Cathode biased amps

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bfitz
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Cathode biased amps

Post by bfitz » Thu Nov 30, 2017 2:13 pm

Recently I had a test drive of a Groove Tubes Soul-O-50 watt amp in one of the local shops in Dublin.

The proprietor of the shop tells me that they can take any type of tube which makes it sound like a really versatile piece of kit.

Then recently on one of the andertons videos or that pedal show (cant remember) but Mick Taylor starts talking about cathode biased amps

and how they are capable of running with any type of tubes as long as they have the same pin config i.e. fat pins á la EL34, 6L6 or even 6V6

my question is does anybody know how true this is or is there only a grain of truth in this statement. I read the manual for the GT Soul-O-50

and it says it can take any type of tube in the EL34 family of tubes. Similarly I thought our ol' Rocker 30 was a cathode biased amp and due to#

this I knew it could take a set of EL34's or an equivalent (6CA7/KT66) and work but by the token of the above, is it then possible to run 6V6 in the R30

and get a decent tone? or would the amp have to be re-configured to have a 6V6 voicing etc etc?
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Jondog
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Re: Cathode biased amps

Post by Jondog » Thu Nov 30, 2017 3:03 pm

They can only take the tubes they are designed for. An amp with EL34’s could not really use 6V6 interchangebly. Could use 6CA7/KT66 because they are in the same family electrically. Different plate currents and voltages, transformer impedance all factor in to what tubes an amp can handle.
As for the Rocker 30, mine is currently running 6V6’s. However, I changed the bias and installed a proper Output Transformer for those tubes. Plate voltages in the Rocker are fine for modern 6V6 tubes.
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Rlw59
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Re: Cathode biased amps

Post by Rlw59 » Thu Nov 30, 2017 3:28 pm

If Mick Taylor was talking about a specific cathode biased amp, he might be correct about that particular amp design. It wouldn't be optimal with all octal base tubes, but it's possible to design one that would function with a wide variety of tube families.

If Taylor said all cathode biased amps are like that, he's simply wrong.

fiveightandten
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Re: Cathode biased amps

Post by fiveightandten » Fri Dec 01, 2017 1:59 pm

There are 3 main things to consider when taking about interchanging tube types:
1) Bias range
2) Plate / Screen grid voltages
3) Impedance load

Bias range - depending on how the amp is set up, a cathode biased amp *may* be able to take a different tube type without overworking it. But swapping tubes like this should only be done by someone who really knows what they're doing, and has the ability to swap out the cathode resistor(s) to proper values if the tubes aren't running within spec.

There are amps like the THD Univalve and Bivalve, which do safely run many different tube types. But those amps were designed for it. Typically, with an amp like the Rocker 30, for instance, the amp is designed with a certain tube type in mind. You can't just swap in a different type and expect it to run right.

Plate and screen grid voltages - These can vary a LOT with octal based amps. There are EL-34 or 6550 based amps that run well over 500V, and you shouldn't throw a 6V6 in an amp like this (a few like the JJ and EH may survive though). You have to make sure the voltages in the amp are within spec for the tube that you want to use.

Impedance load - Every tube compliment has a specific impedance load associated with it. The output transformer windings are designed around this impedance load. The OT is designed with the correct ratio to mate the 4, 8, or 16 ohm speaker load to the 2K, 4K (or whatever) output impedance of the tubes. When you change tube types, you change output section impedance. This can wind up putting stress on the output transformer and/or the tubes themselves. Will the amp run reliably? It all depends on the amp and the tubes you'll be running with it.


So, there are a lot of factors that are involved here. Some changes are more dangerous than others. But I'd say that swapping around to different tube types should only be done by people that understand it. And *most* cathode biased amps certainly can't just have the tubes swapped without any consideration for the above issues.

Cathode biasing adjusts itself over a small range. Even tubes of the same type and manufacturer will run hotter or cooler in the same amp. The existing cathode resistor in the amp is not a fit-all bias range for every tube type with the same number of pins. Unfortunately, it doesn't work like that.
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