/13 bias

[Blind Lemon]

Hey guys

Aint been in a long time, got kind of out of touch. Need some help, was out the other night and got to talking amps with a guy that has a Divided by 13 FRT or FTR 37 and he wanted help biasing it. I said I could help ........well....... its got some kind of cathode/fixed bias set up.

Can anyone point me to some reading on this? I'd like to know how it works and help this guy out at the same time.

TIA

BL

[kagliostro]

The only schematic I've find seems to show a cathode biased amp

http://4.bp.blogspot.com/-I4lAq4UjgEk/T-DZCkyjrYI/AAAAAAAABiE/WWtJcmSkSOY/s1600/CeeJayEleven.jpg



K

[stratele52]

Blind Lemon ,

You don't kneed schematic to bias an amp . Just some knowledge to connect bias probe or voltmeter to power tube Cathode and Plate. Just looking the wiring of the amp at the power tube to see if it is a cathode Bias ( class A ) or grid bias ( fixed bias or class Ab ) . And  this tell what to do with knowing the specs of outputs tubes
This work if it is a point to point circuit , not so easy with printed circuit board

You probably don't have enough knowledge , be careful there is lethal voltage in amp.

[Ed_Chambley]

I am not familiar with 13 amps, but either the cathode on the power tubes leads to a resistor if cathode bias.  If it is switchable, then you will have negative voltage on one of the power tube pins according to the pin out of the power tubes.  Take a DC voltage reading of the power tube pins and post your findings along with the tube type.  6V6, 6L6 or whatever.  This will determine the next step.

[kagliostro]

well....... its got some kind of cathode/fixed bias set up

assuming that it is so

or there is a switchable set between the two options (seen in some amps)


or, may also be, there is a simultaneous cathode + fixed bias arrangement, not frequent but possible

K

[tubenit]

Hmmm,  ............. IF one took a Princeton Reverb and :

- remove reverb and tremolo
- add LarMar PPIMV
- change fixed biased to cathode biased
- remove NFB
- remove 3.3M/10p & instead put 47k resistor to ground

& then changed some of the component values to get more headroom ..................  you'd have a pretty similar amp.  Note cathode resistor and cap values



http://www.el34world.com/charts/Schematics/files/fender/princeton_rev_gz34_aa1164.pdf

With respect,  Tubenit

[Geezer]

http://www.dividedby13.com/ftr37.html

Apparently it is switchable, listed as "class AB1/A" operation.

Is there a "class" switch or something similar on the back panel?

You could call or email Fred, the builder....I understand he's a pretty nice guy.

949-631-1330
info@dividedby13.com

[kagliostro]

A mix of Fender Blackface Deluxe AB763 and the Fender Tweed Deluxe 5E3 circuit

http://revolutiondeux.blogspot.it/2012/06/divided-by-thirteen-cj-11-amplifier.html

K

[tubenit]

This could be an interesting PR conversion .................. along the same lines.

with respect, Tubenit

[Blind Lemon]

Sorry Guys, I thought I made it clear what model the amp was in the first post. No one seems to have any experience with it, I got it.

BL

Steve you still making that thing on the tower go round and round?

[topbrent]

I found these hand drawn schematics of a /13 FTR37 on another forum. 
Not mine, but should help you out a bit.

[sluckey]

Steve you still making that thing on the tower go round and round?

Blind baby, I'm done. I belong to OPM now. No tech evals, TPRs, Peabody, safety stand downs, online bullshit courses, etc. Oh, and no sequester either! Just a check on the first of each month. The hours are great. I'm lovin' it! How much longer for you?

[kagliostro]

@ Topbrent

Thanks for sharing your finding

@ Steve

many people would like to be  where you are now   

here in Italy, perhaps a little 'more than anywhere else 

Ciao 

Franco

[Blind Lemon]

TopBrent - Thanks for the Schem.

Steve ; Got 9.5 as an FLM to go and not sure I'll make it.  I'm happy you made it.

BL

[HotBluePlates]

... a guy that has a Divided by 13 FRT or FTR 37 and he wanted help biasing it. ... its got some kind of cathode/fixed bias set up.

Can anyone point me to some reading on this? I'd like to know how it works and help this guy out at the same time.

Let's assume TopBrent's schematic is correct. Each output tube has a cathode resistor, but each tube also has a bias supply connected to the grids.

Do you see the 270Ω cathode resistors for each output tube in the amp you're looking in? Since the output tubes are 6V6's, and each tube has its own cathode resistor, you'd normally expect those to be 500Ω resistors or higher.

So the output tubes get about half their bias by way of the cathode resistors, and need the other half of the bias voltage from the fixed bias supply. I'm guessing the intent is to have a cathode-biased amp yet still have adjustable bias when swapping output tubes. Therefore, bias them for idle current as you would with any 6V6 amp.

Does it make a difference in sound to have a little bit of both bias methods? I dunno. Maybe a little, but I'd expect that the sonic contribution of the cathode resistors to be minimized somewhat by the bypass caps. That said, I defer to those who've actually tried fixed/cathode bias in one amp, or this type of mix (as I haven't been inclined to try it myself). I know LooseChange had said flipping from fixed- to cathode-bias in his Standard didn't cause a sonic change, when the tubes were biased to the same point with both methods.

[alerich]

The Marshall Studio 15 I used to own had a mix of cathode and fixed bias like this. You measure the voltage at idle across the cathode resistor and do the math to figure the current and then calculate dissipation with the plate voltage. If it is too cold you adjust the fixed bias control and remeasure. Lather, rinse, repeat. I never could figure out why they did it that way (surely there is some benefit to warrant the extra cost) but setting the bias is pretty straight forward.

[HotBluePlates]

The Marshall Studio 15 I used to own had a mix of cathode and fixed bias like this. ... I never could figure out why they did it that way (surely there is some benefit to warrant the extra cost) ...

I don't really know.

But, assume for a moment it's not for some sonic benefit. Cathode bias is inherently safer than fixed bias. If tube current tries to run away, the increased voltage across the cathode resistor means increased bias which reduces tube current.

Fixed bias has no self-correcting mechanism, and with leaky caps or an accidental short, the fixed bias will cheerfully let the tubes draw current until a fuse, tube or OT pops (maybe all three).

Anyway, I don't know if the intent is to have "adjustable cathode bias" or "current-limiting fixed bias". You'd have to ask the designer.

[kagliostro]

@ HotBluePlates

Do you see the 270Ω cathode resistors for each output tube in the amp you're looking in? Since the output tubes are 6V6's, and each tube has its own cathode resistor, you'd normally expect those to be 500Ω resistors or higher.

there are 4 output tubes and I see a 270R resistor for each pair of tubes, no for each tube

K