a center tap. 

[bmccowan]

More specifically, I’m curious about the one leg of the heater circuit being connected to the cathode as compared to two resistors converging at the cathode.  It’s not really a center tap.

I have never done that, but looking at the schematic again, it seems to me that it is a variation of grounding one leg of the heater circuit, sending 6.3Vac through one leg, but now with the DC elevation. Grounding one leg was commonly done in early guitar/hi-fi/PA amps (tweed era) and later on in some smaller amps. Typically more hum is produced by the single leg carrying the heater current. Others may know how much elevating the heaters in that configuration helps reduce hum, but my opinion would be that its still better to center tap and send balanced current to the heaters.

[PRR]

...curious about the one leg of the heater circuit being connected to the cathode...

Oh. Why would you do that?

One leg of heater returned *through chassis* saves a few pennies and was common in old rig, especially radios where 4 of 5 stages are not hum-sensitive. But if you are going to run both AC wires, CT is much better.