Proven Design for Cathode Bias EL34 PP

[rexindigo]

Hi Everybody,
is there a working design out there for EL34 PP amp with Cathode Bias?
I liked the compressed sound of my 18watt clone very much and got some more EL34 lying around.....
I've heard of Trainwreck amp, but could not find a schematic on the web.
Any links to a schematic?

Thanks in Advance!

[Vaughan]

Hi Rexindigo

If you go to the following link you can download a pdf file with the TrainWreck Schematic and loads of variations.
http://blueguitar.org/schems.htm#Trainwreck

Hope this helps

Vaughan

[Wakculloch]

You could try this idosyncratic wee jobbie:-
http://www.vintagehofner.co.uk/selme...atic/zod3.html

[vaia10]

The Trainwreck Express is fixed bias. Look for a schematic for Bad Cat's Hot Cat or Matchless Chieftain (schematic heaven has both of them).

[Robert M. Martinelli]

The Trainwreck Express is fixed bias. Look for a schematic for Bad Cat's Hot Cat or Matchless Chieftain (schematic heaven has both of them).

Seems to me like there' s a little confusion about the terms "fixed", "variable", "cathode", "automatic" "self" bias.

"Fixed" only means that the bias cannot be adjusted - so it can be referred either to a cathode bias ( obtained by means of a fixed resistor between cathodes and GND ) or to a non-adjustable external bias network ( a reverse polarized diode plus a filter cap with no potentiometer, so the voltage is fixed, and the cathodes are at GND potential ).

"cathode", "automatic" and "self" bias are all synonyms, meaning the cathode is lifted from GND by some amount, using the cathode current to create a drop across a fixed resistor, thus making the control grid negative with respect to the cathode without the use of an external negative power supply ( hence the names "self" "cathode" and "automatic" bias - the cathode resistor acts like the second leg of a voltage divider, the tube being the voltage divider's first leg ).

Cathode biasing output tubes is practically applicable to tubes up to the size of EL84s/6BQ5, which need a bias voltage around 10-12 VDC. It becomes inefficient and power-consuming with tubes like EL34s or 6L6s, because the required voltage drop rises in the 35 to 45 VDC range, thus needing a big power resistor which will dissipate a lot of power, with the unwanted side effect of lowering output power, because Vak gets lower ( with a +B of, say, 350 VDC and a cathode bias drop of 40VDC it's like the plates are supplied at 305 VDC ).

To complement this, if you take a look at all of the trainwreck variations, they all use an adjustable SEPARATE bias network, and the output tubes' cathodes are connected to GND, so those aren't neither "fixed" nor "cathode biased" ( "self" or "automatic" if you like ) designs.

Hope this helps

Best regards

Bob

[MWJB]

"Fixed" only means that the bias cannot be adjusted - " Nonsense. Fixed bias can be adjustable, like an AB763 Fender or non-adjustable like an original 5F6A Fender. The presence of a fixed/predetermined negative voltage to the power tube control grids and grounded cathodes identify it as such.

Bias can ALWAYS be adjusted...try a different grade or brand of tubes! Practicality can sometimes decide whether you adjust the circuit or change the tube.

Whether or not you cathode bias EL34/6L6/6550/KT88 is purely a matter of preference. Most push pull EL84 amp currently built are fixed bias. Yes, you get a smaller voltage drop with EL84, but typically EL84s run less voltage anyway...it's a moot point.

"To complement this, if you take a look at all of the trainwreck variations, they all use an adjustable SEPARATE bias network, and the output tubes' cathodes are connected to GND, so those aren't neither "fixed" nor "cathode biased" ( "self" or "automatic" if you like ) designs." - What you have just described is "adjustable fixed bias".

Fixed & cathode methodologies can be combined as on the AC568 Fenders.

[Robert M. Martinelli]

"Fixed" only means that the bias cannot be adjusted - " Nonsense. Fixed bias can be adjustable, like an AB763 Fender or non-adjustable like an original 5F6A Fender. The presence of a fixed/predetermined negative voltage to the power tube control grids and grounded cathodes identify it as such.

Bias can ALWAYS be adjusted...try a different grade or brand of tubes! Practicality can sometimes decide whether you adjust the circuit or change the tube.

Whether or not you cathode bias EL34/6L6/6550/KT88 is purely a matter of preference. Most push pull EL84 amp currently built are fixed bias. Yes, you get a smaller voltage drop with EL84, but typically EL84s run less voltage anyway...it's a moot point.

"To complement this, if you take a look at all of the trainwreck variations, they all use an adjustable SEPARATE bias network, and the output tubes' cathodes are connected to GND, so those aren't neither "fixed" nor "cathode biased" ( "self" or "automatic" if you like ) designs." - What you have just described is "adjustable fixed bias".

Fixed & cathode methodologies can be combined as on the AC568 Fenders.

