Tube rectifier and solid-state rectifier

[paisley_tele]

Is the a way to put in a tube rectifier and a soils state and be able to have a switch to choose either option? It's for 4 6l6s two rock build. What tube would work?

[talbany]

One of the problems in doing this is when you switch from one to the other is your changing the B+ in turn shifting the bias....The other problem is high AC on a switch...Pop Snap..

Tony

[greiswig]

My Bad Cat Hot Cat 30 had such a switch. Might look for schematics of that.

[talbany]

FWIW..If memory serves the Hot Cat is a cathode biased EL34 amp .

BTW..I see they advertise that amp as a Class A amp so how can this be if they add such a switch...Just Sayin..

Tony

[Paul Fawcett]

FWIW..If memory serves the Hot Cat is a cathode biased EL84 amp (Voxish).

BTW..I see they advertise that amp as a Class A amp so how can this be if they add such a switch...Just Sayin..

Tony

Huh? Why would the presence of such a switch imply in any way that it's not a class A amp?

As for how to deal with the change in bias voltage, take a look over at the Metroamp forum or wiki to see how you can wire a multipole switch to handle low/high B+ and change the bias at the same time. In that case, it's changing the transformer taps, but you could use the same idea with tube vs. SS recto. Or if you're slightly more ambitious, you can easily make a three transistor circuit (differential amp controlling a pass element) that automatically scales the bias to the B+

[paisley_tele]

Can u send me some info on the three transistor circuit.

[talbany]

? Why would the presence of such a switch imply in any way that it's not a class A amp?Huh

Because changing between SS and tube rectifier can alter plate voltages..(Depending on Tube up to 30v).. Plate voltage is a critical equation in determining class A load line..
In the case of the Bad Cat the plate voltages are way to high to be running true class A anyway.. So it's rather a moot point..
Naturally if you can find a method to change from SS to tube without changing plate voltages..Problem solved!!...Perhaps switching in a bank of 5w zeners on the CT
EL-34's run in class A recommended from Mullard..

Amplification factor: 11W
Anode voltage: 265V
Anode current: 100mA
Screen voltage: 250V
Bias voltage: -13,5V
Anode resistance: 2k Ohms

Hot Cat looks to be a Hot biased AB amplifier

Tony

[greiswig]

FWIW..If memory serves the Hot Cat is a cathode biased EL84 amp (Voxish).

BTW..I see they advertise that amp as a Class A amp so how can this be if they add such a switch...Just Sayin..

Tony

I think it was a bit of marketing hype on their part, and they're not the only company to advertise class A when technically they are not. Although more truthful, one might expect to not sell more amps if one said "Class A-ish...really closer to AB."

[talbany]

FWIW..If memory serves the Hot Cat is a cathode biased EL84 amp (Voxish).

BTW..I see they advertise that amp as a Class A amp so how can this be if they add such a switch...Just Sayin..

Tony

I think it was a bit of marketing hype on their part, and they're not the only company to advertise class A when technically they are not. Although more truthful, one might expect to not sell more amps if one said "Class A-ish...really closer to AB." :wink:

g-
Yeah just about all the guitar amps advertised as class A really are not..(including AC-30)True class A voltages are to low to really sound good for what we want from them

All The Best!!

Tony

[wjdunham]

Is the a way to put in a tube rectifier and a soils state and be able to have a switch to choose either option? It's for 4 6l6s two rock build. What tube would work?

The Fender Prosonic had such a setup, it can switch between cathode bias with tube rectifier, tube rectifier with fixed bias, and SS rectifier with fixed bias. The switching is fairly involved but it works well. I never got any good sounds out of that thing so I converted it to a D-style amp. Really didn't sound so good to me in anything but the fixed bias SS mode after the conversion, but at least it had one mode that sounded good Anyway, if you want to see how they did it schematics are available on line.
Bill

[Structo]

Here is the way the Prosonic does it.

[selloutrr]

you can always lower the voltage coming off the SS diodes before they tap back into the rest of the circuit so they match (or closer to) the tube rec. or vise versa.

[John_P_WI]

IMHO, trying to balance the voltages between the SS and tube rectifier negate one aspect of the tube rectifier.... the 30 volt drop or so of the tube rectifier darkens the tone due to the lower preamp voltages. Both the voltage drop and sag can easily be achieved in other ways without the tube recto.

[ChrisM]

IMHO, trying to balance the voltages between the SS and tube rectifier negate one aspect of the tube rectifier.... the 30 volt drop or so of the tube rectifier darkens the tone due to the lower preamp voltages. Both the voltage drop and sag can easily be achieved in other ways without the tube recto.

+1

[Paul Fawcett]

Can u send me some info on the three transistor circuit.

Hi PT,

Sorry for the slow response, I don't peek in here very often. You can find an "in context" implementation of a bias tracker in the schematic of an amp I've been working on here: http://www.mediafire.com/?8ar4l4xcsidb861

It's a complicated schematic, but the bias tracker is off by itself in the bottom right hand corner. It's a very minor modification of a circuit from Merlin Blencowe's excellent book on amplifier power supply design. http://stores.lulu.com/valve_wizard

The circuit works well. If you implement this, mind the power rating of the 300K resistors. I use 2W parts in this position.

EDIT: On reflection, I'm not sure such a circuit would be suitable in your particular circumstance. Its operation depends on measuring a difference between a fixed voltage (pre VVR DC node) and a variable voltage (scaled screen voltage). However, in your case both the plate and screen voltages will increase about the same amount in absolute terms, so the ratio between the two will only vary by a small amount. At the very least you would need to modify the response to take this into account.