6L6GC 470ohm screen resistor?

[kingswayguitar]

Been searching the internet and talking to local amp techs (even barflys). No good answers. I have a 77/78 Bassman Ten with ultralinear output. Can someone explain what the 470ohm resistor does between the OT and the screen grid?
thanks

[Chuck H]

They limit current. Tha's my guess. I don't know all the operating conditions in that amp. It may be that the screens tend to overdissapate at voltage unless current is limited. In other words, leave them in there.

[Enzo]

Probably also serve to lower the screen voltage a bit, to get them under the plates. Tapped off the OT, they would otherwise be a little higher than the plates?

[kingswayguitar]

thanks guys. other amps, whether ultralinear or not, often have a screen resistor. never really thought about it but it must limit current. fwiw my bassman runs plates and screens at 500-520V...lower end is the screens, upper end is plates. will certainly leave them in for now. just looking to replace them and wondering how critical they be matched.

[Chuck H]

If you run the amp at a lower volume than it's potential, and typically like the sound your getting, match them. This would help the power amp to be a clean "reference" amplifier for what's happening in the preamp. If you run the amp overdriven I would be inclined to say 'don't bother matching them' because push pull differential is often a key component in good power amp distortion.

[kingswayguitar]

thanks Chuck. and i found my link to the circuit's schematic http://www.el34world.com/charts/Sche...MAN_10_75w.pdf

[pdf64]

According to a thread on another forum http://www.tdpri.com/forum/amp-centr...er-reverb.html Fender UL OTs are tapped at 20% (for max power) rather than 43% (for lowest distortion).
The 470 ohm resistors are very necessary for limiting screen grid current (and therefore screen grid dissipation) which seems to rise exponentially with power output. The very stiff, very high VB+, combined with UL operation, seems to exaserbate screen grid dissipation.
Pete

[jrfrond]

They limit screen grid current, and also lower the voltage potential of the screen grid a bit. They also, to a small degree, affect power output.

The jury seems to be mostly out on this, but the "traditional" screen grid resistor values from the old days just don't work with new issue tubes, for which screen grid failures are rampant. I now DOUBLE the traditional values, and have realized extended tube life with no audible or palpable "feel" differences, across the board, unless the customer is using NOS tubes, which, I am sorry to say, were MUCH better at handling higher SG current and transients.

[kingswayguitar]

According to a thread on another forum Why not a UL Fender Super Reverb? - Telecaster Guitar Forum Fender UL OTs are tapped at 20% (for max power) rather than 43% (for lowest distortion).
The 470 ohm resistors are very necessary for limiting screen grid current (and therefore screen grid dissipation) which seems to rise exponentially with power output. The very stiff, very high VB+, combined with UL operation, seems to exaserbate screen grid dissipation.
Pete

thanks
excellent link at tdpri

[kingswayguitar]

They limit screen grid current, and also lower the voltage potential of the screen grid a bit. They also, to a small degree, affect power output.

The jury seems to be mostly out on this, but the "traditional" screen grid resistor values from the old days just don't work with new issue tubes, for which screen grid failures are rampant. I now DOUBLE the traditional values, and have realized extended tube life with no audible or palpable "feel" differences, across the board, unless the customer is using NOS tubes, which, I am sorry to say, were MUCH better at handling higher SG current and transients.

EL34world.com also suggests 2W or 3W versus the original 1W to protect against overheating. But if I'm correct, a higher wattage resistor only protects the resistor and not the screed grid itself, correct?

[Chuck H]

John is talking about value, not rating. He is suggesting a value of 1k for your application.

[kingswayguitar]

John is talking about value, not rating. He is suggesting a value of 1k for your application.

10-4
and thanks

[pdf64]

Increasing the value to 1k may entail the wattage needing to be increased (from the 1 watt spec) also (or may not).
In amps such as this with a completely unfused B+, I see a benefit in retaining the screen grid resistor power rating at a 'sacrificial' value, so that in the event of some tube failure modes, the screen grid resistor blows open circuit, thereby protecting the PT and OT from fault currents of extended duration.
Whatever, a resistor type that won't promote fire in the case of its overheating should be used, not carbon, but rather eg metal oxide with a flame redardant coating.
Pete