Hi MWJB,
Maybe, due to my poor English, I didn't manage to make things clearer, which was my sole intention, but to me saying "fixed bias can be adjustable" like you say is "more nonsense" than saying that "fixed bias is fixed"

What I wanted to point out is that when bias ( adjustable or not ) is obtained by external means ( a separate negative supply ) then the bias is a "non automatic", "non self", "non cathode" bias - I would refer to it as "external bias" to highlight it is not achieved with a cathode resistor, as IMHO this is much clearer than saying "fixed adjustable" - a thing either is fixed or it is adjustable, can't be both at the same time.

The fact that bias changes with different tubes is due to their different internal characteristics, but if your PS biases at -40VDC fixed (=non adjustable ) , that will still be 40VDC, what will be changing is the tube quiescent current. This is the reason why alot of people tweak their amps adding an ajustable biasing network.

The tube will influence both bias current AND bias voltage only in a self-bias arrangement, because the voltage drop across the cathode resistor will vary ( as I said, in this case the tube is the first leg of a voltage divider ).

So....I would propose to come to standardize some bias technical terms :

Self-bias = formerly a.k.a. cathode bias or automatic bias. Refers to a bias obtained by means of cathode resistor(s) ( or diode(s) ) whose voltage drop makes the grid negative with respect to the cathode.

External bias=Refers to a negative control grid bias that is coming from an external source ( usually PT tap--->reverse biased diode---> filter cap, with or without an adjustment pot )

Grid-leak bias=Refers to a negative grid bias obtained by electrons leaking from the grid via a grid-leak resistor.

Adjustable=something that can be EASILY adjusted, in our case by means of a potentiometer.

Fixed=Something fixed by design, that can ONLY be altered CHANGING components values ( =NOT EASILY ).

Best regards

Bob

[rexindigo]

Hello friends,
Thanks for your helpful informations, (@vaia10 i think the hot cat schematic is perfect to start my next project with.
(@voxrules! I don't mind If I loose some power by cathode biasing, for most occasions 50watts are more than enough and 35watts or so will do just as well.
Thanx again to this forum, allways very helpful postings here!

[MWJB]

Hi Bob,

The stumbling block here is your interpretation of the term "fixed" - rather than pertaining to a fixed circuit/voltage it really means what you refer to as "external bias", whether adjustable or not (cathode bias can easily be made adjustable with a switch to bring in/out parallel resistors or a high wattage rheostat). "Adjustable" & "non-adjustable" bear NO relation whatsoever to bias methodology (whether fixed or cathode, or a combination of both).

"a thing either is fixed or it is adjustable, can't be both at the same time" Yes it can.

The terminology already exists - "Cathode bias" is a universally recognised term, some folks call it 'self biasing', but I HATE that term, if the tube was "self biasing" it would never be under or overbiased - cathode bias can result in either.

Fixed circuits are used by manufacturers primarily to save money & warranty claims (& sometimes to steer customers towards buying the manufacturer's branded tubes). Any amp tweaker/small scale builder would build an adjustable "fixed bias" (negative grid, grounded cathode) supply for convenience.

"The tube will influence both bias current AND bias voltage only in a self-bias arrangement, because the voltage drop across the cathode resistor will vary ( as I said, in this case the tube is the first leg of a voltage divider )." Not true, you may see 20-30v difference on the B+ when subbing tubes in a fixed bias amp (more if looking at extremes of current), as the B+ rises & falls so will negative bias supply voltage. A bigger load on any secondary will cause a fluctuation on the other secondaries/taps. FWIW I only ever use the -ve voltage method with tubes out of the amp, to ensure that when ungraded tubes are installed, that you are reasonably in the 'ball park' when setting plate current in mA.

Cheers, Mark.

[Robert M. Martinelli]

Hi Bob,

The stumbling block here is your interpretation of the term "fixed" - rather than pertaining to a fixed circuit/voltage it really means what you refer to as "external bias", whether adjustable or not (cathode bias can easily be made adjustable with a switch to bring in/out parallel resistors or a high wattage rheostat). "Adjustable" & "non-adjustable" bear NO relation whatsoever to bias methodology (whether fixed or cathode, or a combination of both).

Hi Mark,
I agree, in fact in one of my previous posts I stated that cathode bias can be made adjustable, but that' s not cheap and practical to do because you would need a high wattage pot - As to the "adjustable" and "not adjustable" "querelle" ( oh my God how I love French ), in the terminology I proposed I intentionally separated them, to show they indeed bear no relation whatsoever to bias methodology, as you correctly noted.

All in all I think it' s a matter of different terms to say similar things, and in this respect my rather poor English actually is "the" stumbling block ( OTH I think it' s getting better by the minute ), I really thought "fixed" meant "fixed". - I probably need to learn English better to appreciate its subtleties.

Cheers

Bob

[MWJB]

Hi Bob,

Your English is as good as mine and a million times better than my Italian!

All the best, Mark.