5cg8 triode/pentode amp ?

allsavy · 02-13-2009, 11:14 AM

I have an interesting combination 9 pin noval 5CG8 tube that combines a medium mu triode (mu=40) with a sharp cutoff pentode (2w max). Max plate voltage=200 and has 4.7v heater.

Might make an interesting little guitar amp.

Can anyone sketchout a basic (the simplest possible) circuit?
thanks.

Also, since the pentode only puts out 1.5 to 2w might it be useful in a guitar amp preamp to get something like driven pentode type dist as opposed to the usual triode type?

tubeswell · 02-13-2009, 06:40 PM

If you had a pair you could do a PP with another dual triode.

As it is you might think about using that tube as the driver-output of a low-powered SE amp with maybe an EF86 voltage amplifier stage (or a dual triode in front instead for a gainy style low powered SE lead guitar amp)

As to its tube characteristics? You'll need to find a chart. There is a circuit for an EF86 with an ECL86 (similar sort of thing) here, which you could use as guide, and being careful to set the voltages of your 5CG8 appropriately.

http://www.r-type.org/static/ecl86.htm

Or use it as an all-in-one reverb driver and recovery tube?

allsavy · 02-13-2009, 09:04 PM

A gainy SE lead amp is what I have in mind. Short of putting a dual triode in front of the 5cg8 I thought it might be interesting to have a "one tube amp" and found the attached circuit. Not sure if it would have enough gain tho.

Going back to the the other idea regarding pentode overdrive....Right now I'm using my tube driven reverb circuit as a parallel switchable boost but I'm not so fond of the overdriven triode sound. Might the 5cg8 be used instead? (redesign the circuit to suit)... would the small pentode overdrive sound any different than the reverb circuit's parallel connected 12ax7 triode
(considering we are talking about the reverb driver section and not the recovery tube 1/2 12ax7)? I'm thinking the pentode section would need more driving it than the 40mu single triode contained in the 5cg8.

allsavy · 02-13-2009, 09:07 PM

oh... here is the schem foa a single combination tube amp.

tubeswell · 02-14-2009, 01:13 AM

Build it like that, but if the bias is too hot for your tube, then put a bigger cathode resistor on the pentode (470R or 1K) - or try building it with a 1k cathode resistor and if that sounds too sterile, then go to a 820R, 680R, 470R and so on until it sounds good when dimed without redplating

02-13-2009, 11:14 AM	#1
allsavy Member Join Date: Dec 2008 Posts: 55	5cg8 triode/pentode amp ? I have an interesting combination 9 pin noval 5CG8 tube that combines a medium mu triode (mu=40) with a sharp cutoff pentode (2w max). Max plate voltage=200 and has 4.7v heater. Might make an interesting little guitar amp. Can anyone sketchout a basic (the simplest possible) circuit? thanks. Also, since the pentode only puts out 1.5 to 2w might it be useful in a guitar amp preamp to get something like driven pentode type dist as opposed to the usual triode type?

02-13-2009, 06:40 PM	#2
tubeswell Supporting Member Join Date: Feb 2007 Location: Wellington NZ Posts: 2,302	If you had a pair you could do a PP with another dual triode. As it is you might think about using that tube as the driver-output of a low-powered SE amp with maybe an EF86 voltage amplifier stage (or a dual triode in front instead for a gainy style low powered SE lead guitar amp) As to its tube characteristics? You'll need to find a chart. There is a circuit for an EF86 with an ECL86 (similar sort of thing) here, which you could use as guide, and being careful to set the voltages of your 5CG8 appropriately. http://www.r-type.org/static/ecl86.htm Or use it as an all-in-one reverb driver and recovery tube? __________________ Building a better world (one tube amp at a time) Last edited by tubeswell; 02-13-2009 at 06:54 PM.

02-14-2009, 01:13 AM	#5
tubeswell Supporting Member Join Date: Feb 2007 Location: Wellington NZ Posts: 2,302	Build it like that, but if the bias is too hot for your tube, then put a bigger cathode resistor on the pentode (470R or 1K) - or try building it with a 1k cathode resistor and if that sounds too sterile, then go to a 820R, 680R, 470R and so on until it sounds good when dimed without redplating __________________ Building a better world (one tube amp at a time)

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02-13-2009, 09:04 PM	#3
allsavy Member Join Date: Dec 2008 Posts: 55	A gainy SE lead amp is what I have in mind. Short of putting a dual triode in front of the 5cg8 I thought it might be interesting to have a "one tube amp" and found the attached circuit. Not sure if it would have enough gain tho. Going back to the the other idea regarding pentode overdrive....Right now I'm using my tube driven reverb circuit as a parallel switchable boost but I'm not so fond of the overdriven triode sound. Might the 5cg8 be used instead? (redesign the circuit to suit)... would the small pentode overdrive sound any different than the reverb circuit's parallel connected 12ax7 triode (considering we are talking about the reverb driver section and not the recovery tube 1/2 12ax7)? I'm thinking the pentode section would need more driving it than the 40mu single triode contained in the 5cg8.

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